A lot of conditions should be satisfied. Latest investigations on this problem you can find in following articles.

1. D.W. Hahn, N. Omenetto, Laser-induced breakdown spectroscopy (LIBS), part I: review of basic diagnostics and plasma–particle interactions: still-challenging issues within the analytical plasma community, Appl. Spectrosc. 64 (2010) 335A–366A.

2. G. Cristoforetti, A. De Giacomo, M. Dell'Aglio, S. Legnaioli, E. Tognoni, V. Palleschi, N. Omenetto, Local thermodynamic equilibrium in laser-induced breakdown spectroscopy: beyond the McWhirter criterion, Spectrochim. Acta Part B 65 (2010) 86–95.

thanks alot Mr. llyin,

great help!

There is good paper I'd read carefully for this issue.

H. Griem, Phys. Rev. 131, 1170 (1963).

Most basically McWhirter's Chriterion must be satisfied, that says

ne ≥ 1.6 *10^12 T^(1/2) ∆E^3

The differences in the temperature of different species can be caused by non-LTE conditions, but can have also different reasons.

One of the most probable is a non-local effect, in the sense that the spectrum of different species comes from different regions at different temperatures.

It strongly depends on the way you register the spectra. If you spectra is time integrated, the different temperature can depend on the the different time of the plume evolution.

See for example these papers

http://dx.doi.org/10.1364/AO.42.005963

DOI: 10.1016/S0169-4332(02)01340-5

DOI: 10.1016/j.sab.2010.03.012

doi:10.1088/0963-0252/18/2/025027

For your question, Please read these two papers:

DOI: 10.1016/j.sab.2010.04.015

DOI: 10.1007/s00216-011-4920-8

Plasma is called in LTE condition when it satisfies

McWhirter's criterion that says

ne ≥ 1.6 *10^12 T^(1/2) ∆E^3

I think LTE happen when the electron temperature less than 1ev

Te < @1 ev