Ambient pressure effect on single-bubble sonoluminescence

We report the first results on the influence of ambient pressure on single bubble sonoluminescence. The equilibrium radius as well as the maximum radi us of the bubble increased as the ambient pressure over the liquid head was decreased. The number of photons emitted during a sonoluminescence pulse i ncreased linearly with a decrease of ambient pressure. These results are in good agreement with the theory of the ambient pressure effect [L. Kondic e t al., Phys. Rev. E 57, R32 (1998)] and with the recent dissociation hypoth esis.