W.Th. Wenckebach

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.

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.

Identification and ionization energies of the shallow donor metastable states in GaAs:Si

Transitions from the 1s0 ground state to the shallow donor metastable states in n‐GaAs have been investigated at a temperature of T=4.2 K, magnetic fields up to 6 T, and for far‐infrared transition energies between 50 and 300 cm−1. The final states of these transitions have been identified and characterized by high‐field quantum numbers. The magnetic‐field dependence of the ionization energies of some of these metastable states have been determined.

Dynamic nuclear polarization of proton spins via photoexcited triplet states: the system phenanthrene in fluorene

Abstract The proton spins in a single crystal of fluorene have been polarized by dynamic nuclear polarization via the photoexcited triplet state of phenanthrene guest molecules. It is possible to reach an enhancement of the proton polarization of 90 at 1.2 K where the thermal equilibrium polarization is 0.033%.

Ionization Energies and Lifetime Broadening of Autoionizing States of the Hydrogen Atom in Strong Magnetic Fields: Theory vs. Experiment

Theoretical results for the ionization energies of some autoionizing states of the hydrogen atom in magnetic fields up to 5105 T, as obtained by Friedrich et al. (Phys. Rev. A, 28 (1983) 1423) are compared for the first time with experimental data, derived from farinfrared magneto-optical experiments on shallow donors in n-GaAs. The theoretical calculations are found to give an excellent description of the experimental results in the reduced field range 0.15 < γ = ωcr/2 Ry < 1. Also predictions for the lifetime broadening of these states match the experimental data quite well.

Polarizing proton spins by electron-spin locking of photo-excited triplet state molecules

Abstract The transfer of the electron spin polarization from photo-excited triplet state molecules to proton spins has been studied in single crystals of fluorene doped with 2000 ppm acridine in three isotopically different forms. From spin-locking experiments it appears that the proton spins on the acridine guest molecules play a minor role in this process and that the evolution of the nuclear spin polarization is determined by the second moment of the dipolar interaction with the proton spins on neighbouring fluorene molecules. In contrast to recent reports we do not observe fast Rabi oscillations in the polarization transfer process. This is explained by the fact that the magnetic field in our experiments is large, resulting in a weak contact between the nuclear Zeeman reservoir and the electron dipole-dipole reservoir.

The mechanism of microwave-induced optical nuclear polarization in fluorene doped with phenanthrene: a qualitative analysis

Abstract Experiments are presented on microwave-induced optical nuclear polarization (MIONP) using the photoexcited triplet states in a crystal of fluorene doped with phenanthrene. The effects of modulation of the magnetic field and the microwave frequency on the dynamic nuclear polarization rate were studied. The results show that MIONP must be described in this case by the solid effect theory taking into account the burning of holes in the inhomogeneously broadened ESR line.

Properties of electron-irradiation-produced O2- in Ca(OH)2

O2- centres can be produced in Ca(OH)2 by irradiation with 1.5 MeV electrons. The centres have been identified and their properties were studied by means of ESR, ENDOR, optical absorption and Raman measurements. A simple apparatus for the growth of large single crystals of Ca(OH)2 is described. The results of the measurements of the g tensor can be understood in terms of a model developed for O2- in alkali halides. The authors have also measured the hyperfine interaction with the 17O nuclear spin in a 17O enriched powder and in a single crystal. The results fit the model. The optical spectra show the characteristic O2- absorption at 4.92 eV. From the Raman spectra the vibrational constants were calculated. It was determined from the ENDOR spectra that the O2- centre is located at an OH- site. The interaction of the centre with the neighbouring proton spins can be described in terms of a Hamiltonian with a purely dipolar interaction term and an isotropic hyperfine term. Almost no covalency is expected.

Hot-hole lasers in III–V semiconductors

Following the success of p-Ge hot-hole lasers, there is potential for using other semiconductor materials, notably III–V materials such as GaAs and InSb. Previous analysis had suggested that a large effective mass ratio between the heavy and light holes is advantageous, which implies that InSb would make an excellent hot-hole laser. Using our Monte Carlo simulation of both GaAs and InSb hot-hole lasers in combination with a rate equation model, we see that previously accepted criteria used to predict performance are not always reliable, and we suggest suitable alternatives. The simulation results include gain and gain bandwidth as a function of field strength and laser frequency, and alternative field orientations and photon polarizations are considered. Comparisons are made with bulk p-Ge systems. The optimum conditions predicted by our simulation could then be used in the design of quantum-well hot-hole lasers.