Luminescence from laser-created bubbles in cryogenic liquids

A luminescence pulse is observed from laser-created bubbles in pressurized liquid nitrogen and argon, occurring at the first collapse point of the bubble motion. The duration of the light pulse depends linearly on the bubble size, ranging from $200\phantom{\rule{0.3em}{0ex}}\mathrm{ns}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ for bubbles with maximum diameters between 0.2 and $1\phantom{\rule{0.3em}{0ex}}\mathrm{mm}$. The spectrum of the light consists entirely of atomic lines, which surprisingly turns out to contain strong emission lines from excited states of the neutral chromium atom. It is likely that this results from small microflakes of stainless steel knocked off the walls of the sample cell by the shock waves generated from the laser pulse. The small metallic flakes are preferentially ionized by the laser, resulting in bubbles containing chromium and other metal atoms in their interior. An analysis of the relative intensities of the triplet chromium lines shows that the compression in the bubble collapse is heating the chromium to temperatures of order $4500\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.