Molecular Precursor State Research Articles

Laser spectroscopy of desorbing molecules

In this article the application of tunable dye lasers to desorption phenomena is illuminated. These lasers provide radiation continuously tunable from 105 nm in the vacuum ultraviolet to about 10 μm in the mid-IR. By employing either laser induced fluorescence (LIF) or resonance enhanced multiphoton ionization (REMPI) spectroscopy almost all diatomic and many polyatomic molecules can be probed with the sensitivity required to detect desorbing molecules under UHV conditions. The spectral resolution of the lasers is sufficiently high that rotational state selectivity is achieved. Recent developments permit in addition the velocity distributions of molecules to be determined with internal quantum state resolution. Therefore very detailed information about the molecular dynamics has been obtained. In most experiments so far reactive recombinations off surfaces have been investigated. In this paper special emphasis will be given to the recombination of hydrogen on copper and palladium surfaces. For these systems very detailed data about the internal state populations at various surface temperatures have been obtained. The rotational cooling previously observed in molecular beam scattering has also been established for desorption. Strong vibrational excitation has been observed, which in the case of desorption from copper may be associated with the recombination dynamics, whereas for desorption of D2 from Pd(100) a molecular precursor state might be responsible. By measuring the velocity distribution in each quantum state, the complete energetics of the desorbing molecules has been determined. Some first experiments on laser induced desorption with state selective detection of the desorbing molecules will also be discussed. Finally, making use of the polarization analysis of the signal, alignment effects in the desorption can be observed, permitting observation of molecular dynamics with a “magnifying glass”.

Adsorption kinetics of SiH 4, Si 2H 6 and Si 3H 8 on the Si(111)-(7×7) surface

The rates and mechanisms of chemisorption on the Si(111)-(7×7) surface have been investigated under UHV conditions for SiH 4, Si 2H 6, and Si 3H 8. Temperature programmed desorption of H 2 has been used to quantitate adsorption of the silanes following calibrated exposures. A dramatic enhancement of the adsorption rate occurs with one SiSi bond in the molecule. A second SiSi bond has no effect on adsorption rate on the clean surface, but increases the reactivity with a hydrogen covered surface. The reactive sticking coefficient, S R , for silane is less than 0.001 near zero coverage on Si(111)-(7×7). S R (SiH 4) is independent of temperature from 25 to 275°C. Reactive sticking coefficients of 0.47±0.1 for higher silanes Si 2H 6 and Si 3H 8 are observed at low coverage for 25°C surface temperature. The higher silanes adsorb on the clean surface through molecular precursor states, as evidenced by coverage independent sticking coefficients and by negative activation energies for adsorption. At low coverage, Si 2D 6 and Si 2H 6 exhibit the same sticking coefficient, so that no deuterium kinetic isotope effect is observed for the adsorption of disilane. This observation is consistent with SiSi bond breaking as the rate limiting step for conversion of the molecular precursor to chemisorbed species. The surface residence time of molecular SiH 4 is inferred to be very short, relative to the residence times of molecular Si 2H 6 and Si 3H 8. SiH 4 is relatively unreactive due to its short residence time and because adsorption requires SiH bond scission.

Evidence from coadsorption studies for a molecular precursor state in the oxidation of Zn(0001)

The rate of dissociative chemisorption of dioxygen at a Zn(0001) surface at 110 K increases by a factor of nearly 103 when coadsorbed with ammonia; a model based on a precursor state involving an ammonia–dioxygen complex is suggested.

Isotope effects in hydrogen adsorption on Ni(111): Direct observation of a molecular precursor state

The nature of the interaction of hydrogen with Ni(111) is probed by comparing the adsorption and desorption kinetic behavior of H2 and D2. Pure H2 and pure D2 adsorbed on Ni(111) at 140 K exhibit identical desorption behavior. The adsorption rate law depends upon (1-θ) rather than (1-θ)2, suggesting that the rate determining step involves the interaction of an H2 molecule with a single Ni site. The temperature programmed desorption spectra show two desorption features, β1 and β2, which have maxima in desorption rate at 290 and 370 K. When either pure H2 and pure D2 or a mixture of H2 and D2 interact with the Ni(111) surface at 87 K, evidence for kinetic retardation of D2 adsorption is observed. When the H2:D2 mixture is exposed to the Ni(111) surface, held at 87 K, an additional desorption feature, α, is uniquely observed at 100 K for D2. α-D2 desorbs with first order kinetics exhibiting Ed =11.1 kJ/mol and a preexponential factor of 1.2×105 s−1. It may be due to desorption from an intrinsic molecular precursor state. The observed differences between hydrogen and deuterium adsorption indicate the presence of an activation energy barrier for dissociative chemisorption, and a sensitivity of the adsorption process to differences in zero point vibrational energies. A molecular precursor state is proposed to describe the H2+Ni(111) interaction. Dissociative adsorption of hydrogen on intentionally produced surface defects has been shown to destabilize α-D2.

Adsorption of oxygen on a Pd(111) surface studied by high resolution electron energy loss spectroscopy (EELS)

Oxygen adsorption on a Pd(111) surface has been studied in the temperature range from 30 K to 300 K by high resolution electron energy loss spectroscopy (EELS) and by low energy electron diffraction (LEED). At 30 K oxygen adsorption leads to two roughly equally populated molecular adsorption states characterized by vibrational losses at 850 and 1035 cm −1. These losses are assigned to the O-O stretching frequency of molecularly adsorbed oxygen in a peroxo-like state and a superoxo-like state, respectively. After saturation of these chemisorbed molecular states, a state of physisorbed oxygen with its vibrational frequency close to the gas phase value of 1556 cm −1 is populated. Upon warming the sample above 80 K, an additional loss feature at 650 cm −1 develops which is assigned to a second peroxo-like molecular species. Between 80 and 180 K an interconversion of the different molecular species takes place in which the molecular states are sequentially populated in the order of decreasing vibrational frequency ν O-O as the sample temperature is raised. The dissociation process is completed at T ≈ 200 K leaving a layer of atomic oxygen on the surface which is characterized by a vibrational loss at 480 cm −1 and by a 2 × 2 pattern in LEED. The results demonstrate that the dissociative chemisorption of oxygen on Pd(111) does not proceed in a single step but through a sequence of several well defined molecular precursor states.

Total-energy calculations for N2 dissociation on Fe(111): Characterization of precursor and dissociative states.

The ${\mathrm{N}}_{2}$ dissociation on Fe(111) is investigated by calculating total-energy surfaces within a linear combination of atomic orbitals type formalism. The two molecular precursor states, involving ${\mathrm{N}}_{2}$ bound parallel and perpendicular to the surface, and the dissociative state are characterized by their geometry and total energy. The results of the calculation agree with recent experimental data.

Evidence for multiple states of hydrogen on palladium

Adsorption of hydrogen on polycrystalline palladium has been investigated using ESD techniques. Although a single thermal desorption peak is observed for H 2 and Pd, total ESD cross section measurements suggest the existence of four distinct sources for desorbing H + and H - species. The very large cross section associated with the H - signal along with its behavior during sample heating suggests the possible existence of a molecular precursor state.

Evidence for multiple states of hydrogen on palladium

Adsorption of hydrogen on polycrystalline palladium has been investigated using ESD techniques. Although a single thermal desorption peak is observed for H 2 and Pd, total ESD cross section measurements suggest the existence of four distinct sources for desorbing H + and H − species. The very large cross section associated with the H − signal along with its behavior during sample heating suggests the possible existence of a molecular precursor state.

Adsorption of O2 and N2O on carbon-covered Ru(001) surfaces

The dissociative chemisorption of N2O and O2 on carbon-covered Ru(001) surfaces has been studied by Auger electron spectroscopy. Small amounts of carbon sharply lower the dissociative probability for N2O as compared to O2. Carbon blocks sites for atomic oxygen adsorption and one monolayer of carbon removes the dissociation activity for O2 and N2O. The results are interpreted in terms of a significant effect of carbon on the N2O molecular precursor state and in terms of more restrictive surface site requirements for N2O as compared to O2.

Direct inelastic and trapping-desorption scattering of N 2 from polycrystalline W; Elementary steps in the chemisorption of nitrogen

Abstract Scattering of N 2 from a clean polycrystalline W surface is studied with a time-of-flight molecular beam apparatus. The time-of-flight spectra are used to characterize the N 2 -W energytransfer and condensation, allowing inferences to be made about the initial steps of N 2 chemisorption, thought to proceed via a molecular precursor state. The sticking coefficient on our sample for N 2 to chemisorb to an atomic nitrogen bound state was 0.5 ± 0.1 5 for a 600 K beam and a 450 K surface temperature. Unreacted N 2 scattered into direct and trapping-desorption channels. The direct channel is shown to be entirely inelastic with temperature independent differential energy accommodation coefficients that average 0.46 for normal and specular scattering at 45° incidence angle. The fraction of trapping-desorption scattering diminishes significantly with increasing surface and beam temperature. The observed decrease in sticking coefficient with increase in surface temperature is shown to be due to a diminution of the N 2 condensation coefficient as well as an increase in desorption of the N 2 , recursor relative to its migration-chemisorption.

Direct inelastic and trapping desorption scattering of N2 from polycrystalline W; elementary steps in the chemisorption of nitrogen

Scattering of N 2 from a clean polycrystalline W surface is studied with a time-of-flight molecular beam apparatus. The time-of-flight spectra are used to characterize the N 2-W energytransfer and condensation, allowing inferences to be made about the initial steps of N 2 chemisorption, thought to proceed via a molecular precursor state. The sticking coefficient on our sample for N 2 to chemisorb to an atomic nitrogen bound state was 0.5 ± 0.1 5 for a 600 K beam and a 450 K surface temperature. Unreacted N 2 scattered into direct and trapping-desorption channels. The direct channel is shown to be entirely inelastic with temperature independent differential energy accommodation coefficients that average 0.46 for normal and specular scattering at 45° incidence angle. The fraction of trapping-desorption scattering diminishes significantly with increasing surface and beam temperature. The observed decrease in sticking coefficient with increase in surface temperature is shown to be due to a diminution of the N 2 condensation coefficient as well as an increase in desorption of the N 2, recursor relative to its migration-chemisorption.

The adsorption of oxygen on a stepped platinum single crystal surface

The adsorption of oxygen on the Pt(S)-[12(111) × (111) surface has been studied by Auger electron spectroscopy, low energy electron diffraction and thermal desorption spectroscopy. Two types of adsorbed oxygen have been identified by thermal desorption spectroscopy and low energy electron diffraction: (a) atoms adsorbed on step sites; (b) atoms adsorbed on terrace sites. The kinetics of adsorption into these two states can be modeled by considering sequential filling of the two adsorbed atomic states from a mobile adsorbed molecular precursor state. Adsorption on the step sites occurs more rapidly than adsorption onto the terraces. The sticking coefficient for oxygen adsorption is initially 0.4 on the step sites and drops when the step sites are saturated. The heat of desorption from the step site (45 ± 4 kcal/mole) is about 15% larger than the heat of desorption from the terraces.

Mechanism and speed of initial step of oxygen chemisorption-O2 on W

A direct first step in the mechanism for the initial oxidation of a polycrystalline tungsten surface, the catalyzed dissociation of O2 on it, or the formation of WO X by O2 at intermediate temperatures is shown to occur in ∼10−13 s. Molecular beam experiments demonstrated that the reactivity was greater than 90%, the unreacted O2 was substantially unaccommodated in translational energy to the surface temperature, and the reaction probability was nearly independent of surface temperature from 2800 to 415 K although there was a small increase at 415 K. The general conditions when no molecular surface precursor state contributes to the surface reactivity are discussed.

Surface stoichiometry and structure of GaAs

LEED, HEED and Auger spectroscopy were combined with mass spectrometric studies of Ga and As 2 molecular beam interactions with GaAs (100) and (1̄1̄1̄) surfaces. It was found that surface structural changes could be correlated with gain or loss of As 2 from the surface. A kinetic model involving changes in surface stoichiometry is proposed which gives quantitative agreement with experiment for both desorption rate and surface composition. The model assumes that As 2 is adsorbed in a weakly-bound molecular precursor state from which dissociation into As atoms in As surface sites occurs. Recombination of surface As atoms into the molecular state occurs at a rate whose activation energy decreases rapidly with increasing As concentration in the surface.