High-order harmonic generation from intense laser-driven inner electrons of Rydberg atoms

By solving the time-dependent Schr\"odinger equation of a two-electron system, we demonstrate that efficient high-order harmonics can be emitted by inner electrons of Rydberg atoms exposed to an intense laser pulse, while the Rydberg electron turns out to be stabilized. Due to stabilization of the Rydberg electron, more intense laser fields may essentially interact with inner electrons, which have high ionization potentials, thereby efficiently generating high-order harmonics. The process has the advantage of producing the very high harmonics, seen in intense laser-ion interactions, while maintaining the higher target densities of a neutral Rydberg (n~10) gas.