Infrared lasers, time-resolved

In laser measurements on hydrogen halides, the system under investigation is itself all, or part, of the active laser medium. Similar experiments have not yet been made on other molecules. However, in related experiments on CO, in the author s laboratory [242,432] and elsewhere [433], a CO cw laser has been used to probe the vibrational distributions in a reaction system outside the optical cavity that may act as an amplifier or absorber of lines from the laser. Time-resolved observations can be made for as long as one chooses after the reaction is initiated. If tunable infrared lasers become readily available this technique is likely to be applied more widely. 

Diller R, Malt S, Walker G 0, Cowen B R, Pippenger R, Bogomoini R A and Hochstrasser R M 1995 Femtosecond time-resolved infrared laser study of the J-K transition of bacteriorhodopsin Chem. Rhys. Lett. 241 109-15... 

Yamakata, A., Uchida, T., Kubota, J. and Osawa, M. (2006) Laser-induced potential jump at the electrochemical interface probed by picosecond time-resolved surface-enhanced infrared absorption spectroscopy./. Phys. Chem. B, 110, 6423-6427. 

G. Hungerford, D. J. S. Birch and R. E. Imhof, Spark source infrared fluorometry, in Time-Resolved Laser Spectroscopy in Biochemistry ///(J. R. Lakowicz, ed.), Proc. SPIE 1640, 262-270 (1992). 

Intermediate a-nitrosobenzaldehyde 36 was generated in solution by laser photolysis of 3,5-diphenyl-l,2,4-oxadiazole-4-oxide 37 and its time-resolved infrared (TRIR) spectroscopy has been recorded <2003JA1444>. The second-order rate constants for reaction with diethylamine and 1,3-cyclohexadiene were determined to be (1.3 0.5) x s ... 

In principle, absorption spectroscopy techniques can be used to characterize radicals. The key issues are the sensitivity of the method, the concentrations of radicals that are produced, and the molar absorptivities of the radicals. High-energy electron beams in pulse radiolysis and ultraviolet-visible (UV-vis) light from lasers can produce relatively high radical concentrations in the 1-10 x 10 M range, and UV-vis spectroscopy is possible with sensitive photomultipliers. A compilation of absorption spectra for radicals contains many examples. Infrared (IR) spectroscopy can be used for select cases, such as carbonyl-containing radicals, but it is less useful than UV-vis spectroscopy. Time-resolved absorption spectroscopy is used for direct kinetic smdies. Dynamic ESR spectroscopy also can be employed for kinetic studies, and this was the most important kinetic method available for reactions... 

Laser flash photolysis methods have also been applied to the study of nitrenium ion trapping rates and hfetimes. This method relies on short laser pulses to create a high transient concentration of the nitrenium ion, and fast detection technology to characterize its spectrum and lifetime The most frequently used detection method is fast UV-vis spectroscopy. This method has the advantage of high sensitivity, but provides very little specific information about the structure of the species being detected. More recently, time-resolved infrared (TRIR) and Raman spectroscopies have been used in conjunction with flash photolysis methods. These provide very detailed structural information, but suffer from lower detection sensitivity. 

Terahertz spectroscopy uses continuous wave (CW) and short pulsed laser excitation in the spectrum region between infrared and microwave frequencies. Pulsed laser excitation using pulse widths in the range of 10-100 femtoseconds has enabled the use of time-resolved terahertz spectroscopy, which is capable of capturing dynamic information at subpicosecond time scales. 

While the first experiments of time-resolved IR spectroscopy were conducted with pulse durations exceeding 10 ps, the improved performance of laser systems now offers subpicosecond (12) to femtosecond (13-15) pulses in the infrared spectral region. In addition, the pump-probe techniques have been supplemented by applications of higher-order methods, e.g., IR photon echo observations (16). 

Figure 6.2-27 Time resolved near-infrared measurement (at 8258 5 cm ) of monomer conversion, induced by a single pulse from a KrF excimer laser (248 nm), during ethene polymerization at 190 °C and at 2550 bar (reaction mixture already containing 9.5% polyethylene before this particular pulse was applied).

The experiments described above do not allow us to perform time-resolved measurements and to track the kinetics initiated by irradiation. While the time-resolved infrared spectroscopy was intensively and successfully developed with flash laser photolysis, time-resolved infrared spectroscopy was surprisingly not implemented in radiation chemistry until recently. 

Martin CB, Shi X, Tsao M-L, Karweik D, Brooke J, Hadad CM, Platz MS. (2002) The photochemistry of riboflavin tetraacetate and nucleosides. A study using density functional theory, laser flash photolysis, fluorescence, UV-Vis and time resolved infrared spectroscopy. J Phys Chem B 106 10263-10271. 

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