Role of Excited States In High-order Harmonic Generation
  We investigate the role of excited states in High-order Harmonic Generation by studying the spectral, spatial and temporal characteristics of the radiation produced near the ionization threshold of argon by few-cycle laser pulses. We show that the population of excited states can lead either to direct XUV emission through Free Induction Decay or to the generation of high-order harmonics through ionization from these states and recombination to the ground state. By using the attosecond lighthouse technique, we demonstrate that the high-harmonic emission from excited states is temporally delayed by a few femtoseconds compared to the usual harmonics, leading to a strong nonadiabatic spectral redshift. 
Samuel Beaulieu, Seth Camp, Dominique Descamps, Antoine Comby, Vincent   Wanie, St\'ephane Petit, Fran\c{c}ois L\'egar\'e, K...
physics.atom-ph