  We report consistent ignition of high-pressure (p > 20-30 bar)
hydrogen-oxygen mixtures diluted with helium, using an unfocused Nd:YAG laser.
This corresponds to laser irradiances several orders of magnitude below the
minimum ignition energies reported in the literature. By placing a mirror
inside a cylindrical vessel and filling it up to 100 bar with H2-He or O2-He
non-combustible mixtures, we obtain the pressure-dependent absorptivity of the
combustible He-H2-O2 mixture. We find no measurable absorption of the laser
signal by the medium, for the overall pressure range, to the experimental
apparatus sensitivity (about 1% of the laser irradiance). This leads credence
to the theory that ignition stems from seed electrons created by autofocusing
ionization of dust/impurities in the gas.
