Transient absorption measurements apparatus

The femtosecond transient absorption studies were performed with 387 nm laser pulses (1 khz, 150 fs pulse width) from an amplified Ti Sapphire laser system (Model CPA 2101, Clark-MXR Inc). A NOPA optical parametric converter was used to generate ultrashort tunable visible pulses from the pump pulses. The apparatus is referred to as a two-beam setup, where the pump pulse is used as excitation source for transient species and the delay of the probe pulse is exactly controlled by an optical delay rail. As probe (white light continuum), a small fraction of pulses stemming from the CPA laser system was focused by a 50 mm lens into a 2-mm thick sapphire disc. A schematic representation of the setup is given below in Fig. 7.2. 2.0 mm quartz cuvettes were used for all measurements. 

Laser Flash Photolysis. A sample solution was prepared by vacuum-transfer techniques just before the measurements. Transient-absorption spectra and lifetimes of various intermediates were measured using the previously described apparatus (30). Excitation was with the fourth harmonic of a Nd YAG laser with a pulse width of ca. 30 ps. The solutin was vigorously stirred for 10 s between laser shots. 

The majority of laboratories using the pulse radiolysis technique use the method of kinetic spectrophotometry to detect and measure radiation-induced changes. With such apparatus it is thus possible to use transient as well as permanent optical changes as a measure of the effects of a radiation pulse. A convenient transient absorption is the hydrated electron (6, 11) which has the very high extinction coefficient of 1.85 X 104 cm."1 at 7000 A. Use of the hydrated electron as a dosimeter has already been discussed (4,9), and it has found widespread use among workers in pulse radiolysis as an approximate dosimeter. This work set out to measure the parameters under which the hydrated electron could be used as an accurate dosimeter for low dose pulses. As the amount of hydrated electron present at the end of the irradiation pulse is used as a measure of dose, it is important that the lifetime of the e aq is long compared with the pulse length. 

An apparatus for measuring the time dependence in the xs range of lightscattering intensity has been used to investigate the degradation of poly(phenyl vinyl ketone) (PVK) in solution.205-250 The PVK was irradiated with 25 ns pulses at 347.1 nm, and butyrophenone was also investigated in the same way. In both cases a transient absorption, identified by sensitization measurements as the PVK and butyrophenone triplet state, was shown to have a first-order decay rate-constant of 1.0 0.2 x 107 s-1. The quantum yield of triplet state formation in PVK was estimated to be between 0.1 and 0.3, whereas the quantum yield of main-chain scission was found to be 0.4—0.6, insensitive to the presence of Oa. 

Nanosecond absorbance transients were measured with a single beam instrument (11). Excitation was provided by a 6-8 ns, 1 mJ 610 nm pulse from a NdrYAG pumped rhodamine dye laser. Absorption transients were either detected with a Hamamatsu R928 (A<900 nm) or R406 (A>900 nm) photomultiplier operating with a 2.5 ns response time. Picosecond absorbance transients were measured with a double beam apparatus (11). 1.5 ps, 1 mJ, 610 nm excitation pulses were generated by using the output of a mode-locked Ar" laser to synchronously pump a rhodamine dye laser. 

Time resolved fluorescence measurements were carried out using a single photon counting apparatus. Transient absorption data were taken with a flash photolysis system as described by Durrant et al. 1989a. 

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). 

A tunable pulsed laser Raman spectrometer for time resolved Raman studies of radiation-chemical processes is described. This apparatus utilizes the state of art optical multichannel detection and a-nalysis techniques for data acquisition and electron pulse radiolysis for initiating the reactions. By using this technique the resonance Raman spectra of intermediates with absorption spectra in the 248-900 nm region, and mean lifetimes > 30 ns can be examined. This apparatus can be used to time resolve the vibrational spectral o-verlap between transients absorbing in the same region, and to follow their decay kinetics by monitoring the well resolved Raman peaks. For kinetic measurements at millisecond time scale, the Raman technique is preferable over optical absorption method where low frequency noise is quite bothersome. A time resolved Raman study of the pulse radiolytic oxidation of aqueous tetrafluoro-hydroquinone and p-methoxyphenol is briefly discussed. 

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