Local Mode Effects in the Absorption Spectrum of H2S between 10 780 and 11 330 cm−1

Abstract

The absorption spectrum of H2S (1159 lines) has been recorded between 10 780 and 11 330 cm−1 by intracavity laser absorption spectroscopy (ICLAS). As a result of the increase of sensitivity of ICLAS over previous Fourier transform investigations in the same spectral region, the number of experimental energy levels of the (40+, 1) and (40−, 1) local mode states ((212) and (311) in normal mode notation) has been triplicated for the main isotopic species H232S (198 compared to 62). Moreover, 112 energy levels of H234S and 25 energy levels of H233S, present in natural abundance in the sample, could also be determined. Three rotational levels of the (30±, 3) local mode pair of dark states ((132) and (231) in normal mode notation) of H232S could also be observed through extra lines borrowing their intensities via resonance interactions with line-partners of the dominant bands. The (40+, 1) and (40−, 1) local mode states form a degenerate local mode pair, with the same rotational structure for the two components. The observed energy levels were reproduced by an effective Hamiltonian including the (40±, 1) and (30±, 3) interacting local mode pairs. This Hamiltonian was applied either in the C2v or Cs symmetry groups leading to an rms deviation of 0.0058 and 0.0068 cm−1 respectively, comparable to the experimental accuracy (0.0028 cm−1). The degeneracy of the rotational structure of the (40±, 1) pair is unaffected by the interaction with the nearby (30±, 3) local mode states. On the contrary, an additional interaction with dark states lifts in some cases this degeneracy. The same effective Hamiltonian with scaled parameters was success fully used to reproduce the energy levels of the H233S and H234S isotopic species. Finally, the extremely weak 3ν1+3ν2 band of H232S at 11 097.161 cm−1 could also be detected leading to the determination of 70 levels of the corresponding (21+, 3) upper state ((330) in normal mode notation). The integrated intensities have been estimated for the bands corresponding to the (40+, 1) and (40−, 1) upper states. A total of six transformed dipole moment parameters were derived from the measured line intensities allowing to simulate very satisfactorily the observed spectrum. The transition moment relative to the (40−, 1) upper state is found about twice as large as the one relative to the (40+, 1) state.