High-order harmonic generation in atom clusters interacting with an intense
linearly polarized laser field is studied by means of numerical simulation
s of the time-dependent Schrodinger equation for a two-dimensional model cl
uster in the single-active electron approximation. The well-known first and
second cutoffs (in the atomic and molecular cases) are not clear in cluste
r calculations. In fact, the structure of the spectrum consists of a long p
lateau that extends far beyond the I-p + 3.17 U-p limit and a collection of
high-order harmonics with decreasing but significant intensities up to ext
remely high frequencies. (C) 2001 Optical Society of America.