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

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 Angstrom) and T-e values (by 450-550 cm(-1)) for the lowest singlet states. Measured spin-orbit splittings for the X (3) Sigma(-) and A (3) Pi multiplets are also accurately reproduced in the present calculations and the Omega = 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 (5) Sigma(-) state. Dipole moments mu(upsilon = 0) for the lowest-lying X (3) Sigma(-), a (1) Delta, and b (1) Sigma(+) states, all arising from the ...sigma(2) pi(2) electronic configuration, are computed to have small (e.g., 0.1266 D for X-1 (3) Sigma(0+)(-)) and nearly equal positive values (As-H+ polarity). This finding is used to explain why the partial radiative lifetime for the parallel b-X-1 transition (tau(p) = 44 ms) is much longer than that (tau(p) = 0.95 ms) of the perpendicular b-X-2. The lifetime of the a (1) Delta state is calculated to be 97 ms, significantly longer than that of the b (1) Sigma(+) state, while the A (3) Pi substates have much shorter lifetimes (less than or equal to 1 mu s) for radiative decay to the X (3) Sigma(-) 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. (C) 1998 American Institute of Physics.