Hole-burning in atomic and ionic spectral lines

Abstract

Applying the momentum method, the Fokker-Planck equation is derived from the Boltzmann equation for the r-4 potential between atoms and ions. Two earlier attempts are revealed to be limiting cases of our more general equation, namely for very large and very small mass ratio of the collision partners. The Fokker-Planck equation, after being completed with a decay term and a source term, is solved by the Green's function method. The solution turns out to be composed of modes in the velocity space which decay independently towards a stationary spectrum. The amplitudes of the latter depend on the source term which we assume to be shaped like a delta -function if laser pumping is accounted for. Only for very long relaxation times does the lowest mode prevail in the spectrum which then merges into the Maxwell distribution. Several new lines for ion-atom pairs as well as atom-atom pairs have been measured. They are presented along with the fits using the aforementioned formula. As expected, the agreement is satisfactory only in the first case. In the second case the Fokker-Planck approximation no longer applies.