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

Abstract

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.

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

“…The results of molecular dynamics simulations of the dissociation ClCHO + with two laser pulses are listed in Table . In earlier work, we found that a single short, intense 7 μm laser pulse (90 fs, 0.09 au) perpendicular to the C–H bond successfully promoted the high energy channel for ClCHO + dissociating to HCl + + CO. Hence this wavelength and orientation can serve as reference for the various two pulse sequences in the present study.…”

“…The simulations of dissociation were carried out by classical trajectory calculations on the ground state Born‐Oppenheimer surface for aligned formyl chloride cations in the time varying electric field of the laser pulses. As in our previous studies, the B3LYP/6‐311G(d,p) level of theory was chosen as a suitable compromise between accuracy of the potential energy surface and efficiency in the trajectory calculations. Molecular dynamics calculations were carried out with the development version of the Gaussian series of programs and the PCvelV integrator with a step size of 0.25 fs and Hessian updating for 20 steps before recalculation of the Hessian.…”

“…Since the Cl dissociation channel is significantly lower in energy than the other two dissociation channels, classical trajectory calculations on the ground state Born‐Oppenheimer potential energy surface showed that when the initial kinetic energy is distributed statistically, Cl dissociation channel is dominant . However, an intense, 4 cycle 7 μm pulse perpendicular to the C–H bond direction enhances the higher energy HCl + dissociation channel.…”

Molecular Dynamics of Methylamine, Methanol, and Methyl Fluoride Cations in Intense 7 Micron Laser Fields

“…16 The interaction of the molecule with the laser field was calculated in the semiclassical dipole approximation. (1) μ⃗ is the dipole operator, E ⃗ (t) is the electric field component of the laser, and s⃗ (t) describes the shape of the pulse. The present study used a four-cycle linearly polarized trapezoid pulse (2) or a cosine squared pulse…”

“…1,2 In the present work, we examine the isomerization and fragmentation of CH 3 NH 2 + , CH 3 OH + , and CH 3 F + by short intense 7 μm laser pulses. This work is motivated by recent studies of methanol cations by Yamanouchi and co-workers using pump−probe coincidence momentum imaging.…”