Vibrational Energy Dissipation in Condensed Phases Investigated by Multiple Modes Multiple Dimensional Vibrational Spectroscopy

Abstract

The methodology of ultrafast multiple-mode multiple-dimensional vibrational spectroscopy has been developed and applied to investigate the vibrational energy dissipation in condensed phase. In particular, experiments have been focused on the studies of vibrational energy relaxation and mode-specific vibrational energy transfer in both heterogeneous and homogeneous phases. This thesis presents two distinctive vibrational energy dissipation pathways for molecules absorbed on the typical heterogeneous metal nanoparticle surfaces. On 2-10 nm platinum and palladium nanoparticles, it was found that the electronic excitation-mediated vibrational energy dissipation (~2ps) was at least one order magnitude faster than direct vibration-vibration relaxation (50ps). This electronic energy damping is accompanied by low frequency thermal energy generation on metallic surfaces. This electronic mediated pathway dominates until the electronic property of the particle is altered by reducing size to ~1nm. The energy relaxation pathway also could be altered by changing the chemical nature of the metallic nanoparticle. These findings are of fundamental importance to ultimately understanding the nature of heterogeneous catalysis. This thesis also demonstrates mode-specific vibrational energy exchange between ions in electrolyte solution. (i) Interactions between model molecules representing different building-blocks of proteins and thiocyanate anions in aqueous solutions are studied. The binding affinity between the thiocyanate anions and the charged amino acid residues is about 20 times bigger than that between water molecules and the amino acids, and about 5~10 times larger than that between the anions and neutral backbone amide groups. (ii) Ion segregation was also investigated by mode-specific vibrational energy exchange between thiocyanate anions. In aqueous solutions, it was found that “structure maker” ions, such as F-, would stay in the “water phase” and thereby promote aggregation of the SCN- in an “ionic phase”. “Structure breaker” ions, such as I-, would break the ionic SCN- phase. (iii) Mediated by combination band, vibrational energy flow down from thiocyanate to ammonium was used to confirm that ion pair is formed between ammonium and thiocyanate in aqueous solutions. Investigations of these microscopic structures and dynamics of aqueous salt solutions experiments will add depth to our understanding of general macroscopic properties of electrolyte solutions.