The spectrum of arsenic hydride: An ab initio configuration interaction study employing a relativistic effective core potential

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Abstract
An ab initio configuration interaction (CI) study including the spin–orbit interaction is carried out for numerous valence and Rydberg states of the AsH radical by employing a relativistic effective core potential for the arsenic atom. The computed spectroscopic constants are in good agreement with corresponding experimental data, with a tendency toward a slight overestimation of bond lengths (by 0.01–0.02 Å) and Te values (by 450–550 cm−1) for the lowest singlet states. Measured spin–orbit splittings for the X 3Σ− and A 3Π multiplets are also accurately reproduced in the present calculations and the Ω=0−, 1, and 2 components of the latter state are shown to be strongly predissociated due to spin–orbit interaction with the corresponding components of the repulsive Σ-5 state. Dipole moments μ(v=0) for the lowest-lying X 3Σ−, a 1Δ, and b 1Σ+ states, all arising from the ⋯σ2π2 electronic configuration, are computed to have small (e.g., 0.1266 D for X1 3Σ0+−) and nearly equal positive values (As−H+ polarity). This finding is used to explain why the partial radiative lifetime for the parallel b–X1 transition (τp=44 ms) is much longer than that (τp=0.95 ms) of the perpendicular b–X2. The lifetime of the a 1Δ state is calculated to be 97 ms, significantly longer than that of the b 1Σ+ state, while the A 3Π substates have much shorter lifetimes (⩽1 μs) for radiative decay to the X 3Σ− ground state. A number of other bound states and avoided crossings are indicated in the calculations which may be of relevance in future experimental studies of this system.

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