Stereodynamic Effects in the Adsorption of Ethylene onto a Metal Surface

Abstract

The role of molecular alignment of nonpolar molecules in gas-surface interactions has so far escaped direct experimental investigations. We have employed a novel technique to prepare highly aligned supersonic beams and we show that ethylene molecules that impinge on silver surfaces precovered with molecular oxygen as "helicopters" have a higher sticking probability than "cartwheels" (Figure 1). The sticking probability, S, of reactant gas-phase molecules impinging on a metal surface is a key parameter for the quantitative understanding of heterogeneous catalysis at the gas-solid interface. Herein we report the dependence of S on molecular alignment for ethylene, an apolar molecule, interacting with a metallic surface, specifically O-recovered Ag (001). The observed steric effect is large, thus strongly influencing adsorption. The study of such a prototype system is very important to clarify the dynamics in the entrance channels of the potential energy surfaces controlling the catalytic epoxidation of unsaturated hydrocarbons.Measuring S for a molecule in a well-defined quantum state represents a major goal of physical chemistry.[1] The extensive use of molecular beams in conjunction with the well-known retarded reflector method [2] allowed the determination of the dependence of S on translational energy and angle of incidence and on surface temperature and coverage for a wide set of gas-surface adsorption systems already in the first years of surface science.However, the characterization of the dependence of S on the molecular internal state became accessible only recently, mainly as a result of the availability of high-power lasers, which allowed the preparation of molecules in well-defined Figure 1. Ethylene molecules rotating along the principal axes impinge perpendicularly on Ag (001) metal surface precovered with O 2 and may stick in some favorable cases. A net variation of the coverage dependence of the sticking probability S when changing the geometry of approach of the C = C bond to the surface is observed: "helicopters" (d) stick better than "cartwheels" (b and c) or "cigars" (a).

Hydrogen-induced nanotunnel opening within semiconductor subsurface

“…The entrance could be found by shooting them directly in exploiting the directionality of a supersonic molecular beam in which the molecules are carried by an inert gas. As recently shown, symmetric tops can be aligned with respect to the rotational axis, increasing significantly the probability of entering with the molecular axis oriented upon purpose, either along or from the top, perpendicularly to the nanotunnel axis535455.…”

Role of Rotational Alignment in Dissociative Chemisorption and Oxidation: O2 on Bare and CO‐Precovered Pd(100)

“…Less attention has been devoted to rotations because they involve less energy, [8] and are therefore important only for physisorption. [9,10] We show here that rotations as well as the alignment of the rotational axis play a role in the dissociative chemisorption of O 2 onto a COÀ Pd(100) surface. The effect, which arises from the requirements needed to pass through the CO adlayer, leads to different sticking probabilities and average O À CO distances, and has possible general applications for controlling reactions and film growth.…”

Role of Rotational Alignment in Dissociative Chemisorption and Oxidation: O2 on Bare and CO‐Precovered Pd(100)

“…Vibrational excitations and conversion of translational to vibrational energy were thereby found to account for strong enhancements in the dissociation probability. Less attention has been devoted to rotations because they involve less energy,8 and are therefore important only for physisorption 9. 10 We show here that rotations as well as the alignment of the rotational axis play a role in the dissociative chemisorption of O 2 onto a COPd(100) surface.…”

“…The alignment of rotational angular momentum can be exploited to investigate stereodynamical effects in molecular adsorption processes 9. 11–13 Such an alignment, which consists of a propensity of molecules to populate specific helicity states (defined by the quantum number M , the projection of the rotational angular momentum along the propagation direction) can be naturally induced by collisions in seeded supersonic molecular beams (MBs).…”

“…Diatomic, linear, and linear‐like molecules flying in the MB with M =0 behave as cartwheels when impinging on the surface at normal incidence, while when M is maximum they move in a helicopter‐like manner. In particular, it has been shown that for the interaction of C 2 H 4 with an O 2 ‐precovered Ag(100) surface,9, 11 and for C 3 H 6 /Ag(100),12 molecules with helicopter‐like motion adsorb more efficiently than cartwheeling ones only at intermediate hydrocarbon coverage. The stereo‐insensitivity of the initial sticking probability S 0 and its coverage dependence indicated that the effect is due to the collision between trapped molecules, which still remember about their original alignment, and preadsorbed ones, which lie flat on the surface.…”