Ultrafast TRIR methods

Although very detailed, fundamental information is available from ultrafast TRIR methods, significant expertise in femtosecond/picosecond spectroscopy is required to conduct such experiments. TRIR spectroscopy on the nanosecond or slower timescale is a more straightforward experiment. Here, mainly two alternatives exist step-scan FTIR spectroscopy and conventional pump-probe dispersive TRIR spectroscopy, each with their own strengths and weaknesses. Commercial instruments for each of these approaches are currently available. 

An alternative ultrafast TRIR method, originally developed by Hochstrasser and co-workers [26], avoids the requirement of short IR pulses. Here a single frequency of a continuous wave (cw) IR probe source is overlapped with a femtosecond pump beam (again typically in the visible). Time-resolved detection is accomplished by frequency mixing (in a nonlinear crystal located after the sample) of the cw IR probe beam with a second femtosecond visible pulse that has been time delayed relative to the pump beam. This produces a visible pulse at a frequency equal to the sum of the frequency of the original visible pulse plus the IR probe frequency with an intensity related to the IR absorbance of the photo-excited sample. 

The photoinduced Wolff rearrangement of 5-diazo-2,2-dimethyl-l,3-dioxan-4,6-dione has also been examined by TRIR methods [116], These ultrafast measurements, conducted in a PMMA matrix, revealed that the formation of the ketene rearrangement product was complete within 20 ps a carbonyl carbene was not detected in this study. Other applications of TRIR spectroscopy to the study of carbene-related chemistry include investigations of diazirine to diazo rearrangements [117] and of oxygen and sulfur atom transfer reactions [118]. 

Another method, which allows the structural characterization and elucidation of the reactivity of transient species using infrared spectroscopy, is to observe them in real time, using fast time-resolved infrared (TRIR) spectroscopy. In this section we shall focus on the application of fast (submillisecond) and ultrafast (subnanosecond) TRIR spectroscopy to coordination compounds, and describe experiments that cannot be performed using conventional infrared spectrometers. 

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