| Autor: |
Higgins, J., Conjusteau, A., Scoles, G., Bernasek, S. L. |
| Předmět: |
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| Zdroj: |
Journal of Chemical Physics; 3/22/2001, Vol. 114 Issue 12, p5277, 7p, 2 Diagrams, 3 Graphs |
| Abstrakt: |
The state-selective dissociative sticking coefficient on Pt(111) surfaces for CH[sub 4] in the 2ν[sub 3] J=1,2 ro-vibrational levels has been measured using thermal energy atomic scattering. Continuous wave laser excitation of a molecular beam of CH[sub 4] seeded in He with a 1.5 μm color center laser, tunable around 6000 cm-1 and coupled to the beam by means of a resonant build-up cavity, allows pumping of up to 11% of the molecules to the excited ro-vibrational state. The laser/molecular beam combination allows precise control over the translational energy as well as the vibrational state of the methane that impinges on the clean Pt(111) surface. The intensity of the specular reflection of the incident helium beam is used to quantitatively monitor the coverage of chemisorbed methane on the platinum surface as a function of time (exposure). The sticking coefficient (S[sub 0]) of CH[sub 4] with 5.4 kJ/mol normal translational energy is found to increase from 6.2x10[sup -6] to 1.8x10[sup -4] upon 2ν[sub 3] excitation (the overtone of the asymmetric stretch of methane). This represents a ∼30 fold enhancement in reactivity of the methane with the Pt(111) surface upon vibrational excitation. We also measured the changes of S[sub 0] obtained by varying the nozzle temperature and methane concentration over a tenfold range of energy. We find that 72 kJ/mol of vibrational energy in the excited CH[sub 4] is approximately equivalent to at least 30 kJ/mol of normal translational energy. This corresponds to a utilization efficiency of the 2ν[sub 3] vibrational energy of greater than 40%. In the only other measurement of this kind published in the literature, [L. B. F. Juurlink, P. R. McCabe, R. R. Smith, C. L. DiCologero, and A. L. Utz, Phys. Rev Lett. 83, 868 (1999)] for the fundamental (v=1) excitation of ν[sub 3] of CH[sub 4] on Ni(100) surfaces, a comparable value for the vibrational energy utilization efficiency was found (59%). Further work is... [ABSTRACT FROM AUTHOR] |
| Databáze: |
Complementary Index |
| Externí odkaz: |
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