Ab initio calculations of the electronic structure of the doublet any quartet states of the rubidium trimer

Ab initio quantum chemical calculations of the electronic structure of the ground and low-lying doublet and quartet states of the Rb$_2$Cs molecule were performed, as a result of which 3D potential energy surfaces (PES) were obtained simulating the approach of a Rb atom towards a RbCs dimer aimed at bothe the Cs atom and the Rb atom at different angles of attack varying in the range from 10 to 180 degrees. It is shown that the ground state of the heteronuclear trimer (1)$^2$A' exhibits an avoided crossong with the first excited (2)$^2$A' state near the equilibrium geometry, therefore the ground state of Rb$_2$Cs cannot be described within the framework of the traditional adiabatic approximation. For all 12 electronic states studied, equilibrium parameters corresponding to the $C_{2v}$ point group were determined. The constructed PES can be used for quantum calculation of collision cross sections and rate constants for the reaction of an RbCs dimer with a Rb atom, as well as conducting detailed analyses of the rovibronic structure of the trimer by solving the 3D vibrational-rotational Schrödinger equation in order to find optimal ways for laser synthesis, cooling and manipulation of the ultracold ensemble of this atomic-molecular system.