High-order harmonic generation from polyatomic molecules including nuclear motion and a nuclear modes analysis

We present a generic approach for treating the effect of nuclear motion in the high-order harmonic generation from polyatomic molecules. Our procedure relies on a separation of nuclear and electron dynamics where we account for the electronic part using the Lewenstein model and nuclear motion enters as a nuclear correlation function. We express the nuclear correlation function in terms of Franck-Condon factors which allows us to decompose nuclear motion into modes and identify the modes that are dominant in the high-order harmonic generation process. We show results for the isotopes CH$_4$ and CD$_4$ and thereby provide direct theoretical support for a recent experiment [Baker {\it et al.}, Science {\bf 312}, 424 (2006)] that uses high-order harmonic generation to probe the ultra-fast structural nuclear rearrangement of ionized methane.Comment: 6 pages, 6 figure

Ionization of oriented targets by intense circularly polarized laser pulses: Imprints of orbital angular nodes in the 2D momentum distribution

We solve the three-dimensional time-dependent Schr\"{o}dinger equation for a few-cycle circularly polarized femtosecond laser pulse interacting with an oriented target exemplified by an Argon atom, initially in a $3\text{p}_{x}$ or $3\text{p}_{y}$ state. The photoelectron momentum distributions show distinct signatures of the orbital structure of the initial state as well as the carrier-envelope phase of the applied pulse. Our \textit{ab initio} results are compared with results obtained using the length-gauge strong-field approximation, which allows for a clear interpretation of the results in terms of classical physics. Furthermore, we show that ionization by a circularly polarized pulse completely maps out the angular nodal structure of the initial state, thus providing a potential tool for studying orbital symmetry in individual systems or during chemical reactions

Ionization of 1D and 3D oriented asymmetric top molecules by intense circularly polarized femtosecond laser pulses

We present a combined experimental and theoretical study on strong-field ionization of a three-dimensionally oriented asymmetric top molecule, benzonitrile (C$_7$H$_5$N), by circularly polarized, nonresonant femtosecond laser pulses. Prior to the interaction with the strong field, the molecules are quantum-state selected using a deflector, and 3-dimensionally (3D) aligned and oriented adiabatically using an elliptically polarized laser pulse in combination with a static electric field. A characteristic splitting in the molecular frame photoelectron momentum distribution reveals the position of the nodal planes of the molecular orbitals from which ionization occurs. The experimental results are supported by a theoretical tunneling model that includes and quantifies the splitting in the momentum distribution. The focus of the present article is to understand strong-field ionization from 3D-oriented asymmetric top molecules, in particular the suppression of electron emission in nodal planes of molecular orbitals. In the preceding article [Dimitrovski et al., Phys. Rev. A 83, 023405 (2011)] the focus is to understand the strong-field ionization of one-dimensionally-oriented polar molecules, in particular asymmetries in the emission direction of the photoelectrons.Comment: 12 pages, 9 figure

Lineshapes in carbon 1s photoelectron spectra of methanol clusters

A general protocol for theoretical modeling of inner-shell photoelectron spectra of molecular clusters is presented and applied to C1s spectra of oligomers and medium-sized clusters of methanol. The protocol employs molecular dynamics for obtaining cluster geometries and a polarizable force field for computing site-specific chemical shifts in ionization energy and linewidth. Comparisons to spectra computed from first-principle theories are used to establish the accuracy of the proposed force field approach. The model is used to analyze the C1s photoelectron spectrum of medium-sized clusters in terms of surface and bulk contributions. By treating the surface-to-bulk ratio as an adjustable parameter, satisfactory fits are obtained to experimental C1s spectra of a beam of methanol clusters