On the ultraviolet photofragmentation of hydrogen iodide

Abstract

An ab initio configuration interaction (CI) study including spin-orbit coupling is carried out for the ground and low-lying excited states of the HI molecule by employing a relativistic effective core potential for the iodine atom. The computed spectroscopic constants for the X 1Σ+ ground and b 3ΠΩ Rydberg states are in good agreement with available experimental data, as are the vertical excitation energies for the repulsive a 3Π1, a 3Π0+, and A 1Π1 states of the A band. The a 3Π0+ state is found to possess a shallow minimum of 600 cm−1 depth outside the Franck–Condon region, at ≈5.1 a0. The electric-dipole moments have also been calculated for transitions from the ground to the A band states. Contrary to what is usually assumed, the a 3Π1, A 1Π1←X0+ transition moments are found to depend strongly on internuclear distance. Employing the computed potential energy and transition moment data, partial and total absorption spectra for the A band are calculated and the I* quantum yields, ΦI*(ν), are determined as a function of excitation energy. The maximal ΦI*(ν) values are calculated to be 0.55–0.59 and lie at 39 000–40 000 cm−1, which agrees well with experimental results. The influence of the t 3Σ1+ state and of the nonadiabatic effects on the ΦI*(ν) values is found to be negligible in the essential part of the A band. Finally, it is shown that significantly higher I* quantum yield values (up to 0.8–0.9) may be achieved when vibrationally hot HI molecules are excited in the appropriate spectral range.