Generation of High-Order Harmonics by Gallium Ions in Intense Laser Pulse

The spectrum of high-order harmonics generated in interaction of an intense femtosecond laser pulse with a singly charged gallium ion is theoretically investigated. The results of the calculations reveal enhancement of odd harmonics that are in resonance with the frequency of the transition from the ground state to the odd autoionizing state 3d 9 4s 2 4p 1 P 1 and harmonics that correspond to the multiphoton resonance with the even autoionizing states 3d 10 4p5p 1 S 0 and 1 D 2 (dark states) and are one laser frequency away from them. The effect of resonant enhancement of harmonics was earlier observed in experiments on gallium targets with a plasma plume.