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 isotope effect over a wide range of translational energies. The translational energy threshold for an observable ( > 0.01) probability of chemisorption varies between 6 and 11 kcal/mol for the four molecules. We have also investigated the effect of vibrational energy on dissociative chemisorption by using vibrationally hot molecular beams. The results show 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 chemisorption probability to the asymmetric C-D and C-H stretching modes. Consistent with this interpretation, we observe a lowering of the translational energy threshold for the chemisorption of the vibrationally hot beams by approximately 4 kcal/mol.