Nonperturbative approach to atomic multiphoton processes under intense, short laser pulses

Effects of ultrashort laser pulses on angular distributions of photoionization spectra

We study the photoelectron spectra by intense laser pulses with arbitrary time dependence and phase within the Keldysh framework. An efficient semianalytical approach using analytical transition matrix elements for hydrogenic atoms in any initial state enables efficient and accurate computation of the photoionization probability at any observation point without saddle point approximation, providing comprehensive three dimensional photoelectron angular distribution for linear and elliptical polarizations, that reveal the intricate features and provide insights on the photoionization characteristics such as angular dispersions, shift and splitting of photoelectron peaks from the tunneling or above threshold ionization(ATI) regime to non-adiabatic(intermediate) and multiphoton ionization(MPI) regimes. This facilitates the study of the effects of various laser pulse parameters on the photoelectron spectra and their angular distributions. The photoelectron peaks occur at multiples of 2ħω for linear polarization while  odd-ordered peaks are suppressed in the direction perpendicular to the electric field. Short pulses create splitting and angular dispersion where the peaks are strongly correlated to the angles. For MPI and elliptical polarization with shorter pulses the peaks split into doublets and the first peak vanishes. The carrier envelope phase(CEP) significantly affects the ATI spectra while the Stark effect shifts the spectra of intermediate regime to higher energies due to interference.

Phase-dependent excitation and ionization in the multiphoton ionization regime

We theoretically study the dependence of atomic excitation and ionization on the carrier envelope phase of few-cycle laser pulses in the multiphoton ionization regime. Our theoretical results for the hydrogen atom based on the solution of the 3D time-dependent Schrödinger equation show that the strong phase dependence can be seen in not only total ionization, but also bound-state population under the weak laser intensity regime.

Absorbing boundaries in time-dependent problems with discretized energy continua

We develop a general method for removing artifacts associated with the numerical solution of time-dependent Schrödinger equation (TDSE) involving a (multiple) energy continuum discretization. This method is the equivalent to absorbing boundaries in the case where the space is discretized. By removing the reflected part of the wave function (on the artificial boundaries of the system), one is able to reduce the computational cost of the calculations, with a benefit scaling as the power of the continuum multiplicity. As a demonstration, we apply our method to the TDSE of a hydrogen atom subjected to a laser pulse, the spontaneous emission of a two-level atom in free space, and the interaction of two photons with a two-level atom and a defect mode at the edge of a photonic band gap.

Photoelectron angular distributions

Angle-resolved photoelectron spectroscopy has been performed for more than 70 years in various guises, but recently its potential to help solve in detail problems in the photoionization dynamics and intramolecular dynamics of gas-phase molecules has been recognized. One key development has been the design of experiments in appropriate geometries to extract information that pertains to the molecular frame, another has been the development of imaging spectrometers, and a third is the use of ultrafast lasers to cause photoionization. In this review, which is aimed at experimentalists, simple expressions for photoelectron angular distributions (PADs) in various experimental geometries are given and their applications explained.