[en] The feasibility of hydroxycarbene being an intermediate in the laser photolysis of formaldehyde and the energy barrier in the conversion of formaldehyde to hydroxycarbene were calculated using different theoretical methods. The results are tabulated and the most significant result was that S₀ trans, S₀ cis, and T₁ gauche electronic states of hydroxycarbene all lie below the formaldehyde S₁ state. Therefore all three are energetically accessible and the feasibility of hydroxycarbene being the intermediate state is demonstrated

[en] We have applied the stochastic trajectory method to the scattering of NO from the (001) surface of an LiF crystal. The surface was represented by a 32 atom primary zone and the effects of the rest of the surface atoms were included using the Generalized Langevin Equation method. The forces between the surface atoms were treated using a shell model and the gas--surface interactions included short-range repulsions, attractive dispersion forces, and the interaction of the dipole moment of NO and the surface ions. The dependence of the dipole moment and the dependence of the molecular polarizability on the internuclear separation of NO were also included. The calculated vibrational energy accommodation coefficient was less than 1% when trapping of the gas diatom was negligible. By modifying the vibrational frequency of the scattered diatom we have examined the dependence of vibrational energy transfer on frequency. An exponential dependence on the ratio of the molecular frequency to the surface Debye frequency was found, similar to behavior observed for vibrational relaxation in condensed phases. We have also examined the relative efficiency of the coupling of the vibrational mode to the translational and rotational modes of the gas diatom and to the phonon modes of the surface. We found that the vibrational mode is most strongly coupled to the surface phonons with a much weaker coupling to the rotational and translational modes

[en] Yields of molecular ions desorbed from surfaces by energetic heavy ions have been studied using a thermal spike model. A temperature profile was assumed which is appropriate for high linear energy transfer (LET) ion-solid interactions. The desorption kinetics were assumed to follow a simple Arrhenius rate law which has been found to be useful in understanding desorption due to rapid temperature jumps. With these assumptions, we have fit recent desorption experiments where the exciting ions had a LET in the range of 10--100 MeV cm² mg^-1 and where the desorbed molecules ranged from valine to insulin. The thermal spike model predicted nonlinear dependence of the yield on LET at lower LET and a nearly linear dependence of the yield on LET at high LET. These features are in good agreement with the experimental data. We also considered modified models which included a loss channel due to fragmentation. These models gave somewhat better agreement with the experimental data. The parameters obtained in these fits were analyzed using sensitivity analysis to determine their uncertainties and to determine the interdependencies between parameters. The best fit model has been used to predict desorption yields at higher and lower LET and for different initial energy densities in the ionized track

[en] Classical trajectory calculations of the desorption of large molecules from surfaces are considered. Initial conditions correspond to differing models for the initial excitation mechanism of the molecular adsorbates. A linear chain of anharmonic oscillators is used to model the system. Results for the popcorn and thermal spike mechanism are given. In both cases the essential dynamical feature of the desorption is found to be the coupling of the expansion mode of the system to the desorption mode. Simple dynamical models also will be presented for large molecular adsorbates which include a few important degrees of freedom and an internal heat bath. Applications of these models to large scale simulations will be considered

[en] We discuss a method for systematically correcting results obtained using variational expressions in scattering theory. The approach taken is to compute a sequence of Pade approximants of the form [N/N] for the error in an initial variational estimate obtained using a basis-set expansion. The relationship between the Pade-approximant approach and the iterative Schwinger method for correcting variational estimates is also examined. We discuss a large class of general variational expressions to which the Pade-approximant approach can be applied. The variational expressions considered include those for the wave function, for photoionization transition matrix elements, as well as for scattering matrix (K-matrix) elements. We have applied this approach to the 5sigma photoionization of CO using the frozen-core Hartree-Fock and fixed-nuclei approximations. We find that the Pade-approximant method converges rapidly and reliably. Both total photoionization cross sections and photoelectron angular distributions from threshold to 40 eV are presented and compared to previous experimental and theoretical results. We find major quantitative discrepancies between the present results for the total cross section and previous theoretical results

[en] The results of the application of sensitivity analysis to the simulation of gas--surface scattering are presented. The simulation technique was the three dimensional generalized Langevin method of Tully, and the sensitivity analysis methods were the coupled solution of system and sensitivity equations approach and a new stochastic sensitivity analysis method. We found that the coupled solution approach only converged when trajectories which involved multiple collisions of the gas with the surface were excluded from the Monte Carlo averaging of this simulation approach. The stochastic sensitivity method was found to give convergent results for both single collision trajectories and multiple collision trajectories. In the stochastic sensitivity method the sensitivity coefficients are computed by selecting the parameters of the system randomly from a given distribution and then performing a linear least-squares analysis of the variations of the output variables of the simulation model with respect to variations of the input parameters. The stochastic sensitivity analysis method should also be applicable to completely deterministic trajectory simulations which however exhibit ergodic behavior in individual trajectories. In addition to first-order coefficients, we also used the stochastic sensitivity analysis method to compute higher order coefficients (selected second and third order) and derived sensitivity coefficients. We have applied both the coupled solution and stochastic sensitivity analysis method to the Ar--Pt (111) scattering system, where we found that the surface corrugation and the steepness of the respulsive two-body interactions were essential features of the interaction potential in determining the quantity of energy transfered

[en] Two different models for treating the contributions of dynamical electron-positron correlation forces to the full interaction between low-energy positron beams scattered off polyatomic gases are considered and applied to obtain the elastic integral and differential cross sections (at collision energies below Ps formation) for the methane molecule. The computed quantities are then compared with available experiments, both for integral and differential data, and found to be in good accord with measured quantities. Both models also agree rather closely with each other and the physical reasons for this behavior are briefly discussed

[en] Classical trajectory calculations of the desorption of large molecules from surfaces are considered. Initial conditions correspond to differing models for the initial excitation mechanism of the molecular absorbates. A linear chain of anharmonic oscillators is used to model the system. Results for the popcorn and thermal spike mechanism are given. In both cases the essential dynamical feature of the desorption is found to be the coupling of the expansion mode of the system to the desorption mode. Simple dynamic models also will be presented for large molecular adsorbates which include a few important degrees of freedom and an internal heat bath. Applications of these models to large scale simulations will be considered

[en] We have compared variational functionals for multichannel scattering. The functionals considered were Schwinger-type functionals based on the close-coupling equations, the Schwinger-type variational functionals of Takatsuka and McKoy [Phys. Rev. A 24, 2473 (1981)], and a Kohn-type variational functional. The results for a simple Huck-model potential containing both open and closed channels indicate that the Schwinger-type variational functionals yielded very similar results and that all the methods considered converged at a similar rate with respect to the closed-channel expansion. To obtain good convergence with any of these methods it was essential to include separate trial functions outside the range of the Huck square-well potential in the channels where a correct asymptotic form of the wave function was required, those being only the closed channels for the Schwinger-type functionals and both open and closed channels for the Kohn-type functionals. These asymptotic functions were needed to reproduce the discontinuity in the second derivative of the wave function due to the discontinuities in the model potential. Convergence characteristics of these methods with respect to target-state expansions were also considered

[en] The interaction of a high energy ions with a solid leads to a cylindrical region of highly vibrationally excited atoms. The authors model this excited region as a thermal spike and consider the effects of the spike on molecules absorbed on the surface of the solid. Results will be presented of simulations of the desorption induced by the thermal spike which show that both large and small molecular systems can be desorbed without significant excitation of the internal degrees of freedom of the molecules. The translational energy distributions of the desorbed particles will also be considered. Kinetic models of the desorption process will be compared to available experimental data on desorbed ion yields. These models give good agreement with experiments were variations in the linear energy transfer of the exciting primary ion are considered. The effects of postdesorption collisions on the internal energy and velocity distributions of the desorbed molecules will also be discussed