Liang-You Peng

Attosecond pulse carrier-envelope phase effects on ionized electron momentum and energy distributions: roles of frequency, intensity and an additional IR pulse

The effects of the carrier-envelope phase (CEP) of a few-cycle attosecond pulse on ionized electron momentum and energy spectra are analyzed, both with and without an additional few-cycle IR pulse. In the absence of an IR pulse, the CEP-induced asymmetries in the ionized electron momentum distributions are shown to vary as the 3/2 power of the attosecond pulse intensity. These asymmetries are also found to satisfy an approximate scaling law involving the frequency and intensity of the attosecond pulse. In the presence of even a very weak IR pulse (having an intensity of the order of 10 11 -10 12 W cm 2 ), the attosecond pulse CEP-induced asymmetries in the ionized electron momentum distributions are found to be significantly augmented. In addition, for higher IR laser intensities, we observe for low electron energies peaks separated by the IR photon energy in one electron momentum direction along the laser polarization axis; in the opposite direction, we find structured peaks that are spaced by twice the IR photon energy. Possible physical mechanisms for such asymmetric, low-energy structures in the ionized electron momentum distribution are proposed. Our results are based on single- active-electron solutions of the three-dimensional, time-dependent Schrodinger equation including atomic potentials appropriate for the H and He atoms.

Application of Coulomb wave function discrete variable representation to atomic systems in strong laser fields.

We present an efficient and accurate grid method for solving the time-dependent Schrodinger equation for an atomic system interacting with an intense laser pulse. Instead of the usual finite difference (FD) method, the radial coordinate is discretized using the discrete variable representation (DVR) constructed from Coulomb wave functions. For an accurate description of the ionization dynamics of atomic systems, the Coulomb wave function discrete variable representation (CWDVR) method needs three to ten times fewer grid points than the FD method. The resultant grid points of the CWDVR are distributed unevenly so that one has a finer grid near the origin and a coarser one at larger distances. The other important advantage of the CWDVR method is that it treats the Coulomb singularity accurately and gives a good representation of continuum wave functions. The time propagation of the wave function is implemented using the well-known Arnoldi method. As examples, the present method is applied to multiphoton ionization of both the H atom and the H(-) ion in intense laser fields. The short-time excitation and ionization dynamics of H by an abruptly introduced static electric field is also investigated. For a wide range of field parameters, ionization rates calculated using the present method are in excellent agreement with those from other accurate theoretical calculations.

Enhanced asymmetry in few-cycle attosecond pulse ionization of He in the vicinity of autoionizing resonances

By solving the two-active-electron, time-dependent Schr¨ odinger equation in its full dimensionality, we investigate the carrier-envelope phase (CEP) dependence of single ionization of He to the He + (1s) state triggered by an intense few-cycle attosecond pulse with carrier frequency ! corresponding to the energy ¯! = 36eV. Effects of electron correlations are probed by comparing projections of the final state of the two-electron wave packet onto field-free highly correlated Jacobi matrix wave functions with projections onto uncorrelated Coulomb wave functions. Significant differences are found in the vicinity of autoionizing resonances. Owing to the broad bandwidths of our 115 and 230 as pulses and their high intensities (1-2PWcm 2 ), asymmetries are found in the differential probability for ionization of electrons parallel and antiparallel to the linear polarization axis of the laser pulse. These asymmetries stem from interference of the one- and two-photon ionization amplitudes for producing electrons with the same momentum along the linear polarization axis. Whereas these asymmetries generally decrease with increasing ionized electron

Controlling molecular rotational population by wave-packet interference

We propose a control scheme for selecting populations of molecular rotational states by wave-packet interference. A series of coherent rotational wave packets is created by nonadiabatic rotational excitation of molecules using two strong femtosecond laser pulses. By adjusting the time delay between the two laser pulses, constructive or destructive interference among these wave packets enables the population to be enhanced or suppressed for a specific rotational state. The evolution of the rotational wave packet with selected populations produces interference patterns with controlled spatial symmetries. This method provides an approach to prepare a molecular ensemble with selected quantum-state distributions and controlled spatial distributions under field-free condition.

Nonadiabatic tunneling ionization of atoms in elliptically polarized laser fields

We theoretically investigate the nonadiabatic effects in strong field tunneling ionization of atoms in elliptically polarized laser fields by solving the 3D time-dependent Schrodinger equation (TDSE). Comparing our TDSE results with those of two semi-classical methods, i.e., the quantum-trajectory Monte Carlo simulation (QTMC) and the Coulomb-corrected strong field approximation (CCSFA), we confirm the existence of the nonadiabatic effects with its fingerprint in the nonzero initial lateral velocity at the tunneling exit in the laser polarization plane. Our study shows that these nonadiabatic initial lateral momentum effects become significant in high ellipticity or circularly polarized laser field. These results indicate that the calibration of the experimental laser intensity in this situation should be performed nonadiabatically, which may strongly affect the results of the real tunneling time delay measurements.

Dissociation of H+2 from a short, intense, infrared laser pulse : proton emission spectra and pulse calibration

The proton energy spectrum from photodissociation of the hydrogen molecular ion by short intense pulses of infrared light is calculated. The time-dependent Schrodinger equation is discretized and integrated. For few-cycle pulses one can resolve vibrational structure, arising from the experimental preparation of the molecular ion. We calculate the corresponding energy spectrum and analyse the dependence on the pulse time delay, pulse length and intensity of the laser for λ ~ 790 nm. We conclude that the proton spectrum is a sensitive probe of both the vibrational populations and phases, and allows us to distinguish between adiabatic and nonadiabatic dissociation. Furthermore, the sensitivity of the proton spectrum from H+2 is a practical means of calibrating the pulse. Our results are compared with recent measurements of the proton spectrum for 65 fs pulses using a Ti:Sapphire laser (λ ~ 790 nm) including molecular orientation and focal-volume averaging. Integrating over the laser focal volume, for the intensity I ~ 3 × 1015 W cm−2, we find our results are in excellent agreement with these experiments.The dissociation spectrum of the hydrogen molecular ion by s hort intense pulses of infrared light is calculated. The time-dependent Schrö dinger equation is discretized and integrated in position and momentum space. For few-cycle pu lses one can resolve vibrational structure that commonly arises in the experimental prepara tion of the molecular ion from the neutral molecule. We calculate the corresponding energ y spectrum and analyze the dependence on the pulse time-delay, pulse length, and inten s ty of the laser forλ ∼ 790nm. We conclude that the proton spectrum is a both a sensitive pro be of the vibrational dynamics and the laser pulse. Finally we compare our results with rece nt measurements of the proton spectrum for 55 fs pulses using a Ti:Sapphire laser ( λ ∼ 790nm). Integrating over the laser focal volume, for the intensityI ∼ 3 × 10W cm, we find our results are in excellent agreement with these experiments. To be submitted to J. Phys. B

A discrete time-dependent method for metastable atoms and molecules in intense fields

The full-dimensional time-dependent Schrödinger equation for the electronic dynamics of single-electron systems in intense external fields is solved directly using a discrete method. Our approach combines the finite-difference and Lagrange mesh methods. The method is applied to calculate the quasienergies and ionization probabilities of atomic and molecular systems in intense static and dynamic electric fields. The gauge invariance and accuracy of the method is established. Applications to multiphoton ionization of positronium, the hydrogen atom and the hydrogen molecular ion are presented. At very high laser intensity, above the saturation threshold, we extend the method using a scaling technique to estimate the quasienergies of metastable states of the hydrogen molecular ion. The results are in good agreement with recent experiments.

Photodetachment of H - by a short laser pulse in crossed static electric and magnetic fields

We present a detailed quantum mechanical treatment of the photodetachment of

Attosecond streaking of molecules in the low-energy region studied by a wavefunction splitting scheme

{\mathrm{H}}^{\ensuremath{-}}

Double ionization of He by time-delayed attosecond pulses

by a short laser pulse in the presence of crossed static electric and magnetic fields. An exact analytic formula is presented for the final state electron wave function (describing an electron in both static electric and magnetic fields and a short laser pulse of arbitrary intensity). In the limit of a weak laser pulse, final state electron wave packet motion is examined and related to the closed classical electron orbits in crossed static fields predicted by Peters and Delos [Phys. Rev. A 47, 3020 (1993)]. Owing to these closed orbit trajectories, we show that the detachment probability can be modulated, depending on the time delay between two laser pulses and their relative phase, thereby providing a means to partially control the photodetachment process. In the limit of a long, weak pulse (i.e., a monochromatic radiation field) our results reduce to those of others; however, for this case we analyze the photodetachment cross section numerically over a much larger range of electron kinetic energy (i.e., up to