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...

Adsorption of iso-/n-butane on an anatase thin film: a molecular beam scattering and TDS study

“…Effects of internal excitations on adsorption probabilities have been reported in a few experimental studies [35,36]. For physisorption systems, a suppression of the adsorption by rotational energy has been observed [35].…”

Sticking Probability and Reactivity of Hyperthermal O2 Molecules Impinging on CO Pre-covered Pd(100): Effect of Rotational States with K > 1

“…In our previous investigations [14,15] of the stereodynamical effect in the adsorption and reaction of O 2 with CO pre-covered Pd(100) we established that:…”

“…Data analysis was facilitated by previous direct measurements [8] of the degree of alignment as a function of the final velocity. From such data our experimental conditions could be firmly assessed taking into account the fraction of cartwheels transmitted by our velocity selector [8,14]. Even if, unfortunately, no analogous measurements of the degree of polarisation exist in literature for nozzle temperatures, T N , higher than RT it is undoubtedly interesting to investigate the adsorption at surfaces of aligned molecules in higher rotational states.…”

“…Such direction becomes therefore the ''natural'' axis to define the spatial quantization of the final rotational states and to describe any beam scattering event far from the expansion zone. Stereodynamical effects in collisions have been investigated both in the gas phase [12] and at surfaces [13][14][15] by velocity selected SMB using a slotted disks mechanical device. The tails of the beam velocity distribution are thereby used for uptake experiments in which both the sticking probability and the reactivity with pre-adsorbed species can be directly measured.…”

Communication: Fully alignment-specified O2 chemisorption on vicinal Si(100)

“…Stereodynamics in O 2 -surface interaction is, however, still far from being properly understood despite its great technological importance in heterogeneous catalysis, corrosion, and semiconductor industry. Previous studies employing aligned O 2 beams prepared with collisional alignment [1][2][3] or magnetic hexapolar field [4][5][6] have unveiled steric effects in O 2 chemisorption on Si(100), 4 Al(111), 5 clean 1 and adsorbed Pd(100) 2 surfaces. All of them, however, concern the polar angle effect of the O 2 axis; i.e., the difference in O 2 reactivity between the cases of side-on and end-on collisions.…”