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One- and two-photon ionization

de Meijere, Jacob Louis Frederik

Published Jan 1, 2016• 0 citations
OpenAlex

Abstract

Detailed descriptions of one- and two-photon ionization are presented. We go beyond conventional perturbation theory to investigate the dynamics of both processes for a broad range of field intensities. The general theory of unstable decaying states is applied to the problem of one-photon ionization (the photoeffect). It is found that a non-trivial dependence of the transition dipole moment on the continuum state energy can lead to corrections to exponential decay, and even oscillations in the bound state population. As examples we consider the photoionization of hydrogen and atoms with autoionizing states. Only in the absence of autoionizing states corrections to exponential decay remain inconsequential for any experimentally relevant intensity. Our description of two-photon ionization is formulated in the Heisenberg picture. Besides the usual two-level atom projection operators we introduce explicitly the operators referring to the atomic continuum. By eliminating from the equations of motion the terms related to the electromagnetic self field as well as the terms referring to the continuum, transverse and longitudinal damping corresponding to spontaneous emission and photoionization is obtained. The use of field correlation functions allows us to account for the partial coherence of the applied field. We develop a technique to extract from the equations of motion a unique two-photon ionization rate. Such a characterization of the ionization process is not only instructive because of its simplicity, it is also in many cases completely satisfactory. The resulting general expression for the two-photon ionization rate is the central result of our treatment. From it we recover several familiar results in different limiting cases. In addition, our general rate expression provides the first description of the complete hierarchy of saturation effects that one should expect. We present a number of examples where we calculate the two-photon ionization rate as a function of the detuning, and where we emphasize the saturation and field correlation effects.

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arXiv:2006.02477•Physical Review A•Nov 16, 2020•3 citations
Detailed descriptions of one- and two-photon
ionization are presented. We go beyond conventional perturbation
theory to investigate the dynamics of both processes for a broad
range of field intensities.
The general
theory of unstable decaying states is applied to the problem of
one-photon ionization (the photoeffect). It is found that a
non-trivial dependence of the transition dipole moment on the
continuum state energy can lead to corrections to exponential
decay, and even oscillations in the bound state population. As
examples we consider the photoionization of hydrogen and atoms with
autoionizing states. Only in the absence of autoionizing states
corrections to exponential decay remain inconsequential for any
experimentally relevant intensity.
Our
description of two-photon ionization is formulated in the
Heisenberg picture. Besides the usual two-level atom projection
operators we introduce explicitly the operators referring to the
atomic continuum. By eliminating from the equations of motion the
terms related to the electromagnetic self field as well as the
terms referring to the continuum, transverse and longitudinal
damping corresponding to spontaneous emission and photoionization
is obtained. The use of field correlation functions allows us to
account for the partial coherence of the applied field.
We develop a technique to extract from the
equations of motion a unique two-photon ionization rate. Such a
characterization of the ionization process is not only instructive
because of its simplicity, it is also in many cases completely
satisfactory. The resulting general expression for the two-photon
ionization rate is the central result of our treatment. From it we
recover several familiar results in different limiting cases. In
addition, our general rate expression provides the first
description of the complete hierarchy of saturation effects that
one should expect.
We present a number of
examples where we calculate the two-photon ionization rate as a
function of the detuning, and where we emphasize the saturation and
field correlation effects.
In the theoretical study of photoionization, certain properties of the photoelectrons are not defined in terms of any (even idealized) model for the electron-detection process, in contrast to the well-known Glauber theory of photon detection. Thus, for instance, the energy spectrum of the electron produced in atomic ionization is usually defined simply as the long-time probability for the electron to have a given energy. In this paper a formal model for an electron detector localized to a small region in space, similar to the DeWitt monopole detector, is set up in terms of which results analogous to those obtained for photon detection in the Glauber theory can be obtained. In addition, it is possible to define correlation functions describing the joint probability of detecting an electron and a photon. This correlation function is calculated here in the case of two-photon ionization of an atom modeled as having two discrete energy levels and an ionization continuum. When the atom is subjected to an incident driving field, both a photoelectron and scattered photons arising from the spontaneous decay of the upper of the two discrete levels are produced. Correlations between these photons and the photoelectron are studied here by considering the joint probability of detection of a photoelectron and a scattered photon for a monochromatic driving field.
This paper discusses the general theory of two-photon ionization of a hydrogenic state in the nonrelativistic dipole approximation. An application is made to the photoionization of the metastable 2S2S2S state of atomic hydrogen by optical radiation, for example, by the light from a ruby laser. The sum over intermediate states is carried out exactly by means of an implicit technique. The results are given for a range of wave-lengths from the threshold at 7290 down to 4556 \AA{}. At 6943 \AA{}, the total cross section per unit intensity is found to be 5.35\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}29}$ cm4{\mathrm{cm}}^{4}cm4 ${\mathrm{W}}^{\ensuremath{-}1}$. The lowest order damping corrections to the perturbation-theory results are worked out.
A treatment is given of two-photon ionization in which particular attention is paid to the influence of the bandwidth of the ionizing light on the ionization rate. The model adopted for the atom is a common one, including two bound levels and a continuum. The model of the laser is general enough to allow short-term temporal fluctuations of amplitude or phase, and these fluctuations give rise to the laser bandwidth. It is found that the fluctuations have the most interesting effects on the ionization rate when the laser coherence time is shorter than the atomic memory time, as could be expected. In addition to laser bandwidth, we consider the influence on the ionization rate of the detuning of the laser from the intermediate-state resonance, the laser power, and the finite lifetime of the intermediate state. Comparisons with related earlier calculations are made and similarities and differences are pointed out.
We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.
The strong-field multiphoton ionization of atoms is considered and a theoretical approach dealing nonperturbatively with the radiation field formulated. The general computational scheme is the conventional perturbation theory, but the intermediate states are dressed by the field. We present in detail a method to dress the continuum states and to study the dipole transitions within the continuum. In the high-frequency domain, the proposed procedure rapidly converges over a wide range of field intensity and offers an interesting framework for calculating ionization rates for arbitrary numbers of absorbed (above-threshold) photons and field polarization.
We have developed a perturbative method to model the resonant ionization of atomic systems in fluctuating laser fields. The perturbative method is based on an expansion in terms of the multitime cumulants, a suitable combination of moments (field's coherence functions), used to represent the field's statistical properties. The second-order truncated expansion is expressed in terms of the radiation's power spectrum and the intensity autocorrelation function. We investigate the range of validity of the model in terms of the field's coherence temporal length and peak intensity and have compared the results with conventional Monte Carlo calculations. We apply the theory in the case of a near-resonant ionization of the helium 2s2p2s2p2s2p autoionizing state with a self-amplified spontaneous emission free-electron laser pulse with square-exponentially dependent first-order coherence function. The ionization lineshape profile acquires a Voight profile; the degree of the Gaussian or Lorentzian character of the lineshape will depend crucially on the field's coherence time.