The Negligible Role of C–H Stretch Excitation in the Physisorption of CH₄ on Pt(111)

Abstract

We report a molecular beam study of the effect of vibrational excitation on the physisorption of methane on a Pt(111) surface. Our experiments use a continuous molecular beam of CH 4 , prepared in its antisymmetic C−H stretch mode ν 3 by infrared laser pumping via rapid adiabatic passage. Physisorbed CH 4 (ads) is detected on a Pt(111) surface by reflection absorption infrared spectroscopy. At a surface temperature of 77 K, the desorption lifetime of CH 4 (ads) is observed to be 0.4 ± 0.2 s. Trapping probabilities for the incident CH 4 are measured by the King and Wells beam reflectivity method with and without vibrational excitation. Vibrational excitation of the incident CH 4 with one quantum of ν 3 vibration does not produce a measurable effect (less than 0.2% change) on the trapping probability, in sharp contrast to the dissociative chemisorption process. The effect of resonant vibrational excitation of physisorbed CH 4 (ads) on its dissociation rate was also investigated. ■ INTRODUCTIONThe trapping of a gas-phase molecule into a weakly bound physisorbed state on a solid surface plays a central role in condensation processes as well as the initial step of chemisorption reactions via a precursor-mediated mechanism. It is well-established that the trapping probability decreases with increasing incident translational energy because any excess energy beyond the physisorption well depth needs to be transferred to surface degrees of freedom during the surface impact for trapping to occur. In the simplest case, the initial trapping probability depends only on the incident translational energy normal to the surface (normal energy scaling with E t · cos 2 θ) as described by the Baule model. 1 Madix et al. 2,3 have measured trapping probabilities for Ar and CH 4 on Pt(111) and concluded that in these two systems normal energy scaling is obeyed.The role of vibrational excitation of the incident molecule on the physisorption (trapping) probability is not yet wellunderstood, and experimental results reported to date in the literature differ for different molecules and surfaces. Sibener et al.4 studied the condensation coefficients of molecular beams of CCl 4 and SF 6 on their respective condensed phases at cryogenic temperatures. They observed a (small) decrease in the trapping probability with increasing rotational and vibrational excitation at low incident translational energy consistent with simple models of the trapping process. 5 On the other hand, Wodtke et al.6 studied the trapping of the diatomic NO in v = 0 and 2 on a Au(111) surface at 300 and 480 K. Using quantum state resolved detection of surface scattered NO by resonant multiphoton ionization they concluded that there was no measurable difference between the trapping probabilities for NO(v = 0) and NO(v = 2) for incident kinetic energies in the range of 0.1−0.75 eV. The fact that trapping was found to be insensitive to NO vibrational states was interpreted as being due to weak mechanical coupling between the high-frequency NO stretch and the low-f...

Methane dissociation on Pt(111): Searching for a specific reaction parameter density functional

“…The experiments we compare to here that were done for low collision energies used a low surface temperature (120 K) [40] so that the RAIRS detection technique [104] could be used [24,33,73,105]. However, reaction probabilities can also be measured at higher surface temperatures based on either Auger spectroscopy [13,14,19,38,106] Table 3: Transition state energies calculated with various density functionals for methane reacting on Pt(111).…”

“…In very few trajectories of all the AIMD trajectories computed using vdW-corrected density functionals (6 out of more than 15000), the molecule remains trapped in the physisorption well after having collided with the surface. After a maximum propagation time of 1 ps these molecules are considered scattered, on the basis of the negligible reactivity of physisorbed molecules on Pt(111) observed experimentally [73]. Error bars on the sticking probability values have been estimated using the Wilson method [74] and represent 95% confidence interval unless otherwise stated.…”

“…We note in passing that accurately modeling the vdW well for methane interacting with metal surfaces should be relevant for describing two recent sets of experimental data [73,88]. In a first study, Beck and coworkers investigated the physisorption of CH 4 and the dissociation of CH 4 physisorbed on Pt(111), without finding significant differences between laser-off and vibrationally excited beams [73].…”

“…In a first study, Beck and coworkers investigated the physisorption of CH 4 and the dissociation of CH 4 physisorbed on Pt(111), without finding significant differences between laser-off and vibrationally excited beams [73]. In a second study, Utz and coworkers, suggested a precursor-mediated dissociation channel for CH 4 impinging at very low collision energies on Ir(111) [88].…”

Quantum state resolved molecular beam reflectivity measurements: CH4 dissociation on Pt(111)

“…39-45. More recently, the K&W technique has been used for quantum state resolved sticking coefficient measurements to study the sticking of molecules prepared in an initial rovibrational quantum state. No difference between laser-on and laser-off sticking was detected for the physisorption of CH 4 on Pt(111) 28 or for D 2 O on ice 46 which demonstrated that the trapping probability into a physisorbed state is insensitive to vibrational excitation of the incident molecule. Utz et al have used K&W to investigate the dissociative chemisorption of methane on Ni(111) with quantum state resolution.…”