Quantitative Rescattering Theory for high-order harmonic generation from molecules

The Quantitative Rescattering Theory (QRS) for high-order harmonic generation (HHG) by intense laser pulses is presented. According to the QRS, HHG spectra can be expressed as a product of a returning electron wave packet and the photo-recombination differential cross section of the {\em laser-free} continuum electron back to the initial bound state. We show that the shape of the returning electron wave packet is determined mostly by the laser only. The returning electron wave packets can be obtained from the strong-field approximation or from the solution of the time-dependent Schr\"odinger equation (TDSE) for a reference atom. The validity of the QRS is carefully examined by checking against accurate results for both harmonic magnitude and phase from the solution of the TDSE for atomic targets within the single active electron approximation. Combining with accurate transition dipoles obtained from state-of-the-art molecular photoionization calculations, we further show that available experimental measurements for HHG from partially aligned molecules can be explained by the QRS. Our results show that quantitative description of the HHG from aligned molecules has become possible. Since infrared lasers of pulse durations of a few femtoseconds are easily available in the laboratory, they may be used for dynamic imaging of a transient molecule with femtosecond temporal resolutions.Comment: 50 pages, 15 figure

Diffraction in low-energy electron scattering from DNA: bridging gas phase and solid state theory

Using high-quality gas phase electron scattering calculations and multiple scattering theory, we attempt to gain insights on the radiation damage to DNA induced by secondary low-energy electrons in the condensed phase, and to bridge the existing gap with the gas phase theory and experiments. The origin of different resonant features (arising from single molecules or diffraction) is discussed and the calculations are compared to existing experiments in thin films.Comment: 40 pages preprint, 12 figures, submitted to J. Chem. Phy

New Fragmentation Pathways in K-THF Collisions as Studied by Electron Transfer Experiments: Negative Ion Formation

Time-of-flight (TOF) negative ion mass spectra have been obtained in collisions of 20−100 eV neutral potassium atoms with tetrahydrofuran (C4H8O), an analogue for the sugar unit in DNA/RNA. Major enhancements in O− and C2H3O− production were observed compared with earlier dissociative electron attachment (DEA) experiments. In further contrast with DEA, no evidence was observed for dehydrogenated parent anions, and three new fragment anions were detected: CH−, C2−, and C2H−. These contrasting results for potassium impact and DEA highlight significant differences in the reaction pathways initiated by the two electron delivery processes

Electron impact calculations of total elastic cross sections over a wide energy range – 0.01 eV to 2 keV for CH₄, SiH₄ and H₂O

In this paper we report the results of a new theoretical methodology for determining the total elastic electron scattering cross section, Qel, over a wide range of incident energies between 0.01 eV and 2 keV. We have combined results from the UK molecular R-matrix code using Quantemol-N software to determine Qel for incident energies between 0.01 eV and the ionization threshold of the target with calculations based on the spherical complex optical potential formalism for higher energies up to 2 keV. We present results for three selected molecular targets; CH4, SiH4 and H2O as exemplars of the methodology. The present results were found to be in good agreement with previous experimental and theoretical results. The total elastic cross sections for such a wide energy range are reported perhaps for the first time

Electron scattering from pyridine

We have calculated cross sections for elastic and inelastic electron scattering from pyridine in the energy range 1 eV to 1 keV. The R-matrix and IAM-SCAR methods have been used for low and higher collision energies, respectively. Agreement with available theoretical data is good. We have also examined the formation of shape resonances and compared our results with existing experimental data. We compare the results with data for electron scattering from pyrimidine