The precision of nuclear magnetic resonance spectroscopy 1 (NMR) is limited by the signal-to-noise ratio, the measurement time T m and the linewidth ν = 1/(πT 2 ). Overcoming the T 2 limit is possible if the nuclear spins of a molecule emit continuous radio waves. Lasers 2,3 and masers 4-13 are self-organized systems which emit coherent radiation in the optical and microwave regime. Both are based on creating a population inversion of specific energy states. Here we show continuous oscillations of proton spins of organic molecules in the radiofrequency regime (raser 5 ). We achieve this by coupling a population inversion created through signal amplification by reversible exchange (SABRE) 14-16 to a high-quality-factor resonator. For the case of 15 N labelled molecules, we observe multi-mode raser activity, which reports di erent spin quantum states. The corresponding 1 H-15 N J-coupled NMR spectra exhibit unprecedented sub-millihertz resolution and can be explained assuming two-spin ordered quantum states. Our findings demonstrate a substantial improvement in the frequency resolution of NMR.Radio-wave masers (rasers) arise from the radiofrequency Zeeman splittings of nuclear spins such as 1 H, 3 He, 29 Al or 129 Xe. Rasers using 3 He and 129 Xe gas as the rasing medium employ spin exchange optical pumping 17 (SEOP) at T > 400 K to create sufficient population inversion 6-8 , whereas 29 Al solid 9 or 1 H Zeeman liquidstate masers 10 rely on dynamic nuclear polarization (DNP) techniques 18 or photochemical excitation 11 to invert populations. Solidstate maser action has been observed in pulsed mode at room temperature with pentacene 12 , and a continuous-mode solid-state maser based on nitrogen-vacancy centres in diamond has been proposed 13 .Here we report the observation of a liquid-state para-hydrogen pumped molecular raser that operates at 300 K with protons of organic molecules in solution and thereby avoids costly high magnetic fields, high vacuum, optical pumping or DNP techniques. We continuously supply para-hydrogen (p-H 2 ) gas into a solution containing the raser active molecules and an iridium-based SABRE catalyst [14][15][16] . This spin-order transfer catalyst creates population inversion, equivalent to a negative spin temperature, on target molecules without altering their molecular structure. Coupling of these hyperpolarized molecules to a high-Q resonator 19 produces a sustained raser signal comprised of frequencies that originate from the scalar couplings of nuclei within the molecule.The operating principles for the 3 He Zeeman maser 6-8 are a starting point to assess challenges associated with the design of a SABRE-pumped room-temperature proton raser working at low frequencies. Masing starts once the radiation-damping rate 1/τ rd , which quantifies the coupling between the resonator and the nuclear spins, satisfies the conditionHere the apparent transverse relaxation rate 1/T * 2 = 1/T 2 + 1/τ p is the sum of the transverse relaxation rate 1/T 2 and the pumping rate 1/τ p . The radiation-dampi...
The development of powerful sensors for the detection of weak electromagnetic fields is crucial for many spectroscopic applications, in particular, for nuclear magnetic resonance (NMR). Here we present a comprehensive, new theoretical model for boosting the signalto-noise ratio, validated by liquid-state 1 H, 129 Xe, and 6 Li NMR experiments at low frequencies (20-500 kHz), using an external resonator with a high quality factor combined with a low-quality-factor input coil. In addition to a very high signal-to-noise ratio, this external high quality-factor enhanced NMR exhibits striking features such as a large flexibility with respect to input coil parameters, and a square-root dependence on the sample volume, and signifies an important step towards compact NMR spectroscopy at low frequencies with small and large coils.Nuclear magnetic resonance spectroscopy preferably operates at high magnetic fields (10-24 T) to benefit from high sensitivity and high chemical shift dispersion. The signal is detected by nuclear induction in coils, which are part of a resonant circuit [1][2][3] . NMR operating at high frequencies (~500 MHz) is particularly in demand for microcoil-based spectroscopy with sample volumes smaller than 1 L (ref. 4). Microcoils, with their small inductance (L < 1 nH), cannot readily be tuned to lower resonance frequencies (<1 MHz) owing to the large capacity (C > 1 mF) necessary to fulfil the resonance condition L C . Sillerud et al. 5 were the first to address this problem by adding an external inductor to a microcoil for NMR detection at 40 MHz. Coffey et al. 6 reported a very weak frequency dependence of the signal-to-noise ratio (SNR) when comparing hyperpolarized NMR/MRI at 0.047 T and 4.7 T (keeping the polarization constant), with a possibly higher SNR of hyperpolarized low-field NMR/MRI. So far, NMR spectroscopy with microcoil detection and a large SNR have not be realized at very low Larmor frequencies -1000 kHz, where is the gyromagnetic ratio of the nuclear spin species and B0 is the strength of the static magnetic field. Instead, other unconventional schemes for NMR detection are being explored at low frequencies. Such lowfrequency NMR field sensors are superconducting quantum interference devices (SQUIDs, refs 7,8) and atomic magnetometers 9,10 with sensitivities better than 1 fT/Hz 1/2 . Moreover, a single nitrogen vacancy (NV) centre in diamond can detect 100 out of 10 4 proton spins, which are
High-field nuclear magnetic resonance (NMR) spectroscopy is an indispensable technique for identification and characterization of chemicals and biomolecular structures. In the vast majority of NMR experiments, nuclear spin polarization arises from thermalization in multi-Tesla magnetic fields produced by superconducting magnets. In contrast, NMR instruments operating at low magnetic fields are emerging as a compact, inexpensive, and highly accessible alternative but suffer from low thermal polarization at a low field strength and consequently a low signal. However, certain hyperpolarization techniques create high polarization levels on target molecules independent of magnetic fields, giving low-field NMR a significant sensitivity boost. In this study, SABRE (Signal Amplification By Reversible Exchange) was combined with high homogeneity electromagnets operating at mT fields, enabling high resolution 1H, 13C, 15N, and 19F spectra to be detected with a single scan at magnetic fields between 1 mT and 10 mT. Chemical specificity is attained at mT magnetic fields with complex, highly resolved spectra. Most spectra are in the strong coupling regime where J-couplings are on the order of chemical shift differences. The spectra and the hyperpolarization spin dynamics are simulated with SPINACH. The simulations start from the parahydrogen singlet in the bound complex and include both chemical exchange and spin evolution at these mT fields. The simulations qualitatively match the experimental spectra and are used to identify the spin order terms formed during mT SABRE. The combination of low field NMR instruments with SABRE polarization results in sensitive measurements, even for rare spins with low gyromagnetic ratios at low magnetic fields.
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