Ultrastrong Laser Fields Research Articles

Atomic stabilization in ultrastrong laser fields.

One- and two-photon ionization of atomic hydrogen by an ultrashort hyperbolic secant laser pulse is analyzed in detail by means of the essential-states method. We consider two frequency regimes. For photon energies close to the ionization potential, we show that the excitation of atomic hydrogen, initially in its ground state, leads, at low and moderate laser intensities, to an np-state population distribution that, for ultrashort pulses, is strongly shifted down to low-lying Rydberg states. At very high intensities in this frequency regime, we show that the time evolution of the Rydberg-state population follows adiabatically the pulse shape and does not lead to population trapping in the Rydberg states. This contrasts with the results obtained in the low-frequency regime. We demonstrate that, when hydrogen is excited from the 2s state, a substantial inhibition of ionization occurs at high field intensities. The stabilization is caused by the creation of a spatially extended wave packet that results from the Raman mixing of intermediate Rydberg states.

Relativistic effects in potential scattering of electrons in an ultrastrong laser field.

The T matrix is derived for the potential scattering of electrons in an elliptically polarized ultra- strong laser pulse. Special attention has been paid to the scattering of electrons that are initially nonrelativistic, but, due to the effects of the strong laser pulse, the full relativistic treatment has been applied. Possible energy ranges of the scattered electrons are discussed versus the laser field intensity. The detailed expressions in the first Born approximation are derived for linearly and circularly polarized laser fields. Numerical results for the angular distributions and energy spectra of the scattered electrons on the Yukawa-type potential in circularly polarized laser fields are presented.

Coulomb correction for strong-field multiphoton free-free absorption

A correction factor, which arises from the logarithmic phase factor in Coulomb waves, is shown to be necessary in the treatment of multiphoton free-free absorption in ultra-strong laser fields.

Atomic transitions in ultrastrong laser fields

We have calculated the ionization rate of hydrogen by an intense electromagnetic field in the dipole approximation and in an approximation in which this field dominates the electron-proton interaction in intermediate states. We find the transition rate behaves as ${E}^{\ensuremath{-}1}$ for linearly polarized fields and ${E}^{\ensuremath{-}2}\mathrm{ln}E$ for circularly polarized fields where $E$ is the amplitude of the electric field. We have also calculated the transition rate for excitation of the $2s$ state and a Raman photon. The result behaves as (${E}^{\ensuremath{-}2}W$) for small $W$ where $W$ is the energy of the Raman photon. We estimate that the method is a good one for electric field strengths of the order of 2\ifmmode\times\else\texttimes\fi{}${10}^{11}$ V/cm or higher. This is larger than available currently.

Atomic ionization by ultra—strong laser fields

The problem of multi-photon ionization of hydrogen by ultra-strong electromagnetic fields is solved in the limit where the field energy is greater than the Coulomb energy. The transition rate goes to zero as the reciprocal of the field strength.