(2+1) resonant enhanced multiphoton ionization of H_2 via the E, F^(1)Σ^+_g state

In this paper, we report the results of ab initio calculations of photoelectron angular distributions and vibrational branching ratios for the (2+1) REMPI of H_2 via the E, F^(1)Σ^+_g state, and compare these with the experimental data of Anderson et al. [Chem. Phys. Lett. 105, 22 (1984)]. These results show that the observed non‐Franck–Condon behavior is predominantly due to the R dependence of the transition matrix elements, and to a lesser degree to the energy dependence. This work presents the first molecular REMPI study employing a correlated wave function to describe the Rydberg–valence mixing in the resonant intermediate state

DNA Damage by Ionizing Radiation: Tandem Double Lesions by Charged Particles

Oxidative damages by ionizing radiation are the source of radiation-induced carcinogenesis, damage to the central nervous system, lowering of the immune response, as well as other radiation-induced damages to human health. Monte Carlo track simulations and kinetic modeling of radiation damages to the DNA employ available molecular and cellular data to simulate the biological effect of high and low LET radiation io the DNA. While the simulations predict single and double strand breaks and base damages, so far all complex lesions are the result of stochastic coincidence from independent processes. Tandem double lesions have not yet been taken into account. Unlike the standard double lesions that are produced by two separate attacks by charged particles or radicals, tandem double lesions are produced by one single attack. The standard double lesions dominate at the high dosage regime. On the other hand, tandem double lesions do not depend on stochastic coincidences and become important at the low dosage regime of particular interest to NASA. Tandem double lesions by hydroxyl radical attack of guanine in isolated DNA have been reported at a dosage of radiation as low as 10 Gy. The formation of two tandem base lesions was found to be linear with the applied doses, a characteristic of tandem lesions. However, tandem double lesions from attack by a charged particle have not been reported

Electronic excitation of oriented molecules by low-energy electrons: An application to H2

We report inelastic differential cross sections for electronic excitation of an oriented molecule by low-energy electrons. Specifically we look at the dependence of these cross sections for the X1Σg+→b3Σu+ transition in H2 on both incident and scattered angles as well as on impact energy. These electron scattering cross sections exhibit a pronounced dependence on the incident and scattered angles, which suggests that related electron-energy-loss spectroscopy studies can be a useful probe of adsorbate-substrate structure

Studies of electron–polyatomic-molecule collisions: Applications to e-CH4

We report the first application of the Schwinger multichannel formulation to low-energy electron collisions with a nonlinear polyatomic target. Integral and differential cross sections are obtained for e-CH4 collisions from 3 to 20 eV at the static-plus-exchange interaction level. In these studies the exchange potential is directly evaluated and not approximated by local models. An interesting feature of the small-angle differential cross section is ascribed to polarization effects and not reproduced at the static-plus-exchange level. Our differential cross sections are found to be in reasonable agreement with existing measurements at 7.5 eV and higher energies

Application of the Schwinger multichannel formulation to electron-impact excitation of the B 1Σu+ state of H2

In this paper we report cross sections for electron-impact excitation of the X 1Σg+→B 1Σu+ transition in H2 for collision energies of 15, 20, and 30 eV. For this dipole-allowed transition with its associated long-range potential, the contributions of the more strongly scattered low-angular-momentum partial waves to the cross section were obtained from a two-state Schwinger multichannel calculation, and a modified Born-closure scheme was used to include the contributions from the remaining weakly scattered partial waves. Agreement between the calculated differential cross sections and available experimental data is encouraging

Studies of electron-molecule collisions: Applications to e-H2O

We report elastic differential and momentum transfer cross sections for the elastic scattering of electrons by H2O for collision energies from 2 to 20 eV. These fixed-nuclei static-exchange cross sections were obtained using the Schwinger variational approach. In these studies the exchange potential is directly evaluated and not approximated by local models. The calculated differential cross sections, obtained with a basis set expansion of the scattering wave function, agree well with available experimental data at intermediate and larger angles. As used here, the results cannot adequately describe the divergent cross sections at small angles. An interesting feature of the calculated cross sections, particularly at 15 and 20 eV, is their significant backward peaking. This peaking occurs in the experimentally inaccessible region beyond a scattering angle of 120°. The implication of this feature for the determination of momentum transfer cross sections is described

Electron-Impact Excitation Cross Sections for Modeling Non-Equilibrium Gas

In order to provide a database for modeling hypersonic entry in a partially ionized gas under non-equilibrium, the electron-impact excitation cross sections of atoms have been calculated using perturbation theory. The energy levels covered in the calculation are retrieved from the level list in the HyperRad code. The downstream flow-field is determined by solving a set of continuity equations for each component. The individual structure of each energy level is included. These equations are then complemented by the Euler system of equations. Finally, the radiation field is modeled by solving the radiative transfer equation

On the possibility of using differential cross section measurements for the electronic excitation of adsorbates by an electron beam, to determine the adsorbate orientation

We show, by detailed electron–molecule scattering calculations, that the angular dependence of electron energy loss spectra in which an adsorbate is electronically excited can be used to identify the orientation of the molecule with respect to the surface and the nature of the final states. The calculations are exploratory and were carried out for an H2 molecule. The transition amplitude for electron–molecule scattering is calculated by using the Schwinger variational principle with two open channels. The effects of the surface were introduced through a semiquantitative model which treats the surface as a partly reflecting, flat mirror

Rotational branching ratios in (1+1) resonant-enhanced multiphoton ionization of NO via the A 2Σ+ state

The rotational branching ratios resulting from (1+1) resonant-enhanced multiphoton ionization of NO via the A 2Σ+ Rydberg state are analyzed. Theoretical results using ab initio molecular parameters agree reasonably well with recent experimental data. More importantly, the analysis underscores the importance of the molecular nature of the problem and its resulting complexities. It is shown that, for photoionization of a Σ state that leaves the ion in a Σ state, the allowed rotational states of the ion satisfy the selection rule ΔN+l=odd, where ΔN is the difference in (electronic + rotational) quantum numbers for the neutral and for the ion, and l is the partial wave of the electron. Based on this selection rule, it follows that the predominantly gerade 3sσ Rydberg orbital of the A 2Σ+ state couples only to the ungerade channel in the continuum (l odd), thereby suppressing the ΔN=±1 peaks, in agreement with experiment. The molecular nature of the ionic potential leads to strong l mixing in electronic continuum orbitals. In fact, the influence of a nearby shape resonance causes the f wave to be dominant in the σ channel