DIRECT OBSERVATION OF THE (2) 3-PI(U) STATE OF CS2 BY RESONANCE-ENHANCED 2-PHOTON IONIZATION SPECTROSCOPY IN A VERY COLD MOLECULAR-BEAM

A very cold molecular beam of Cs2 is generated using a high temperature pulsed nozzle. We observe new absorption bands of CS2 with an origin near 648 nm by resonance enhanced two photon ionization. From the simulation of the rotational band contours, the bands are assigned to the (2) 3PI(u)(1u)-X1SIGMA(g)+(0g+) and (2) 3PI(u)(0u+)-X1SIGMA(g)+(0g+) transitions. The electronic term values and the dissociation energies are determined as T(e) = 15456.9 +/- 0.1 cm-1 and D(e) = 2692.1 +/- 0.9 cm-1 for the 1u state, and T(e) = 15432.3 +/- 0.1 cm-1 and D(e) = 2716.7 +/- 0.9 cm-1 for the 0u+ state. These values as well as the vibrational frequency and rotational constant show good agreement with those from the relativistic quantum mechanical calculations. We show that the potential minimum of the (2) 3PI(u) state lies lower than those of the C1PI(u) and D1SIGMA(u)+ state. The potential energy curve of the (2) 3PI(u) state crosses that of the C1PI(u) state on the inner branch.