Surface Enhancement in Ultrafast 2D ATR IR Spectroscopy at the Metal-Liquid Interface

We investigate surface enhancement in two-dimensional attenuated total reflectance infrared (2D ATR IR) spectroscopy from organic monolayers (MLs) at metal–liquid interfaces. We consider MLs from both aromatic and aliphatic organic samples equipped with nitrile and azide functional groups, both of which are widely used as local vibrational probes in ultrafast spectroscopy. Polarization-dependent 2D ATR IR spectroscopy indicates the excitation of local hot spots formed between gold (Au) nanoparticles as the dominant origin of signal enhancement. The highest enhancement factors (∼50) are observed in the case of aromatic nitrile MLs, whereas modest values (<10) are found for aliphatic azide and nitrile groups. Different contributions to signal enhancement are evaluated systematically and indicate the presence of both electromagnetic enhancement and contributions from molecular properties. The obtained enhancement factors are promising to allow 2D ATR IR spectroscopy to become applicable as a versatile technique for the detection of ultrafast structural dynamics in even low-absorbing organic MLs at solid–liquid interfaces.