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We describe a novel ultrahigh vacuum state-to-state molecule/surface scattering apparatus with quantum state preparation of the incident molecular beam and angle-resolved quantum state detection of the scattered molecules. State-resolved detection is accomplished using a tunable mid-infrared laser source combined with a cryogenic bolometer detector and is applicable to any molecule with an infrared-active vibrational transition. Results on rotationally inelastic scattering of CH4 methane from a Ni(111) surface and NiO(111)/Ni(111) oxide film, obtained by the new apparatus, are presented. Molecules scattering from the oxidized surface, compared to those scattering from the bare nickel surface, are more highly excited rotationally and scatter into a broader distribution of angles. The internal alignment of molecular rotation is in addition found to be stronger in molecules scattering from the bare surface. Furthermore, the maxima of the state-resolved angular distributions shift toward and away from surface normal with increasing rotational quantum number J for the oxidized and bare surface, respectively. Finally, the rotational state populations produced in scattering from the oxidized surface are well-described by a Boltzmann distribution, while those produced in scattering from the bare surface exhibit large deviations from their best-fit Boltzmann distributions. These results point toward a marked enhancement in molecule–surface collisional energy exchange induced by oxidation of the nickel surface. [ABSTRACT FROM AUTHOR]

Stepped transition metal surfaces, including the reconstructed Pt(110)-(2 × 1) surface, can be used to model the effect of line defects on catalysts. We present a combined experimental and theoretical study of CHD3 dissociation on this surface. Theoretical predictions for the initial sticking coefficients, S0, are obtained from ab initio molecular dynamics calculations using the specific reaction parameter (SRP) approach to density functional (DF) theory, while the measured sticking coefficients were obtained using the King and Wells method. The SRP DF used here had been previously derived for methane dissociation on Pt(111) so that the experiments test the transferability of this SRP DF to methane + Pt(110)-(2 × 1). The agreement between the experimental and calculated S0 is poor, with the average energy shift between the theoretical and measured reactivities being 20 kJ/mol. There are two factors which may contribute to this difference, the first of which is that there is a large uncertainty in the calculated sticking coefficients due to a large number of molecules being trapped on the surface at the end of the 1 ps propagation time. The second is that the SRP32-vdW functional may not accurately describe the Pt(110)-(2 × 1) surface. At the lowest incident energies considered here, Pt(110)-(2 × 1) is more reactive than the flat Pt(111) surface, but the situation is reversed at incident energies above 100 kJ/mol. [ABSTRACT FROM AUTHOR]

The dissociative chemisorption of singly deuterated methane (CH3D) has been studied on the steps and terraces of a Pt(211) surface by quantum state resolved molecular beam methods. At incident translational energy (Et) below 50 kJ/mol, CH3D dissociates only on the more reactive steps of Pt(211), where both C–H and C–D cleavage products CH2D(ads) and CH3(ads) can be detected by reflection absorption infrared spectroscopy. Vibrational excitation of a slow beam of CH3D (Et = 10 kJ/mol), prepared with one quantum of antisymmetric C–H stretch excitation by infrared laser pumping, allows for fully bond- and site-selective dissociation forming exclusively CH2D(ads) on the step sites. At higher kinetic energies (Et > 30 kJ/mol), bond selective dissociation by C–H bond cleavage is observed on the terrace sites for stretch excited CH3D (ν4) while on the steps, the C–H/C–D cleavage branching ratio approaches the statistical 3/1 limit. Finally, at Et > 60 kJ/mol, both C–H and C–D cleavages are observed on both step and terrace sites of Pt(211). Our experiments show how careful control of incident translational and vibrational energy can be used for site and bond selective dissociation of methane on a catalytically active Pt surface. [ABSTRACT FROM AUTHOR]

Methane dissociation on the step and terrace sites of a Pt(211) single crystal was studied by reflection absorption infrared spectroscopy (RAIRS) at a surface temperature of 120 K. The C—H stretch RAIRS signal of the chemisorbed methyl product species was used to distinguish between adsorption on step and terrace sites allowing methyl uptake to be monitored as a function of incident kinetic energy for both sites. Our results indicate a direct dissociation mechanism on both sites with higher reactivity on steps than on terraces consistent with a difference in an activation barrier height of at least 30 kJ/mol. State-specific preparation of incident CH4 with one quantum of antisymmetric (ν3) stretch vibration further increases the CH4 reactivity enabling comparison between translational and vibrational activation on both steps and terraces. The reaction is modeled with first principles quantum theory that accurately describes dissociative chemisorption at different sites on the surface. [ABSTRACT FROM AUTHOR]

The infrared spectra of protonated and lithiated valine with varying degrees of hydration in the gas phase is reported and interpreted with the help of DFT calculations at the B3LYP/6-31++G level. It is seen that infrared spectroscopy provides valuable information on the structure of solvated biological molecules in the gas phase that complements data obtained by blackbody infrared radiative dissociation studies.

Quantum state resolved reactivity measurements probe the role of vibrational symmetry on the vibrational activation of the dissociative chemisorption of CH4on Ni(111). IR-IR double resonance excitation in a molecular beam was used to prepare CH4 in three different vibrational symmetry components, A1, E, and F2, of the 2ν3 antisymmetric stretch overtone vibration as well as in the ν1+ν3symmetric plus antisymmetric C-H stretch combination band of F2 symmetry. The quantum state specific dissociation probability S0 (sticking coefficient) was measured for each of the four vibrational states by detecting chemisorbed carbon on Ni(111) as the product of CH4 dissociation by Auger electron spectroscopy. We observe strong mode specificity, where S0 for the most reactive state ν1+ν3 is an order of magnitude higher than for the least reactive, more energetic 2ν3-E state. Our first principles quantum scattering calculations show that as molecules in the ν1 state approach the surface, the vibrational amplitude becomes localized on the reacting C-H bond, making them very reactive. This behavior results from the weakening of the reacting C-H bond as the molecule approaches the surface, decoupling its motion from the three non-reacting C-H stretches. Similarly, we find that overtone normal mode states with more ν1 character are more reactive: S0 (2ν1) > S0 (ν1 + ν3) > S0 (2ν3). The 2ν3eigenstates excited in the experiment can be written as linear combinations of these normal mode states. The highly reactive 2ν1 and ν1 + ν3 normal modes, being of A1 and F2 symmetry, can contribute to the 2ν3-A1 and 2ν3-F2 eigenstates, respectively, boosting their reactivity over the E component, which contains no ν1 character due to symmetry. [ABSTRACT FROM AUTHOR]

The King and Wells molecular beam reflectivity method has been used for a quantum state resolved study of the dissociative chemisorption of CH4 on Pt(111) at several surface temperatures. Initial sticking coefficients S0 were measured for incident CH4 prepared both with a single quantum of ν3 antisymmetric stretch vibration by infrared laser pumping and without laser excitation. Vibrational excitation of the ν3 mode is observed to be less efficient than incident translational energy in promoting the dissociation reaction with a vibrational efficacy ην3 = 0.65. The initial state resolved sticking coefficient Sν30 was found to be independent of the surface temperature over the 50 kJ/mol to 120 kJ/mol translational energy range studied here. However, the surface temperature dependence of the King and Wells data reveals the migration of adsorbed carbon formed by CH4 dissociation on the Pt(111) surface leading to the growth of carbon particles. [ABSTRACT FROM AUTHOR]

Highly efficient preparation of molecules in a specific rovibrationally excited state for gas/surface reactivity measurements is achieved in a molecular beam using tunable infrared (IR) radiation from a single mode continuous wave optical parametric oscillator (cw-OPO). We demonstrate that with appropriate focusing of the IR radiation, molecules in the molecular beam crossing the fixed frequency IR field experience a Doppler tuning that can be adjusted to achieve complete population inversion of a two-level system by rapid adiabatic passage (RAP). A room temperature pyroelectric detector is used to monitor the excited fraction in the molecular beam and the population inversion is detected and quantified using IR bleaching by a second IR-OPO. The second OPO is also used for complete population transfer to an overtone or combination vibration via double resonance excitation using two spatially separated RAP processes. [ABSTRACT FROM AUTHOR]

We present a stereodynamics study of the dissociative chemisorption of vibrationally excited methane on the (100), (110), and (111) planes of a nickel single crystal surface. Using linearly polarized infrared excitation of the antisymmetric C-H stretch normal mode vibration (ν3), we aligned the angular momentum and C-H stretch amplitude of CH4(ν3) in the laboratory frame and measured the alignment dependence of state-resolved reactivity of CH4 for the ν3 = 1, J = 0-3 quantum states over a range of incident translational energies. For all three surfaces studied, in-plane alignment of the C-H stretch results in the highest dissociation probability and alignment along the surface normal in the lowest reactivity. The largest alignment contrast between the maximum and minimum reactivity is observed for Ni(110), which has its surface atoms arranged in close-packed rows separated by one layer deep troughs. For Ni(110), we also probed for alignment effects relative to the direction of the Ni rows. In-plane C-H stretch alignment perpendicular to the surface rows results in higher reactivity than parallel to the surface rows. The alignment effects on Ni(110) and Ni(100) are independent of incident translational energy between 10 and 50 kJ/mol. Quantum state-resolved reaction probabilities are reported for CH4(ν3) on Ni(110) for translational energies between 10 and 50 kJ/mol. [ABSTRACT FROM AUTHOR]

The reactivity of methane (CH4) on Pt(110)-(1×2) has been studied by quantum state-resolved surface reactivity measurements. Ground state reaction probabilities, S0(v=0)≅S0(laser-off), as well as state-resolved reaction probabilities S0(2ν3), for CH4 excited to the first overtone of the antisymmetric C–H stretch (2ν3) have been measured at incident translational energies in the range of 4–64 kJ/mol. We observe S0(2ν3) to be up to three orders of magnitude higher than S0(v=0), demonstrating significant vibrational activation of CH4 dissociation on Pt(110)-(1×2) by 2ν3 excitation. Furthermore, we explored the azimuthal and polar incident angle dependence of S0(2ν3) and S0(v=0) for a fixed incident translational energy Et=32 kJ/mol. For incidence perpendicular to the missing row direction on Pt(110)-(1×2) and polar angles θ>40°, shadowing effects prevent the incident CH4 molecules from impinging into the trough sites. Comparison of this polar angle dependence with reactivity data for incidence parallel to the missing rows yields state-resolved site specific reactivity information consistent with a Pt(110)-(1×2) reactivity that is dominated by top layer Pt atoms located at the ridge sites. A comparison of S0(v=0) measured on Pt(110)-(1×2) and Pt(111) yields a lower average barrier for Pt(110)-(1×2) by 13.7±2.0 kJ/mol. [ABSTRACT FROM AUTHOR]

The quantum state-resolved reactivity S0 of SiH4 on Si(100)-2×1 has been measured for the first time for two vibrationally excited Si–H stretch local mode states (|2000> and |1100>) as well the ground state S0 as a function of translational energy En and surface temperature Ts. We observe evidence for both direct and precursor-mediated chemisorption pathways. As expected, increasing En (or Ts) decreases S0 for the precursor-mediated reaction and increases S0 for the direct chemisorption. However, vibrational excitation of the incident SiH4 increases S0 for both the direct and the precursor-mediated pathway with a higher S0 for the |2000> state than for the |1100> state, indicating a nonstatistical reaction mechanism. [ABSTRACT FROM AUTHOR]

We report state resolved sticking coefficients for highly vibrationally excited CH[SUB4] on Ni(100) at well-defined kinetic energies in the range of 12-72 kJ/mol. Incident methane molecules are prepared by pulsed laser radiation in single rovibrational levels of the first overtone of the antisymmetric stretch (2 v[SUB3]) at 6004.69 cm[SUP-1] and collided at normal incidence with a clean Ni(100) single crystal. We find that the vibrational excitation enhances the reaction probability by a factor 100 at an incident translational energy of 72 kJ/mol, but this enhancement increases to more than 4 orders of magnitude at low kinetic energy. Despite this large increase in the sticking coefficient, vibrational energy in 2 v[SUB3] appears to be about 80% as effective as an equivalent amount of translational energy in promoting the chemisorption reaction. [ABSTRACT FROM AUTHOR]

Fragmentation of various fullerenes was studied by surface impact on highly oriented pyrolytic graphite at collision energies Ecol of 150–1050 eV/molecule. The projectiles C+60, C+70, C+76, C+84, and C+94 were formed by laser desorption of chromatographically separated samples, while large carbon clusters C+94, C+110, C+164 were produced by laser-induced coalescence reactions. Except at the highest impact energies, the fragment distributions consist of even numbered C+n species with abundance maxima similar to those observed in fullerene synthesis. With increasing Ecol, we observe a size evolution in the fragment distributions characteristic of a sequential fragmentation process. Simulated fragment distributions based on statistical rate theory and a sequential C2 loss mechanism reproduce the experimental data well up to a maximum Ecol. They are used to determine the mean energy transfer during surface impact as a function of collision energy as well as its dependence on several experimental parameters such as the nature (cleanliness) of the target surface, the internal energy of the incident ion, and the incident fullerene size. Both internal and kinetic energy of the incident ion are found to contribute to the observed fragmentation although with different efficiencies. For the higher fullerenes we find a tendency towards increasing transfer efficiency of incident kinetic to internal energy with increasing projectile size. Finally, above a size-dependent impact energy threshold, a transition to a different high energy fragmentation process is indicated by changes in the fragment distributions. These go from exclusively even numbered fullerene fragments at low impact energy to smaller even and odd numbered C+n fragments at high Ecol. It is suggested that this change indicates the formation of high energy, nonfullerene isomers. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Laser desorption mass spectra of thin films of pure C60O were recorded in a range of equilibrium sample temperatures from 300 to 1100 K. On heating of the sample above 650 K a new peak corresponding to C-119 is detected in the anion mass spectrum which persists even after the sample is cooled to 300 K. Possible solid state reactions leading to the formation of C119 are discussed based on the mass spectrometric observations as well as temperature programmed decomposition measurements which were carried out for C60 and C60O samples. The results are discussed in connection with recently reported polymerization reactions of fullerene materials. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

C60Ox, x=1–3 were prepared by exposing C60 solutions to ozone. After high performance liquid chromatography purification, these materials were studied by laser desorption time of flight mass spectroscopy. These mass spectra suggest that fullerene oxides undergo laser desorption induced coalescence more efficiently than pure C60. A correlation between the fragmentation of the desorbed parent species and the observed coalescence products both in yield and distribution suggests that efficient gas phase coalescence involves at least one reactive species, such as C58, produced by fragmentation of the desorbed fullerenes or fullerene derivatives. [ABSTRACT FROM AUTHOR]

Neutral and positively charged clusters Mx(C60)z, where M=K/Rb and z=1–3 were prepared in pulsed molecular beams by laser desorption (266 nm) from preformed alkali metal fulleride thin films. Experimental results encompassed mass spectral characterization of beam abundance and in selected cases, determination of rough ionization potential brackets and elucidation of fragmentation pathways upon (i) unimolecular dissociation in gas phase and (ii) surface collision induced dissociation from Si(111). Results are contrasted with electrostatic model calculations as well as ab initio self-consistent field computations. [ABSTRACT FROM AUTHOR]

Positively and negatively charged coalescence products of C60 and C70 were characterized following pulsed laser desorption from fullerene thin films. Coalescence occurs by gas-phase reactions. Positive ions are generated by thermoionization of hot neutrals, while negative ions likely form by electron attachment to neutral species in the outermost region of the desorption plume. Thermoionization rate and surface-induced dissociation (SID) measurements are consistent with fullerene-like structures for coalescence products detected as positive ions. [ABSTRACT FROM AUTHOR]

High resolution stimulated Raman spectroscopy (SRS) has been used to examine N2 and N2/He free jet expansions and also equilibrium samples of N2 from 15 to 110 K. The jet spectra show the formation of large liquid clusters which supercool and subsequently freeze to form crystalline β-N2 solid and, in He expansions, undergo a further transformation to a partially annealed α-N2 form. CW-SRS frequency and linewidth data obtained for equilibrium samples of the condensed phases of N2 yielded frequency–temperature relations used in deducing internal temperatures for the clusters produced in the expansion experiments. Analysis of the cooling curves indicates a mean cluster diameter of 35 nm and favors a prompt freezing process rather than a gradual conversion of liquid to solid in a single cluster on the microsecond time scale of the experiments. Supercooling limits of 34 to 44 K are deduced for the liquid, far below the triple point temperature of 63.2 K at which equilibrium samples freeze. Some evidence for surface versus bulk contributions to the spectra is seen in the asymmetric line shapes observed for liquid clusters in the condensation region. The results show that the high spectral and spatial resolution of nonlinear Raman methods such as SRS and CARS provide a unique probe of the condensation processes in free jet expansions. [ABSTRACT FROM AUTHOR]