27 March 1998 Ž . Chemical Physics Letters 285 1998 373–378 Quasi-discrete resonances observed in photoionization to the A 2 P state of the CS q molecule u 2 A. Karawajczyk a , P. Erman a , E. Rachlew a , M. Stankiewicz b , K. Yoshiki Franzen a ´ a Fysik I, Kungl. Tekniska Hogskolan, S-10044 Stockholm, Sweden ¨ b Instytut Fizyki, Uniwersytet Jagiellonski, ulica Reymonta 4, 30-059 Krakow, Poland ´ ´ Received 13 October 1997 Abstract We report total and dispersed fluorescence studies of the A 2 P – X 2 P emission system in the CS q molecule. The u g 2 single-electron Rydberg series converging to the B 2 S q and C 2 S q states have been observed in the undispersed fluorescence u g spectrum as pronounced window-type resonances. Autoionization from these states gives rise to a non-Franck–Condon q Ž 2 . distribution of the vibrational population of the final CS A P state. It is shown that the vibrational branching ratios 2 u primarily depend on the l-quantum number of the Rydberg state and they may give information about perturbations of the Rydberg states. At 20–21.5 eV excitation energy the undispersed fluorescence spectrum reveals a previously unobserved structure which has been tentatively attributed to a 2p y1 shape resonance. No deviations of the branching ratio have been u found in agreement with the predictions of multichannel theory. q 1998 Elsevier Science B.V. 1. Introduction Two types of quasi-discrete resonances, the Ryd- berg autoionizing states and the shape resonances, affect the photoionization dynamics of the valence shell electrons. The Rydberg states appear in the absorption spectra as regular progressions of rela- tively narrow resonances, while the shape resonances extend over a much larger energy region. Recently a new type of states, the so-called non-Rydberg doubly Ž . excited resonances NRDERs have been discovered wx 1. Although the absorption cross-sections to NRDERs are very small, the decay by autoionization from these resonances into lower lying states of the ion may strongly influence vibrational branching ra- tios of the latter states. Many works have been devoted to studies of ionization processes of linear triatomic molecules in the valence shell excitation region and special atten- tion has been paid to the formation of shape reso- w x nances. Various mechanisms 2,3 may give rise to trapping of the ionizing electron in a potential barrier which enhances the absorption probability over a broad energy range. Transitions to quasi-discrete states formed in the potential barrier are observed as ‘shape’ resonances in the partial cross-section spec- trum. It has been extensively discussed in the litera- ture whether shape resonances are atomic or molecu- w x lar in nature 4–9 . If the shape resonance wavefunc- tion is delocalized over the entire molecule the exci- tation cross-section is expected to be sensitive to changes of the internuclear distances. This in turn should cause a deviation of the vibrational branching ratio from the expected Franck–Condon level in the region of a shape resonance. However, for a local- ized wavefunction of the shape resonance the transi- 0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž . PII S0009-2614 98 00078-5