Spectral persistent

Basche T, Ambrose W P and Moerner W E 1992 Optical spectra and kinetics of single impurity molecules in a polymer spectral diffusion and persistent spectral hole burning J. Opt. See. Am. B 9 829-36... 

W. E. Moemer, ed.. Persistent Spectral Hole Burning Science andMpplications, Spriuger-Vedag, Heidelberg, Germany, 1988. 

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

We used short broadband pump pulses (spectral width 200 cm 1, pulse duration 130 fs FWHM) to excite impulsively the section of the NH absorption spectrum which includes the ffec-exciton peak and the first three satellite peaks [4], The transient absorbance change signal shows pronounced oscillations that persist up to about 15ps and contain two distinct frequency components whose temperature dependence and frequencies match perfectly with two phonon bands in the non-resonant electronic Raman spectrum of ACN [3] (Fig. 2a, b). Therefore the oscillations are assigned to the excitation of phonon wavepackets in the ground state. The corresponding excitation process is only possible if the phonon modes are coupled to the NH mode. Self trapping theory says that these are the phonon modes, which induce the self localization. 

Despite the obvious power of quantum line shape calculations for the analysis of measured collision-induced absorption spectra, a need persists for simple but accurate model line profiles, especially for extrapolating experimental spectra to both low and high frequencies for an accurate determination of the spectral moments. Reliable model profiles are also useful for line shape analyses, i.e., for representing complex spectra as a superposition of lines (where this is possible). 

Such a short spin-equilibrium relaxation time raises the question of whether discrete spin state isomers exist. Their existence is affirmed by two observations. One is the persistence of electronic spectral bands typical of the low-spin 2E state over a wide temperature range in solid samples (98). The other is the observation of EPR signals characteristic of the 2E state in both solids and solutions between 4 and 293 K (98,139). At very low temperatures EPR signals of both spin states can be observed simultaneously (98). At low temperatures hyperfine splitting into eight lines is observed from coupling with the 1 = 7/2 Co nucleus. As the temperature is raised the spectral features broaden and the hyperfine resolution is lost. This implies a relaxation process on the EPR time scale of 1010 sec-1, or a relaxation time of the order 0.1 nsec, consistent with the upper limit set by the ultrasonic experiments. 

The optical emission spectrum of uranium, while exceedingly rich in lines, cannot be depended upon in the detection and identification of this element. None of its lines is sufficiently persistent, and unless a comparatively large quantity of uranium is present in the zone of excitation, no spectral lines will be observed. On the other hand, it is possible to make use of a simple photoluminescence reaction which will enable the chemist to detect small traces of uranium. 

At present, two problems persist for the clinical application of IR spectral imaging methods. One of them is the enormous size of the datasets. In order to image an entire lymph node section, about 5x5 mm2 in size, on the order of 200 x 200, or 40000, individual spectra are collected. Although the data acquisition presents little problem and could be performed with existing array detector-based spectrometers within a few minutes, the data reduction by HCA takes over a day, even when using powerful personal computers such as the 64 bit AMD processor in our laboratory. The situation will undoubtedly improve tremendously when true 64 bit, WINDOWS-based operating system will be available to address the enormous... 

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