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

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.