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

Ab Initio Direct Dynamics

The reactivity and dynamics of molecular systems can be explored computationally by classical trajectory calculations. The traditional approach involves fitting a functional form of a potential energy surface (PES) to the energies from a large number of electronic structure calculations and then integrating numerous trajectories on this fitted PES to model the molecular dynamics. The ever-decreasing cost of computing and continuing advances in computational chemistry software have made it possible to use electronic structure calculations directly in molecular dynamics simulations without first having to construct a fitted PES. In this "on-the-fly" approach, every time the energy and its derivatives are needed for the integration of the equations of motion, they are obtained directly from quantum chemical calculations. This approach started to become practical in the mid-1990s as a result of increased availability of inexpensive computer resources and improved computational chemistry software. The application of direct dynamics calculations has grown rapidly over the last 25 years and would require a lengthy review article. The present Account is limited to some of our contributions to methods development and various applications. To improve the efficiency of direct dynamics calculations, we developed a Hessian-based predictor-corrector algorithm for integrating classical trajectories. Hessian updating made this even more efficient. This approach was also used to improve algorithms for following the steepest descent reaction paths. For larger molecular systems, we developed an extended Lagrangian approach in which the electronic structure is propagated along with the molecular structure. Strong field chemistry is a rapidly growing area, and to improve the accuracy of molecular dynamics in intense laser fields, we included the time-varying electric field in a novel predictor-corrector trajectory integration algorithm. Since intense laser fields can excite and ionize molecules, we extended our studies to include electron dynamics. Specifically, we developed code for time-dependent configuration interaction electron dynamics to simulate strong field ionization by intense laser pulses. Our initial application of ab initio direct dynamics in 1994 was to CH₂O → H₂ + CO; the calculated vibrational distributions in the products were in very good agreement with experiment. In the intervening years, we have used direct dynamics to explore energy partitioning in various dissociation reactions, unimolecular dissociations yielding three fragments, reactions with branching after the transition state, nonstatistical dynamics of chemically activated molecules, dynamics of molecular fragmentation by intense infrared laser pulses, selective activation of specific dissociation channels by aligned intense infrared laser fields, angular dependence of strong field ionization, and simulation of sequential double ionization.

Theoretical and experimental studies on hydrogen migration in dissociative ionization of the methanol monocation to molecular ions H3+ and H2O+

The dissociative ionization processes of the methanol monocation CH₃OH⁺ to H₃⁺ + CHO and H₂O⁺ + CH₂ are studied by ab initio method, and hydrogen migration processes are confirmed in these two dissociation processes. Due to the positive charge assignment in dissociation processes, the fragmentation pathways of CH₃OH⁺ to H₃ + CHO⁺ and CH₃OH⁺ to H₂O + CH₂⁺ are also calculated. The calculation results show that a neutral H₂ moiety in the methanol monocation CH₃OH⁺ is the origin of the formation of H₃⁺, and the ejection of fragment ions H₃⁺ and H₂O⁺ is more difficult than CHO⁺ and CH₂⁺ respectively. Experimentally, by using a dc-slice imaging technique under an 800 nm femtosecond laser field, the velocity distributions of fragment ions H₃⁺, CHO⁺, CH₂⁺, and H₂O⁺ are calculated from their corresponding sliced images. The presence of low-velocity components of these four fragment ions confirms that the formation of these ions is not from the Coulomb explosion of the methanol dication. Hence, the four hydrogen migration pathways from the methanol monocation CH₃OH⁺ to H₃⁺ + CHO, CHO⁺ + H₃, H₂O⁺ + CH₂, and CH₂⁺ + H₂O are securely confirmed. It can be observed in the time-of-flight mass spectrum of ionization and dissociation of methanol that the ion yields of fragment ions H₃⁺ and H₂O⁺ are lower than CHO⁺ and CH₂⁺ respectively, which is consistent with the theoretical results according to which dissociation from the methanol monocation to H₃⁺ and H₂O⁺ is more difficult than CHO⁺ and CH₂⁺ respectively.

Hydrogen migration processes of methanol monocation CH₃OH⁺ to H₃⁺, COH⁺, H₂O⁺ and CH₂⁺ were studied theoretically and experimentally.

Mechanisms and time-resolved dynamics for trihydrogen cation (H3+) formation from organic molecules in strong laser fields

Strong-field laser-matter interactions often lead to exotic chemical reactions. Trihydrogen cation formation from organic molecules is one such case that requires multiple bonds to break and form. We present evidence for the existence of two different reaction pathways for H₃⁺ formation from organic molecules irradiated by a strong-field laser. Assignment of the two pathways was accomplished through analysis of femtosecond time-resolved strong-field ionization and photoion-photoion coincidence measurements carried out on methanol isotopomers, ethylene glycol, and acetone. Ab initio molecular dynamics simulations suggest the formation occurs via two steps: the initial formation of a neutral hydrogen molecule, followed by the abstraction of a proton from the remaining CHOH²⁺ fragment by the roaming H₂ molecule. This reaction has similarities to the H₂ + H₂⁺ mechanism leading to formation of H₃⁺ in the universe. These exotic chemical reaction mechanisms, involving roaming H₂ molecules, are found to occur in the ~100 fs timescale. Roaming molecule reactions may help to explain unlikely chemical processes, involving dissociation and formation of multiple chemical bonds, occurring under strong laser fields.

Molecular dynamics of methylamine, methanol, and methyl fluoride cations in intense 7 micron laser fields

Fragmentation and isomerization of methylamine (CH3NH2(+)), methanol (CH3OH(+)), and methyl fluoride (CH3F(+)) cations by short, intense laser pulses have been studied by ab initio classical trajectory calculations. Born-Oppenheimer molecular dynamics (BOMD) on the ground-state potential energy surface were calculated with the CAM-B3LYP/6-31G(d,p) level of theory for the cations in a four-cycle laser pulse with a wavelengths of 7 μm and intensities of 0.88 × 10(14) and 1.7 × 10(14) W/cm(2). The most abundant reaction path was CH2X(+) + H (63-100%), with the second most favorable path being HCX(+) + H2 (0-33%), followed by isomerization to CH2XH(+) (0-8%). C-X cleavage after isomerization was observed only in methyl fluoride. Compared to random orientation, CH3X(+) with the C-X aligned with the laser polarization gained energy nearly twice as much from laser fields. The percentage of CH3(+) + X dissociation increased when the C-X bond was aligned with the laser field. Alignment also increased the branching ratio for H2 elimination in CH3NH2(+) and CH3OH(+) and for isomerization in CH3OH(+).