Time-resolved infrared absorption measurements

Millisecond Time-Resolved Infrared Absorption Measurements... 

For microsecond to nanosecond time-resolved infrared absorption measurements, three types of spectroscopic methods have been developed (i) a method using an infrared laser, (ii) a method using a dispersive spectrometer, and (iii) a method using an FT-IR spectrometer. The time resolution of each of these is limited to the fastest time-response capability of the detector used. 

In ultrafast, time-resolved infrared absorption measurements by the pump-probe method, the sample is first excited by an ultrashort pump pulse, and then irradiated by an ultrashort infrared pulse (probe pulse) after a certain delay time from the excitation by the pump pulse. The delay time of the probe pulse from the pump pulse is usually changed by the difference in the optical path lengths of the pump and probe pulses (a delay time of 1 ps arises from a path difference of about 0.3 mm). When the infrared spectrum of a molecule in an excited electronic state is measured, pulses in the ultraviolet to visible region are used for the pump purpose, and pulses in the infrared region are used for the probe purpose. When a vibrationally excited molecule is the target of such a measurement, pulses in the infrared region are used for both the pump and probe purposes. The transient (or time-resolved) infrared absorption spectra by this method are usually measured as the difference in absorption intensities for the probe pulses between the measurements with the pump pulses and those without the pump pulses. 

Picosecond to femtosecond time-resolved infrared absorption measurements were initiated in the middle of the 1980s. In 1984, Heilweil et al. [17] studied the dynamics of vibrational relaxation by using picosecond infrared pulses obtained from an OPA (LiNb03) excited by a mode-locked Nd YAG laser. 

Since about the end of the 1990s, generation of ultrashort pulses has become easier due to the progress of laser technology, and, as a consequence, a measuring method based on femtosecond Ti sapphire regenerative amplifier with a kilohertz repetition rate has become the mainstream of fast time-resolved infrared absorption measurements. 

Taatjes, C. A. (1999). Time-resolved infrared absorption measurements of product formation in Cl atom reactions with alkenes and alkynes. Int. Rev. Phys. Chem. 18,419. 

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