Laser flash photolysis spectrometers

Room-temperature singlet oxygen phosphorescence can be detected at 1,270 nm with the help of an appropriate detector (e.g., Hamamatsu R5509-42 photomultiplier cooled to 193 K in a liquid nitrogen chamber), and following laser excitation (at 266, 355 or 532 nm) of aerated solutions of the samples in a laser flash photolysis spectrometer [11]. In addition, the interposition of a 600-line diffraction grating, instead of the standard spectrometer grating (1,200-line), is needed to extend spectral response to the infrared. 

Laser flash photolysis of Pis in the cavity of an ESR spectrometer is often accompanied by chemically induced dynamic electron polarization, CIDEP, that is, by the formation... 

In a similar way, anthracene triplet (4>,gj3=0.71, z =6A,700Mr cmr ) and the naphthalene triplet (4>jg = 0.75, e j = 24,500 M" cm" ) in cyclohexane solution have been introduced as transient chemical actinometers for the third-harmonic (355 run) and fourth-harmonic (266 nm) output of Nd YAG lasers, respectively (44). In summary, transient chemical actinometers are ideal for accurately measuring the energy of single laser pulses, provided the quantum yields and extinction coefficients of the transients are well known (45 7). Thus, the well-established benzophenone actinometer (42-44) has been used as a reliable reference to calibrate the azobenzene actinometer (see section "Laser Intensity Measurements with the Azobenzene Actinometer" Doherty S, Hubig SM, unpublished results) and the Aberchrome 540 actinometer (48,49) for intensity measurements with pulsed Nd YAG and/or XeCl excimer lasers. However, such actinometer can only be used when a complete set of laser flash photolysis equipment including a kinetic spectrometer is available. 

Nanosecond Flash Photolysis Measurements.—A computer-controlled ns flash photolysis spectrometer has been described. " The system was employed in a study of the photochemistry of xanthene dyes in solution. A nitrogen laser was used to provide 2—3 mJ excitation pulses at 337.1 nm for a ns flash photolysis study of electron-transfer reactions of phenolate ions with aromatic carbonyl triplets. " A PDP II computer was used to control the transient digitizer employed for detection, and to subsequently process the data. A nanosecond transient absorption spectrophotometer has been constructed using a tunable dye laser in a pulse-probe conflguration with up to 100 ns probe delayA method for reconstructing the time-resolved transient absorption was discussed and results presented for anthracene in acetonitrile solution. The time-resolution of ns flash photolysis may be greatly increased by consideration of the integral under the transient absorption spectrum. Decay times comparable to or shorter than the excitation flash may be determined by this method. 

Flash Photolysis Spectrometers. Picosecond timescale measurements were carried out at 355 nm (3rd Harmonic, Nd YAG) with 20 ps half width pulses from the passively mode locked laser system at the Canadian Center for Picosecond Laser Flash Photolysis (11.). The pulse energy was 3 mj and the wavelength range of the probe pulse used for observation of spectra extended from 400 to 700 nm. 

Bohne, C., Boch, R., and Scaiano, J. C., Exploratory studies of the photochemistry of N-hydroxy-pyridine-2-thione esters generation of excited radicals by laser flash photolysis and in a conventional fluorescence spectrometer, /. Org. Chem., 55, 5414,1990. 

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. 

An ingenious tunable vacuum u.-v. photofragment spectrometer has been constructed by Jackson et al. The photolysis source is provided by a flash lamp, the electrodes of which constitute the entrance slit of a vacuum monociiromator product state distributions from selectively excited parent molecules have been monitored using laser induced fluorescence. 

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