Intra–inter polyad mixing and breaking of symmetric–antisymmetric selection rule in the vibrational spectra of CS2 molecule

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Abstract
A laser system composed of tunable lasers pumped by a copper vapor laser (Oxford Lasers Cu60) is described in this paper. The high resolution obtained with this system has allowed excitation of selective rotational levels of the 15 V and 10 V vibrational bands of the V 1 B 2excited electronic state of the CS2 molecule in its vapor phase (∼100 mTorr). The rotational assignment of the excitation spectra was accomplished by observing the dispersed fluorescence. We show that it is not necessary to use a supersonic jet in order to assign the emission spectra of CS2. The goal of this work is to study the highly excited vibrational states of the ground electronic state of CS2 up to the first dissociation limit. For our purpose, there are two important consequences of the particular geometry of the 15 V excitation, which is just below and close to the bending potential barrier of the V 1 B 2 state. First, a very good Franck–Condon overlap in the excitation and a large Franck–Condon access to high vibrational states, as high as 20 000 cm−1, allow observation of the dispersed fluorescence through a high resolution monochromator with an OMA detector. This avoids the need for more complicated techniques, like SEP spectroscopy. Moreover, we show for the first time, that, in the V 1 B 2excited state, a strong Coriolis interaction Q 3 2 Q 3 J c couples the bending (0,3,0) and antisymmetric stretching levels (0,0,1). This breaking of the symmetric–antisymmetric selection rule gives access to the antisymetric stretching levels of the ground electronic stateX̃ Σ+ g from the 15 V excitation. We also show that, below 12 000 cm−1, the vibrational couplings of CS2 can be described by a model of 2 degrees of freedom, which includes a strong 1:2 Fermi resonance and accidental resonant perturbations between adjacent polyads which is probably a first step in the transition to a chaotic regime in CS2.