Exchange-narrowed spectrum

The quantum theory must describe not only the shape of a resolved rotational structure of the Q-branch but its transformation with increase of pressure to a collapsed and well-narrowed spectrum as well. A good example of such a transformation is shown in Fig. 4.6. The limiting cases of very low and very high pressures are relatively easy to treat as they relate to slow modulation and fast modulation limits of frequency exchange. 

This single line (go = 1.935 0.005, AH 140 G) is usual for V4+ centers, which are coupled by strong spin-exchange interaction [23, 164], These V4+ ions are localized in some areas with high local concentration (V4+)s (the so called nano-phases - because of their probable size), which can be estimated from the exchange narrowing of a spectrum by the equation [23] ... 

How fast should one spin the sample to achieve line narrowing Just as with chemical exchange narrowing, the criterion for motion to narrow the spectrum is that the rate of... 

The problem of exchange narrowing involving a correlation time spectrum was first tackled by Bloembergen et al. (1948) in the original theory for the narrow N.M.R. lines observed in liquids. The theory has... 

The spectrum is said to be exchange narrowed since the electron spins are exchanging so fast that the time average of the hyperfine field is close to zero. Electron spin exchange must be avoided if narrow lines are desired in solution and this is readily achieved if the concentration is maintained below ca. lO" mol/L. 

Aerobic fermentation of a common medium by Streptomyces griseus produces an antibiotic called stenothricin which was isolated by ion-exchange chromatography and purified by gel filtration. This antibiotic has a narrow spectrum and inhibits cell wall biosynthesis (79). 

The quantum theory of spectral collapse presented in Chapter 4 aims at even lower gas densities where the Stark or Zeeman multiplets of atomic spectra as well as the rotational structure of all the branches of absorption or Raman spectra are well resolved. The evolution of basic ideas of line broadening and interference (spectral exchange) is reviewed. Adiabatic and non-adiabatic spectral broadening are described in the frame of binary non-Markovian theory and compared with the impact approximation. The conditions for spectral collapse and subsequent narrowing of the spectra are analysed for the simplest examples, which model typical situations in atomic and molecular spectroscopy. Special attention is paid to collapse of the isotropic Raman spectrum. Quantum theory, based on first principles, attempts to predict the. /-dependence of the widths of the rotational component as well as the envelope of the unresolved and then collapsed spectrum (Fig. 0.4). 

As discussed in [91], the shape of a static spectrum determines significantly the spectral transformation as frequency exchange increases. In particular, spectral narrowing will take place only if the second moment of the spectrum is finite. In our case... 

Consequently, when the gas density increases, the increase of the frequency exchange rate causes the narrowing of the isotropic scattering spectrum. 

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