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Impulsive stimulated thermal scattering (ISTS) allows one to access the structural relaxation dynamics in supercooled molecular liquids on a time scale ranging from nanoseconds to milliseconds. Till now, a heuristic semi-empirical model has been commonly adopted to account for the ISTS signals. This model implicitly assumes that the relaxation of specific heat, C, and thermal expansion coefficient, γ, occur on the same time scale and accounts for them via a single stretched exponential. This work proposes two models that assume disentangled relaxations, respectively, based on the Debye and Havriliak-Negami assumptions for the relaxation spectrum and explicitly accounting for the relaxation of C and γ separately in the ISTS response. A theoretical analysis was conducted to test and compare the disentangled relaxation models against the stretched exponential. The former models were applied to rationalize the experimental ISTS signals acquired on supercooled glycerol. This allows us to simultaneously retrieve the frequency-dependent specific heat and thermal expansion up to the sub-100 MHz frequency range and further to compare the fragility and time scale probed by thermal, mechanical, and dielectric susceptibilities. ispartof: JOURNAL OF CHEMICAL PHYSICS vol:155 issue:16 ispartof: location:United States status: published

This PhD-project deals with the development and experimental validation of an all-optical thermometry method that enables to detect remotely and very high temporal resolution (10 nanoseconds, 100MHz bandwidth). The technique was used to tackle the challenge to investigate the relaxation behavior of the heat capacity of the glassformer glycerol, and thus supply information that can be compared with the relaxation features of other, classically investigated physical properties of glassformes such as the dielectric constant and the mechanical properties. The developed method is exploiting the temperature dependence and rapid response time of the fluorescence characteristics of a probe material. In the investigated application, a transient temperature jump was induced by irradiating the sample with a nanosecond laser pulse, whose energy was optically absorbed and converted into heat. Also a configuration in which the sample was periodically heated by optical excitation was implemented. Contents Acknowledgements Abstract Nederlandstalig abstract Abbreviations/Symbols Chapter 1 Introduction 1.1 Background and state of the art 1.2 Motivation and research objectives 1.3 Outline of the dissertation References Chapter 2 Temperature dependent fluorescence 2.1 Calibration setup for investigation of temperature dependent fluorescence 2.2 Experimental results and discussion 2.3 Conclusion References Chapter 3 Temperature retrieval by neural network (NN)recognition 3.1 Introduction 3.2 NN implementation of fluorescence-based thermometry 30 3.3 Performance of temperature reconstruction by NN 3.4 Conclusion References Chapter 4 Development of ultrafast fluorescence based thermometry 4.1 Introduction 4.2 CW probe-pulsed pump implementation 4.3 Pulsed probe-pulsed pump implementation 4.4 Conclusion References Chapter 5 Spatially resolved photothermal fluorescence spectroscopy in frequency domain 5.1 Experimental setup 5.2 Results and discussion 5.3 Conclusion References Chapter 6 Lock-in photothermal photoluminescence spectroscopy in frequency domain 6.1 Introduction 6.2 Temperature dependence of the QD photoluminescence 6.3 Lock-in detection of photothermal photoluminescence signals 6.4 Experimental results and discussion 6.5 Conclusion References Chapter 7 Temperature dependent time-resolved photothermal spectroscopy of glycerol 7.1 Introduction • Relaxation dynamics in supercooling systems • Relaxation of specific heat capacity 7.2 Time-resolved thermal lens spectroscopy of supercooled glycerol • Experimental setup • Results and discussion 7.3 Photothermal fluorescence spectroscopy of supercooled glycerol • Simulation of C effect on the impulsive temperature response • Experiment results and discussion 7.4 Conclusion References Chapter 8 General conclusion and perspective 8.1 Conclusion 8.2 Perspective Curriculum Vitae List of publications nrpages: 149 status: published