Thomas Kim Kjeldsen

Strong-field ionization of diatomic molecules and companion atoms: Strong-field approximation and tunneling theory including nuclear motion

We present a detailed comparison of strong-field ionization of diatomic molecules and their companion atoms with nearly equal ionization potentials. We perform calculations in the length and velocity gauge formulations of the molecular strong-field approximation and with the molecular tunneling theory, and in both cases we consider effects of nuclear motion. A comparison of our results with experimental data shows that the length gauge strong-field approximation gives the most reliable predictions.

Influence of molecular symmetry on strong-field ionization: Studies on ethylene, benzene, fluorobenzene, and chlorofluorobenzene

Using the molecular strong-field approximation we consider the effects of molecular symmetry on the ionization of molecules by a strong, linearly polarized laser pulse. Electron angular distributions and total ionization yields are calculated as a function of the relative orientation between the molecule and the laser polarization. Our studies focus on ethylene (C{sub 2}H{sub 4}), benzene (C{sub 6}H{sub 6}), fluorobenzene (C{sub 6}H{sub 5}F), and ortho chlorofluorobenzene (1,2 C{sub 6}H{sub 4}ClF), the molecules representing four different point groups. The results are compared with experiments, when available, and with the molecular tunneling theory appropriately extended to nonlinear polyatomic molecules. Our investigations show that the orientational dependence of ionization yields is primarily determined by the nodal surface structure of the molecular orbitals.

Solving the m-mixing problem for the three-dimensional time-dependent Schroedinger equation by rotations: Application to strong-field ionization of H{sub 2}{sup +}

We present a very efficient technique for solving the three-dimensional time-dependent Schroedinger equation. Our method is applicable to a wide range of problems where a fully three-dimensional solution is required, i.e., to cases where no symmetries exist that reduce the dimensionality of the problem. Examples include arbitrarily oriented molecules in external fields and atoms interacting with elliptically polarized light. We demonstrate that, even in such cases, the three-dimensional problem can be decomposed exactly into two two-dimensional problems at the cost of introducing a trivial rotation transformation. We supplement the theoretical framework with numerical results on strong-field ionization of arbitrarily oriented H{sub 2}{sup +} molecules.

Vibrational Excitation of Diatomic Molecular Ions in Strong Field Ionization of Diatomic Molecules

A model based on the strong-field and Born-Oppenheimer approximations qualitatively describes the distribution over vibrational states formed in a diatomic molecular ion following ionization of the neutral molecule by intense laser pulses. Good agreement is found with a recent experiment [X. Urbain et al., Phys. Rev. Lett. 92, 163004 (2004)]. In particular, the observed deviation from a Franck-Condon-like distribution is reproduced. Additionally, we demonstrate control of the vibrational distribution by a variation of the peak intensity or a change of frequency of the laser pulse.

Effects of orientation and alignment in high-order harmonic generation and above-threshold ionization

When molecules interact with light sources of femtosecond or shorter duration the rotational degrees of freedom are frozen during the response to the strong nonperturbative interaction. We analytically derive how the frozen degrees of freedom affect the measurable signals in high-order harmonic generation and above-threshold ionization. High-order harmonic generation exhibits optical coherence in the signal from different orientations of the molecule. For ionization, the contributions from different orientations are added incoherently. The consequences of these findings are illustrated by numerical results.

Three-dimensional time-dependent Hartree-Fock approach for arbitrarily oriented molecular hydrogen in strong electromagnetic fields

We present a theoretical framework for the electronic dynamics of arbitrarily oriented molecular hydrogen in strong and short electromagnetic fields. The ground state of H{sub 2} is obtained by propagating the time-dependent Schroedinger equation in imaginary time by assuming the Hartree-Fock ansatz for the interaction between the electrons. The interaction of H{sub 2} with the radiation field is considered in the single-active-electron approximation, with the continuum electron subject to Hartree-Fock radial potentials. We propagate the wave function by a split-operator scheme projected on a spherical harmonics basis. Alignment-dependent yields and angular distributions for one- and two-photon ionization induced by an external femtosecond light source are presented and compared with available theoretical results.

Spectral and partial-wave decomposition of time-dependent wave functions on a grid: Photoelectron spectra of H and H{sub 2}{sup +} in electromagnetic fields

We present a method for spectral (bound and continuum) and partial-wave analysis of a three-dimensional time-dependent wave function, defined on a grid, without projecting onto the field-free eigenstates of the system. The method consists of propagating the time-dependent Schroedinger equation to obtain its autocorrelation function C(t)= after the end of the interaction, at time T, of the system with an external time-dependent field. The Fourier spectrum of this correlation function is directly related to the expansion coefficients of the wave function on the field-free bound and continuum energy eigenstates of the system. By expanding on a spherical harmonics basis we show how to calculate the contribution of the various partial waves to the total photoelectron energy spectrum.

Strong-field ionization of atoms and molecules : The two-term saddle-point method

We derive an analytical formula for the ionization rate of neutral atoms and molecules in a strong monochromatic field. Our model is based on the strong-field approximation with transition amplitudes calculated by an extended saddle-point method. We show that the present two-term saddle-point method reproduces even complicated structures in angular resolved photoelectron spectra.

Comment on 'Generalization of Keldysh's theory'

We reassess some mathematical aspects of the theory by Keldysh [Sov. Phys. JETP 20, 1307 (1965)]. In particular, we discuss in detail what closed contour is associated with the symbol {integral} introduced by Keldysh and we expose how the contour involved is conveniently deformed to calculate the integral by the saddle-point method. The integral cannot be reduced to a contribution of a single pole that could be evaluated via the residue theorem, contrary to the statement in the recent work by Mishima et al. [Phys. Rev. A 66, 033401 (2002)].

Atoms and molecules in intense attosecond fields: beyond the dipole approximation

The exact non-dipole minimal-coupling Hamiltonian for an atomic system interacting with an explicitly time- and space-dependent laser field is transformed into the rest frame of a classical free electron in the laser field, i.e., into the Kramers-Henneberger frame. The new form of the Hamiltonian has been used to study the non-dipole dynamics of atoms and molecules in intense XUV laser pulses. The time-dependent Schrodinger equation is solved without any simplifications.