15min:

YOUNGJEE YOON, YONGHOON LEE, SUN JONG BAEK, JEONGYE CHOI AND BONGSOO KIM, Department of Chemistry, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea.

The rovibrational spectrum of RbCs molecule in the range of 520 nm is observed by the resonance enhanced two-photon ionization (RE2PI) method. A very cold pulsed molecular beam that contains RbCs, Rb₂, Cs₂, etc is generated by a high temperature pulsed nozzle. Only RbCs⁺ ion could be detected using the time-of-flight (TOF) mass spectrometer (mass resolution 1300). Vibrational bands (v' = 6 31) are rotationally resolved by the high resolution dye laser with an intracavity etalon (laser resolution 0.02 cm^-1).

The excited electronic state is assigned to the 5 ^{1 +} state that dissociates into Rb(5s ²S_1/2) + Cs(7s ²S_1/2). By the pseudopotential calculation, the 5 ^{1 +} state has adiabatic potential curve with a shelf as a result of the avoided crossing with ionic pair state at long internuclear distances. Observed in the Franck-Condon region, however, the G_v curve shows slightly positive curvature for v' > 20, which may result from the avoided crossing with the 6 ^{1 +} state at 6 Å.

By the selection rules of J = \pm 1 for ¹ \to ¹ transitions, only P and R lines are observed. The rotational constants, B_v, and the vibrational term value, T_v, are determined from the analysis of the rotationally resolved spectra. From the vibrational energy level spacing, G_v, the spectroscopic parameters for the 5 ^{1 +} state are determined as T_e = 18560.12(8) cm^-1, _e = 40.83(1) cm^-1, and _ex_e = 0.2465(6) cm^-1.

The dissociation energy, D_e, is 3811.5 cm^-1. Measured isotope shifts confirm the absolute vibrational numbering. The B_e and _e obtained from the B_v vs. v plot are 0.013468(8) cm^-1 and 7.48(4)× 10^-5 cm^-1, respectively. Using these molecular constants, potential energy curve of the 5 ^{1 +} state is constructed by the RKR method.