Laser-induced reaction spectroscopy

As mentioned in Section 9.2, there is an important distinction between action spectroscopies based on the absorption of multiple photons, as is possible with FEL excitation, and those based on single photon absorption, such as messenger spectroscopy [26,27], laser-induced reaction spectroscopy [73] or indeed direct absorption experiments. When the signal (in this case, the depletion of the parent ion and creation of product ious) depends on the absorption of multiple photons, the signal intensity may not be compared directly to computed linear (single-photon) absorption spectra, as is the normal fashion. There are several reasons why intensities may differ. Thus, baudpositiou (frequency) is considered usually to be more reliable than band intensity when comparing computed and experimentally-measured spectra. 

Monitoring the formation of Na atoms by laser induced fluorescence spectroscopy (at another wavelength) as a function of time shows a stepwise growth in the Na atom intensity, following the implications of the periodic vibration of the [Na—I] species exactly. Thus product formation depends on both a vibrational mode and a curve crossing both play a role along the reaction coordinate. 

The energy released as heat in the course of the nonradiative decay of P to the ground state and detected as a pressure wave by laser-induced optoacoustic spectroscopy (LIOAS) exhibits positive deviations (i.e., a> 1 cf. Eq. (1)) from the values which were calculated on the basis of the absorption spectrum of Pr alone (Figure 15) [90,115]. This indicates that already within the 15-ns duration of the excitation flash, one or several intermediates must have been formed. These in turn, within the same interval, may again absorb light from an intense laser flash and (at least in part) dissipate heat upon their return to the ground state of the same species (internal conversion) and/or to Pr (photochemical back reaction). The formation of primary photoproducts within the nanosecond flash duration was of course to be expected in view of the much shorter lifetimes of the photochromic fluorescence decay compo-... 

PCAH have been observed in a flame using laser induced fluorescence spectroscopy by injecting individual species into the post-reaction zone. While the spectra are broadened by the elevated temperature, the spectra are qualitatively similar to low temperature (100 C) spectra and are indicative of the particular species injected. Thus, the injection procedure appears to be a feasible method of determining PCAH spectra at flame temperatures. These spectra will be used as a data base to determine individual PCAH concentrations in flames from their LIF spectra. 

Donohoue et al. [31] has reported two other kinetic data sets for Cl and Br reactions using a pulsed laser photolysis-pulsed laser induced fluorescence spectroscopy. These data sets are obtained using pseudo-first order conditions with respect to halogens or mercury and experiments were performed at a broad range of temperatures. The authors of these studies indicate an uncertainty estimation of 50% in the rate coefficients due to the determination of absolute concentrations of chlorine and bromine atoms [31]. Sumner et al. [20] reinvestigated both reactions using a 17.3 m environmental chambers equipped with fluorescent lamps and sun lamps to mimic environmental reactions, and evaluated the rate constants... 

Hydrogen. The reaction of O ( D2) with H2 takes place on the ground state potential surface of water, HiOf Ai). On the basis of trajectory calculations, (Whitlock et al., 1982) it has been suggested that, as is true for the hydrocarbons, parallel mechanisms involving insertion/elimination and direct abstraction govern the course of this reaction. The observation using laser induced fluorescence spectroscopy (Luntz et al., 1979 Smith and Butler, 1980) of a highly excited, non-Boltzmann rotational distribution and a nearly statistical vibrational distribution for v" = 1 and v" = 0 is consistent with the insertion/elimination... 

Apart from desorption, surface reaction with adsorbate can be stimulated by the laser irradiation. In this chapter we will demonstrate the formation of new surface species by the CO2 laser induced reaction of CDF3 with the surface of SIO2 (17,18). In order to elucidate the mechanism of the reaction especially to determine the surface species, ir spectroscopy was used. A systematic investigation was performed Including the determination of reaction yields as a function of the laser frequency, the laser intensity and the gas pressure as well as the reaction products, and the determination of the correlation between the excited species and the reaction path. 

Bennett et al, (55) investigated the hydrolysis reaction of PuOt in 0.1 Af NaC104 solution with laser-induced photoacoustic spectroscopy (LIPAS). The effect of hydrolysis on the absorption spectrum of Pu02 (569 nm 19 Af" cm" ) between 530 and 590 nm was studied in Ae pH range from 3 to 10.5. No hydrolysis was observed below pH 8, and the PuOj concentration was far below the detection limit above pH 10. The data were fitted to the hydrolysis reaction ... 

In order to determine ion temperatures more directly, laser based techniques have been used and new schemes of action spectroscopy are under development. The Doppler profile of an observed transition provides direct information on the ion velocity distribution along the laser beam. In addition a quantitative analysis of the population of rotational or other suitable internal states can provide information about how cold the ions are internally. The first successful application based on the laser induced reaction... 

Asvany O, Hugo E, MiiUer F, Kuhnemann F, Schiher S, Tennyson J, Schlemmer S. (2007) Overtone spectroscopy of H2D" and D2H using laser induced reactions. J. Chem. Phys. 127 154317-1-154317-11. 

Photodecarboxylation of 2-(3-benzoyl)phenylpropionate, the ketoprofen anion, was studied in water by time-resolved, laser-induced optoacoustic spectroscopy (LIOAS). The various transient species involved in the title reaction were identified and their Hfetimes measured. The intrinsic photoreactivity of the 2-benzoylthiophene chromophore of the photosensitizing drug, tiaprofenic acid, was also studied. The observed photoprocesses account readily for the biological photosensitization reactions such as membrane damage and protein modification. The transient photochemistry in photodecarboxylation of rufloxacin (a fluoroquinolone antibacterial drug), fenofibric acid (used in the treatment of hyperlipidemia), and tohnetin has also been reported. 

In addition, a 532 (visible) or 355 (UV region) nm laser-induced photoisomerization of allylic alcohols to aldehydes catalyzed by [Fe3(CO)i2] or [Fe(CO)4PPh3] was developed by Fan [176]. In this reaction, key intermediates such as the 7i-allyl hydride species [FeH(CO)3(q -C3H3ROH)] (R = H, Me) were detected by pulsed laser FTIR absorption spectroscopy. These results strongly support the 7i-allyl mechanism of photoisomerization of allyl alcohols. 

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. ... 

The pump and probe pulses employed may be subjected to a variety of nonlinear optical mixing processes they may be prepared and characterized by intensity, duration, spectral band width, and polarization. They may arrive in the reaction chamber at a desired time difference, or none. The probe pulse may lead to ionizations followed by detections of ions by mass spectrometry, but many alternatives for probing and detection have been used, such as laser-induced fluorescence, photoelectron spectroscopic detection, absorption spectroscopy, and the like. 

Under UV-laser irradiation, photosensitive multifunctional acrylate resins become rapidly cross-linked and completely insoluble. The extent of the reaction was followed continuously by both UV and IR spectroscopy in order to evaluate the rate and quantum yield of the laser-induced polymerization of these photoresist systems. Two basic types of lasers emitting in the UV range were employed, either a continuous wave (C.W.) argon-ion laser, or a pulsed nitrogen laser. 

The use of T2D-IR spectroscopy in its various modes is not limited to MLCT. It is applicable to all kinds of photo-triggered processes. Besides the investigation of electronically excited states, photo-chemical reactions like isomerizations and dissociations can be explored. Application to a photo-switchable peptide has already been demonstrated [10]. Conformational dynamics of biomolecules that are triggered by laser-induced T-jump or the control of pH by photo-acids, can also be investigated. 

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