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By applying a sufficiently intense beam of off-resonant light, simultaneously with a conventional excitation source beam,
the efficiencies of one- and two-photon absorption processes may be significantly modified. The nonlinear mechanism
that is responsible, known as laser modified absorption, is fully described by a quantum electrodynamical analysis. The
origin of the process, which involves stimulated forward Rayleigh-scattering of the auxiliary beam, relates to higher
order terms which are secured by a time-dependent perturbation treatment. These terms, usually inconsequential when a
single beam of light is present, become prominent under the secondary optical stimulus – even with levels of intensity
that are moderate by today’s standards. Distinctive kinds of behaviour may be observed for chromophores fixed in a
static arrangement, or for solution- or gas-phase molecules whose response is tempered by a rotational average of
orientations. In each case the results exhibit an interplay of factors involving the beam polarisations and the molecular
electronic response. Special attention is given to interesting metastable states that are symmetry forbidden by one- or
two-photon absorption. Such states may be accessible, and thus become populated, on input of the auxiliary beam. For
example, in the one-photon absorption case, terms arise that are more usually associated with three-photon processes,
corresponding to very different selection rules. Other kinds of metastable state also arise in the two-photon process, and
measuring the effect of applying the stimulus beam to absorbances of such character adds a new dimension to the
information content of the associated spectroscopy. Finally, based on these novel forms of optical nonlinearity, there
may be new possibilities for quantum non-demolition measurements.
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