Dynamics at surfaces


Molecular Spectroscopy
Molecular Dynamics and Kinetics at Surfaces
Current Research: Surface Scattering at Free Electron Lasers

PhD Research
My graduate research under Bob Field at MIT focused on the spectroscopy and dynamics of small polyatomic molecules. Particular themes include the development of highly multiplexed, high-resolution spectroscopic instrumentation as well as techniques that can be used to study molecules in highly excited states that sample chemically interesting regions of the potential energy surface. Specifically, I focused on dynamics that occur proximal to reaction barriers or that involve Born-Oppenheimer breakdown in the vicinity of conical intersections and avoided crossings.

Chirped Pulse Millimeter-Wave Spectroscopy
Early in my PhD research, I developed the first millimeter-wave implementation of the broadband Chirped-Pulse excitation technique, invented by Brooks Pate for microwave spectroscopy [1]. This was the first instrument capable of broadband (>10 GHz), high-resolution (sub-MHz) Fourier transform spectroscopy in the millimeter-wave region. (See Fig. 1 for an example spectrum.) As part of a collaboration with the Arthur Suits group, I helped implement chirped pulse millimeter-wave spectroscopy as a broadband, single-shot, molecule-specific and state-specific detector for studying the kinetics of cold bimolecular reactions. I also took advantage of the frequency flexibility and phase stability of the spectrometer to design new multiplexed, free induction decay (FID)-detected millimeter-wave/optical double resonance (mmODR) techniques, which had not previously been possible. I recently wrote a review paper that looks back on the first ten years of Chirped Pulse Microwave and Millimeter-wave Spectroscopy.

Figure 1. Chirped pulse Fourier-transform millimeter wave spectrum of the 193 nm photolysis products of vinyl cyanide (CH2CHCN). The J = 0−1 transitions of more than 25 HCN and HNC vibrational levels (indicated by blue and red arrows, respectively) are obtained with resolved hyperfine structure. This was the first example of broadband Fourier transform spectroscopy in the millimeter wave region, which allowed broadband spectra to be acquired at unprecedented rates and with meaningful relative intensities.

Large-Amplitude Local Bending Eigenstates of Acetylene
The shape of the potential energy surface (PES) near the transition state plays a key role in influencing chemistry. As energy in the bending vibrational modes of acetylene increases, large-amplitude local bending eigenstates emerge along the acetylene ↔ vinylidene isomerization path that have their classical turning point at half-linear geometries near the transition state. Although these local bending states had been predicted by effective Hamiltonian models [2], they had never been observed. I developed methodology for calculating Franck-Condon factors for the linear-to-bent à ↔ X̃ transition, which were used to guide stimulated emission pumping (SEP) experiments that succeeded in gaining spectroscopic access to these eigenstates. Increasing disagreement between the observed and predicted frequencies at higher local bending energies indicates the failure of current effective Hamiltonian models at describing the region near the transition state.

Why does the C̃ (1B2) state of SO2 have unequal bond lengths?
The C̃ (1B2) state of SO2 is vibronically distorted so that it has a double minimum potential energy surface along the antisymmetric stretch (q3) direction, which results in nonequivalent SO bond lengths at the minimum energy geometry. The vibronic distortion gives rise to a staggered energy level pattern between states with even and odd quanta of v3. However, this staggering had never been directly observed because transitions to levels with odd quanta of v3 are rigorously forbidden from the ground state. We employed a two-photon IR-UV technique to directly observe the dark levels, allowing a detailed force field analysis of the C̃ state and a characterization of the vibronic interaction. Our results provide direct experimental evidence for interaction with a perturbing D̃ (1A1) state, which has a very low effective ω3 frequency, and which has a conical intersection with the C̃ state at a wide bond angle of approximately 145–150°. (See Fig. 2.)

Figure 2. The staggering of the v3 progression of the C̃ state of SO2 increases linearly with quanta of bending vibration, v2 (left panel), which indicates that the vibronic distortion is stronger at wider bond angles. This is a signature of a conical intersection with the perturbing D̃ (1A1) state, which has been predicted by ab initio calculations [3] at a bond angle of 145–150°. As the bond angle is increased, the energy separation between the C̃ and D̃ states decreases due to the approach to the conical intersection, resulting in stronger vibronic interaction (center and right panel). Our vibronic coupling model qualitatively reproduces the observed effect (dotted line, left panel).

Post-Doctoral Research
During my post-doc with Alec Wodtke in Göttingen, Germany, I changed my focus to molecular dynamics at surfaces and interfaces. The problem of dynamics at surfaces fascinates me, not only because of its broad impact on our society, but also because it presents grand challenges for researchers and lies at the edge of what chemical theory is currently capable of describing.

Dynamics of Polyatomic Molecules at Surfaces
The adsorption of reactant molecules onto surfaces is the important first step in a wide variety of surface chemistry. However, in order for a molecule to stick to a surface, the incident kinetic energy must be dissipated during the first encounter, which motivates a detailed understanding of the energy exchange mechanisms between molecules and surfaces. To date, almost all detailed state-to-state scattering studies have focused on atoms or diatomic molecules, and almost nothing is known about energy dissipation in polyatomic molecule-surface collisions, where energy storage during the trapping process might be mode-specific or rotational axis-specific. My first project in Göttingen involved the elucidation of the adsorption mechanism of the environmentally and industrially relevant polyatomic molecule, formaldehyde, on the Au(111) surface. After developing a new resonance-enhanced multiphoton ionization (REMPI) scheme necessary for sensitive, rovibrationally specific detection of formaldehyde, we performed state-to-state scattering experiments from the Au(111) surface. The rotationally resolved scattering distribution indicates that formaldehyde scatters into a non-Boltzmann rotational distribution with a strong propensity for ~0.13 eV (13 kJ/mol) of rotational energy about the CO axis (a-axis rotation). See Fig. 3. We have performed model classical scattering simulations, which reproduce this effect and allow us to interpret it as a "rotational rainbow", which arises from steric effects that cause a singularity in the classical angular momentum distribution about a specific rotational axis. The results indicate that temporary storage of energy in molecular rotation plays a dominant role in the trapping mechanism and significantly enhances the trapping probability of formaldehyde to the Au(111) surface at high incident kinetic energies (> 50 kJ/mol).

Figure 3. The total population in each Ka manifold (divided by the nuclear spin degeneracy, gns) is plotted as a function of a-axis rotational energy, Ea, for formaldehyde directly scattered from the Au(111) surface over a range of incident kinetic energies, Ei. A rotational rainbow at Ka ≈ 10 grows in strongly as the incident energy is increased. Evidence suggests that the high degree of a-axis rotation plays a key role in the dissipation of incident kinetic energy during the trapping process.

Ion Imaging the Velocity-Resolved Kinetics of Surface Reactions
Right now, I am working on the development of off-axis ion imaging as a tool for direct measurement of the kinetics of surface reactions. This technique offers major advantages over previous methods [4] for studying the kinetics of reaction product desorption from surfaces and can be used to unravel complex reaction mechanisms. My current project focuses on the decomposition of methanol on the stepped Pt(332) surface. Methanol decomposition to CO+2H2 is a potentially important source of chemical energy in fuel cell applications and serves as an alternative source of synthesis gas. By varying the incidence kinetic energy of the molecular beam, a short pulse of methanol can be either molecularly physisorbed or dissociatively chemisorbed on Pt surfaces, which enables both the forward and reverse kinetics of the H-atom abstraction reaction to be determined from the flux of the methanol or CO product desorption channels, respectively. Results are still preliminary.

Ion Imaging
Figure 4. A schematic of the surface scattering/ion imaging geometry is shown in the left panel. A molecular beam impinges upon a surface. Scattering products are ionized by a laser (red dot) in the region between a set of repeller and grid electrodes. The velocity of the ions is imaged in the plane perpendicular to the surface, which provides a precise measurement of the kinetic trace of reaction products desorbing from the surface. The voltage across the MCP stack is pulsed to select a single mass/charge and to slice out a single plane of the velocity map. The right panel shows images of CO2 products obtained from the oxidation of CO on a Pt(111) (0.5% step density) and Pt(332) (16% step density) surface. The (111) surface gives rise to two different velocity components due to competing reaction mechanisms from terraces and steps. The ion imaging technique allowed the complete reaction mechanism to be unraveled and allowed the reaction rate on the (111) terraces to be measured for the first time.

[1] G. G. Brown, B. C. Dian, K. O. Douglass, S. M. Geyer, S. T. Shipman and B. H. Pate, Rev. Sci. Instrum. 79, 053103, (2008).
[2] M. P. Jacobson and R. W. Field, J. Phys. Chem. A 104, 3073, (2000).
[3] O. Bludský, P. Nachtigall, J. Hrušák, and P. Jensen, Chem. Phys. Lett. 318, 607, (2000).
[4] J. Libuda and H.-J. Freund, Surf. Sci. Rep. 57, 157, (2005).