The direct ultraviolet absorption spectrum of the ¹∑ _g⁺ → ¹B₂(¹∑ _u⁺) transition of jet-cooled CS₂

The absorption spectrum of the linear→bent ¹∑ _g⁺→¹B₂ (¹∑ _u⁺) transition of jet-cooled CS₂ is reported in the 47 750-53 200 cm^-1 region. Adjustment of diluent gas composition enables the preparation of rotationally cold (< 10 K)/vibrationally hot (∼300 K) and rotationally cold/vibrationally cold samples. The collapse of rotational envelopes on cooling facilitates the accurate determinations of frequencies and relative absorption intensities of transitions both from the zero point level and from excited vibrational levels of the ground electronic state. At 49 000-51 000 cm^-1, in the region of the Franck-Condon maximum, a simple vibronic pattern emerges involving two upper state vibrational modes containing both stretching and bending contributions. In this region of the spectrum, the upper state rotational structure diverges from that of a nearly symmetric top and approaches that of the linear or quasilinear molecule. At higher energies, the vibronic structure grows in complexity, suggesting increased anharmonic mixing of these nearly degenerate modes.