Transient infrared absorption

Figure 4. Transient infrared absorption spectrum obtained at 400 ns following the 351 nm photolysis of Cr(CO)6- [Cr(CO)6] = 0.020 torr, [CO] = 0.400 torr, [He] = 20.0 torr.

Dougherty TP, Heilweil EJ. Ultrafast transient infrared absorption studies of M(CO)6 (M = Cr, Mo, W) photoproducts in n-hexane solution. Chem Phys Lett 1994 227 19-25. 

Transient Infrared Absorption (TRISP) and laser-induced fluorescence. Because the CJ temperatures are only 2000-3000 K, most of the molecular products are in the ground electronic state. Emission spectroscopy looks selectively at only a few extraordinary molecules which are scarcely representative of most of the products. Infrared absorption, on the other hand is ideal for probing the vibrotational states of the ground state molecules, and the fast response time of TRISP makes it ideal for detonations. The technique has not been applied extensively and is difficult to implement, but our preliminary attempts have shown that we can do it with the proper laser apparatus. Broadband CARS is an alternative approach if the instrumental difficulties of TRISP cannot be overcome. 

The transient infrared absorption spectra of the co-ordinatively unsaturated Cr(CO) species [x = 5, 4, 3 and 2), generated via excimer laser photolysis of gas phase Cr(C0)6, have been described. The results indicate that the gas-phase structures of these species are similar to those observed in condensed-phase experiments. Reactions, including some kinetic data, have been reported on several of these species. " Several photolyses experiments have been carried out on Fe(C0)5, " and rate constants for the reaction of Fe(C0)p (n = 4, 3 or 2) with CO determined. The generation and some reac-... 

The transient infrared absorption spectra of molecules in excited electronic states were measured by Elsaesser and Kaiser [18]. The probe pulses used were (i) picosecond infrared pulses in the wavenumber region of 3500-3000 cm generated by an OPA in LiNb03 excited by the Nd YAG laser line, and (ii) picosecond infrared pulses in the region of 3000-1400 cm i obtained by the difference-frequency generation in AgGaS2 between the Nd YAG laser line and near-infrared dye-laser lines excited by the same Nd YAG laser line. 

Figure 3 Examples of transient infrared spectra obtained at 40 ps time delay for W(CO)6 in n -hexane using (top) positively chirped, (middle) no chirp, and (bottom) negatively chirped IR excitation pulses centered at 1983 cm-1. Note the suppression and increase in v = 1 —> 2 excited state absorption near 1950 cm-1. 

Figure 6 Transient infrared kinetics risetime for injected electrons originating from visible excitation (590 nm) Ru(4,4,-(COOH)2-2,2 -bipyridine)2(NCS)2 absorbed on nanostructured Ti02 thin films. The filled points arise from the infrared transient absorption at 5.4 pm, while the open points represent the single-sided VIS-IR cross correlation. An upper limit for the injection risetime is 350 fs. The horizontal arrow indicates the level of infrared signal induced by two-photon excitation of the substrate.

The IR spectra of carbazole and carbazole-(H20) ( =l-3) clusters in a supersonic jet, measured by IR dip spectroscopy, show vibrational structures of both the monomer and the clusters in the 2900-3800 cm frequency region, assigned to the NH stretch of carbazole and the OH stretches of H2O molecules in the clusters <2001PCA8651>. In the first excited singlet and triplet states A -(4-cyanophenyl)carbazole gives rise to transient bands at 2090 cm and 2060 cm detected by time-resolved infrared absorption spectroscopy and attributed to the CN stretch modes of the molecule <2002CL340>. 

Furthermore, it is possible to increase the sensitivity of the IR technique, and thus the probability of detecting transient surface species characterized by (very) low absorption coefficients (such as the intermediate species present during the first steps of the polymerization reaction on the Phillips catalyst) by exploiting the surface-enhanced infrared absorption (SEIRA) effect. It is known that molecules adsorbed on metal island films or particles exhibit 10-1000 times more intense infrared absorption than would be expected from conventional measurements without the metal (253-256). The possibility of performing SEIRA spectroscopy should therefore be expected to provide an opportunity to better investigate the nature of the intermediate species, not only for ethene polymerization on Cr(II)/ Si02, but for other reactions on solid catalysts. 

The copper(I) alkynyls displayed rich photochemistry and particularly strong photoreducing properties. The transient absorption difference spectrum of [Cu3(dppm)3(/X3-) -C=CPh)2]+ and the electron acceptor 4-(methoxycarbonyl)-A-methylpyridinium ion showed an intense characteristic pyridinyl radical absorption band at ca. 400 nm. An additional broad near-infrared absorption band was also observed and it was assigned as an intervalence-transfer transition of the mixed-valence transient species [Cu Cu Cu (dppm)3(/x3- -C=CPh)2] +. The interesting photophysical and photochemical properties of other copper(I) alkynyl complexes such as [Cu(BTA)(hfac)], 2 [Cui6(hfac)8(C=C Bu)8], and [Cn2o(hfac)8(CsCCH2Ph)i2] have also been studied. 

A historical perspective on these developments is given in the first chapter by Jonah. Janata offers a detailed account of the key technique of electron pulse radiolysis, then firmly placed on the modern stage of ultrafast techniques in the chapter by Belloni et al. By far the most common detection scheme is that of transient optical absorption, however chapters by Warman and de Haas (on microwave conductivity) and Le Caer et al. (on infrared spectroscopy) illustrate alternative approaches. Others, not explicitly addressed, but key to... 

To confirm that the transient fluorescence is indeed an action signal due to resonant infrared absorption, the fluorescence intensity in the image was measured as a function of IR wavelength (TFD-IR spectroscopy, [27]). Figure 29.4e shows the TFD-IR spectrum of Rhodamine-6G. When the IR wavelength was scanned over the... 

Another approach to the problem of the identification of transient species in irradiated polyethylene is through the use of infrared spectroscopy. Figure 4 illustrates some of the infrared absorption bands pertinent to this work. The spectra illustrated in Figure 4 were taken of polyethylene irradiated at 77 °K., dotted line, and then after heating to... 

Fig. 5. CO concentrations measured by infrared absorption spectroscopy at the inlet (top) and outlet (bottom) of a catalytic converter during a lean-to-rich transient. The A/F setting was switched from a lean setting of 15.1 to a rich setting of 14.1.

Fig. 6. Solid line CO response measured by infrared absorption spectroscopy at the outlet of a converter containing a Pt/Rh/Al 0 pelleted catalyst. Dashed line computed instantaneous response. The area between the two curves shows that transient chemical processes in the catalyst resulted in enhanced conversion of 44 micro-mol of CO, on average, per gram of catalyst (ref.8).

Also, Fourier transform infrared absorption spectroscopy provides relevant information regarding the specific interactions of different probes within substrates [17], especially in the diffuse-reflectance mode when applied to the study of powdered opaque surfaces that disperse the incident radiation. The extension of this technique to obtain time resolved transient absorption spectra in the IR wavelength range (laser flash-photolysis with IR detection) will certainly play in the near future an important role in terms of clarifying different reaction mechanisms in the surface photochemistry field [17c, 18]. 

The advantage of CARS compared to infrared absorption ion spectroscopy is the higher sensitivity and the fact that nonpolar molecules, such as N2, can also be studied [459]. With pulsed CARS short-lived transient species produced by photo dissociation in molecular beams can also be investigated [460,461]. 

Generally, from spectroscopic data such as frequency position, band shape, intensity and dichroism of specific absorption bands conclusions can be derived in terms of the applied mechanical stress and the state of order and orientation of the polymer under investigation. An extremely powerful method for the study of transient phenomena in polyuMr deformation and relaxation is rheo-optics which describes the relation between stress, strain and an optical quantity (for example birefringence, infrared absorption, light scattering. X-ray diffraction) measured simultaneously with stress and strain as a function of time In a given rheo-optical method therefore, a mechanical test is combined with one of these various types of optical measurements. 

Becaii.se of the phenomenon of self-absorption the ideal sample for conventional emission studies is a thin layer (e.g.. a polymer film), on both metal and semiconductor. surfaces [81]. A sample is usually heated from below the emitting surface, the lower surface thus having a higher temperature than the upper one. Therefore, radiation emitted from below the upper surface is absorbed before it reaches the surface, and this self-absorption of previously emitted light severely truncates and alters features in the emission spectra of optically thick samples. This problem is overcome by using a laser for controlled heat generation within a thin surface layer of the sample, self-absorption of radiation thus being minimized. These methods, known as laser-induced thermal emission (LITE) spectroscopy [85], [86] and transient infrared emission spectroscopy (TIRES) [87], [88] can produce analytically useful emission spectra from optically thick samples. Quantitative applications of infrared emission spectroscopy are described in [89]-[91]. 

In most experiments, ultraviolet or infrared absorption, resonance fluorescence, or laser-induced fluorescence (LIF) is used to follow how transient concentrations change after the photolysis pulse. These optical techniques vary considerably in their sensitivity and hence to the extent to which they isolate the primary reaction. LIF is extremely sensitive, enabling one to follow decays of concentrations from an initial value of 10 ° cm , but its use is restricted to species with a bound-bound electronic transition within the range of tunable dye lasers. LIF has been used to follow the kinetics of reactions of, inter alia, the radicals OH [12-14], CN [15] and CH3O [16,17]. It is more difficult to apply to radical atoms vihich usually have allowed electronic transitions only in the vacuum ultraviolet. Some LIF measurements utilising two-photon excitation of atoms have been reported [18]. 

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