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Nucleus excitation by transient bremsstahlung emission of femtosecond laser plasma

The investigation of nuclear processes in femtosecond laser plasma is under intensive study in the last time [1,2]. The first successful observation of excitation and decay of 6.238 keV Ta-181 isomeric level [3,4] has stimulated interest to this specific nuclear transition [5,6]. The theory of nucleus excitation by blackbody x-ray emission of stationary laser plasma has been discussed earlier [3,4,7]. However the experimentally registered efficiency of excitation exceeds the theoretical calculations. There are a number of possible reasons that explain this difference. The first possible reason can be associated with the different mechanisms of nuclear transition broadening. Indeed, the expansion of laser plasma results in the appearance of the quasi-static electric and magnetic fields. The simple estimations show that the inhomogeneous line-width due to the quasi-static fields in laser plasma can exceed the Doppler broadening. However, the blackbody emission is s-correlated and integration over the inhomogeneously broaden spectrum of nuclear transition results in the efficiency of excitation which is quite closer to the efficiency of excitation for homogeneously broaden transition. Here we study the influence of the non-stationary manner of bremsstrahlung by hot electron component of laser plasma on the efficiency of radiative channel of nucleus excitation in laser plasma. The main idea of proposed explanation is in the following. The process of inelastic collision of electron and ion in laser plasma has a finite duration. It is natural to assume that it is this time that determines the spectral width of x-ray bremsstrahlung emission. For the hot electron component the characteristic temporal interval of inelastic collision is less than 0.1 fs. Thus, the coherent bandwidth is comparable with the frequency of bremsstrahlung peak intensity. As a result the account of the finiteness of the bremsstrahlung correlation time can significantly affect on the calculated efficiency of excitation.