Laser flash

Figure Bl.15.7. Transient EPR. Bottom time-resolved EPR signal of the laser-flash-indueed triplet state of pentaeene in /j-terphenyl. BpO.085 mT. Top initially, the transient magnetization M is aligned along B z. In the presenee of a MW magnetie field B the magnetization preeesses about BJ x (rotating frame representation).

Figure Bl.16.15. TREPR spectmm after laser flash photolysis of 0.005 M DMPA (5) in toluene, (a) 0.7 ps, 203 K, RE power 10 mW O, lines CH (8), spacing 22.8 G , benzoyl (6), remaining lines due to (7). (b) 2.54 ps, 298 K, RE power 2 mW to avoid nutations, lines of 7 only. Reprinted from [42].

Contradictory evidence regarding the reaction to fonn 8 and 9 from 7 led the researchers to use TREPR to investigate the photochemistry of DMPA. Figure B1.16.15A shows the TREPR spectrum ofthis system at 0.7 ps after the laser flash. Radicals 6, 7 and 8 are all present. At 2.54 ps, only 7 can be seen, as shown in figure B1.16.15B. All radicals in this system exliibit an emissive triplet mechanism. After completing a laser flash intensity sPidy, the researchers concluded that production of 8 from 7 occurs upon absorption of a second photon and not tiiemially as some had previously believed. 

Figure Bl.16.16. TREPR spectrum of TEMPO radicals in 1,2-epoxypropane solution with benzophenone, 1 ps after 308 inn laser flash. Reprinted from [45],...

Work by Koga et aJ [62] demonstrates how the polarization iiiechanism can change upon alteration of the chemical enviromnent. Upon laser flash photolysis, excited xanthone abstracts a proton from an alcohol... 

FT-EPR spectra of the ZnTPPS/DQ system in a solution of cetyltriinethylaininonium chloride (CTAC), a cationic surfactant, are shown in figme BE 16.21. As in the TX100 solution, both donor and acceptor are associated with the micelles in the CTAC solution. The spectra of DQ at delays after the laser flash of less than 5 ps clearly show polarization from the SCRP mechanism. While SCRPs were too short-lived to be observed in TXlOO solution, they clearly have a long lifetime in this case. Van Willigen and co-workers... 

Closs G L and Miller R J 1979 Laser flash photolysis with NMR detection. Microsecond time-resolved CIDNP separation of geminate and random-phase polarization J. Am. Chem. Soc. 101 1639—41... 

Miller R J and Closs G L 1981 Application of Fourier transform-NMR spectroscopy to submicrosecond time-resolved detection in laser flash photolysis experiments Rev. Sc/. Instrum. 52 1876-85... 

Blattler C and Paul H 1991 CIDEP after laser flash irradiation of benzil in 2-propanol. Electron spin polarization by the radical-triplet pair mechanism Res. Chem. Intermed. 16 201-11... 

Many experimental methods may be distinguished by whether and how they achieve time resolution—directly or indirectly. Indirect methods avoid the requirement for fast detection methods, either by detemiining relative rates from product yields or by transfonuing from the time axis to another coordinate, for example the distance or flow rate in flow tubes. Direct methods include (laser-) flash photolysis [27], pulse radiolysis [28]... 

A recent example of laser flash-lamp photolysis is given by Hippier etal [ ], who investigated the temperature and pressure dependence of the thennal recombmation rate constant for the reaction... 

B2.5.4.2 LASER FLASH PHOTOLYSIS AND PUMP-PROBE TECHNIQUES... 

Figure B2.5.8. Schematic representation of laser-flash photolysis using the pump-probe technique. The beam splitter BS splits the pulse coming from the laser into a pump and a probe pulse. The pump pulse initiates a reaction in the sample, while the probe beam is diverted by several mirrors M tluough a variable delay line.

This technique with very high frequency resolution was used to study the population of different hyperfme structure levels of the iodine atom produced by the IR-laser-flash photolysis of organic iodides tluough multiphoton excitation ... 

Figure B2.5.11. Schematic set-up of laser-flash photolysis for detecting reaction products with uncertainty-limited energy and time resolution. The excitation CO2 laser pulse LP (broken line) enters the cell from the left, the tunable cw laser beam CW-L (frill line) from the right. A filter cell FZ protects the detector D, which detennines the time-dependent absorbance, from scattered CO2 laser light. The pyroelectric detector PY measures the energy of the CO2 laser pulse and the photon drag detector PD its temporal profile. A complete description can be found in [109].

The conmron flash-lamp photolysis and often also laser-flash photolysis are based on photochemical processes that are initiated by the absorption of a photon, hv. The intensity of laser pulses can reach GW cm or even TW cm, where multiphoton processes become important. Figure B2.5.13 simnnarizes the different mechanisms of multiphoton excitation [75, 76, 112], The direct multiphoton absorption of mechanism (i) requires an odd number of photons to reach an excited atomic or molecular level in the case of strict electric dipole and parity selection rules [117],... 

Although modem laser teelmiques ean in prineiple aehieve mueh narrower energy distributions, optieal exeitation is frequently not a viable method for the preparation of exeited reaetive speeies. Therefore ehemieal aetivation—often eombined with (laser-) flash photolysis—still plays an important role in gas-phase kmeties, in partieular of unstable speeies sueh as radieals [ ]. Chemieal aetivation also plays an important role in energy-transfer studies (see chapter A3.13). 

A recent study of the vibrational-to-vibrational (V-V) energy transfer between highly-excited oxygen molecules and ozone combines laser-flash photolysis and chemical activation with detection by time-resolved LIF [ ]. Partial laser-flash photolysis at 532 mn of pure ozone in the Chappuis band produces translationally-... 

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

Tlie existence of the ylide 19, which can formally be interpreted as the deprotonation product from the corresponding salt 7a, has been claimed by trapping chlorocarbene with pyridine during the laser-flash photolysis of e do-7-chlorodibenzo[n,c]bicyclo[4.1.0]heptane (18) (96JPC18426). Bromination of l-vinyl-2-pyridone (20) yields the bicyclic pyridinium bro-... 

Sporadic reports of the 2//-thiopyran transformation are still available. The electrocyclic ring-opening of 2-benzyl-2,4,6-triphenyl-2//-thiopyran (131, X = PhCH2) was detected by laser flash spectroscopy (86MI1). 2-Amino-2//-thiopyran was predicted to be more stable than its valence-bond isomer on the basis of semiempirical MINDO/3 and MNDO calculations (84JPR955). 

Nano- and microsecond laser flash spectroscopy was employed to clarify photophysical effects on benzyl group migration in 2//-thiopyran 60 (R = PhCH2, 86MI1). 

At its best, the study of solvent kies by the formalism given can be used to learn about proton content and activation in the transition state. For this reason it is known as the proton inventory technique. The kinetics of decay of the lowest-energy electronic excited state of 7-azaindole illustrates the technique.25 Laser flash photolysis techniques (Section 11.6) were used to evaluate the rate constant for this very fast reaction. From the results it was suggested that, in alcohol, a double-proton tautomerism was mediated by a single molecule of solvent such that only two protons are involved in the transition state. In water, on the other hand, the excited state tautomerism is frustrated such that two water molecules may play separate roles. Diagrams for possible transition states that can be suggested from the data are shown, where of course any of the H s might be D s. 

Photolytic methods are used to generate atoms, radicals, or other highly reactive molecules and ions for the purpose of studying their chemical reactivity. Along with pulse radiolysis, described in the next section, laser flash photolysis is capable of generating electronically excited molecules in an instant, although there are of course a few chemical reactions that do so at ordinary rates. To illustrate but a fraction of the capabilities, consider the following photochemical processes ... 

After the laser flash, one then monitors the progress of events by some rapidly responding method. Conductivity, absorption spectroscopy, and fluorescence spectroscopy are the methods most commonly used. If a reaction product has a characteristic absorption band of sufficient intensity, one can monitor its buildup with time. This might be a UV, visible, or IR band. The need for a band with a high molar absorptivity arises because the reactive transient is usually present at a relatively low concentration, KT6-lCr5 M being typical. If the species of interest is phosphorescent, then the timed decay of its phosphorescence intensity can be recorded. 

Langmuir adsorption isotherm, 93 Laser flash photolysis, 263-266 Least-squares regression, 37-40 linear, 37-39 nonlinear, 39-40 unweighted, 38 weighted, 38-39 Lifetime, 16... 

The rapid desulfonylation of PhCH2SO 2 has been used successfully as a probe for laser flash photolytic investigations of various reactions generating sulfonyl radicals from sulfonyl halides26,28. 

The fastest reliable PMC transients recorded at electrodes (ZnO single crystals24) were limited by the lifetime of a 10-ns laser flash. It was apparent from the nondeconvoluted signal at shorter time scales that much faster decay processes took place and would be accessible with faster laser pulses. 

Carbocations are intermediates in several kinds of reactions. The more stable ones have been prepared in solution and in some cases even as solid salts, and X-ray crystallographic structures have been obtained in some cases. An isolable dioxa-stabilized pentadienylium ion was isolated and its structure was determined by h, C NMR, mass spectrometry (MS), and IR. A P-fluoro substituted 4-methoxy-phenethyl cation has been observed directly by laser flash photolysis. In solution, the carbocation may be free (this is more likely in polar solvents, in which it is solvated) or it may exist as an ion pair, which means that it is closely associated with a negative ion, called a counterion or gegenion. Ion pairs are more likely in nonpolar solvents. 

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

Reactivity of Bovine Whey Proteins, Peptides, and Amino Acids toward Triplet Riboflavin as Studied by Laser Flash Photolysis. Journal of Agricultural and Food Chemistry, Vol. 52, No. 21, (October 2004), pp. 6602-6606, ISSN 0021-8561. 

Lu, C. Y. Lui, Y.Y. (2002). Electron transfer oxidation of tryptophan and tyrosine by triplet states and oxidized radicals of flavin sensitizers a laser flash photolysis study. Biochimica et Biophysica Acta (BBA) - General Subjects, Vol. 1571, No.l, (May 2002), pp. 71-76, ISSN 0304-4165... 

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