Computational Study of Resonantly Ionizing Rubidium Vapor by Nanosecond Laser Pulses

Mohamed Abd El-Hameid Mahmoud, Yosr Ez El Din Gamal

Department of Physics, Faculty of Science, Sohag University, Sohag, Egypt.
Department of Physics, Girls’ Faculty of Science, King Abdul-Aziz University, Jeddah, KSA.

DOI: 10.4236/jmp.2012.39121                

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 (5S_1/2 → 5P_3/2 or 5S_1/2 → 5P_1/2, λ_D1 = 780 nm, λ_D2 = 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 5S_1/2 → 5P_3/2 transition is due to photoioization process. In contrast, for 5S_1/2 → 5P_1/2 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.

Keywords

Laser; Plasma; Collisional Ionization; Electron Energy Distribution Function; Photoionization; Rubidium

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	A. N. Klyucharev, “Chemi-Ionization Processes,” Physics-Uspekhi, Vol. 36, No. 6,1993, pp. 486-512.
[2]	T. B. Lucatorto and T. J. McIlrath, “Laser Excitation and Ionization of Dense Atomic Vapors,” Applied Optics, Vol. 19, No. 23, 1980, pp. 3948-3956. doi:10.1364/AO.19.003948
[3]	A. N. Klucharev, N. N. Bezuglov, A. A. Matveev, A. A. Mihajlov, Lj. M. Ignjatovic and M. S. ,Dimitrijevic, “Rate Coefficients for the Chemi-Ionization Processes in Sodium- and other Alkali-Metal Geocosmical Plasmas,” New Astronomy Reviews ,Vol. 51, No. 7, 2007, pp. 547-562.
[4]	M. Cheret, A. Spielfiedel, R. Durand and R. Deloche, “Collisional Ionisation of Highly Excited Rubidium S State,” Journal of Physics B: Atomic and Molecular Physics, Vol. 14, No. 20, 1981, pp. 3953-3959. doi:10.1088/0022-3700/14/20/019
[5]	L. Barbier and M. Cheret, “Energy Pooling Process in Rubidium Vapour,” Journal of Physics B: Atomic and Molecular Physics, Vol. 16, No. 17, 1983, pp. 3213-3228. doi:10.1088/0022-3700/16/17/014
[6]	L. Barbier and M. Cheret, “Experimental Study of Penning and Hornbeck-Molnar Ionization of Rubidium Atoms Excited in a High s or d Level (5d ≤ nl ≤ 11s),” Journal of Physics B: Atomic and Molecular Physics, Vol. 20, No. 6, 1987, pp. 1229-1248. doi:10.1088/0022-3700/20/6/011
[7]	M. Cheret and L. Barbier, “Collisional Ionization of Excited Rubidium Atoms,” Journal De Physique, Vol. 46, No. 1, 1985, pp. 193-197.
[8]	R. M. Measures, P. L. Wizinowich and P. G. Cardinal, “Fast and Efficient Plasma Heating through Superelastic Laser Energy Conversion,” Journal of Applied Physics, Vol. 51, No. 7, 1980, pp. 3622-3628. doi:10.1063/1.328142
[9]	J. L. Bobin and M. A. Zaibi, “Inversions de Population lors de la Création, par Interaction Laser Résonnante, de Plasmas Complètement Ionisés,” Journal de Physique Lettres, Vol. 46, No. 16, 1985, pp. 737-744. doi:10.1051/jphyslet:019850046016073700.
[10]	T. Stacewicz and W. Latek, “Ionization of Sodium Vapour by Nanosecond Resonant Laser Pulses Tuned to 3s to 4p Transition,” Physica Scripta, Vol. 42, No. 6, 1990, pp. 658-660.
[11]	P. Bicchi, “Energy Pooling Reactions,” La Rivista del Nuovo Cimento, Vol. 20, No. 7, 1997, pp. 1-74. doi:10.1007/BF02879250
[12]	W. C. Stwalley and J. T. Bhans, “Atomic, Molecular, and Photonic Processes in Laser-Induced Plasmas in Alkali Metal Vapors,” Laser and Particle Beams, Vol. 11, No. 1, 1993, pp. 185-204.
[13]	E. W. McDaniel, “Atomic Collisions,” Wiley, New York, 1998.
[14]	T. J. Moseley, “Photodissociation and Photoionization Processes,” Wiley, New York, 1994.
[15]	S. A. Bakhramov, E. V. Vaganov, A. M. Kokhkharov and O. V. Parpiev, “Laser-Induced Resonant Multiphoton and Collisional of Rubidium Atoms,” Proceeding of SPIE, Vol. 4748, 2002, pp. 205-210. doi:10.1117/12.468948
[16]	M. A. Mahmoud, “Electron Energy Distribution Function in Laser-Excited Rubidium Atoms,” Journal of Physics B: Atomic, Molecular & Optical physics, Vol. 38, No. 10, 2005, pp. 1545-1556.
[17]	M. A. Mahmoud, “Kinetics of Rb2+ and Rb+ Formation in Laser-Excited Rubidium Vapor ,” Central European Journal of Physics,Vol. 6, No. 3, 2008, pp. 530-538. doi:10.2478/s11534-008-0074-5
[18]	H. W. Drawin, “Rapport Euratom CEA.92 Fontenay-aux-Roses (France) FC ,1967.
[19]	J. L. Vriens, “Energy Balance in Low-Pressure Gas Discharges,” Journal of Applied Physics, Vol. 44, No. 9, 1973, pp. 3980-3989.
[20]	N. N. Bezuglov, A. N. Klyucharev and V. A. Sheverev, “Associative Ionization Rate Constants Measured in Cell and Beam Experiments,” Journal of Physics B: Atomic and Molecular Physics, Vol. 20, No. 11, 1987, pp. 2497-2513. doi:10.1088/0022-3700/20/11/018
[21]	M. Aymar, E. Luc-Koenig and F. Combet-Farnoux, “Theoretical Investigation on Photoionization from Rydberg States of Lithium, Sodium and Potassium,” Journal of Physics B: Atomic and Molecular Physics, Vol. 9, No. 8, 1976, pp. 1279-1291.
[22]	M. Aymar, O. Robaux and S.Wane, “Central-Field Calculations of Photoionisation Cross Sections of Excited States of Rb and Sr+ and Analysis of Photoionisation Cross Sections of Excited Alkali Atoms Using Quantum Defect Theory,” Journal of Physics B: Atomic and Molecular Physics, Vol. 17, No. 6, 1984, pp. 993-1007.