Mobility in Single Crystals Studied by Optical Nuclear Polarization

Abstract

By optical excitation of molecular triplet states along with selective population of their spin sublevels nuclear spin polarization can be generated up to a level far above the Boltzmann equilibrium. This effect known as Optical Nuclear Polarization (ONP) has been applied for the investigation of dynamical processes in solid state matrices in the following three ways: (i) The efficiency of the ONP process and its dependency on an external magnetic field is affected by the fluctuation of local fields which is correlated to molecular motions. (ii) By pulsed irradiation of light and radio‐frequencies one can achieve a temporal resolution of the polarization process; in this way the time evolution of the spin order of the molecular states involved in the optical excitation cycle can be mapped out. (iii) The high nuclear polarization makes time resolved NMR measurements on solid state systems feasible which cannot be studied by conventional techniques because of insufficient sensitivity, e.g. guest molecules in low concentrations or isotopes with small natural abundance. The three methods, which cover together a time range between 10−12 and 103 seconds are illustrated by experiments on various organic crystals.

Magnetic-field cycling instrumentation for dynamic nuclear polarization-nuclear magnetic resonance using photoexcited triplets

“…Filed-cycling by sample shuttling has been reported in papers on zero-field NMR and nuclear quadruple resonance, [23][24][25][26] nuclear-spin relaxation studies, [27][28][29] coherence transfer in chemically induced dynamic nuclear polarization, 30 and ONP. 17,[31][32][33][34] As schematically shown in Fig. 2, a split-electromagnet and a solenoidal superconducting magnet ͑SCM͒ are employed for DNP and NMR measurement, between which the sample is shuttled.…”

A Time-Resolved Stimulated Nuclear Polarization Study of Biradicals in Low Magnetic Field

“…9 The photochemical reaction was carried out at the low magnetic field of a current-controlled electromagnet. The solution under investigation was irradiated by a Lambda-Physik excimer laser at 308 nm.…”

Temperature dependence of low field CIDNP and time resolved SNP in cyclic ketones

“…The 1H CIDNP and SNP experiments were performed on a custom-built FT NMR spectrometer (7 T) allowing magnetic field cycling by use of a pneumatic probehead transfer system [6] with a transfer time of 1-2 s. The photochemical reaction was carried out at the low magnetic field of a current-controlled electromagnet in the range of 0-0.13 T. The sample was irradiated for 1-2 s at a pulse repetition frequency of 10-20 Hz by a Lambda-Physik excimer laser at 308 nm. The laser pulse duration was 15 ns (fwhm) and the energy per pulse, measured at the sample position, was 5 mJ.…”