Coherent Polarization Transfer Effects Are Crucial for Interpreting Low-Field CIDNP Data

In this work we demonstrate that low-field chemically induced dynamic nuclear polarization (CIDNP) is strongly affected by re-distribution of polarization, which is formed in the course of spin evolution in transient radical pairs, in diamagnetic reaction products. This phenomenon is of importance when the spins of the reaction product are coupled strongly meaning that spin–spin interactions between them are comparable to the differences in their Zeeman interactions with the external magnetic field. In this case, polarization transfer relies on a coherent mechanism; as a consequence, spins can acquire significant polarization even when they have no hyperfine coupling to the electron spins in the radical pairs, i.e., cannot be polarized directly by CIDNP. This is demonstrated by taking CIDNP of n-butylamine as an example: in this case only the α-CH₂ protons are polarized directly, which is confirmed by high-field CIDNP, whereas the β-CH₂, γ-CH₂ and δ-CH₃ protons get polarized only indirectly due to the transfer of polarization from the α-CH₂ protons. These results show that low-field CIDNP data should be interpreted with care to discriminate between the effects of spin evolution in transient radical pairs and in diamagnetic reaction products.     

基于散射实验极化靶的2.5T/1.3K的动态核极化(DNP)系统的开发

动态核极化法(Dynamic Nuclear Polarization, DNP)是利用热平衡下的电子在磁场中的高自旋极化率转移到原子核自旋的技术,从而极大的提高原子核自旋极化率。多种动态极化靶材料已广泛的用于自旋物理散射实验。本文介绍一种简单实用,共同开发的日本山形大学DNP系统,包括超导磁场,氦4蒸发恒冷器,微波系统以及NMR核磁共振检测系统,测得中子靶材料氘带丁醇(D-butanol)中氘核的极化率在2.5T/1.3K达到+6.5%。     

Dynamic nuclear polarization in pulsed ENDOR experiments

Properly prepared pulse sequences of microwave and radio frequency have been employed to investigate the effect of polarization transfer from the polarized photo excited triplet state of pentacene in p-terphenyl crystals to the surrounding protons in pulsed ENDOR experiments. The ENDOR signal, measured as the change of electron spin echo (ESE) amplitude, is affected by the mode of RF pulses. When B0 parallelx (the long molecular axis), the ESE amplitude of the high-field transition of the triplet state changes from the maximum positive to zero with a pi RF pulse, and to the maximum negative with a 2pi pulse, while that of the low-field transition changes from nearly zero to the maximum negative as the RF pulse width increases. The effect is attributed to the strong electron spin polarization produced in the creation of the photoexcited triplet state and the subsequent efficient electron- nuclear polarization transfer process.     

Theoretical Study of Dipolar Relaxation of Coupled Nuclear Spins at Variable Magnetic Field

A theoretical study was made of magnetic field-dependent dipolar relaxation in two- and three-spin systems. The results for the nuclear magnetic relaxation dispersion (NMRD) curves were compared with those for the simpler model of fluctuating local fields. For both models it was found that at low fields spins tend to relax with a common T ₁-relaxation time. Sharp features in the NMRD curves coming from nuclear spin level anti-crossings are also predicted by both models. However, the simple model fails to describe the behavior of so-called long-lived spin states (LLS). We have studied the LLS as function of magnetic field and molecular geometry and simulated experimental results for the LLS in histidine amino acid obtained at the laboratory of Prof. H.-M. Vieth (Free University Berlin, Germany). In addition, we described polarization transfer in a three-spin system where two spins are protons, which are initially hyperpolarized by para-hydrogen induced polarization (PHIP), while the third spin is a spin ½ hetero-nucleus, which acquires polarization in the course of cross-relaxation.     

The theory of nuclear orientation via electron spin locking (NOVEL)

Nuclear orientation via electron spin locking (NOVEL) is a technique to orient nuclear spins embedded in a solid. Like other methods of dynamic nuclear polarization (DNP) it employs a small amount of unpaired electron spins and uses a microwave field to transfer the polarization of these unpaired electron spins to the nuclear spins. Traditional DNP uses CW microwave fields, but NOVEL uses pulsed electron spin resonance (ESR) techniques: a 90 degree pulse–90 degree phase shift–locking pulse sequence is applied and during the locking pulse the polarization transfer is assured by satisfying the Hartmann–Hahn condition. The transfer is coherent and similar to coherence transfer between nuclear spins. However, NOVEL requires an extension of the existing theory to many, inequivalent nuclear spins and to arbitrary, i.e. high electron and nuclear spin polarization. In this paper both extensions are presented. The theory is applied to the system naphthalene doped with pentacene, where the proton spins are polarized using the photo-excited triplet states of the pentacene molecules and found to show excellent agreement with the experimentally observed evolution of the polarization transfer during the locking pulse.     

High-resolution study of nuclear magnetic relaxation dispersion of purine nucleotides: effects of spin-spin coupling

By combining magnetic field cycling in the range from 0.1mT to 7T with high-resolution NMR detection the T(1) relaxation dispersion (nuclear magnetic relaxation dispersion (NMRD)) of protons in the nucleotides adenosine mono-phosphate and guanosine mono-phosphate was measured in a site-specific way. While at high field the individual spins have distinctly different T(1) times, their scalar spin-spin interaction fulfills at low field the condition of strong coupling and leads to convergence of their T(1) dispersion curves. In addition, the spin-spin coupling can lead to oscillatory components in the relaxation kinetics traceable to a coupling between spin polarization and coherence in the relaxation process. As a consequence the NMRD curves do not directly reflect the spectral density function of the motional processes, but the effects of motion and spin coupling must be separated for a reliable evaluation. A theoretical approach is described allowing such an analysis.     

Spin transfer from an optically pumped alkali vapor to a solid

We report enhancement of the spin polarization of 133Cs nuclei in CsH salt by spin transfer from an optically pumped cesium vapor. The nuclear polarization was 4.0 times the equilibrium polarization at 9.4 T and 137 degrees C, with larger enhancements at lower fields. This work is the first demonstration of spin transfer from a polarized alkali vapor to the nuclei of a solid, opening up new possibilities for research in hyperpolarized materials.     

Spin-labeled gel for the production of radical-free dynamic nuclear polarization enhanced molecules for NMR spectroscopy and imaging

Dynamic nuclear polarization (DNP) has recently received much attention as a viable approach to enhance the sensitivity of nuclear magnetic resonance (NMR) spectroscopy and the contrast of magnetic resonance imaging (MRI), where the significantly higher electron spin polarization of stable radicals is transferred to nuclear spins. In order to apply DNP-enhanced NMR and MRI signal to biological and in vivo systems, it is crucial to obtain highly polarized solution samples at ambient temperatures. As stable radicals are employed as the source for the DNP polarization transfer, it is also crucial that the highly polarized sample lacks residual radical concentration because the polarized molecules will be introduced to a biological system that will be sensitive to the presence of radicals. We developed an agarose-based porous media that is covalently spin-labeled with stable radicals. The loading of solvent accessible radical is sufficiently high and their mobility approximates that in solution, which ensures high efficiency for Overhauser mechanism induced DNP without physically releasing any measurable radical into the solution. Under ambient conditions at 0.35 T magnetic field, we measure the DNP enhancement efficiency of (1)H signal of stagnant and continuously flowing water utilizing immobilized stable nitroxide radicals that contain two or three ESR hyperfine splitting lines and compare them to the performance of freely dissolved radicals.     

Polarized proton collisions at 205 GeV at RHIC

The Brookhaven Relativistic Heavy Ion Collider (RHIC) has been providing collisions of polarized protons at a beam energy of 100 GeV since 2001. Equipped with two full Siberian snakes in each ring, polarization is preserved during acceleration from injection to 100 GeV. However, the intrinsic spin resonances beyond 100 GeV are about a factor of 2 stronger than those below 100 GeV making it important to examine the impact of these strong intrinsic spin resonances on polarization survival and the tolerance for vertical orbit distortions. Polarized protons were first accelerated to the record energy of 205 GeV in RHIC with a significant polarization measured at top energy in 2005. This Letter presents the results and discusses the sensitivity of the polarization survival to orbit distortions.     

~(129)Xe核自旋极化转移增强分子的核磁共振信号

激光极化的12 9Xe核具有极高的非平衡极化度和长的弛豫时间 ,这一特点使得它能够极化转移增强液体、固体或者固体表面分子中原子核自旋极化。因而 ,提高了它们的核磁共振探测灵敏度和扩展了在材料和表面科学研究中的应用。综述激光极化12 9Xe核与其它分子中原子核之间的极化转移研究与进展 ,介绍相关物理机制和参数的测量。     

Effect of Electric Field on Spin Polarized Current in Ferromagnetic/Organic Semiconductor Systems

Considering the special carriers in organic semiconductors, the spin polarized current under electric field in a ferromagnetic/organic semiconductor system is theoretically studied. Based on the spin-diffusion theory, the current spin polarization under the electric field is obtained. It is found that electric field can enhance the current spin polarization.     

Field cycling by fast NMR probe transfer: Design and application in field-dependent CIDNP experiments

A novel field-cycling unit with fast digital positioning of a high-resolution nuclear magnetic resonance probe in a spatially varying magnetic field is described and used to measure CIDNP spectra of the amino acid-dye (histidine-bipyridyl) photoreaction system in the range between 0 and 7 T. The pattern of nuclear polarization varies with the magnetic field. In particular, strong polarization with an emission/absorption pattern (multiplet effect) is found at low field for two histidine ringprotons with scalar coupling below 3 Hz visible only because of the high resolution made possible by the new field-cycling setup. Also for the CH₂ protons in the β-position a multiplet effect is observed having a pattern changing with magnetic field. By analysis of the spin nutation the non-Boltzmann population differences of the nuclear levels have been determined.     

Two examples of in-flight spin flippers

In precise experiments with polarized beam it’s very often appear a necessity to change beam polarization on opposite. If such operation does not change other beam parameters, it helps to avoid or minimize some systematic errors. It is especially important in experiments, where spin dependent effect is small enough. This paper describes two set of equipments, that make spin flip for extracted beams. In both cases, these devices are absolutely distinct, because they are appropriate for different particles and at different energy range. The first of them is intended for future muon (g-2) experiment, which is under preparation now at JPARC. Here, the muon spin flip will be done by chain of electrostatic and magnetic bends at the kinetic energy 340 keV. A beam matching is provided by a number of short solenoids. The other flipper (or Siberian snake) will rotate spin of protons or antiprotons, which come from Λ-meson decay with the energy up to 40 GeV. This experiment (no. 24) is planed at IHEP, Protvino. In this case, two superconducting helical magnets with opposite helicities and magnetic field 4.5 T will be used. To correct beam trajectory, additional dipole correctors are required.     

Optical polarization of nuclei and ODNMR in GaAs/AlGaAs quantum wells

Optical orientation of electrons was used to polarize the crystal lattice nuclei in quantum-size heterostructures and to study the effect of the conduction band spin splitting on the spin states of quasi-two-dimensional (2D) electrons drifting in an external electric field. High (～1%) nuclear polarization was registered using polarized luminescence and ODNMR in single GaAs/AlGaAs quantum wells. Measurement was made of the hyperfine interaction fields created by polarized nuclei on electrons and by electrons on nuclei. The spin-lattice relaxation of nuclei on the non-degenerate 2D electron gas was calculated. A comparison of the theoretical and experimental longitudinal relaxation times permitted the conclusion that the localized charge carriers are responsible for nuclear polarization in quantum wells in the temperature range of 2–77 K. A new effect has been studied, i.e. induction of an effective magnetic field acting on 2D electron spins when electrons drift in an external electric field in the quantum well plane. This effective field B_eff is due to the spin splitting of the conduction band of 2D electrons. The paper discusses possible registration of an ODNMR signal when the field B_eff is modulated by an electric current during optical orientation.     

用于指导仲氢诱导核极化状态保存的己烯分子中五自旋的单重态制备和寿命研究

仲氢诱导核极化（PHIP）技术能极大地增强核磁共振（NMR）信号的灵敏度,已被应用于磁共振成像、原位化学反应监测等领域．除了不断提高不同分子极化后的灵敏度外,延长和保存高极化度状态对PHIP技术的应用也至关重要,其中将极化后的状态制备成核自旋单重态是目前被研究较多的一种方法．本文以能被PHIP技术极化的己烯分子为研究对象,通过设计优化控制脉冲,对分子中的一个五自旋体系进行操控,制备了多种核自旋单重态,结果表明：己烯分子的碳-碳双键上存在三种不同的核自旋单重态,它们的寿命均长于仲氢极化后产生的初始态的寿命,可以作为延缓极化度衰减的一种中间态;通过对比单重态的寿命与相应自旋的纵向弛豫时间发现,将极化后己烯的状态转化为纵向磁化可能也是一种保存极化度的有效方法．     

Investigation of ultrafast nuclear spin polarization induced by short laser pulses

We theoretically investigate the dynamics of nuclear spin induced by short laser pulses and show that ultrafast nuclear spin polarization can take place. Combined use of the hyperfine interaction together with the static electric field is the key for that. Specifically we apply the idea to unstable isotopes, (27)Mg and (37)Ca, with nuclear spin of 1/2 and 3/2, respectively, and show that 88% and 62% of nuclear spin polarization can be achieved within a few to tens of ns, which is 2-3 orders of magnitude shorter than the time needed for any known optical methods. Because of its ultrafast nature, our scheme would be very effective not only for stable nuclei but also unstable nuclei with a lifetime as short as mus.     

Nuclear spin nanomagnet in an optically excited quantum dot

Linearly polarized light tuned slightly below the optical transition of the negatively charged exciton (trion) in a single quantum dot causes the spontaneous nuclear spin polarization (self-polarization) at a level close to 100%. The effective magnetic field of spin-polarized nuclei shifts the optical transition energy close to resonance with photon energy. The resonantly enhanced Overhauser effect sustains the stability of the nuclear self-polarization even in the absence of spin polarization of the quantum dot electron. As a result the optically selected single quantum dot represents a tiny magnet with the ferromagnetic ordering of nuclear spins-the nuclear spin nanomagnet.     

Alignment and orientation of charge exchange excitation with polarized projectiles

The alignment and orientation of hydrogenlike excited atoms produced by electron capture in collisions of spin polarized bare atomic nuclei with atomic targets is described in terms of Stokes parameters. TheH(2p)^* excited state formed by spin polarized protons making charge exchange collisions on atomic targets is dealt with extensively for the purpose of illustration. The linear polarization and circular polarization of resulting Lyman-α when polarized protons make electron capture collision on atomic hydrogen target are plotted to compare observable aspects of different theories of electron capture.     

Highly spin-polarized atoms and molecules from rotationally state-selected molecules

It is shown that large nuclear polarizations in isolated molecules may be created via the hyperfine interaction following excitation to selected rotational states |JM>. Explicit time-dependent nuclear polarization expressions for pulsed rotational excitation are presented for the case of one nuclear spin I, and for the case of two nuclear spins I1 and I2 in the hierarchical coupling limit. Photodissociation of the polarized molecules allows the production of polarized atoms, on short time scales if pulsed, at densities close to the parent molecule density.