6533b831fe1ef96bd12997ae

RESEARCH PRODUCT

Radiative lifetimes of the(1–3)Π1states in NaCs: Experiment and theory

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The radiative lifetimes of the $(3)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}$ and $D(2)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}$ states of the NaCs molecule have been directly measured in a thermal cell from fluorescence kinetics after modulated laser excitation. The experimental ${\ensuremath{\tau}}_{(3)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}}^{\mathrm{rad}}$ values of the $(3)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}({v}^{\ensuremath{'}}∊[3,25];{J}^{\ensuremath{'}}∊[25,106])$ levels decrease from $29\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}21\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ as the ${v}^{\ensuremath{'}}$ values increase. The measured ${\ensuremath{\tau}}_{(3)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}}^{\mathrm{rad}}$ values for two rotational ${J}^{\ensuremath{'}}=45$ and 106 levels of the ${v}^{\ensuremath{'}}=3$ state are 29 and $25\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$, respectively, showing the effect of rotation. The experimental ${\ensuremath{\tau}}_{(2)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}}^{\mathrm{rad}}$ value of the $D(2)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}({v}^{\ensuremath{'}}=0;{J}^{\ensuremath{'}}=16)$ level is about $37\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$. The average measured effective quenching collision cross section of excited NaCs molecule with Cs atoms $\ensuremath{\sigma}=(3.5\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}$. The theoretical radiative lifetimes for the lowest $(1--3)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}$ states have been evaluated in the framework of a pure $\mathbf{a}$ Hund's coupling case using the approximate sum rule over the lower-lying vibronic states. The required quasirelativistic transition dipole moments were calculated by many-body multipartitioning theory. Propagation of the systematic errors of ab initio transition moments and potential energy curves (PECs) into the ${\ensuremath{\tau}}^{\mathrm{rad}}$ values is discussed. The best lifetime estimates predicted for particular rovibronic levels of the $(3)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}$ and $D(2)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}$ states agree with their experimental counterparts within 2--8 % which can be compared with the 1--3 % uncertainties in the measured ${\ensuremath{\tau}}^{\mathrm{expt}}$ values. The lifetimes of the $B(1)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}$ and $(3)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}$ states are almost exclusively determined by a single transition to the ground $X\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$ state while a contribution of the $D(2)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}\ensuremath{\rightarrow}A\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Sigma}^{+}$ transition into the ${\ensuremath{\tau}}_{(2)\phantom{\rule{0.2em}{0ex}}^{1}\ensuremath{\Pi}}^{\mathrm{rad}}$ values reaches up to 10--15 %.