Alec M. Wodtke

Coherent cavity ring down spectroscopy

Abstract In a cavity ring down experiment the multi-mode structure of a short resonant cavity has been explicitly manipulated to allow a high spectral resolution, which is advantageous for the overall detection sensitivity as well. Coherent cavity ring down spectroscopy is performed around 298 nm on OH in a flame.

Conversion of large-amplitude vibration to electron excitation at a metal surface

Gaining insight into the nature and dynamics of the transition state is the essence of mechanistic investigations of chemical reactions, yet the fleeting configuration when existing chemical bonds dissociate while new ones form is extremely difficult to examine directly. Adiabatic potential-energy surfaces—usually derived using quantum chemical methods that assume mutually independent nuclear and electronic motion—quantify the fundamental forces between atoms involved in reaction and thus provide accurate descriptions of a reacting system as it moves through its transition state. This approach, widely tested for gas-phase reactions, is now also commonly applied to chemical reactions at metal surfaces. There is, however, some evidence calling into question the correctness of this theoretical approach for surface reactions: electronic excitation upon highly exothermic chemisorption has been observed, and indirect evidence suggests that large-amplitude vibrations of reactant molecules can excite electrons at metal surfaces. Here we report the detection of ‘hot’ electrons leaving a metal surface as vibrationally highly excited NO molecules collide with it. Electron emission only occurs once the vibrational energy exceeds the surface work function, and is at least 10,000 times more efficient than the emissions seen in similar systems where large-amplitude vibrations were not involved. These observations unambiguously demonstrate the direct conversion of vibrational to electronic excitation, thus questioning one of the basic assumptions currently used in theoretical approaches to describing bond-dissociation at metal surfaces.

Electronically non-adiabatic interactions of molecules at metal surfaces: Can we trust the Born–Oppenheimer approximation for surface chemistry?

When neutral molecules with low levels of vibrational excitation collide at metal surfaces, vibrational coupling to electron-hole pairs (EHPs) is not thought to be strong unless incidence energies are high. However, there is accumulating evidence that coupling of large-amplitude molecular vibration to metallic electron degrees of freedom can be much stronger even at the lowest accessible incidence energies. As reaching a chemical transition-state also involves large-amplitude vibrational motion, we pose the basic question: are electronically non-adiabatic couplings important at transition states of reactions at metal surfaces? We have indirect evidence in at least one example that the dynamics and rates of chemical reactions at metal surfaces may be strongly influenced by electronically non-adiabatic coupling. This implies that theoretical approaches relying on the Born–Oppenheimer approximation (BOA) may not accurately reflect the nature of transition-state traversal in reactions of catalytic importance. Developing a predictive understanding of surface reactivity beyond the BOA represents one of the most important challenges to current research in physical chemistry. This article reviews the experimental evidence and underlying theoretical framework concerning these and related topics.

Vibrational-state-specific self-relaxation rate constant. Measurements of highly vibrationally excited O2(ν = 19–28)

Abstract The technique of stimulated emission pumping (SEP) has made the state-specific study of highly vibrationally excited molecules increasingly appealing. A sophisticated spectroscopic probe SEP may also be used as a preparative technique for species with “chemically significant” amounts of vibrational energy. The sensitivity of the approach allows even relatively improbable processes, such as vibrational energy transfer and chemical reaction, to be studied. Recently, highly vibrationally excited oxygen O2(ν) has received attention as a possible source of stratospheric O3 via an autocatalytic mechanism first proposed by Slanger. To evaluate this mechanism, a complete set of vibrational-state-specific vibrational relaxation rate constants are needed for highly vibrationally excited O2(ν) colliding with atmospheric gasses. This information is required in order to determine if solar photodissociation of O2(ν) can compete with collision-induced vibrational re-equilibration. This article will describe recent SEP studies of the vibrational quantum number dependence of the vibrational relaxation of highly vibrationally excited O2 with O2 in the ground vibrational state at 295 and 460 K. These data permit a comparison with recent theoretical models of vibrational relaxation rates and provide indirect evidence for the influence of the vibrationally enhanced endothermic reaction O2(ν>26) + O2(ν = 0) → O3 + O(3P) on the vibrational relaxation rates. The possible implications of these measurements on the autocatalytic stratospheric ozone production model are discussed.

Ultrafast low-energy electron diffraction in transmission resolves polymer/graphene superstructure dynamics.

Probing interfaces with electrons When molecules move on surfaces, they behave differently from when inside a solid. But surface layers give off limited signals, so to probe these systems, scientists need to act fast. Gulde et al. developed an ultrafast low-energy electron diffraction technique and used it to study how a polymer moved and melted on a graphene substrate (see the Perspective by Nibbering). After hitting the sample with a laser pulse, energy transferred across the graphene-polymer interface, the polymer film became less orderly, and an amorphous phase appeared. Science, this issue p. 200; see also p. 137 Time-resolved low-energy electron diffraction resolves picosecond structural dynamics in a polymer-graphene bilayer. [Also see Perspective by Nibbering] Two-dimensional systems such as surfaces and molecular monolayers exhibit a multitude of intriguing phases and complex transitions. Ultrafast structural probing of such systems offers direct time-domain information on internal interactions and couplings to a substrate or bulk support. We have developed ultrafast low-energy electron diffraction and investigate in transmission the structural relaxation in a polymer/graphene bilayer system excited out of equilibrium. The laser-pump/electron-probe scheme resolves the ultrafast melting of a polymer superstructure consisting of folded-chain crystals registered to a free-standing graphene substrate. We extract the time scales of energy transfer across the bilayer interface, the loss of superstructure order, and the appearance of an amorphous phase with short-range correlations. The high surface sensitivity makes this experimental approach suitable for numerous problems in ultrafast surface science.

Pd-Sensitized Single Vanadium Oxide Nanowires: Highly Responsive Hydrogen Sensing Based on the Metal−Insulator Transition

Exceptionally sensitive hydrogen sensors were produced using Pd-nanoparticle-decorated, single vanadium dioxide nanowires. The high-sensitivity arises from the large downward shift in the insulator to metal transition temperature following the adsorption on and incorporation of atomic hydrogen, produced by dissociative chemisorption on Pd, in the VO(2), producing approximately 1000-fold current increases. During a rapid initial process, the insulator to metal transition temperature is decreased by >10 degrees C even when exposed to trace amounts of hydrogen gas. Subsequently, hydrogen continues to diffuse into the VO(2) for several hours before saturation is achieved with only a modest change in the insulator to metal transition temperature but with a significant increase in the conductivity. The two time scales over which H-related processes occur in VO(2) likely signal the involvement of two distinct mechanisms influencing the electronic structure of the material one of which involves electron-phonon coupling pursuant to the modification of the vibrational normal modes of the solid by the introduction of H as an impurity.

Site-specific photocatalytic splitting of methanol on TiO2(110).

Clean hydrogen production is highly desirable for future energy needs, making the understanding of molecular-level phenomena underlying photocatalytic hydrogen production both fundamentally and practically important. Water splitting on pure TiO2 is inefficient, however, adding sacrificial methanol could significantly enhance the photocatalyzed H2 production. Therefore, understanding the photochemistry of methanol on TiO2 at the molecular level could provide important insights to its photocatalytic activity. Here, we report the first clear evidence of photocatalyzed splitting of methanol on TiO2 derived from time-dependent two-photon photoemission (TD-2PPE) results in combination with scanning tunneling microscopy (STM). STM tip induced molecular manipulation before and after UV light irradiation clearly reveals photocatalytic bond cleavage, which occurs only at Ti4+ surface sites. TD-2PPE reveals that the kinetics of methanol photodissociation is clearly not of single exponential, an important characteristic of this intrinsically heterogeneous photoreaction.

Universal crossed molecular beams apparatus with synchrotron photoionization mass spectrometric product detection

Vacuum ultraviolet radiation was generated from an undulator at the Advanced Light Source Synchrotron facility and used for photoionization detection of reaction products in a new universal crossed molecular beams machine. A description of the machine and its performance is presented. Initial experiments on the photodissociation of methylamine (CH3 NH2), ozone (O3), oxalyl chloride [ (OCCl)2] as well as the reactive scattering of Cl with C3 H8 show many of the advantages of photoionization in comparison to electron impact ionization, which has been exclusively used in such instruments in the past. “Momentum matching” of reaction products is much more easily accomplished than in electron impact studies due to suppression of dissociative ionization. The tunability of the vacuum ultraviolet radiation can be used to suppress background from residual gases especially when it is desired to detect free radical reaction products. Even when the tunability cannot be used to suppress background, the fact that little...

Tin-oxide-nanowire-based electronic nose using heterogeneous catalysis as a functionalization strategy.

An electronic nose (e-nose) strategy is described based on SnO(2) nanowire arrays whose sensing properties are modified by changing their operating temperatures and by decorating some of the nanowires with metallic nanoparticles. Since the catalytic processes occurring on the metal nanoparticles depend on the identity of the metal, decorating the semiconducting nanowires with various metal nanoparticles is akin to functionalizing them with chemically specific moieties. Other than the synthesis of the nanowires, all other steps in the fabrication of the e-nose sensors were carried out using top-down microfabrication processes, paving the way to a useful strategy for making low cost, nanowire-based e-nose chips. The sensors were tested for their ability to distinguish three reducing gases (H(2), CO, and ethylene), which they were able to do unequivocally when the data was classified using linear discriminant analysis. The discriminating ability of this e-nose design was not impacted by the lengths or diameters of the nanowires used.

Vibrational energy transfer of very highly vibrationally excited NO

The dependence of vibrational energy transfer on vibrational excitation has been studied using the stimulated emission pumping technique to efficiently prepare a large range of specific vibrational states of the nitric oxide molecule in its ground electronic state. Laser induced fluorescence was used to detect collisionally relaxed NO. The self‐relaxation rate constants of NO(v≫1) were up to 200 times larger than that of NO(v=1). Multiquantum relaxation was found to be important at high energy and was quantified at 3.8 eV. Self‐relaxation rate constants of 15N18O as well as 14N16O were measured and a large isotope effect was observed. Relaxation of NO(v‘=22) with H2 was also investigated. Theoretical explanations of our experimental results were attempted and it is shown that at vibrational energy up to ≊3 eV the qualitative trends observed in these experiments such as the mass effect and the multiquantum relaxation can be explained by Schwartz–Slawsky–Herzfeld theory. A simple explanation of the anomalou...