Adiabatic and fast passage ultra-wideband inversion in pulsed EPR

• Published in: Journal of magnetic resonance (1997) 2013-05, Vol.230, p.27-39
• Main Authors: Doll, Andrin, Pribitzer, Stephan, Tschaggelar, René, Jeschke, Gunnar
• Format: Article
• Language: English
• Subjects: Adiabatic flow; Adiabatic pulses; Algorithms; AWG; Bandwidth; Computer Simulation; Copper; DEER; Electron Spin Resonance Spectroscopy - methods; Electrons; Excitation; Excitation bandwidth; Group delay; Hardware modeling; Inversion recovery; Inversions; Models, Chemical; Modulation; Pulse compensation; Sensitivity enhancement; Signal Processing, Computer-Assisted; Spectral diffusion; Ultrawideband
• ISSN: 1090-7807; 1096-0856
• DOI: 10.1016/j.jmr.2013.01.002

[Display omitted] ► Extension of ordinary pulsed EPR spectrometer by 12GS/s AWG for chirped excitation. ► Pulses compensating for bandwidth effects to exceed actual bandwidth limitations. ► Suppression of spectral diffusion in IR by almost complete inversion of nitroxide. ► Threefold enhancement of DEER modulation depth when pumping on a metal center. We demonstrate that adiabatic and fast passage ultra-wideband (UWB) pulses can achieve inversion over several hundreds of MHz and thus enhance the measurement sensitivity, as shown by two selected experiments. Technically, frequency-swept pulses are generated by a 12 GS/s arbitrary waveform generator and upconverted to X-band frequencies. This pulsed UWB source is utilized as an incoherent channel in an ordinary pulsed EPR spectrometer. We discuss experimental methodologies and modeling techniques to account for the response of the resonator, which can strongly limit the excitation bandwidth of the entire non-linear excitation chain. Aided by these procedures, pulses compensated for bandwidth or variations in group delay reveal enhanced inversion efficiency. The degree of bandwidth compensation is shown to depend critically on the time available for excitation. As a result, we demonstrate optimized inversion recovery and double electron electron resonance (DEER) experiments. First, virtually complete inversion of the nitroxide spectrum with an adiabatic pulse of 128ns length is achieved. Consequently, spectral diffusion between inverted and non-inverted spins is largely suppressed and the observation bandwidth can be increased to increase measurement sensitivity. Second, DEER is performed on a terpyridine-based copper (II) complex with a nitroxide-copper distance of 2.5nm. As previously demonstrated on this complex, when pumping copper spins and observing nitroxide spins, the modulation depth is severely limited by the excitation bandwidth of the pump pulse. By using fast passage UWB pulses with a maximum length of 64ns, we achieve up to threefold enhancement of the modulation depth. Associated artifacts in distance distributions when increasing the bandwidth of the pump pulse are shown to be small.