Robert R. Lucchese

Applications of the Schwinger variational principle to electron-molecule collisions and molecular photoionization

In this article we present a detailed overview of our studies of molecular photoionization and electron-molecule collisions in which we have used Schwinger-like variational principles and several important extensions of these principles. The various variational functionals and formulations, the interrelationships between these formulations, and a detailed discussion of the numerical and computational procedures which have been used in applications are presented.

Monte Carlo simulations of gas‐phase collisions in rapid desorption of molecules from surfaces

We have examined the effects of collisions among the molecules desorbing from solid surfaces by means of Monte Carlo simulations, and have identified the conditions under which and to what extent these collisions would influence the experimentally observed product distributions. By simulating the experiment performed by Cowin e t a l. [Surf. Sci. 7 8, 545 (1978)] on the laser induced desorption of D2 from tungsten, we have found that at high coverages each desorbate makes on average 2.9 collisions which decreases to no collisions at very low coverages. These collisions affect the product distribution at high coverages to such an extent that even if the nascent desorbed flux is under thermal equilibrium, these post‐desorption collisions could lead to nonequilibrium distributions. The effect of the post‐desorption collisions is influenced by the rate of heating the surface and the kinetics of the desorption process.

Strong-field rescattering physics—self-imaging of a molecule by its own electrons

When an atom or molecule is exposed to a short intense laser pulse, electrons that were removed at an earlier time may be driven back by the oscillating electric field of the laser to recollide with the parent ion, to incur processes like high-order harmonic generation (HHG), high-energy above-threshold ionization (HATI) and nonsequential double ionization (NSDI). Over the years, a rescattering model (the three-step model) has been used to understand these strong field phenomena qualitatively, but not quantitatively. Recently we have established such a quantitative rescattering (QRS) theory. According to QRS, the yields for HHG, HATI and NSDI can be expressed as the product of a returning electron wave packet with various field-free electron–ion scattering cross sections, namely photo-recombination, elastic electron scattering and electron-impact ionization, respectively. The validity of QRS is first demonstrated by comparing with accurate numerical results from solving the time-dependent Schrodinger equation (TDSE) for atoms. It is then applied to atoms and molecules to explain recent experimental data. According to QRS, accurate field-free electron scattering and photoionization cross sections can be obtained from the HATI and HHG spectra, respectively. These cross sections are the conventional tools for studying the structure of a molecule; thus, QRS serves to provide the required theoretical foundation for the self-imaging of a molecule in strong fields by its own electrons. Since infrared lasers of duration of a few femtoseconds are readily available today, these results imply that they are suitable for probing the dynamics of molecules with temporal resolutions of a few femtoseconds.

CALCULATION OF LOW-ENERGY ELASTIC CROSS SECTIONS FOR ELECTRON-CF4 SCATTERING

A new computational approach has been used to evaluate the rotationally summed, vibronically elastic integral cross sections from the scattering of slow electrons (energy ranging from 1.0 eV up to 40.0 eV) by tetrafluoromethane molecules in the gas phase. The various symmetry components have been analyzed using the exact static exchange approximation and also by including a nonempirical, model polarization potential employed before by our group. A comparison with earlier calculations and with existing experiments allows us to assign the symmetries of the shape resonances in the 5–30 eV energy region which are seen by experiments and are also shown by the present calculations.

Trajectory studies of vibrational energy transfer in gas–surface collisions

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...

One-electron resonances in electron scattering from polyatomic molecules

Abstract One-electron resonances in electron scattering from polyatomic molecules were examined using set of interconnected models. We compared resonant states predicted from the virtual orbitals of a minimum-basis-set self-consistent-field (MBS-SCF) calculation with scattering resonances found using both a purely local model potential for the electron-molecule interaction based on an adiabatic separation of the angular and radial motion and a more accurate exact-static-exchange-plus-model-correlation—polarization interaction potential. Considering electron scattering from N2, SF6, and C6H6, we found that the MBS-SCF virtual orbitals were an excellent predictor of the symmetry and approximate location of one-electron resonances. The adiabatic radical potentials were very useful in understanding the mechanism for resonant trapping, although strong non-adiabatic coupling sometimes required more than one adiabatic potential to be considered to accurately represent the resonant dynamics. The essential feature...

Cross section and asymmetry parameter calculation for sulfur 1s photoionization of SF6

We use the polyatomic Schwinger variational method with Pade corrections to calculate the cross section and asymmetry parameter for the sulfur 1s core level photoionization of SF6, for photon energies from threshold up to 2600 eV. Our results show very good agreement with experimental cross sections. Our asymmetry parameter show good qualitative agreement with experiment. A resonant feature at a photon energy of ≈2550 eV is found to be due to a nonvalence type resonant state which is trapped by an l=9 angular momentum barrier.

Laser induced thermal desorption from surfaces

Nonresonant laser induced desorption of adsorbed molecules from surfaces has been simulated using the stochastic trajectory technique. An NO molecule is initially bound to a cold LiF(100) surface. Rapid heating of the surface is then simulated via random forces applied to the edges of the 32 atom surface slabs. When the rate of heating is rapid compared to the rates of thermalization of the degrees of freedom of the molecule, it is found that the mean energies of the translational, rotational, and vibrational degrees of freedom of the desorbing molecule are significantly lower than those corresponding to the temperature of the surface at the instant of desorption. Additionally, the angular distribution of the desorbing molecules is found to peak towards the surface normal, and the rotational angular momentum vector is preferentially aligned parallel to the surface plane. These results shed light on recent experimental observations.

Radiation damage of biosystems mediated by secondary electrons: Resonant precursors for uracil molecules

Calculations are presented for the energy locations and spatial structures of low-energy resonant states describing transient negative ions (TNIs) of the uracil molecule in the gas phase. The resonant states are modeled using scattering calculations of low energy electrons interacting with isolated molecules in their equilibrium geometry. The interaction forces used in this model are described in detail. Examination of the spatial densities of the excess resonant electrons for the various TNIs found by the calculations allows one to associate the metastable anions with specific features of the experimentally observed fragmentation patterns.

Effects of gas‐phase collisions in rapid desorption of molecules from surfaces in the presence of coadsorbates

The effects of gas‐phase collisions in mixtures of gases rapidly desorbed from surfaces are studied using direct Monte Carlo techniques. The results are compared with the effects observed in the desorption of pure gases under similar conditions. The translational energy distribution of the desorbed particles are found to deviate from the Boltzmann distribution and are found to be well represented by ellipsoidal Boltzmann distributions. In this respect the rapid desorption process is found to have similarities to the expansion of gases in nozzle sources. The influence of mass, internal degrees of freedom, and surface coverage of the adsorbates on the focusing, accelerating, and cooling effects due to gas‐phase collisions are analyzed. The presence of molecules with active internal degrees of freedom is found to increase the average number of collisions experienced by the rapidly desorbed molecules. However, the influence of this increased number of collisions on the focusing effects due to gas‐phase collisions is less pronounced compared to the focusing effects due to collisions between the desorbed atoms. In a gas mixture containing molecules as the minor constituents (10%) and atoms as the major constituents (90%), atoms are found to be more focused towards the surface normal than the molecules and the mean translational energies of the molecules are found to be less than those calculated in the desorption of pure molecules under similar conditions. The presence of atoms in the desorbed gas mixture is found to increase the most probable speed of the desorbing molecules and this accelerating effect increases with decrease in the mass of the coadsorbed atoms. The light atoms are found to be more efficient than heavy atoms in cooling the internal degrees of freedom.