▪ Abstract Nonresonant two-photon electronic spectroscopy of polyatomic molecules is reviewed for the period since 1979. Emphasis is placed on studies that expose patterns in the two-photon fluorescence (also ionization, optoacoustic) excitation spectra of aromatic hydrocarbons and the effect of vibrations and substitution, particularly within the framework of pseudoparity rules. A section is devoted to biological molecules and the emerging use of two-photon–induced fluorescence anisotropy. Relevant theoretical results are discussed, with emphasis on quantum chemical predictions of vibronic coupling and substituent effects on two-photon absorptivity and tensor properties of individual molecules. This chapter includes higher-order spectroscopy, and a limited number of three- and four-photon studies are discussed.

http://nathan.instras.com/documentDB/paper-410.pdf

Two-photon-induced fluorescence.

The quasistationary quasienergy state approach (QQES) is applied to the analysis of partial (n-photon) decay rates and angular distributions (ADs) of photoelectrons produced by an elliptically polarized laser field. The problem is formulated for a weakly bound electron with an energy E0 in the threedimensional δ-model potential (which approximates the short-range potential of a negative ion) interacting with a strong monochromatic laser field having an electric vector F (ωt) .T he resu lts presented cover weak (perturbative), strong (nonperturbative), and superstrong field regimes as well as a wide interval of frequencies ω extending from the tunnelling (¯ hω �| E0|) and multiphoton (¯ ω |E0|). For a weak laser field, exact equations for the normalization factor and for the Fourier coefficients of the QQES wavefunction at the origin (|r |→ 0) (that are key elements of the QQES approach for a δ-model potential) as well as for the detachment amplitudes are analysed analytically using both standard Rayleigh– Schr¨ odinger perturbation theory (PT) in the intensity, I ,o f the lase rfi eld and Brillouin–Wigner PT expansions involving the exact (complex) quasienergy � . The lowest-order perturbative results for the n-photon ADs are presented in analytic form, and the parametrization of ADs in terms of polarization- and angular-independent atomic parameters is discussed for the general case of elliptical polarization. The major emphasis is on the analysis of an ellipticity induced distortion of three-dimensional ADs and, especially, on the elliptic dichroism (ED) effect, i.e. the dependence of the photoelectron yield in a fixed direction n on the sign of the ellipticity (or on the helicity) of a laser field. The dominant role of binding potential effects for a correct description of ED and threshold effects is demonstrated, and the intimate relationship between atomic ED factors and scattering phases o ft hedetached electron is established for multiphoton detachment, including the above-threshold case.

/pdf/multiphoton-detachment-of-a-negative-ion-by-an-elliptically-172uinfvwc.pdf

Multiphoton detachment of a negative ion by an elliptically polarized, monochromatic laser field

The summation of all the higher-order contributions to multiphoton absorption in atoms is presented. The nonperturbative expressions for the transition amplitudes are expressed in terms of continued fractions. They account for the whole perturbation originated by the radiation field on the atom but do not exhibit explicit expression for the level shift. In the case of radiation fields of high intensity the only reliable determination of the level shift involves a careful location of the poles of the resolvent operator. The theory has been tested in considering a two-level atom. An extremely rapid convergence of the continued fractions has been observed, and accurate results concerning high-order resonance shifts have been obtained. The expressions for the transition amplitudes being nonperturbative with regard to the field strength, they can be considered as analytic continuations of the perturbation series. This provides a useful approach to the problem of ultraintense fields.

Exact summation of higher-order terms in multiphoton processes

Polarization Effects in High-Order Multiphoton Ionization

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 of perturbation theory in atomic multiphoton ionization

We evaluate the maximum allowed value of the ratio between the total $N$-photon ionization rates for circularly and linearly polarized incident light. It appears that this maximum increases very rapidly with $N$.

Circular versus Linear Polarization in Multiphoton Ionization

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Circular versus Linear Polarization in Multiphoton Ionization