Surface-Sensitive and Surface-Specific Ultrafast Two-Dimensional Vibrational Spectroscopy

Abstract

Ultrafast two-dimensional infrared spectroscopy (2D IR) has been advanced in recent years toward measuring signals from only a monolayer of sample molecules at solid-liquid and solid-gas interfaces. A series of experimental methods has been introduced, which in the chronological order of development are 2D sum-frequency-generation (2D SFG), transmission 2D IR, and reflection 2D IR, the latter in either internal, attenuated total reflection (ATR), or external reflection configuration. The different variants of 2D vibrational spectroscopy are based on either the even-order or the odd-order nonlinear susceptibility, and all allow resolving similar molecular temporal and spectral information. In this review, we introduce the basic principles of the different methods of 2D vibrational spectroscopy at surfaces along with a balanced overview on the technological aspects as well as benefits and shortcomings. We furthermore discuss the current scope of applications for 2D vibrational surface spectroscopy, which spans an impressively broad range of samples from biological molecules to heterogeneous catalysts. The emphasis is on the ultrafast structural dynamics of molecules at interfaces, environmental interactions, and intermolecular interactions. We furthermore consider important recent technological developments of 2D vibrational surface spectroscopy, which employ (i) surface enhancement, (ii) methods for studying electrochemical interfaces, and (iii) extensions for resolving nonequilibrium processes (transient 2D IR). A detailed outlook is finally given regarding important future applications and technological developments of 2D vibrational surface spectroscopy.

Structure from Dynamics: Vibrational Dynamics of Interfacial Water as a Probe of Aqueous Heterogeneity

“…A comprehensive review on time-resolved SFG can be found in the literature 27 as well as a review on 2D vibrational spectroscopies at surfaces. 28 Note that, additionally, there exists a wealth of literature reporting studies using bulk-sensitive techniques of the dynamics of confined, non-bulk-like water in reverse micelles (see, e.g., refs (29−32)).…”

Molecule-specific interactions of diatomic adsorbates at metal-liquid interfaces

“…By correlating vibrational signals of an excitation and a detection frequency, the coupling between oscillators 20 and the dynamics of energy transfer 21 or chemical exchange 22 have been resolved with sub-picosecond temporal resolution. 2D IR has been technologically advanced in recent years to also enable investigations of only monolayers of sample molecules at surfaces and interfaces 16,23–28 . Particularly for adsorbates, the observation of different types of interactions is extremely valuable, since the associated dynamics may be used to extract information about molecular conformations and intermolecular distances under the influence of spatial confinement 19 …”

“…Here, we employ 2D attenuated total reflectance (ATR) IR spectroscopy 16,31,34 in combination with isotope-labelling of adsorbates at metal-liquid interfaces to investigate intermolecular interactions between diatomic molecules as well as substrate-adsorbate interactions. For this purpose, we employ 12 C 16 O/ 13 C 18 O mixtures of carbon monoxide and 12 C 14 N − / 13 C 15 N − mixtures of cyanide adsorbed to a thin Platinum (Pt) layer.…”

Infrared Spectroscopy as Molecular Probe of the Macroscopic Metal-Liquid Interface

“…Second-order (or even-order, in general) nonlinear effects are advantageous for studying surfaces and interfaces [11]. The even-order susceptibility is zero in the bulk of a fluid, and hence only the molecules in the optically anisotropic interfacial layer contribute to the signal, in particular when the molecules at the interface are oriented in a certain manner.…”

“…In other words, the signal of even-order methods is highly surface-specific, while methods utilizing odd-order effects like infrared absorption and Raman scattering may be biased by signals from the bulk. Second-order vibrational spectroscopy can, for instance, be performed in terms of sum-frequency generation (SFG) [11,12]. However, when the IR radiation must travel through a highly absorbing medium like water, the resulting signal levels may be low.…”