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Journal of Physics B Atomic Molecular and Optical Physics

Bremsstrahlung in collisions of a slow charged particle with an excited hydrogen

Andrei V. Korol, Michael Yu. Kuchiev, Andrey V. Solov’yov

Published Aug 14, 1992• 4 citations
Publisher

Abstract

The authors demonstrate in this work that there is a peculiar source of low-frequency photon emission when a slow massive charged particle collides with an excited hydrogen atom. This radiation is generated by the rotating dipole moment of the hydrogen during the collision. The specific feature of the problem is in the appearance of the dipole moment of the excited hydrogen due to mixing of degenerated levels with different parity in the field of the projectile particle (linear Stark effect). The radiation associated with transitions between components of the Stark multiplet is also considered. Its contribution to the total radiation spectrum is shown to be important.

DOI
10.1088/0953-4075/25/15/018

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Journal of Experimental and Theoretical Physics•May 1, 2008•1 citation
The authors demonstrate in this work that there is a peculiar source of low-frequency photon emission when a slow massive charged particle collides with an excited hydrogen atom. This radiation is generated by the rotating dipole moment of the hydrogen during the collision. The specific feature of the problem is in the appearance of the dipole moment of the excited hydrogen due to mixing of degenerated levels with different parity in the field of the projectile particle (linear Stark effect). The radiation associated with transitions between components of the Stark multiplet is also considered. Its contribution to the total radiation spectrum is shown to be important.
The features of the bremsstrahlung appearing during a collision of a fast charged particle with a hydrogen atom (or hydrogenic ion) in an excited state are investigated. It is shown that the emission spectrum of photons with energies greater than the ionization potential of a given excited state (except the 2 s state) displays narrow lines, which are caused by de-excitation of the atom in an intermediate state. It is demonstrated that the scattering of a charged particle on an excited hydrogen atom produces a feature which is not observed in the case of scattering on a ground-state hydrogen atom. Expressions are obtained for the generalized dynamic polarizability of the hydrogen atom and hydrogenic ions in the 1 s , 2 s , and 3 s states. A method is developed for deriving expressions for the generalized dynamic polarizabilities of other excited states through the use of the Coulomb Green’s function and representation of the electronic wave function in terms of the differentiation of the generating functions of Laguerre polynomials. The bremsstrahlung cross sections for electrons and positrons colliding with hydrogen atoms in the 1 s , 2 s , and 3 s states are calculated.
The theory of coherent radiation emission in two-level quantum systems with permanent dipole moments at the multiphoton resonant excitation by moderately strong laser fields is presented. A simple analytic formula for the expectation value of a dipole moment operator is obtained. The spectrum corresponding to harmonic generation and low-frequency radiation on the generalized Rabi frequency is investigated both analytically and numerically. The effect of dynamic Stark shift on a dipole spectrum under direct multiphoton resonant excitation is considered.
Second harmonic generation (SHG) in free atoms is one of the unsolved problems in nonlinear optics. The most probable mechanism is due to Stark mixing between nearby excited states through an electric field caused by charge separation through the process of three-photon ionization. To test the mechanism, atomic hydrogen appears to be the best candidate because of its well-known electronic structure and large dipole moments of its excited states. In this work we report on the observation of SHG in atomic hydrogen in the Lyman-α region. Pulsed dye laser radiation was doubled and tuned to 243 nm for excitation of the 2s ← 1s transition at 121.6 nm by a two-photon resonance process. A cw atomic beam was used with hydrogen atoms generated in a dc discharge of H2 gas. The laser beam was focused ~1 mm below the nozzle where the hydrogen atom density was estimated to be 1013 cm−3. The second harmonic radiation was strongly enhanced by adding an external dc field of up to 10 kV/cm. Stark splitting was clearly observed at higher fields.
A study is made of the spectra of a highly excited hydrogen atom in crossed external l-mixing electric and magnetic fields. A semiclassical model is suggested for the radiative transitions between the perturbed Rydberg levels and the low-lying levels of the atom. The emitted light is shown to be a superposition of radiation of two effective linear vibrators symmetrically placed with respect to the external fields' plane. The interference between the vibrators is the source of rapid oscillations of the observed line intensities and polarization accompanying even small variation of the external fields. The averaged spectral intensity is shown to be unchanged by the presence of the magnetic field, being the same as in Stark splitting.
The alignment of atomic states as the result of collisions with a beam of slow charged particles is investigated. The problem of the collisional reorientation of atomic moments is solved by numerical integration of the equations of the impact parameter method taking into account charge--quadrupole interaction. Calculations are made of the alignment of the /sup 2/P/sub 3//sub ///sub 2/ levels of alkali atoms populated by collisions with selective excitation of the /sup 2/P/sub 1//sub ///sub 2/ level. The linear polarization of light emitted by the /sup 2/P/sub 3//sub ///sub 2/ level attains some tens of percents and corresponds to the predominance of polarization in directions perpendicular to the beam. The efficiency of the resulting alignment depends on the volume density but not on the velocity of the beam of charged particles. Thus the slow drift of ions (and electrons) in a gas-discharge plasma can compete with alignment as the result of fluxes of charged particles. We shall formulate the sum rules for anisotropic collisional relaxation of atomic polarization moments. It follows from these that for the appearance of alignment in intramultiplet mixing under the action of anisotropic collisions we require a nonuniform population of multiplet levels that differs from the more » ratio of their statistical weights. « less
The effect of collisions on the resonance fluorescence spectrum of two-level atoms excited by monochromatic resonance radiation is studied. Analysis is performed for systems where the Doppler broadening is small compared to the collision frequency (high buffer gas pressures) and for the general case where collisions arbitrarily change the phase of the radiation-induced dipole moment (from completely interrupted to completely unaffected phase of the dipole moment). Both at a relatively low and at a high excitation radiation intensity, the resonance fluorescence spectrum is shown to depend on whether the two-level system is closed or open. This is especially true for the narrow unshifted Rayleigh scattering line. It is shown that, although the absorption line is homogeneously broadened, the resonance fluorescence spectrum exhibits a clearly pronounced anisotropy. In a direction close to the direction of propagation of the excitation radiation, the Rayleigh scattering line is maximally narrowed. Under certain conditions (that can easily be created in experiments), the width of this line is proportional to the diffusion coefficient of atoms interacting with the radiation.