A RESONANCE-ENHANCED MULTIPHOTON IONIZATION STUDY OF THE CS2 MOLECULE - THE 4P RYDBERG STATES

The resonance enhanced multiphoton ionization (REMPI) spectrum of jet-cooled CS, has been recorded in the one-photon wavelength range 460-500 nm, corresponding to the three photon excitation energy range 60 000-65 000 cm(-1). A previous assignment of one photon forbidden transitions in this region to 3d Rydberg states is shown to be incorrect and reassigned to the 4p pi (1,3)Delta(u) states. In fact all the observed states in this region can be assigned to 4p Rydberg states; the 4p sigma (IIu)-I-1,3 states at 62 768 and 62 083 cm(-1), respectively, and the 4p pi (1,3)Delta(u) states at 64 214 and 63 698 cm(-1), respectively. Another band at 64 374 cm(-1) may be due to a three photon excitation to the 4p pi (1) Sigma(u)(+) Rydberg state. Our resolution is sufficient to resolve band shapes enabling symmetry assignments when coupled with their polarization behavior. The fact that the origin bands are not degraded and that Delta v=0 sequence bands are strongly excited whereas Delta v not equal 0 transitions are either absent or very weak implies that the upper states have a linear geometry similar to that of the ground state. A comparison of siglet-triplet splittings suggests the 4p pi Delta(u) states have stronger Hund's case (c) character than the 4p sigma IIu state. While CS2+ was generally the predominant ion formed, resonance ionization through the (1) Delta(u) <-- X(1) Sigma(g)(+) unusual predominance of S+ and CS+ ions. This is explained by multiphoton fragmentation of CS2+ via an accidental one-photon resonance from the X (II1/2)-I-2 (v=0) ionic ground state. (C) 1995 American Institute of Physics.