Multichannel chemical state

K. Takatsuka, Temperature, Geometry, and Variational Stmcture in Microcanonical Ensemble for Stmctural Isomerization Dynamics of Clusters A Multichannel Chemical Reaction Beyond the Transition-State Concept, Adv. Chem. Phys. Part B 130, 25 (2005). 

TEMPERATURE, GEOMETRY, AND VARIATIONAL STRUCTURE IN MICROCANONICAL ENSEMBLE FOR STRUCTURAL ISOMERIZATION DYNAMICS OF CLUSTERS A MULTICHANNEL CHEMICAL REACTION BEYOND THE TRANSITION-STATE CONCEPT... 

Current developments in AES are mainly in the areas of improved electron guns with higher brightness and smaller spot sizes, and multichannel detectors with improved sensitivities. Improvements of energy resolution will enable chemical states to be studied in more detail, leading to better analysis of complex mixtures with partly overlapping peaks. 

In summary, we have combined state of the art optical multichannel analyzer techniques with well established low repetition rate picosecond laser technology to construct an instrument capable of measuring transient spectra with unprecedented reliability. It is, in its present form, a powerful tool for the investigation of ultrafast processes in biological, chemical, and physical systems. We foresee straightforward extension of the technique to the use of fourth harmonic excitation (at 265 nm) and also a future capability to study gaseous as well as condensed phase samples over a more extended spectral range. 

A state-of-the-art description of broadband ultrafast infrared pulse generation and multichannel CCD and IR focal plane detection methods has been given in this chapter. A few poignant examples of how these techniques can be used to extract molecular vibrational energy transfer rates, photochemical reaction and electron transfer mechanisms, and to control vibrational excitation in complex systems were also described. The author hopes that more advanced measurements of chemical, material, and biochemical systems will be made with higher time and spectral resolution using multichannel infrared detectors as they become available to the scientific research community. 

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. 

MV2+ acceptors and SCN electron donors in solution [43], Colloidal semiconductor particles, typically of ca. 10-100 nm diameter, in aqueous sols may be treated as isolated microelectrode systems. Steady-state RRS experiments with c.w. lasers can be used to study phototransients produced at the surfaces of such colloidal semiconductors in flow systems [44], but pulsed laser systems coupled with multichannel detectors are far more versatile. Indeed, a recent TR3S study of methyl viologen reduction on the surface of photoex-cited colloidal CdS crystallites has shown important differences in mechanism between reactions occurring on the nanosecond time scale and those observed with picosecond Raman lasers [45]. Thus, it is apparent that Raman spectroscopy may now be used to study very fast interface kinetics as well as providing sensitive information on chemical structure and bonding in molecular species at electrode surfaces. 

There are several potential sources of radioactive materials that can contaminate water (see Chapter 4, Section 4.14). Radioactive contamination of water is normally detected by measurements of gross P activity and gross a activity, a procedure that is simpler than detecting individual isotopes. The measurement is made from a sample formed by evaporating water to a very thin layer on a small pan, which is then inserted inside an internal proportional counter. This setup is necessary because P particles can penetrate only very thin detector windows, and a particles have essentially no penetrating power. More detailed information can be obtained for radionuclides that anit y-rays by the use of gamma spectrum analysis. This technique employs solid-state detectors to resolve rather closely spaced y peaks characteristic of specific isotopes in the sample s spectra. In conjunction with multichannel spectrometric data analysis, it is possible to determine a number of radionuclides in the same sample without chemical separation. This method requires minimal sample preparation. 

Study of transient chemical species or excited electronic states via their Raman spectra, often with pulsed lasers and multichannel photon detectors. 

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