An ab initio study of the C H 3 I photodissociation. I. Potential energy surfaces

• Published in: The Journal of chemical physics 2007-06, Vol.126 (23), p.234102-234102-11
• Main Authors: Alekseyev, Aleksey B., Liebermann, Heinz-Peter, Buenker, Robert J., Yurchenko, Sergei N.
• Format: Article
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.2736695

The multireference spin-orbit (SO) configuration interaction (CI) method in its Λ - S contracted SO-CI version is employed to calculate two-dimensional potential energy surfaces for the ground and low-lying excited states of C H 3 I 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 ν 2 umbrella and ν 3 symmetric stretch modes, are found to be in good agreement with available experimental data. The Q 0 + 3 state converging to the excited I ( P 1 ∕ 2 o 2 ) limit is found to possess a shallow minimum of 850 cm − 1 strongly shifted to larger internuclear distances ( R C - I ≈ 6.5 a 0 ) relative to the ground state. This makes a commonly employed single-exponent approximation for analysis of the C H 3 I fragmentation dynamics unsuitable. The 4 E ( A 1 3 ) 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 Q 1 3 , Q 0 + 3 , and Q 1 states indicate that the A -band spectrum must lie approximately between 33000 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 ( 6 s 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.