Compression of spin-polarized hydrogen bubbles to thermal explosion

We have compressed spin-polarized atomic hydrogen gas in small, ≳${10}^{\mathrm{-}6}$-${\mathrm{cm}}^3$ bubbles to densities higher than ${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$ using a liquid${\mathrm{-}}^4$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)×${10}^{\mathrm{-}39}$ ${\mathrm{cm}}^6$/s for the gas phase and $K_{\mathrm{bbb}}^s$=8(2)×${10}^{\mathrm{-}25}$ ${\mathrm{cm}}^4$/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↓ 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.