Narrowing of Spectral Lines beyond the Natural or Dephasing Line Width

In spectroscopy one is always faced with the problem of spectral resolution. One wishes to determine the frequency of the quantized transition with highest accuracy and one is interested in detecting fine structure or neighboring transitions. Disregarding experimental factors, the ultimate resolution is dictated by the inherent linewidth of the transitions. In optical spectroscopy we know various line-broadening processes. We point to the Doppler effect or to collision broadening in gases, to dephasing processes in condensed phases and to the population lifetime which gives rise to the natural l inewidth. 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 [1 ] . Even measurements beyond the natural line width 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 [9,10]. The lines were broadened by vibrational dephasing. It is the aim of this article to show new possibilities of line narrowing in coherent transient experiments. In our treatment we include dephasing processes. Without dephasing our results are applicable to line narrowing beyond the natural linewidth.