Time-resolved absorption techniques, laser

Much attention has been devoted to the development of methods to generate quinone methides photochemically,1,19-20 since this provides temporal and spatial control over their formation (and subsequent reaction). In addition, the ability to photogenerate quinone methides enables their study using time-resolved absorption techniques (such as nanosecond laser flash photolysis (LFP)).21 This chapter covers the most important methods for the photogeneration of ortho-, meta-, and para-quinone methides. In addition, spectral and reactivity data are discussed for quinone methides that are characterized by LFP. 

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... 

The data presented are in the form of time-resolved absorption and emission spectra obtained by the use of the experimental system shown in Fig. 1. This optical system is based upon a cw-mode locked Nd/YAG and dye laser. The pulse duration can be tuned from 100 to 0.1 ps by means of etelons and compression techniques. 

The time-dependent change in the populations of the various vibrational levels due to collisions can be detected in different ways. In many cases the detection of the fluorescence, emitted from these levels, is a simple and sensitive technique. In some cases absorption methods may be more suitable. Here two lasers irradiate the sample simultaneously a strong pulsed pump laser which populates the initial vibrational level, and a weak probe laser which probes the time-dependent populations N (t). Either the time-resolved absorption of a cw probe laser is monitored or the total time-Integra ted fluorescence induced by a pulsed probe laser is measured as a function of the delay time between pump and probe [12.20]. 

A useful and common way of describing the reorientation dynamics of molecules in the condensed phase is to use single molecule reorientation correlation functions. These will be described later when we discuss solute molecular reorientational dynamics. Indirect experimental probes of the reorientation dynamics of molecules in neat bulk liquids include techniques such as IR, Raman, and NMR spectroscopy. More direct probes involve a variety of time-resolved methods such as dielectric relaxation, time-resolved absorption and emission spectroscopy, and the optical Kerr effect. The basic idea of time-resolved spectroscopic techniques is that a short polarized laser pulse removes a subset of molecular orientations from the equifibrium orientational distribution. The relaxation of the perturbed distribution is monitored by the absorption of a second time-delayed pulse or by the time-dependent change in the fluorescence depolarization. 

Time-resolved absorption spectra of samples of BZP/C12- 1700/EtOH and Cl 2-I5OO/H2O samples were obtained by the use of diffuse reflectance laser flash photolysis technique, developed by Wilkinson et al. [2-4]. In this study, the use of an intensified charge-coupled device as a detector allowed us to obtain time-resolved absorption spectra with nanometer spectral spacing (i.e., where the 200-900 scale is defined by the 512 pixels used for recording spectra in the array of the ICCD) [1,8-14]. 

The rate coefficients of Cox et al. (1976), Jenkin and Cox (1987) and Burkholder et al. (1992) are given in table VIII-I-1 and shown in figure VIII-I-1. Cox et al. (1976) measured their value relative to fc(OH + Hg). Jenkin and Cox (1987) and Burkholder et al. (1992) determined the rate coefficient as function of temperature using, respectively, molecular modulation combined with time-resolved absorption and pulsed laser photolysis-laser-induced fluorescence techniques. While the different ambient temperature measurements are in reasonable agreement, there are significant differences with respect to the temperature dependence between Jenkin and Cox (1987) and Burkholder et al. (1992). The preferred value at 298 K is an average of the three determinations. 

Time-resolved X-ray absorption is a very different class of experiments [5-7]. Chemical reactions are triggered by an ultrafast laser pulse, but the laser-induced change in geometry is observed by absorption rather than diffraction. This technique permits one to monitor local rather than global changes in the system. What one measures in practice is the extended X-ray absorption fine structure (EXAFS), and the X-ray extended nearedge strucmre (XANES). 

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. 

Very recently, white light continuum pulses of duration ca. 200 fsec, pulse energy ca. 1 / J, and peak wavelength of ca. 780 nm have been generated at repetition rates up to 250 kHz by commercially available Ti sapphire regenerative amplified laser systems. Such systems are very expensive, but the expected easier use, as compared with homemade systems, should open up new research applications for time-resolved fluorescence and absorption techniques in the near-IR. 

Time-resolved laser flash ESR spectroscopy generates radicals with nonequilibrium spin populations and causes spectra with unusual signal directions and intensities. The signals may show absorption, emission, or both and be enhanced as much as 100-fold. Deviations from Boltzmann intensities, first noted in 1963, are known as chemically induced dynamic electron polarization (CIDEP). Because the splitting pattern of the intermediate remains unaffected, the CIDEP enhancement facilitates the detection of short-lived radicals. A related technique, fluorescence detected magnetic resonance (FDMR) offers improved time resolution and its sensitivity exceeds that of ESR. The FDMR experiment probes short-lived radical ion pairs, which form reaction products in electronically excited states that decay radiatively. ... 

A promising recent development in the study of nitrenium ions has been the introduction of time-resolved vibrational spectroscopy for their characterization. These methods are based on pulsed laser photolysis. However, they employ either time resolved IR (TRIR) or time-resolved resonance Raman (TRRR) spectroscopy as the mode of detection. While these detection techniques are inherently less sensitive than UV-vis absorption, they provide more detailed and readily interpretable spectral information. In fact, it is possible to directly calculate these spectra using relatively fast and inexpensive DFT and MP2 methods. Thus, spectra derived from experiment can be used to validate (or falsify) various computational treatments of nitrenium ion stmctures and reactivity. In contrast, UV-vis spectra do not lend themselves to detailed structural analysis and, moreover, calculating these spectra from first principles is still expensive and highly approximate. 

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