Direct covariance spectroscopy

In liquid solutions, nuclear spins are correlated through J-couplings, whereas in solids dipolar interactions come into play as well. The first report on the application of covariance to soHd-state NMR was published by Hu et al. in which an indirect covariance homonuclear correlation (HOMCOR) spectrum was obtained from a 2D-FT heteronuclear correlation (HETCOR) spectrum. The indirect covariance scheme, which will be discussed in Section 5.1, contrasts with the original direct covariance spectroscopy, where the covariance processing is performed over an array of one-dimensional spectra. 

Hu et al. also reported on the first appHcation of direct covariance spectroscopy to sohd-state NMR to reveal correlations in amino acids. 

Indirect covariance spectroscopy was proposed by Zhang and Briischweiler [85] soon after publication of their pioneering work on covariance spectroscopy [58]. Its apphcation to soHd-state NMR was reported a few years later by Hu et al. [86]. The procedure of the indirect covariance spectroscopy starts with a 2D Fourier-transformed FIETCOR spectrum. This is in contrast to the original direct covariance spectroscopy, in which covariance processing is appHed to an array of one-dimensional spectra so that Fourier transformation is performed only with respect to the direct dimension. From a single I—S 2D FiETCOR spectrum, a pair of HOMCOR spectra are... 

Perhaps the most interesting application of electron-electron covariance mapping relates to the question of the major mode of multiple ionization of atoms and molecules. Luk et al. [34] studied the multiple ionization process in Xe using a laser of 193 nm wavelength and 10 ps pulse length and conventional ion TOP spectroscopy they suggested that it was direct (a collective, instantaneous emission of many electrons). Lambropoulos [35] pointed out that, with a laser of such modest rise time, the ionization must proceed sequentially. In fact L Huillier et al. [36] had also studied Xe at 532 nm and observed a knee in the curve of log (ion counts) vs log (laser intensity) for Xe that they attributed to direct double ionization. 

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