Ab initio calculations on the X 2B1 and A 2A1 states of AsH2, and Franck–Condon simulation, including anharmonicity, of the A(0,0,0)-X single vibronic level emission spectrum of AsH2

Restricted-spin coupled-cluster single-double plus perturbative triple excitation {RCCSD(T)} calculations were carried out on the X 2B1 and A 2A1 states of AsH2 employing the fully relativistic small-core effective core potential (ECP10MDF) for As and basis sets of up to the augmented correlation-consistent polarized valence quintuple-zeta (aug-cc-pV5Z) quality. Minimum-energy geometrical parameters and relative electronic energies were evaluated, including contributions from extrapolation to the complete basis set limit and from outer core correlation of the As 3d10 electrons employing additional tight 4d3f2g2h functions designed for As. In addition, simplified, explicitly correlated CCSD(T)-F12 calculations were also performed employing different atomic orbital basis sets of up to aug-cc-pVQZ quality, and associated complementary auxiliary and density-fitting basis sets. The best theoretical estimate of the relative electronic energy of the A 2A1 state of AsH2 relative to the X 2B1 state including zero-point energy correction (T0) is 19 954(32) cm-1, which agrees very well with available experimental T0 values of 19 909.4531(18) and 19 909.4910(17) cm-1 obtained from recent laser induced fluorescence and cavity ringdown absorption spectroscopic studies. In addition, potential energy functions (PEFs) of the X 2B1 and A 2A1 states of AsH2 were computed at different RCCSD(T) and CCSD(T)-F12 levels. These PEFs were used in variational calculations of anharmonic vibrational wave functions, which were then utilized to calculate Franck–Condon factors (FCFs) between these two states, using a method which includes allowance for anharmonicity and Duschinsky rotation. The A(0,0,0)-X single vibronic level (SVL) emission spectrum of AsH2 was simulated using these computed FCFs. Comparison between simulated and available experimental vibrationally resolved spectra of the A(0,0,0)-X SVL emission of AsH2, which consist essentially of the bending (2n) series, suggests that there is a significant loss in intensity in the low emission energy region of the experimental spectrum. [ABSTRACT FROM AUTHOR]