Ann E. Orel

Elastic scattering of low-energy electrons by tetrahydrofuran

We present the results of ab initio calculations for elasticelectron scattering by tetrahydrofuran (THF) using the complex Kohnvariational method. We carried out fixed-nuclei calculations at theequilibrium geometry of the target molecule for incident electronenergies up to 20 eV. The calculated momentum transfer cross sectionsclearly reveal the presence of broad shape resonance behavior in the 8-10eV energy range, in agreement with recent experiments. The calculateddifferential cross sections at 20 eV, which include the effects of thelong-range electron-dipole interaction, are alsofound to be in agreementwith the most recent experimental findings.

Time‐dependent calculations of molecular photodissociation resonances

The molecular photodissociation dynamics of a model, collinear CO2 system is investigated using the time‐dependent wave packet method. Resonance structures in the absorption line shape, found previously in time‐independent studies, are correlated to particular oscillatory motions in the dissociating wave packet dynamics. Dramatic changes in the absorption line shape are predicted for this system for short pulse excitation due to the removal of the effects of one class of resonances. Three different methods of solving the time‐dependent Schrodinger equation were tested and the optimal scheme was used in these calculations.

Wave packet studies of predissociation in H3 Rydberg states

The predissociation of H3 Rydberg states by the two lowest electronic states of H3 is studied using time‐dependent wave packet techniques. The final vibrational state distribution function and branching ratios between two‐ and three‐body channels are calculated within a two‐dimensional approximation. The effect of initial vibrational excitation on these distributions is studied, and comparison is made to recent experiments.

Calculation of the photoionization cross section of the 1Σu+ excimer state of Ar2

The vertical photoionization profiels of the bound 1Σu+ excimer state of Ar2 are reported.(AIP)

The Complex Kohn Variational Method

Though substantial progress has been made in the theoretical study of electron collisions with molecules and molecular ions, most work has been restricted to diatomic or linear targets. Electron- and photon-molecule collision cross sections are needed in such diverse areas such as advanced laser development, pollution control, the design of highspeed space re-entry vehicles, the manufacture of semiconductor devices and plasma driven chemical synthesis. For example, the photoionization of polyatomic radicals, which plays an important role in combustion1, requires a description of electron scattering from a polyatomic molecular ion. Such studies are scarce. In the area of plasma enhanced chemical vapor deposition and etching2, studies indicate a subtle interplay between the neutrals, ions, electrons, and the surface. A critical lack of fundamental cross sections is hindering our understanding of these processes. Reliable theoretical methods are exceptionally important because of the extreme difficulty of experiments in this area.

Quantum‐mechanical calculations of the dissociation of H3 Rydberg states

We present three‐dimensional, time‐dependent quantum‐mechanical calculations of the dynamics of the dissociation of H3 Rydberg states at total energies up to 6 eV. The method used in this work employs a Chebychev propagator in time, and computes the kinetic‐energy operators in the discrete variable representation. We calculate the total dissociation cross section, as well as partial vibrational and rotational cross sections, and compare our results to previous two‐dimensional calculations and to experiment. The results display clear three‐dimensional effects, and indicate the importance of including both sheets of the H3 ground potential‐energy surface in the dynamics.

Interchannel coupling and ground state correlation effects in the photoionization of CO

We describe a general procedure for applying the complex Kohn variational method to the calculation of molecular photoionization cross sections and asymmetry parameters. In this initial application of the method, we examine the effects of interchannel coupling and ground state correlation on the X 2Σ+(5σ−1), A 2Π(1π−1), and B 2Σ+(4σ−1) partial photoionization cross sections and asymmetry parameters for the CO molecule. We find that the dominant effect of interchannel coupling is to remove a spurious π→π* resonance feature from the continuum that appears at the frozen‐core Hartree–Fock level. We also find that it appears to be important to combine the effects of final channel coupling with a correlated initial target state to achieve quantitatively correct cross sections.

Dissociative recombination of H3+: progress in theory

Dissociative recombination is the main destruction process for ground–state H3+ in diffuse interstellar medium. Experiments agree on relatively large cross–sections for this reaction. Time–dependent two–dimensional calculations confirm the experimental results at high energy as well as the observed predissociation rates of H3 Rydberg states, due to non–adiabatic interactions. However, the value for low–energy crosssection, deduced from the predissociation rates by an extrapolation procedure, is about four orders of magnitude lower than the measured one. A calculation based on multichannel quantum defect theory suggests that an indirect non–adiabatic process may prevail in this case. The cross–section increases by orders of magnitude compared with the extrapolated value when indirect couplings via apparently ineffective channels are properly considered. We discuss how this channel–mixing mechanism can be effective in the case of H3+, and show encouraging results stressing the role of Rydberg series or ‘closed channels’. We also discuss possible three–dimensional effects that could enhance the process at low energy.

Imaging molecular isomerization using molecular-frame photoelectron angular distributions

We report the results of an ab initio theoretical study of K-shell photoionization of the monocation of acetylene in its ground (X) and electronically excited (A) states using the complex Kohn variational method. We show how the molecular-frame photoelectron angular distributions, which are sensitive to the instantaneous positions of the nuclei, can be used to monitor the isomerization of the excited cation from an initial linear geometry to a final vinylidene-like structure.

Theoretical study of radiative electron attachment to CN, C2H, and C4H radicals.

A first-principle theoretical approach to study the process of radiative electron attachment is developed and applied to the negative molecular ions CN(-), C4H(-), and C2H(-). Among these anions, the first two have already been observed in the interstellar space. Cross sections and rate coefficients for formation of these ions by direct radiative electron attachment to the corresponding neutral radicals are calculated. For the CN molecule, we also considered the indirect pathway, in which the electron is initially captured through non-Born-Oppenheimer coupling into a vibrationally resonant excited state of the anion, which then stabilizes by radiative decay. We have shown that the contribution of the indirect pathway to the formation of CN(-) is negligible in comparison to the direct mechanism. The obtained rate coefficients for the direct mechanism at 30 K are 7 × 10(-16) cm(3)/s for CN(-), 7 × 10(-17) cm(3)/s for C2H(-), and 2 × 10(-16) cm(3)/s for C4H(-). These rates weakly depend on temperature between 10 K and 100 K. The validity of our calculations is verified by comparing the present theoretical results with data from recent photodetachment experiments.