Line broadening mechanisms

Lorentzian line shapes are expected in magnetic resonance spectra whenever the Bloch phenomenological model is applicable, i.e., when the loss of magnetization phase coherence in the xy-plane is a first-order process. As we have seen, a chemical reaction meets this criterion, but so do several other line broadening mechanisms such as averaging of the g- and hyperfine matrix anisotropies through molecular tumbling (rotational diffusion) in solution. 

The hne-shape function gives the profile of the optical absorption (and emission) band and contains important information about the photon-system interaction. Let us briefly discuss the different mechanisms that contribute to this function, or the different line-broadening mechanisms. 

In this application of the BWR theory, Hudson and Lewis assume that the dominant line-broadening mechanism is provided by the modulation of a second rank tensor interaction (i.e., ZFS) higher rank tensor contributions are assumed to be negligible. R is a 7 X 7 matrix for the S = 7/2 system, with matrix elements written in terms of the spectral densities J (co, rv) (see reference [65] for details). The intensity of the i-th transition also can be calculated from the eigenvectors of R. In general, there are four transitions with non-zero intensity at any frequency, raising the prospect of a multi-exponential decay of the transverse magnetization. There is not a one-to-one correspondence between the... 

On the basis of the Q-band spectrum giving better resolution than that the X- and S-bands, we concluded that the dominant line-broadening mechanism at Q-band EPR was g-strain and that there was broadening in the X- and S- band spectra. From spectra simulation in the Q-band spectrum, we obtained the g principal values of gxx = 5.20, gyy = 4.75, and gzz = 2.24, and the g-strain tensor principal values <7gxx =1.11 (0.2), dgyy = 1.11 (0.2), and dgzz = 0.19(0.01). 

B. Gd(III) EPR. The literature on Gd(III)EPR in solution is not extensive. Hudson and Lewis (27) have presented a theory for the electron spin relaxation of S ions (e.g., gadoliniumdll)) in solution. These authors assumed that the dominant line broadening mechanism for an ion is provided by the modulation of the zero-field splitting by a process with a characteristic time X. The transverse relaxation rate is given by... 

Pure dephasing has proven to be the dominant line-broadening mechanism in most liquids. However, the diversity among systems is great enough that potential contributions from other mechanisms must be considered for each system studied. 

As already mentioned, the lack of molecular motion in solids gives rise to broad resonances and the received spectral pattern consist of overlapped lines, hiding the valuable analytical information available from the isotropic chemical shifts. In principle, there are three line broadening mechanisms, described in the following (13). 

Thus, the occurrence of broad C resonances under H high-power decoupling and MAS conditions should neither lead to the automatic conclusion of lack of crystallinity of the sample, nor be considered a nuisance in contrast line broadening mechanisms under MAS are a valuable source of information for the extraction of kinetic parameters and activation energies of thermally activated processes. It should also be mentioned that further line broadening effects may arise from interference between ooi and the MAS frequency, leading to so-called rotary-resonance recoupling of dipolar... 

Derivation of zero- and first-order contributions to the effective Hamiltonian. Comparison of various sequences based on first-order contributions and spectral simulations to obtain a broad perspective of the line-broadening mechanism in spectra. 

Owing to the line broadening mechanisms, the physical widths of spectral lines in most radiation sources used in optical atomic spectrometry are between 1 and 20 pm. This applies both for atomic emission and atomic absorption line profiles. In reality the spectral bandwidth of dispersive spectrometers is much larger than the physical widths of the atomic spectral lines. 

The absorption profile of the atoms in the atom reservoir is a function of the different line broadening mechanisms and contibutions to the physical line width. 

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