Near infrared region, absorption frequencies

Very few experiments have been performed on vibrational dynamics in supercritical fluids (47). A few spectral line experiments, both Raman and infrared, have been conducted (48-58). While some studies show nothing unique occurring near the critical point (48,51,53), other work finds anomalous behavior, such as significant line broadening in the vicinity of the critical point (52,54-60). Troe and coworkers examined the excited electronic state vibrational relaxation of azulene in supercritical ethane and propane (61-64). Relaxation rates of azulene in propane along a near-critical isotherm show the three-region dependence on density, as does the shift in the electronic absorption frequency. Their relaxation experiments in supercritical carbon dioxide, xenon, and ethane were done farther from the critical point, and the three-region behavior was not observed. The measured density dependence of vibrational relaxation in these fluids was... 

Figure 2.2-2 The visible, near, middle and far infrared region of the spectrum drawn in a scale linear in wavenumbers. The infrared (IR) and far-infrared (FIR) spectrum is recorded by absorption of light from a continuous spectrum in the range of A = 2.5. .. 100 pm = i) = 4000. .. 100 cm and A = 100. .. 1000 pm = 100. .. 10 cm". Raman spectra can be excited by monochromatic radiation, emitted by different lasers in the visible (VIS) or near-infrared range (NIR). Molecules emit Raman lines with a frequency difference AF to that of the exciting frequency >o between 0 and -f 4000 or - 4000 cm. Usually only the Raman spectrum which is shifted to. smaller wavenumbers, the Stokes Raman spectrum, is recorded. Its range is indicated by bars for different exciting lines Ar" " laser at 488 and 515 nm, HeNe laser at 623 nm, GaAs laser at 780 nm, and Nd YAG laser at 1064 nm.

The authors found that the in situ electrochemical potentiokinetic reactivation (EPR) data obtained for the monocation I" " showed that the two Mn centers had identical spin densities, and hence the cation showed delocalized mixed valence the near-infrared spectrum supported this postulate. If such a delocalized system existed on the infrared timescale, then two bands would be expected in the carbonyl region of the infrared spectrum of 1+ at frequencies intermediate between those of the Mn(I)/Mn(I) neutral complex, (1861 and 1934 cm ), and those of the fully oxidized Mn(II)/Mn(II) species, which were predicted to be ca. 1966 and 2048 cm on the basis of the bands observed for CpMn(II)(CO)2PPh3 +, (Cp = cyclopentadienyl). In contrast, four intense CO absorptions were observed near 1888, 1931, 1952, and 2003 cm , which were typical of the trapped valent Mn(II)/Mn(I) species. The variation in the response obtained on the EPR timescale, ca. 10 s, to that observed on the infrared timescale, ca. 10 s, was taken by the authors as implying a time-dependent localization process. 

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