Laser-induced breakdown inH2O andD2O

Recently Smith, Liu, and Bloembergen reported that the laser-induced breakdown of ${\mathrm{H}}_2$O required a significantly higher field than for ${\mathrm{D}}_2$O. In this comment it is suggested that two quite different mechanisms could account for the difference, (i) the viscosity-dependent solvation time for free electrons, and (ii) opticalphonon emission by hot electrons. The two mechanisms should be easily distinguishable by the temperature dependence of the breakdown field.