Sensitivity enhancement by population transfer in Gd(iii) spin labels

Abstract

In order to enhance echo signals observed with selective pulses, equilibrium populations of the energy levels of S = 7/2 Gd(III) spin labels are rearranged with frequency-swept passage pulses. To transfer population from as many energy levels as possible, the 2 μs long passage pulses range over more than 1 GHz. Application of this technique at Q-band frequencies to three different Gd(III) complexes and spin dynamics simulations reveal large signal enhancements beyond 100% for Gd(III) complexes with zero-field splitting parameters below 1 GHz. For complexes with larger splittings, experimental enhancements are on the order of 90%. Moreover, population transfer is combined with distance measurements on a model system with a pair of Gd(III) ions. As a result, a signal enhancement of 85% is achieved without inducing changes in the obtained distance information. Besides this enhancement by population transfer, a dipolar modulation depth of 9% is demonstrated, which results in a total enhancement of 3.3 with respect to data obtained with monochromatic rectangular pulses. The limitations of the population transfer technique are discussed. In particular, the extraordinary broad pulse bandwidths caused heating effects and pulse distortions, which constrain the pulse length and thus the achievable signal enhancement.

Gd(III)–Gd(III) distance measurements with chirp pump pulses

“…All experiments were performed on a customized Bruker Elexsys E580 Q-band spectrometer [35] with a previously described extension by a 12 GS/s AWG (Agilent M8190A) [30]. The temperature was 10 K and the repetition rate of the pulse sequences was 342 µs in the case of Gd-ruler 1 3 and 716 µs for all the other Gd-rulers.…”

“…An exception is the acquisition of resonator profiles ν 1 ( f ) based on transient nutation experiments, which were all acquired at a repetition time of 342 µs. The timing and sequence parameters for these transient nutation experiments were identical to the ones described previously in [30].…”

“…S22 in the SI. For pre-polarized DEER experiments [30], two chirp pulses, with pulse duration of 2 µs each, preceded the above DEER sequence. The time gap between the two pre-polarization pulses was 300 ns and the time gap between the last pre-polarization pulse and the DEER sequence was 3 µs.…”

“…In this work, we investigate how Gd-Gd DEER experiments can be improved with chirp pump pulses synthesized by an arbitrary waveform generator (AWG). A key advantage of arbitrarily shaped pulses is the improved excitation bandwidth, as demonstrated in a number of recent studies [25][26][27][28][29][30][31][32]. For chirp pulses, pulse performance can be quantified by the critical adiabaticity factor Q crit using the Landau-Zener equation [31].…”

“…We have demonstrated recently that the sensitivity of Gd-Gd DEER can be enhanced by rearranging the equilibrium populations and by using a chirp pump pulse [30]. The focus of this previous study was on the long and intense chirp pulses required for efficient population transfer over frequency ranges beyond 1 GHz.…”