Accessing Multiple Conical Intersections in the 3s and 3p sub(x) Photodissociation of the Hydroxymethyl Radical

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2013-01, Vol.117 (46), p.12049-12059-12049-12059
• Main Authors: Rodrigo, Chirantha P, Zhou, Chuanchuan, Reisler, Hanna
• Format: Article
• Language: English
• Subjects: Dynamics; Excitation; Formaldehyde; Internal energy; Intersections; Photodissociation; Rotational; Rydberg states
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp404552g

The photodissociation dynamics of the hydroxymethyl radical (CH sub(2)OH, CH sub(2)OD, and CD sub(2)OD) following excitation to the 3s and 3p sub(x) Rydberg states is studied using time-sliced velocity map imaging of hydrogen photofragments. Dissociation takes place on the ground potential energy surface reached via conical intersections from the excited states, and formaldehyde and hydrxymethylene are identified as reaction products. The major product, formaldehyde, has a bimodal internal energy distribution. The largest fraction has high kinetic energy (KE), modest rotational excitation, and vibrational excitation mainly in the CO stretch and the CH(D) sub(2) deformations modes (scissors, wag, and rock). The minor fraction has lower KEs and a higher rovibrational excitation that is unresolved. A bimodal internal energy distribution in the formaldehyde fragment has been predicted by Yarkony [J. Chem. Phys. 2005, 122, 084316] for a conical intersection along the O-H bond coordinate. The hydroxymethylene product state distributions depend strongly on the nature of the excited state. In dissociation via the 3s state, the hydroxymethylene products have broad rovibrational state distributions and are produced at low yield. As suggested by Yarkony, they may be produced in the same dissociation events that give rise to low KE formaldehyde. In these events, the bound region of the PES is sampled following the conical intersection along O-H(D). The hydroxymethylene yield is low near its threshold and increases slowly with excitation energy to the 3s state, but its internal energy distribution remains broad and the contributions of the cis- and trans-isomers cannot be resolved. The mechanism changes markedly when exciting to the 3p sub(x) state. The hydroxymethylene products have less rotational excitation and show separate contributions of cis- and trans-isomers. The trans-isomer is found to be a minor product relative to the higher-energy cis-isomer, as predicted by Yarkony for conical intersections along the C-H coordinate. It appears that the efficiency of dissociation via conical intersections along the O-H and C-H coordinates depends on the initial excited state. While the O-H conical intersection seam (vertical cone) provides an efficient route to the ground state following excitation via the 3s or the 3p sub(x) Rydberg states, conical intersections along the C-H bond coordinate (tilted cone) are sampled more efficiently via 3p sub(x) excitation and proceed