Quantum state specific reactant preparation in a molecular beam by rapid adiabatic passage

2016

Chem. Soc. Rev.

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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.

“…Moreover, using stepwise excitation via RAP, complete population transfer from an initial level 1 to a final level 3 can be achieved as described in ref. 10.…”

Section: Reactant State Preparation By Infrared Pumpingmentioning

confidence: 99%

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

Chadwick

2016

Chem. Soc. Rev.

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“…Details of the RAP excitation have been described earlier. 16 A pyroelectric detector could be moved into the molecular beam past the excitation region to detect and quantify the flux of vibrationally excited molecules in order to verify that complete population inversion was achieved in both excitation steps.…”

Section: Methodsmentioning

confidence: 99%

“…The molecular beam/surface science apparatus, the state specific reactant preparation by rapid adiabatic passage (RAP), and the product detection by AES have been described in previous publications. 16,17 Briefly, a continuous molecular beam of methane was generated by expanding pure CH 4 or 3% CH 4 seeded in helium with a backing pressure of 2 bar through a temperature controlled nozzle of 30 µm diameter. A 1 mm diameter nickel skimmer was used to extract a molecular beam from the supersonic expansion.…”

Section: Methodsmentioning

confidence: 99%

Quantum-state-resolved reactivity of overtone excited CH4 on Ni(111): Comparing experiment and theory

Hundt

Reijzen

The Journal of Chemical Physics

et al. 2017

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Quantum state resolved reactivity measurements probe the role of vibrational symmetry on the vibrational activation of the dissociative chemisorption of CH 4 on Ni(111). IR-IR double resonance excitation in a molecular beam was used to prepare CH 4 in three different vibrational symmetry components, A 1 , E, and F 2 , of the 2ν 3 antisymmetric stretch overtone vibration as well as in the ν 1 + ν 3 symmetric plus antisymmetric C-H stretch combination band of F 2 symmetry. The quantum state specific dissociation probability S 0 (sticking coefficient) was measured for each of the four vibrational states by detecting chemisorbed carbon on Ni(111) as the product of CH 4 dissociation by Auger electron spectroscopy. We observe strong mode specificity, where S 0 for the most reactive state ν 1 + ν 3 is an order of magnitude higher than for the least reactive, more energetic 2ν 3 -E state. Our first principles quantum scattering calculations show that as molecules in the ν 1 state approach the surface, the vibrational amplitude becomes localized on the reacting C-H bond, making them very reactive. This behavior results from the weakening of the reacting C-H bond as the molecule approaches the surface, decoupling its motion from the three non-reacting C-H stretches. Similarly, we find that overtone normal mode states with more ν 1 character are more reactive: S 0 (2ν 1 ) > S 0 (ν 1 + ν 3 ) > S 0 (2ν 3 ). The 2ν 3 eigenstates excited in the experiment can be written as linear combinations of these normal mode states. The highly reactive 2ν 1 and ν 1 + ν 3 normal modes, being of A 1 and F 2 symmetry, can contribute to the 2ν 3 -A 1 and 2ν 3 -F 2 eigenstates, respectively, boosting their reactivity over the E component, which contains no ν 1 character due to symmetry. Published by AIP Publishing.

“…RAP uses controlled frequency tuning of the excitation field to achieve a complete population transfer from an initial to a final quantum state. 49 A room temperature pyroelectric detector was used to determine the f exc of CH 4 in the molecular beam prepared by the laser.…”

Section: Experimental Methodsmentioning

confidence: 99%

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

Chadwick

Gutiérrez-González

The Journal of Chemical Physics

2016

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The King and Wells molecular beam reflectivity method has been used for a quantum state resolved study of the dissociative chemisorption of CH4 on Pt(111) at several surface temperatures. Initial sticking coefficients S0 were measured for incident CH4 prepared both with a single quantum of ν3 antisymmetric stretch vibration by infrared laser pumping and without laser excitation. Vibrational excitation of the ν3 mode is observed to be less efficient than incident translational energy in promoting the dissociation reaction with a vibrational efficacy ην3 = 0.65. The initial state resolved sticking coefficient S0ν3 was found to be independent of the surface temperature over the 50 kJ/mol to 120 kJ/mol translational energy range studied here. However, the surface temperature dependence of the King and Wells data reveals the migration of adsorbed carbon formed by CH4 dissociation on the Pt(111) surface leading to the growth of carbon particles.