Transient Absorption with Nanosecond Resolution

The typical nowadays system for the detection of transient absorption in solution has the optical scheme represented in Fig. 8.1. It is based on single beam spec-trophotometric time resolved detection of light transmitted by the sample at single wavelengths (kinetic spectrophotometry). 

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Figure 12. a) Transient absorption spectra obtained upon nanosecond pulsed laser excitation of 1) cw-[Ru"(dcbpy)2(NCS)2] dye in ethanolic solution, and 2) a sensitized Ti02 transparent film. Spectra were recorded 50 ns (la, 2a) and 0.5 ps (lb, 2b) after the laser excitation pulse (A = 605 nm, 5 ns pulse duration), b) Transient absorption spectra recorded 6 ps after ultrafast laser excitation (A = 605 nm, 150 fs pulse duration) of 1) c -[Ru (dcbpy)2(NCS)2] dye in ethanol, and 2) a fresh sensitized titanium dioxide film. Insert, the temporal behavior of the absorbance of the latter system, measured at A = 750 nm with sub-picosecond time resolution. 

The transient absorption method utilized in the experiments reported here is the transient holographic grating technique(7,10). In the transient grating experiment, a pair of polarized excitation pulses is used to create the anisotropic distribution of excited state transition dipoles. The motions of the polymer backbone are monitored by a probe pulse which enters the sample at some chosen time interval after the excitation pulses and probes the orientational distribution of the transition dipoles at that time. By changing the time delay between the excitation and probe pulses, the orientation autocorrelation function of a transition dipole rigidly associated with a backbone bond can be determined. In the present context, the major advantage of the transient grating measurement in relation to typical fluorescence measurements is the fast time resolution (- 50 psec in these experiments). In transient absorption techniques the time resolution is limited by laser pulse widths and not by the speed of electronic detectors. Fast time resolution is necessary for the experiments reported here because of the sub-nanosecond time scales for local motions in very flexible polymers such as polyisoprene. 

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. 

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. 

Optical absorption spectrophotometry is probably the most commonly used technique [4,a]. Reaction cells are similar to those used in flash work. Photomultipliers cover the uv-visible range the initial photoelectric signal is amplified internally, by an amoimt controlled by selection of the number of dynodes. Nanosecond equipment is commercially available. Picosecond time-resolution has been achieved [l,h]. For the infrared and Raman region, semiconductor photodiodes cover the range 400-3000 nm the vibrational spectra yield structural information about transient species much more detailed and precise than that from electronic spectra. Resonance enhancement of Raman spectra increases their intensity by a factor of 10, and makes them attractive for detection and monitoring [4,b]. They can be recorded with time-resolution down to sub-nanoseconds. Fluorescence detection is sensitive, and fast with single-photon counting or a streak camera (Section 4.2.4.2), it has been used for times down to 30 ps after an electron pulse. Conductivity also provides a fast and sensitive technique [4,c,d,l,m], especially in hydrocarbon solutions, where... 

Lifetimes shorter than a nanosecond can be measured using picosecond lasers with suitable detectors (streak camera) [3], bearing in mind that, as a rule of thumb, the cost of the equipment is inversely proportional to its time resolution. However, the measurement of lifetimes shorter than a nanosecond is most commonly performed with a single photon apparatus (see Sect. 7.2.3). Lasers with pulse duration shorter than 100 femtoseconds (1 fs = 1 x 10 s) are also available, but with such equipment the sample emission eannot be monitored for technical reasons, and transient absorption must be measured instead (see Chap. 8). 

The conditions which determine whether flash photolysis can be used to smdy a given chemical system are (i) a precursor of the species of kinetic interest has to absorb light (normally from a pulsed laser) (ii) this species is produced on a timescale that is short relative to its lifetime in the system. Current technical developments make it easy to study timescales of nanoseconds for production and analysis of species, and the use of instrumentation with time resolution of picoseconds is already fairly common. In certain specific cases, as we will see in the last part of this chapter, it is possible to study processes on timescales greater than a few femtoseconds. Once the species of interest has been produced, it is necessary to use an appropriate rapid detection method. The most common technique involves transient optical absorption spectroscopy. In addition, luminescence has been frequently used to detect transients, and other methods such as time-resolved resonance Raman spectroscopy and electrical conductivity have provided valuable information in certain cases. 

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