Method for calculating multiphoton above-threshold processes in atoms: Two-photon above-threshold ionization
- PDF / 337,746 Bytes
- 14 Pages / 612 x 792 pts (letter) Page_size
- 53 Downloads / 199 Views
LECULES, OPTICS
Method for Calculating Multiphoton Above-Threshold Processes in Atoms: Two-Photon Above-Threshold Ionization N. L. Manakov, S. I. Marmo, and S. A. Sviridov Voronezh State University, Universitetskaya pl. 1, Voronezh, 394006 Russia e-mail: [email protected] Received February 18, 2008
Abstract—The two-photon above-threshold ionization of atoms is calculated using numerical algorithms of the Padé approximation in the model-potential method with the Coulomb asymptotics. The total and differential cross sections of the above-threshold ionization of helium and alkali metal atoms by elliptically polarized radiation are presented. The dependence of the angular distribution of photoelectrons on the sign of the ellipticity of radiation (the elliptic dichroism phenomenon) is analyzed in the above-threshold frequency range. PACS numbers: 32.80.-t, 32.80.Rm DOI: 10.1134/S1063776109040025
1. INTRODUCTION The above-threshold ionization consists in detaching a bound electron (with the binding energy |Ei |) upon absorption of photons so that the number of these photons N (N = K + S, S = 1, 2, …) is larger than their minimum necessary number in accordance with the law of conservation of energy: K = [|Ei |/ω + 1], where [x] is the integer part of x. Investigations of the above-threshold ionization were initiated in 1979, when measurements of the energy spectrum of photoelectrons generated upon ionization of xenon atoms by second-harmonic radiation from a neodymium laser (ω = 2.34 eV) with the intensity I ≈ 1013 W/cm2 revealed electrons with the energy Ei + 7ω for the threshold number of photons K = 6 [1]. Subsequently, similar experiments were repeatedly performed with other atoms at considerably higher intensities of optical radiation (see, for example, references in the monograph [2]), and the additional number S of absorbed photons at K ≈ 10 reached a rather large value. Attempts to theoretically describe these results in terms of the perturbation theory for interaction of an atom with the field did not lead to agreement between theory and experiment. Furthermore, when Paulus et al. [3] found that the spectra of above-threshold electrons exhibit pronounced plateau effects (nondecreasing ionization cross sections over a wide range of additional numbers S), it became clear that the above-threshold ionization process in an intense field at optical frequencies (ω |Ei |) has a substantially nonperturbative character and is determined by the motion of a free electron in a strong light field rather than by the structure of the spectrum of a particular atom.
sources in the ultraviolet spectral region with a photon energy of up to several tens of electron-volts and an intensity of 1014–1016 W/cm2 (in particular, sources operating with the use of higher harmonics of intense radiation from femtosecond lasers and free-electron lasers). In this case, the ionization has become possible already due to the absorption of one photon; moreover, when the photon energy is considerably higher than the binding energy |Ei |, the
Data Loading...
