Dynamic Nuclear Polarization Studies of Charred Carbohydrates

The dynamic enhancement of proton spin polarization by pumping on ``forbidden'' electron-nuclear transitions is studied as a function of microwave power, O2 pressure, and magnetic field for a pair of low-temperature paramagnetic sucrose chars. The dependence of the electron spin-lattice relaxation time on O2 pressure is obtained for both samples. Comparison of the enhancement for a 405°C char with theory indicates that the electron spin resonance line is inhomogeneously broadened, and yields absolute values for the spin-spin and spin-lattice relaxation times of the spin packets. No definite fit could be made to the 605°C char data. The difference between the magnetic fields for maximum positive and negative polarization enhancement is anomalously small for the 405°C char.