6.4 Soft X-ray Spectra for Each Collision
of C4+ ions with He target at collision energy less than 2.5 keV/u [82, 83].
However, in the present collision energy of 5.0 keV/u, the single electron capture dominants over the double electron capture and therefore the C3+
ions is a main emission source in this measurements.
In the He target spectrum, the relative intensities of the 1s2s(3,1S)2p 2P lines (line no. 7, 8) are higher than that of the 1s2s2p 4P line (line no. 9).
Strohscheinet al. published a paper showing enhancements for the metastable 1s2s2p 4P state following single and double electron capture in collisions of He-like and H-like C ions with He and Ne targets [22], but the present spec-trum is different from their Auger emission specspec-trum. This might be due to branching ratios of autoionization to radiation in each 1s2s2p state and the observation efficiency of the 1s2s2p 4P transition. The lifetime of the 1s2s2p 4P state of C3+ ions is approximately 10−6 s (Tab. 6.5). It is much longer than a flight time of the ions to pass by the detection area of our setup (∼6×10−9 s). Therefore, the observation efficiency of the 1s2s2p 4P state is lower than those of other states with very short lifetimes. In the present C4+
+ He measurement, the line intensity of the 1s22s–1s2s2p 4P transition was relatively small and new lines were also observed at 3.716(3)and 4.030(5‡) nm.
The former corresponds to the 1s–2p transitions of the 1s2p(1P)3s state and the latter corresponds to the 1s–2p and TEOP transitions of the 1s2p(1P)3d state.
For collisions with the Xe targets, additional emission lines at wavelengths of 3.637(1), 4.006(4†)and 4.046(6) nm were observed in addition to the lines as observed on the He target. Each line corresponds to the 1s–3p transition from the 1s2s(1S)3p state, the TEOP transition from the 1s2s(3S)4d state and the 1s–2p transition from the 1s2p(1P)3s and 1s2p(1P)3p states, respectively.
The relative intensity of the TEOP line at 4.006(4†) nm increases with increasing atomic number of the rare gas targets, which is equivalent to decreasing of the ionization potentials of these targets. This arises because the observed TEOP transition needs electron capture into a high energy level, that indicates the 4d subshell. However, a significant difference in its line intensity between the spectra on the Xe and O2 targets was observed in spite of their similar ionization potentials. This might be due to the spin-orbit interaction present in the heavy atomic gas targets. In the potential curve crossing model, the initial and final channels are approximated by the
polarization potential and Coulomb potential as below, Vinitial(R)≈ −α0q2
2R4 (6.6)
Vfinal(R)≈+(q−s)s
R −Q (6.7)
where α0 is the polarizability volume, R is the internuclear distance between the projectile ion and target atom, q is the charge of the projectile, s is the number of the captured electrons, and Q is the energy balance for each reaction. The spin-orbit interaction induces splitting fine structure. For example, the energy differences between outermost np5 2P3/2 and np5 2P1/2 levels for Ar+, Kr+ and Xe+ ions are 0.178, 0.666 and 1.306 eV. This leads to different crossing points of the potential curves between initial and final channels in contrast with He and Ne.
In addition, differences of polarizabilities of the targets might induce the spectral changes. It has been demonstrated that relativistic effects increase polarizabilities of heavy rare gases [84, 85]. In the present case, Xe has signifi-cant polarizability and there is a large difference between those of Xe and O2, namely 4.044 and 1.598 ˚A3, respectively [86, 87]. Therefore, the differences of the polarizabilities of the targets also induce the different electron capture levels even if the targets have similar ionization potential values. There may be other possible causes but the details are not understood yet.
N
5+Ion Experiments
Figure 6.4 shows emission spectra observed in collisions of metastable N5+
ions with several neutral targets. Prominent transitions are the same as those observed in the C4+ ion experiments (line no. 2, 4–6 in Fig. 6.4). The emission lines at 2.919(4‡), 2.948(5) and 2.997(6) nm are due to the 1s–2p of the 1s2s2p2,4P, 1s2p(3P)2p and 1s2p(3P)3d states and the TEOP transitions of the 1s2s(1S)3d. Unlike the carbon ion spectra, the 1s2s2p 4P line is the strongest regardless of its long lifetime. This might be due to an increasing branching ratio of the radiative 1s22s–1s2s2p 4P transition and a shortening of the lifetime of the 1s2s2p 4P state. This branching ratio increases in the order of C3+, N4+ and O5+ ions [88] and thus the relative line intensity of the quartet state increases in the same order. The emission line at 2.607(2) nm corresponds to the 1s–3p transitions from the 1s2s(3S)3p2,4P and 1s2p(3P)3p
states. The relative intensity of this line depends significantly on the target gas because of the difference of the dominant electron capture levels.
Among the molecular targets, small differences on line intensity ratios are found, but spectral differences are significant between the Xe and O2 gas targets. A weak emission line at 2.505(1) nm was observed only on the Xe target, which is due to the 1s–4p transitions from the 1s2s(3S)4p state.
Moreover, an emission line at 2.834(3†) nm was clearly observed on the heavy rare gas Xe collisions. This line corresponds to the TEOP transitions of the 1s23s–1s2s(3S)4p. However, these two lines were not observed with the O2 target in spite of the similar ionization potentials.
O
6+Ion Experiments
Figure 6.5 shows observed spectra in collisions of metastable O6+ ions with He and Xe targets in the wavelength region of 1.5–5 nm. The four first-order diffraction lines are resolved into seven second-order diffraction lines. In the present O6+ion experiments, we focus on the second-order spectrum for clear separation of lines. Hence, wavelengths of the observed lines are exactly two times longer than the true transition wavelengths.
Figure 6.6 shows soft X-ray spectra observed in collisions of metastable O6+ ions with eight neutral targets. Prominent emission lines at 4.371(5‡), 4.416(6)and 4.477(7)nm are due to the 1s–2p transitions of the 1s2s(1,3S)2p2,4P, 1s2p(3P)2p and 1s2p(3P)3d states and the TEOP transitions of the 1s2s(1S)3d states. Strong 1s–np (n ≥ 3) lines had been expected to be observed due to the high dominant electron capture levels. In fact, for the rare gas targets, both the 1s–3p transitions were observed, but on the molecular targets, they are almost negligible. This means that the dominant capture levels are the highest angular momentum states and yrast transitions might be dominant in the molecular target cases.
Additionally, four emission lines at 3.826(1†), 3.876(2), 3.901(3)and 4.334(4)nm were also observed for collisions on the rare gas targets. According to the theoretical calculations, the first three lines correspond to the 1s–3p transi-tions of the 1s2s(1,3S)3p and 1s2p(3P)3p states, and the TEOP transitions of the 1s2p(3P)3s states. The fourth line corresponds to the 1s–2p transitions of the 1s2p(1P)3l states. Unlike the C4+ and N5+ ion experiments, distinct TEOP lines derived from the 1s2s(3S)4l states were not observed. The emis-sion line at 4.334(4) nm were observed only on the rare gas targets and its relative intensity depends on the choice of targets.
Gu et al. observed the same emission line from O5+ ions at 2.1672 nm by using an EBIT and GIS and identified it as the 1s23s–1s2p3d and 1s23d–
1s2p3d transitions with the wavelengths of 2.1600 and 2.1763 nm [89]. How-ever, our calculated results and the previously reported calculations [81]
show that this line is due to only the resonance 1s–2p transitions from the 1s2p(1P)3l states produced by the transfer-excitation, and this identification is in better agreement with the experimental data.
Figure 6.3: Soft X-ray spectra in collisions of metastable C4+ ions with rare gas (left) and molecular (right) gas targets at collision energy of 60 keV. The resolved emission lines labeled as 1–9 are listed in Tab. 6.5.
Figure 6.4: Soft X-ray spectra in collisions of metastable N5+ ions with rare gas (left) and molecular (right) gas targets at collision energy of 75 keV. The resolved emission lines labeled as 1–6 are listed in Tab. 6.6.
Figure 6.5: Soft X-ray spectra in collisions of metastable O6+ ions with He (upper) and Xe (lower) targets at collision energy of 90 keV. These include both the first and second-order diffraction lines. The four first-order lines are separated into the seven second-order lines.
Figure 6.6: Soft X-ray spectra in collisions of metastable O6+ ions with rare gas (left) and molecular (right) gas targets at collision energy of 90 keV. In the spectra, the observed second-order diffraction lines are shown in order to separate the overlapped lines. The resolved emission lines labeled as 1–7 are listed in Tab. 6.7.