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Journal of Physics B Atomic and Molecular Physics

The infinite summations of perturbation theory in atomic multiphoton ionization

G Laplanche, A. Durrieu, Y. Flank, M. Jaouen, A. Rachman

Published Jun 1, 1976• 51 citations
Publisher

Abstract

The infinite summations over the complete set of unperturbed atomic states appearing in the Nth-order time-dependent perturbation theory are performed using the Green's function formalism. In order to test the method extended calculations of the cross section for two- and three-photon ionization for atomic hydrogen are made and numerical comparison is shown with the values obtained by different methods. The resonant character of the process is made evident since the poles of the radial part of the Green's function represent the actual energy spectrum of the atom. The relevant transition matrix elements are written in closed form and then computed exactly without averaging over the intermediate states. The cross sections vanish for circularly polarized light near some energies which explains the form of the cross sections for circularly and linearly polarized light.

DOI
10.1088/0022-3700/9/8/012

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Physical Review A•May 1, 1989•65 citations
The infinite summations over the complete set of unperturbed atomic states appearing in the Nth-order time-dependent perturbation theory are performed using the Green's function formalism. In order to test the method extended calculations of the cross section for two- and three-photon ionization for atomic hydrogen are made and numerical comparison is shown with the values obtained by different methods. The resonant character of the process is made evident since the poles of the radial part of the Green's function represent the actual energy spectrum of the atom. The relevant transition matrix elements are written in closed form and then computed exactly without averaging over the intermediate states. The cross sections vanish for circularly polarized light near some energies which explains the form of the cross sections for circularly and linearly polarized light.
The infinite summations over the complete set of unperturbed atomic states appearing in the NNNth-order perturbation theory for multiphoton ionization are performed using the Dalgarno-Lewis method. The relevant transition matrix elements are written in a closed integral form exhibiting all the analytic properties of the amplitude as a function of incident photon energy. The cross sections for two- and three-photon ionization for atomic hydrogen are calculated for both linearly and circularly polarized light with a wide range of photon energy spectrum including the near resonance, and numerical comparison is made with the values obtained by different methods.
The cross section for multiphoton ionization of hydrogen is computed using the momentum translation method (MTA) in the cases of linearly and circularly polarized light, alternatively. Extended numerical comparison is made with well-established results obtained from the Nth-order time-dependent perturbation theory in the weak intensity limit. The MTA does not reproduce the values given by perturbation theory and considerable differences are observed. Higher order iterations than the lower one are introduced and an analysis of the leading terms of the development is made: at weak intensities the zeroth-order term that defines the MTA cancels with higher order terms and the final result obtained after making (N-1 iterations is none other than the well-known transition amplitude deduced from standard perturbation theory, defined by the remaining terms of the last iteration after mutual cancellation with lower-order ones. According to the results obtained in this work, it is not possible to affirm that the MTA is adequate to calculate atomic multiphoton ionization cross sections.
The general theory of multiphoton ionization of hydrogen atoms by simultaneous absorption of several photons is discussed. In the nonrelativistic dipole approximation, the calculations lead to a recursion relationship concerning angular functions and involve radial matrix elements containing many summations over intermediate states. A general method for exactly evaluating several sums is described in detail and applied to the photo-ionization of atomic hydrogen in the ground state. Numerical results are presented as dispersion curves of the transition rate $\frac{{\ensuremath{\sigma}}_{n}}{{I}^{N\ensuremath{-}1}}$ for a range of wavelengths from 1000 up to 7200 \AA{}.
Two-photon ionization of atomic hydrogen with an excess photon is revisited. The non-relativistic dipole approximation and Coulomb Green function (CGF) formalism are applied. Using the CGF Sturmian expansion straightforwardly, one gets the radial transition amplitude in the form of an infinite sum over Gauss hypergeometric functions which are polynomials. It is convergent if all intermediate states are in the discrete spectrum. In the case of two-photon ionization with an excess photon, when photoionization is also possible, intermediate states are in the continuum. We performed the explicit summation over intermediate states and got a simple general expression for the radial transition amplitude in the form of a finite sum over Appell hypergeometric functions, which are not polynomials. An Appell function may be expressed as an infinite sum over Gauss functions. In the case of ionization by an excess photon, Gauss functions are transformed to give a convergent radial transition amplitude for the whole region. The generalized cross sections for two-photon above-threshold ionization of atomic hydrogen in the ground state and excited states calculated by us agree very well with results of previous calculations. Generalized cross sections for two-photon ionization of positronium in the ground state are obtained by scaling those for atomic hydrogen.
In the present thesis we have formulated the Dalgarno-Lewis procedure\nfor two-and three-photon processes and an elegant alternate expressions\nare derived. Starting from a brief review on various multiphoton processes\nwe have discussed the difficulties coming in the perturbative treatment\nof multiphoton processes. A small discussion on various available methods\nfor studying multiphoton processes are presented in chapter 2. These\ntheoretical treatments mainly concentrate on the evaluation of the higher\norder matrix elements coming in the perturbation theory. In chapter 3 we\nhave described the use of Dalgarno-Lewis procedure and its implimentation on second order matrix elements. The analytical expressions for twophoton\ntransition amplitude, two-photon ionization cross section, dipole\ndynamic polarizability and Kramers-Heiseberg are obtained in a unified\nmanner. Fourth chapter is an extension of the implicit summation technique\npresented in chapter 3. We have clearly mentioned the advantage of\nour method, especially the analytical continuation of the relevant expressions\nsuited for various values of radiation frequency which is also used for\nefficient numerical analysis.\nA possible extension of the work is to study various multiphoton processcs\nfrom the stark shifted first excited states of hydrogen atom. We\ncan also extend this procedure for studying multiphoton processes in alkali\natoms as well as Rydberg atoms. Also, instead of going for analytical expressions,\none can try a complete numerical evaluation of the higher order\nmatrix elements using this procedure.
The variationally stable procedure for Nth-order perturbative-transition-matrix elements introduced by the authors [Phys. Rev. Lett. 61, 404 (1988)] is presented here in detail. Its key features are that it is noniterative, involves only two unknown functions regardless of the value of N, is stable even near intermediate-state resonances (due to the presence of energy numerators rather than energy denominators), and uses the inverse of the perturbation operator for N\ensuremath{\ge}3. Explicit formulas are presented for application to high-order multiphoton processes in atomic hydrogen. Numerical results for multiphoton-ionization cross sections (for two, three, and seven photons), for the frequency dependence of the nonlinear susceptibilities for harmonic generation (for the third, fifth, and seventh harmonics), and for harmonic-generation transition rates up to 11th order for \ensuremath{\lambda}=1064 nm are presented for the atomic hydrogen ground state and compared with results of others by more standard procedures. We also present detailed analyses of the Z scaling of our results for hydrogenic systems, of the use of the imaginary part of the appropriate 2N-photon nonlinear susceptibility to obtain N-photon-ionization cross sections, and of the relation of the variational principle presented here to variational principles for scattering processes of Nuttall and Cohen [Phys. Rev. 188, 1542 (1969)] and of Schwinger [Phys. Rev. 72, 742 (1947)].