Direct probing of electronic density distribution of a Rydberg state by high-order harmonic generation in a few-cycle laser pulse

We demonstrate that the electronic density distribution of a Rydberg state can be probed directly using a broadband harmonic spectrum in a few-cycle laser pulse. Taking advantage of the large orbital radius of a Rydberg state, we show that the Rydberg electron can be accelerated directly toward the core under the influence of a few-cycle laser pulse, leading to recombination and emission of harmonic photons. In this case the initial position of the electron and the harmonic order form a one-to-one correspondence, resulting in a realization of direct probing of the density distribution of the initial Rydberg state.