摘要:

Two-photon free-free transitions take place in such processes as the ionization with excess photons and bremsstralung. In nonrelativistic dipole approximation it is convenient to use the Green’s function formalism. Up to now in the configuration space free-free transition amplitudes have not been written in the closed form [1]. The modified Coulomb Green’s function (CGF) Sturmian expansion is derived and analytic expressions for two-photon bound-free and free-free transition amplitudes are obtained. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291921

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Calculations of high-order above-threshold-ionization (ATI) spectra for a zero-range model potential are interpreted in terms of quantum orbits as they formally result from a saddle-point evaluation of theS-matrix element. The spectrum is built up by the superposition of these orbits and exhibits distinctive interference patterns. For a laser with elliptical polarization, the ATI plateau becomes a stair whose individual steps can be attributed to particular quantum orbits. The real parts of the orbits are depicted. They are closely related to electronic space-time orbits. For parameters around an ATI channel closing, the spectrum shows resonant behavior. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291922

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We report on the relevance of quantum effects in above threshold ionization for low and high energy photoelectrons. High repetition rate lasers are used to study energy resolved electron yields as a function of ellipticity as well as the angular distribution of the photoelectrons. In comparison with theory it is possible to identify quantum effects. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291923

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We present calculations of multiphoton ionization of inert gases using an improved version of theR-matrix Floquet method. We use a new radial basis for theR–matrix internal region which avoids the use of non-variational corrections to the calculatedR-matrix. This provides the freedom to choose either the length or velocity gauge to describe the laser–atom interaction in this region. Differences between results calculated using length and velocity gauges are discussed. We present new results for argon at 248 nm, the fundamental mode of the KrF laser. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291924

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

The development of a new class of laser systems, capable of producing intense radiation in the mid-infrared (MIR) regime (photon energies between 0.3 and 0.4 eV), opens the possibility of observing multiphoton processes in a new class of systems with lower ionization potentials than those previously studied. Of particular interest are the alkali metal atoms, which are true one-(valence)-electron systems. We present theoretical calculations of above threshold ionization (ATI) and high harmonic generation (HHG) from alkali metal atoms subject to 3-4 &mgr;m laser irradiation. The ATI calculations, which use a multiple gauge propagation method, show a striking dependence in the production of high-order photoelectrons on the electron-ion potential. The HHG calculations illustrate the importance of the strong ground-to-first excited state coupling in multiphoton processes in the alkali metals. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291925

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We examine strong field atomic physics in a wavelength region (3-4 microns) where very little work has previously been done. The soft photon energy allows the exploration of one-electron atoms with low binding energies (alkali metals). We find that photoionization spectra differ from rare gas studies at shorter wavelengths due to more complex ion core potentials. Harmonic generation is studied, and we find that harmonic bandwidths are consistent with theory and the possibility of compression to pulse widths much shorter than that of the driving pulse. Harmonic yields in the visible and UV are sufficient for a complete study of their amplitude and phase characteristics. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291926

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

We present a detailed comparison of experiment and calculation for photoelectron kinetic energy spectra in argon using high intensity 800 nm, 120 fs laser light. The calculation utilizes only single electron physics and matches the experimental data very well. For a wide range of laser intensities and electron kinetic energies we find that multielectron effects are absent. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291927

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Multiphoton detachment rates have been obtained forNa−using theR-matrix Floquet (RMF) approach. In the low intensity limit agreement has been found with recent theoretical calculations of the total photodetachment cross section. At intensities of1010 W cm−2two-photon above threshold detachment rates are presented and similarities with earlier calculations inLi−are discussed. Also at this intensity, three-photon rates are investigated for energies ranging from below the3p&hthinsp;2P°threshold to the4p&hthinsp;2P°threshold. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291928

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

Floquet states represent intrinsic modes of ionization of an atom in a monochromatic field of constant intensity. To describe atomic wave packets evolving in realistic laser pulses, linear superpositions of Floquet states are required (“multistate Floquet theory”). This gives the possibility of following the evolution of wave packets in terms of the Floquet states that are populated during the pulse. We study here the way in which the Floquet states present in the representation of the wave packet manifest themselves in the excess-photon ionization spectra (EPI/ATI). For the purpose of illustration we choose a 1D atomic model with a soft-core Coulomb potential. We calculate the totality of the Floquet states, at all intensities needed, and generate the corresponding “Floquet map”. We then calculate the EPI spectra for wave packets evolving from the ground state under different types of pulses. By analyzing the location of the lines in the spectrum, and their shapes, we show that they can be associated, in a clear cut and predictable way, to Floquet states responsible for the emission. The understanding of the underlying physics can lead to tailoring laser pulses, such as to obtain EPI signals in a controlled way. Whereas our analysis is applied to theoretical spectra, it would apply, just as well, to experimental ones. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291929

出版商:AIP    

年代:1900

数据来源: AIP

摘要:

This overview presents basic notions on atomic stabilization, some new illustrative results, as well as the recent controversy it has stirred. We start with quasistationary (adiabatic) stabilization (QS), the original form in which the concept has emerged from high-frequency Floquet theory (HFFT). QS designates the property of the high-frequency ionization rates to decrease with intensity beyond some critical high value of the latter. Other forms of Floquet theory (Sturmian basis diagonalization, close coupling in the angular momentum basis, R-matrix) have confirmed the existence of QS, and obtained concordant numerical values for the rates of H. The experimental manifestation of QS can be obtained with adiabatically varying pulses. Dynamic stabilization (DS), on the other hand, is the general form of the phenomenon, covering also the case of rapidly turned-on pulses. It designates the fact that, for wave-packet solutions of the time-dependent Schro¨dinger equation, the ionization probability of an atomic electron at the end of a laser pulse of given shape and length, starts decreasing (albeit in an oscillatory manner) beyond a certain critical value of the peak intensity. At still higher intensities, a “destabilization” regime was found, in which the ionization probability increases slowly to 1. We illustrate this behavior with recent results. Further, we give an interpretation of DS based on the expansion of Schro¨dinger wave packets in terms of Floquet eigenstates (“multistate Floquet theory”). The interpretation relies on the fact that the Floquet states involved manifest QS, that several of them may be populated during the turn-on of the pulse (“shake-up”), and that, if the turn-on is very rapid, the initial population can be projected directed directly into the continuum (“shake-off”). We also comment on the controversy around DS in recent years, originating, on the one hand, in numerical results disagreeing with mainstream calculations, and, on the other, in results from mathematical physics. The experimental evidence on stabilization is presented subsequently: two state of the of the art experiments have been performed on low-lying Rydberg states. Finally, perspectives are considered, and the advent of VUV-FEL light sources is signalled as a crucial step towards the experimental study of the ground state stabilization of H. The message of the overview is that, on the basis of existing information, (nonrelativistic) atomic stabilization is in good shape, and further research is desirable to reveal its potential. ©2000 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1291930

出版商:AIP    

年代:1900

数据来源: AIP