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Physical Review A

Photoionization of atomic hydrogen dressed by a circularly polarized CO2 -laser field

J. L. Madajczyk, Marcel Pont, R. M. Potvliege, Robin Shakeshaft, Howard S. Taylor

Published Apr 1, 1992• 12 citations
Publisher

Abstract

We calculate the two-color ionization profile of atomic hydrogen in the presence of both a weak tunable high-frequency laser field and a low-frequency laser field of moderate intensity. The high-frequency spectroscopic light is used to probe the structure of Rydberg states dressed by the low-frequency field. We construct the photoionization spectrum from the sum of the resonance contributions by calculating the transition dipole moments between the initial state and the dressed Rydberg states, analogous to the study of photoionization in the presence of a dc field carried out by Alvarez and Silverstone [Phys. Rev. Lett. 63, 1364 (1989)].

DOI
10.1103/physreva.45.4848

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We calculate the two-color ionization profile of atomic hydrogen in the presence of both a weak tunable high-frequency laser field and a low-frequency laser field of moderate intensity. The high-frequency spectroscopic light is used to probe the structure of Rydberg states dressed by the low-frequency field. We construct the photoionization spectrum from the sum of the resonance contributions by calculating the transition dipole moments between the initial state and the dressed Rydberg states, analogous to the study of photoionization in the presence of a dc field carried out by Alvarez and Silverstone [Phys. Rev. Lett. 63, 1364 (1989)].
To study the physical mechanism of atomic Rydberg state excitation (RSE) in two-color laser fields, we solve the time-dependent Schrödinger equation (TDSE) to calculate the ionization and excitation of hydrogen atoms. The relationship between the excited state population and the excitation rate of hydrogen atoms and the intensity of the fundamental frequency (FW) field can be observed by adjusting the laser intensity of the second harmonic (SH) field. The results demonstrate that multiphoton resonance absorption aligns with the physical mechanism of atomic resonant state excitation in two-color laser fields.
A numerical method is presented to treat atoms in strong time-dependent electric fields. The method is applied to calculate ionization probabilities of H Rydberg atoms in strong monochromatic and bichromatic microwave fields. Recent experimental data on the microwave ionization of H Rydberg atoms are well reproduced. Ionization probabilities for microwave field parameters not yet covered experimentally are predicted. We propose to use the setup of existing two-frequency waveguide experiments to reveal the structure of the quasi-energy spectrum of microwave-driven Rydberg atoms. In analogy to dressed-state spectroscopy of atoms in strong laser fields, the quasi-energies of a periodically driven quantum system are mapped out by sweeping the frequency of a second (weak) additional rf probe field.
High-frequency Floquet theory (HFFT) is again applied to photoionization in a linearly polarized field of the hydrogen atom in its ground state. The new feature is the systematic use of a two-pole approximation to represent both the dressed potential (with two positive charges) and the coupling responsible for one-photon ionization (with two charges of opposite sign). Within such a scheme, both the initial bound and the final continuum states can be very accurately determined. The final continuum state is obtained by deriving, from Floquet theory, an equation involving a source term. The model accounts reasonably well, even at rather low frequencies, for the stabilization of the atom in an intense field and supports the claim that HFFT represents the photoionization rate with a golden rule expression involving dressed states.
We consider the excitation of atomic hydrogen in its ground state by an intense radiation field tuned close to the ionization threshold. Results are presented for the ac Stark shift of the 1s and 2s states, the width of the 1s state, and the photoelectron energy spectrum. In the first instance, our nonperturbative approach includes the 1s state, the p series of excited states, and the p continuum: we demonstrate that the p Rydberg states generate a rich structure in the photoelectron energy spectrum through the action of the counter-rotating wave dynamics when the laser frequency is tuned below the ionization threshold. We then go on to analyze in detail how the interaction of the p states with the other Rydberg series and continua increases the width of the p states; it is shown, however, that at the field intensities considered here (\ensuremath{\approxeq}1013{10}^{13}1013 W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$), this effect is not sufficient to smear out the structure in the photoelectron energy spectrum.
The strong-field multiphoton ionization of atoms is considered and a theoretical approach dealing nonperturbatively with the radiation field formulated. The general computational scheme is the conventional perturbation theory, but the intermediate states are dressed by the field. We present in detail a method to dress the continuum states and to study the dipole transitions within the continuum. In the high-frequency domain, the proposed procedure rapidly converges over a wide range of field intensity and offers an interesting framework for calculating ionization rates for arbitrary numbers of absorbed (above-threshold) photons and field polarization.