Time-resolved IR and Raman spectroscopy

In conclusion, the global analysis of spectral data is a very useful tool to validate a proposed model, when used with proper understanding and caution. However, statistical analysis in general is not able to eliminate any systematic errors that may be hidden in experimental data. On the contrary, it will emphasize any such deviations from a chosen model and might thereby insinuate false complexity of the system investigated. No mathematical treatment can ever make up for less than optimal methods of data collection. 

In picosecond time-resolved Raman spectroscopy, the sample is pumped and probed by energetically well-defined optical pulses, producing a full vibrational spectrum over a 1000 2000 cm 1 window.207 One would expect vibrational spectroscopy to be restricted to the picosecond time domain and above by the Heisenberg uncertainty principle (Equation 2.1), because a 1 ps transform-limited pulse has an energy width of 

To obtain IR spectra on a time scale of nanoseconds, the sample cell in conventional spectrometers is usually excited by an Nd YAG laser. Flow cells with a pathlength of at least 0.1 mm must be used for photoreactive samples and the pulse repetition frequency is then limited to 1 Hz. In step scan FTIR spectroscopy,211 the time evolution is collected at single points of the interferogram, which is then reconstructed point-by-point and subsequently transformed to time-resolved IR spectra. Alternatively, dispersive instruments equipped with a strong IR source can be used.212 The time resolution of both methods is about 50 ns. FTIR instruments provide a triggerable fast-scan mode to collect a complete spectrum within a few milliseconds.213 

Quantum yields are fundamental quantities that define the photonic economics of processes induced by light absorption. They are required to determine rate constants of photophysical and photochemical processes (Section 3.9.7). Many different techniques are used to measure quantum yields depending on the process studied. In the following, we describe some procedures commonly used in the chemical laboratory. The measurement of quantum yields is an art that has a number of pitfalls. The experimenter has few options to double-check his or her own results other than reproducibility, which will not reveal any repeated systematic errors. Therefore, it is prudent to reproduce the quantum yield of a related, well-known process in the laboratory before determining an unknown one. 

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Among approaches in vibrational spectroscopy are differential and time-resolved IR and Raman spectroscopy, coherent anti-Stokes Raman scattering (CARS), Fourier transform infrared spectroscopy (FT-IR) multidimensional IR and RR spectroscopy, two-dimensional infrared echo and Raman echo [56], and ultrafast time-resolved spontaneous and coherent Raman spectroscopy the structure and dynamics of photogenerated transient spedes [50, 57]. 

Fluorescence spectroscopy is also useful in detecting the excimer or the exciplex state associated with the molecular motion. Time-resolved IR and Raman spectroscopies, which give useful information on vibration of the intermediate, have also been employed to these smdies. The time resolution of these vibration spectroscopies has been enhanced to the femtosecond domain recently. The enhancement in the Raman signal in the presence of resonance and the local electronic field is very useful in detecting the intermediate in each environment. Time-resolved electron spin resonance (ESR) is also a useful technique, which gives information on the spin of the intermediate even when detection by other methods is difficult. The weak point of the time-resolved ESR is usually the time resolution. The above methods have been mainly employed in the investigations on the topics summarized in this chapter. 

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