ISOTOPE EFFECTS FOR THE DIRECT DISSOCIATIVE CHEMISORPTION OF METHANE AND ETHANE ON IR(110) AND VIBRATIONALLY ASSISTED CHEMISORPTION

We have investigated the direct dissociative chemisorption of methane and ethane and their fully deuterated isotopes on the Ir(110) surface using supersonic molecular beams. There is a substantial kinetic isoto pe effect over a wide range of translational energies. The translation al energy threshold for an observable ( > 0.01) probability of chemiso rption varies between 6 and 11 kcal/mol for the four molecules. We hav e also investigated the effect of vibrational energy on dissociative c hemisorption by using vibrationally hot molecular beams. The results s how that there is substantial vibrational assistance for CD4 and C2D6 (approximately 5%), but no measurable effect for CH4 and C2H6 (< 2%). These results are most consistent with attributing the enhanced chemis orption probability to the asymmetric C-D and C-H stretching modes. Co nsistent with this interpretation, we observe a lowering of the transl ational energy threshold for the chemisorption of the vibrationally ho t beams by approximately 4 kcal/mol.