Time-resolved fluorescence spectroscopy excitation sources

The instrumentation for time-resolved fluorescence spectroscopy shares some similarities with that used for steady-state measurements, in the sense that an excitation source and a detection systems are used. The nature of these components may, and usually does, vary significantly. Namely, a pulsed light source is used in time-resolved spectroscopy, which is at variance with the continuous light source used in steady-state fluorescence. 

In the previous sections, it has been shown how powerful the time-resolved fluorescence techniques are in real time probing of photoinduced processes and in allowing the determination of reaction rates from fluorescence lifetimes. The present section is devoted to the method of UV/vis transient absorption spectroscopy, which is a key method in probing non emissive species and is thus crucial to detect photoreaction products or intermediates following optical excitation of molecules in their electronic excited states. When carried out on short time scales, i.e. with femtosecond to subnanosecond excitation sources, fluorescent species can also be detected by their stimulated emission. Combining time-resolved fluorometry and transient absorption spectroscopy is ideal for the study of photochemical and photophysical molecular processes. 

Experimentally, commercial steady-state fluorescence spectrometers can be equipped with polarizer attachments, either sheet or Glan-Thompson polarizers. Alternatively, sheet polarizers are usually easily incorporated into the sample cavity in the excitation and emission pathways. Likewise, for time-resolved spectroscopy, polarizers may simply be introduced into the excitation and detection paths. Frequently, the excitation source in time-resolved experiments is a laser which will be inherently polarized. 

The absorption and fluorescence measurements of molecular gases with high spectral resolution by use of CW lasers or time-resolved spectroscopy with pulsed laser sources that enable the determination of the life time of short-lived excited molecular levels and radicals. 

A very widely employed method for the measurement of spin-orbit state-specific rate constants is the time-resolved measurement of the concentrations of individual atomic levels after formation of these species from a suitable precursor, either by flash photolysis [13], or, more recently, by laser photodissociation. The concentrations of the various atomic reactant states are monitored by atomic absorption or fluorescence spectroscopy using atomic emission sources [14], or, for spin-orbit-excited states, by observation of the spontaneous infrared emission [15-18]. Recently, Leone and co-workers have utilized gain/absoiption of a colour centre and diode infrared laser to probe the relative populations of ground and spin-orbit excited halogen atoms produced in a chemical reaction [19] and also by photodissociation [20],... 

From a scientific perspective, absorption and fluorescence spectra are a source of fundamental information in many chemical-physical molecular features [1-3]. First, they are two of the most straightforward methods of identifying a compound and thus represent key analytical techniques. Furthermore, advances in pump-probe time-resolved experiments have made absorption and fluorescence spectroscopy ideal tools to monitor the time evolution and outcome of many reactive processes [4]. These techniques can also provide indications of the electronic stmcture of molecules, their ground- and excited-state geometry, and the most relevant vibrational features, just to name a few basic properties. 

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