  We report a study of high harmonic generation from noble gas clusters of
xenon atoms in a gas jet. Harmonic spectra were investigated as a function of
backing pressure, showing spectral shifts due to the nanoplasma electrons in
the clusters. At certain value of laser intensity this process may oppose the
effect of the well-known ionization-induced blueshift. In addition, these
cluster-induced harmonic redshifts may give the possibility to estimate cluster
density and cluster size in the laser-gas jet interaction range.
