Marc Smits

Ultrafast two dimensional-infrared spectroscopy of a molecular monolayer.

The study of vibrational coupling and energy flow in bulk (bio)molecular systems using two-dimensional infrared spectroscopy, has dramatically broadened our ability to elucidate structures and their dynamic evolution on ultrafast timescales. For molecules at surfaces, however, these insights have been lacking. In our study, vibrational coupling in a molecular monolayer is revealed by ultrafast two-dimensional vibrational spectroscopy, with interface specificity and (sub)monolayer sensitivity. This technique provides information on vibrational coupling and energy transfer at surfaces and interfaces with subpicosecond time resolution rendering it a unique tool for the investigation of both structural and dynamical surface processes in a wide variety of disciplines.

Ultrafast energy flow in model biological membranes

We report on the energy flow dynamics in model membranes, investigated by surface-specific time-resolved (femtosecond) sum frequency generation spectroscopy. This recently developed technique allows us to probe energy dynamics selectively at the water/lipid interface. We report vibrational relaxation dynamics of C–H stretch modes in the lipid alkyl chains, and reveal that incoherent energy transfer occurs from the excited CH2 groups to the terminal CH3 groups. We also find evidence for strong anharmonic coupling between different CH2 and CH3 modes. Relaxation and the energy transfer processes within the lipid alkyl chain occur on (sub-)picosecond timescales. Studies of the dynamics on different lipid phases (gel or liquid crystalline phase) reveal a marked independence of the dynamics on the precise molecular conformation of the lipids. In addition, we report the energy transfer dynamics between membrane-bound water and lipids, and find that the transfer of heat between water and lipids occurs remarkably fast: heat is transferred across the monolayer, from the polar head group region of the lipid to the end of the alkyl chain, within 1 ps. These results demonstrate the potential of using ultrafast surface-specific spectroscopies to elucidate biomolecular dynamics at membrane surfaces.

Femtosecond time-resolved and two-dimensional vibrational sum frequency spectroscopic instrumentation to study structural dynamics at interfaces.

We present a novel setup to elucidate the dynamics of interfacial molecules specifically, using surface-selective femtosecond vibrational spectroscopy. The approach relies on a fourth-order nonlinear optical interaction at the interface. In the experiments, interfacial molecules are vibrationally excited by an intense, tunable femtosecond midinfrared (2500-3800 cm(-1)) pump pulse, resonant with the molecular vibrations. The effect of the excitation and the subsequent relaxation to the equilibrium state are probed using broadband infrared+visible sum frequency generation (SFG) light, which provides the transient vibrational spectrum of interfacial molecules specifically. This IR pump-SFG probe setup has the ability to measure both vibrational population lifetimes as well as the vibrational coupling between different chemical moieties at interfaces. Vibrational lifetimes of interfacial molecules are determined in one-dimensional pump-SFG probe experiments, in which the response is monitored as a function of the delay between the pump and probe pulses. Vibrational coupling between molecular groups is determined in two-dimensional pump-SFG probe experiments, which monitor the response as a function of pump and probe frequencies at a fixed delay time. To allow for one setup to perform these multifaceted experiments, we have implemented several instrumentation techniques described here. The detection of the spectrally resolved differential SFG signal using a combination of a charge-coupled device camera and a piezocontrolled optical scanner, computer-controlled Fabry-Perot etalons to shape and scan the IR pump pulse and the automated sample dispenser and sample trough height corrector are some of the novelties in this setup.

Ultrafast vibrational dynamics of interfacial water

Abstract We report investigations of the vibrational dynamics of water molecules at the water–air and at the water–lipid interface. Following vibrational excitation with an intense femtosecond infrared pulse resonant with the O–H stretch vibration of water, we follow the subsequent relaxation processes using the surface-specific spectroscopic technique of sum frequency generation. This allows us to selectively follow the vibrational relaxation of the approximately one monolayer of water molecules at the interface. Although the surface vibrational spectra of water at the interface with air and lipids are very similar, we find dramatic variations in both the rates and mechanisms of vibrational relaxation. For water at the water–air interface, very rapid exchange of vibrational energy occurs with water molecules in the bulk, and this intermolecular energy transfer process dominates the response. For membrane-bound water at the lipid interface, intermolecular energy transfer is suppressed, and intramolecular relaxation dominates. The difference in relaxation mechanism can be understood from differences in the local environments experienced by the interfacial water molecules in the two different systems.

Interfacial Self-Assembly and Oriented Attachment in the Family of PbX (X = S, Se, Te) Nanocrystals

The realization of materials with new optoelectronic properties draws much scientific attention toward the field of nanocrystal superstructures. Low-dimensional superstructures created by interfacial assembly and oriented attachment of PbSe nanocrystals are a striking example because theory showed that PbSe sheets with a honeycomb geometry possess non-trivial flat bands and Dirac cones in the valence and conduction bands. Here, we report on the formation of one-dimensional linear and zigzag structures and two-dimensional (2D) square and honeycomb structures for the entire lead chalcogenide family: PbX (X = S, Se, Te). We observe that PbTe, with a lower bulk melting temperature and enthalpy of formation than those of PbSe, shows a higher nanocrystal surface reactivity, such that the surface must be passivated and the reaction conditions moderated to obtain reasonably ordered superstructures. The present findings constitute a step forward in the realization of a larger family of atomically coherent 2D superstructures with variable IV–VI and II–VI compositions and with electronic properties dictated by the nanogeometry.

Ultrafast 2D-IR spectroscopy of a molecular monolayer

We report on ultrafast 2-dimensional vibrational surface spectroscopy, providing information on coupling and energy transfer between vibrations of surface molecules. As a 4th order technique, it is bulk-forbidden in centrosymmetric materials and hence surface specific.

2-Dimensional Non-Linear Surface Vibrational Spectroscopy

We present 2-dimensional infrared spectra in the C - H stretch region of a self-assembled monolayer of dodecanol on water. This novel surface-specific, fourth-order non-linear spectroscopic tool provides detailed information on coupling between vibrational modes at surfaces.