Time-resolved spectroscopies transient

Morita N and Yajima T 1984 Ultrafast-time-resolved coherent transient spectroscopy with incoherent light Rhys. Rev. A 30 2525-36... 

Relaxation kinetics may be monitored in transient studies tlirough a variety of metliods, usually involving some fonn of spectroscopy. Transient teclmiques and spectrophotometry are combined in time resolved spectroscopy to provide botli tire stmctural infonnation from spectral measurements and tire dynamical infonnation from kinetic measurements that are generally needed to characterize tire mechanisms of relaxation processes. The presence and nature of kinetic intennediates, metastable chemical or physical states not present at equilibrium, may be directly examined in tliis way. 

Most importantly, the careful kinetic analysis of the rise and decay of the transient species in equation (69) shows that the decarboxylation of Ph2C(OH)CO occurs within a few picoseconds (kc c = (2-8) x 1011 s-1). The observation of such ultrafast (decarboxylation) rate constants, which nearly approach those of barrier-free unimolecular reactions, suggests that the advances in time-resolved spectroscopy can be exploited to probe the transition state for C—C bond cleavages via charge-transfer photolysis. 

Electron-transfer activation. Time-resolved spectroscopy shows that the activation of the [ArH, PyNO ] complex by the specific irradiation of the CT absorption band results in the formation of transient aromatic cation radical... 

The Wheland intermediate in equation (87) is identified by time-resolved spectroscopy as follows.247 Laser excitation of the EDA complex of NO+ with hexamethylbenzene in dichloromethane immediately generates two transient species as shown in the deconvoluted spectrum in Fig. 20. The absorption band at lmax = 495 nm is readily assigned to the cation radical of... 

FLASH IRRADIATION TIME-RESOLVED SPECTROSCOPIES FOR THE DETECTION OF THE TRANSIENT PHENOMENON... 

Time-resolved spectroscopy is performed using a pump-probe method in which a short-pulsed laser is used to initiate a T-jump and a mid-IR probe laser is used to monitor the transient IR absorbance in the sample. A schematic of the entire instrument is shown in Fig. 17.4. For clarity, only key components are shown. In the description that follows, only those components will be described. A continuous-wave (CW) lead-salt (PbSe) diode laser (output power <1 mW) tuned to a specific vibrational mode of the RNA molecule probes the transient absorbance of the sample. The linewidth of the probe laser is quite narrow (<0.5 cm-1) and sets the spectral resolution of the time-resolved experiments. The divergent output of the diode laser is collected and collimated by a gold coated off-axis... 

The photocyanation and the photohydrolysis of 4-chloroanisole and 4-fluoroanisole have been studied with time-resolved spectroscopy and by measurement of the photoconductivity in anhydrous and aqueous acetonitrile and tert-butyl alcohol solutions632. The mechanism is depicted in equation 167. The transient species which have been... 

Finally, time-resolved spectroscopy with femtosecond pulses was recently carried out by Gale and coworkers on a similar HD0 D20 sample (125). Due to the notably wider bandwidth of the applied IR pulses in the latter investigations, no details on reshaping of the transient spectra in dependence of the excitation frequency were accessible. A time-dependent position of the peak position of the induced sample bleaching was interpreted in terms of a shift within the statistical distribution of OH frequencies with a time constant of 1 ps. However, because only the parallel signal of the induced sample transmission was detected, the measured dynamics corresponds to a superposition of vibrational, reorientational, and structural relaxation. The data are interpreted by the help of a model of with random (bell-shaped) distribution of OH oscillators, quite different from the results of other groups. 

Coupling pulse radiolysis with time-resolved spectroscopy also allowed the determination of the transient absorption spectra of hydrated silver atom and of the first silver clusters showing that the absorption properties of silver atoms and metal aggregates in solution are different from that of bare clusters in gas phase. Silver atom presents an absorption maximum at 360 nm in water,while it absorbs at substantially shorter wavelengths in argon (292-310 nm) or in xenon (322-333 nm) atmosphere. ... 

The reactive intermediates leading to the (charge-transfer) photodecomposition of the 6w(arene)iron(II) acceptor are revealed by picosecond time-resolved spectroscopy. For example, photoexcitation of the CT absorption band of the ferro-cene-(HMB)2Fe complex (HMB = hexamethylbenzene) with the second harmonic output (at 532 nm) of a mode-locked Nd YAG laser (25-ps pulse width) generates a transient spectrum with an absorption maximum at 580 nm (see Figure 11 A). Careful deconvolution of this absorption spectrum reveals the superposition of the absorption bands of ferrocenium (Imax = 620 nm, e = 360 cm [162]) and (HMB)2Fe+ (2 ,ax = 580 nm, = 604 M" cm" [163]). 

The reactive intermediates in the charge-transfer photoreactions of carbonyl metallate salts are examined by time-resolved spectroscopy applying a 10-ns pulse of a Q-switched Nd YAG laser at 532 nm [118]. Thus, the charge-transfer excitation of various manganate and cobaltate salts results in the formation of intense transient absorption centered at 800 nm and 780 nm, respectively, which are readily assigned to the 17-electron radicals Mn(CO)5 and Co(CO)4 , respectively (see Figure 12) [168- 170]. 

The direct characterization of an eT mechanism requires a much more complicated technique time-resolved spectroscopy. The solution containing the system under investigation is irradiated by a laser pulse, and the absorption spectra of the solution are consecutively recorded at chosen and very short time intervals (e.g. every 10 ns). If, in the envisaged two-component system F1 M, an M-to-Fl eT process takes place upon illumination, one should be able to measure the absorption spectra of Fl and M" ", as well as their decay, which allows the determination of the lifetime of the transient species F1 M. It goes without saying that very sophisticated and expensive instrumentation is required to carry out this type of experiment. Moreover, the smaller the fluorophore lifetime and the faster the back-electron transfer process, the more rapid and expensive the data acquisition equipment required. In particular, narrow laser pulses and especially fast data collections are needed for systems such as 1, where a short-living polyaromatic fluorophore (anthracene, r = 5 ns) is linked to the electron donor (or acceptor) group by a rather short carbon chain. 

These interpretations were supported by time-resolved spectroscopy. Laser flash photolysis of 4 in CH2C12 produces a transient spectrum (Figure 11) very similar to that obtained by flash photolysis of phenyl azide (Figure 8) and is therefore attributed to pentafluorodehydroazepine. Similar results were obtained in acetonitrile and tetrahydrofuran. However LFP of 4 in methanol gives an entirely different transient spectrum. (Figure 12). The... 

Robert B, Nabedryk E and Lutz M (1989) Vibrational spectroscopy of transient states in photosynthetic bacterial reaction centers. In Clark RJH and Hester RE (eds) Time-resolved spectroscopy, pp 301-333. John Wiley and Sons, New York... 

Pulse radiolysis of a metal ion solution is a powerful method of generating isolated metal atoms and then studying their coalescence, and the redox properties of the transient clusters formed, by time-resolved spectroscopy or conductimetry. 

Walters has designed a complete system for time-resolved spectroscopy that is useful for transients from 0.2 to 90 /xsec over a wavelength range of 1000 A per exposure, using either photographic or photoelectric detection. The system utilizes a rotating mirror with photocell synchronization. 

In 2000, Hubig and Kochi claimed to have made the first observation of a true Wheland intermediate of an S Ar when studying nitrosation of methyl-substituted benzenes by time-resolved spectroscopy [49]. Irradiation of a suspension of NOBF " in dichloromethane containing mesitylene with a laser pulse of 355 nm generated a transient species, which was identified as the radical ion pair. This species rapidly converted to another transient, which was assigned as the Wheland intermediate. However, theoretical studies have indicated that the reaction does not proceed via a radical ion pair and that the Wheland structure is a transition state rather than an intermediate of the nitrosation reaction [50]. This interpretation also found support in the analysis of the x-complex of nitrosonium ion and henzene hy IRMPD [44]. 

Charge-transfer oxidations of a series of dialkyl and mixed alkylaryl sulphides leads to high yields of sulphoxides by the same photochemical procedure. Intermediates formed in the charge-transfer activation of the thioether complexes were probed by time-resolved spectroscopy. The spectra transient with =500 nm in tetrahydrothiophene were attributed to the corresponding radical cations ... 

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