Quasiclassical theory of Th229m excitation by laser pulses via electron bridges

Abstract

In the quasiclassical approximation it is studied excitation of the 8.2 eV isomer $^{229m}\mathrm{Th}$ in the electron bridges (EB), initiated by a laser pulse. While the nucleus and atomic electron are treated quantum mechanically, the laser pulse is described by a time-dependent wave packet, formed by classic electromagnetic waves. Two types of the EB are taken into consideration: via the continuous electron spectrum and via discrete atomic levels. In calculations of the excitation probability of $^{229m}\mathrm{Th}$ by a laser pulse I employ the generalized formalism of Floquet functions, which allows one to solve the time-dependent Schr\"odinger equation by the methods of stationary scattering theory. The task is solved for weak lasers as well as for the conventional lasers of arbitrary intensity. In the first case the results well correlate with those obtained previously when treating the laser pulse as a bunch of uncorrelated photons. If conventional lasers generate EB through an atomic level, its population suffers Rabi oscillations. Therefore application of the $\ensuremath{\pi}$ pulses may strongly enhance the effect of the isomer excitation. For the same EB the excitation probability of the isomer as a function of the laser carrier frequency is shown to have two peaks associated with two resonant levels.