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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Coherent effects in free-bound transitions in hydrogen/antihydrogen atom under the action of ultra-short strong-field laser pulse

Maria V. Ryabinina, Leonid A. Melnikov

Published Jun 21, 2007• 0 citations
Publisher

Abstract

Using discrete approximation of the continuous levels in H-atom the nonperturbative probability amplitude equations for 1s state and groups of continuous levels coupled via laser resonant pulse were solved numerically. The coherent Rabi-like population oscillations of 1s -state are observed. The role of free-free transitions, optimum pulse area and other parameters for recombination are discussed.

DOI
10.1117/12.753909

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Using discrete approximation of the continuous levels in H-atom the nonperturbative probability amplitude equations for 1s state and groups of continuous levels coupled via laser resonant pulse were solved numerically. The coherent Rabi-like population oscillations of 1s -state are observed. The role of free-free transitions, optimum pulse area and other parameters for recombination are discussed.
The dynamics of two-photon 1s↔2s transition in (anti)hydrogen atom in the field of femtosecond linearly polarized standing wave laser pulse is investigated. The density matrix equations for three levels 1s, 2s, and 2p were solved numerically out of frames of rotating wave approximation and perturbation approach. The coherent Rabi oscillation at different spatial points, velocity effect on level populations, saturation behavior of 2p-level population, and spectra of multiphoton resonances are demonstrated.
The dynamics of populations of 4s and 3p states in hydrogen atom is investigated under the action of ultra high laser single frequency linear-polarized pulse at one-, twoand three-photon resonance and at large detuning out of frame of perturbation theory and rotating wave approximation. It was shown the existence of low frequency modulation of optical oscillations,which frequency becomes zero at some values of laser field amplitude. For the transition 3s ^ 2p the population dynamics of discrete states 3s and 2p is investigated, and also of the states in continuum which are bounded with the state 3s via optical transition. Coherent-like oscillations of the population of the states in continuum are demonstrated.
A nonperturbative analysis is given for an nnn-level atom or molecule dressed by a Stark field and in single-photon or multiphoton (near) resonant interaction with coherent optical field modes, which may be intense. All the field amplitudes and phases may have nonadiabatic time variation. New formulas for probability amplitudes of states with finite lifetimes are given for atom-field levels swept or optical Stark shifted through resonance or into close approach. One of these includes the Landau-Zener formula as a special case. Power broadening is contained in these formulas. They are useful for both weak- and strong-field interaction. Further, it is shown that many familiar single-photon coherent transient phenomena have their analogs in the intense-field and the multiphoton domains. Two new effects, pulse-shape dependence in final transition probability and population inversion due to the nonadiabatic passage of an asymmetric pulse, are demonstrated with numerical results for the process $\mathrm{Na}(3s)+2\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\rightarrow}\mathrm{Na}(5s)$. Finally, double resonance (single-photon and/or multiphoton) is analyzed, with discussion of adiabatic following and a derivation of a formula for level crossing. Generalization of an earlier procedure for solving the adiabatic eigenvalue problem of the whole atom-field system is given.
We investigate resonant strong field ionization of atomic hydrogen with respect to the 1s–2p transition. By 'strong' we understand that Rabi periods are executed on a femtosecond time scale. Ionization and AC Stark shifts modify the bound state dynamics severely, leading to nonperturbative signatures in the photoelectron spectra. We introduce an analytical model, capable of predicting qualitative features in the photoelectron spectra such as the positions of the Autler–Townes peaks for modest field strengths. Ab initio solutions of the time-dependent Schrödinger equation show a pronounced shift and broadening of the left Autler–Townes peak as the field strength is increased. The right peak remains rather narrow and shifts less. This result is analysed and explained with the help of exact AC Stark shifts and ionization rates obtained from Floquet theory. Finally, it is demonstrated that in the case of finite pulses as short as 20 fs the Autler–Townes duplet can still be resolved. The fourth generation light sources under construction worldwide will provide bright, coherent radiation with photon energies ranging from a tenth of an meV up to tens of keV, hence covering the regime studied in the paper so that measurements of nonperturbative, relative AC Stark shifts should become feasible with these new light sources.
We investigate the third order harmonic generation for model atoms interacting with intense short laser pulses. We focus on the cases where the ponderomotive energy is not big enough to generate high-order harmonics, and where resonant transitions between bound states are also as important as multi-photon ionization and low-order harmonic generation. We find that the third order harmonic intensity from a model hydrogen atom can decrease as the laser frequency draws near a resonance between bound states. We explain this result by looking into the ionization and Rabi oscillation from the numerical calculation of the time-dependent 1-D Schrödinger equation. In order to support our explanation we also investigate the populations of bound states that play an important role in this atomic process.
We present ionization probability and lineshape calculations for the two-step three-photon ionization process, , of the ground state of hydrogenic atoms in a non-monochromatic laser field with a time-dependent amplitude. Within the framework of a three-level model, the AC Stark shifts and non-zero ionization rates of all states involved were taken into account, together with spatial and temporal inhomogeneities of the laser signal. In contrast with the usual perturbative technique, the time evolution of the atomic states was simulated by directly solving the system of coupled time-dependent inhomogeneous differential equations numerically, the equations being equivalent to the appropriate non-stationary Schrödinger equation. Particular numerical results were obtained for typical parameters of the pulsed laser field that are employed in a new experiment to measure the 1S-2S energy separation with muonium at the Rutherford Appleton Laboratory. The shifts and asymmetries of the photoionization lineshapes revealed may be of relevance for ultra-high-precision experiments in hydrogen in CW laser fields.