Direct measurement of fast ortho-para conversion of molecularly chemisorbed H2 on Pd(210)

Ortho-para conversion of molecularly chemisorbed ${\mathrm{H}}_{2}$ on a Pd(210) surface at a surface temperature of 50 K is reported. A combination of a pulsed molecular beam, photostimulated desorption, and resonance-enhanced multiphoton ionization techniques was used for probing the change in the rotational states of molecularly chemisorbed ${\mathrm{H}}_{2}$ on the surface. Our result shows that fast ortho-para conversion of chemisorbed ${\mathrm{H}}_{2}$ occurs. The conversion time was experimentally determined to be about 2 s, which is in good agreement with a previous theoretical calculation. This agreement supports that the ortho-para conversion mechanism of the molecularly chemisorbed ${\mathrm{H}}_{2}$ on Pd(210) is a two-step process based on the hyperfine-Coulomb excitation.