Two-dimensional-vibrational spectroscopy: Development and testing of a two-dimensional ultrafast Raman spectrometer with Time-Frequency Detection.

摘要

The major emphasis of this dissertation will be given toward the theoretical tools necessary to acquire high resolution femtosecond Raman spectra from broadband femtosecond pulses. The theory of simultaneous Time-Frequency Detection (TFD) will be discussed and demonstrated to be a robust technique to acquire the vibrational coherence information. Finally, two experimental cases that demonstrate the feasibility of femtosecond TFD-CARS for acetonitrile and nitrobenzene will be presented.;In the introductory first chapter, the motivation and fundamentals for developing 2D-vibrational spectroscopy using femtosecond Raman detection is presented. For coherent Raman spectroscopies, common femtosecond pulses often lie in an intermediate regime: their bandwidth is too wide for measurements in the frequency domain, but their temporal width is too broad for homodyne measurements in the time domain. A recent paper [Phys. Rev. Lett. 97 , 267401 (2006)] showed that complete Raman spectra can be recovered from intermediate length pulses by using simultaneous time and frequency detection (TFD). Heterodyne detection and a phase-stable local oscillator at the anti-Stokes frequency are not needed with TFD. Phase-control, pulse shaping or pulses of widely differing duration are not required. To demonstrate the TFD method, a high resolution Raman spectrum of nitrobenzene obtained from 60 fs pulses is discussed theoretically and experimentally in the second chapter.;In the third chapter model calculations illustrate how information on the Raman spectrum is smoothly transferred from the frequency domain to the time domain as the pulse width shortens. When data is collected in both dimensions, the Raman spectrum is completely determined to high resolution, regardless of the probe pulse width.;The TFD method is tested on experimental CARS data from acetonitrile in the fourth chapter. Compared to theoretical models, experimental data are complicated by noise and incomplete knowledge of the pulse structure. Despite these complications, most of the information in the Raman spectrum is recovered from the data; weak transitions are detected and natural-linewidth resolution is achieved across an 800 cm-1 spectral range. However, circumstances in which experimental limitations result in missed features or ambiguities in the recovered spectrum are also identified. These results suggest where improvements in measurement and data analysis can be made.