Energy and radiative properties of the low-lying NaRb states

Abstract

Many-body multipartitioning perturbation theory (MPPT) was applied to calculate the potential energy of 11 lowest electronic states of the NaRb molecule, $A,C{}^{1}{\ensuremath{\Sigma}}^{+}\ensuremath{-}X{}^{1}{\ensuremath{\Sigma}}^{+},$ $B,D{}^{1}\ensuremath{\Pi}--{X}^{1}{\ensuremath{\Sigma}}^{+},$ $D{}^{1}\ensuremath{\Pi}--A{}^{1}{\ensuremath{\Sigma}}^{+}$ and $D{}^{1}\ensuremath{\Pi}--B{}^{1}\ensuremath{\Pi}$ transition dipole moments, as well as nonadiabatic L-uncoupling matrix elements between the examined ${}^{1}\ensuremath{\Pi}$ and four lowest ${}^{1}{\ensuremath{\Sigma}}^{+}$ states for both ${}^{23}{\mathrm{Na}}^{85}\mathrm{Rb}$ and ${}^{23}{\mathrm{Na}}^{87}\mathrm{Rb}$ isotopomers. The relevant MPPT ab initio matrix elements and energy curves were converted by means of the approximate sum rule to radiative lifetimes and $\ensuremath{\Lambda}$-doubling constants $(q$ factors) for the particular rovibronic levels of the $B{}^{1}\ensuremath{\Pi}$ and $D{}^{1}\ensuremath{\Pi}$ states. The theoretical lifetimes agree well with their experimental counterparts for both $B{}^{1}\ensuremath{\Pi}$ and $D{}^{1}\ensuremath{\Pi}$ states. The q factor estimates obtained in the singlet-singlet approximation are in good agreement with the experimental ones for the $D{}^{1}\ensuremath{\Pi}(1<~{v}^{\ensuremath{'}}<~12;7<~{J}^{\ensuremath{'}}<~50)$ levels, exhibiting a pronounced difference for the $B{}^{1}\ensuremath{\Pi}$ state. Considerably better agreement was achieved by accounting for the spin-orbit perturbation effect caused by the near-lying ${c}^{3}{\ensuremath{\Sigma}}^{+}$ state. Relative intensity distributions in the $D{}^{1}\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\Pi}}X{}^{1}{\ensuremath{\Sigma}}^{+}$ dispersed fluorescence spectra excited by fixed ${\mathrm{Ar}}^{+}$ laser lines were measured for ${v}^{\ensuremath{'}}{(J}^{\ensuremath{'}})=0(44),$ $1(104),$ $4(25),$ $6(44,120),$ $10(36),$ and $12(50)$ $D{}^{1}\ensuremath{\Pi}$ levels. The experimental intensities and term values were simultaneously embedded in the nonlinear least-square fitting procedure to refine the $D{}^{1}\ensuremath{\Pi}$ potential.