Evelyn M. Goldfield

Nonadiabatic interactions in the photodissociation of ICN

Photodissociation of ICN in the A continuum has been modeled using classical trajectories assuming that all absorption from the linear ground state is to a single linear diabatic excited state which dissociates to form I*(2P1/2) and CN(2Σ+,v=0). It is also assumed that, in nonlinear excited state configurations, nonadiabatic transitions occur to a bent surface which correlates diabatically to ground state I(2P3/2) and CN(2Σ+,v=0). Empirical potential surfaces with frozen CN bond lengths are employed, while transitions between the surfaces are treated using either the Miller–Meyer classical electron model or a simple diabatic version of the Tully–Preston surface‐hopping model. With the above assumptions, the Miller–Meyer method is found to give much better agreement with the experimental results. Theoretical results obtained with the Miller–Meyer method are compared with recent experimental data on the I*/I branching ratio, the average CN rotational energies, and the product CN rotational distributions as...

Quantum dynamics of Renner--Teller vibronic coupling: The predissociation of HCO

A Hamiltonian model and parity‐adapted wave packet representation are developed to describe a rotating triatomic system with two Renner–Teller coupled potential surfaces, and HCO predissociation is studied. New configuration interaction calculations on HCO are performed to determine its excited A(2A‘) potential surface, and Bowman, Bittman, and Harding’s X(2A’) ground potential surface is employed. The properties of many resonances, correlating with stretch/bend excitations on the A‘ surface, are determined. Resonance energies and decay constants are in good agreement with experimental results of Houston and co‐workers, but CO rotational product distributions tend to be hotter and narrower than experiment, particularly for pure bend excitations. Wave packet dynamics involves growth of amplitude on the A’ surface near collinear geometries via Renner–Teller coupling, and subsequent adiabatic evolution to determine product distributions. The wave packets probe a previously untested part of the A’ surface, ...

Anatomy of a vertical metal halide discharge

Experimental measurements are compared with model calculations for a vertical metal halide discharge. The 400‐W arctube has a gap of 4.3 cm, an inside diameter of 2.0 cm, and operates at about 5‐atm Hg pressure with NaI and ScI3 additives. Emission spectroscopy is used to measure the discharge temperature and the densities of additive species. The model is based on the assumption of local thermodynamic equilibrium, and predicts the discharge behavior from first principles. Coupled equations describing the convective fluid flow, chemical equilibrium and diffusion, radiation transport, and discharge current flow are solved simultaneously so as to provide a self‐consistent description of the discharge. Model results are compared with the experimental measurements. Discharge features such as the hot contraction regions near the electrode tips, and the depletion of additives from the upper parts of the discharge are discussed. Particular attention is paid to quantitative details of the Na diffusion.

Ab initio studies of low‐lying 3Σ−, 3Π, and 5Σ− states of NH. I. Potential curves and dipole moment functions

Configuration interaction wave functions, potential energy curves, and dipole moment functions have been calculated for the four lowest 3Σ− and the three lowest 3Π states and 5Σ− states of NH. The electronic wave functions were constructed to give a balanced description of valence–Rydberg interactions. Two repulsive states have been identified as important photodissociation pathways. We present spectroscopic constants for the bound states and compare our results to other theoretical and to experimental work. The possible predissociation of the A 3Π state by the 1 5Σ− state is discussed.

Trajectory studies of OH vibrational excitation propensities in the reaction of O(1D) with H2

The reaction of excited oxygen atoms with hydrogen molecules has been explored using classical trajectory methods, with particular emphasis on those features of the dynamics responsible for the OH vibrational distribution. Plots of final OH vibrational action vs. initial H2 phase are analyzed. A Monte Carlo ensemble of 6300 trajectories is also studied. Two methods are used to classify trajectories: (1) the amount of exponential separation of nearby trajectories and (2) the number of minimum bond length exchanges during the course of the trajectory. Trajectories which undergo different numbers of minimum exchanges are found to have markedly different propensities for producing OH in a given vibrational state. Direct insertions are found to play an important role in the reaction; two very direct types of trajectories are identified, one preferentially populating OH in v=0; the other leading to highly vibrationally excited OH (v=4 or 5). Slightly less direct trajectories are found to result preferentially i...

CO product distributions from the visible photodissociation of HCO

The final state distribution of carbon monoxide produced in the photodissociation of the formyl (HCO) radical has been studied both experimentally and theoretically. Renner–Teller coupling between the excited HCO A state and the ground state leads to dissociation and yields H and CO. Vibrational and rotational distributions have been measured for CO produced after excitation to specific vibrational levels on the A←X transition of HCO cooled in a supersonic expansion. The strongest transitions are for excitation to vibrational states with six to 16 quanta in the bending mode, and dissociation from these states produces inverted CO rotational distributions with average rotational quantum numbers <J≳ in the 22–33 range. The value of <J≳ increases monotonically with the vibrational quantum number describing the bend of the excited triatomic. Experiments involving excitation of one quantum of the C–H stretching motion have revealed that this vibration results in increased rotational excitation of the product CO with values of <J≳ as high as 41. In contrast, experiments indicate that the C–O stretching mode of HCO acts nearly as a spectator during the dissociation process. Excitation of HCO states with one quantum of C–O stretch yields vibrationally excited CO as the dominant dissociation product, but with a rotational distribution similar to that for CO(ν=0) produced following the excitation of HCO states without the quantum of C–O stretch. Classical trajectory calculations on an ab initiopotential energy surface have modeled many of the experimental features and trends of the CO product distributions. There are, however, some discrepancies in the positions of rotational maxima and in the efficiency of the coupling of the C–O vibration of HCO to the dissociation coordinate. It is not clear whether these are due to approximations made in the modeling or inaccuracies in the potential energy surface.

Theoretical study of the radiative properties of the triplet states of the NH radical: Transition dipole moments, radiative lifetimes, photodissociation cross sections

Ab initio transition dipole moments between the X 3Σ− and the A 3Π states of NH and the 2 3Σ− and 2 3Π dissociative states have been computed. These transition dipole moments have been used to compute photodissociation cross sections and interstellar photodissociation rates for NH. Photodissociation rates for NH in the interstellar radiation field range from 1.9 to 4.7×10−10 s−1 depending on the field used. Direct dissociation via the 2 3Σ− and 2 3Π states is found to be the only important pathway for photodestruction of NH in diffuse interstellar clouds. A large photodissociation cross section is found for the A 3Π–2 3Σ− transition and a method for photolyzing NH in the laboratory is suggested. Einstein A coefficients and radiative lifetimes have also been computed for the A–X transition and are compared with recent theoretical and experimental work.

Wave packet dynamics of vibrational quenching in collisions of Kr and O2

Collisional quenching of vibrationally excited O+2 by Kr atoms is studied using time‐dependent quantum mechanics with an emphasis on exploring the underlying mechanisms. Three‐dimensional solutions to the time‐dependent Schrodinger equation with zero total angular momentum are obtained at two values of the average collision energy, 0.5 and 0.1 eV. At 0.5 eV, the 1→0 quenching probability is computed to be 0.091. The rotational distributions of the vibrationally quenched O+2 are highly bimodal. An analysis of ‘‘nascent’’ v=0 probability density reveals the origin of this bimodality and yields insight into the mechanisms of vibrational deexcitation. Comparisons are made to a corresponding classical study. Scattering at 0.1 eV is complicated by the existence of overlapping resonances in addition to the direct scattering. Due to these resonances, the quenching probability is an extremely sensitive function of energy. An averaged quenching probability of 0.078 at 0.1 eV is computed. Average resonance lifetimes...