Transient absorption spectroscopy laser

A suitable method for a detailed investigation of stimulated emission and competing excited state absorption processes is the technique of transient absorption spectroscopy. Figure 10-2 shows a scheme of this technique. A strong femtosecond laser pulse (pump) is focused onto the sample. A second ultrashort laser pulse (probe) then interrogates the transmission changes due to the photoexcita-lions created by the pump pulse. The signal is recorded as a function of time delay between the two pulses. Therefore the dynamics of excited state absorption as... 

Figure 8.7. Delayed fluorescence and diffuse reflectance transient absorption spectroscopy on scattering substrates. Example terthicnyl on silica gel excited with = 354 nm (neodymium/yttrium-aluminum-garnet) (Nd/YAG) laser pulse of 10 nsec, 20 mj), recorded with a gated diode array spectrometer.

In the experiment, ZnCcP is produced by laser photolysis, then transient absorption spectroscopy follows the formation (k ) and decay (/Ceb) of ZnCcP + in wild-type and mutant crystals. Kang and Crane have studied the effects of interface mutations on electron transfer rates in single crystals using complexes between a zinc-substituted cytochrome c peroxidase (ZnCcP) and site-directed mutants of yeast cytochrome c (yCc).The mutants replaced the... 

Only three experiments involving transient absorption were found, probably because the populations required for a good signal to noise ratio require more powerful and therefore more expensive lasers. One of these used transient absorption spectroscopy to study ozone formation (26), making this an... 

The UV-visible absorption and fluorescence spectra of NKX-2311 in solution are measured. The absorption and fluorescence maxima are located at 490 and 555 nm, respectively. The fluorescence spectrum is shown in Fig. 1. This figure also shows the transient absorption spectrum of NKX-2311 in deuterated methanol (CD3OD) solution measured by the nanosecond laser system. The spectrum is ascribed to the excited singlet state (dye ). The absorption spectrum of the oxidized form of NKX-2311 (dye+) was measured by nanosecond transient absorption spectroscopy after adding electron acceptor, 1,4-benzoquinone, in NKX-2311 solution of CD3OD. As shown in Fig. 1, there are two characteristic peaks at around 875 and 1010 nm. 

The dynamics of the interfacial electron-transfer between Dye 2 and TiOz were examined precisely by laser-induced ultrafast transient absorption spectroscopy. Durrant et al.38) employed subpicosecond transient absorption spectroscopy to study the rate of electron injection following optical excitation of Dye 2 adsorbed onto the surface of nanocrystalline Ti02 films. Detailed analysis indicates that the injection is at least biphasic, with ca. 50% occurring in <150 fsec (instrument response limited) and 50% in 1.2 0.2 psec. 

The primary steps of the photolysis of aqueous monuron and diuron were investigated by Canle et al. by means of transient absorption spectroscopy using an ArF laser (X = 193 nm) for excitation [89]. Under these conditions, photoionization occurred with a quantum yield of about 10%. Radical cations were detected after the laser pulse and found to deprotonate to yield neutral radicals [89]. 

The P +-Q state can be observed by transient absorption spectroscopy because the absorption spectrum of the charge-separated state differs from that of the porphyrin first excited singlet state. Measurement of the transient absorption spectrum of a benzonitrile solution of 8 as a function of time following excitation with a 150 to 200-fs laser pulse at 590 nm allowed determination of the rate constant for decay of P" -Q back to the ground state (step 3 in Figure 3). The value obtained for kj, is 5.3 X 10" s- [61]. 

The absorption spectrum of 22 is nearly identical to the sum of the spectra of unlinked model compounds. The long wavelength band of the free base porphyrin is observed at 650nm in chloroform, whereas that of the zinc porphyrin is at 590nm. Excitation of a chloroform solution of 22 with a 15 ns pulse of 650 nm laser light leads to the formation of a carotenoid radical cation which can be detected by transient absorption spectroscopy (Figure 14). This ion arises from the charge separated state C -Pzh-P-Qa-Qb, and is formed with a quantum yield of 0.83. The lifetime of the species is 55/iS. 

Here, we briefly describe the laser ablation dynamics of phthalocyanine film where the primary species was confirmed, by femtosecond transient absorption spectroscopy [25], to be its electronically excited state. The exdton absorption band was replaced in 20 ps by a hot band of the ground electronic state. This rapid decay was ascribed to mutual interactions between densely formed excitons leading to sudden temperature elevation. The elevated temperature was estimated by comparing transient absorption spectra with the temperature difference ones. It is worth noting that quite normal dynamics of the excited states are detected, even under ablation conditions, which is the reason why we do not accept the plasma mechanism. Then we considered how to correlate these electronic processes of phthalocyanine films with their fragmentation. 

Fig. 7.16. Characterization of the photoinduced charge-separated state of a porphyrin-fullerene diads with flexible polyether linkers by transient absorption spectroscopy. The differential absorption spectrum is given for a 50 ns delay after excitation at 532 nm with a 10 ns laser. (Reprinted with permission from ref. [41]).

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

The photodissociation of bromobenzene in solution has been investigated with ultrafast transient absorption spectroscopy, following excitation at 266 nm. The main kinetic feature in acetonitrile was a 9 ps decay that was assigned to predissociation similar decays were observed in hexane, dichloromethane and tetrachloromethane. Laser-aligned iodobenzene have been photodissociated into phenyl radicals and iodine atoms with a 1.5 ps laser pulse at 266 nm, and the yield of iodine photoproducts detected by resonant multiphoton ionization." Significant yield enhancements were observed when the dissociation laser was polarized parallel instead of perpendicular to the alignment laser polarization. 

Important mechanistic insights have been obtained in the last 5 years, particularly owing to the availability of such modern techniques as laser-flash time-resolved transient absorption spectroscopy [22,23], photo-CIDNP [21c, 24], and so on. It is thus the particular purpose of this review to analyze this recent work on mechanisms for photodecomposition of iodonium and sulfonium salts, and to evaluate its implications for practical applications of onium salt photochemistry. 

The homolysis product, Phl"+, has been observed directly by laser-flash photolysis probed with transient absorption spectroscopy on both the nanosecond [23,69] and picosecond [22] time scales. Klemm et al. [23] and Hacker and Dektar [70] suggest the possibility of electron transfer between the homolysis fragments leading to products identical with those of heterolysis. DeVoe et al. found no evidence for this process [22], Electron transfer... 

Laser flash photolysis is one of the most efficient methods for the direct spectroscopic observation of free radicals and for monitoring the kinetics of formation and decay in real-time. This method is an extension of conventional flash photolysis method [26] that was invented by Norrish and Porter in 1949, and who were awarded by the Nobel Prize in 1967. We have used this approach to investigate the generation and reactions of free radicals with DNA. In this technique, a laser light pulse is used to produce short-lived intermediates in solution contained in an optical cuvette, and the kinetics of their formation and decay are monitored by transient absorption spectroscopy. The apparatus we used is shown in Figure 4.1. 

Intramolecular electron transfer in cytochrome c has been investigated by attaching photoactive Ru complexes to the protein surface. Ru(bpy)2(C03) (bpy = 2,2 -bipyridine) has been shown to react with surface His residues to yield, after addition of excess imidazole (im), Ru(bpy)2(im)(His) +. The protein-bound Ru complexes are luminescent, but the excited states ( Ru ) are rather short lived (r 100 ns). When direct electron transfer from Ru to the heme cannot compete with excited-state decay, electron-transfer quenchers (e.g., Ru(NH3)6 + ) are added to the solution to intercept a small fraction (1-10%) of the excited molecules, yielding (with oxidative quenchers) Ru ". If, before laser excitation of the Ru site, the heme is reduced, then the Fe to Ru + reaction (ket) can be monitored by transient absorption spectroscopy. The ket values for five different modified cytochromes have been reported (Ru(His-33), 2.6(3) xlO Ru(His-39), 3.2(4) xlO Ru(His-62), 1.0(2) x 10 Ru(His-... 

Truly incoherent white light sources convenient for cw and transient absorption spectroscopy include cw, high pressure arc lamps ( 2 kW, spectral irradiance 3 mW/ (cm l cm2 sr)) and flashlamps ( 1 spectral irradiance of a 1 mW He-Ne laser (Av 1 MHz, dQ = 10-6sr, beam waist w 0.1 cm) is 3 x 109 W/(cm-1 cm2- sr), approximately 1012 times larger than that of the most robust incoherent cw radiation source. 

Scaiano and co-workers photogenerated the diphenylmethyl radical from laser flash photolysis of 1,1-diphenylacetone. Electron transfer from the excited state radical to aromatic electron acceptors such as p-dicyanobenzene gave the diphenylmethyl cation observed by transient absorption spectroscopy [Eq. (9)] [92]. The diphenylmethyl radical was also photogenerated by irradiation of 1,1,3,3-tetraphenylacetone in acetonitrile. Electrochemical oxidation gave the diphenylmethyl cation [93] ... 

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