Variational treatment of electron-polyatomic molecule scattering calculations using adaptive overset grids

The Complex Kohn variational method for electron-polyatomic molecule scattering is formulated using an overset grid representation of the scattering wave function. The overset grid consists of a central grid and multiple dense, atom-centered subgrids that allow the simultaneous spherical expansions of the wave function about multiple centers. Scattering boundary conditions are enforced by using a basis formed by the repeated application of the free particle Green's function and potential, $\hat{G}^+_0\hat{V}$ on the overset grid in a "Born-Arnoldi" solution of the working equations. The theory is shown to be equivalent to a specific Pad\'e approximant to the $T$-matrix, and has rapid convergence properties, both in the number of numerical basis functions employed and the number of partial waves employed in the spherical expansions. The method is demonstrated in calculations on methane and CF$_4$ in the static-exchange approximation, and compared in detail with calculations performed with the numerical Schwinger variational approach based on single center expansions. An efficient procedure for operating with the free-particle Green's function and exchange operators (to which no approximation is made) is also described

Polarization and ellipticity of high-order harmonics from aligned molecules generated by linearly polarized intense laser pulses

We present theoretical calculations for polarization and ellipticity of high-order harmonics from aligned N$_2$, CO$_2$, and O$_2$ molecules generated by linearly polarized lasers. Within the rescattering model, the two polarization amplitudes of the harmonics are determined by the photo-recombination amplitudes for photons emitted parallel and perpendicular to the direction of the {\em same} returning electron wave packet. Our results show clear species-dependent polarization states, in excellent agreement with experiments. We further note that the measured polarization ellipse of the harmonic furnishes the needed parameters for a "complete" experiment in molecules.Comment: 4 pages, 4 figure

Probing molecular frame photoionization via laser generated high-order harmonics from aligned molecules

Present photoionization experiments cannot measure molecular frame photoelectron angular distributions (MFPAD) from the outermost valence electrons of molecules. We show that details of the MFPAD can be retrieved with high-order harmonics generated by infrared lasers from aligned molecules. Using accurately calculated photoionization transition dipole moments for fixed-in-space molecules, we show that the dependence of the magnitude and phase of the high-order harmonics on the alignment angle of the molecules observed in recent experiments can be quantitatively reproduced. This result provides the needed theoretical basis for ultrafast dynamic chemical imaging using infrared laser pulses.Comment: 5 pages, 4 figure

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

Dissociative photoionization of NO across a shape resonance in the XUV range using circularly polarized synchrotron radiation.

We report benchmark results for dissociative photoionization (DPI) spectroscopy and dynamics of the NO molecule in the region of the σ* shape resonance in the ionization leading to the NO+(c3Π) ionic state. The experimental study combines well characterized extreme ultraviolet (XUV) circularly polarized synchrotron radiation, delivered at the DESIRS beamline (SOLEIL), with ion-electron coincidence 3D momentum spectroscopy. The measured (N+, e) kinetic energy correlation diagrams reported at four discrete photon energies in the extended 23-33 eV energy range allow for resolving the different active DPI reactions and underline the importance of spectrally resolved studies using synchrotron radiation in the context of time-resolved studies where photoionization is induced by broadband XUV attosecond pulses. In the dominant DPI reaction which leads to the NO+(c3Π) ionic state, photoionization dynamics across the σ* shape resonance are probed by molecular frame photoelectron angular distributions where the parallel and perpendicular transitions are highlighted, as well as the circular dichroism CDAD(θe) in the molecular frame. The latter also constitute benchmark references for molecular polarimetry. The measured dynamical parameters are well described by multichannel Schwinger configuration interaction calculations. Similar results are obtained for the DPI spectroscopy of highly excited NO+ electronic states populated in the explored XUV photon energy range

Density Functional Theory for the Photoionization Dynamics of Uracil

Photoionization dynamics of the RNA base Uracil is studied in the framework of Density Functional Theory (DFT). The photoionization calculations take advantage of a newly developed parallel version of a multicentric approach to the calculation of the electronic continuum spectrum which uses a set of B-spline radial basis functions and a Kohn-Sham density functional hamiltonian. Both valence and core ionizations are considered. Scattering resonances in selected single-particle ionization channels are classified by the symmetry of the resonant state and the peak energy position in the photoelectron kinetic energy scale; the present results highlight once more the site specificity of core ionization processes. We further suggest that the resonant structures previously characterized in low-energy electron collision experiments are partly shifted below threshold by the photoionization processes. A critical evaluation of the theoretical results providing a guide for future experimental work on similar biosystems

Probing autoionizing states of molecular oxygen with XUV transient absorption: Electronic symmetry dependent lineshapes and laser induced modification

The dynamics of autoionizing Rydberg states of oxygen are studied using attosecond transient absorption technique, where extreme ultraviolet (XUV) initiates molecular polarization and near infrared (NIR) pulse perturbs its evolution. Transient absorption spectra show positive optical density (OD) change in the case of $ns\sigma_g$ and $nd\pi_g$ autoionizing states of oxygen and negative OD change for $nd\sigma_g$ states. Multiconfiguration time-dependent Hartree-Fock (MCTDHF) calculation are used to simulate the transient absorption spectra and their results agree with experimental observations. The time evolution of superexcited states is probed in electronically and vibrationally resolved fashion and we observe the dependence of decay lifetimes on effective quantum number of the Rydberg series. We model the effect of near-infrared (NIR) perturbation on molecular polarization and find that the laser induced phase shift model agrees with the experimental and MCTDHF results, while the laser induced attenuation model does not. We relate the electron state symmetry dependent sign of the OD change to the Fano parameters of the static absorption lineshapes.Comment: 15 pages, 8 figure

Retrieving transient conformational molecular structure information from inner-shell photoionization of laser-aligned molecules

Citation: Wang, X., Le, A. T., Yu, C., Lucchese, R. R., & Lin, C. D. (2016). Retrieving transient conformational molecular structure information from inner-shell photoionization of laser-aligned molecules. Scientific Reports, 6, 11. doi:10.1038/srep23655We discuss a scheme to retrieve transient conformational molecular structure information using photoelectron angular distributions (PADs) that have averaged over partial alignments of isolated molecules. The photoelectron is pulled out from a localized inner-shell molecular orbital by an X-ray photon. We show that a transient change in the atomic positions from their equilibrium will lead to a sensitive change in the alignment-averaged PADs, which can be measured and used to retrieve the former. Exploiting the experimental convenience of changing the photon polarization direction, we show that it is advantageous to use PADs obtained from multiple photon polarization directions. A simple single-scattering model is proposed and benchmarked to describe the photoionization process and to do the retrieval using a multiple-parameter fitting method

Peatland Restoration: An Ecohydrological Assessment

Pristine peatlands store approximately one-third of the world's soil carbon through the long-term accumulation of carbon as peat (Gorham, 1991). In Canada and Europe, peatlands are exploited for peat fuel and horticultural peat, which has an impact on the hydrological conditions and carbon balance of these ecosystems. Recent advances in peatland restoration techniques (e.g., Rochefort, 2000) have succeeded in the revegetation of Sphagnum moss on previously cutover surfaces. However, a peatland can only be considered functionally 'restored' after the newly formed moss layer has achieved a thickness such that the water table position in a drought year does not extend into the underlying formerly cutover peat surface (i.e. an acrotelm is developed). This study determines ecohydrological and hydrophysical properties of a newly formed peat layer, compares them to those of a nearby natural site and a naturally revegetated site and examines the spatio-temporal development of a new peat layer at a restored peatland, and from this, estimates of when the newly developing moss layer in a restored peatland will become a functional acrotelm are made. The properties of the new peat layer differed significantly between the sites, especially for the lower (8-12 cm) layer. Lower samples for the natural and naturally revegetated sites had a bulk density of 43 ± 5 kg m-3 and 41 ± 11 kg m-3 respectively, almost twice as high as the value for lower samples from the restored site (24 ± 4 kg m-3 ). Sphagnum rubellum capitula density (ρc) was significantly higher (p -2 ) compared to the natural site (26050 # m-2 ). Residual moisture content at 200 mb (- 200 cm in soil tension) (θr ) was significantly lower (p Sphagnum rubellum in a natural peatland is able to hold onto more moisture under increasing soil-tension than the same species growing in a restored site likely due to its higher bulk density and relatively more decomposed state. The new moss layer thickness increased from 2.3 ± 1.7 cm in 2003 to 13.6 ± 6.5 cm in 2007 at the restored site. For the cutover (unrestored) portion of the peatland, the mean thickness values were significantly lower than the mean values for the restored portion of the site for each year (p -2 in 2000 to 1692 ± 932 g m-2 in 2005. The cutover (unrestored) portion of the site showed higher biomass accumulation for ericaceous vegetation, but lower Sphagnum, other mosses and other vascular biomass accumulation. A simple hydrological model was developed and determined that for the Bois-des-Bel peatland, given the mean summer water deficit at the site (-64 mm) and the storativity properties of the new moss layer (Sy = 0.34), a 19 cm thick moss layer would be required to offset summer deficit induced water table drop. elymo's (1984) model for acrotelm growth was parameterized to estimate how long it would take to develop a 19 cm moss layer at the restored site. Model results coupled to a GIS database for the site suggest that within 17 years post-restoration, more than 50% of the site would be above the 19 cm thickness threshold, an indication that peatland ecosystem restoration from a carbon accumulation and hydrologic perspective may be achieved in the medium-term. This ecohydrological approach will aid in designing a sampling strategy that can be useful in assessing the long-term impact of restoration on peatland ecohydrology and modelling carbon sequestration.Master of Science (MSc

High order harmonic generation from SF6: Deconvolution of macroscopic effects

Citation: Wilson, B. P., Fulfer, K. D., Mondal, S., Ren, X., Tross, J., Poliakoff, E. D., . . . Trallero-Herrero, C. (2016). High order harmonic generation from SF6: Deconvolution of macroscopic effects. Journal of Chemical Physics, 145(22), 11. doi:10.1063/1.4971244We measure high order harmonics from the molecule SF6 over a large range of phase matching conditions and observe several features in the harmonics that are largely independent of such macroscopic conditions. The experimental data are then compared to the quantitative rescattering theory for the generation of harmonics from three orbitals. With this comparison, we are able to assign spectroscopic features in the harmonics to contributions from 1t(1g) (HOMO) and 5t(1u) (HOMO-1) orbitals. Published by AIP Publishing