  We present a theoretical analysis of two-photon absorption of classical and
squeezed light valid when one-photon absorption to an intermediate state is
either resonant or far-detuned from resonance, and in both the low and high
intensity regimes. In this paper we concentrate on continuous-wave excitation,
although the approach we develop is more general. We calculate the energy
removed from an incident field for typical experimental parameters and consider
the limiting cases when the photon pairs are narrowband or broadband compared
to the molecular linewidths. We find an enhancement of the two-photon
absorption due to resonant contributions from the large squeezed light
bandwidth and due to photon bunching in the low intensity regime. However, in
both cases, for the parameters we choose, the one-photon absorption is the
dominant process in the region of parameter space where a large enhancement of
the two-photon absorption is possible.
