Line shapes and signal enhancement in 2DIR spectroscopy of molecules under plasmonic fields

Plasmonic nanostructures made of noble metals are frequently used to manipulate electric fields on the nanoscale. When molecules are located close to the surface of the nanostructure, the interaction with the local fields induced at plasmon resonance frequencies can lead to various fascinating phenomena ranging from the enhancement of spectroscopic signals to the hybridization of molecular quantum states with the plasmon-induced electric field. Two-dimensional infrared (2DIR) spectroscopy provides crucial insights into molecular structure and ultrafast dynamics and nowadays is being extended to studies of molecules confined to nanostructure topography and subject to plasmonic fields. Two aspects most important for the corresponding studies are the mechanism (and magnitude) of signal enhancement and the origin of spectroscopic line shapes. Studies of bulk samples by 2DIR frequently rely on elimination of the dispersive components of the 2D signal. The interaction of narrow band molecular vibrational transitions with broad band plasmon resonance leads to the lineshape asymmetry, known as Fano resonance. We systematically study these dispersive line shapes in a series of 2DIR spectra of a thin polymer layer deposited on plasmonic structures, nearly resonant with the carbonyl stretching vibration of the polymer molecules. The mechanism of signal enhancement is studied in a series of 2DIR experiments, where the polymer thickness (and the number of vibrational chromophores interacting with the plasmonic field) is systematically varied. The outcome of both series of experiments are rationalized by the combination of numerical electromagnetic simulations with the physically-intuitive model based on coupled oscillators. Our results can be particularly helpful in rational design of plasmonic substrates for 2DIR spectroscopy and studies of molecular dynamics and structure in nano-confined environment.