An ab initio study of the CH3I photodissociation. II. Transition moments and vibrational state control of the I* quantum yields

Abstract

Multireference spin-orbit configuration interaction calculations of transition moments from the X A1 ground state to the 3Q0+, 3Q1, and 1Q excited states responsible for the A absorption band of CH3I are reported and employed for an analysis of the photofragmentation in this system. Contrary to what is usually assumed, the 3Q0+(A1), 3Q1(E), and 1Q(E)<--X A1 transition moments are found to be strongly dependent on the C-I fragmentation coordinate. The sign of this dependence is opposite for the parallel and perpendicular transitions, which opens an opportunity for vibrational state control of the photodissociation product yields. The computed absorption intensity distribution and the I* quantum yield as a function of excitation energy are analyzed in comparison with existing experimental data, and good agreement between theory and experiment is found. It is predicted that significantly higher I* quantum yield values (>0.9) may be achieved when vibrationally hot CH3I molecules are excited in the appropriate spectral range. It is shown that vibrational state control of the I*/I branching ratio in the alkyl (hydrogen) iodide photodissociation has an electronic rather than a dynamic nature: Due to a different electron density distribution at various molecular geometries, one achieves a more efficient excitation of a particular fragmentation channel rather than influences the dynamics of the decay process.

The photodissociation of CH3I in the red edge of the A-band: Comparison between slice imaging experiments and multisurface wave packet calculations

“…47 for v =0, 1, and 2 are attributed mainly to the fact that the present 3D treatment considers the CH 3 I bending mode, while the 2D treatment of Ref. 47 does not. The spectra of Fig.…”

“…6 reflect also the shape of the 0i functions along R, namely, an increasing function with increasing R for the 3 Q 0 ← X 1 A 1 transition ͑maximizing the distribution intensity at large R͒, and a decreasing function with increasing R for the 1 Q 1 ← X 1 A 1 and 3 Q 1 ← X 1 A 1 transitions ͑maximizing the distribution intensity at short R͒. 47 These radial distributions will be useful to interpret the absorption spectra for the different initial vibrational states v discussed below.…”

“…The electric-dipole moment functions ge ͑R C-I ͒ which couple radiatively the X 1 A 1 ground state with the three excited electronic states were taken from the ab initio calculations of Alekseyev et al 47,52 Actually, the ab initio ge ͑R C-I ͒ functions were fitted to analytical forms. In particular, the ab initio ge ͑R C-I ͒ function for the 3 As we shall see below, there is evidence in the present experiments that excitation of CH 3 I from X 1 A 1 to the three excited electronic states takes place from vibrational states v Ն 0 of the C-I stretch mode, for the excitation wavelengths of current interest.…”

“…A good theoretical description of the red edge of the A absorption band of CH 3 I is a challenge both for electronic structure calculations of the potential energy surfaces involved 47 and for quantum dynamics calculations. In this work, we present an effort directed to understand the involvement of different potential energy surfaces with different parallel and perpendicular characters and the presence of a conical intersection influencing the dynamics in a region of a very weak absorption cross section.…”

“…45͒ and, more recently, focusing on time-resolved properties. 2,46 Recently, Alekseyev et al 1,47 employed larger atomic basis sets and a larger configuration interaction treatment to recalculate the X 1 A 1 ground state and the low-lying excited states, 3 Q 1 , 3 Q 0 , and 1 Q 1 , together with the transition moments relevant to the photodissociation of CH 3 I in the A band, with the aim of improving the agreement between the experimental and calculated absorption spectra.…”