短寿命化学種の赤外ダイオードレーザー分光研究

九州大学学術情報リポジトリ

Kyushu University Institutional Repository

短寿命化学種の赤外ダイオードレーザー分光研究

住吉, 吉英

九州大学理学研究科化学専攻

https://doi.org/10.11501/3081161

出版情報：Kyushu University, 1994, 博士（理学）, 課程博士 バージョン：

権利関係：

)

Infrared Diode Laser Spectroscopy of Transient Molecules

A Thesis Presented

by

YOSHIHIRO SUMIYOSHI

to

Department of Chernistry Faculty of Science Kyushu University

January, 1995

Acknowledg1nent

The author would like to express his sincere gratitude to Professor Takehiko Tanaka and Professor Keiichi Tanaka for their guidance and encouragement throughout the course of this work. He wishes to thank Dr. Takashi Imajo for his technical advice on electronics and programming, and all the members of Quantum Chemistry Laboratory for their friendly discussions and experimental help.

He is grateful to Professor Eizi Hirota and Dr. Misaki Okunishi of Institute for Molecular Science for making their apparatus available in the course of the diode laser spectroscopic study of the v3 band of the SiH3 radical.

11

1-1

1-2

2-1

Introduction References

Contents

Chapter 1.

Introduction

Diode Laser Spectrometer References

Tables and Figures

Chapter 2.

2 8 10

14 15

Infrared Diode Laser Spectroscopy of Transeint Molecules Generated in Discharge Plastnas.

Infrared Diode Laser Spectroscopy of the v3 Band of

the SiH3 Radical. 20

Abstract 21

2-1-1 Introduction 22

2-1-2 Experimental 25

2-1-3 Observed Spectrum and Analysis 27

2-1-4 Discussion 34

2-1-5 Application to Plasma Processes 38

References 40

Tables and Figures 42

111

2-2

3-1

2-2-1 2-2-2 2-2-3 2-2-4

Infrared Diode Laser Spectroscopy of the GeF Radical in the X2Il112 State.

Abstract Introduction Experimental

Observed Spectrum and Analysis Discussion

References

Tables and Figures

Chapter 3.

Infrared Diode Laser Spectroscopy of Transient Molecules Generated by UV Laser Photolysis.

Infrared Diode Laser Spectroscopy of the v5 Band of H2CCN.

Abstract 3-1-1 Introduction 3-1-2 Experimental

3-1-3 Observed Spectrum and Analysis 3-1-4 Discussion

References

Tables and Figures

lV

57 58 59 60 61 66 68 69

75 76 77 79 82 88 93 95

3-2 Infrared Diode Laser Spectroscopy of the Propargyl Radical Produced in Supersonic Jet Expansion by

UV Laser Photolysis.

Abstract 3-2-1 Introduction 3-2-2 _Apparatus 3-2-3 Experimental 3-2-4 Observations 3-2-5 _Analysis 3-2-6 Discussion

References

Tables and Figures

Appendices.

Appendix a Hund's Coupling Cases

Appendix b Rotational Energy for an Oblate Symmetric Top Molecule

v

111 112 113 116 119 120 124 126 131 133

150

153

Chapter 1.

Introduction

1-1 Introduction.

2

During the course of a chemical reaction, intermediate transient species with

"incomplete" chemical bonds are often generated. These transient species are generally called

"free radicals" which are characterized by short life time. To elucidate chemical reactions, structural information on these intermediates must be quantitatively studied. High resolution spectroscopy with sufficient resolution for observing the rotational structure, such as microwave and infrared laser spectroscopies, is a direct method for this purpose, because the analysis of the rotational structure yields diagonal components of inertia tensor which directly includes information on the molecular structure.

Microwave spectroscopy of transient molecules started in 1950s with the detection of the OH radical by Dousmanis et al. [1]. They observed transitions between A-type doublets of the OH radical. Because of difficulty in detection of low energy photons, a few successful results were only reported in the initial stage of the research [2,3]. About twenty years later instruments were drastically improved and microwave spectroscopy was extended to shorter wavelength regions such as millimeter and submillimeter regions. Because the absorption coefficient of a rotational transition is proportional to the cube of the frequency, sensitive detection became possible, and microwave spectroscopy has been actively applied to the detection of transient molecules. The recent results of the free radical study by microwave spectroscopy were summarized in a review paper by Hirota [ 4].

Basically microwave spectroscopy can not be applied to molecules with no permanent dipole moments. On the other hand infrared spectroscopy which monitors rovibrational transitions is free from the restriction. The first important contribution of infrared spectroscopy to the free radical study is the work of Milligan and Jacox using the matrix isolation method [5]. In the matrix isolation method, discharge products were trapped in a low-temperature matrix and the infrared absorption spectra of the transient species surviving in the cold matrix were observed. They also produced free radicals by photolysis in which a precursor trapped in a low-temperature matrix was irradiated by the Lyman-a line. The

3

results of matrix-isolation studies of transient molecules were com piled by Jacox

[ 6, 7].

Rotational structures can not be observed because of the interaction with the matrix.

Therefore, only qualitative information has been obtained on the structure of the transient species from the isotopic shifts of vibrational frequencies. It is nesscesarry to assess the matrix effect by comparing the matrix data with those in the gaseous phase, because the frequencies obtained in low-temperature matrices are subject to the matrix effect. The data obtained by the matrix isolation method are incomplete as structural information on free radicals in the gas phase, although they are very useful for observing gas phase infrared spectra of transient molecules.

Perhaps the most serious intrinsic limitation on gas phase high resolution spectroscopy in the infrared region is of a practical nature, i.e., lack of light sources. In

1968

_{Evenson et al.}

developed a spectroscopic method called laser magnetic resonance

(LMR)

to observe paramagnetic molecules

[8]

in the far-infrared region (FIR). The first application of

FIR LMR

was to the 02 molecule, where the HCN laser was employed as a light source. Gradually, the method was extended to the infrared region, where the C02 and N20 lasers at 10 )lm and the CO laser at S)lm were used as light sources. These lasers have high output power and excellent mode quality, although they oscillate at fixed frequencies and can not be continuously tuned. In order to observe transient molecules, the molecule must be tuned into resonance with the fixed frequency laser. The laser systems modified to provide tunable radiation were reported

[9

�

13].

The technique realized very sensitive detection because of a very stable light source and it is suitable for the high resolution spectroscopic study of free radicals

[14,16].

The main difficulty in the laser n1agnetic resonance method is in the assignment of the complex spectra because of the limited wavelength coverage. Because of this difficulty, infrared laser magnetic resonance spectroscopy has been replaced by tunable laser spectroscopy such as diode laser spectroscopy.

4

Three types of tunable laser sources have been developed around the late 1970s in the

infrared region, namely the diode laser, the color-center laser and the difference frequency laser. They have been prosperously applied to the high resolution spectroscopy of free radicals and the observed radicals are listed in previous articles [ 4, 16, 17]. The color-center laser covered the wavelength region shorter than about 3.5 _�m [18]. Therefore, fundamental band of high frequency hydride (CH, NH, OH) vibrations and weaker overtones of lower frequency vibrations are only accessible. For example, NH2 spectra have been detected by infrared kinetic spectroscopy with a color center laser [ 19]. The difference frequency laser is available in the infrared region of 2,.., 5.5 �m by mixing the outputs of a dye laser and an Ar+

ion laser in a nonlinear crystal [17]. The linewidth of the infrared radiation is mainly determined by the jitter of the Ar+ ion laser of about 50 MHz. The observed radicals with a difference frequency laser are listed in Ref.16. For example, Oka detected the v2 band of the H3 ion by the source modulation technique [20]. Amano ^et al. observed the V3 band (C-H degenerate stretch) of the CH3 radical with the Zeeman modulation technique [21].

The diode laser has attracted much attention in high resolution molecular spectroscopy in the mid-infrared region because of its narrow linewidth (15MHz), ease of tuning the oscillation frequency and wide coverage of the infrared region. Actually, most high resolution spectroscopic studies of free radicals in the infrared region have been performed by diode laser spectroscopy [ 4, 1₆, 17]. Semiconductor diode laser sources provide tunable infrared radiation from 360 _to 3000 cm-1 [22]. A diode oscillates in the region of typically 100 _cm-1 with the coverage of about 30o/o, therefore several laser diodes must be prepared for the study of a new radical. There have been steady improvement of the device in recent years. For example, the application of molecular beam epitaxial growth techniques allowed the production of diodes that could be operated above the liquid nitrogen temperature 77K. In fact, the improvement brought about superiority of the diode laser source to other laser light sources in the infrared region in terms of coverage, output power, linewidth, and mode

5

quality. Therefore, diode laser spectroscopy will be widely used for absorption spectroscopy

of transient molecules.

Fourier transform infrared (FTIR) spectroscopy has been applied to high resolution spectroscopy of stable molecules as summarized in a review paper [23]. Convenient advantages of FTIR, including wide spectral coverage and accurate wavenumber scale, will ensure its survival. For example, the Bruker HR 120 and Bomem DA3/DA8 spectrometers cover from the far infrared to ultraviolet region. They provide very high resolution, for example, the Bomem machine has a maximum optical path difference of 2.5 m, which corresponds to 0.002 cm-1 resolution [24]. However, for spectroscopy of transient molecules, resolution is usually second to sensitivity in importance. For example, Carlisle ^et al. [25]

demonstrated quantum noise-limited frequency modulation spectroscopy with a sensitivity (.1I/I) of l0-7 for diode laser spectroscopy. On the other hand, .1I/1 of 10-3 is typical for FTIR absorption spectroscopy. The restriction of sensitivity must be overcome for the application of FTIR to high resolution spectroscopy of free radicals. The recent efforts were summarized in Ref.16.

The author is interested in the study of intermediate species in chemical reactions such as free radicals and ions and has been studying the structure of free radicals by infrared diode laser spectroscopy. The advantage of diode laser spectroscopy for detection of transient species over other spectroscopic methods in the infrared region has already been mentioned.

In this thesis, the author describes the study of high resolution spectroscopy of transient molecules. In this study, abundant information of transient molecules about structure, electronic state and reactivity was obtained.

Chapter 2 consists of two sections; Section 2-1 is devoted to the study of the SiH3 radical and Section 2-2 to the study of the GeF radical. These radicals play important roles in chemical vapor deposition (CVD) processes. The SiH3 radical is also a very attractive radical

6

for chemists because the bending motion of the SiH3 radical in the electronic ground state is governed by a double minimum potential like that in NH3.

Chapter 3 also consists of two sections; Section 3-1 is devoted to the study of the cyanomethyl radical and Section 3-2, which is the highlight of this thesis, to the study of the propargyl radical. These radicals are very interesting as interstellar molecules. In section 3-2 the propargyl radical produced by UV laser photolysis in a supersonic jet is described. From the comparison with the study under the room temperature, rotational temperature of the propargyl radical was determined to be 16 K. The production and the cooling process of propargyl are discussed in Section 3-2.

7

References

[1] G.C.Dousmanis, T.M.Sanders, and C.H.Townes, Phys. Rev. 100 (1955) 1753.

[2] F.X.Powell and D.R.Lide, J.Chem.Phys. 41 (1964) 1413.

[3] M.Winnewisser, K.V.L.N.Sastry, R.L.Cook, and W. Gordy, J.Chem.Phys. 41 (1964) 1687.

[4] E.Hirota, Chern. Rev. 92 (1992) 141.

[5] D.E.Millian and M.E.Jacox, J.Chem.Phys. 47 (1967) 5167.

[6] M.E. Jacox, J.Phys.Chem.Ref.Data, 13 (1984) 945.

[7] M.E. Jacox, J.Phys.Chem.Ref.Data, 17 ( 1988) 269.

[8] K.M.Evenson, H.P.Broida, J.S.Wells, R. J.Mahler, and M.Mizushima, Phys. Rev. Lett. 21 (1968) 1038.

[9] R.S.McDowell, "In Vibrational Spectra and Structure" ed. J. R. Duuring, Amsterdam: Elsevier, 10 (1980) 1.

[10] A. Van Lerberghe, S.Avrillier, and C.J.Borde, IEEE J.Quantum Electron.

QE-14 (1976) 481.

[11] G. Magerl and E.Bonek, J.Appl.Phys. 47 (1976) 4901.

[12] G.Magerl, W.Schupita, and E.Bonek, IEEE J.Quantum Electron. QE-18 (1982) 1214.

[13] C.K.N.Patel and E.D.Shaw, Phys.Rev.Lett. 24 (1970) 451.

[14] P.B.Davies, J.Phys.Chem. 85 (1981) 2599.

[15] G.W.Hill, Magn.Reson. Rev. 9 (1984) 15.

[16] P.F. Bernath, Annu.Rev.Phys.Chem. 41 (1990) 91.

[17] E.Hirota, "High-Resolution Spectroscopy of Transient Molecules. ",Springer­

Verlag, Berlin/New York, 1985.

[18] L.F.Mollenauer, Methods Exp. Phys. 15^-p^{a r}t B (1979) 1.

8

[19] _{H.Adams, J.L.Hall,} L.A.Russell, J.V.V.Kasper, F.K.Tittel, and R.F.Curl, J.Opt.Soc.Am. B2 (1985) 776.

[20] _T.Oka, Phys.Rev.Lett. 45 (1980) 531.

(21] T.Amano, P.F.Bernath, C. Yamada, and E.Hirota, J.Chem.Phys. 77 (1982) 5284.

[22] J.C.Hill and G.P.Montgomery, Appl.Opt. 15 (1976) 748.

(23] A.G.Robitte and J.L.Duncan, Annu.Rev.Phys.Chem. 34 (1983) 245.

[24] M.Birk and J.W.Brault, Microchim. Acta. 2 (1988) 243.

9

1-2 The Diode Laser Spectrometer.

10

In this section the set up of the diode laser spectrometer adapted to the present high resolution spectroscopic study of transient molecules is described [ 1 ,2]. Figure 1 illustrates

the block diagram of the spectrometer. The spectrometer consists of three parts, which are the laser light source, the production system, and the detection system. For the production system, three types of production techniques,i.e., generation in discharge plasma, UV laser photolysis, and UV laser photolysis in supersonic jet, were used to generate transient species.

These production techniques are described in detail in the following chapters.

The diode laser emits radiation corresponding to the energy gap between the conduction and valence bands, and the frequency is principally determined by the current across the junction. A higher current leads to a higher frequency of radiation. The gain profile is quite wide and a several cavity modes can be radiated. For this reason, a monochromator with resolution of about 0.2 cm-1 need to be placed in the beam path to select one of the cavity modes. The range of the diode laser may be extended somewhat by changing the temperature of the crystal, because this operation causes the change in the band gap and the index of refraction of the material. Usually, a higher temperature causes a higher radiation frequency.

Although current is eventually transformed into heat, current control is much more convenient in fine tuning and modulating the oscillation frequency. The typical output power is about 0.1 m W for multimode operation and the frequency jitter is about 10 MHz.

The laser diodes used in this study are of Spectra Physics model SP5600 type.

Oscillation of the laser diodes is controlled by model SP5820 laser current module and model SP5720 cryogenic temperature stabilizer. Laser diodes currently available for spectroscopic study in this laboratory are listed in table 1. Performance characteristics of each diode are also listed. The diodes are mounted on the cold finger with a heater coil. The temperature of the mounting surface is monitored by a Si diode which provides a feedback signal to the laser current module. Up to four laser diodes can be mounted at a time in the cold head. The laser radiation is taken out through a KRS5 window mounted on the front surface of the cold head.

11

The laser beam after passing through the KRSS window is shown in Fig. 1. Since the diode laser beam is radiated from a limited area on a facet of the diode, it is very divergent.

Therefore, the laser beam is collimated into a parallel beam of about 12mm diameter by a parabola of 3.175 em focal length. As shown in Fig. 1 a 50 em monochromator (model CT- 50) is used to select a single mode from among others. The laser beam is then split into three beams by two ZnSe plates (They are denoted by ^I BS ^I in Fig.l ). The transmittance of the ZnSe plate is about 70%. The beam selected by the first ZnSe plate is introduced into the reference cell. The reference cell is filled with a reference gas whose rovibrational transitions have already been measured by high-resolution spectroscopy as FTIR. Table 2 lists typical reference molecules used in this spectroscopic study[3 ^� 6]. The beam selected by the second plate is passed through a vacuum spaced etalon such as the one shown in Fig. 2. It is composed by two Ge menisci which are fitted to the both ends of a 25cm quartz tube to form a conforcal resonator. As shown in Fig.2, incident laser beam is partially reflected in the order of 1 � 2 � 3 � 4 and then interfered with the unreflected beam. The free spectral range of the etalon was calibrated to be 0.009979 ± 0.000003 cm-1 using the lines of OCS [3] as standard. The rovibrational spectrum of the reference gas and interference fringes of the etalon are used for wavenumber calibration. The diode is easily modulated by simply superimposing a small amplitude sinusoidal wave on the injection current. The reference spectrum and etalon fringes are both detected by phase sensitive detectors synchronized with the laser source modulation signal. A phase sensitive detector (Brookdeal Co. model 411) is used for detecting the etalon signal and a single phase lock-in amplifier (NF Electronic Instruments Co. model 5600) is used for the detection of the reference spectrum. The beam is often reflected back to the source diode, resulting in instability of the laser output power.

Sometimes, the sensitivity attained by source frequency modulation is limited by the instability of the laser power.

12

The third beam which passes through the two ZnSe plates is introduced into an absorption cell for the detection of transient species produced by discharge or UV laser photolysis. The beam is detected by a HgCdTe detector which is used at a very low temperature of 77K (New England Research Center, Inc). The detector made from semiconductor has a very low noise equivalent power (NEP) of about

10-10 _W, therefore the sensitivity of this system is limited by the laser power fluctuation rather than the detector noise. Absorption cells used for the present spectroscopic studies of transient molecules which were produced in discharge plasmas or by UV laser photolysis will be described in detail in the following chapters.

In the case of experiments using discharge plasma, the signal from the preamplifier is detected by the phase sensitive detection technique which is synchronized with the frequency of the ac high voltage for discharge. A lock-in-amplifier (Princeton Applied Research Co.

model 5209) is used for detecting the absorption spectrum of transient molecules.

In this spectroscopic system, the power fluctuation due to the vibration of the cold head caused by the action of a helium compressor frequently disturbs sensitive detection and high resolution, and this problem must be solved partly by optimizing the beam alignment and partly by using high frequency modulation, so that the low frequency vibration can be easily separated.

13

References

[1] _K.Tanaka, Y.Akiyama, and T.Tanaka, J. Mol. Spectrosc. 137 (1989) 55

[2] Y.Akiyama, K.Tanaka, and T.Tanaka, Chern. Phys. Lett. 155 (1989) 15 [3] G.Guelachvili and K.Narahari Rao, "Handbook of Infrared Standards,"

Academic Press, New York, 1986.

[ 4] A.R.W.McKellar, private communication.

[5] G.Guelachvili, private communication.

[6] J.Chagalas, J.Pliva, A. Valentin, and L.Henrry, J. Mol. Spectrosc. 110 (1985) 326.

[7] N.Hunt, S.C.Foster, J.W.C.Johns, and A.R.W.Mckellar, "High Resolution Spectroscopy of 16 Bands of OCS in the 1975- 2140 cm-1 Region for Diode Laser Cilibration." Herzberg Institute of Astrophysics. National Reserch Council of Canada, Ottawa, Ontario, Canada K 1 _{A OR6}

14

Tablel Laser Diodes

Model Tuning Range/cm-1 Power/mW

L5615-600 600 � 670 0.16

L5615-660 650 � 710 0.11

L5615-740 740 � 820 0.27

L5615-850 758 � 880 0.62

SP5615-860 820 � 870 0.16

L5615-930 � 990 0.46

SP5615-1000 970 � 1090 0.12

SP5616-1120 1120 � 1135 2.0

L5615-1200 1200 � 1215 0.86

SP5615-1250 � 1310 0.19

SP5615-1450 1325 � 1410 0.79

L5622-1550 1550 � 1565 0.25

SP5616-1670 1670 � 1685 4.5

L5622-1740 1740 � 1755 2.4

SP5622-1850 1850 � 1865 0.36

L5622-1950 1990 � 2080 0.15

�102-42-74 � 2000

L5622-2000 2000 � 2200 0.20

L5622-2100 2100 � 2115 0.16

L5622-2240 2182 � 2248 0.15

L5622-2360 2360 � 2375 0.26

15

Table2

Reference Gases Used as Wavelength Standards Molecule

H20 N20

C02

NH3 CD3Fa CH3Cla

C3l4a ocs

co

a See reference [ 4 ^� 6].

b See reference [7].

16

Wavenumber/em-!

400 ^� 500 1200 ^� 2094

540 ^� 630 1120 ^� 1343 1831 ^� 2830 590 ^� 725 2236 ^� 2388

718 ^� 1194 890 ^� 1230 960 ^� 1600 1920 ^� 1979

823 ^� 889 1975 ^� 2140b

1940 ^� 2264

REF. CELL

,.._.

-...}

� I ;J � ___!L___A__

�

COMPUTER

Fig. 1 Block diagram of a diode laser spectroscopic system

i--' 00

D.L.

IN

�---·

I I I I

�

R

^emeniscus =250

I ---� _I

25 em

OUT

t

Quartz tube spacer

Fig.2 Vacuum spaced etalon

Capter 2.

Infrared Diode Laser Spectroscopy of Transient Molecules Generated in Discharge Plasmas

19

2-1

INFRARED DIODE LASER SPECTROSCOPY OF THE V3 BAND OF THE SiH3 RADICAL.

20

Abstract

The SiH3 radical was investigated by high resolution infrared diode laser absorption spectroscopy. The radical was generated by the discharge of phenyl silane diluted with argon in a multi-pass absorption cell. The spectrum was recorded by the discharge modulation technique. About seventy absorption lines in the 4.5 IJ.m region were unambiguously identified as belonging to the V3 (Si-H degenerate stretch) vibrational band of SiH3, and molecular constants in the V3 state were first determined.

Observed lines are useful for monitoring the SiH3 radical in silane plasmas.

21

2-1-1. Introduction

The SiH3 radical is an oblate symmetric top molecule which belongs to point group c3v at the equilibrium structure and has four normal vibrational modes including two degenerate modes as listed in Table 1. One of the most interesting properties of the SiH3 radical for the spectroscopic study is that an inversion motion is feasible like in NH3, that is, the two equivalent potential minima are separated by a surmountable barrier. As a result, each vibrational level, which is single in the limit of unsurmountable barrier, splits into two components of the inversion doubling. These components are designated by "s"

and "a", which mean symmetry and antisymmetry, respectively, of the vibration­

inversion wavefunction with respect to the operation £* of inversion of all particles (nuclei and electrons) through the center of mass of the molecule. Of the two components, the "s" component is normally lower in energy than the "a" component.

Yamada and Hirota first observed the rovibrational spectrum of the v2 band of SiH3

by the infrared diode laser spectroscopic method rll. They observed a pair of parallel bands of an oblate symmetric top molecule and determined their band origins as 721.0468 _and 727.9438 cm-1 as shown in Table 1. These two bands were ascribed to two components of the inversion doubling, 1 + f- o- and 1-f-Q+, where + and - mean the same as "s" and "a", respectively, and the numbers are quantum numbers of the v 2

vibration. These two bands may be also described as the sf- a and a f- s components of the v2 band. They derived a double minimum potential function for the inversion motion by using the band origins determined for 1 + f- o- and 1-f- Q+ and assuming the following functional form,

V(z) ⁼ kz2j2 + 1:1£/2 - ( 1:1£2 + 4a2z2) , (1)

22

where flE was fixed to 50000 cm-1. The inversion coordinate is denoted by z, k i the unperturbed quadratic force constant, and a is a parameter. The barrier height at the planar configuration was calculated to be 1868 cm-1 and the a-s splittings of the ground and v2 state were calculated as 0.1444 _and 6.751 cm-1, respectively. Johnson III et al.

performed multiphoton ionization spectroscopy of SiH3 [2]. By assuming a quartic potential, they calculated the barrier height to be 1935 _cm-1. Other works of the diode laser spectroscopic studies on the v2 band of the SiH3 radical are quoted in Section 2-1-5.

High resolution spectrum of SiH3 in the wavelength region corresponding to the Si­

H stretching bands has not been reported. Milligan and Jacox observed the products by the VUV light photolysis of silane in argon matrix [3]. They assigned infrared absorptions observed at 1955 _and 1999 cm-1 to the v 1 and v3 bands of the SiH3 radical.

An ab initio calculation of vibrational frequencies and intensities of infrared transitions for all vibrational modes was reported for the ground electronic state of the SiH3 radical by Allen and Schaefer [4], _{and the VI} (symmetric stretch) and the V3 (degenerate stretch) band origins are calculated to be 2150 _and 2180 cm-1, respectively.

The calculated values are listed in Table 1. According to the calculation, the V3 band is the strongest among the four fundamental bands, the transition intensity of the v3 band being about three times stronger than that of the v2 band and about thirty times stronger than that of the VI band. Calculated transition intensities are also listed in Table 1. _They concluded in their report that the 1955 _and 1999 cm-1 absorptions observed by Milligan and Jacox [3] are not due to the SiH3 radical. But it was not known which result is correct until the present study was performed.

According to the ab initio calculation [ 4] it would be very hard to observe the V3 band in spite of its strong intensity under the condition in which SiH4 is present, because

23

the transition frequency of the V3 band is very close to that of SiH4 which is usually used

as precursor. In the present experiment we generated the SiH3 radical by the glow discharge of phenyl silane in order to escape the interference of absorption by SiH4 and succeeded in observing the V3 band of the SiH3 radical by infrared diode laser spectroscopy for the first time. The band origin of the v3 band and the molecular constants in the v3 state were first determined.

24

2-1-2. Experimental

The absorption cell used for the present measurement is shown in Fig. 3. _{It was} 100 em long and had an inner diameter of 5 em in its central 85 em part, in which stainless­

steel short cylindrical electrodes for discharge were installed near the both ends. Each electrode was 5 em long and had an inner diameter of 4.5 em. The separation between the two electrodes was 65 em. Ac high voltage of about 50 Hz generated by a 1.4 kW amplifier and stepped up by a transformer (1 :30) was applied between the electrodes.

Glow discharge occurred twice in each cycle of the ac voltage, in alternating directions, and the signal was detected by a phase sensitive detector (PSD) operated at twice the frequency of the ac high voltage. The technique of ac discharge combined with phase sensitive detection for low noise detection is known as the discharge modulation technique.

White-type multireflection optical system which consisted of three concave mirrors of 1000 mm radius of curvature was set up in the absorption cell. The laser beam traveled ten round trips, yielding an effective path length of about 20m.

A solenoid coil was wound outside of the cell for the observation of Zeeman effect due to unpaired electrons. Hence, paramagnetic species can be easily distinguished from diamagnetic species. The coil consisted of resin-coated copper wire of 2x3 mm cross section wound in six layers around an 80 em long fiber reinforced plastic tube with a diameter of 66 mm. It was immersed in an oil tank cooled by circulating water. Magnetic field can be applied up to about 600 G.

The absorption cell was evacuated through a 1 inch i.d. port (denoted by "PUMP"

in Fig. 3) by a Roots blower (SHINKO SEIKI, SMB-200) backed up by a rotary pump (TOKUDA, RP-600Z) .

25

The absorption lines of the SiH3 radical in the 700 � 800 cm-1 region have been observed by the discharge of Sif4 [1]. _{In the} 4.5 11-m region, however, it is very hard to

distinguish the absorption lines of SiH3 from those of SiH4. When phenyl silane was used as parent gas, the interference of absorption by SiH4 was almost perfectly suppressed.

Therefore the SiH3 radical was generated by the discharge of phenyl silane (0.1 _Torr) mixed with argon (0.3 Torr) in the present study. The optimum ac frequency for discharge was 50 Hz. Discharge current of 100 rnA peak to peak gave the best signal to noise ratio. Increase in the discharge frequency resulted in loss of the signal intensity, presumably because of a rather long lifetime of SiH3 which is estimated to be about 10 msec.

26

2-1-3. Observed Spectrum and Analysis

Searches in the region near 2080 ^� 2250 em-1 yielded a number of characteristic doublet lines with a separation of around 0.04 em ^-1^. Observed doublet signals are visualized in Fig. 4 as a Loomis-Wood diagram. In this figure, each doublet is indicated by a stick placed at the average wavenumber of the doublet components. Doublet signals in the region 2200 � 2250 cm-1 were found to be organized as several groups, each of which consisted of signals repeated almost regularly with an interval of about 9.5 cm-1.

In Fig. 4, the sticks marked with open circles each correspond to the starting member of a group. These signals were assigned to 'R-branch transitions in the fundamental band of the Si-H degenerate stretching vibration (v3), and the groups correspond to the K- subbands. Figure 5 shows a typical trace of the observed lines recorded by the discharge modulation technique. The components of the doublet signal were assigned to the s � s and a� a components of the 'R3(5) transitions. The s � s and a � a transitions are illustrated in Fig. 2. The spectrum of N20 (top) and etalon fringes (bottom) were used for wavenumber calibration. Spin-rotation splitting was not observed in 'R-branch lines.

Figure 6 shows 'R9(10) and 'R2(7) transitions observed around 2247 cm-1 . These lines are also split into s � s and a� a components. The peak heights of 'R9(10) and 'R2(7) were observed to have the ratio 3.7: 1. When the absorption intensity is weak such as in the present case, the intensity of the absorption line is proportional to the line strength factor and the Boltzmann factor. We must also take into account the spin statistical weight, which is twice as large for 'R9(10) as for rR2(7). Assuming a rotational temperature of 300 K, the ratio thus calculated is 3.3: 1. The observed intensity ratio is slightly larger than the calculated, however, considering that the laser power in the region of 'R9(10) was slightly larger than in the region of rR2(7), the observed and calculated

27

ratios are in good agreement and this result confirms the present assignment. Several Pp_

branch transitions in the region of 2160 cm-1 repeated with a separation of about 10 cm-1 and they were assigned as matched with the R branch.

Q-branch transitions consisted of four lines, which were ascribed as a four components caused by the inversion doubling and spin-rotation doubling. Figure 7 shows the 'Q3(6) transition observed in the 2169.20 cm-1 region.

Zeeman effect could only be observed in Q-branch lines. Figure 8 shows the 'Q3(6) transition under magnetic field of 0 G (upper trace) and 600 G (lower trace). The absorption line width became clearly broader when magnetic field was applied.

Table 2 lists the observed wavenumbers of the signals identified as belonging to the v3 fundamental band of the SiH3 radical, but for Q-branch transitions, averaged wavenumbers of spin-rotation doubling components were listed. The observed wavenumbers of the Q-branch lines including the spin-rotation splittings are listed in Table 4.

The observed wavenumbers in Table 2 were subjected to a least-squares fitting. In the analysis, two components of the inversion doubling were separately fitted. Spin­

rotation splittings were neglected in the analysis. The analysis of the spin-rotation splitting will be described later.

In the case that the electronic angular momentum L is completely quenched in the electronic state considered, Hund's coupling case (b) (see Appendix b) is the most appropriate for the expression of rovibrational spectra, namely the electronic spin angular momentum S is decoupled from the molecular axis and the rotational angular momentum N is well preserved. In this scheme, N is combined with S to give the total angular momentum];

28

1= _N +S. (2)

In any case 1 is well preserved. The overall base function is written as

INKS J M1 > (3)

where K corresponds to the molecule-fixed components of Nand M1 corresponds to the space-fixed components of 1. The rotational energy W v(N, K) in a vibrational state is obtained from the expectation value of the rotational Hamiltonian H r of an oblate symmetric top molecule (see Appendix b).

Wv(N, K) = <N K S J M1 IHr INKS J M1 > (4)

The following energy level expressions are obtained;

Wo(N,K)=BoN· (N + l)-(B0-C0)K2

- DNr}/2 (N +If-DNKoN· (N + l)K2 - DKoK4 + HNoN3 (N + 1)3

+ HNNKoN2 (N + 1)2K2

+ HNKKoN (N + l)K4 + H KoK 6 (5)

for the ground state, and

29

+ 17 N 3N (N

+ 1)

K L

+

17

K

3 K 3!

- DNyV2

(N

+1)2- DNK� (N + l)K2

+ HNNK3N2 (N

+1)2K2

+ HNKK3N (N +

l)K4 +

H

K3K6 (6)

for the V3 excited state. In equation

(6),

we added v3, which is the vibrational energy of the V3 excited state. The terms of-2C(3Kl+ 17N� (N +l)KL + 17K3K3L in the v3 excited state represent first- and third-order corrections of the Coriolis term. The molecular constants in equations

(5)

and

(6)

are defined in Appendix b. The difference W3(N+l, K+l,l)- Wo(N,

K)

corresponds to the wavenumber for the transition

rRK(N), W3(N-1 , K-1,-1)-

Wo(N,

K)

forPPK(N) andW3(N,

K+l,l)-

Wo(N,

K)

forrQK(N).

The spectroscopic constants optimized by the least-squares fitting are listed in Table 3, where L1B ⁼ B3 - Bo, etc. The constants fixed to zero are omitted. The observed

wavenumbers in Table 2 are reproduced by these constants within several thousandths of cm-1. The standard deviations of the fit are 0.0049 and 0.0055 cm-1 for the s� s and a � a components, respectively. The corresponding uncertainties in the constants are given in parentheses.

From the classical view of magnetic interaction between the unpaired electron spin magnetic moment and the magnetic field caused by the molecular rotation, the operator for the spin-rotation splitting is proportional to a scalar product of

NS

. This operator is rewritten by using equation

(2)

as follows,

NS

⁼

(12 - N2 - S2 )/2 (7)

30

For an oblate symmetric top molecule, the effective spin-rotation Hamiltonian is written

as,

HsR =Ebb(NxSx +NySy) + EccNzSz

� EbbNS- (Ebb- Ecc) N zS z (8)

where N ^{25 z} denotes the product of molecular axis components of N and S. Ebb and Ecc are the effective spin-rotation interaction constants defined in equation (8). _These constants include the second order contribution of the spin-orbit term AsoLS ^through electronic Coriolis interaction as the most important contribution. Matrix elements of the operators appearing in equation (8) are calculated using the wave function (3) as a basis function and they are expressed as follows,

<.N K S 1 ^M1 INS INKS 1 ^{M1 > =} [1(1+ 1)-N(N+ 1) -3/4]/2 (9)

<.N K S 1 ^M1 INzSz INKS 1 ^{M1 > =} [1(1+1)- N(N+l) -3/4]K2/{2N(N+1)} (10)

From equations (9) _� (11) it was realized that matrix elements exist between the basis functions with the same 1 value. The fine structure components caused by spin-rotation splitting are illustrated in the following scheme. The pairs of fine structure components which are combined by the operator N ²⁵ z are indicated by arrows.

31

J

N+1

N+312 FI

N+l/2 _F2

~

N+l/2 FI

N ^{.. :::}

·-.

N-112 F2

=

N-1/2 F1

..

N-1

N-312 _F2

The fine structure components F1 and F2 are defined such that J = N+S for F1 and J =N -5. for F2 (S = 112). Matrix elements of the spin-rotation Hamiltonian H SR calculated using equations (9) � (1 1) are written as follows;

<NKSJM1 IHsR INKSJM1 >

=-21[1( 1+1)-N(N+1) -l]xfEhb-( Ebb-Ecc) K2 ] (12)

4 _- N(N+ 1)

for diagonal terms and

<N K S 1 M1 IHsR IN-1 K S J M1 >

K(N2-K2) 112

=( Ebb- Ecc) 2N (1 3)

for off-diagonal terms. In this analysis only diagonal contributions of equation ( 12) were considered. Spin-rotation splitting of a rotational level is obtained by taking the difference of the matrix element expressed by equation ( 12) between the F1 and F2

32

components. The spin-rotation splitting .dEsR(N,K) of the rotational state of INKS J M1 >

is expressed as follows,

(14)

Spin-rotation splittings of the Q-branch lines are listed in Table 4. In the analysis ^Ecc was

fixed to zero because it is too small to explicitly determine. The values of the spin­

rotation constant Ebb was determined to be -0.036 (4) cm-1, which agrees well with the reported value of -0.036(1) cm-1 [1].

33

2-1-4. Discussion

The band origins for the sf- s and a f- a components of the inversion doubling are 2185.218 and 2185.266 cm-1, respectively. These values are very close to the calculated value of 2180 cm-1 by Allen and Schaefer III [4]. The observed values 1955 and 1999 cm-1 by Milligan and Jacox [3] are too low to be ascribed to the SiH3 radical even if matrix shifts are considered, therefore it is concluded that the absorptions at 1955 and 1999 cm-1 are not due to the SiH3 radical.

The term expressed by equation (b-15) in Appendix b has matrix elements between v1 first excited state and Y3 first excited state. The interaction is ascribed to the Coriolis coupling coefficients

�{,3x

^and

��.3y·

In the Y3 state, where l =±^l, the K = ±1, l = ±1 levels are degenerate to the first and third order, as equation (6) indicates, but they are split in the second order by the !J.K =±2, !J.l =±2 matrix elements (the so-called 2, 2 interaction), resulting in a doublet referred to as the l-type doublet. The splitting is given by

!J.E/hc ⁼ ql(l+ 1) (15)

where q is the l-type doubling constant. The rRo(N) transition may deviate from equation (6) because of the l-type doubling. In the present experiment, rRo(N) transitions could not be observed, therefore q could not be determined.

Figure 9 shows probable interactions between the Y3 state and other vibrational excited states. In Fig. 9 the vibrational energies concerning the v1 and ^V4 modes are based on an ab initio calculation [ 4]. As shown in Fig. 9 several perturbations will exist. For example, Corio lis interaction couples the Y3 state with the v 1 and 3v2 states. Fermi

34

interaction may also exist between the v3, 2v2+ ^V4, and 2v4 states. But in this case Fermi

interaction is very small, because the 2v4 state is far from the V3 state by about 400 _cm-1, and interaction between v3 and 2v2+V4 will be also small because the interaction is caused by the higher anharmonic potential term k2234. Coriolis interaction between the VI and v3 states was also neglected as mentioned above, even though they are supposed to have similar energies, because the v 1 transition could not be observed, which has transition intensity one thirtieth of the v3 band according to Ref. 4. Standard deviations in the present least-squares analysis are 0.0049 _and 0.0055 cm-1 for the s � s and a� a components, respectively. The standard deviations show that the spectrum is not much affected by Coriolis resonance. However, the fit which is slightly worse than that expected for a well-behaved band may be ascribed to the Coriolis coupling which necessitates small corrections to equation (6).

Spin-rotation splitting was not observed In 'R- and PP-branch lines but only observed in 'Q-branch lines. As mentioned in the previous section, spin-rotation splitting of a symmetric top molecule is approximately expressed by equation (14). _{In this} equation, the dominant diagonal matrix element of spin-rotation interaction in Hund's case (b) is considered. With assumptions, Ebb= -0.036 _{cm-1 and Ecc =} 0, splitting of each rotational level caused by spin-rotation interaction can be calculated. Figure 10 illustrates the splitting thus calculated as a function of the quantum numbers Nand K. Then, if the vibrational change of the spin-rotation coupling constant is negligible, the spin-rotation splitting for a vibronic transition is obtained by considering an arrow in the figure. As shown, for the perpendicular band such as the v3 band Q-branch transitions correspond to vertical arrows (denoted by a and b in the figure), therefore the spin-rotation splitting is of the order of about 0.04 cm-1. On the other hand, P- and R-branch transitions correspond to nearly horizontal arrows (denoted by d and ^c in the figure), therefore the

35

splitting is of the order of about 0.007 cm-1, and it is too small to be observed in the 4.5 )lm region. On the contrary, in the case of the v2 band which is a parallel band, spin- rotation splittings were only observed in P-and R-branch transitions

ll].

These results are also understood from Fig. 10. In this case P- and R-branch transitions correspond to

arrows of oblique directions, therefore the splitting is large enough to be observed. On the other hand a Q-branch line corresponds to a point, therefore it is reasonable that the splitting was not observed.

Spin-rotation interaction consists of the first-order and second order terms, the latter arising from mixing with excited electronic states. Normally the second order term is overwhelmingly important, and the second-order contribution to Ebb is expressed by

(16)

where Aso denotes the spin-orbit coupling constants

[5].

Usually the first excited state makes a dominant contribution to the spin-rotation interaction in the ground state. The spin-orbit interaction constant Aso may be estimated from that in the Si atom, which belongs to 3pJ in the ground electronic state. Spin-orbit Hamiltonian for the Si atom is written as follows,

(17)

where � is the spin-orbit coupling constant for 3p electrons. The spin-orbit coupling constantA

ffb

occurring in Lande's interval rule is expressed by,

36

lj2 (18)

where I3P2 >denotes a wave function of the Si atom. When the value of the Aso constant for SiH3 is assumed to be the same as the value of �' 146.16 cm-1 [6] and pure precession model is assumed as follows,

1(01Lbln)12:::: i(L=1,ML=11LbiL=1,ML=0)12

= I(L=1,ML=11

t

{L++L_}IL=1,ML=0)12 = 1 , (19)

the electronic excited energy of SiH3 is calculated to be 77196 em -1. According to the vanishing integral rule, the first excited state must belong to 2£' symmetry because the ground electronic state and the b component Lb of the electronic orbital angular momentum belong to 2A" and E" species of D3h point group. The calculated value of 77196 cm-1 is consistent with the fact that SiH3 gives no emission in the visible region.

37

2-1-5. Application to Plastna Processes

High resolution spectroscopic data of silicon hydride radicals are of crucial importance for the monitoring of photo-excited processes as well as plasma processes, because they may play as important intermediates in those processes. In the infrared region, the SiH radical has been studied by FTIR emission spectroscopy [7, 8] and by tunable diode laser absorption spectroscopy [9]. The v2 band of SiH2 near 1000 cm-1 has been observed by diode laser kinetic spectroscopy [ 1 0].

The absorption signals of SiH3 in 700 - 800 cm-1 region have been used for the infrared diode laser absorption spectroscopy measurements of radical density [11, 12], diffusion coefficient and reaction rate [ 1³], and spatial distribution [ 14]. Infrared absorption spectroscopy is especially useful for the measurement of non-emissive species such as SiH3, to which emission spectroscopic techniques including laser induced fluorescence (LIF) are inapplicable. Observed data in the present experiment are highly desirable for application to an improved method for monitoring the radical in plasma processes. In this section the present result will be discussed from the view point of application of the observed data.

The spectroscopic constants in Table 3 reproduce the wavenumbers observed in the present study within 0.01 cm-1, which is roughly equal to the full linewidth. The positions of transitions which have not been observed can be predicted from the constants in Table 3, with varying reliability depending on the transition. Relatively correct prediction is expected for the rRK(N) and PPK(N) transitions with (K, N) not very different from those for the corresponding observed transitions. However, the positions of the rRo(N) transitions will not be correctly calculated, because of inadequacy of the energy level expression of equation (6), in which the /-type doubling effect is not considered.

38

The significance of the spectroscopic data of SiH3 in the 4.5 �m region from the viewpoint of improving the diode laser absorption monitoring method is discussed as

follows: (1) The infrared absorption in the Si-H tretching region is expected to be more intense than that in the region near 700 cm-1, and measurement with higher sensitivity will be realized. (2) The SiH and SiH2 radicals also have absorption bands in this region, and simultaneous and comparative monitoring of SiH, SiH2, and SiH3 will be facilitated.

(3) Measurement with the higher spatial resolution is possible at the shorter wavelength.

(4) Commercially available diode lasers in this region are generally better than those in the other regions in terms of output power, beam quality, frequency stability, lifetime etc.

Laser diodes operating at the liquid nitrogen temperature are also available in this region, implying a low cost of the monitoring apparatus.

The Y3 band of the SiH3 radical is located close to the stretching bands of SiH4, and it is probable that some transitions in Table 2 are overlapped by SiH4 lines. Such transitions of course cannot be used for monitoring. The published spectroscopic data of SiR4 [15 ^� 17] give us a rough estimate of the density of SiH4 lines, in which isotopic species, hot bands and perturbation induced transitions are also taken into account. A large number of SiB4 lines are concentrated in the region 2180 - 2190 em -1, so that a SiH3 line in this region would be almost certainly interfered by Sifi4 lines. As for the transitions in Table 2, which are all located outside this region, we estimate that half, at least, of them are free from the interference. In fact, we confirmed by observing the diode laser spectrum of SiH4 that no SiH4 lines overlap the PP6(9) transition near 2119.95 _cm-1.

This transition is also one of the most intense among the entry of Table 2, and may be recommended for the application to monitoring. However, it remains probable that other transitions are more convenient, depending on the laser diodes available; the check by the diode laser observation has been performed for only few of the transitions in Table 2.

39

References

[1] C. Yamada and E.Hirota, Phys.Rev.Lett. 56 (1986) 923.

(2] R.D.Johnson III, B.P.Tsai, and J.W.Hudgens, J.Chem.Phys. 91 (1989) 3340.

[3] D.E.Milligan and M.E.Jacox, J.Chem.Phys., 52 (1970) 2594.

[4] W.D.Allen and H.F.Schaefer III, Chem.Phys. 108 (1986) 243.

(5] E.Hirota, "High-Resolution Spectroscopy of Transient Molecules", Springer­

Verlag, Berlin/New York,1985.

[6] "ATOMIC ENERGY LEVELS" Vol. 1. National Bureau of Standards.

[7] J.C.Nights, J.P.M.Schmitt, J.Perrin, and G.Guelachvili, J.Chem.Phys. 76 (1982) 3414.

[8] M.Betrencourt, D.Boudjaadar, P.Chollet, G.Guelachvili, and M.Morillon­

Chapey, J.Chem.Phys. 84 (1986) 4121.

[9] P.B.Davies, N.A.Isaacs, S.A.Johnson, and D.K.Russell, J.Chem.Phys. 83 (1985) 2060.

[10] C.Yamada, H.Kanamori, E.Hirota, N.Nishiwaki, N.ltabashi, K.Kato, and T.Goto,

J.Chem.Phys. 91 (1989) 4582.

[11] N.Itabashi, K.Kato, N.Nishiwaki, T.Goto, C. Yamada , and E. Hirota, Jpn.J.Appl.Phys.Part 2, 27 (1988) L 1565.

[12] N.Itabashi, N.Nishiwaki, M.Magane, T.Goto, A.Matsuda, C. Yamada, and E.Hirota, Jpn.J.Appl.Phys.Part 1, 29 (1990) 585.

[13] N.Itabashi, K.Kato, N.Nishiwaki, T.Goto, C. Yamada, and E.Hirota, Jpn.J.Appl.Phys.Part 2, 28 (1989) L325.

[14] N.Itabashi, N.Nishiwaki, M.Magane, S.Naito, T.Goto, A.Matsuda, C.Yamada,

40

and E.Hirota, Jpn.J.Appl.Phys. Part 2, 29 (1990) L505.

[15] B.Lavorel, G.Millot, Q.L.Kou, G.Guelachvili, K.Bouzouba, P.Lepage, Vl.G.Tyuterev, and G.Pierre, J.Mol.Spectrosc. 143 (1990) 35.

[16] A.Cabana, D.L.Grey, A.G.Robiette, and G.Pierre, Mol.Phys. 36 (1978) 1503.

[17] A.Cabana, L.Lambert, and C.Pepin, J.Mol.Spectrosc. 43 (1972) 429.

41

Tablel

Vibrational Modes of SiH3

Band Mode Ab initio3

VI Sym-stretch 2J50C

Y2 Sym-deform 773C

V3 Deg-stretch 2J80C

V4 Deg-deform 933C

a) In cm-1 units.

b) Calculated values in km/mol units [ 4].

c) Calculated values of Ref. [4].

d) Observed values in Ref. [1].

e) Observed values in the present study.

42

Observed3

1955

a � s 721.0486d s � a 727 .9438d

s � s 2185.2J8C

a � a 2185.266C

Intensityb

3.7

35.4

109.0

59.6

Table 2

Observed Spectrum of the V3 Band of SiH3a

Transition Obs.

s f- s

IR1 (2) 2207.0264

rR1 (4) 2225.1600

rR2(5) 2230.1272 rR2(7) 2247.5649 rR3(3) 2208.3210 rR3(4) 2217.3490 rR3(5) 2226.2645 rR3(6) 2235.0600 fR4(4) 2213.5341

�(6) 2231.2738 fR4(7) 2239.9567 rRs(5) 2218.7204 IRs(6) 2227.5364 rRs(8) 2244.8194

fR(j(6) 2223.8566

rR6(7) 2232.5845 rR6(8) 2241.1806 rR7(7) 2228.9719 rRs(8) 2234.0348 rR9(9) 2239.0548

a) In em -1 units.

0.-C.b Obs.

af-a

-6. 2207.0748

2. 2225.2060

14. 2230.1708

-6. 2247.6053

0. 2208.3672

2. 2217.3936

6. 2226.3074

2. 2235.1037

-9. 2213.5816

1. 2231.3205

-20. 2240.0004

42. 2218.7671

-69. 2227.5832

23. 2244.8684

-47. 2223.9025

-11. 2232.6311

33. 2241.2279

49. 2229.0181

25. 2234.0864

-50. 2239.1056

b) Observed minus calculated frequency in 1 o-4 cm-1.

43

0.-C.b

-1.

3.

12.

-14.

-8.

-9.

-8.

11.

3.

24.

-22.

50.

-65.

43.

-51.

-23.

12.

33.

44.

-51.

Table

²

(continued)

Observed Spectrum of the V3

Band

of SiH3a

Transition Obs. 0.-C.b Obs.

s (- s a(- a

rR9(lO)

^{2247.4455 -11. 2247.4984}

fR1oClO)

2244.0614 22. 2244.1102

rQ

³

(

⁵

)

^{2169.8042 -34. 2169.8339}

fQ

³

(

⁶

)

2169.2679 40. 2169.2949

f

Q

³⁽⁷

)

^{2168.6449 70. 2168.6694}

fQ

³

(

⁸

)

2167.9300 -66. 2167.9543

PP

³

(

³

)

^{2166.6789 75. 2166.7259}

PP

³

(

⁴

)

^{2156.8856 -67. 2156.9272}

PP4(4)

^{2161.1401 -28. 2161.1855}

PP4(9)

^{2111.2835 16. 2111.3288}

PP6(6)

^{2150.0626 16. 2150.1056}

PP6(9)

2119.9470 -12. 2119.9663

PP

7

(

¹³

)

2084.1403 1. 2084.1734

a) In cm-1 units.

b) Observed minus calculated frequency in 1 o-4 cm-1.

44

0.-C.b

-10.

20.

-43.

50.

81.

-79.

86.

-75.

-34.

19.

18.

-14.

1.

Table 3

Spectroscopic Constants for the v3 Band of SiH3

Constants s a

Bo 4.7577(14) 4.7586(16) cm-1

Co 2.82 (Fixed) 2.82 (Fixed) cm-1

DNO 1.394(188) 1.296(211) 10-4 cm-1

DNKO -4.56(81) -4.77(91) l0-4 cm-1

HNO -0.751(141) -0.914( 159) 1 o-6 cm-1

HNNKO 2.329(576) 2.976(649) 1 o-6 cm-1

HNKKO -4.255(1150) -4.4 70( 1296) 10-6 cm-1

�B -4.634(40) -4.659( 45) 10-2 cm-1

�c -1.557(55) -1.532(62) I0-2 cm-1

mNK -8.93( 177) -1 0.26( 199) 10-5 cm-1

�DK 5.81 (224) 6.95(253) 1Q-5 cm-1

�HN 1.660(352) 2.284(396) 10-7 cm-1

�NNK -9.36(173) -12.26(195) 10-7 cm-1

�HNKK 1.541(239) 1.674(270) 10-6 cm-1

c�3 0.1185(8) 0.1183(9) cm-1

llN3 -0.994(215) -1.179(243) I0-3 cm-1

V3 2185.218(5) 2185.266(6) cm-1

45

Table 4

Observed Q-branch Transitions in the v3 Band of SiH33

Transition Obs. Obs.

s f.- s a f-a

fQ3(5) ^F1 2169.7839 2169.8137

F2 2169.8244 2169.8540

fQ3(6) ^Fl 2169.2484 2169.2759

F2 2169.2874 2169.3139

fQ3(7) ^Fl 2168.6269 2168.6506

F2 2168.6630 2168.6883

fQ3(8) ^Fl 2167.9151 2167.9362

F2 2167.9449 2167.9724

a) In cm-1 units.

46

2

-+::>.

-...)

,---� I ,--------

�

1

₁₈₆₈ I

v ₌

0 '\: 1 j_ \J�

Fig.

¹

Double minimum potential for the inversion motion.

The values of the barrier height and inversion splittings are taken from Ref.

^1.

6.7

^cm-1

1-

(a)

1 +

( s)

0.14

^{em- 1}

o- (a)

o+ c s)

a

s

a

s

v3

state

Ground state

Fig. 2

Inversion doubling can be observed in the v3 band because

splittings of inversion doubling vary with the excitation of the v3

mode. The transitions of ·s � s· and a �a are allowed in the v3

band.

48

�

\()

�

I I

1 A.C. AMP I

ELECTRODE

_r

I

-

V//.iJ �iLl

;//II

^{1//// V///} 1////

ljj!J lf/!J

I( I�

^j

^z

^{·. \ I}

�I

PUMP COIL OIL

Fig. 3. Multiple-reflection discharge cell for discharge modulation.

Ac high voltage is applied between two electrodes to produce the SiH3 radical in ^a glow discharge. The discharge current is typically 100 rnA.

Magnetic field of about 600 Gauss can be applied by the solenoid coil.

lr GAS·

11�

,.. _'

u

�

M ,

[J�

-,

I

40

55

I

II ^II

11 ^o I I ⁵⁰

^I

r ^{8 I}

^I

r ⁹

^I

^I

I ⁶ r ⁷

K=·4.

i ⁵

^I

.r� K=.1 3

2200

Pp· K=3

r I ^I (aa

_I

1 ⁴

I

^{I .}

cni 1 SiH3 Vs 2160

Fig. 4. Loomis- Wood diagram of the observed lines. The lines denoted by open circles indicate the starting members of the K-subbancls.

so

2226.20 2226.30 2226.40

_cm-1

Fig. 5. Observed spectrum for a rR-branch transition in the v3 band of SiH3.

The doublet structure is due to the inversion doubling, the spin-rotation doubling being unresolved.

51

2247.40 2247.50 2247.60

^cm-1

Fig. 6. Observed lines which were assigned to the rR9(10) _and rR2(7) transitions, respectively. Nuclear spin statistics dictates that the rR9(10) transition has the spin weight twice as large as that for the rR2(7) transition.

52

2169.20 2169.30

^cm-1

Fig. 7. Observed spectrum for a Q-branch transition in the v3 band of SiH3.

The quartet spectrum arises from the in version doubling into the s- s and a- a components and from the spin-rotation doubling.

The wavenumber calibration was performed using the N20 specrum (upper) and etalon fringes (lower).

53

s a

SiH3

69.25 2169.30

Fig. 8. Zeeman effect was observed in the

rQ3(6)

transition.

54

0

Gauss

600

em

-1

cm-1

230

220

200

180

Fig. 9

2v2

^{+ V4}

E'

F

vl

^c

a

�

At'

^s

c a

3v2

s

v3

E'

F

A

1' +

E'

a

s

a s

a s

The vibrational states of the SiH3 radical. The vibrational frequencies ^v1 and ^{v 4} are taken from ab initio values of Ref. 4.

The inversion splittings are exaggerated. ^"C^" and "F" denote Coriolis and Fermi interactions, respectively.

55

0.2

0.1

0.0

.1E/cm·1

d: Pp

N

0 2 4 6 8

Fig. 10

Splitting of the rotational levels (N ,K) due to spin-rotation interaction.

56

2-2

INFRARED DIODE LASER SPECTROSCOPY OF THE GeF RADICAL IN THE X2flt/2 STATE

57