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Chinese Physics

Photoexcitation of atom with ultrashort laser pulses

Dai Chang-Jian, Zidong Chen

Published May 1, 2001• 3 citations
Publisher

Abstract

The uncoupled basis set is used to describe the interaction between the ultrashort pulses and the Na atom. Several polarization configurations of laser pulses have been investigated, which produce different effects on the two-step photoexcitation process. The study shows that the final states of the transitions driven by ultrashort laser pulses have a strong dependence on the laser polarization configuration. A comparison of the two different basis sets shows that the uncoupled basis set may provide more information than the coupled basis set, such as angular distribution of the atom in the final states of the transitions.

DOI
10.1088/1009-1963/10/5/308

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The uncoupled basis set is used to describe the interaction between the ultrashort pulses and the Na atom. Several polarization configurations of laser pulses have been investigated, which produce different effects on the two-step photoexcitation process. The study shows that the final states of the transitions driven by ultrashort laser pulses have a strong dependence on the laser polarization configuration. A comparison of the two different basis sets shows that the uncoupled basis set may provide more information than the coupled basis set, such as angular distribution of the atom in the final states of the transitions.
The dynamical process of photoexcitation and photoionization of alkali atoms is studied with three-step laser pulses, focusing on the similarities and differences between Li and Cs atoms on their properties by making a comparison of them. Based on several excitation schemes, the present work not only establishes the rate equations of atom population for all related transition states, but also obtains the analytical solutions of photoexcitation and photoionization process. The mathematical solutions are simplified significantly by restricting the most general case to the several special cases, either designed or selected carefully, in order to highlight the main factors and obtain the physical insight underlying the complicated mathematical expressions. With self-programming, the possible impact of time configuration of laser pulses on the three-step laser excitation process of the photoexcitation and photoionization is calculated and studied systematically. Variation of the ionization efficiency with the laser parameters is investigated and discussed. With the same time configuration of laser pulses, the dependences of atom population for all the related transition states on the two different excitation paths are compared and analyzed, reflecting the impact of changing the atomic parameters. Under the similar excitation conditions, the differences of ionization efficiency between Li and Cs atoms are explored. Finally, based on the present study, several means for optimizing ionization efficiency are proposed.
In this paper we investigate how Rydberg atoms respond to perturbation by two contrapropagating ultrashort laser pulses. We consider the case where the durations of both pulses ${\ensuremath{\tau}}_{1}$ and ${\ensuremath{\tau}}_{2}$ are shorter than the inverse of the initial-state energy ${\ensuremath{\epsilon}}_{i}^{\ensuremath{-}1}$. When acting alone such a pulse passes through the atom without noticeable alteration in the atomic state. The situation is different if two such pulses interfere in the region of atom localization. In this case the atomic response is significantly enhanced. This is due to the nonzero momentum transferred to the electron by the interplay of the electric field of one pulse and the magnetic field of the other. The sudden perturbation approximation is used to evaluate the transition probabilities. They are shown to depend on the atom position with respect to the pulse interference region. This dependence is determined by the relationship between the atomic diameter di{d}_{i}di​ and the interference-region size $l=c({\ensuremath{\tau}}_{1}+{\ensuremath{\tau}}_{2})$ (ccc is the speed of light). If ${d}_{i}\ensuremath{\ll}l$ this dependence is sensitive to the function form of the pulses. For sufficiently strong fields the atoms can be ionized completely. In the opposite case of ${d}_{i}\ensuremath{\gg}l$ the transition probabilities are sensitive to the electron density distribution along the propagation direction. The probabilities of the initial-state destruction and atom ionization drop as l∕dil∕{d}_{i}l∕di​ irrespective of the characteristics of the pulses.
The paper is devoted to the investigation of the absorption of ultrashort laser pulses on atoms in plasmas, accounting for the different broadening mechanisms of atomic resonant transitions. The analysis is made in terms of the absorption probability during the entire interaction between the laser pulse and atom. Attention is mainly given to dependence of probability upon the pulse duration and the carrier frequency of the pulse. The results are presented via dimensionless parameters and functions describing the effect of finite pulse duration on atomic spectra for different broadening mechanisms, namely Doppler, Voigt, Holtsmark and their combinations, as well as the Stark line broadening of Rydberg atomic lines.
We investigate the two-photon double ionization of atomic magnesium induced by ultrashort pulses. Though the initial and final state symmetries are comparable to the same process in helium, in stark contrast the range of photon energies for which nonsequential ionization is the only open pathway is narrow (less than 1 eV) in magnesium. Thus several sequential ionization pathways feature heavily in these processes. Nonetheless, it is found that for pulse durations between 0.25 and 2.0 fs, the joint angular dependence of the ejected electrons can depend sensitively on pulse length, varying between the strictly back-to-back ejection characteristic of nonsequential ionization to other distributions. The significance of excited-state correlating configurations in representing the initial state of magnesium is discussed in the light of their consequences for the resulting angular distributions at photon energies where sequential ionization can access intermediate states that lie nearby in energy, particularly for longer pulses.
We have considered the induced processes that occur in a driven two-level atom, where a laser photon is absorbed and emitted by the ground and by the excited states of the atom, respectively. In the low-intensity limit of the laser field, the induced spectra arising when a laser photon is absorbed by the ground state of the atom consist of two peaks describing induced-absorption and stimulated-emission processes, respectively, where the former prevails over the latter. Asymmetry of the spectral lines occurs at off-resonance and its extent depends on the detuning of the laser field. The physical, process where a laser photon is emitted by the excited state is the reverse of that arising from the absorption of a laser photon by the ground state of the atom. The former differs from the latter in that the emission of a laser photon by the excited state occurs in the low-frequency regime and that the stimulated-emission process prevails over that of the induced absorption. In this case, amplification of ultrashort pulses is likely to occur without the need of population inversion between the optical transitions. The computed spectra are graphically presented and discussed.