Ab initio calculations on the X˜²B₁and òA₁states of AsH₂, and Franck--Condon simulation, including anharmonicity, of the Ã(0,0,0)-X˜ single vibronic level emission spectrum of AsH₂

Abstract

Restricted-spin coupled-cluster single-double plus perturbative triple excitation {RCCSD(T)} calculations were carried out on the X˜²B₁and òA₁states 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 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 òA₁state of AsH₂ relative to the X˜²B₁state 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˜²B₁and òA₁states 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 à (0,0,0)-X˜ 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)-X˜ (SVL) emission of AsH₂, which consist essentially of the bending (2[sub n]) series, suggests that there is a significant loss in intensity in the low emission energy region of the experimental spectrum.