Franck–Condon simulation of the photoelectron spectrum of AsF2 and the photodetachment spectrum of AsF2− using ab initio calculations: Ionization energy and electron affinity of AsF2

Abstract

RCCSD(T) and/or CASSCF/MRCI calculations were carried out on the ²B₁ state of AsF₂, the ¹A₁, ã³B₁ and ùB₁ states of AsF₂⁺, and the ¹A₁ state of AsF₂⁻ employing the fully-relativistic small-core effective core potential (ECP10MDF) for As and basis sets of up to 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. In addition, simplified, explicitly correlated RHF/UCCSD(T)-F12x calculations were also performed employing different atomic orbital basis sets, and associated complementary auxiliary and density-fitting basis sets. The best theoretical estimates of the adiabatic ionization energies (AIE₀) of AsF₂(²B₁) to the ¹A₁ and ã³B₁ states of AsF₂⁺, including corrections for zero-point vibrational energy (ΔZPE), are 9.099(8) and 13.290(22) eV respectively. The best estimated electron affinity (EA₀) of AsF₂ is 1.182(16) eV, also including Δ(ZPE). These are currently the most reliable AIE₀ and EA₀ values for AsF₂. Potential energy functions (PEFs) of the ²B₁ state of AsF₂, the ¹A₁ and ã³B₁ states of AsF₂⁺ and ¹A₁ state of AsF₂⁻ were computed at RCCSD(T)/aug-cc-pV5Z, RCCSD(T)/aug-cc-pCV5Z and RHF/UCCSD(T)-F12a/aug-cc-pCVTZ levels. These PEFs were employed in variational calculations of anharmonic vibrational wavefunctions, which were then utilised to calculate Franck–Condon factors (FCFs), using a method which includes allowance for anharmonicity and Duschinsky rotation. The computed FCFs were used to simulate the first two bands in the photoelectron spectrum of AsF₂ and the first band in the photodetachment spectrum of AsF₂⁻, both yet to be recorded. The simulated spectra obtained using different sets of PEFs were found to be almost identical, suggesting that the simplified explicitly correlated UCCSD(T)-F12x method with a relatively small basis set can be a reliable alternative to the conventional RCCSD(T) correlation methods with a relatively large basis set, but at a significantly lower cost.