A TECHNIQUE FOR GENERATING HIGH-FREQUENCY, SINGLE-ENERGY PHONONS has been devised, making it feasible to perform phonon spectroscopy, the study of how matter absorbs and radiates phonons. Phonons are discrete bundles of acoustical or thermal energy in solids. Researchers at Utrecht University in the Netherlands (contact H.W. de Wijn, H.W.deWijn@phys.uu.nl) produce a directional beam of monochromatic phonons in a molybdate (PbMoO4) crystal by shining a pair of interfering, narrow-frequency-band dye lasers on a deposited gold film. The interfering lasers periodically create a deformation or "strain" in the film. The periodic deformations produce a phonon beam whose frequency is equal to the difference in frequency between the two lasers. The beam then travels into the crystal, where studies of its properties can be performed. The phonon frequency can be tuned by changing the frequency of one of the lasers. The narrowness of the beam is limited by a classical effect known as Fraunhofer diffraction, in which a series of parallel rays that reach a small opening (the spaces in the crystal, in this case) spread out by a small amount (on the order of 0.01 radians, in the experiments). (E.P.N. Damen et al, Physical Review Letters, upcoming article.)