RESONANCE ENHANCED TWO-PHOTON IONIZATION (RE2PI) SPECTRUM OF THE 520 NM SYSTEM OF RbCs

Author Institution: Department of Chemistry, Korea Advanced Institute of Science and TechnologyThe 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, $Rb_{2}$, $Cs_{2}$, 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 $\sim$ 1300). Vibrational bands $(v^{\prime}= 6 \sim 31)$ are rotationally resolved by the high resolution dye laser with an intracavity etalon (laser resolution $\sim 0.02 cm^{-1})$. The excited electronic state is assigned to the $5^{1}\Sigma^{+}$ state that dissociates into $Rb(5s ^{2}S_{1/2})+ Cs(7s ^{2}S_{1/2})$. By the pseudopotential calculation, the $5^{1}\Sigma^{+}$ state has adiabatic potential curve with a shelf as a result of the avoided crossing with ionic pair state al long internuclear distances. Observed in the Franck-Condon region, however, the $\Delta G_{v}$ curve shows slightly positive curvature for $v^{\prime}>20$, which may result from the avoided crossing with the $6^{1}\Sigma^{+}$ state at $\sim$ 6 \AA. By the selection rules of $\Delta J =\pm 1$ for $^{1}\Sigma \rightarrow {^{1}\Sigma}$ transitions, only P and R lines are observed. The rotational constants. $B_{\nu}$, and the vibrational term value, $T_{\nu}$, are determined from the analysis of the rotationally resolved spectra. From the vibrational energy level spacing, $\Delta G_{\nu}$, the spectroscopic parameters for the $5^{1}\Sigma^{+}$ state are determined as $T_{e}= 18560.12(8) cm^{-1}$, $\omega_{e}=40.83(1) cm^{-1}$, and $\omega_{e}x_{e}=0.2465(6) cm^{-1}$. The dissociation energy, $D_{e}$, is $3811.5 cm^{-1}$. Measured isotope shifts confirm the absolute vibrational numbering. The $B_{e}$ and $\alpha_{e}$ obtained from the $B_{v}$ vs. v plot are $0.013468(8) cm^{-1}$ and $7.48(4) \times10^{-5} cm^{-1}$, respectively. Using these molecular constants, potential energy curve of the $5^{1}\Sigma^{+}$ slate is constructed by the RKR method