Spectral Resolution Within the Natural Linewidth

Another interesting effect that is now accessible to measurements is the quantum electrodynamic energy shift of atomic levels when the atoms pass between two parallel metal plates (Casimir-Polder effect) [1302]. 

Assume that all other line-broadening effects except the natural linewidth have been eliminated by one of the methods discussed in the previous chapters. The question that arises is whether the natural linewidth represents an insurmountable natural limit to spectral resolution. At first, it might seem that Heisenberg s uncertainty relation does not allow outwit the natural linewidth (Vol. 1, Sect. 3.1). In order to demonstrate that this is not true, in this section we give some examples of techniques that do allow observation of structures within the natural linewidth. It is, however, not obvious that all of these methods may realty increase the amount of information about the molecular structure, since the inevitable loss in intensity may outweigh the gain in resolution. We discuss under what conditions spectroscopy within the natural linewidth may be a tool that really helps to improve the quality of spectral information. 

---

Level-crossing experiments with time-resolved detection following pulsed excitation may even allow a spectral resolution within the natural linewidth. If only those fluorescence photons are detected which have been emitted at times t > ax after the excitation process (a l) the spectral profile of the signal is narrowed [10.92]. This technique allows one to reach a spectral resolution beyond the natural linewidth (see Sect.13.5). 

With this technique the Doppler width could be reduced by two orders of magnitude below the natural linewidth, and spectral structures within the Doppler width could be resolved. Examples are the resolution of hyperfine structure components in an 12-beam using a single-mode argon laser (tunable within a few gigahertz) or the investigation of the upper state hfs-splitting in the atomic... 

As a host crystal a p-terphenyl crystal, with a few micrometers in diameter, has been used, which has been doped with a low concentration (10 ) of ter-rylene molecules. The tip of the optical fiber has been cooled to 1.4 K in order to avoid broadening of the fluorescence line via collective phenomena such as interaction with the phonons of the host crystal. The observed linewidth in fact is the natural linewidth (a few tens of MHz Fig. 9.9). The spectral response of fhese individual molecules can now be used for a local analysis of the surface, albeif with relatively low resolution (180 nm) (Fig. 9.10). Further experimental tricks such as electrical field induced Stark shifts can be applied to determine at least the position of the investigated molecules to within a few 10 nm. 

Assume that all other line-broadening effects except the natural linewidth have been eliminated by one of the methods discussed in the previous chapters. The question that arises is whether the natural linewidth represents an insurmountable natural limit to spectral resolution. In this section we give some examples of techniques which allow observation of structures within the... 

---