A. Henstra

High dynamic nuclear polarization at room temperature

Abstract The highly polarized photo-excited triplet state of pentacene in a naphthalene crystal is used for pulsed dynamic nuclear polarization at room temperature. Thus far an enhancement of 5500 of the naphthalene proton polarization has been reached. For this purpose, a newly developed technique, the integrated solid effect, performed while obeying the Hartmann-Hahn condition, is used to transfer the triplet polarization efficiently to the nuclear spin system.

Enhanced dynamic nuclear polarization by the integrated solid effect

Abstract A spectacular gain in efficiency of DNP by the differential solid effect is demonstrated in a Si:B sample using inversion of the polarization of electron spins. The best results are obtained using an adiabatic fast passage through the ESR line.

Nuclear spin orientation via electron spin locking (NOVEL)

In this communication we present what we believe to be the first observation of resonant transfer of electron spin polarization to a nuclear spin system. Our method is called nuclear spin orientation via electron spin locking (NOVEL) and is, in some aspects, similar to the technique of dynamic nuclear polarization (I). The nuclear spin system is present in a solid doped with paramagnetic centers and microwave irradiation is used to transfer the electron spin polarization of these paramagnetic centers to the nuclear spins. However, instead of cw microwave irradiation, we apply pulsed microwave fields in the NOVEL experiment. Our experimental procedure is closely related to the method of Hartmann and Hahn (2) for transferring spin polarization from one nuclear spin species to another. As we will explain in more detail below, our method consists of bringing the electron spins in a so-called spin-locked state in the rotating frame where they have the same resonance frequency as the nuclear spins in the laboratory frame. The presence of dipolar interaction between the two spin species then allows for the resonant transfer of the electron spin polarization to the nuclear spins. The sample in our experiment is a single crystal of silicon doped with 1017 cms3 boron acceptors. The paramagnetic centers are the holes bound to these acceptors. We apply a uniaxial stress of 4 kbar along the (111) axis of the crystal. Then the ground state of the holes forms a Kramers doublet described by an effective electron spin S = 1 with an axially symmetric g tensor. The principal values of this tensor are gll = 1.21 and g, = 2.43 for the directions parallel and perpendicular to the direction of the uniaxial stress (3). The nuclear spin system is formed by the 29Si I = 1 nuclei having a gyromagnetic ratio y1 = 2~8.458 MHz/T and present at a natural abundance of 4.7%. Our NOVEL experiments are performed at 1.2 K with an electron spin-echo (ESE) spectrometer operating at 9.5 GHz equipped with a specially designed double, splitring resonator which allows NMR measurements to be performed and the uniaxial stress to be applied to the sample. The details of the construction of this resonator will be described elsewhere. The magnetic field & is oriented perpendicular to the (111) axis of the crystal and has an amplitude of 0.28 T. In these circumstances the electron spins reach a thermal equilibrium polarization Pp = (&)/S = 0.19 with a time constant Ti, = 20 ms, as a result of electron spin-lattice relaxation processes. With a repetition time long compared with Tr, we apply composite microwave pulses

Transient oscillations in pulsed dynamic nuclear polarization

Abstract We have measured the time evolution of the cross-polarization processes between the proton spins of a naphthalene host crystal and the photo-excited triplet electron spins of pentacene guest molecules in a pulsed DNP experiment. The observed oscillatory behaviour is explained as arising from dominant dipolar interactions between the nuclear and electron spins.

Dynamic nuclear polarisation via the integrated solid effect I: theory

In the hyperpolarisation method known as dynamic nuclear polarisation (DNP), a small amount of unpaired electron spins is added to the sample containing the nuclear spins and the polarisation of these unpaired electron spins is transferred to the nuclear spins by means of a microwave field. Traditional DNP uses weak continuous wave (CW) microwave fields, so perturbation methods can be used to calculate the polarisation transfer. A much faster transfer of the electron spin polarisation is obtained with the integrated solid effect (ISE) which uses strong pulsed microwave fields. As in nuclear orientation via electron spin locking, the polarisation transfer is coherent, similar to the coherence transfer between nuclear spins. This paper presents a theoretical approach to calculate this polarisation transfer. ISE is successfully used for a fast polarisation transfer from short-lived photo-excited triplet states to the surrounding nuclear spins in molecular crystals. These triplet states are strongly aligned in the photo-excitation process and do not require the low temperatures and strong magnetic fields needed to polarise the electron spins in traditional DNP. In the following paper, the theory is applied to the system naphthalene-h8 doped with pentacene-d14 which provides the photo-excited triplet states, and compared with experimental results.

Dynamic nuclear polarisation via the integrated solid effect II: experiments on naphthalene-h8 doped with pentacene-d14

In dynamic nuclear polarisation (DNP), also called hyperpolarisation, a small amount of unpaired electron spins is added to the sample containing the nuclear spins, and the polarisation of these unpaired electron spins is transferred to the nuclear spins by means of a microwave field. Traditional DNP polarises the electron spin of stable paramagnetic centres by cooling down to low temperature and applying a strong magnetic field. Then weak continuous wave microwave fields are used to induce the polarisation transfer. Complicated cryogenic equipment and strong magnets can be avoided using short-lived photo-excited triplet states that are strongly aligned in the optical excitation process. However, a much faster transfer of the electron spin polarisation is needed and pulsed DNP methods like nuclear orientation via electron spin locking (NOVEL) and the integrated solid effect (ISE) are used. To describe the polarisation transfer with the strong microwave fields in NOVEL and ISE, the usual perturbation methods cannot be used anymore. In the previous paper, we presented a theoretical approach to calculate the polarisation transfer in ISE. In the present paper, the theory is applied to the system naphthalene-h8 doped with pentacene-d14 yielding the photo-excited triplet states and compared with experimental results.

Microwave-induced optical nuclear polarization in Si:P:B

Abstract The first successful experiments of microwave-induced optical nuclear polarization in compensated silicon are presented. Bound holes are created at the boron sites using band-gap light of 1.047 μm. Subsequently, the high spin polarization of these bound holes is transferred to the 29 Si nuclei using microwave irradiation, resulting in an enhancement of the nuclear magnetic resonance signal. It is shown that a long lifetime of the nuclear spin polarization thus created is obtained, once the excitation light is shut off.

Muon spin rotation (μSR) detected electron spin echo envelope modulation (ESEEM)

Abstract The recently developed method of NOVEL, in which very rapid cross polarization between electron spins and nuclear spins is induced, is proposed to be extended to the field of μSR. This would provide a method to unravel the ESR and ESEEM spectra of muonated radicals. The proposal is backed up by an experiment in a molecular crystal, where the ESEEM spectrum of an excited triplet state is observed indirectly via NMR.

Resonant polarization transfer from electron spins to nuclear spins-or to muon spins-in semiconductors

A review is given of newly developed pulsed Electron Spin Resonance (ESR) methods for dynamic polarization of nuclear spins. The application of two of these methods, Nuclear Orientation Via Electron spin Locking (NOVEL) and the Integrated Solid Effect (ISE), for the polarization of nuclear spins in semiconductors is discussed in more detail. It is proposed to use these methods to study the ESR spectrum of unpaired electrons in the vicinity of muons that are bound in a solid. Thus, ESR would be observed with a sensitivity which is enhanced by about ten orders of magnitude compared to conventional ESR.