Millisecond time-resolved infrared

In this chapter, millisecond time-resolved infrared measurements are first described in Section 20.2 for this time scale, time resolution is set by the time needed to measure (scan) a spectrum. Then, microsecond to nanosecond time-resolved measurements, which are limited by the detector response time are described in Section 20.3, and finally, picosecond to femtosecond time-resolved measurements, the time resolution for which is determined by the width of the laser pulse used for the measurement, are described in Section 20.4. 

Millisecond Time-Resolved Infrared Absorption Measurements... 

Figure 20.1 Millisecond time-resolved infrared spectra measured during a reaction of CpCo(CO)2 with CINO [3], (a) Spectra at room temperature and (b) spectra at 10°C. Cp, cyclopentadienyl. (Source Reproduced with permission from [3], Copyright 2010, SAS.)...

The rate of polymerization and the cure penetration can be further increased by employing powerful lasers as radiation source, the exposure time dropping then into the millisecond range. Polymer relief images were thus obtained at micronic resolution by simply scanning the photosensitive plate with a highly focused laser beam. The kinetic profiles of such ultrafast reactions were directly record by using time-resolved infrared spectroscopy which allows instantaneous evaluation at any moment of the actual rate of polymerization and the precise amount of residual unsaturation in the cured polymCT. 

The detection of short-lived transient species is often achieved by flash photolysis where an extremely short flash of UV/Vis radiation from a laser generates a high concentration of transient species, and a second probe beam monitors any changes that occur after the flash. Traditionally, UVA is spectroscopy has been used as a detection method. However, time-resolved infrared spectroscopy (TRIR), a combination of UV flash photolysis and fast IR detection, also has a long history. There are several different approaches to fast IR spectroscopy and the method of choice depends upon the timescale of the reaction. Measurements on the nanosecond to millisecond timescale are obtained using point-by-point techniques or by step-scan FTIR. In the point-by-point approach, a continuous wave IR laser (GO or diode) or globar is used as the IR source, which is tuned to one particular IR frequency (Figure 3). ... 

As demonstrated, different time-resolved FTIR techniques allow to study the complete photocycle of bacteriorhodopsin in the entire range from picoseconds to several milliseconds. Infrared difference spectra trace reactions which take place in different parts of the protein molecule. Isotopically labeled proteins or proteins with mutations at specific sites... 

Slow dissociation rates (10 -10 s ) have been measured in Dunbar s laboratory by time-resolved photodissociation, which consists of trapping ions in an ICR cell during a variable delay time after a phot-odissociating photon pulse. The technique called time-resolved photoionization mass spectrometry , developed by Lifshitz, consists of trapping photoions in a cylindrical trap at very low pressure to avoid bimolecular collisions, and then ejecting them into a mass filter after a variable delay covering the microsecond to millisecond range. When the dissociation rate constant becomes lower than ca. 10 s competition with infrared fluorescence takes place and limits the lifetime of the decomposition process. This has to be taken into account to extract the dissociation rate constant from the experimental data. 

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