Compression of spin-polarized hydrogen bubbles to thermal explosion

We have compressed spin-polarized atomic hydrogen gas in small, {}10^-6-cm³ bubbles to densities higher than 10¹⁸ cm^-3 using a liquid-⁴He 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 ⁴He surface to 1.15 K, which then yields the third-order dipolar recombination rate constants Kbbbᵛ=2.7(7)×{}10^-39 cm⁶/s for the gas phase and Kbbbˢ=8(2)×{}10^-25 cm⁴/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.