Alexei Lagutchev

Vibrational sum frequency generation studies of the (2×2)→(√19×√19) phase transition of CO on Pt(111) electrodes

The potential-dependent (2x2)-3CO-->(radical19x radical19)R23.4 degrees-13CO adlayer phase transition on Pt(111) with 0.1M H(2)SO(4) electrolyte was studied using femtosecond broadband multiplex sum frequency generation (SFG) spectroscopy combined with linear scan voltammetry. Across the phase boundary the SFG atop intensity jumps, and at the same time the SFG spectrum of threefold CO sites is transformed into a bridge site spectrum with a small decrease in integrated SFG intensity. The SFG atop intensity jump and three fold-to-bridge intensity drop are noticeably different from what would be expected for these structures on the basis of coverage alone. This occurs because the SFG signal is sensitive to both the coverage and changes in the local field that result from a changing adlayer structure. We derive an equation that allows us to correct the SFG intensities for these effects using information derived from infrared absorption-reflection spectroscopy (IRAS) and second-harmonic generation (SHG) measurements. With this correction, the SFG results agree well with what would be expected for a transition between perfect adlattices. A small (approximately 20%) discrepancy in the SFG determination of atop coverage is attributed to either a small amount of surface disorder or uncertainties in the SFG, SHG, and IRAS measurements. SFG is also used to examine the reversibility hysteresis and kinetics of the phase transition and its dependence on electrolyte composition. The phase transition is reversible with an approximately 150 mV anodic overpotential and the forward (2x2)-->(radical19x radical19) transition is slower than the reverse. Repeated cycles of phase transition indicate that the 25 microm electrolyte layer used here does not appreciably distort the potential-coverage relationships.

Compact broadband vibrational sum-frequency generation spectrometer with nonresonant suppression

A compact broadband vibrational sum-frequency spectroscopy (SFG) apparatus is described to study molecules at surfaces and interfaces. Using an étalon as the frequency narrowing device, the visible pulse has a time-asymmetric profile that allows the user to deeply suppress nonresonant background signals that hinder detection of molecular vibrational resonances. Several features of the spectrometer that, in aggregate, improve signal-to-noise ratios by a large factor are described. The spectrometer features a series of interchangeable prealigned sample holders for different applications. Examples of applications are presented where nonresonant suppression greatly improves the ability to study adsorbates on single-crystal surfaces as a function of rotation about the azimuth, and where the rapid data acquisition abilities of the spectrometer are used to study electrochemical transformations on single-crystal electrodes.

Vibrational energy in molecules probed with high time and space resolution

This article reviews experimental measurements of vibrational energy in condensed-phase molecules that simultaneously provide time resolution of picoseconds and spatial resolution of ångströms. In these measurements, ultrashort light pulses are used to input vibrational energy and probe dynamical processes. High spatial resolution is obtained using vibrational reporter groups in known locations on the molecules. Three examples are discussed in detail: (1) vibrational energy flow across molecules in a liquid from an OH–group to a CH3–group; (2) vibrational energy flow across a molecular surfactant monolayer that separates an aqueous and a non-polar phase in a suspension of reverse micelles; and (3) vibrational energy input by laser-driven shock waves to a self-assembled monolayer of long-chain alkane molecules. These experiments provide new insights into the movement of mechanical energy over short length and time scales where ordinary concepts of heat conduction no longer apply, where the concepts of quantum mechanical energy transfer reign supreme.

Sum-Frequency Spectroscopy of Molecular Adsorbates on Low-Index Ag Surfaces: Effects of Azimuthal Rotation

Vibrational sum-frequency generation spectroscopy (SFG) lineshapes of p-cyanobenzenethiol on low-index Ag crystal surfaces are studied as a function of azimuthal rotation by angle phi. A broadband multiplex SFG method is used, with a new technique that variably suppresses the non-resonant (NR) background using time-asymmetric time-delayed picosecond laser pulses. When both resonant (R) and NR signals are present, the amplitude and phase of the R line shape can vary significantly with phi, leading to dramatic phi-dependent variations of the SFG spectrum. The CN-stretch transition of p-cyanobenzenethiol modified Ag(111) and Ag(110) surfaces has an SFG spectrum consisting of a single vibrational resonance R atop a NR background that originates from the metal surface. Using the NR suppression technique, it was found that the R amplitude of the CN-stretch was independent of phi on both Ag(111) and Ag(110), which proves that the CN dipole moment is parallel to the surface normal in both cases. We show that it is possible to accurately extract the phi-dependence of the R amplitude, the NR amplitude, and the phase difference from SFG spectra by suppressing the NR signal during sample roation, thereby proving that the R contribution from the CN-stretch transition evidence phi-invariant behavior on both Ag surfaces.

Shock compression spectroscopy with high time and space resolution

Femtosecond laser‐driven shock compression experiments are described using nonlinear coherent vibrational sum‐frequency generation spectroscopy (SFG) probing of molecular materials. SFG selectively monitors molecular groups at surfaces and interfaces, providing a high degree of spatial resolution. In initial experiments a self‐assembled monolayer of long‐chain alkane molecules is studied, where SFG sees only the methyl (−CH3) head groups. The plane of methyl groups is just 1.5A thick. Shock‐induced bending of the chain and shock‐induced rotations around carbon‐carbon bonds are observed. Possible future directions are discussed.

Shock Compression of Molecules with 1.5 Angstrom Resolution

The passage of a steep shock front across a plane of methyl (−CH3) groups approximately 1.5A thick is monitored by 200 fs vibrational spectroscopy. The methyl groups are the terminal groups of an ordered monolayer of long‐chain molecules bonded to a gold surface. When the shock front arrives, it compresses the chains, which become disordered by rotation around C–C bonds within 4 ps. Molecular dynamics of the methyl groups can be seen during the subsequent ∼500 ps reordering phase.

Nitro stretch probing of a single molecular layer to monitor shock compression with picosecond time resolution

Ultrafast shock compression vibrational spectroscopy experiments with molecular monolayers provide atomic-scale time and space resolution, which enables critical testing of reactive molecular simulations. Since the origination of this project, we have greatly improved the ability to detect shocked monolayers by nonlinear coherent vibrational spectroscopy with nonresonant suppression. In this study, we show new results on a nitroaromatic monolayer, where the nitro symmetric stretch is probed. A small frequency blue-shift under shock conditions compared to measurements with static high pressure shows the shock is ~1 GPa. The ability to flash-preheat the monolayer by several hundred K is demonstrated. In order to observe shock monolayer chemistry in real time, along with pre-heating, the shock pressure needs to be increased and methods to do so are described.

Ultrafast shock wave coherent dissociation and spectroscopy of materials

We present the possibility of using femtosecond laser‐generated shock waves to effect a new kind of material transformation, a coherent dissociation of a material into two slabs. Coherent dissociation is fundamentally different from fracture. Every atom at the interface would react synchronously, which would greatly enhance our ability to understand fundamental mechanisms and to identify and characterize transition states.

ULTRAFAST VIBRATIONAL SPECTROSCOPY OF SHOCK COMPRESSION WITH MOLECULAR RESOLUTION

Ultrafast shock compression and heat shock experiments using molecular monolayers provide atomic‐scale time and space resolution. Heat shock refers to flash‐heating a metal substrate by 600–800 K causing heat to flow into different probed functionalities of adsorbed monolayer molecules. We have developed methods to greatly improve monolayer spectra using vibrational sum‐frequency generation with nonresonant suppression. Monolayer engineering with atomic precision is used to study shock‐molecule interactions in energetic material simulants. Monolayer vibrational spectra at high pressures are obtained using diamond‐anvil cells with photonic substrates.