  We experimentally and numerically study the atomic response and pulse
propagation effects of high-order harmonics generated in Xe, Kr, and Ar driven
by a 2.1-\mu m infrared femtosecond light source. The light source is an
optical parametric chirped-pulse amplifier, and a modified strong-field
approximation and 3-dimensional pulse propagation code are used for the
numerical simulations. The extended cutoff in the long-wavelength driven
high-harmonic generation has revealed the spectral shaping of high-order
harmonics due to the atomic structure (or photo-recombination cross-section)
and the macroscopic effects, which are the main factors of determining the
conversion efficiency besides the driving wavelength. Using precise numerical
simulations to determine the macroscopic electron wavepacket, we are able to
extract the photo-recombination cross-sections from experimental high-order
harmonic spectra in the presence of macroscopic effects. We have experimentally
observed that the macroscopic effects shift the observed Cooper minimum of Kr
from 80 eV to 60-70 eV and wash out the Cooper minimum of Ar. Measured
high-harmonic conversion efficiencies per harmonic near the cutoff are ~10^{-9}
for all three gases.
