Hongli Tao

First Principles Dynamics and Minimum Energy Pathways for Mechanochemical Ring Opening of Cyclobutene

We use ab initio steered molecular dynamics to investigate the mechanically induced ring opening of cyclobutene. We show that the dynamical results can be considered in terms of a force-modified potential energy surface (FMPES). We show how the minimal energy paths for the two possible competing conrotatory and disrotatory ring-opening reactions are affected by external force. We also locate minimal energy pathways in the presence of applied external force and show that the reactant, product, and transition state geometries are altered by the application of external force. The largest effects are on the transition state geometries and barrier heights. Our results provide a framework for future investigations of the role of external force on chemical reactivity.

Ab initio multiple spawning dynamics using multi-state second-order perturbation theory

We have implemented multi-state second-order perturbation theory (MS-CASPT2) in the ab initio multiple spawning (AIMS) method for first-principles molecular dynamics including nonadiabatic effects. The nonadiabatic couplings between states are calculated numerically using an efficient method which requires only two extra energy calculations per time step. As a representative example, we carry out AIMS-MSPT2 calculations of the excited state dynamics of ethylene. Two distinct types of conical intersections, previously denoted as the twisted-pyramidalized and ethylidene intersections, are responsible for ultrafast population transfer from the excited state to the ground state. Although these two pathways have been observed in prior dynamics simulations, we show here that the branching ratio is affected by dynamic correlation with the twisted-pyramidalized intersection overweighting the ethylidene-like intersection during the decay process at the AIMS-MSPT2 level of description.

Conformationally Controlled Chemistry: Excited-State Dynamics Dictate Ground-State Reaction

Ion imaging reveals distinct photodissociation dynamics for propanal cations initially prepared in either the cis or gauche conformation, even though these isomers differ only slightly in energy and face a small interconversion barrier. The product kinetic energy distributions for the hydrogen atom elimination channels are bimodal, and the two peaks are readily assigned to propanoyl cation or hydroxyallyl cation coproducts. Ab initio multiple spawning dynamical calculations suggest that distinct ultrafast dynamics in the excited state deposit each conformer in isolated regions of the ground-state potential energy surface, and, from these distinct regions, conformer interconversion does not effectively compete with dissociation.

Control of 1,3-cyclohexadiene photoisomerization using light-induced conical intersections.

We have studied the photoinduced isomerization from 1,3-cyclohexadiene to 1,3,5-hexatriene in the presence of an intense ultrafast laser pulse. We find that the laser field maximally suppresses isomerization if it is both polarized parallel to the excitation dipole and present 50 fs after the initial photoabsorption, at the time when the system is expected to be in the vicinity of a conical intersection that mediates this structural transition. A modified ab initio multiple spawning (AIMS) method shows that the laser induces a resonant coupling between the excited state and the ground state, i.e., a light-induced conical intersection. The theory accounts for the timing and direction of the effect.

Ultrafast internal conversion in ethylene. II. Mechanisms and pathways for quenching and hydrogen elimination

Through a combined experimental and theoretical approach, we study the nonadiabatic dynamics of the prototypical ethylene (C(2)H(4)) molecule upon π → π(∗) excitation with 161 nm light. Using a novel experimental apparatus, we combine femtosecond pulses of vacuum ultraviolet and extreme ultraviolet (XUV) radiation with variable delay to perform time resolved photo-ion fragment spectroscopy. In this second part of a two part series, the XUV (17 eV < hν < 23 eV) probe pulses are sufficiently energetic to break the C-C bond in photoionization, or to photoionize the dissociation products of the vibrationally hot ground state. The experimental data is directly compared to excited state ab initio molecular dynamics simulations explicitly accounting for the probe step. Enhancements of the CH(2)(+) and CH(3)(+) photo-ion fragment yields, corresponding to molecules photoionized in ethylene (CH(2)CH(2)) and ethylidene (CH(3)CH) like geometries are observed within 100 fs after π → π(∗) excitation. Quantitative agreement between theory and experiment on the relative CH(2)(+) and CH(3)(+) yields provides experimental confirmation of the theoretical prediction of two distinct conical intersections and their branching ratio [H. Tao, B. G. Levine, and T. J. Martinez, J. Phys. Chem. A. 113, 13656 (2009)]. Evidence for fast, non-statistical, elimination of H(2) molecules and H atoms is observed in the time resolved H(2)(+) and H(+) signals.

Ab initio multiple spawning on laser-dressed states: a study of 1,3-cyclohexadiene photoisomerization via light-induced conical intersections

We extend the ab initio multiple spawning method to include both field-free and field-induced nonadiabatic transitions. We apply this method to describe ultrafast pump-probe experiments of the photoinduced ring-opening of gas phase 1,3-cyclohexadiene. In the absence of a control field, nonadiabatic transitions mediated by a conical intersection (CoIn) lead to rapid ground state recovery with both 1,3-cyclohexadiene and ring-opened hexatriene products. However, application of a control field within the first 200 fs after photoexcitation results in suppression of the hexatriene product. We demonstrate that this is a consequence of population dumping prior to reaching the CoIn and further interpret this in terms of light-induced CoIns created by the control field.

Conformationally selective photodissociation dynamics of propanal cation.

We have previously reported experimental evidence for conformationally selective dissociation of propanal cation that was interpreted, on the basis of ab initio multiple spawning calculations, as arising from distinct dynamics in the excited state manifold of the cation. Two conical intersections (CIs) are accessible from Franck-Condon points on the dark state; however, different conformers prefer different CIs and quench to different regions on the ground state. In this paper, we extend our initial report to include experimental results for the partially deuterated propanal cation as well as detailed characterization of the ground state potential energy surface and statistical calculations of the ground state dissociation dynamics. The DC slice imaging experiments show a bimodal velocity distribution for H elimination with the observed branching ratio of the two channels different for the cis and gauche conformers. H(D)-elimination experiments from deuterated propanal cation support the dissociation mechanism proposed in the earlier report. We further investigate reaction rates on the ground state using Rice-Ramsperger-Kassel-Marcus theory. We find that the experimental results are consistent with a mechanistic picture where the ground state dissociation is statistical, and conformer specificity of the dissociation products arises because of the different populations in distinct ground state isomers after photoexcitation due to ultrafast quenching to the ground state.

Enhancement of strong-field multiple ionization in the vicinity of the conical intersection in 1,3-cyclohexadiene ring opening

Nonradiative energy dissipation in electronically excited polyatomic molecules proceeds through conical intersections, loci of degeneracy between electronic states. We observe a marked enhancement of laser-induced double ionization in the vicinity of a conical intersection during a non-radiative transition. We measured double ionization by detecting the kinetic energy of ions released by laser-induced strong-field fragmentation during the ring-opening transition between 1,3-cyclohexadiene and 1,3,5-hexatriene. The enhancement of the double ionization correlates with the conical intersection between the HOMO and LUMO orbitals.