Fourier transform spectroscopy, relativistic electronic structure calculation, and coupled-channel deperturbation analysis of the fully mixed $A^1{\mathrm{\Sigma }}_u^{+}$ and $b^3{\mathrm{\Pi }}_u$ states of Cs${}_2$

The 4503 rovibronic term values belonging to the mutually perturbed $A^1{\mathrm{\Sigma }}_u^{+}$ and $b^3{\mathrm{\Pi }}_u$ states of Cs${}_2$ were extracted from laser induced fluorescence (LIF) $A\sim b\to X^1{\mathrm{\Sigma }}_g^{+}$ Fourier transform spectra with the 0.01 cm${}^{-1}$ uncertainty. The experimental term values of the $A^1{\mathrm{\Sigma }}_u^{+}\sim b^3{\mathrm{\Pi }}_u$ complex covering the rotational levels $J\in [4,395]$intheexcitationenergyrange$[9655,13630]$cm$^{-1}$ were involved into coupled-channel (CC) deperturbation analysis. The deperturbation model takes explicitly into account spin-orbit coupling of the $A^1{\mathrm{\Sigma }}_u^{+}(A{0}_u^{+})$ and $b^3{\mathrm{\Pi }}_{0_u}^{+}(b{0}_u^{+})$ states as well as spin-rotational interaction between the $\mathrm{\Omega }=0$, $1$ and $2$ components of the $b^3{\mathrm{\Pi }}_{{\mathrm{\Omega }}_u}^{+}$ state. The \emph{ab initio} relativistic calculations on the low-lying electronic states of Cs${}_2$ were accomplished in the framework of Fock space relativistic coupled cluster (FSRCC) approach to provide the interatomic potentials of the interacting $A{0}_u^{+}$ and $b{0}_u^{+}$ states as well as the relevant $A\sim b$ spin-orbit coupling function. To validate the present CC deperturbation analysis solely obtained by energy-based data, the $A\sim b \to X(v^{\prime\prime}_X)$ LIF intensity distributions were measured and compared with their theoretical counterparts obtained by means of the non-adiabatic vibrational wave functions of the $A\sim b$ complex and the FSRCC $A\sim b\to X$ transition dipole moments calculated by the finite-field method.