  We study the effect of gas pressure on the generation of high-order harmonics
where harmonics due to individual atoms are calculated using the recently
developed quantitative rescattering theory, and the propagation of the laser
and harmonics in the medium is calculated by solving the Maxwell's wave
equation. We illustrate that the simulated spectra are very sensitive to the
laser focusing conditions at high laser intensity and high pressure since the
fundamental laser field is severely reshaped during the propagation. By
comparing the simulated results with several experiments we show that the
pressure dependence can be qualitatively explained. The lack of quantitative
agreement is tentatively attributed to the failure of the complete knowledge of
the experimental conditions.
