Quantum state resolved gas–surface reaction dynamics experiments: a tutorial review

Abstract

We present a tutorial review of our quantum state resolved experiments designed to study gas-surface reaction dynamics. The combination of a molecular beam, state specific reactant preparation by infrared laser pumping, and ultrahigh vacuum surface analysis techniques make it possible to study chemical reactivity at the gas-surface interface in unprecedented detail. We describe the experimental techniques used for state specific reactant preparation and for detection of surface bound reaction products developed in our laboratory. Using the example of the reaction of methane on Ni and Pt surfaces, we show how state resolved experiments uncovered clear evidence for vibrational mode specificity and bond selectivity, as well as steric effects in chemisorption reactions. The state resolved experimental data provides valuable benchmarks for comparison with theoretical models for gas-surface reactivity aiding in the development of a detailed microscopic understanding of chemical reactivity at the gas-surface interface. Key learning points(1) How to prepare beams of quantum state selected molecules through rapid adiabatic passage. (2) Physisorption vs. chemisorption. (3) What is state and mode specificity? (4) What is vibrational bond selectivity? (5) Orientation vs. alignment and alignment dependent reactivity.

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.…”

“…The molecular beam is produced by expanding a 1% CH 4 in H 2 mix through a nozzle with a 50 µm diameter hole, with backing pressures between 1.4 bar and 2 bar. The nozzle can be heated resistively to temperatures above 800 K, with temperatures between 298 K and 700 K used in the experiments presented here.…”

“…Utz et al have used K&W to investigate the dissociative chemisorption of methane on Ni(111) with quantum state resolution. 16 Here we report the first application of the K&W technique to the dissociative chemisorption of CH 4 on Pt(111), a system where vibrational excitation enhances the reactivity less strongly than for CH 4 on Ni(111). 15,47 The remainder of the paper is organized as follows.…”

“…Madix et al used this technique extensively for studying the physisorption of CH 4 27,28 and other small alkanes on Pt(111) [29][30][31][32] and Pd(111) surfaces, 33,34 as well as to obtain sticking coefficients for the dissociative chemisorption of a) Email address: rainer.beck@epfl.ch methane on Pt(111). 35 Other groups have also applied it to study the dissociative chemisorption of CH 4 on Pt(110)-(1 × 2), 36 Ru(0001), 37 and Ni(100) 38 as well as studying the chemisorption in other systems, see, for example, Refs. 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.…”

“…[1][2][3][4] Observations include mode specificity, [5][6][7][8][9] bond selectivity, [10][11][12] and steric effects 13,14 all of which demonstrate the non-statistical reaction dynamics of methane chemisorption. First principles theory combined with classical and quantum dynamics simulations is able to reproduce at least qualitatively many of the experimental observations.…”