The curious case of CO2 dissociation on Cu(110).

Dissociation of CO2 on copper surfaces is an important model system for understanding the elementary steps in catalytic conversion of CO2 to methanol. Using molecular beam-surface scattering methods, we measure the initial dissociation probabilities (S0) of CO2 on a flat, clean Cu(110) surface under ultrahigh vacuum conditions. The observed S0 ranges from 3.9 × 10-4 to 1.8 × 10-2 at incidence energies of 0.64-1.59 eV. By extrapolating the trend observed in the incidence energy dependence of S0, we estimate the lower limit of the dissociation barrier on terrace sites to be around 2 eV. We discuss these results in the context of what is known from previous studies on this system using different experiments and theoretical/computational methods. These findings are anticipated to be valuable for correctly understanding the elementary steps in CO2 dissociation on Cu surfaces.

Vibrational energy transfer near a dissociative adsorption transition state: State-to-state study of HCl collisions at Au(111).

In this work we seek to examine the nature of collisional energy transfer between HCl and Au(111) for nonreactive scattering events that sample geometries near the transition state for dissociative adsorption by varying both the vibrational and translational energy of the incident HCl molecules in the range near the dissociation barrier. Specifically, we report absolute vibrational excitation probabilities for HCl(v = 0 → 1) and HCl(v = 1 → 2) scattering from clean Au(111) as a function of surface temperature and incidence translational energy. The HCl(v = 2 → 3) channel could not be observed-presumably due to the onset of dissociation. The excitation probabilities can be decomposed into adiabatic and nonadiabatic contributions. We find that both contributions strongly increase with incidence vibrational state by a factor of 24 and 9, respectively. This suggests that V-T as well as V-EHP coupling can be enhanced near the transition state for dissociative adsorption at a metal surface. We also show that previously reported HCl(v = 0 → 1) excitation probabilities [Q. Ran et al., Phys. Rev. Lett. 98, 237601 (2007)]-50 times smaller than those reported here-were influenced by erroneous assignment of spectroscopic lines used in the data analysis.

CO desorption from a catalytic surface: elucidation of the role of steps by velocity-selected residence time measurements.

Directly measuring the rate of a surface chemical reaction remains a challenging problem. For example, even after more than 30 years of study, there is still no agreement on the kinetic parameters for one of the simplest surface reactions: desorption of CO from Pt(111). We present a new experimental technique for determining rates of surface reactions, the velocity-selected residence time method, and demonstrate it for thermal desorption of CO from Pt(111). We use UV−UV double resonance spectroscopy to record surface residence times at selected final velocities of the desorbing CO subsequent to dosing with a pulsed molecular beam. Velocity selection differentiates trapping-desorption from direct scattering and removes influences on the temporal profile arising from the velocity distribution of the desorbing CO. The kinetic data thus obtained are of such high quality that bi-exponential desorption kinetics of CO from Pt(111) can be clearly seen. We assign the faster of the two rate processes to desorption from (111) terraces, and the slower rate process to sequential diffusion from steps to terraces followed by desorption. The influence of steps, whose density may vary from crystal to crystal, accounts for the diversity of previously reported (single exponential) kinetics results. Using transition-state theory, we derive the binding energy of CO to Pt(111) terraces, D(0)(terr) (Pt−CO) = 34 ± 1 kcal/mol (1.47 ± 0.04 eV) for the low coverage limit (≤0.03 ML) where adsorbate−adsorbate interactions are negligible. This provides a useful benchmark for electronic structure theory of adsorbates on metal surfaces.

C-H···N hydrogen-bonding interaction in 7-azaindole:CHX3 (X=F, Cl) complexes.

The C-H···Y (Y=hydrogen-bond acceptor) interactions are somewhat unconventional in the context of hydrogen-bonding interactions. Typical C-H stretching frequency shifts in the hydrogen-bond donor C-H group are not only small, that is, of the order of a few tens of cm(-1) , but also bidirectional, that is, they can be red or blue shifted depending on the hydrogen-bond acceptor. In this work we examine the C-H···N interaction in complexes of 7-azaindole with CHCl3 and CHF3 that are prepared in the gas phase through supersonic jet expansion using the fluorescence depletion by infra-red (FDIR) method. Although the hydrogen-bond acceptor, 7-azaindole, has multiple sites of interaction, it is found that the C-H···N hydrogen-bonding interaction prevails over the others. The electronic excitation spectra suggest that both complexes are more stabilized in the S1 state than in the S0 state. The C-H stretching frequency is found to be red shifted by 82 cm(-1) in the CHCl3 complex, which is the largest redshift reported so far in gas-phase investigations of 1:1 haloform complexes with various substrates. In the CHF3 complex the observed C-H frequency is blue shifted by 4 cm(-1). This is at variance with the frequency shifts that are predicted using several computational methods; these predict at best a redshift of 8.5 cm(-1). This discrepancy is analogous to that reported for the pyridine-CHF3 complex [W. A. Herrebout, S. M. Melikova, S. N. Delanoye, K. S. Rutkowski, D. N. Shchepkin, B. J. van der Veken, J. Phys. Chem. A- 2005, 109, 3038], in which the blueshift is termed a pseudo blueshift and is shown to be due to the shifting of levels caused by Fermi resonance between the overtones of the C-H bending and stretching modes. The dissociation energies, (D0), of the CHCl3 and CHF3 complexes are computed (MP2/aug-cc-pVDZ level) as 6.46 and 5.06 kcal mol(-1), respectively.

Incidence energy dependent state-to-state time-of-flight measurements of NO(v = 3) collisions with Au(111): the fate of incidence vibrational and translational energy.

We report measurements of translational energy distributions when scattering NO(vi = 3, Ji = 1.5) from a Au(111) surface into vibrational states vf = 1, 2, 3 and rotational states up to Jf = 32.5 for various incidence energies ranging from 0.11 eV to 0.98 eV. We observed that the vibration-to-translation as well as the translation-to-rotation coupling depend on translational incidence energy, EI. The vibration-to-translation coupling, i.e. the additional recoil energy observed for vibrationally inelastic (v = 3 → 2, 1) scattering, is seen to increase with increasing EI. The final translational energy decreases approximately linearly with increasing rotational excitation. At incidence energies EI > 0.5 eV, the slopes of these dependencies are constant and identical for the three vibrational channels. At lower incidence energies, the slopes gradually approach zero for the vibrationally elastic channel while they exhibit more abrupt transitions for the vibrationally inelastic channels. We discuss possible mechanisms for both effects within the context of nonadiabatic electron-hole pair mediated energy transfer and orientation effects.

Electron hole pair mediated vibrational excitation in CO scattering from Au(111): incidence energy and surface temperature dependence.

We investigated the translational incidence energy (Ei) and surface temperature (Ts) dependence of CO vibrational excitation upon scattering from a clean Au(111) surface. We report absolute v = 0 → 1 excitation probabilities for Ei between 0.16 and 0.84 eV and Ts between 473 and 973 K. This is now only the second collision system where such comprehensive measurements are available - the first is NO on Au(111). For CO on Au(111), vibrational excitation occurs via direct inelastic scattering through electron hole pair mediated energy transfer - it is enhanced by incidence translation and the electronically non-adiabatic coupling is about 5 times weaker than in NO scattering from Au(111). Vibrational excitation via the trapping desorption channel dominates at Ei = 0.16 eV and quickly disappears at higher Ei.

The importance of accurate adiabatic interaction potentials for the correct description of electronically nonadiabatic vibrational energy transfer: a combined experimental and theoretical study of NO(v = 3) collisions with a Au(111) surface.

We present a combined experimental and theoretical study of NO(v = 3 → 3, 2, 1) scattering from a Au(111) surface at incidence translational energies ranging from 0.1 to 1.2 eV. Experimentally, molecular beam-surface scattering is combined with vibrational overtone pumping and quantum-state selective detection of the recoiling molecules. Theoretically, we employ a recently developed first-principles approach, which employs an Independent Electron Surface Hopping (IESH) algorithm to model the nonadiabatic dynamics on a Newns-Anderson Hamiltonian derived from density functional theory. This approach has been successful when compared to previously reported NO/Au scattering data. The experiments presented here show that vibrational relaxation probabilities increase with incidence energy of translation. The theoretical simulations incorrectly predict high relaxation probabilities at low incidence translational energy. We show that this behavior originates from trajectories exhibiting multiple bounces at the surface, associated with deeper penetration and favored (N-down) molecular orientation, resulting in a higher average number of electronic hops and thus stronger vibrational relaxation. The experimentally observed narrow angular distributions suggest that mainly single-bounce collisions are important. Restricting the simulations by selecting only single-bounce trajectories improves agreement with experiment. The multiple bounce artifacts discovered in this work are also present in simulations employing electronic friction and even for electronically adiabatic simulations, meaning they are not a direct result of the IESH algorithm. This work demonstrates how even subtle errors in the adiabatic interaction potential, especially those that influence the interaction time of the molecule with the surface, can lead to an incorrect description of electronically nonadiabatic vibrational energy transfer in molecule-surface collisions.

C-H···Y hydrogen bonds in the complexes of p-cresol and p-cyanophenol with fluoroform and chloroform.

In this work, we present spectroscopic investigations of hydrogen bonded complexes of CHF3 and CHCl3 with p-cresol and p-cyanophenol. The systems were chosen as the potential candidates bound by C-H···Y type hydrogen bonds that are known to exhibit unconventional blue shifts in the C-H stretching frequency. The two phenol derivatives chosen offer multiple hydrogen bonding acceptor sites. They also differ from each other in regard to the electron-donating/withdrawing ability of the para substituents which could dictate the global minimum structure in each case. The complexes were formed using the supersonic jet expansion method and were investigated using a variant of the IR-UV double resonance technique, namely fluorescence depletion IR (FDIR) spectroscopy. It was found that in the case of p-cresol the complexes were C-H···π bound in which the C-H stretch was blue-shifted. In the case of p-cyanophenol the complexes were C-H···N bound. In its fluoroform complex the C-H frequency was blue-shifted by 27 cm(-1), whereas the chloroform complex gave an example of zero-shifted hydrogen bond. The ab initio computational studies indicated that for the CHCl3 complexes it is necessary to optimize the structures on the BSSE-corrected PES using the counterpoise method to correctly predict the magnitudes of the C-H frequency shift.

O-H···S hydrogen bonds conform to the acid-base formalism.

Hydrogen bonding interaction between the ROH hydrogen bond donor and sulfur atom as an acceptor has not been as well characterized as the O-H···O interaction. The strength of O-H···O interactions for a given donor has been well documented to scale linearly with the proton affinity (PA) of the H-bond acceptor. In this regard, O-H···O interactions conform to the acid-base formalism. The importance of such correlation is to be able to estimate molecular property of the complex from the known thermodynamic data of its constituents. In this work, we investigate the properties of O-H···S interaction in the complexes of the H-bond donor and sulfur containing acceptors of varying proton affinity. The hydrogen bonded complexes of p-Fluorophenol (FP) with four different sulfur containing acceptors and their oxygen analogues, namely H2O/H2S, MeOH/MeSH, Me2O/Me2S and tetrahydrofuran (THF)/tetrahydrothiophene (THT) were characterized in regard to its S1-S0 excitation spectra and the IR spectra. Two-color resonantly enhanced multiphoton ionization (2c-R2PI), resonant ion-dip infrared (RIDIR) spectroscopy, and IR-UV hole burning spectroscopic techniques were used to probe the hydrogen bonds in the aforementioned complexes. The spectroscopic data along with the ab initio calculations were used to deduce the strength of the O-H···S hydrogen bonding interactions in these system relative to that in the O-H···O interactions. It was found that, despite being dominated by the dispersion interaction, the O-H···S interactions conform to the acid-base formalism as in the case of more conventional O-H···O interactions. The dissociation energies and the red shifts in the O-H stretching frequencies correlated very well with the proton affinity of the acceptors. However, the O-H···S interaction did not follow the same correlation as that in the O-H···O H-bond. The energy decomposition analysis showed that the dissociation energies and the red shifts in the O-H stretching frequencies follow a unified correlation if these two parameters were correlated with the sum of the charge transfer and the exchange component of the total binding energy.

State-to-state time-of-flight measurements of NO scattering from Au(111): direct observation of translation-to-vibration coupling in electronically nonadiabatic energy transfer.

Translational motion is believed to be a spectator degree of freedom in electronically nonadiabatic vibrational energy transfer between molecules and metal surfaces, but the experimental evidence available to support this view is limited. In this work, we have experimentally determined the translational inelasticity in collisions of NO molecules with a single-crystal Au(111) surface-a system with strong electronic nonadiabaticity. State-to-state molecular beam surface scattering was combined with an IR-UV double resonance scheme to obtain high-resolution time-of-flight data. The measurements include vibrationally elastic collisions (v = 3→3, 2→2) as well as collisions where one or two quanta of molecular vibration are excited (2→3, 2→4) or de-excited (2→1, 3→2, 3→1). In addition, we have carried out comprehensive measurements of the effects of rotational excitation on the translational energy of the scattered molecules. We find that under all conditions of this work, the NO molecules lose a large fraction (∼0.45) of their incidence translational energy to the surface. Those molecules that undergo vibrational excitation (relaxation) during the collision recoil slightly slower (faster) than vibrationally elastically scattered molecules. The amount of translational energy change depends on the surface temperature. The translation-to-rotation coupling, which is well-known for v = 0→0 collisions, is found to be significantly weaker for vibrationally inelastic than elastic channels. Our results clearly show that the spectator view of the translational motion in electronically nonadiabatic vibrational energy transfer between NO and Au(111) is only approximately correct.

A simple and low-cost setup for part per billion level frequency stabilization and characterization of red He-Ne laser.

This work describes the frequency stabilization of a dual longitudinal mode, red (632.8 nm) He-Ne laser, implemented using an open source low-cost microcontroller (Arduino Uno) and its performance characterization using a simple interferometric method. Our studies demonstrate that frequency stability up to 0.42 MHz (3σ, 17 h) can be achieved using this setup. This simple and low-cost system can serve as an excellent part per billion level frequency reference for high-resolution spectroscopy based applications.

A compact and highly collimated atomic/molecular beam source.

We describe the design, characterization, and application of a simple, highly collimated, and compact atomic/molecular beam source. This source is based on a segmented capillary design, constructed using a syringe needle. Angular width measurements and free molecular flow simulations show that the segmented structure effectively suppresses atoms traveling in off-axis directions, resulting in a narrow beam of Helium atoms having a width of 7 mrad (full width half maximum). We demonstrate an application of this source by using it for monitoring real-time changes in surface coverage on a clean Cu(110) surface exposed to oxygen by measuring the specular reflectivity of the Helium beam generated using this source.

A versatile and narrow linewidth infrared radiation source for ro-vibration state-selected preparation of molecules in molecular beams.

We describe the design and characterization of a versatile pulsed (5 ns, 10 Hz repetition rate) optical parametric oscillator and amplifier system capable of generating single longitudinal mode, narrow linewidth (0.01 cm-1) radiations in the wavelength range of 680-870 nm and 1380-4650 nm. Using a combination of power-normalized photoacoustic signal and a Fizeau interferometer-based wavemeter, we are able to actively stabilize the output wavenumber to within 0.005 cm-1 (3σ) over a timescale longer than 1000 s. We demonstrate an application of this system by performing ro-vibration state-selected preparation of CO in the v = 2 state, via direct overtone excitation (v = 0 → 2 at 2346 nm) and subsequent state-selected detection in an internally cold molecular beam.

Structure of the indole-benzene dimer revisited.

The structure of the indole-benzene dimer has been investigated using experimental techniques, namely, UV spectroscopy and infrared-ultraviolet (IR/UV) double resonance spectroscopy, combined with quantum chemical calculations such as MP2 and dispersion corrected DFT methods. The red shift of the indole N-H stretch frequency in the dimer provides direct evidence that the experimentally observed indole-benzene dimer is an N-H···π bound hydrogen bonded complex. Theoretical investigations suggest that the potential energy surface (PES) of the complex is rather flat along the coordinate describing the tilt angle between the molecular planes of indole and benzene, with several minima of similar energies, namely, parallel displaced (PD), right-angle T-shaped (T), and other intermediate structures which can be categorized as tilted T-shaped (T') and tilted parallel displaced (PD') structures. Three different computational methods, namely, RI-MP2, RI-B97-D, and PBE1-DCP, are used to arrive at a new structural assignment after assessing their performance in predicting the structure of the pyrrole dimer, for which accurate experimental data are available. By comparing the computed IR spectra of PD, T, and T'/PD' structures with the experimental IR spectrum, the tilted T-shaped (T') structure was assigned to the indole-benzene dimer. The empirically dispersion-corrected functionals (RI-B97-D and PBE1-DCP) correctly reproduce the experimental IR spectrum whereas the popular post-Hartree-Fock, MP2 method gives disappointing results. These results are also in agreement with the experimental dissociation energy (D(0)) reported in the literature. The N-H stretch frequency of the indole-benzene dimer has been found to be a more pertinent parameter for the structural assignment than the dissociation energy (D(0)).

Blue shifted hydrogen bond in 3-methylindole.CHX(3) complexes (X = Cl, F).

Although the first experimental report on the blue shifted hydrogen bond in gas phase appeared about a decade ago, not many examples have been reported in the literature thereafter. Computational studies on systems exhibiting such blue shifted hydrogen bond however have been abundant since then. Many of these theoretical predictions remain to be verified experimentally. In this work we present an example of blue shifted hydrogen bond observed in the weakly bound complexes of 3-methylindole and CHX(3) (X = F, Cl). The complexes were prepared using the supersonic jet expansion method and studied using the laser spectroscopic methods. The key findings were that these complexes exhibit C-Hpi type hydrogen bonding interaction and the CH is bound to the phenyl part of the aromatic plane. The CH stretch was found to be blue shifted by 2 and 16 cm(-1) in the case of CHCl(3) and CHF(3), respectively. Ab initio calculations along with atoms-in-molecule analysis and natural bond orbital analysis support the experimental findings. The computed results at the DFT/MP2 level also indicated that the IR intensity of the H-bond donor CH-stretch increases by two to three orders of magnitude for the CHCl(3) complex whereas for the fluoroform complex the same decreases by an order of magnitude, which are consistent with the trend reported in the case of C-HO type of blue shifting hydrogen bonds.

O-H...O versus O-H...S hydrogen bonding. 2. Alcohols and thiols as hydrogen bond acceptors.

In this paper, the effect of alkyl substitution at the hydrogen bond acceptor and its chain length on the strength and nature of hydrogen bonding is presented. In the present study we combine both experimental and computational methods to investigate the characteristics of O-H...O and O-H...S hydrogen bonding in the complexes of p-cresol (p-CR) with methanol (MeOH), ethanol (EtOH), methanethiol (MeSH), and ethanethiol (EtSH). The results indicate that, with an increase in the alkyl chain length, both O-H...O hydrogen bonding and O-H...S hydrogen bonding become stronger. Energy decomposition analysis emphasizes the dispersive nature of O-H...S hydrogen bonding. In addition, it revealed that the dispersion energy contribution in O-H...O hydrogen bonding increases with an increase in the alkyl chain length of the hydrogen bond acceptor. In the case of O-H...S hydrogen bonding, however, the dispersion energy contribution decreased from 68% for the H(2)S complex to 53% in the case of the MeSH complex; it remained unchanged with a further increase of the alkyl chain length. It was also observed that the red shifts in the OH stretching frequency did not correlate with the proton affinities of the O-centered acceptor vs the S-centered H-bond acceptor, in contrast with the known trend for the conventional H-bonded complexes. The IR/UV double resonance study enabled the assignments of the anti and gauche conformers of p-CR-EtOH and p-CR-EtSH.

Following the microscopic pathway to adsorption through chemisorption and physisorption wells.

Adsorption involves molecules colliding at the surface of a solid and losing their incidence energy by traversing a dynamical pathway to equilibrium. The interactions responsible for energy loss generally include both chemical bond formation (chemisorption) and nonbonding interactions (physisorption). In this work, we present experiments that revealed a quantitative energy landscape and the microscopic pathways underlying a molecule's equilibration with a surface in a prototypical system: CO adsorption on Au(111). Although the minimum energy state was physisorbed, initial capture of the gas-phase molecule, dosed with an energetic molecular beam, was into a metastable chemisorption state. Subsequent thermal decay of the chemisorbed state led molecules to the physisorption minimum. We found, through detailed balance, that thermal adsorption into both binding states was important at all temperatures.

O-H...O versus O-H...S hydrogen bonding I: Experimental and computational studies on the p-cresol x H2O and p-cresol x H2S complexes.

The weak hydrogen bonding ability of sulfur-containing hydrides makes it difficult to study their complexes and has not been characterized experimentally so far. In this work, the hydrogen-bonded complexes of H(2)S and H(2)O with p-cresol (p-CR) were studied using a variety of techniques such as two-color resonant two-photon ionization (2c-R2PI) spectroscopy, single vibronic level fluorescence (SVLF) spectroscopy, resonance ion dip infrared spectroscopy (RIDIRS), and fluorescence dip infrared spectroscopy (FDIRS), with an aim of comparing the nature and strength of their respective hydrogen bonding abilities. The intermolecular stretch (sigma) and the shift in the O-H stretching frequency of p-CR in the complex were taken as the measures of the O-H...O and O-H...S hydrogen bonding strength. The experiments were complemented by the ab initio calculations, atoms in molecules (AIM), natural bond orbital (NBO), and energy decomposition analyses carried out at different levels of theory. The experimental data indicates that in the p-CR x H(2)S complex, the phenolic OH group acts as a hydrogen bond donor, and sulfur as the acceptor. Further, it indicates that the p-CR x H(2)S complex was about half as strong as the p-CR x H(2)O complex. The AIM and NBO analyses corroborate the experimental findings. The energy decomposition analyses for the O-H...S hydrogen bond in the p-CR x H(2)S complex reveal that the dispersion interaction energy has the largest contribution to the total interaction energy, which is significantly higher than that in the case of the p-CR x H(2)O complex.

Observation of the adsorption and desorption of vibrationally excited molecules on a metal surface.

The most common mechanism of catalytic surface chemistry is that of Langmuir and Hinshelwood (LH). In the LH mechanism, reactants adsorb, become thermalized with the surface, and subsequently react. The measured vibrational (relaxation) lifetimes of molecules adsorbed at metal surfaces are in the range of a few picoseconds. As a consequence, vibrational promotion of LH chemistry is rarely observed, with the exception of LH reactions occurring via a molecular physisorbed intermediate. Here, we directly detect adsorption and subsequent desorption of vibrationally excited CO molecules from a Au(111) surface. Our results show that CO (v = 1) survives on a Au(111) surface for ~1 × 10-10 s. Such long vibrational lifetimes for adsorbates on metal surfaces are unexpected and pose an interesting challenge to the current understanding of vibrational energy dissipation on metal surfaces. They also suggest that vibrational promotion of surface chemistry might be more common than is generally believed.

Activated Dissociation of HCl on Au(111).

We report zero-coverage reaction probabilities (S0) for HCl dissociative adsorption on Au(111) obtained by the seeded molecular beam hot-nozzle method. For measurements at normal incidence with mean translational energies ranging from 0.94 to 2.56 eV (nozzle temperatures 296 to 1060 K), S0 increased from 6 × 10(-6) to 2 × 10(-2). S0 also increased with increasing nozzle temperature for fixed incidence energy associated with the motion normal to the surface. Accounting for the influence of the vibrational state population and translational energy distributions in the incident beam, we are able to compare the experimental results to recent theoretical predictions. These calculations, performed employing 6-D quantum dynamics on an electronically adiabatic potential energy surface obtained using density functional theory at the level of the generalized gradient approximation and the static surface approximation, severely overestimate the reaction probabilities when compared with our experimental results. We discuss some possible reasons for this large disagreement.