abstract submission

[Victor Kwong <vhs@physics.unlv.edu>]

Abstract submitted to the 2006 NASA Labastro Workshop

Victor Kwong

Metastable State Populations in Laser Induced Plasmas


        
      
       Victor H.S. Kwong, Chrysanthos Kyriakides, Wayn Ward

      Physics Department, UNLV






Laser ablation plasma has been used as a source of neutrals and ions. The purity of state 
of this source is critical to the measurement of  collisional parameters such as the charge 
transfer rate coefficients between ions and neutrals used in the modeling of astrophysical 
plasmas. However, there appears to be some uncertainty on the presence of metastable 
state population in this source. We address this issue in this paper by reviewing 
theoretical and experimental evidences to show that the temperature of the laser-induced 
plasma is a rapidly decreasing function of time and that the temperature of the plasma is 
initially high but cools off rapidly by collision with the expanding plasma electrons as the 
neutrals and ions streams into to the vacuum.  Similar to a supersonic jet, this rapid 
expansion of the plasma drastically lowers the internal energy of the neutrals and ions.  
Measurements on the time evolution of the population ratio of metastable state to ground 
state indicate that the population ratio freezes out at 3.5 ?s after the plasma is produced.  
The freeze-out population ratio suggests Te ? 1000 K. This measurement is consistent 
with the observations (1,2) that the charge transfer rate coefficient is independent of the 
power of the laser used in the production of the ions. We conclude that the metastable 
fraction in both the neutral and ion source must be negligibly small if the metastable state 
is ? 0.4 eV above the ground state.      


(1)	Z. Fang and Victor H.S. Kwong, Phys. Rev.A51 ,1321 (1995)
(2)	Jiebing Wang and Victor H.S. Kwong, Rev. Sci. Instru. 68, 3712 (1997)