  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.
