An ab initio study of the CH3I photodissociation.: I.: Potential energy surfaces

The multireference spin-orbit (SO) configuration interaction (CI) method in its Lambda-S contracted SO-CI version is employed to calculate two-dimensional potential energy surfaces for the ground and low-lying excited states of CH3I relevant to the photodissociation process in its A absorption band. The computed equilibrium geometry for the X A(1) ground state, as well as vibrational frequencies for the nu(2) umbrella and nu(3) symmetric stretch modes, are found to be in good agreement with available experimental data. The (3)Q(0)(+) state converging to the excited I(P-2(1/2)o) limit is found to possess a shallow minimum of 850 cm(-1) strongly shifted to larger internuclear distances (RC-I approximate to 6.5a(0)) relative to the ground state. This makes a commonly employed single-exponent approximation for analysis of the CH3I fragmentation dynamics unsuitable. The 4E((3)A(1)) state dissociating to the same atomic limit is calculated to lie too high in the Franck-Condon region to have any significant impact on the A-band absorption. The computed vertical excitation energies for the (3)Q(1), (3)Q(0)(+), and (1)Q states indicate that the A-band spectrum must lie approximately between 33 000 and 44 300 cm(-1), i.e., between 225 and 300 nm. This result is in very good agreement with the experimental findings. The lowest Rydberg states are computed to lie at >= 49 000 cm(-1) and correspond to the ...a(1)(2)n(3)a(1)(6s(I)) leading configuration. They are responsible for the vacuum ultraviolet absorption lines found experimentally beyond the A-band spectrum at 201.1 nm (49 722 cm(-1)) and higher. (c) 2007 American Institute of Physics.