Dominik Hammer

Sonoluminescence: How bubbles glow

Abstract We review recent attempts to elucidate the phenomenon of sonoluminescence in terms of fundamental principles. We focus mainly on the processes which generate the light, but other relevant facts, such as the bubble dynamics, must also be considered for the understanding of the physics involved. Our emphasis is on single bubble sonoluminescence which in recent years has received much attention, but we also look at some of the excellent work on multiple bubble sonoluminescence and its spectral characteristics for clues. The weakly ionized gas models were recently studied most thoroughly and are remarkably successful when combined with a hydrodynamic bubble model, in terms of reproducing observed spectral shapes, intensities, optical pulse widths and the dependencies of these observables on the experimental parameters. Other radiation models, such as proton tunnelling radiation and the confined electron model, were not combined with hydrodynamic models and/or have freely adjustable parameters so that their relevance to sonoluminescence studies is at present less critically tested.

Emission by collisional H2–He pairs at temperatures from 2 to 20 kK

The dipole surface of H2–He collisional pairs is computed from first principles for 20 intramolecular spacings of the H2 molecule, from 0.6 to 4 Bohr; 11 separations of the H2–He pair, from 2 to 6 Bohr; and five values for the angle subtended by the intramolecular and intermolecular axes. From this dipole surface, the dipole matrix elements are obtained for all possible rotovibrational transitions |vJ〉→|v′J′〉, with v and v′=0,… ,14. Subsequently, the collision-induced emission spectra are computed for frequencies from 100 to 100 000 cm−1, at temperatures of thousands of Kelvin—a range believed to be important for various light sources of high gas densities, such as the atmospheres of “cool” stars, shockwaves, flames, rocket jets, etc. We find that at the lower temperatures considered, radiation is emitted mostly in the fundamental band of H2, while at high temperatures the collision-induced emission spectra extend into the visible, with overtone transitions involving large Δv=v−v′.

Light emitting processes of sonoluminescence

We consider elementary radiative processes that play a role under conditions such as encountered in sonoluminescence studies. These are electron‐atom and electron‐ion bremsstrahlung; electron‐atom polarization bremsstrahlung; electron‐ion recombination radiation; radiative attachment of electrons to atoms; ion‐atom bremsstrahlung and charge exchange collisions; and ion‐atom association radiation. We include these processes in a hydrodynamic‐chemical model of the expanding and collapsing bubble and compute spectral profiles, spectral intensities, and duration of the light pulses for the rare gas bubbles. We generally obtain good agreement with observations for photon numbers, pulse durations, and in most cases for the spectral shapes. Some calculated spectral profiles agree, however, less well with the experiment, especially if water temperatures near freezing are involved, and in the case of helium bubbles. We show that by accounting for a somewhat non‐uniform heating across the bubble diameter, agreement...

The spectra of sonoluminiscent rare gas bubbles

We calculate sonoluminescence emission spectra of the heavy rare gases by combining the hydrodynamic model of Hilgenfeldt et al. (Phys. Fluids 11, 1318 (1999)) with quantum line shape calculations of electron-neutral atom bremsstrahlung radiation (L. Frommhold, Phys. Rev. E 58, 1899 (1998)). For the heavier rare gases, good agreement between theoretical and experimental spectra with respect to shape and absolute intensity is obtained by choosing values for the experimental parameters as input that are compatible with those used in the experiment.