Ab initio calculations on the X̃ 2B1 and à 2A1 states of AsH2, and Franck-Condon simulation, including anharmonicity, of the à (0,0,0) - X̃ 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 X̃2B1and Ã2A1states of AsH2employing 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 3 d10 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 Ã2A1state of AsH2relative to the X̃B1state including zero-point energy correction (T0) is 19 954 (32) cm-1, which agrees very well with available experimental T0values 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 X̃2B1and Ã2A1states of AsH2were 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 à (0,0,0) - X̃ single vibronic level (SVL) emission spectrum of AsH2was simulated using these computed FCFs. Comparison between simulated and available experimental vibrationally resolved spectra of the à (0,0,0) - X̃ 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.