Ultrafast Temperature Jump Study of Energy Transfer in Dye-Doped Polymer Films

123 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1993.Ultrafast laser spectroscopy of condensed matter samples containing dyes functioning as molecular optical heaters and optical thermometers is developed. A known amount of heat is deposited in a molecular solid sample and the temperature is accurately measured. This technique, termed ultrafast optical calorimetry, permits the generation and measurement of molecular hot spots of several hundred degrees and bulk temperature jumps exceeding one hundred degrees at a heating rate of $10\sp $ degrees/second.A unique dye, IR165, was found to act as both a heater and a thermometer. The properties of IR165 containing polymers such as stability, absorption coefficient, saturation intensity, and absorption recovery time, under the intense optical pumping conditions required to obtain large and uniform temperature jumps are experimentally determined and discussed.As one of the many potential applications of ultrafast optical calorimetry, nonequilibrium mechanical energy transfer processes, including mechanical energy flow into or out of molecules by vibrational cooling and multiphonon up-pumping, are studied experimentally in poly-(methyl methacrylate) films (PMMA) doped with low concentrations of heater and thermometer dyes. A near-infrared picosecond pulse induces a temperature jump in the sample. The subsequent intra and intermolecular heat flows are monitored via visible hot band absorption spectra of the heater and thermometer dyes, e.g., IR165 and Rhodamine 590 (R6G).The PMMA-IR165 system was used to study the properties of polymers with a sizable temperature jump. The data could not be fit by a model which assumes the polymer heats up in a single stage. A quasitemperature model with two-stage heating gave excellent quantitative agreement. The data at several values of temperature jump were simultaneously fit using three parameters: the molecular thermal conductivity for vibrational cooling of the heater dye; the molecular thermal conductivity for multiphonon up pumping; and the polymer heat capacity. The value of molecular thermal conductivity for vibrational cooling was of the same magnitude as the thermal conductivity of the polymer. The value of molecular thermal conductivity for multiphonon up pumping was two orders of magnitude smaller than the thermal conductivity of the polymer.The PMMA-IR165-R6G system was used to study up pumping of the second dye R6G by phonons generated by IR165. This experiment led to the first observation of the distribution of excess vibrational energy in large molecules in the solid state during multiphonon up pumping. (Abstract shortened by UMI.)U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD