Raman spectroscopy with ultrashort coherent excitation. Narrowing of spectral lines beyond the dephasing linewidth

Spectroscopists are constantly faced with the task of improved
spectral resolution. Two points are of major interest: (i) The
precise frequency of the quantized transition and (ii) the
detection of new neighboring transitions. Besides experimental
factors the ultimate spectral resolution is determined by the
inherent linewidth of the transition. Optical spectroscopists have
to deal with different line-broadening processes; for instance with
the Doppler effect or with collision broadening in gases, with
dephasing processes in condensed systems and with the population
relaxation which results in the natural linewidth. In recent years,
different novel techniques have been devised which provide spectral
resolution beyond the transition linewidth. For instance, Doppler
broadening can be eliminated by saturation spectroscopy or by two
counter-propagating beams for two-photon transitions/I/. Even
measurements beyond the natural linewidth have been performed
taking biased signals from the fluorescent decay /2-5/. Techniques
have been proposed where the difference between the decay rates of
the two states rather than their sum determines the linewidth
/6,7/, and narrowing of the natural linewidth by decaying-pulse
excitation has been discussed /8/. Very recently, we have
demonstrated substantial line narrowing of Raman type transitions
in condensed phases /9-11/. The lines were broadened by vibrational
dephasing. New information was obtained in congestedspectral
regions.