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Journal of Modern Physics

Computational Study of Resonantly Ionizing Rubidium Vapor by Nanosecond Laser Pulses

Mohamed Mahmoud, Yosr E. E.-D. Gamal

Published Jan 1, 2012• 1 citation
Publisher

Abstract

An investigation of the resonant interaction of the rubidium atoms with an intensity (10 kWcm-2 ≤ I ≤ 2 MWcm-2) and a wavelength close to that of the D1 and D2 transitions of the rubidium atom (5S1/2 → 5P3/2 or 5S1/2 → 5P1/2, λD1 = 780 nm, λD2 = 795 nm), which has passes through rubidium vapor with density (1011 - 1014 cm-3) been studied theoretically. The system of equations describing the processes of Collisional ionization and multiphoton ionization of rubidium vapour resonantly excited with nanosecond pulsed laser is solved. The dependence of the ion density on the laser intensity and the atomic density of rubidium are considered. The result of calculations revealed that, both quadratic ion density dependence on laser intensity and linear behaviour of the ion density versus rubidium density for 5S1/2 → 5P3/2 transition is due to photoioization process. In contrast, for 5S1/2 → 5P1/2 transition, the ion density dependence is nonlinear and indicates that the collisional processes play the major contribution in the total ionization. Also, the obtained results showed reasonable agreement with the experimentally measured values of the ion density dependence given by Bakhramov et al. In addition, the analysis of mutual competition between the different ionization processes considered for the ion yield as a function of both laser intensity and atoms density are also presented this work.

DOI
10.4236/jmp.2012.39121

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An investigation of the resonant interaction of the rubidium atoms with an intensity (10 kWcm-2 ≤ I ≤ 2 MWcm-2) and a wavelength close to that of the D1 and D2 transitions of the rubidium atom (5S1/2 → 5P3/2 or 5S1/2 → 5P1/2, λD1 = 780 nm, λD2 = 795 nm), which has passes through rubidium vapor with density (1011 - 1014 cm-3) been studied theoretically. The system of equations describing the processes of Collisional ionization and multiphoton ionization of rubidium vapour resonantly excited with nanosecond pulsed laser is solved. The dependence of the ion density on the laser intensity and the atomic density of rubidium are considered. The result of calculations revealed that, both quadratic ion density dependence on laser intensity and linear behaviour of the ion density versus rubidium density for 5S1/2 → 5P3/2 transition is due to photoioization process. In contrast, for 5S1/2 → 5P1/2 transition, the ion density dependence is nonlinear and indicates that the collisional processes play the major contribution in the total ionization. Also, the obtained results showed reasonable agreement with the experimentally measured values of the ion density dependence given by Bakhramov et al. In addition, the analysis of mutual competition between the different ionization processes considered for the ion yield as a function of both laser intensity and atoms density are also presented this work.
Rubidium vapor (1011 - 1014 cm-3) was irradiated by a dye-laser beam (10 kW(DOT)cm-2 - 2 MW(DOT)cm-2, pulse duration approximately 30 nsec, linewidth <EQ 0.25 cm-1), which was tuned to the D1,2-resonance lines. The mutual competition of the two photon ionization, associative ionization and Penning ionization of the laser excited rubidium atoms was investigated. The data of laser intensities and a rubidium vapor concentrations, which results in domination of the each ionization channels were founded. On the basis of the received experimental results, the cross-section values for the multiphoton and collisional ionization channels of laser-excited atoms were determined.
Abstract We have studied the formation of the molecular ion Rb2+ and the atomic ion Rb+. These are created in laser excited rubidium vapor at the first resonance, 5s–5p and 5p-nl transitions. A theoretical model is applied to this interaction to explain the time evolution and the laser-power dependence of the population density of Rb+ and Rb2+. A set of rate equations which describe: the temporal variation of the population density of the excited states; the atomic ion density; and the electron density, were solved numerically under the experimental conditions of Barbier and Cheret. In their experiment the Rb concentration was 1×1013cm−3 and the laser power was taken to be 50–500 mW at vapor temperature = 450 K. The results showed that the main processes for producing Rb2+ are associative ionization and Hornbeck-Molnar ionization. The calculations have also showed that, the atomic ions Rb+ are formed through the Penning Ionization (PI) and photoionization processes. Moreover, a reasonable agreement between the experimental results and our calculations for the ion currents of the Rb+ and Rb2+ is obtained.
The electron energy distribution function (EEDF) in laser-excited Rb vapour at the first resonance transition 5S–5P and 5p– nl transitions is calculated numerically from the Boltzmann equation under the experimental conditions of Barbier and Cheret ( 1987 J. Phys. B: At. Mol. Phys. 20 1229 ). The calculations included all the interactions between electrons and ground or excited states of atomic rubidium. We have also studied the time evolution and the laser power dependences of the EEDF and the electron density which is created during the interactions. The energy spectra of the electrons emerging from the interaction contain a number of peaks corresponding to the low-energy electrons produced by the associative and Hornbeck–Molnar ionization in addition to the electrons created from the various excited states by the ionizing collisions (Penning ionization). The low-energy electrons are then heated by one or more superelastic collisions leading to further ionization. However the calculations indicated that, under the conditions of low power laser intensity and relatively low atom density, the dominant processes are collisional ionization and collisional excitation.
We have studied two-photon resonant three-photon ionization of atomic sodium vapor. A dye laser tuned to either the 4d or 5s two-photon allowed transition was used to excite the atom and a second more intense laser (λ = 7000 Å) was used for ionization. The ion yield was measured as a function of resonant laser intensity and detuning, the nonresonant laser intensity, and the temporal overlap of the exciting and ionizing laser pulses. We have found1 that at high laser intensities the photoionization yield decreases as the nonresonant laser intensity is increased. When tuned to the 5s level a value of ~−0.6 was obtained for the exponential index even with the resonant laser tuned to maximize the ion yield. A theoretical model including the effects of Stark shift of the ground state, power broadening of the intermediate level due to saturation of the two-photon transition and by rapid transitions to the ionization continuum was used to predict the experimental results. Good agreement between theory and experiment required the use of a photoionization cross section for the 5s level which was ~20 times greater than the accepted value. Results for photoionization from the 4d level are also presented.
We investigate the propagation of ultrashort laser pulses in atomic rubidium vapor. The pulses are intensive enough to ionize the atoms and are directly resonant with the 780-nm D2{D}_{2}D2​ line. We derive a relatively simple theory for computing the nonlinear optical response of atoms and investigate the competing effects of strong resonant nonlinearity and ionization in the medium using computer simulations. A nonlinear self-channeling of pulse energy is found to produce a continuous plasma channel with complete ionization. We evaluate the length, width, and homogeneity of the resulting plasma channel for various values of pulse energy and initial focusing to identify regimes optimal for applications in plasma-wave accelerator devices such as that being built by the AWAKE collaboration at CERN. Similarities and differences with laser pulse filamentation in atmospheric gases are discussed.
Diode laser excitation of rubidium atoms at the D2 line followed by (1 + 1) ionization using a pulsed dye laser was examined. Mass-resolved D2 spectra were recorded for both natural isotopes. Excitation using two sequential diode laser photons was accomplished by counterpropagating the beam from the 780 nm laser and one from a 775.8 laser; the latter excites Rb from the {sup 5}P{sub 3/2} level to the {sup 5}D{sub 5/2} level at 25704 cm{sup {minus}1}. A photon from the R6G laser then ionized the atom. Spectra resulting from scanning the second diode laser -- with the first an an off-resonance fixed frequency revealed both two-photon and sequential (1 + 1) processes.