Time-Resolved IR Measurements Technical Considerations

Transient absorption techniques now have a venerable history. The development of flash kinetic spectroscopy was the work of Norrish and Porter (62). This technique typically employed a flash lamp to produce 

It has been long recognized that vibrational spectra often have more easily accessible structural information than similar spectra in the visible or uv regions. This is due to the finite IR spectral shifts associated with even 

A modern variation on the rapid scan spectrometer, which is under development, uses a laser-generated plasma as a high intensity broad-band IR source (65). This method has been used to probe the vc—o absorption of W(CO)6. Another technique TRISP (time-resolved IR spectral photography), which involves up-conversion of IR radiation to the visible, has also been used to probe transients (66). This method has the enormous advantage that efficient phototubes and photodiodes can be used as detectors. However, it is a technically challenging procedure with limitations on the frequency range which depend on the optical material used as an up-converter. 

Both Porter s original flash photolysis apparatus and Pimentel s rapid scan spectrometer recorded the whole spectral region in a time which was short compared to the decay of the transient species. Kinetic information was obtained by repeatedly firing the photolytic flash lamp and making each spectroscopic measurement at a different time delay after each flash. The decay rate could then be extracted from this series of delayed spectra. Such a process clearly has limitations, particularly for IR measurements, where the decay must be slow compared to the scan rate of the spectrum. 

Modern time-resolved IR measurements typically use a different-strategy. For each uv flash, kinetic measurements are made at one IR 

---