The sharp edge of the phonon "bremsstrahlung" spectrum of a superconducting tunnel junction was isolated by modulation. This yielded a monochromatic, voltage-tunable phonon source which was applied for phonon spectroscopy on V 3+ ions in A1 2 0 3 . At a frequency corresponding to the ground-state splitting of V 3+ an absorption dip was obtained due to resonant scattering. achieved.A frequency resolution of about 3% of the energy gap WasIn 1967 Eisenmenger and Dayem 1 used a symmetric superconducting tunnel junction as a source of monochromatic phonons in the range above 100 GHz. The phonons originate on recombination of quasiparticles which are excited in the junction by single-particle tunneling. Though this is a promising device in a frequency range previously inaccessible, except by thermal phonons, only a few applications have so far been proposed. This is mainly because the recombination process produces phonons at a fixed frequency equal to 2A, the energy gap of the superconductor. Tuning the energy gap was attempted by Narayanamurti and Dynes using a magnetic field. 2 In the presence of a magnetic field the density of states is smeared out, however, so that a substantially broadened spectrum of emitted phonons was observed. In addition the detector sensitivity decreases rapidly on lowering the mean energy gap. Therefore this method does not appear to be readily suitable for phonon spectroscopy.In this Letter, we present a new method which relies on the phonon "bremsstrahlung" of the superconducting junction, rather than on the recombination radiation. A monochromatic source of phonons is obtained which can be tuned simply by adjusting the battery voltage. Using this source we demonstrate the feasibility of quantitative phonon spectroscopy with V 3+ impurities in sapphire.Following single-particle tunneling at a voltage exceeding 2A, the quasiparticles attain a continuous energy distribution up to a maximum energy E M = eV~. A. These "fast" quasiparticles will decay before recombination, being gradually slowed down by phonon emission. Thus a continuous phonon spectrum is generated with a maximum frequency given by io M -(eV-2A)//z. We call this the "phonon bremsstrahlung spectrum" (PBS) of the junction in analogy with the x-ray bremsstrahlung spectrum generated by fast electrons. The PBS exhibits a discontinuous edge at the maximum frequency oo M which is mathematically due to the confluence of two BCS squareroot singularities. This was demonstrated in a previous communication 3 which was based on theoretical work by Tewordt. 4 The simplified two-level model proposed by Dayem, Miller, and Wiegand 5 does not reproduce the step edge.In order to obtain a monochromatic phonon source we make use of this sharp edge. This is possible in view of the unique voltage dependence of the PBS: A small change in the voltage (i.e., a modulation) will shift the edge while leaving the rest of the spectrum essentially unaffected. This is shown quantitatively in Fig. 1. The upper traces show the spectral intensities of the p...