Optical control of ground-state atomic orbital alignment: Cl(2P3/2) atoms from HCl(v=2,J=1) photodissociation.

H(35)Cl(v=0,J=0) molecules in a supersonic expansion were excited to the H(35)Cl(v=2,J=1,M=0) state with linearly polarized laser pulses at about 1.7 microm. These rotationally aligned J=1 molecules were then selectively photodissociated with a linearly polarized laser pulse at 220 nm after a time delay, and the velocity-dependent alignment of the (35)Cl((2)P(32)) photofragments was measured using 2+1 REMPI and time-of-flight mass spectrometry. The (35)Cl((2)P(32)) atoms are aligned by two mechanisms: (1) the time-dependent transfer of rotational polarization of the H(35)Cl(v=2,J=1,M=0) molecule to the (35)Cl((2)P(32)) nuclear spin [which is conserved during the photodissociation and thus contributes to the total (35)Cl((2)P(32)) photofragment atomic polarization] and (2) the alignment of the (35)Cl((2)P(32)) electronic polarization resulting from the photoexcitation and dissociation process. The total alignment of the (35)Cl((2)P(32)) photofragments from these two mechanisms was found to vary as a function of time delay between the excitation and the photolysis laser pulses, in agreement with theoretical predictions. We show that the alignment of the ground-state (35)Cl((2)P(32)) atoms, with respect to the photodissociation recoil direction, can be controlled optically. Potential applications include the study of alignment-dependent collision effects.

Reaction of Cl with CD4 excited to the second C-D stretching overtone.

The effects of vibrational excitation on the Cl+CD(4) reaction are investigated by preparing three nearly isoenergetic vibrational states: mid R:3000 at 6279.66 cm(-1), |2100> at 6534.20 cm(-1), and |1110> at 6764.24 cm(-1), where |D(1)D(2)D(3)D(4)> identifies the number of vibrational quanta in each C-D oscillator. Vibrational excitation of the perdeuteromethane is via direct infrared pumping. The reaction is initiated by photolysis of molecular chlorine at 355 nm. The nascent methyl radical product distribution is measured by 2+1 resonance-enhanced multiphoton ionization at 330 nm. The resulting CD(3) state distributions reveal a preference to remove all energy available in the most excited C-D oscillator. Although the energetics are nearly identical, the authors observe strong mode specificity in which the CD(3) state distributions markedly differ between the three Cl-atom reactions. Reaction with CD(4) prepared in the |3000> mode leads to CD(3) products populated primarily in the ground state, reaction with CD(4) prepared in the |2100> mode leads primarily to CD(3) with one quantum of stretch excitation, and reaction with CD(4) prepared in the |1110> mode leads primarily to CD(3) with one quantum of C-D stretch excitation in two oscillators. There are some minor deviations from this behavior, most notably that the Cl atom is able to abstract more energy than is available in a single C-D oscillator, as in the case of |2100>, wherein a small population of ground-state CD(3) is observed. These exceptions likely result from the mixings between different second overtone stretch combination bands. They also measure isotropic and anisotropic time-of-flight profiles of CD(3) (nu(1)=1,2) products from the Cl+CD(4) |2100> reaction, providing speed distributions, spatial anisotropies, and differential cross sections that indicate that energy introduced as vibrational energy into the system essentially remains as such throughout the course of the reaction.

H + CD4 abstraction reaction dynamics: product energy partitioning.

This paper presents experimental and theoretical studies of the product energy partitioning associated with the H + CD4 (nu = 0) --> HD + CD3 reaction for the collision energy range 0.5-3.0 eV. The theoretical results are based on quasiclassical trajectories from (1) first principles direct dynamics calculations (B3LYP/6-31G), (2) an empirical surface developed by Espinosa-García [J. Chem. Phys. 2002, 116, 10664] (EG), and (3) two semiempirical surfaces (MSINDO and reparametrized MSINDO). We find that most of the energy appears in product translation at energies just above the reactive threshold; however, HD vibration and rotation become quite important at energies above 1 eV, each accounting for over 20% of the available energy above 1.5 eV, according to the B3LYP calculations. The barrier on the B3LYP surface, though being later than that on EG, predicts significantly higher HD vibrational excitation than EG. This deviation is contradictory to what would be expected on the basis of the Polanyi rules and derives from modest differences in the potential energy surfaces. The CD3 internal energy is generally quite low, and we present detailed rotational state distributions which show that the CD3 rotational distribution is largely independent of collision energy in the 0.75-1.95 eV range. The most populated rotational levels are N = 5 and 6 on B3LYP, with most of that excitation being associated with motion about the C2 axes, rather than C3 axis, of the CD3 product, in good agreement with the experimental results. Through our extensive studies in this and previous work concerning the scattering dynamics, we conclude that B3LYP/6-31G provides the best available description of the overall dynamics for the title reaction at relatively high collision energies.

H + CD4 abstraction reaction dynamics: excitation function and angular distributions.

We compare experimental photoloc measurements and quasi-classical trajectory calculations of the integral cross sections, lab-frame speed distributions, and angular distributions associated with the CD3 products of the H + CD4(nu = 0) --> CD3 + HD reaction at collision energies ranging from 0.5 to 3.0 eV. Of the potential energy surfaces (PES) we explored, the direct dynamics calculations using the B3LYP/6-31G** density functional theory PES provide the best agreement with the experimental measurements. This agreement is likely due to the better overall description that B3LYP provides for geometries well removed from the minimum energy path, even though its barrier height is low by approximately 0.2 eV. In contrast to previous theoretical calculations, the angular distributions on this surface show behavior associated with a stripping mechanism, even at collision energies only approximately 0.1 eV above the reaction barrier. Other potential energy surfaces, which include an analytical potential energy surface from Espinosa-García and a direct dynamics calculation based on the MSINDO semiempirical Hamiltonian, are less accurate and predict more rebound dynamics at these energies than is observed. Reparametrization of the MSINDO surface, though yielding better agreement with the experiment, is not sufficient to capture the observed dynamics. The differences between these surfaces are interpreted using an analysis of the opacity functions, where we find that the wider cone of acceptance on the B3LYP surface plays a crucial role in determining the integral cross sections and angular distributions.

A reinterpretation of the mechanism of the simplest reaction at an sp3-hybridized carbon atom: H + CD4 --> CD3 + HD.

A comparison between theory and experiment for the benchmark H + CD4 --> HD + CD3 abstraction reaction yields a reinterpretation of the reaction mechanism and highlights the unexpected contribution of a stripping mechanism. Whereas the best analytic surface fails to reproduce experiment, a first-principles direct-dynamics (on the fly) treatment is in good agreement, showing that the H + CD4 reaction exhibits extreme sensitivity to modest differences in the potential energy surface. We find that bent H-D-C transition state geometries play an important role in the dynamics. A simple model that relates the scattering angle impact parameter and cone of acceptance accounts well for the overall reaction dynamics.