INDUCED INFRARED-SPECTRA OF H-2 ADSORBED ON ALKALI-HALIDE SURFACES - SEPARATION OF ORTHO-H-2 AND PARA-H-2 BY DESORPTION

Induced infrared absorption by H-2 (around 4100 cm(-1)) on NaCl(film)/ NaCl(100) has been monitored under ultrahigh vacuum conditions using h igh sensitivity Fourier transform infrared spectroscopy. The adsorbate s are studied at sample temperatures 10 less than or equal to T less t han or equal to 22 K and hydrogen pressures 10(-8) less than or equal to p less than or equal to 10(-4) mbar. In all spectra two different s ites have been observed according to adsorption on the (100) faces of the NaCl crystallites and on their defects. For each site a doublet sp litting of 8 and 9 cm(-1) appears, attributed to adsorption of ortho-H -2 and para-H-2, respectively. Additional absorptions at higher freque ncies are due to combinations between the internal and external vibrat ions. The frequencies of the external vibrations are 54 cm(-1) for ort ho- and para-H-2 at both defect and (100) terrace sites. Ortho- and pa ra-hydrogen are separated by desorption on the defined NaCl surfaces p romising a theoretical understanding of the process. In isobaric desor ption all IR absorptions attributed to para-H-2 disappear, while the a bsorptions of ortho-H-2, are still detected.