VOLUME 86, NUMBER 21 PHYSICAL REVIEW LETTERS 21 MAY 2001 Spectrum of Luminescence from Laser-Created Bubbles in Water Ohan Baghdassarian, Han-Ching Chu, Bernd Tabbert, and Gary A. Williams Department of Physics and Astronomy, University of California, Los Angeles, California 90095 (Received 4 February 2001) The spectrum of the luminescence emitted at the collapse of single laser-induced bubbles in water is measured for different maximum bubble radii. Bubbles as large as 2 mm show a molecular OH  band at 310 nm in the spectrum, which otherwise can be fitted approximately with a blackbody curve at a temperature of 7800 K. This finding provides a connection between the light emission of single bubbles and multibubble sonoluminescence, since in the latter case the same molecular band is observed. Surface instabilities are observed in the larger bubbles, and may be connected with the OH  emission. DOI: 10.1103/PhysRevLett.86.4934 PACS numbers: 78.60.Mq, 47.20.Ma, 47.40.Nm, 47.55.Dz The energy focusing of a collapsing bubble in water can lead to the emission of photons of energy 6 eV and higher, the phenomenon observed in single-bubble sonolumines- cence (SBSL) [1,2]. The exact mechanism of the lumines- cence from single bubbles trapped in a sound field is still debated [3,4], in part because of the challenges presented in probing the light-emitting region, which is thought to be less than a few micrometers in diameter. It is of inter- est to scale up the bubble size in order to compress more mass and study consequently larger hot spots. There is also the puzzle of understanding the relationship of SBSL to multibubble sonoluminescence (MBSL) [5,6], which is light emission from the cloud of bubbles in a fluid cavi- tated continuously with an intense sound field. The spec- trum of MBSL [6] shows an emission band from the OH  molecular complex which starts at 306.4 nm and peaks near 310 nm, while SBSL shows only a smooth contin- uum spectrum that increases into the ultraviolet without any obvious lines [7,8]. We report measurements of the spectrum of the light emission from single collapsing laser-induced bubbles in water that have different maximum radii. Figure 1 shows the OH  spectral band emerging from a continuous spec- trum with increasing bubble size, the same band previously found in the optical spectra of MBSL. In the remainder of the paper we compare the features of these spectra of laser-created bubbles with the spectra of MBSL and SBSL, and discuss the possibility that instabilities in the bubble shape play a role in the occurrence of spectral lines for large bubbles. The bubbles are created from a focused laser pulse that initially vaporizes the water, giving rise to a bubble that first expands, reaches a maximum radius, and then collapses in a time of order 100 400 ms that is directly proportional to the maximum radius [9,10]. The exact composition of the gas being compressed in the bubble collapse is not known, but probably consists primarily of atomic and molecular hydrogen and oxygen, and any water vapor that is unable to condense onto the bubble wall. The luminescence pulse is emitted precisely at the minimum-radius collapse point, and the width of the pulse is several nanoseconds, increasing as the maximum bubble size increases [10]. The range of bubble radii that can be created by this method (0.2–2 mm) is much larger than the bubbles of SBSL, which have a maximum radius about 50 mm. A Nd:YAG laser producing 6 ns pulses with a maximum energy of 600 mJ at 1064 nm is collimated and focused to a point about 10 mm in diameter to cavitate the high purity water sample, as previously described in Ref. [10]. The laser energy is adjusted to the threshold for creating just a single bubble, estimated to be of order 100 mJ. The water is inside a sealed cell having quartz windows for monitoring the emitted light with a photomultiplier, and the radius of the bubble versus time is monitored by a shadowgraph technique and by pulsed-laser photography through a long-distance microscope. To obtain a spectrum, light emitted from the bubble region is also collected and collimated by a MgF 2 -coated paraboloidal mirror. A sec- ond paraboloid mirror focuses the collimated beam into the entrance slits of a 0.3 m spectrometer with a gateable FIG. 1. Spectra of the collapse luminescence as the bubble size is increased, showing the growth of the OH  band at 310 nm. These are averaged over 40–50 bubbles, having maximum radii in the range indicated, and have been offset in the vertical direc- tion to separate them (they are normalized to all coincide with the lowest curve between 500–700 nm). 4934 0031-9007 01  86(21)  4934(4)$15.00 © 2001 The American Physical Society