Physical and Analytical Chemistry Seminar
Lecturer: Bar Cohn
Location: Faculty Seminar Room
Molecular vibrational excitations provide important information on molecular structure and dynamics. Linear vibrational spectroscopy is commonly used focusing on localized vibrational modes as sensitive probes of molecular structure and its environment. Small perturbations can be detected through vibrational frequency shifts and changes in the transition lineshape. However, vibrational excitations are often dominated by inhomogeneous broadening, making the corresponding studies extremely challenging. Two-dimensional ultrafast vibrational spectroscopy (2DIR) meets these challenges. Here, the spectrum is spread in two dimensions, such that the correlation between the excitation and detection frequencies can be detected. This approach separates between the inhomogeneous and homogeneous lineshape components, reveals intra- and inter-molecular coupling, and generally greatly simplifies interpretation of the congested spectra. The waiting time dependence of the vibrational frequency correlations obtained from the series of 2DIR spectra provides direct access to the observables associated with ultrafast dynamics.
Small transition dipole strengths of vibrational excitations and poor sensitivity of detectors in the mid-infrared limit the applications of vibrational spectroscopy to those cases, where large amounts of analyte are available. One example is studies of thin films of molecules on surfaces of nanostructures, which are important building blocks in many nano-technology applications. One way to improve the signal-to-noise ratio in these experiments is by invoking phenomena of surface enhancement by localized plasmons, i.e. enhancement of local electric field by collective excitation of the conduction electrons on surface of metal nanostructures. Here, the nanostructures’ material, size, and shape can be engineered to manipulate the near-fields on the length scale several orders of magnitude below the wavelength of the excitation light.
We explore the 2DIR signal enhancement by localized plasmons, which will allow studies of ultrafast structural dynamics in minute amounts of molecules located within the reach of the enhanced near-fields. The magnitude of 2DIR signal enhancement was found to be up to a five orders of magnitude. Despite characteristic signatures observed in molecular lineshapes that could be interpreted as coupling between plasmonic and molecular excitations, we observed that molecular quantum dynamics have not been altered, as compared to the non-enhanced case. At the same time, among the consequences of signal amplification by plasmon resonances are dispersive lineshapes of the vibrational transitions, competition between the near-field coupling and the radiation damping enhancement mechanisms, and lack of polarization selectivity.