Controlling the strong field fragmentation of ClCHO+ using two laser pulses -an ab initio molecular dynamics simulation.

For a single, intense 7 µm linearly polarized laser pulse, we found that the branching ratio for the fragmentation of ClCHO+ → Cl + HCO+ , H + ClCO+ , HCl+ +CO depended strongly on the orientation of the molecule (J. Phys. Chem. Lett. 2012, 3 2541). The present study explores the possibility of controlling the branching ratio for fragmentation by using two independent pulses with different frequencies, alignment and delay. Born-Oppenheimer molecular dynamics simulations in the laser field were carried out with the B3LYP/6-311G(d,p) level of theory using combinations of 3.5, 7 and 10.5 µm sine squared pulses with field strengths of 0.03 au (peak intensity of 3.15×1013 W/cm2 ) and lengths of 560 fs. A 3.5 µm pulse aligned with the C-H bond and a 10.5 µm pulse perpendicular to the C-H bond produced a larger branching ratio for HCl+ +CO than a comparable single 7 µm pulse. When the 10.5 µm pulse was delayed by one quarter of the pulse envelope, the branching ratio for the high energy product, (HCl+ +CO 73%) was a factor of three larger than the low energy product (Cl + HCO+ , 25%). By contrast, when the 3.5 µm pulse was delayed by one quarter of the pulse envelope, the branching ratio was reversed (HCl+ +CO 38%; Cl + HCO+ , 60%). Continuous wavelet analysis was used to follow the interaction of the laser with the various vibrational modes as a function of time. © 2018 Wiley Periodicals, Inc.

How Is C-H Vibrational Energy Redistributed in F + CHD3(ν1 = 1) → HF + CD3?

The effects of CH stretching excitation on the F + CHD3 → HF + CD3 reaction are studied experimentally using crossed-beam and time-sliced velocity map imaging techniques at the collision energy of 9.0 kcal/mol. The fraction of the vibrationally excited CHD3 reagent in the crossed-beam region was determined accurately, allowing us to investigate quantitatively the effects of CH stretching excitation on the title reaction. Experimental data show that the vibrational energy in the excited CH bond of CHD3 is almost exclusively deposited into the HF product vibration, and hence, the HF products from the excited-state reaction are about one vibrational quantum hotter than those of the ground-state reaction, while the vibrational state distribution of the CD3 products is only slightly affected. The reaction is suppressed by the CH stretching excitation, and the overall reactivity of the vibrationally excited reaction is 74 ± 4% of that of the ground-state reaction for CD3(ν2 = 0, 1, 2, 3) product channels.

A Reaction Accelerator: Mid-infrared Strong Field Dissociation Yields Mode-Selective Chemistry.

Mode-selective chemistry has been a dream of chemists since the advent of the laser in the 1970s. Despite intense effort, this goal has remained elusive due to efficient energy randomization in polyatomic molecules. Using ab initio molecular dynamics calculations, we show that the interaction of molecules with intense, ultrashort mid-infrared laser pulses can accelerate and promote reactions that are energetically and entropically disfavored, owing to efficient kinetic energy pumping into the corresponding vibrational mode(s) by the laser field. In a test case of formyl chloride ion photodissociation, the reactions are ultimately complete under field-free conditions within 500 fs after the laser pulse, which effectively overcomes competition from intramolecular vibrational energy redistribution (IVR). The approach is quite general and experimentally accessible using currently available technology.