Compression of spin-polarized hydrogen bubbles to thermal explosion.

Abstract

We have compressed spin-polarized atomic hydrogen gas in small, \ensuremath{\gtrsim}${10}^{\mathrm{\ensuremath{-}}6}$-${\mathrm{cm}}^{3}$ bubbles to densities higher than ${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ using a liquid${\mathrm{\ensuremath{-}}}^{4}$He piston at temperatures from 0.3 to 0.7 K and in magnetic fields from 4.5 to 7.5 T. A best fit to the compression sweeps is obtained by fixing the binding energy of the adsorbed H atom on the liquid $^{4}\mathrm{He}$ surface to 1.15 K, which then yields the third-order dipolar recombination rate constants ${K}_{\mathrm{bbb}}^{v}$=2.7(7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}39}$ ${\mathrm{cm}}^{6}$/s for the gas phase and ${K}_{\mathrm{bbb}}^{s}$=8(2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}25}$ ${\mathrm{cm}}^{4}$/s for the adsorbed surface layer. Because of inadequate transfer of the recombination heat to the surrounding He bath, the compression sweeps terminate in a spontaneous thermal explosion of the H\ensuremath{\downarrow} bubble. The pressure at the onset of the explosive recombination event is investigated as a function of ambient temperature, bubble volume, and magnetic field. Qualitative agreement with a simple thermally activated explosion model is found.