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Physical Review A

Radiation from intense-laser-stabilized states of hydrogen

Marvin H. Mittleman

Published Oct 1, 1992• 7 citations
Publisher

Abstract

Metastable states of atomic hydrogen in an ideal linearly polarized laser have been shown to exist. These states, in the laboratory frame, have the electron oscillating with a large amplitude. The electron will radiate both even and odd harmonics of the laser frequency at a rate calculated here. The electron may also radiate nonharmonic radiation, thereby leaving the atom in an excited state. This process contributes to the width of the ground state. Both processes are evaluated at an upper limit of intensity imposed upon the theory by the dipole approximation.

DOI
10.1103/physreva.46.4209

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Metastable states of atomic hydrogen in an ideal linearly polarized laser have been shown to exist. These states, in the laboratory frame, have the electron oscillating with a large amplitude. The electron will radiate both even and odd harmonics of the laser frequency at a rate calculated here. The electron may also radiate nonharmonic radiation, thereby leaving the atom in an excited state. This process contributes to the width of the ground state. Both processes are evaluated at an upper limit of intensity imposed upon the theory by the dipole approximation.
Recent calculations on the bound states of atomic hydrogen in a laser field in the nonrelativistic dipole approximation have produced metastable states with decreasing binding energy as a function of ${\mathrm{\ensuremath{\alpha}}}_{0}$\ensuremath{\sim}I1/2{\mathit{I}}^{1/2}I1/2${\mathrm{\ensuremath{\omega}}}^{\mathrm{\ensuremath{-}}2}$ (where I is the laser intensity and \ensuremath{\omega} is its frequency). We extend the calculations by removing these restrictions. We find the limit of validity of the previous calculations and find that beyond this limit of ${\mathrm{\ensuremath{\alpha}}}_{0}$ the lowest bound state persists but that its character is completely changed.
This paper discusses the general theory of two-photon ionization of a hydrogenic state in the nonrelativistic dipole approximation. An application is made to the photoionization of the metastable 2S2S2S state of atomic hydrogen by optical radiation, for example, by the light from a ruby laser. The sum over intermediate states is carried out exactly by means of an implicit technique. The results are given for a range of wave-lengths from the threshold at 7290 down to 4556 \AA{}. At 6943 \AA{}, the total cross section per unit intensity is found to be 5.35\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}29}$ cm4{\mathrm{cm}}^{4}cm4 ${\mathrm{W}}^{\ensuremath{-}1}$. The lowest order damping corrections to the perturbation-theory results are worked out.
Electron-impact excitation of the hydrogen metastable 2s state to the 3s, 4s, 3d, and 4d states is studied in the presence of a resonant laser field using the rotating-wave approximation. The frequency of the circularly polarized laser field is chosen to match the 2s-3p transition frequency in the hydrogen atom. The role of the laser field in individual magnetic substate (mf{\mathit{m}}_{\mathit{f}}mf​=0,\ifmmode\pm\else\textpm\fi{}1,\ifmmode\pm\else\textpm\fi{}2) excitation is analyzed. It is found that for these optically forbidden transitions (s-s, s-d), a significant increase in the excitation cross sections is obtained through joint collisions with electrons and photons.
Current trends in laser technology have reached the regime of studying atoms stabilized against ionization, going beyond the perturbation theory. In this work, properties of a laser-dressed sulfur atom are examined in this stabilization regime. The electronic structure of a sulfur atom changes dramatically as it interacts with strong high-frequency laser fields. Degenerate molecularlike states are obtained for the ground state triplet of the laser-dressed sulfur atom for high-frequency and moderate intensity laser parameters. The degenerate ground state is obtained for a laser intensity which is smaller by more than one order of magnitude than the intensity required for hydrogen atoms due to many electron screening effects. An infinitesimally weak static field mixes these degenerate states to give rise to asymmetric states with large permanent dipole moments. Hence, a strong linear Stark effect rather than the usual quadratic one is obtained.