Pulse optical detection

Vibronic relaxation of XeF in solid Ar at 25 K was studied by pumping vibronic transitions with a subpicosecond UV pulse, and detecting frequency-resolved emission with a fast optical gate [25]. XeF has two sites in Ar, one... 

It is pertinent that S20g accepts an electron generated by pulse radiolysis of water to give optically detectable S04 within 1.5 x 10 sec . ... 

Many methods of investigation of protein-ligand binding kinetics that are based on linear processes are of a pump-probe type. In this approach an optical pulse, called a pump, starts a photoreaction (such as dissociation of MbCO into Mb and CO), and its progress is probed a time At later. The probe could be, for example, a weak laser pulse, which detects the spectral changes in the heme during the protein-ligand recombination, or an x-ray pulse, which allows determination of the protein structure at a particular instant in time. 

It is clearly seen that negative lines are much stronger then the noise. Besides that, the negative lines are always situated at the same places. The invariabihty of the spectral positions provides the evidence that they are not connected with fluctuations of the laser pulses and detection system. Thus it may be concluded that we a deaUng with a reabsorption mechanism. The optical absorption spectra of natural apatites in the range 600-900 nm contain several lines and bands connected with Nd ", Pr ", Mn ", SOj (Gorobets 1975 ... 

The subpicosecond pulse radiolysis [74,77] detects the optical absorption of short-lived intermediates in the time region of subpicoseconds by using a so-called stroboscopic technique as described in Sec. 10.2.2 ( History of Picosecond and Subpicosecosecond Pulse Radiolysis ). The short-lived intermediates produced in a sample by an electron pulse are detected by measuring the optical absorption using a very short probe light (a femtosecond laser in our system). The time profile of the optical absorption can be obtained by changing the delay between the electron pulse and the probe light. 

Actually, the kinetic study of the cluster redox potential by pulse radiolysis [31] (Section 20.3.2) somewhat mimics the process of the black-and-white photographic development, except that clusters are free in the solution (not fixed on AgBr crystals), and that they are produced by ionizing radiation (as in radiography and not by visible photons but the last choice had been incompatible with the time-resolved optical detection in the visible. Beyond the critical nuclearity, they receive electrons without delay from the developer already present (actually, the photographic development is achieved in a delayed step). 

This mechanism leads to a highly spin-polarized triplet state with a characteristic intensity pattern in the EPR spectrum, which is observed by time-resolved techniques (either transient or pulse EPR). The zero field splitting (ZFS) of the triplet state, which dominates the EPR spectrum, is an important additional spectroscopic probe. It can also be determined by optical detection of magnetic resonance (ODMR), for a review of the techniques involved and applications see reference 15. These methods also yield information about dynamical aspects related to the formation, selective population and decay of the triplet states. The application of EPR and related techniques to triplet states in photosynthesis have been reviewed by several authors in the past15 22-100 102. The field was also thoroughly reviewed by Mobius103 and Weber45 in this series. 

Details of the picosecond pulse radiolysis system for emission (7) and absorption (8) spectroscopies with response time of 20 and 60 ps, respectively, including a specially designed linear accelerator (9) and very fast response optical detection system have been reported previously. The typical pulse radiolysis systems are shown in Figures 1 and 2. The detection system for emission spectroscopy is composed of a streak camera (C979, HTV), a SIT... 

It is from these perspectives that we have reviewed the pulse radiolysis experiments on polymers and polymerization in this article. The examples chosen for discussion have wide spread interest not only in polymer science but also in chemistry in general. This review is presented in six sections. Section 2 interprets the experimental techniques as well as the principle of pulse radiolysis the description is confined to the systems using optical detection methods. However, the purpose of this section is not to survey detail techniques of pulse radiolysis but to outline them concisely. In Sect. 3, the pulse radiolysis studies of radiation-induced polymerizations are discussed with special reference to the initiation mechanisms. Section 4 deals with applications of pulse radiolysis to the polymer reactions in solution including the systems related to biology. In Sect. 5 reaction intermediates produced in irradiated solid and molten polymers are discussed. Most studies are aimed at elucidating the mechanism of radiation-induced degradation, but, in some cases, polymers are used just as a medium for short-lived species of chemical interest We conclude, in Sect. 6, by summarizing the contribution of pulse radiolysis experiments to the field of polymer science. 

The importance of this technique to chemistry and biology has been far less widely accepted than it deserves. The essential technical problem involves the generation of short pulses of ionizing radiation followed generally by optical detection of transient species (Swallow, 1973 von Sonntag, 1987 Kiefer, 1990). 

Samples were irradiated by a 10 ps single or 2 ns electron pulse from a 35 MeV linear accelerator for pulse radiolysis studies (17). The fast response optical detection systems of the pulse radiolysis system for absorption spectroscopy (18) is composed of a very fast response photodiode (R1328U, HTV.), a transient digitizer (R7912, Tektronix), a computer (PDP-11/34) and a display unit. The time resolution is about 70 ps which is determined by the rise time of the transient digitizer. 

Fig. 9. ODMR investigations at T = 1.4 K of Pd(2-thpy)2 dissolved in an n-octane Shpol skii matrix. Concentration = 10 mol/1 cw excitation Ag c = 330 nm (30.3 x 10 cm 0- Detection of the emission at 18418 cm (Tj —> Sq transition), (a) Zero-field ODMR (optically detected magnetic resonance) spectrum (b) Zero-field microwave recovery ODMR signal after pulsed microwave excitation with a microwave frequency of 2886 MHz. The best fit of the recovery signal is obtained with Eq. (4). (Compare Ref. [61])...

Optical Detection Systems for Ultrafast Pulse Radiolysis... 

In supercritical krypton the formation of excimers has also been time resolved but the results contrast with those for xenon.As in xenon, electron-ion recombination should occur rapidly. Again, electrons remain hot for many nanoseconds in krypton and the mobility of hot electrons is in the range of 150 to 400 cm /Vs. This leads to a theoretical range for of 1.4 to 3.6 x 10 m" s" at a density of 0.48 g/cm. In pulse radiolysis studies using optical detection, the concentration of intermediates is around 0.5 to 1 pM, thus the first half-life for recombination of electrons with ions is less than 10 ps in krypton. What has been observed is that an excimer species (A), the... 

Ultrasonic absorption measurements were carried out over the frequency range from 0 2 to 65 MHz. Three different techniques were used depending on the frequency ranges from 0 3 to 1.7 MHz, we used the plano-concave resonator method coupled with the optical detection techniques, from 2 to 7 MHz range we used the cylindrical resonator method and from 15 MHz to 65 MHz we used the pulse method. The details of the experimental system are described elsewhere. ... 

The experimental arrangement consists of a simple molecular beam apparatus, a pulsed tunable dye laser for exciting fluorescence, and a gated optical detection system. 

The chemical dosimeter that is used most frequently is the thiocyanate dosimeter [119]. Other chemical dosimeters for pulse radiolysis are ferrocyanide [119], modified Fricke (Super-Fricke) [119], hydrated electron [120], 02-saturated solutions of potassium iodide [112], and N20-saturated solutions of methylviologen and formate [118]. The C(N02)4 (tetranitromethane, TNM) dosimeter is used in pulse radiolysis experiments with simultaneous optical and conductometric detection [121-124]. The composition and characteristics of the various chemical dosimeters used for pulse radiolysis with optical detection are listed in Table 8. 

For low dose per pulse, a large value of Ge is desirable when the response time of the optical detection system is slow, the product should be relatively stable. 

In pulse radiolysis experiments with optical detection, the measured quantity is optical absorbance, so any measured absorbance A can be quantified as a value of Ge with Eq. 67 ... 

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