Nuclear Isomer Excitation in 229Th Atom by Super-Intense Laser Field

Excitation of the isomeric nuclear state 229m Th in the process of thorium atom irradiation by two-color femtosecond pulse of Ti: Sa laser at the fundamental wavelength and second harmonic is analyzed. It is shown that the rate of isomeric state excitation can be enhanced significantly with respect to other nucleus excitation processes in laser plasma or by an external coherent source at the resonance wavelength. This enhancement is due to the process of nonlinear laser nuclear excitation based on the following. First, the atomic current associated with the motion of valence electrons in strong field is a nonlinear function of the field amplitude. Secondly, the energy of photons at frequency of the Ti:Sa laser fifth harmonic coincides with the energy of the 229m Th state. Yield of this harmonic is much higher if the two-color (1st and 2nd harmonics of the fundamental) is used. Thirdly, the intensity of the field produced by valence electrons at atomic nucleus reaches the near-atomic value, which exceeds significantly the field strength associated with any other mechanism of laser plasma emission.