Pulsed-laser photolysis, apparatus

A schematic of the apparatus is shown in Figure 1. OH was produced by 248 nm (or 266 nm in some experiments) pulsed laser photolysis of H2O2 and detected by observing fluorescence excited by a pulsed tunable dye laser. Fluorescence was excited in the 0H(a2e+ - X tt) 0-1 band at 282 nm and detected in the O-O and 1-1 bands at 309+5 nm. Kinetic data was obtained by electronically varying the time delay between the photolysis laser and the probe laser. Sulfide concentrations were measured in situ in the slow flow system by UV photometry at 228.8 nm. 

A(0 curves observed with a ns-laser photolysis apparatus and a pulsed magnet at 525 nm in the photoreduction reactions of BP (a) in an SDS micellar solution and (b) in a Brij 35 micellar solutions at 293 K. (Reproduced from Ref [8] by permission from Elsevier Science B. V.)... 

The limit of conventional, cryogenically cooled pulsed laser photolysis experiments is 80 K, and the technique suffers from the problem noted for flow tube experiments on ion + neutral reactions, viz. freezing out of reactants or precursors on the cold walls of the reaction cell or the pipes leading into the cell. The CRESU technique has been applied to neutral + neutral reactions by Smith and co-workers to overcome this problem. A diagram of the apparatus is shown in Fig. 3.3. Temperatures as low as 13 K have been obtained. An alternative approach is to introduce the gas mixture into the nozzle via a pulsed valve. This is less demanding on the pumping capacity, but produces less stable flows. It is employed in a number of laboratories. Mullen and Smith [55], for example, have studied NH - - NO at temperatures down to 53 K. 

Many optical studies have employed a quasi-static cell, through which the photolytic precursor of one of the reagents and the stable molecular reagent are slowly flowed. The reaction is then initiated by laser photolysis of the precursor, and the products are detected a short time after the photolysis event. To avoid collisional relaxation of the internal degrees of freedom of the product, the products must be detected in a shorter time when compared to the time between gas-kinetic collisions, that depends inversely upon the total pressure in the cell. In some cases, for example in case of the stable NO product from the H + NO2 reaction discussed in section B2.3.3.2. the products are not removed by collisions with the walls and may have long residence times in the apparatus. Study of such reactions are better carried out with pulsed introduction of the reagents into the cell or under crossed-beam conditions. 

For the investigation of triplet state properties a laser flash photolysis apparatus was used. The excitation source was a Lambda Physik 1 M 50A nitrogen laser which furnished pulses of 3.5 ns half-width and 2 mJ energy. The fluorescence decay times were measured with the phase fluorimeter developed by Hauser et al. (11). 

One important difference in the design of a is conventional flash photolysis apparatus and the ns laser flash photolysis system is the size of the sample. The energy of laser pulses is usually very much lower than that of photographic flashes, typically 0.1 J as against 103 J. For this reason the laser light must be focussed on very small samples (0.1 ml for example). 

Figure 8.1 Schematic diagram of a spectrographic ps flash photolysis apparatus. If laser flash pulse C, sample cell S, continuum pulse generator g, diffraction grating P, photographic plate or diode array...

In the above measurement, the pulse duration is longer than the rates of the cyclization and ring-opening reactions. To determine the rates precisely, it is desirable to employ a shorter laser pulse. We measured the response time of the following dithienylethene in 1,2-dichloroethane by using a femto laser (180 fs) photolysis apparatus.20... 

The absorption spectra of the dyes were measured with a Shimadzu UV-3101 PC spectrophotometer (Japan) in a cell with a 1-cm optical path length. The fluorescence and fluorescence excitation spectra were studied with the use of a Shimadzu RF-5301 PC spectrofluorimeter. To study the triplet state of the dyes, apparatuses of flash photolysis with xenon lamp excitation (with an energy of 50 J and a pulse length at half maximum of xi/2 = 7 ps) [6] was used. To detect the triplet state of the dyes, the solutions were deoxygenated using a vacuum unit or purged with argon for experiments on the laser flash photolysis apparatus. A... 

Although such a 2-MHz ESR apparatus was very sophisticated, its time resolution was not enough for measurement of CIDEP. In 1973, Fessenden [5] found that the direct ESR measurement without field modulation improved the time resolution, observing CIDEP signals in solution with pulse radiolysis. This method was applied for laser-photolysis measurements in solids [6] and in solution [7]. A spin-echo ESR technique was also found to be useful for CIDEP [8]. Since then, CIDEP experiments with cw-ESR and pulsed-ESR spectrometers without field modulation have become much more popular than before. Through such transient ESR measurements, CIDEP due to not only the radical pair mechanism but also several other mechanisms have been observed in many chemical reactions including biologically important ones such as photosynthesis reactions. In this chapter, we will show several mechanisms for CIDEP with several typical examples. 

Figure 3.5 Apparatus for preparative photolysis with pulsed lasers...

Fig. 15. Schematic representation of the time-resolved infrared (TRIR) flash photolysis apparatus used at Nottingham. The UV pulse laser generates transient species the continuous IR laser monitors the change in transmission at a particular IR frequency, producing a trace showing the IR absorbance as a function of time. The experiment is repeated at different IR frequencies so that a complete IR spectrum of the transient can be built up [reproduced with permission from (97), p. 103],...

Fig. 4.2. Schematic diagram of apparatus for nanosecond flash photolysis. The components are (A) pulsed laser source (B) beam splitter (C) movable mirror (D) scintillation solution (E) reaction vessel (F) spectrograph. (—) Path of exciting pulse. (--) Path of pulse to scintillation solution (varied by moving C). ( ) Path of analyzing pulse.

Fig. 3.3 Sketch of a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus configured for the study of radical-neutral reactions. In this arrangement, radicals are generated by photolysis of a suitable precursor using radiation from a fixed-frequency pulsed laser operating at one of the three wavelengths, 226, 248, or 193 nm, and are detected by laser-induced fluorescence excited by tuneable radiation from a dye laser or a master oscillator parametric oscillator (MOPO) [56]...

Figure 3.19 Time-resolved spectra for the system anthraquinone -I- trimethoxybenzene in acetonitrile, excited at 355 nm, under conditions where the quinone absorbs. The spectra were recorded 60, 100 and 300 nsec after irradiation with the laser pulse. Results obtained on an Apphed Photophysics LKS 60 flash photolysis apparatus in the Coimbra Chemistry Centre. (Courtesy of C. Serpa.)...

The apparatus consists of a pulsed molecular beam, a pulsed ultraviolet (UV) photolysis laser beam, a pulsed vacuum ultraviolet (VUV) probe laser beam, a mass spectrometer, and a two-dimensional ion detector. The schematic diagram is shown in Fig. 1. 

The photolyzing light pulse is produced by a dye laser and enters the sample at about 10° to the axis of the sample beam. The observation beam originates from a 75-W xenon arc lamp. The apparatus is supplied by OLIS, Athens, Georgia USA. Reproduced with permission from C. A. Sawicki and R. J. Morris, Flash Photolysis of Hemoglobin, in Methods in Enzymology (E. Antonini, L. R. Bernardi, E. Chiancone eds.), 76, 667 (1981). 

Janata E (1992b) Instrumentation of kinetic spectroscopy. 10. A modular data acquisition system for laser flash photolysis and pulse radiolysis experiments. Radiat Phys Chem 40 437-443 Janata E, Lilie J, Martin M (1993) Instrumentation of kinetic spectroscopy. 11. An apparatus for AC-conductivity measurements in laser flash photolysis and pulse radiolysis experiments. Radiat Phys Chem 43 353-356... 

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