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 B-0\\x, (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 2 pi 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. (C) 1999 Academic Press.