Molecular Dynamics of Methanol Monocation (CH₃OH⁺) in Strong Laser Fields

Experimental studies by Yamanouchi and co-workers indicate that an intense 40 fs 800 nm laser pulse can cause CH₃OH⁺ to isomerizes during the pulse. The potential energy surfaces of methanol neutral, monocation, and singlet and triplet dication were explored using the CBS-APNO, CBS-QB3, CAM-B3LYP, and B3LYP levels of theory. Ab initio classical trajectories were calculated in the presence of a 2.9 × 10¹⁴ W/cm² 800 nm laser field for methanol monocation on the ground state potential energy surface using the CAM-B3LYP/6-31G­(d,p) level of theory. With only zero point energy, CH₃OH⁺ gained less than 15 kcal/mol from the 40 fs laser pulse, which was not enough to overcome any of the barriers for isomerization or fragmentation. To simulate extra energy deposited during the ionization process, 75, 100, and 125 kcal/mol of vibrational energy was added to the initial structures. After 400 fs, the distribution of product was CH₂OH⁺ + H (79–81%), HCOH⁺ + H₂ (9–13%), CH₂OH₂⁺ (1–3%), CH₃⁺ + OH (1–3%), and CH₂⁺ + H₂O (<0.5%). The estimated kinetic energy releases are in accord with experimental findings. Experimental results using a probe pulse to ionize CH₃OH⁺ to the dication showed substantial fraction C–O dissociation in both CH₃OH⁺ and CH₂OH₂⁺ after the pulse. Because very few CH₂OH₂⁺ → CH₂⁺ + H₂O trajectories were seen in the simulation, the calculations suggest that some of the processes observed experimentally must occur on excited state surfaces or may be due to coupled nuclear-electron dynamics during the pump pulse.