Desorption associative

Recently, a quantitative lateral interaction model for desorption kinetics has been suggested (103). It is based on a statistical derivation of a kinetic equation for the associative desorption of a heteronuclear diatomic molecule, taking into account lateral interactions between nearest-neighbor adatoms in the adsorbed layer. Thereby a link between structural and kinetic studies of chemisorption has been suggested. 

Here we shall be concerned with the interaction of inacming diatomic molecules (H-/ 0.) with either types of potential energy wells The molecular InteractJjon (responsible for elastic and direct-inelastic scattering with extremely short residence times of the irpinglng molecules in the potential) and the chemisorptive interaction (leading to dissociative adsorption and associative desorption, reflectively, and associated with H (D) atoms trapped in the chemisorption potential for an appreci le time). 

From the point of view of associative desorption, this reaction is an early barrier reaction. That is, the transition state resembles the reactants.46 Early barrier reactions are well known to channel large amounts of the reaction exoergicity into product vibration. For example, the famous chemical-laser reaction, F + H2 — HF(u) + H, is such a reaction producing a highly inverted HF vibrational distribution.47-50 Luntz and co-workers carried out classical trajectory calculation on the Born-Oppenheimer potential energy surface of Fig. 3(c) and found indeed that the properties of this early barrier reaction do include an inverted N2 vibrational distribution that peaks near v = 6 and extends to v = 11 (see Fig. 3(a)). In marked contrast to these theoretical predictions, the experimentally observed N2 vibrational distribution shown in Fig. 3(d) is skewed towards low values of v. The authors of Ref. 44 also employed the electronic friction theory of Tully and Head-Gordon35 in an attempt to model electronically nonadiabatic influences to the reaction. The results of these calculations are shown in... 

Fig. 3. Vibrational population distributions of N2 formed in associative desorption of N-atoms from ruthenium, (a) Predictions of a classical trajectory based theory adhering to the Born-Oppenheimer approximation, (b) Predictions of a molecular dynamics with electron friction theory taking into account interactions of the reacting molecule with the electron bath, (c) Born—Oppenheimer potential energy surface, (d) Experimentally-observed distribution. The qualitative failure of the electronically adiabatic approach provides some of the best available evidence that chemical reactions at metal surfaces are subject to strong electronically nonadiabatic influences. (See Refs. 44 and 45.)...

One of the most significant recent insights in surface chemical dynamics is the idea that the principle of detailed balance may be used to infer the properties of a dissociative adsorption reaction from measurements on an associative desorption reaction.51,52 This means, for example, that the observation of vibrationally-excited desorption products is an indicator that the dissociative adsorption reaction must be vibrationally activated, or vice versa the observation of vibrationally-cold desorption products indicates little vibrational promotion of dissociative adsorption. In this spirit, it is... 

By extending the FT model to the formation of two kinds of products—olefins and paraffins—and including secondary olefin reactions, the kinetic schemes shown in Figure 9.15 are obtained. In parallel primary reactions (from the growth sites), paraffins and alpha-olefins are desorbed—by irreversible associative desorption (the paraffins) and by dissociative desorption (the olefins) (upper scheme in Figure 9.15). 

Figure 3.1. Schematic of bond making/breaking process considered in this chapter (a) atomic adsorption/desorption/scattering, (b) molecular adsorption/desorption/scattering, (c) direct dissocia-tion/associative desorption, (d) precursor-mediated dissociation/associative desorption, (e) Langmuir-Hinschelwood chemistry, (f) Eley-Rideal chemistry, (g) photochemistry/femtochemistry, and (h) single molecule chemistry. Solid figures generally represent typical intial states of chemistry and dashed figures the final states of the chemistry.

For much of the discussion in this chapter, the BOA is assumed valid so that the bond making/breaking is simply described by motion of nuclei on a multidimensional ground state PES. For example, dissociation of a molecule from the gas phase is described as motion on the PES from a region of phase space where the molecule is far from the surface to one with the adsorbed atoms on the surface. Conversely, the time-reversed process of associative desorption is described as motion on the PES from a region of phase space with the adsorbed atoms on the surface to one where the intact molecule is far from the surface. For diatomic dissociation/associative desorption, this PES is given as V(Z, R, X, Y, ft, cp, < ), where Z is the distance of the diatomic to the surface, R is the distance between atoms in the molecule, X and Y are the location of the center of mass of the molecule within the surface unit cell, ft and cp are the orientation of the diatomic relative to the surface normal and represent the thermal distortions of the hh metal lattice atom... 

Figure 3.7. Schematic showing that associative desorption induced by thermal excitation of the lattice can create e-h pairs (hot electrons) via non-adiabatic damping of nuclear coordinates (described here...

This section introduces the principal experimental methods used to study the dynamics of bond making/breaking at surfaces. The aim is to measure atomic/molecular adsorption, dissociation, scattering or desorption probabilities with as much experimental resolution as possible. For example, the most detailed description of dissociation of a diatomic molecule at a surface would involve measurements of the dependence of the dissociation probability (sticking coefficient) S on various experimentally controllable variables, e.g., S 0 , v, J, M, Ts). In a similar manner, detailed measurements of the associative desorption flux Df may yield Df (Ef, 6f, v, 7, M, Ts) where Ef is the produced molecular translational energy, 6f is the angle of desorption from the surface and v, J and M are the quantum numbers for the associatively desorbed molecule. Since dissociative adsorption and... 

A typical application is the use of the (2 + 1) REMPI scheme for measuring the (v,./) distribution of H2 produced in associative desorption from a surface. When the laser is tuned to a spectroscopic transition between individual quantum states in the X -> E electronic band, resonant two-photon absorption populates the E state and this is subsequently ionized by absorption of another photon. The ion current is proportional to the number in the specific (v,./) quantum state in the ground electronic state that is involved in the spectroscopic transition. Tuning the laser to another spectroscopic feature probes another (v, J) state. Therefore, recording the ion current as the laser is scanned over the electronic band maps out the population distribution of H2(v, J) produced in the associative desorption. Ef of the (v, J) state can also often be simultaneously measured using field - free ion TOF or laser pump - probe TOF detection techniques. The (2 +1) REMPI scheme for detecting H2 is almost independent of the rotational alignment and orientation f(M) of molecules so that only relative populations of the internal states... 

For molecular desorption, laser spectroscopic studies of the desorbing molecule can give full internal state distributions, Df Ef, 6f, v, J, f M ), Ts), where f M ) is some distribution function describing the rotational orientation/alignment relative to the surface normal. For thermal desorption in non-activated systems, most atoms/molecules have only modest (but important) deviations from a thermal distribution at Ts. However, in associative desorption of systems with a barrier, the internal state distributions reveal intimate details of the dynamics. Associative desorption results from the slow thermal creation of a transition state, with a final thermal fluctuation causing desorption. Partitioning of the energy stored in V into... 

They also first proposed the use of detailed balance to relate dissociative adsorption to associative desorption. In hindsight, both the experiments and the PES derived by them to fit the experiments were in significant quantitative error, but this in no way minimizes the major contribution of this early work to the development of reactive gas-surface dynamics. 

Figure 3.22. Dissociation probability S of D2 on Cu(lll) plotted logarithmically vs. the normal component of incident energy En for various vibrational temperatures (Tv = Tn) as listed in the figure. The x labeled pure D2 is S for simply increasing Tv and En simultaneously of a pure D2 beam by increasing Tn. From Refs. [33,225]. Lines are S(En,Tv) calculated from associative desorption experiments measuring S(En, v, J) and detailed balance.

A key new element in the associative desorption experiments is the ability to probe different rotational states and rotational alignment. Assuming that the dominant effect of rotation is to make the thresholds dependent upon rotational state,... 

Figure 3.23. State-resolved associative desorption of D2 from Cu(l 11). (a) average desorbing kinetic energy (Ey) as a function of v,J quantum state, (b) state-resolved desorbing flux Df(v, J, Ts = 925 K) normalized by the rotational degeneracy and plotted in a manner such that a Boltzmann distribution is linear. The straight lines correspond to a rotational temperature T3 = Ts for each v state. From Ref. [33].

Using (1 + 1) REMPI, the quadrupole alignment Aq2) (E, v, J, Ts = 925 ) has also been measured for D2 associative desorption from Cu(lll) [229] and some of the results are given in Figure 3.24. Aq2) favors helicoptering alignment, in agreement with a PES that favors dissociation of molecules preferentially parallel to the surface (and detailed balance). It increases substantially with J but decreases with Ef. The... 

Figure 3.25. Probability of a given energy loss into e-h pairs of magnitude eh vs. A/icM occurring in associative desorption of a diatomic from a metal surface from 3D non-adiabatic dynamics, (a) is for H2 associative desorption from Cu(lll), with ( ) 0.02 eV and (b) is N2 associative desorption from Ru(0001), with (A/i ch) 0.5 eV. From Ref. [68].

Since both adsorption and desorption experiments have been performed, it is possible to determine if the experiments satisfy detailed balance and probe the same phase space. They do not, probably because sticking at low , is dominated by dissociation at the steps while associative desorption at higher 0N principally measures desorption from the terraces [244]. There is also ambiguity as to whether energy loss to the lattice and e-h pairs is the same in the two different types of experiments. 

Figure 3.28. N2 vibrational state distribution in associative desorption from Ru(0001). (a) Observed in experiment. From Ref. [126]. (b) From 3D (Z, R, q) first principles quasi-classical dynamics, with the solid triangles pointing upward being adiabatic dynamics and the squares from molecular dynamics with electronic frictions also from DFT. Based on the PES and frictions of Ref. [68]. The open triangles pointing downward are the results of 6D first principles adiabatic quasi-classical dynamics from Ref. [253].

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