Photolysis of Methane at 1236‐Å: Quantum Yield of Hydrogen Formation

Effect of hydrocarbon molecular decomposition on palladium-assisted laser-induced plasma ablation

The Q-switched Nd:YAG laser is focused on a palladium target in the control chamber filled with various hydrocarbon atmospheres (C1-C4) to investigate their effect on the palladium ablated mass, gas reaction products, and corresponding plasma parameters (such as electron density N_e and plasma temperature T_e) during molecular decomposition. The plasma parameters arise mainly from the Pd nanocatalytic activity during the laser-induced plasma process. We compare synthetic air atmosphere to hydrocarbon media to understand how the latter generates excess heat via oxygen-free exothermic (recombination) reactions. Subsequently, this gives rise to more energetic plasma and higher temperature, regarding the large amount of nanoparticles released into the plasma. The dynamics of the decomposition/recombination events accompany the nanocatalyst activity, leading to soot deposition all over the chamber.

First ultraviolet absorption band of methane: Anab initiostudy

Quantum mechanical calculations of the cross sections for photodissociation of CH4 and CD4 in the 1t2-->3s band are presented. The potential energy surfaces for the three states correlating with the 1 1T2 state at tetrahedral geometries are calculated. The elements of the (3x3) matrix representing the electronic Hamiltonian in the diabatic basis are expanded in powers of nuclear coordinates, up to the second order. The expansion coefficients are based on accurate multireference configuration interaction calculations. The electronically nonadiabatic dynamics is treated with the multiconfiguration time-dependent Hartree approach. All nine internal degrees of methane are included in the quantum dynamics simulations. The calculated cross section agrees well with experiment. Semiclassical calculations using the reflection principle suggest that the peaks in the spectrum correspond to the three adiabatic electronic states correlating with the 1 1T2 state at Td geometries. However, the non-Born-Oppenheimer terms in the Hamiltonian have a strong effect on the positions of the peaks in the absorption spectrum. The results of semiclassical calculations, which neglect these terms, are therefore quite different from the accurate quantum results and experiment.

Photodissociation of methane: Exploring potential energy surfaces

The potential energy surface for the first excited singlet state (S(1)) of methane is explored using multireference singles and doubles configuration interaction calculations, employing a valence triple zeta basis set. A larger valence quadruple zeta basis is used to calculate the vertical excitation energy and dissociation energies. All stationary points found on the S(1) surface are saddle points and have imaginary frequencies for symmetry-breaking vibrations. By studying several two-dimensional cuts through the potential energy surfaces, it is argued that CH(4) in the S(1) state will distort to planar structures. Several conical intersection seams between the ground state surface S(0) and the S(1) surface have been identified at planar geometries. The conical intersections provide electronically nonadiabatic pathways towards products CH(3)((approximately)X (2)A"(2))+H, CH(2)((approximately)a (1)A(1))+H(2), or CH(2)((approximately)X (3)B(1))+H+H. The present results thereby make it plausible that the CH(3)((approximately)X (2)A"(2))+H and CH(2)((approximately)a (1)A(1))+H(2) channels are major dissociation channels, as has been observed experimentally.

Primordial Oil Slick

Calculations and some preliminary experiments suggest that an early methane atmosphere would have been polymerized by solar ultraviolet radiation in geologically short periods of time. An oil slick 1 to 10 meters thick could have been produced in this way and might well have been of considerable importance in the development of life.