Imaging covalent bond formation by H atom scattering from graphene

Viewing the atomic-scale motion and energy dissipation pathways involved in forming a covalent bond is a longstanding challenge for chemistry. We performed scattering experiments of H atoms from graphene and observed a bimodal translational energy loss distribution. Using accurate first-principles dynamics simulations, we show that the quasi-elastic channel involves scattering through the physisorption well where collision sites are near the centers of the six-membered C-rings. The second channel results from transient C–H bond formation, where H atoms lose 1 to 2 electron volts of energy within a 10-femtosecond interaction time. This remarkably rapid form of intramolecular vibrational relaxation results from the C atom’s rehybridization during bond formation and is responsible for an unexpectedly high sticking probability of H on graphene

Vacuum ultraviolet photodissociation of hydrogen bromide

Photodissociation dynamics of HBr at a series of photolysis wavelengths in the range of 123.90-125.90 nm and at around 137.0 nm have been studied using the H atom Rydberg "tagging'' time-of-flight technique. The branching fractions between the channels forming ground Br(P-2(3/2)) and spin-orbit excited Br(P-2(1/2)) atoms together with the angular distributions of the products corresponding to these two channels have been measured. The photolysis wavelengths in this work excited the HBr molecule from the ground state X (1)Sigma(+) to various Rydberg states and the V (1)Sigma(+) ion-pair valence state. Predissociation via these states displays rich behavior, indicating the influence of the nature of initially excited states and the coupling to other bound or repulsive states on the predissociation dynamics

Competition between Direct and Indirect Dissociation Pathways in Ultraviolet Photodissociation of HNCO

Photodissociation dynamics of HNCO at photolysis wavelengths between 200 and 240 nm have been studied using the H-atom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been determined. At low photon energy excitation, the product translational energy distribution is nearly statistical and the angular distribution is isotropic, which is consistent with an indirect dissociation mechanism, i.e., internal conversion from S-1 to S-0 surface and dissociation on S-0 surface. As the photon energy increases, a direct dissociation pathway on S-1 surface opens up. The product translational energy distribution appears to be quite nonstatistical and the product angular distribution is anisotropic. The fraction of direct dissociation pathway is determined to be 36 +/- 5% at 202.67 nm photolysis. Vibrational structures are observed in both direct and indirect dissociation pathways, which can be assigned to the NCO bending mode excitation with some stretching excitation