Two-dimensional electron spin echo spectroscopy and slow motions

• DOI: 10.1063/1.447316
• Publication Date: 2003-02-28T17:34:05Z
• Authors: Glenn L. Millhauser; Jack H. Freed
• Keywords: 02 engineering and technology; 0210 nano-technology

A technique is described for the study of slow molecular motions that employs two-dimensional electron spin echo (2D-ESE) spectroscopy. An ‘‘echo-induced ESR spectrum’’ is obtained with a two pulse 90° (τ)/() 180° sequence by recording the echo height as the dc field is swept. A series of sweeps with different τ are Fourier transformed to yield a two-dimensional spectrum comprised of an echo-induced spectrum along the x axis and the homogeneous widths along the y axis. A convenient theoretical analysis appropriate for slow motions is given and it is shown that the 2D spectrum may be regarded as a graph of the widths vs the resonance positions of the individual ‘‘dynamics spin packets’’ that constitute the spectrum. An experimental study of the spin probe Tempone in 85% glycerol/H2O solvent for slow motions (i.e., rotational correlational times of order 10−5–10−6 s) shows variations in homogeneous widths (i.e., T−12) over the spectrum. This is analyzed in terms of canonical models of rotational reorientation—Brownian, free, and jump diffusion, and it is found that the variations of T−12 across the spectrum show features similar to a Brownian model. A special normalized contour plot of the 2D spectra is introduced which clearly emphasizes the variation of T−12 across the spectrum. The enhanced information content of these 2D spectra is discussed, and it is shown how 2D-ESE spectroscopy is both an improvement on and a complement to cw techniques for studying motional dynamics. Experimental extensions and limitations are also discussed.