Two-photon bremsstrahlung of free atoms

Abstract

The two-photon bremsstrahlung emitted in collisions of 7-15-keV electrons with free (gaseous) argon, krypton, and xenon atoms was observed. The agreement with recent theoretical calculations is poor.PACS numbers: 34.80.-iThe acceleration of electrons in the nuclear field of target atoms may give rise to the emission of a continuous spectrum of electromagnetic radiation. Since its first discovery by Rontgen in 1895, ! this "bremsstrahlung" has been recognized as one of the most fundamental physical processes; besides, it has been proven to be of considerably practical relevance to various disciplines. In following decades, the production and the properties of bremsstrahlung radiation were explored quite thoroughly; 2 " 4 today, it is believed that the bremsstrahlung process is well understood and that the corresponding continuous spectrum can be calculated with fair accuracy. This is certainly true for the one-photon process; in contrast, little is known about two-photon or multiphoton bremsstrahlung processes. While other multiphoton processes are well known, for example, in atomic photoionization processes 5 and in free-free transitions, 6 the corresponding bremsstrahlung processes have attracted comparatively little interest over the years. A main reason for this may be found in the relatively small probability for emitting two or more bremsstrahlung photons in a single collision. To our knowledge, the only experimental attempt to investigate the two-photon bremsstrahlung process was performed by Altman and Quarles, 7 who investigated the two-photon process during 75-keV electron impact on solid gold. These authors observed an appreciable fraction of two-photon bremsstrahlung processes, more than 2 orders of magnitude larger than predicted by calculations based on different theoretical approaches. 8,9

Results from the nonrelativistic dipole-approximation theory of two-photon electron bremsstrahlung in the Coulomb field

“…New experimental data came out from the measurements made by Hippler [8] using gaseous targets, and by Kahler, Liu, and Quarles [9].…”

“…The new experimental results of Hippler [8] and of Kahler, Liu, and Quarles [9] have been compared only partially with theory, because of lack of theoretical data. It is the purpose of our paper to present new results from the nonrelativistic theory of two-photon bremsstrahlung in the Coulomb field of a fixed charge Z.…”

Correlated two-photon emission by transitions of Dirac-Volkov states in intense laser fields: QED predictions

“…Probably the most well known process of this kind is double bremsstrahlung [21,22,23,24,25,26,27,28,29], but also photon pair creation in a magnetic field [30,31,32], and in a crossed field [33] has been considered. The process under investigation in this paper is complementary to those mentioned above, and may provide for better control of the properties of the produced photons by adjusting the laser parameters.…”

Two-photon bremsstrahlung processes in atoms: Polarization effects and analytic results for the Coulomb potential

“…The cross sections of the above-mentioned processes are also determined by ME (7). For example, the differential (with respect to the angles of a scattered electron) induced 2BrS cross section has the form (9) where the electron energy Ᏹ in the intermediate state is connected to the energies in the initial (E = p 2 /2) and final (E' = p' 2 /2) states through the relation Ᏹ = E -ω = E' + ω. Similarly, the double bremsstrahlung absorption cross section is given by (10) where…”

“…In 1985, spontaneous 2BrS was detected experimentally (see [7] and subsequent publications [8], where the differential cross sections of emission of two bremsstrahlung photons during scattering of electrons with an energy of about 70 keV by thin targets were measured by the coincidence method). In experiments [9], spontaneous 2BrS was observed for electrons with energy of the order of 10 keV. The first theoretical calculations of the 2BrS cross section for electron scattering by a nucleus were made in the framework of the relativistic Born approximation [10].…”