It is well known that, in the adiabatic approximation of non-relativistic quantum mechanics, the ionization rate can be calculated according to the Landau-Dykhne approach [1]:

Here E_i is the binding energy of the initial atomic state, and E_f(t) is the energy of the final continuum state taking into account the field of laser radiation. Here and below, the atomic system of units is used, as a rule, where e = m_e = ħ = 1 and the speed of light is c = 137.02 . Finally, E_f(t) = -E_i.

Adiabatic approximation is valid when the energy of laser photon is small, i.e. ħω << E_i. Eq. (1) is correct as well as in the relativistic theory, since the classical action S = P_idxⁱ is relativistic invariant quantity, and the coordinate part of this action influences only the pre-exponential factor in the ionization rate, Eq. (1). We neglect this factor both in the non-relativistic energy distribution of ejected electrons [2] and in the relativistic approach. Thus, we can apply Eq. (1) in the relativistic case using the relativistic expression for the energy E_f(t) of a free electron in the field of laser radiation.

In the tunneling ionization limit, the Keldysh adiabaticity parameter γ is [3] small, i.e. (see details in review [4]):

Here F and ω are the amplitude and the frequency of laser field, respectively. We studied the influence of relativistic effects on the angular distribution of ejected electrons in the field of laser radiation in Refs. [5,6].

Now we apply this approach to derive the relativistic energy distribution of ejected electrons in the field of linearly polarized low-frequency laser radiation. We restrict ourselves by the case of electron ejection along the electric field strength vector only, since the most part of electrons are emitted along this direction in a strong laser field. Then the classical relativistic energy E_f(t) of an electron in laser field is given by well-known expression [7]

Here p = p(t) is the electron momentum for arbitrary time moment t, and p ₀ = p(0) is the initial value of this momentum for t = 0. The value of p is determined from the cubic equation [7]:

Substituting Eq. (3) into Eq. (1) and taking into account Eq. (4), we obtain the relativistic energy distribution of ejected electrons in the tunneling ionization of an atom (with exponential accuracy). We restrict ourselves by the moderate values of kinetic energies of ejected electrons

i.e. by the case when these energies are less than the rest energy of the electron. Then the calculation of the integral in Eq. (1) is simplified, and we find after simple, but cumbersome calculations

Here the quantity ω ₀ is the non-relativistic total tunneling ionization rate which can be calculated, foe example, in the frames of ADK theory [8].

The first term in the exponent of the expression (6) is the non-relativistic energy distribution of ejected electrons in the linearly polarized electromagnetic field [2,9]. Thus, the non-relativistic rate of electron ejection with the kinetic energy E_e exponentially decreases with E_e. The non-relativistic width of this distribution is

It is seen from Eq. (6) that the relativistic effect is important under the condition

which does not contradict the above condition E_e < c ² since γ ² << 1.

Thus, it follows from Eq. (6) that the relativistic width of the energy distribution is

For example, at γ = 0.1 the conditions c ² > E_e > γ ² c ² are required for 500 keV > E_e > 5 keV. According to (9) the value of ΔE_e(rel) is equal to 20 keV for radiation of CO₂ - laser.

In conclusion, it follows from the suggested approach that the radiation of linearly polarized CO₂ - laser pulse with the intensity exceeding 10¹⁵ W/cm² produces relativistic distribution of ejected electrons with keV-energies along the polarization axis for most kinds of atoms, though the kinetic energies of these electrons are less than their rest energy mc ². The results of this work can be used in the analysis of experimental data of Rochester group [10].

This work was supported partially by Russian Council of Fundamental Research (grant N 96-02-18299) and by Soros Foundation (grant N 22p).