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Optics and Spectroscopy

On the development of the method of two-photon absorption for detection of atomic particles

Andrey G. Smolin, Oleg S. Vasyutinskii

Published Jan 1, 2006• 0 citations
Publisher

Abstract

New expressions for the intensity of two-photon absorption by atomic photofragments are derived for the cases of linearly and circularly polarized light and for all possible spin and orbital states of photofragments. It is shown that these expressions can be used in detailed experimental analysis of products of collisions and photochemical reactions since, in particular, they make it possible to determine explicitly the higher order polarization moments of photofragments.

DOI
10.1134/s0030400x06010036

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New expressions for the intensity of two-photon absorption by atomic photofragments are derived for the cases of linearly and circularly polarized light and for all possible spin and orbital states of photofragments. It is shown that these expressions can be used in detailed experimental analysis of products of collisions and photochemical reactions since, in particular, they make it possible to determine explicitly the higher order polarization moments of photofragments.
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The two-photon interference effect of polarized light holds significant research importance in quantum optics. Different types of polarized light exhibit distinct interference behaviors, making the systematic study of these behaviors crucial. This paper introduces the concept of the polarization two-photon coherence matrix and provides a theoretical explanation for the differences in two-photon interference phenomena among linear, circular, and elliptically polarized light. Using a polarization Michelson interferometer based on two-photon absorption detection, second-order interference images of circularly polarized and elliptically polarized light were experimentally captured. The differences in these two-photon interference phenomena can be elucidated by visualizing the polarization coherence matrix. Furthermore, this theory was applied to the study of sub-wavelength interference phenomena of circularly polarized light, confirming the universality of the theoretical model and offering new perspectives and tools for the study of quantum optics.
It is well known that irradiation with intense laser light can lead to nonlinear absorption processes by individual molecules. However, the mutual interaction between two molecules can lead to cooperative nonlinear processes which result in entirely new features in the absorption spectrum. In this paper, the theory of cooperative two-photon absorption is developed using the principles of quantum electrodynamics, and expressions are derived for the rate of cooperative absorption for transitions which are forbidden in the absence of any such interaction. Several different cases are examined in detail, including crystalline or matrix-isolated species, gaseous and liquid mixtures, and van der Waals molecules. It is shown that the cooperative absorption rate can be substantially increased by choosing materials with suitable energy levels where one of several resonance enhancement mechanisms can be exploited. Finally, methods of observation are discussed, and a specific photochemical example is given for the case of a mixture of formaldehyde and deuterioformaldehyde.
The first analysis is made of stimulated Raman scattering and two-photon absorption (emission) processes resulting in a change in the states of two colliding particles. These processes are caused the by interaction between dipole moments induced as a result of polarization of the particles by the radiation. Equations are derived to describe multiphoton transitions during a collision between two particles in a radiation field. Using the approximation of rectilinear trajectories, expressions are derived and analyzed for the cross section of collisional two-photon transitions in weak and strong fields. It is shown that in fields of ≈103 cgs esu, the cross section of these transitions may reach 10–4 cm2
The advent of the laser has brought about a revolution in the kinds of experimental phenomena that can be investigated. In particular, ultra-high power monochromatic, collimated sources have intensities of sufficient magnitude to enable studies to be carried out in the regime where 'non-linear' effects are important, even dominant. Important among these are processes which are characterised by the simultaneous absorption of two or more optical photons. While multiple photon processes are not completely new, those in the optical part of the spectrum, like Raman scattering, have, until the advent of the laser, been thought of as linear in the incident intensity, or, like the two-quantum decay of the metastable 2s states of hydrogen-like atoms and ions, been confined to theory. Of the multiple photon, non-linear optical processes, multiquantum ionization is one of the more fruitful for detailed, quantitative measurements, and for comparison with theory, since (1) one does not require precise coincidence between the laser light and an absorption line (2) the ionizations can be detected unambiguously with 100% efficiency (3) atomic wave functions are simple, and sufficiently accurate to enable one to make reasonable precise calculations.