Ultralow-energy excitations and prospects for spatially resolved spectroscopy.

The key contribution of electron microscopy methods to condensed matter spectroscopy is undoubtedly spatial resolution. So far this has mainly been manifest through electron energy loss spectroscopy in the 1-eV to 10-keV energy range and has not seriously challenged the dominance of optical, X-ray, and neutron spectroscopy methods over most of the vast field at lower energies. At frequencies up to a few megahertz, corresponding to energies of a few nanoelectron volts and below, direct excitation by pulsed electron beams or electric fields has proved effective. Prospects are discussed for extending spatially resolved spectroscopy to the intermediate energy region, mainly by combining the advantages of electrons with those of photons.