Showing papers by "Andrey Popov published in 1994"
29 Aug 1994
TL;DR: In this article, the relativistic velocities of an atomic electron in a strong laser field were investigated, and it was shown that high harmonics emerge at the tuming points where the electron suddenly reverses its velocity from essentially the speed of light, in particular if these occur in the vicinity of the nucleus.
Abstract: Advances in short pulse laser technology allow one to generate super-intense laser fields with peak intensities exceeding the atomic unity of intensity. As a consequence, we expect relativistic velocities of an atomic electron in such a strong laser field and relativistic features (see e.g., Refs. 1, 2) in the ionization process and the high harmonic spectra. We here investigate relativistic classical electron trajectories of an electron in a binding potential and superstrong laser field and evaluate the radiation spectrum by averaging the initial conditions of the electrons over a microcanonical ensemble. The dynamics here is primarily govemed by the strong laser field, and the nucleus only gives a small though important deviation from the free motion. From the electron trajectories we evaluate the emitted spectrum and show that high harmonics emerge at the classical tuming points where the electron suddenly reverses its velocity from essentially the speed of light, in particular if these occur in the vicinity of the nucleus? Particular emphasis is put on the magnetic field component of the laser field, which cannot be neglected because of the relativistic velocities of the electronP In this context we will discuss the problem of ionization in the direction of propagation of the applied laser field, and we will point out that stabilization at high frequencies may not occur as put forward in one-dimensional considerations. This problem of ionization in propagation direction can also be understood quantum mechanically as arising from the momentum transfer of the incoming photons due to multiple absorption and emission. In this formulation, a possible breakdown of stabilization has been pointed out recently in a non-relativistic approach? For an observation in the forward direction, we have to take account of retardation effects, which may lead to a reduction in high harmonic generation. These effects arise due to the inclusion of the magnetic field component of the laser field and are shown to be reduced in crossed (standing wave) laser fields with cancellation of the magnetic field component at the origin of the atom. Other effects of the magnetic field are possible Doppler shifts of the harmonic lines. Because of the magnetically induced motion of the electron in the propagation direction of the laser pulse, the electron sees a shifted frequency in its rest frame. In this frame, the electron emits multiples of this shifted frequency. In the case of an observation perpendicular to the propagation direction, we can indeed see this Doppler shifted frequency, while in the forward direction the transformation back into the frame of the non-moving detector in the laboratory frame results in cancellation of the Doppler shift. In general, we will show that magnetic field effects are more significant in directions other than the forward direction. This can be explained by the fact that the magnetic field induces in particular a motion in the forward direction with the characteristic radiation due to this motion just perpendicular to this motion. However, the reduction in high harmonic generation due to increased ionization in forward direction is visible in all directions. 1. J. 2. Kaminski, Z. Phys. D16, 153 (1990). 2. E H. M. Faisal, T. Radozycki, Phys. Rev. A47,4464 (1993); ibid. A48, 554 (1993). C. H. Keitel, P. L. Knight, K. Burnett, Eur. Phys. Lett. 24, 539 (1993). C. H. Keitel, P. L. Knight, in preparation. A. Bugacov, M. Pont, R. Shakeshaft, Phys. Rev. A48, R4027 (1993). 3.
29 Aug 1994
Abstract: The atomic potential was chosen as a rectangular potential well of height V, = 6 eV and width d = 4 A. In such a well there exists two bound states, and the ionization potential is I = 4.8 eV. The laser quantum energy changed from 0.12 to 1.17 eV, which corresponds to the frequency band from CO, to Nd lasers. The laser pulse was assumed to have a Gaussian form with duration T = 150 fs. The maximum intensity was Po = 3 10'* 10" W/cm2. Figure 1 shows the ionization probability dependence on intensity Po for different frequencies (1 hw = 0.12 eV, 2 hw = 0.24 eV, 3 hw = 1.17 eV) obtained numerically. The value Ig(W) in cases 1 and 2 for Po 5 lo" W/cm2 is a linear function of Pi''', that is in agreement with the expression (1). For Po 2 10" W/cm2 saturation occurs and the ionization probability approaches 1. The perturbation theory, used in Ref. 1 in this case is invalid. As it follows from the theory of tunnel ionization," the ionization probability does not depend on radiation frequency. The theoretical dependency obtained by integrating of (1) over the time of laser action is shown in Fig. 1. (dashed line) and well coincides with curves 1 and 2. Another picture takes place for Nd laser radiation (hw = 1.17 eV). In this case Keldyshs parameter y > 1 and transitions to continuum occur as the result of multiquantum photoeffect. The transition from tunnel ionization regime to multiquanta photoeffect is displayed in Fig. 1 as a dependence of the ionization probability on laser frequency. 1. L. V. Keldysh, Sov. Phys. JETP, 20, 1307 (1965). 2. N. B. 'Delone, V. P. Kraynov, Atoms in Strong Light Fields. (Berlin Springer, 1985).
29 Aug 1994
Abstract: oms, and both laser beams act on the beam of Ba atoms. Resonance maxima in yield dependences of Ba' ions on the dye laser radiation frequency (at presence and absence of YAG laser radiation) are caused by excitation of above mentioned states. In the yield dependence of Ba" on dye laser frequency (at presence of YAG laser radiation) a number of minima were observed and besides, the frequencies of these manima coincide with frequencies of resonance maxima in Ba' yield. The maximal amplitude of minimum takes place on the frequency 18 060 cm-', when the probability of Ba' ion formation is maximal. The analysis shows that the observation of these minima is one of the p m f s for the two-electron mechanism of doubly charged Ba" ion formation, as in the case of the step-wise mechanism the realization of similar minima in yield dependences can't be realized.