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

Abstract

The adsorption of gas-phase molecules that approach solid surfaces in well-defined quantum states represents a fundamental step for the understanding of heterogeneous chemical reactions [1] and for better control over the growth of selfassembled layers.[2] This objective has been achieved only in a few cases to date (NO, [3,4] 6,7] ). 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 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.H 2, [5] CH 4 [The alignment of rotational angular momentum can be exploited to investigate stereodynamical effects in molecular adsorption processes. [9,[11][12][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 helicopterlike 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 helicopterlike motion adsorb more efficiently than cartwheeling ones only at intermediate hydrocarbon coverage. The stereoinsensitivity 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. Rotational alignment was found to be ineffective for more strongly bound systems such as s-bonded C 2 H 4 /Pd(100) [13] and O 2 /Ag(100) (interactions mediated via a chemisorbed molecular precursor). This result was attributed to steering forces turning the incoming molecule into the most favorable configuration and causing it to lose memory of its initial state.For O 2 on CO-precovered Pd(100), however, we find that molecules moving as cartwheels stick better than those with helicopter-like motion and that molecular alignment also affects the initial sticking probability. The system under investigation is important because of the effective rate of CO oxidation on Pd(100) when exposed to molecular oxygen. Herein we focus on stereodynamical effects of O 2 on the S 0 value and on the production of...

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“…The elementary steps of simple chemical reactions1–4 can be studied by performing state‐resolved experiments with supersonic molecular beams (SMBs). As recently demonstrated, this technique also allows the exploration of the influence of the rotational alignment of the reactants5–6 by performing an accurate velocity selection of the molecules that are seeded in lighter inert carriers 7…”

“…The CO 2 yield and O 2 sticking probability versus time for O 2 exposures on CO pre‐covered Pd(001) at two different surface temperatures T S are shown in Figures 1 a and b, respectively. The pre‐coverage was set at Θ CO =0.04 ML, so that most of the O 2 molecules interacted with the bare surface21 and the sticking probability was large 6. If T S is high enough for CO to be mobile and for the reaction barrier to be overcome, the O adatoms react and give rise to CO 2 desorption.…”