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Laser photoselective ionization of atoms

Vladilen S Letokhov

Published Feb 15, 2007• 0 citations
Publisher

Abstract

Abstract In this and subsequent chapters, we consider are other laser methods for controlling atoms and molecules that, in contrast to all the methods discussed above in Chapters 2–8, are destructive. They result in the ionization of the atoms or molecules being controlled and the dissociation and fragmentation of molecules. All these methods are united by the capability of the particles to be ionized or dissociated in a photo selective manner, which makes it possible to detect and separate atoms or molecules of certain species. They are based on the fact that all the structural details of atoms and molecules manifest themselves in their spectra, for example, some details manifest in the spectra of atoms are the atomic weight (the number of nucleons), the number of protons (the nuclear charge and the type of element), the number of neutrons (the isotopic state), and even the excitation energy of the nucleus (the isomeric composition). This enables one to excite, and, in some cases, ionize atoms and molecules in a photo selective way and thus accomplish their photo selective excitation and separation. Figure 9.1 lists all types of photo selectivity. All of them have been successfully realized by the method of resonance multistep ionization of atoms that is considered in the present chapter. Our primary attention here is concentrated on the realization of laser control of the photo selectivity of ionization. A detailed description of this method can be found in the monographs by Letokhov (1987) and Hurst and Payne (1988).

DOI
10.1093/oso/9780198528166.003.0009

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Abstract In this and subsequent chapters, we consider are other laser methods for controlling atoms and molecules that, in contrast to all the methods discussed above in Chapters 2–8, are destructive. They result in the ionization of the atoms or molecules being controlled and the dissociation and fragmentation of molecules. All these methods are united by the capability of the particles to be ionized or dissociated in a photo selective manner, which makes it possible to detect and separate atoms or molecules of certain species. They are based on the fact that all the structural details of atoms and molecules manifest themselves in their spectra, for example, some details manifest in the spectra of atoms are the atomic weight (the number of nucleons), the number of protons (the nuclear charge and the type of element), the number of neutrons (the isotopic state), and even the excitation energy of the nucleus (the isomeric composition). This enables one to excite, and, in some cases, ionize atoms and molecules in a photo selective way and thus accomplish their photo selective excitation and separation. Figure 9.1 lists all types of photo selectivity. All of them have been successfully realized by the method of resonance multistep ionization of atoms that is considered in the present chapter. Our primary attention here is concentrated on the realization of laser control of the photo selectivity of ionization. A detailed description of this method can be found in the monographs by Letokhov (1987) and Hurst and Payne (1988).
Abstract Rather different problems can be lumped together under the general term 'laser control of atoms and molecules'. They include the laser selection of atomic and molecular velocities for the purpose of Doppler-free spectroscopy, laser control of the position and velocity of atoms (i.e. laser trapping and cooling of atoms), and laser control of atomic and molecular processes (ionization, dissociation) with a view of detecting single atoms and molecules and particularly separating isotopes and nuclear isomers. Over the last decades the principal problems posed have been successfully solved, and many of them have evolved remarkably in the subsequent investigations of the international research community. For example, the solution of the problem of laser cooling and trapping of atoms has given birth to the new field of the physics of ultracold matter, i.e. quantum atomic and molecular gases. The laser non-coherent control of uni-molecular processes has found an interesting extension in the field of laser coherent control of molecules. The concept of laser control of position has been successfully demonstrated with microparticles (optical tweezers), concurrently with investigations into atomic control. The laser photo-ionization of molecules on surfaces has led to the development of novel techniques of laser-assisted mass spectrometry of macromolecules, and so on. The aim of this book is to review these topics from a unified or 'coherent' point of view. It will be useful for many readers in various fields of laser science and its applications.
As light passes an atom, it exerts forces on the charges, electrons, and nuclei that alter the atomic structure. These may be slight distortions (perturbations) of the electron cloud or they may be more severe, as described in subsequent sections of this monograph. We wish to determine those changes, given the radiation field, or to devise a radiation field that will produce specified changes. The changes to the atomic structure also affect the radiation that subsequently passes the atom. To describe those effects we must consider wave equations for radiation in the presence of altered atomic structure. More generally we must find self-consistent equations for the atoms and the field together, as discussed in Chaps. 21 and 22. Here we consider mathematical descriptions of the influence of coherent radiation on individual atoms, molecules, or other single quantum systems. Individual atoms Traditional sources of emission and absorption spectra, though revealing energy states of the constituent atoms and molecules, are macroscopic samples. One observes averaged characteristics of many individual particles, see Chap. 16. Quantum theory offers the basic formalism for dealing with individual atoms exposed to controlled radiation fields. Several experimental techniques provide acceptable approximations to this ideal. The following paragraphs note some of these examples. Vapors Neutral atoms or molecules in a vapor move freely along straight-line paths, interrupted by brief collisions that redirect the two collision partners. When the kinetic energies of the two partners are small, there can be no transfer of kinetic energy into internal energy of either particle – the collision is elastic .
The method of sub-Doppler spectroscopy is theoretically elaborated, which is based on the specific dynamics of a number of optically excited atomic particles (atoms or molecules) of a rarefied gas medium in a thin cell after the action of the resonance pulse of the monochromatic radiation. Corresponding calculations are carried out on the basis of density matrix equations for the resonance optical transition between Zeeman degen- erate ground and excited quantum levels of particles in case of the linear polarization of the laser pulse at its normal incidence on the cell. The situation is considered when the radiative lifetime of the excited level is much more than the characteristic transit time of particles between nearest plane-parallel walls of the cell. Then the distribution of a number of excited particles versus the pulse frequency detuning narrows in the process of par- ticles collisions with walls of the cell after action of the laser pulse. The factor of such a narrowing (in compar- ison with the Doppler broadening of the spectral line of the resonance transition) may be more than the ratio of the characteristic transverse size of the thin gas cell to its inner thickness. We discuss possible use of given sub- Doppler resonances (of the number of excited particles) in the high-resolution spectroscopy and also in high- selective processes of photo-ionization and photo-dissociation, especially, for isotope (or isomer) separation and detection of rare (in particular single) atoms or molecules of a gas medium.
This review deals with the problem of selective detection of single atoms by laser radiaton. It begins with a classification of the detection methods (fluorescence, photoionization, deflection), discusses detection conditions (atomic vapor, atomic beam, buffer gas), and gives estimates of the parameters of laser radiation ensuring effective detection. Next, a detailed comparision is made of each of the detection methods. It is shown that in the fluorescence method the maximum efficiency of the detection process is achieved for cyclic interaction of atoms with laser radiation. In the photoionization method the most suitable technique is multistage photoionization of atoms by laser radiation. The highest ionization selectivity and a high efficiency are obtained by multistage excitation of an atom either to a Rydberg state followed by ionization with an electric field pulse or to a narrow autoionizing state with a large excitation cross section. Both these techniques are considered from the point of view of the sources of background and ways of discriminating against it, detection selectivity, and absolute sensitivity; the experimental results are analyzed. In discussing the deflection method consideration is given to deflection of an atom in a magnetic field after excitation with laser radiation and to deflection of atoms by resonant optical pressure. The review concludes with a brief discussion of the prospective applications of these detection methods in nuclear physics, atomic physics, and chemistry.