State selective vibrational (2ν3) activation of the chemisorption of methane on Pt (111)

The state-selective dissociative sticking coefficient on Pt(111) surfaces for CH4 in the 2ν3 J=1,2 ro-vibrational levels has been measured using thermal energy atomic scattering. Continuous wave laser excitation of a molecular beam of CH4 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 (S0) of CH4 with 5.4 kJ/mol normal translational energy is found to increase from 6.2×10−6 to 1.8×10−4 upon 2ν3 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 S0 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 CH4 is approximately equivalent to at least 30 kJ/mol of normal translational energy. This corresponds to a utilization efficiency of the 2ν3 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 ν3 of CH4 on Ni(100) surfaces, a comparable value for the vibrational energy utilization efficiency was found (59%). Further work is necessary to determine if this result is general and if and how it may change by changing the vibrational mode excited by the laser.