Knowledge Bank

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 $\sim$ 1300). Vibrational bands $(v\prime =6\sim 31)$ are rotationally resolved by the high resolution dye laser with an intracavity etalon (laser resolution $\sim 0.02cm-1)$. The excited electronic state is assigned to the $51\Sigma +$ state that dissociates into $Rb(5s2S1/2)+Cs(7s2S1/2)$. By the pseudopotential calculation, the $51\Sigma +$ 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 $\Delta Gv$ curve shows slightly positive curvature for $v\prime >20$, which may result from the avoided crossing with the $61\Sigma +$ state at $\sim$ 6 \AA. By the selection rules of $\Delta J=\pm 1$ for $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma \to \mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma$ 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 $51\Sigma +$ state are determined as $Te=18560.12(8)cm-1$, $\omega e=40.83(1)cm-1$, and $\omega exe=0.2465(6)cm-1$. The dissociation energy, $De$, is $3811.5cm-1$. Measured isotope shifts confirm the absolute vibrational numbering. The $Be$ and $\alpha e$ obtained from the $Bv$ vs. v plot are $0.013468(8)cm-1$ and $7.48(4)\times 10-5cm-1$, respectively. Using these molecular constants, potential energy curve of the $51\Sigma +$ slate is constructed by the RKR method.