Resonant enhancement of the harmonic-generation spectrum of beryllium

Abstract

The high-order harmonic-generation (HHG) spectrum of Be is investigated in the multiphoton regime by solving the full-dimensional, two-active-electron, time-dependent Schrödinger equation in an intense (≈10 13 W/cm 2 ), 30-cycle laser field. As the laser frequency ω L varies from 1.7 to 1.8 eV (which is in the tunable range of a Ti:sapphire laser), the seventh harmonic becomes resonant sequentially with the transition between the ground state and two doubly excited autoionizing states, 2p4s( 1 P ) (at ω L = 1.734 eV) and 2p5s( 1 P ) (at ω L = 1.785 eV), while the third harmonic becomes resonant with the 2s2p( 1 P ) singly excited state (at ω L = 1.766 eV). At each of these resonant frequencies, the HHG power spectrum is found to increase by an order of magnitude over a range of harmonics that form a plateau, extending from the resonant harmonic up to a cutoff at the 25th harmonic. In contrast to the well-known rescattering plateau cutoff law appropriate in the tunneling regime (which predicts a cutoff at the fifth or seventh harmonic), the multiphoton regime plateau we find for Be originates from atomic resonance effects. Off resonance, the Be HHG spectrum decreases monotonically with harmonic order. By taking the ratio of the integrated harmonic power of the seventh harmonic to that of the fifth harmonic, one can isolate the resonant effects of the two doubly excited states in the HHG spectrum from those of singly excited resonance states. These ratios exhibit resonance profiles for driving laser-pulse durations much longer than the lifetimes of these autoionizing states. The energy widths of these resonance features are comparable to the widths of the laser pulse and are much smaller than the autoionizing state widths. These results demonstrate an important role for electron correlations in enhancing harmonic-generation rates in the multiphoton regime.

Photoionization of few electron systems: a hybrid coupled channels approach

“…With the addition of 1s 3s 2 and 1s 3p 2 states, a structure similar to the one predicted by the benchmark calculation develops around 10 eV. This structure may be identified with the lowest resonance 1s 2p3s 2 at 10.71 eV [6]. The coupled channels calculations with the number of ionic states considered here, however do not reproduce the structure on the second peak exactly.…”

“…This allows us to have a benchmark for our spectra by adapting the simple Coulomb potential to an effective potential in our two electron code. We use the effective potential given in [6] for our benchmark calculations. We refer to these as 'effective-2e' calculations.…”

“…While simple single electron models or low dimensional models have been partially successful in explaining laser matter interactions, there have been several cases reported where a more elaborate description of electronic structure becomes important. Some of the examples include inter-channel coupling leading to an enhancement in high harmonic generation (HHG) from xenon [3], modification of angle resolved ionization yield of CO 2 [4] and photoionization cross-sections in SF 6 [5], enhancement in HHG due to participation of doubly excited states in beryllium [6], influence of nuclear motion [7], presence of conical intersections [8] and so on. All these instances need a more involved description of the electronic structure.…”

Nonlinear Dichroism in Back-to-Back Double Ionization of He by an Intense Elliptically Polarized Few-Cycle Extreme Ultraviolet Pulse

“…In these works for atoms the three-dimensional time-dependent Schrödinger equation (TDSE) is solved using the single-active-electron approximation. General formulations for single ionization of an atom [25] and double ionization of He [29] by an arbitrarily polarized, few-cycle XUV pulse using perturbation theory (PT) have been validated numerically only for the case of a linearly polarized pulse [29][30][31][32] owing to its axial symmetry, which reduces the numerical effort. None of these many prior numerical investigations has addressed the challenging six-dimensional problem of a two-electron system interacting with an arbitrarily polarized XUV pulse.…”

Generation of isolated ultra-short attosecond pulses by coherent control of the population of excited states

“…It has been shown that excited states play a role in high harmonic generation and enhancement [22][23][24][25]. In this work, we explore the feasibility of selective control over the domination of the two quantum pathways by populating different excited states of single He + ion to significantly enhance the HHG yield and cutoff extension, leading to the generation of intense and isolated ultra-short attosecond pulses.…”