Mims Electron-Nuclear Double Resonance (ENDOR) with chirp microwave pulses

Area of Science:

• Magnetic Resonance Spectroscopy
• Physical Chemistry
• Spectroscopic Techniques

Background:

• Electron Paramagnetic Resonance (EPR) spectroscopy is crucial for studying paramagnetic species.
• Traditional Mims ENDOR experiments can be time-consuming and require orientation-selective measurements.
• Broadband excitation and advanced data processing are needed to improve EPR and ENDOR capabilities.

Purpose of the Study:

• To develop a modified Mims ENDOR experiment using microwave chirp pulses for broadband excitation in W-band EPR.
• To correlate hyperfine couplings with the EPR spectrum using 2D spectra obtained from Fourier transform (FT) of stimulated echoes.
• To demonstrate a more efficient and informative alternative to standard ENDOR measurements.

Main Methods:

• Utilized microwave (MW) chirp pulses for broadband excitation in W-band (94GHz) EPR.
• Applied Fourier transform (FT) to stimulated echoes generated by the chirp pulses.
• Analyzed the effect of chirp pulses on the Mims blind spot function using CHirp Echo Epr SpectroscopY (CHEESY).
• Demonstrated the method on a nitroxide-¹⁹F model system.

Main Results:

• Achieved broadband excitation and FT of stimulated echoes in W-band EPR.
• Generated 2D spectra correlating hyperfine couplings with the EPR spectrum.
• Obtained increased resolution in the EPR dimension, providing insights into hyperfine coupling tensor orientation.
• Observed chemical shielding anisotropy at W-band frequency.
• Showcased the utility of FT of spin echoes even with rectangular MW pulses at Q-band (34GHz).

Conclusions:

• The combination of Mims ENDOR with FT of MW echoes offers a valuable method for detailed electron-nuclear spin interaction tensor analysis.
• This approach simplifies data acquisition, requiring only one experiment at a single EPR resonance field.
• The technique is potentially useful for resolving complex, multi-component ENDOR spectra and provides enhanced spectral resolution.