The vibrational dynamics of Pt-H on a nanostructured platinum surface has been examined by ultrafast infrared spectroscopy. Three bands are observed at 1800, 2000, and 2090 cm, which are assigned to Pt-CO in a bridged and linear configuration and Pt-H, respectively. Lifetime analysis revealed a time constant of (0.8 ± 0.1) ps for the Pt-H mode, considerably shorter than that of Pt-CO because of its stronger coupling to the metal substrate. Two-dimensional attenuated total reflection infrared spectroscopy provided additional evidence for the assignment based on the anharmonic shift, which is large in the case of Pt-H (90 cm), in agreement with the density functional theory calculations. The absorption cross section of Pt-H is smaller than that of the very strong Pt-CO vibration by only a modest factor of ∼1.5-3. Because Pt-H is transiently involved in catalytic water splitting on Pt, the present spectroscopic characterization paves the way for in-operando kinetic studies of such reactions.

The asymmetry of Fano line shapes observed for metal-adsorbate systems is reflected in twodimensional infrared (2D IR) spectroscopy as a distorted spectrum. A phenomenological correction scheme is proposed that transforms distorted 2D IR spectra into conventional spectra. To that end, a phase correction factor is first derived from the IR absorption spectrum of the sample by symmetrizing the asymmetric line shape and subsequently applied to the distorted 2D IR spectra. The concept is illustrated for a model system consisting of an organic molecule (p-mercaptobenzonitrile) adsorbed on a sputter-coated metal layer (Au). The correction scheme reveals conventional, easily interpretable 2D IR spectra.

The surface-enhanced counterparts of Raman scattering (SERS) and infrared (IR) absorption (SEIRAS) are commonly used to probe and identify nanoscale matter and small populations of molecules. The contrasting selection rules offer complementary vibrational information of bulk solids or solutions. In this study, a complementary surface-enhanced vibrational spectroscopy approach is presented to probe the vibrational signature of metal-bound molecular monolayers. Nanocavities are designed and produced with sharp and tunable visible (VIS) and mid-IR gap resonances by placing nanorods on a mirror that is coated with a thin dielectric spacer. Their VIS resonances are tuned to match a 1.61 eV (770 nm) resonant excitation for SERS, while their mid-IR resonances span the 1500-2800 cm −1 range (6.5-3.5 µm) in high resolution for SEIRAS, targeting CN bond vibrations at 2220 cm −1 . Both the VIS and mid-IR gap modes support spatially overlapping and highly enhanced near-fields ensuring strong SERS and SEIRAS signals from the same monolayer molecular population. The differences in the vibrational information obtained with the two surface-enhanced spectroscopies when probing coupled molecular vibrations are highlighted and the advantages of using such a platform for investigating cavity-modified chemical reactions are discussed.

Resonant and off-resonant mid-infrared pump-probe spectroscopy is used to measure the vibrational dynamics of CO adsorbed to thin (0.2 nm, 2 nm, and 10 nm) heterogeneous Pt layers in an aqueous solution. The transient signals observed with resonant pumping are dominated by vibrational relaxation of the CO internal stretch vibration with a lifetime of T1 3 ps in all cases. Off-resonant pumping suppresses that contribution to the signal and singles out a signal, which is attributed to heating of the metal layer as well as transient desorption of the CO molecules. Due to the small photon energy (0.2 eV) used as pump pulses, the mechanism of desorption must be thermal, in which case the desorption yield depends exclusively on the fluence of absorbed light and not its wavelength. The thin Pt layers facilitate CO desorption, despite a relatively low pump pulse fluence, as they concentrate the absorbed energy in a small volume.