We have used Raman scattering to study roton pairs in liquid helium at 1.2°K. We find that the energy required to create two rotons is less than twice the energy of a single roton. This result can be explained by the existence of a two-roton bound state. Comparison of our spectra with theoretical calculations gives a binding energy of (0.37 ±0.10)°K and shows that the pair is in a D state (L=2) of angular momentum.We have been using Raman scattering as a probe of the elementary excitations in superfluid helium. 1,2 In the scattering process two elementary excitations of equal and opposite wave vector are created in the helium. 3,4 The energy difference between the incident and scattered photons is equal to the energy required to create the pair. Therefore, the spectrum of the scattered light is determined by the density of pair states of the excitations as a function of energy, and by the coupling between the pairs and the light. The most prominent feature in our initial spectra was a peak due to the creation of two rotons. Equally noticeable was the absence of a peak corresponding to the creation of two excitations near the maximum of the dispersion curve. Ruvalds and Zawadowski 5 and Iwamoto 6 have shown that this absence is due to a depletion of the density of pair states in this region caused by an interaction between the elementary excitations. They predict that the same interaction, if it is present between rotons, would be attractive and could result in a two-roton bound state. We report here the results of higher-resolution studies of the roton portion of the spectrum. We find that at 1.2°K the two-roton peak occurs at an energy shift which is less than twice the energy of a single roton at the same temperature. This result, which cannot be explained on the basis of noninteracting rotons, provides direct evidence for the existence of a two-roton bound state. Our measurement of the depolarization ratio of the scattered light, when compared with the theory of the coupling between the light and the excitations, indicates that the bound state which we observe is a D state (angular momentum L ~ 2). The exact position and shape of the peak in the Raman spectrum depends on the entire density of D states, unbound as well as bound, for the interacting roton pair. We here analyze our data in terms of a density of states based upon the simplest possible form for the roton-roton interaction which gives rise to a bound D state, and we arrive at a binding energy of (0.37±0.10)°K for the roton pair.The experimental arrangement is the same as that employed in our initial work except in place of the grating monochromator we now use a Fabry-Perot spectrometer whose free spectral range, 48.6°K, is about three times the shift of the two-roton Raman-scattered light. Figure 1 shows a typical experimental trace taken at a scattering angle of 90° with the electric field of the incident 4880-A laser light pointing toward the spectrometer. The sharp spikes on the trace are interferometrically generated marker pulses used to obtai...
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