SUB-DOPPLER HIGH-RESOLUTION SPECTROSCOPY OF 15V BAND OF AND THE ZEEMAN EFFECT

Excitation spectra and the Zeeman spectra of CS2 in the region of 31320-31445 cm - were measured with sub-Doppler resolution. Observed rotational lines were classified to 14 series of lines (vibronic bands) and were named by the wavenumbers of their band origins. The 31344.9 band is the strongest one and is assigned as the main band of the VB20u20(K O)- XE 000 transition. Most of the extra bands may be allowed by the vibronic interaction between the VB2020(K 0) level and singlet levels, which are mostly high vibrational levels of the A(XZ) state. The Zeeman splittings are observed for several lines in almost all bands. These may be originating from the spin-orbit interaction between the rotational levels, which have accidentally nearly the same energy, of the A(XlE) state and the 3A2(3Au) or/and 3B(3Au) state.

than 0.0001 cm crossed to a molecular beam of CS2, Nishizawa et al. [6] observed the rotationally resolved excitation spectra and the Zeeman spectra with sub-Doppler resolution for the 6V, 10V and 13V band systems. Many more lines were identified with higher resolution, and the magnetic moments of the upper levels were obtained from the magnitude of the Zeeman splittings. We have extended the observation of the excitation spectra and the Zeeman spectra with sub-Doppler resolution to the 15V band system. The results and the analysis are reported in this article.

II. EXPERIMENTAL
The experimental setup is almost the same with the one in our previous report, [6] and the diagram is shown in Fig. 1. A single-frequency UV light was produced by installing an angle tuned frequency doubler (a LilO3 crystal) within the cavity of a ring laser (CR699-29). When the frequency doubler is installed, the machine (CR699-29) loses its autoscan operation capability. The autoscan operation in the UV region was made possible by the same method as given in a previous report. [7] The laser power was about 1 mW and tae linewidth was about 2 MHz. The frequency of the laser light was calibrated by using the fundamental light; the excitation spectrum of 12 and frequency marks of an etalon with 150 MHz free spectral range were recorded simultaneously. Some parts of the excitation spectrum and the Zeeman spectrum are shown in Figs. 2 and 3. We measured the excitation spectrum  vz, )2-2, )2-4 1 or 0, where )2 is the vibrational quantum number. We shall use the notations J, K, and M to specify the quantum numbers of, respectively, the total angular momentum J of a rotational level, the projection of the rotational angular momentum N along the molecule-fixed top axis, and the projection of J along the space-fixed Z axis. K in the X1E. state. Because of the zero nuclear spin of the 32S nuclei in 12C32S2, only even J" levels are allowed for 0 by the nuclear spin statistics. [8] Hence, absorption lines only from even J" levels are observed for v 0.
The spectral lines whose band head positions are at 31339-31360 and 31424 cm are classified as 15V band system. [2] We measured the excitation spectra and the Zeeman spectra in the region of 31320-31445 cm-. By using the molecular constants of the ground state [9] and by referring the line intensities, we first searched for the P and R branch lines having a common upper rotational level from the combination difference. [8] The Zeeman splitting of the X1Z state is small and can be neglected.
Hence the Zeeman splitting of a spectral line is attributed to the one of the excited level. The Zeeman spectra are useful to confirm the assignments, because the Zeeman splittings of lines of a common upper level should be of the same magnitude, i.e. the splittings of P(J' + 1), Q(J'), and R(J' 1) lines should be the same.
We identified 14 series of lines (14 vibronic bands) in this region, and we named these bands by the wavenumbers of the band origins. The line energies and the line intensities are listed in Table I. The accuracy of the intensity is low (about +20%). The Fortrat diagrams are shown in Fig. 4. All the bands except the 31392.3 band are composed of only the P and R branches of even J" and are identified as the transitions from the XE 000 level by the spacing between the P(J" + 1) and R(J'-1) lines. These bands are assigned as Z(K 0) E(/= 0) transitions. The 31392.3 band is composed of the P, Q, and R lines of even and odd J" and is identified as the transition from the XZ. 010 level by the spacing between the P(J' + 1), Q(J'), and R(J'-1) lines. The intensities of Q lines are smaller than those of the P and R lines. Hence, this band is assigned as a I-I(K 1) H(/= 1) transition.
All the observed bands are found to be transitions of AK --0, i.e. the transition moments are along the top axis (parallel band). [8] Table II. The grouping of rotational lines to each band is simple and unique, but multiple lines are assigned for a given J' in several bands. The multiple lines can appear by intensity borrowing induced by perturbation between the levels close in energy. When only two lines were observed for a given J' in a band, the deperturbed term energies were calculated by the same way as our previous report. [6] When more than two lines were observed for a given J', the deperturbed term energy Eo(aJ', J') was approximated by the center of gravity; E00a', J') where Ii(aJ," J" "o", J") is the line intensity of the Vi(aJ," J') xEa(aJ/ ", J") transition and EiOJ', J') is the term energy of an excited level ViOJ', J'). The term energy is a + level, which is calculated sum of the line energy and the energy of the XE0(J J") from the molecular constants in Ref. 9. The resulting term energies are listed in Table   136 ATSUSHI DOI et al.

II. By expressing the term energy of the excited level approximately as G + B[J(J + 1) KZ], we calculated the values of G and B for each band by a least-squares
fitting to the term energies. The results are listed in Table II. The Zeeman energy of a JM level is expressed by [10] Ez(JM) = -Oj#BMH, where gs is a g-factor of a JM level, /B is the Bohr magneton, and the magnetic field H is along the Z axis. The gs values greater than 0.01 are listed in Table I. The magnetic moment #s of a J level can be evaluated by [J(J + 1)]/2aj in units of #n, and the values are listed in Table I.  The 31344.9 band is the strongest one in the 15V band system. Two or three lines with small random spacings are observed for each J'. The line intensity of an allowed (ag', J') (", J' + 1) transition at a temperature T is proportional to [8] C(J' + 1) exp[-B,(J" + 1)(J" + 2)hc/kT], where C is a constant proportional to a square of the transition moment and k is the Boltzmann's constant. The sum of the line intensities of the multiple lines for a given J' is shown by a full bar in Fig. 5. The dependence of J' is similar to the one of an allowed transition (open bars in Fig. 5). Therefore, the lines of minor intensity for each J' may be allowed through the intensity borrowing, which is induced by the perturbation with nearby levels (the selection rule is AJ = 0). [8] The Zeeman splittings are observed to be small except P(8) and R(6) lines. Therefore, the 31344.9 band is confirmed as a transition to a singlet state. The lines of major intensity for each J" in the 31344.9 band are assigned as the VB2 020(K 0)-XE. 000 transition. Although the vibrational assignments are not conclusive, [4] Jungen et al. [2] assigned ag 1 for the 15V band. As we can see in Figs -(see Fig. 2). We found two extra R(4) lines at 31345.5988 and 31345.9246 cm-1. If the dispersed fluorescence spectra with exciting the extra lines were observed, we would be able to confirm the interpretation by Pique et al . 11 In the 31340.5 band, appreciable Zeeman splittings are observed for all the assigned lines. Hence, the band may be assigned to the transition to the triplet state 3A2(3Au) which is allowed by the spin-orbit interaction with the VB2 020 (K 0) level. In the other extra bands, the Zeeman splittings are observed to be small, and appreciable Zeeman splittings are observed only for several lines. Hence, most of the excited states may be identified as singlet states. The Renner-Teller interaction can be neglected in the levels of K 0. [8] Because the quantum number a92 of the 31344.9 band is expected to be small, the number of levels which can interact by the Fermi resonance within the VaB2 state is estimated to be small. Most of the extra bands may be allowed by the vibronic interaction between the B 0v0 (K 0) level and singlet states. CS2 has normal vibrations of the symmetry A(vl and v2) and B2(v3). Therefore, only B2 and IA states can mix with the B2 state by the vibronic interaction. A number of extra bands can be allowed only by interactions with a state of high level density. The dissociation limit of the IAI(X1Z) state is estimated to be 35970 + 130 cm-l. [11] Therefore, most of the extra bands may be allowed by the vibronic interaction between the VIB2 0a920 (K 0) level and a high vibrational level of the A(XE) state. The 31392.3 band, which is observed to be a transition from the XZ 01 0 level, may be allowed by the vibronic interaction between the VB20D20(K 1) level and a high vibrational level of the 1A(XE.) state.
It should be noted that the Zeeman splittings are observed for several lines in almost all bands. This may be originating from the spin-orbit interaction between a triplet state and a high vibrational level of the 1A(X1E) state. If the spin-orbit interaction between a triplet state and the B2 0u20 (K 0) level is responsible for the Zeeman splitting, the Zeeman splittings should be observed for all the lines of the same J, but the observed results are different. Symmetry allowed perturbing triplet states are 3A 2, 3B1, and 3B 2. Among the states which arise from the configuration (rro)3rr [3] 3A2(3), 3A2(3Zu), 3B2(3Au), and 3B2(3u+ states can be candidates of the perturbing states. The level of the perturbing triplet state must be close in energy to the high vibrational level of the 1A(XZ.) state, and the level density must be high because the Zeeman splittings are observed for several lines in almost all bands. Hence, the most probable perturbing triplet state is the 3A2(3Au) or/and 3B2(3Au) state.
The V system is strongly perturbed. Merer et al. 12] classified the long wavelength part as "shattered band" and the short wavelength part as "quantum chaos". By observing the excitation spectra and the Zeeman spectra with sub-Doppler resolution, 14 vibronic bands are classified in the energy range of the 15V band system. The 31344.9 band, which is the strongest band of the 15V band system, is found to be perturbed by nearby levels, and the deperturbation analysis is performed. The magnetic character of the excited states are made clear and the origins of the extra bands are estimated. However, the deperturbation analysis of the vibrational levels is not yet performed, and even the assignment of the vibrational quantum number of the main band is not yet conclusive. We do hope that we would be able to understand this complicated band system by extending these high-resolution spectroscopy to whole bands of the V system.