  One key parameter in the high-order harmonic generation (HHG) phenomenon is
the exact frequency of the generated harmonic field. Its deviation from perfect
harmonics of the laser frequency can be explained considering (i) the
single-atom laser-matter interaction and (ii) the spectral changes of the
driving laser. In this work, we perform an experimental and theoretical study
of the causes that generate spectral changes in the HHG radiation. We measured
the driving laser spectral shift after high harmonic generation in long medium
using a correction factor to take into account the multiple possible HHG
initiation distances along the laser path. We separate out the contribution of
laser spectral shift from the resultant high harmonic spectral shift in order
to elucidate the microscopic effect of spectral shift in HHG. Therefore, in
some cases we are able to identify the dominant electron trajectory from the
experimental data. Our investigations lead to valuable conclusions about atomic
dipole phase contribution to a high harmonic spectral shift. We demonstrate
that the significant contribution of a long electron path leads to a high
harmonic shift, which differs from that expected from the driving laser.
Moreover, we assess the origin of the high-order harmonics spectral broadening
and provide an explanation for the narrowest high harmonic spectral width in
our experiment.

Atomic Physics