Velocity Distribution Of Excited Atoms Research Articles

Kinetic energy measurement of hydrogen in LHD peripheral plasma with a multi-wavelength-range fine-resolution spectrometer

We have simultaneously measured high resolution emission spectra of the hydrogen atomic Balmer-α, -β, -γ lines and molecular Fulcher-α band for a LHD peripheral plasma generated under a central magnetic field strength of 0.4 T. It is found that the velocity distributions of excited atoms calculated from the Balmer-α, -β, and -γ line shapes show similar profiles to each other. The translational kinetic energy corresponding to the average velocity is about 13 eV, which is about 300 times larger than the rotational energy of hydrogen molecules estimated from the line intensities in the Fulcher-α band. The velocity distributions differ from Maxwellian and have a high velocity tail over 1×10 5 m/s. A correlation between the high velocity tail and the electron temperature and density is seen and suggesting the excited atoms having such high velocities to be generated by the charge exchange collisions from high velocity protons in the peripheral region.

Thermalization of fast cesium5D3∕2atoms in collisions with ground-state cesium atoms

We have investigated collisions involving fast, excited Cs atoms produced by photodissociating Cs{sub 2} molecules with a pulsed dye laser. The velocities of the atoms in the 5D state formed by the process Cs{sub 2}(X {sup 1}{sigma}{sub g}{sup +})+({Dirac_h}/2{pi}){omega}{sub pump}{yields}Cs{sub 2}{sup *}{yields}Cs(5D)+Cs(6S) are much greater than typical thermal velocities associated with the cell temperature. Using a narrow-band cw probe laser to observe the increased Doppler broadening of the 5D{sub 3/2}{yields}5F{sub 5/2} excitation line shape, we are able to monitor the time evolution of the velocity distribution of these 5D atoms. We analyze the data using a model that predicts the time-dependent excitation line shape of the fast atoms. Because the photons used to dissociate the molecules have a well-defined energy, the velocity distribution of the excited atoms in the early time after they are produced can be fairly well determined. Over time, velocity-changing collisions with ground-state Cs atoms cause the velocity distribution of excited atoms to approach the thermal limit. An analysis based on the strong-collision model leads to a prediction that the observed line shape at intermediate times will be a linear combination of contributions from distinct 'fast' and 'thermalized' atomic populations. By fitting our data to this model,more » a rate coefficient for velocity-changing collisions of fast Cs(5D{sub 3/2}) atoms with ground-state Cs atoms has been determined. The result k{sub VCC}=(6.1{+-}1.2)x10{sup -10} cm{sup 3} s{sup -1} corresponds to an effective velocity-changing collision cross section of {sigma}{sub VCC}{sup Cs,eff}=(1.2{+-}0.2)x10{sup -14} cm{sup 2}.« less

On the velocity distribution of excited Fe-atoms by sputtering of iron

From the velocity distribution of excited sputtered particles detailed information on the excitation process can be obtained. In the present paper the first direct measurement of velocity distribution of excited atoms sputtered from a metal target is presented. The irradiation of the Fe-target was performed with 10keV Ar+-ions. The sputtered atoms were detected using pulsed laser induced fluorescence (LIF). The sputtered Fe atoms in the metastable statea5F5 at 0.86 eV shows a much broader distribution, than found for the ground-state atoms, but no energy threshold, implied in the statistical excitation models, was found.

Theory of gain and saturation for collision-induced lasing transitions

Theoretical expressions are given for the inversion density, small-signal-gain coefficient, and saturation-energy flux which pertain to stimulated collision-induced emission, that is, the free-free and free-bound dipole emission from unbound excited states formed during a collision of an atomic metastable species and another atom. The probability of stimulated emission during a collision is assumed to remain much less than unity so that a Maxwellian velocity distribution of excited atoms is maintained. The gain and saturation are described in terms of a profile function k (ν), a frequency-dependent two-body rate coefficient per unit frequency. The final expressions are analogous to those for a homogeneously broadened two-level atom. The theory is applied to the collision-induced green continuum from oxygen 1S0 in argon.

Use of the Scattering Kernel Approach in Certain Gas Kinetic Problems

The collision gain term of the Boltzmann equation may at times be transformed to a triple integral over the product of the unknown distribution and a scattering kernel in the case of a minor constituent interacting through collisions with a major constituent (scatterer). If isotropy in velocity space may be assumed, the term may be further reduced to an integral over the particle speed. The scattering kernel for hard-sphere collisions has been used in neutron slowing down and thermalization problems, but this approach does not seem to have been used widely in other gas kinetic applications. The transformation is discussed and cast in a form that is applicable to collision models other than hard-sphere scattering and to arbitrary velocity distributions of the scatterers. Kernels are obtained for isotropic constant mean-free-time scattering, and it is shown that kernels for monoenergetic velocity distributions of the scatterers are useful as well as those for Maxwellian and stationary distributions. Use of the scattering kernel approach is illustrated by finding the steady-state velocity distribution of excited atoms that are being produced at a steady rate and that are losing kinetic energy via collisions and their excitation energy by radiation. The solution for Maxwellian scatterers is well approximated at high velocities by the solution for stationary scatterers, and at low velocities by the solution for monoenergetic scatterers.