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A New, Non-local Quantum Electrodynamics: Its Effect on Multiphoton Single and Double Ionization

Krishna Rai Dastidar

Published Jan 1, 2002• 0 citations

Abstract

By considering a new, non-local quantum electrodynamics, wc have shown that the non-sequential double ionization of aloms occur due to the phase correlated two-photon excitation o f electrons.For this phase conelated excitation, two-photon rate depend.slinearly on laser intensity.However, the local nature of the radiation field gives rise to conventional multiphoton transitions, where the two-photon rate is proportional to the square of the laser intensity Wc have shown that n-photon transitions in an experiment can be modeled as a series o f these two types of transitions and hence several channels will run parallel to each other The channels for w-photon transitions are (i) a scries of phase correlated transitions giving rise to non-sequential double ionization, (ii) a senes of conventional dipole transitions giving rise to sequential ionization and (in) a mixture of phase correlated and phase non-corrclated transitions We have shown that, with the increase in laser intensity the channel comprising of only the phase correlated excitations becomes significant and gets saturated before the sequential channel can dominate, giving rise to a kink-like structure in the intensity variation of double inoi/ation 1 ins feature has been observed in the experiments on double ionization of He Wc have also provided a physical basis for the damping of non-sequcntial double inoization at the smaller wavelength, which is also an experimentally observed feature.

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We have performed an experimental study of the atomic system ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$, for which the ground state 5${\mathit{s}}^{2}$ is coupled to the autoionizing state 5${\mathit{p}}_{1/2}$6s by means of a strong resonant laser field. The investigations are based on irradiation of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ with intense (I\ensuremath{\le}1.5\ifmmode\times\else\texttimes\fi{}${10}^{10}$ W/${\mathrm{cm}}^{2}$) laser pulses of duration \ensuremath{\sim}1 ns. Single-photon detachment of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ is suppressed due to the Fano interference between transitions directly to the continuum and transitions via the autoionizing state. In contrast, two-photon absorption is at the same time resonantly enhanced since no such destructive interference is present in this process. These observations constitute experimental evidence of general features predicted from theoretical considerations of autoionizing states in strong laser fields. Furthermore, we discuss the role of autoionizing states in multiphoton double-ionization processes. Electron spectroscopy has been used to show that double ionization of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ primarily is the result of a sequential removal of the two electrons via an excited Rb state. The perspectives for direct two-electron ionization in negative ions are briefly discussed.

Physical Review A•Aug 1, 1994•17 citations

By considering a new, non-local quantum electrodynamics, wc have shown that the non-sequential double ionization of aloms occur due to the phase correlated two-photon excitation o f electrons.For this phase conelated excitation, two-photon rate depend.slinearly on laser intensity.However, the local nature of the radiation field gives rise to conventional multiphoton transitions, where the two-photon rate is proportional to the square of the laser intensity Wc have shown that n-photon transitions in an experiment can be modeled as a series o f these two types of transitions and hence several channels will run parallel to each other The channels for w-photon transitions are (i) a scries of phase correlated transitions giving rise to non-sequential double ionization, (ii) a senes of conventional dipole transitions giving rise to sequential ionization and (in) a mixture of phase correlated and phase non-corrclated transitions We have shown that, with the increase in laser intensity the channel comprising of only the phase correlated excitations becomes significant and gets saturated before the sequential channel can dominate, giving rise to a kink-like structure in the intensity variation of double inoi/ation 1 ins feature has been observed in the experiments on double ionization of He Wc have also provided a physical basis for the damping of non-sequcntial double inoization at the smaller wavelength, which is also an experimentally observed feature.

The non linear interaction between an intense laser radiation and atoms leads to ionization through the absorption of N photons from the laser radiation via laser-induced virtual states. The multiphoton ionization rate varies as a function of the laser intensity I as IN. We discuss the two most important effects which govern multiphoton ionization processes : resonance effects and laser-coherence effects. In a moderate laser intensity range (107 - 109 W cm-2) corresponding to the two, three or four-photon ionization of atoms, resonance effects play a dominant role while the photon statistics of the laser radiation are relatively unimportant. On the contrary, in the very high intensity range (1012 - 1013 W cm-2) required to observe large Nth - order ionization processes, coherence effects play a dominant role through N! enhancement in the N-photon ionization rate, while resonance effects are completely damped due to the high laser intensity. We conclude by reviewing the possible applications of multiphoton absorption processes in other fields.

Singly and doubly charged Ca ions have been produced by multiphoton excitation of Ca vapour with 5 ns, low intensity (⩽3 × 10 11 W cm −2 ) dye-laser pulses in the 675–685 nm wavelength range, including the four-photon 4s 2 1 S 0 → 4p 2 1 S 0 excitation. The intensity and wavelength dependence of the ion yields was recorded as well as the fluorescence emission from excited states of the Ca ion in an effort to identify the excitation pathways leading to single and double ionization. Unambiguous evidence for the absorption of at least two photons above the first ionization threshold was recorded, in agreement with earlier results for Mg and Sr obtained under similar conditions. However, certain characteristics of the process differ significantly from those of the earlier results, despite the apparent similarity in both the atomic structure and the excitation scheme. The most striking and unexpected finding is that for a certain wavelength, the Ca 2+ yield is observable well before the saturation intensity of Ca + and, moreover, it grows with intensity and saturates in parallel with the Ca + yield. Possible mechanisms behind this outcome are discussed in detail as well as their implications for the multiphoton multiple ionization of complex atoms. Our tentative conclusion is that the occurrence of doubly charged ion production before the singly charged ion saturation should not be considered as 'synonymous' with direct (or non-sequential) multiphoton double ionization.

Studies of multiphoton ionization of atoms have revealed several unexpected characteristics. The confluence of the experimental evidence leads to the hypothesis that the basic character of the atomic response involves highly organized, coherent motions of entire atomic shells. The important regime, for which the radiative field strength is greater than an atomic unit (e/a(2)(0)), can be viewed in approximate correspondence with the physics of fast (approximately 10 MeV per atomic mass unit) atom-atom scattering. This physical picture provides a basis for the expectation that stimulated emission in the x-ray range can be produced by direct, highly nonlinear coupling of ultraviolet radiation to atoms.

Despite inherently complex multiphoton dynamics, our observations show that Ba double ionization with an infrared laser (8800--8920 cm${}^{\ensuremath{-}1}$) resembles a single-photon process; namely, its probability is proportional to the laser intensity. In this regime, single-electron ionization is due to a four-photon resonant transition through the highly perturbed state

6{p}^{2}\phantom{\rule{0.16em}{0ex}}{}^{1}{D}_{2}

, whereas double ionization is realized by the two-electron mechanism. Furthermore, we argue that these conclusions are valid for other alkaline-earth-metal atoms and other parameters of laser radiation.

In view of recent papers on two-photon double ionization of helium, pertaining to a perceived anomaly in the behavior of the sequential process and its influence on the direct process, as well as the dependence of both on the laser pulse duration, we show that upon the proper formulation, the sequential is well defined and free of divergence, and that the dependence on pulse duration is considerably more intricate. We also argue that the apparent sharp rise of the cross section for the direct process, around

54.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}

, is due to the unintended inclusion of the sequential process, which in any case is not properly describable in terms of a single cross section.

We have performed an experimental study of the atomic system ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$, for which the ground state 5

{\mathit{s}}^{2}

is coupled to the autoionizing state 5${\mathit{p}}_{1/2}$6s by means of a strong resonant laser field. The investigations are based on irradiation of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ with intense (I\ensuremath{\le}1.5\ifmmode\times\else\texttimes\fi{}

{10}^{10}

W/

{\mathrm{cm}}^{2}

) laser pulses of duration \ensuremath{\sim}1 ns. Single-photon detachment of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ is suppressed due to the Fano interference between transitions directly to the continuum and transitions via the autoionizing state. In contrast, two-photon absorption is at the same time resonantly enhanced since no such destructive interference is present in this process. These observations constitute experimental evidence of general features predicted from theoretical considerations of autoionizing states in strong laser fields. Furthermore, we discuss the role of autoionizing states in multiphoton double-ionization processes. Electron spectroscopy has been used to show that double ionization of ${\mathrm{Rb}}^{\mathrm{\ensuremath{-}}}$ primarily is the result of a sequential removal of the two electrons via an excited Rb state. The perspectives for direct two-electron ionization in negative ions are briefly discussed.