Stochastic ENDOR

Enhanced ENDOR Spectroscopy

Two common artifacts present in pulse ENDOR spectra, baseline distortion and asymmetric intensities, can be eliminated via the stochastic excitation mode available with the E560 DICE unit. Baseline distortions are a result of the heating of sample and resonator by the RF pulses causing changes in resonator coupling and matching.  Asymmetric ENDOR intensities are a phenomenon arising from the interplay of the various relaxation processes (T1e, T1n, and T1x) in the system.

Stochastic ENDOR: The Setup

Enabling 'Stochastic Mode' in Xepr, directs the DICE unit to use a set of random RF frequencies for the defined RF pulse. For each scan, the sequence of RF frequencies applied is random and unrelated to the previous scans resulting in a truly stochastic excitation of the sample.

stochasti endor enhancement
Single shot, single scan 31P ENDOR spectrum of phosphorous doped silicon under linear excitation (top) and stochastic excitation (bottom). Sample courtesy Marco Fanciulli.

Sensitivity Enhancement for Slow Relaxing Species

The slow relaxation present in some EPR systems present challenges for the timely acquisition of ENDOR spectra.  For maximum ENDOR effect, saturation of the NMR transitions should be avoided.  One route is to use long repetition times which result in long accumulation times to achieve reasonable signal-to-noise levels.  Alternatively, this saturation can be minimized with stochastic excitation and short repetition times to significantly increase the signal-to-noise in shorter accumulation times.

 

Faithful Spectrum Representation

stochastic endor intensities
W-Band 1H ENDOR spectrum with stochastic (top) and linear (bottom) RF excitation. In collaboration with Robert Bittl and Susanne Pudollek.

 The relationship between the electronic relaxation time, T1e, the nuclear relaxation time, T1n, and cross relaxation, T1x, can lead to asymmetric line intensities in ENDOR spectra. This phenomenon can be frequently observed in low temperature W-band pulse-ENDOR experiments. While these asymmetries can be used to provide the sign of the hyperfine coupling, what is often desired is an ENDOR spectrum free of such asymmetry. Stochastic excitation minimizes the effects of relaxation which result in negative intensities with linear excitation.