Ultrafast, Multidimensional Attenuated Total Reflectance Spectroscopy of Adsorbates at Metal Surfaces

Abstract

Ultrafast dynamics of molecules at solid-liquid interfaces are of outstanding importance in chemistry and physics due to their involvement in processes of heterogeneous catalysis. We present a new spectroscopic approach to resolve coherent, time-resolved, two-dimensional (2D) vibrational spectra as well as ultrafast vibrational relaxation dynamics of molecules adsorbed on metallic thin films in contact with liquids. The setup is based on the technique of Attenuated Total Reflectance (ATR) spectroscopy which is used at interfaces between materials that exhibit different refractive indices. As a sample molecule we consider carbon monoxide adsorbed in different binding configurations on different metals and resolve its femtosecond vibrational dynamics. It is presented that mid-infrared, multi-dimensional ATR spectroscopy allows for obtaining a surface-sensitive characterization of adsorbates' vibrational relaxation, spectral diffusion dynamics and simple inhomogeneity on the femtosecond timescale. ABSTRACTUltrafast dynamics of molecules at solid-liquid interfaces are of outstanding importance in

2D attenuated total reflectance infrared spectroscopy reveals ultrafast vibrational dynamics of organic monolayers at metal-liquid interfaces

“…25 In brief, a chirped-pulse amplifier system (Spectra Physics, Spitfire), which delivers ≈90 fs pulses centered at 800 nm, is operated at 5 kHz to pump a two-stage optical parametric amplifier (OPA). 35 The OPA delivers ≈90-100 fs pulses with a bandwidth (full-width at half maximum (FWHM)) of ≈250 cm −1 and a total of ≈1.3 µJ per pulse at typical central frequencies around 2100 cm −1 .…”

“…be a useful tool to unravel changes in molecular structure and dynamics, which occur down to the femtosecond time scale. [16][17][18][19] In particular, multi-dimensional techniques 17,[24][25][26][27] have been developed in recent years as powerful methods to tackle the issues mentioned above. These techniques are sensitive to, e.g., structural heterogeneity and flexibility, energy transfer, or intermolecular interactions, signatures of which are challenging to extract unambiguously from conventional (1D) vibrational spectra.…”

“…28,29 We recently developed a new method to acquire twodimensional (2D) IR spectra from molecules at interfaces by means of ultrafast attenuated total reflectance (ATR) IR spectroscopy. 25 The 2D ATR IR method is a straight-forward to implement technique, which exploits the evanescent electric field that occurs under total reflectance conditions 30,31 if light impinges on an interface that is composed of materials with different refractive indices (n 1/2 , Fig. 1).…”

Surface-enhanced, multi-dimensional attenuated total reflectance spectroscopy

“…18,19 . ATR spectroscopy is based on the reflecting properties of interfaces between materials of different refractive indices when certain experimental conditions are fulfilled.…”

“…18,19 Here, we explore the capability of the method further by providing evidence that 2D ATR IR spectroscopy can be used to study ultrafast dynamics of samples with intrinsically low IR extinction coefficients (< 200 M -1 cm -1 ) at substantial sub-ML coverage. In particular, we study MLs and mixed MLs (MMLs) of organic nitrile (CN) stretch vibrations covalently attached to thin films of gold (Au).…”