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where X" 1 is the mean lifetime of the excited nuclear state (the lower state is assumed to be stable). The quantity a=u> 0 x 0 /c is called the modulation index. The origin of time has been chosen as a zero of the sine, and the decaying state was formed at t =t 0 . Irrelevant normalization and phase factors have been omitted. In what follows, t 0 is never measured and must be averaged over. If one forms the average, (I^KMo)! 2 )^' the vari " able t disappears, so that, as expected, there is no time dependence of the intensity. The spectrum corresponding to Eq. (1) is obtained by squaring its Fourier transform and averaging over 1 0 . It is the familiar sum of Lorentzianshaped carrier and sidebands, first shown with Mossbauer radiation by Ruby and Bolef l and observed and discussed by others since. 2 " 6where the J n are Bessel functions of the first kind.If we now interpose a resonant absorber between the vibrating source and the y-ray detector, so that there are alterations in phase or ampli-Recoil-free y rays from the decay of 57 Co are frequency modulated by vibrating the source with a piezoelectric crystal and one of the lines of the resulting multiplet emission spectrum is absorbed. The remaining radiation displays a time-dependent counting rate whose harmonic composition and relative phases are sensitive to small energy shifts and can be used for their measurement.When ay-ray emitter is vibrated sinusoidally with amplitude x 0 along the direction of observation, and with angular frequency £2, the time dependence of the radiation field can be expressed as S exp[-X(£ -t 0 )/2 +i(u 0 t +a sinQt)], t^ t 0 , (1) 0, t

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A 100-MeV 32S beam was used to pump a He-Ar gas laser. Laser action of the 1.79-μm line was observed above a threshold of 33-W beam power input. The light conversion efficiency is about 1×10−4 with a (99%)He-(1%)Ar gas mixture at 200-mbar pressure and 132-W input power.