The branching ratio of intercombination $A^1\Sigma^+\sim b^3\Pi\to a^3\Sigma^+/X^1\Sigma^+$ transitions in the RbCs molecule: measurements and calculations

TL;DR

This study combines experimental laser-induced fluorescence measurements with theoretical calculations to determine the branching ratios and transition dipole moments of specific electronic transitions in the RbCs molecule, aiding ultracold molecule formation.

Contribution

It provides new experimental data and ab initio calculations of transition dipole moments for RbCs, enhancing understanding of its electronic transition probabilities.

Findings

Branching ratio of 1-5 x 10^{-4} in the bound region.

Relative systematic error in TDM functions estimated as a few percent.

Calibrated spectral sensitivity of the FT system using K2 and KCs molecules.

Abstract

We observed the $A^1\Sigma^+\sim b^3\Pi\to a^3\Sigma^+/X^1\Sigma^+$ laser-induced fluorescence (LIF) spectra of the RbCs molecule excited from the ground $X^1\Sigma^+$ state by the Ti:Sapphire laser. The LIF radiation from the common perturbed levels of the singlet-triplet $A\sim b$ complex was recorded by the Fourier-transform (FT) spectrometer with the instrumental resolution of 0.03~cm$^{-1}$. The relative intensity distribution in the rotationally resolved $A\sim b\to a^3\Sigma^+(v_a)/X^1\Sigma^+(v_X)$ progressions was measured, and their branching ratio was found to be about of 1$\div$5$ \times$10$^{-4}$ in the bound region of the $a^3\Sigma^+$ and $X^1\Sigma^+$ states. The experiment was complemented with the scalar and full relativistic calculations of the $A/b - X/a$ transition dipole moments (TDMs) as functions of internuclear distance. The relative systematic error in the resulting \emph{ab initio} TDM functions evaluated for the strong $A - X$ transition was estimated as few percent in the energy region, where the experimental LIF intensities are relevant. The relative spectral sensitivity of the FT registration system, operated with the InGaAs diode detector and CaF beam-splitter, was calibrated in the range $[6~500,12~000]$~cm$^{-1}$ by a comparison of experimental intensities in the long $A\sim b\to X(v_X)$ LIF progressions of the K$_2$ and KCs molecules with their theoretical counterparts evaluated using the \emph{ab initio} $A - X$ TDMs. Both experimental and theoretical transition probabilities can be employed to improve the stimulated Raman adiabatic passage process, $a\to A\sim b \to X$, which is exploited for a laser assembling of ultracold RbCs molecules.