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Nanosecond Time Resolution of Electron-Nuclear Cross Polarization within the Optical Nuclear-Polarization (ONP) Process

Buntkowsky, G. and Stehlik, D. and Vieth, H. M. and Salikhov, K. M. (1991):
Nanosecond Time Resolution of Electron-Nuclear Cross Polarization within the Optical Nuclear-Polarization (ONP) Process.
3, In: Journal of Physics-Condensed Matter, (32), pp. 6093-6111, [Online-Edition: http://apps.webofknowledge.com/full_record.do?product=WOS&se...],
[Article]

Abstract

The spin dynamics of the generation of high nuclear spin polarization as a result of optical excitation (ONP) is studied with time resolution extending into the nanosecond range with the help of synchronized light and RF pulses. The well-characterized system - acridine doped into a crystalline fluorene matrix - has been used for the present dynamics study. With a short laser pulse, a selective sublevel population of the lowest acridine triplet state is generated. Resonant RF pulses of variable length initiate transfer of the electronic to nuclear spin polarization; the ONP-amplitude as a function of the RF pulse-length shows oscillating behaviour closely related to nutations of the electronic spins due to the resonant RF field. The ONP amplitude at the oscillation maximum is more than twice as high as the ONP level obtainable by CW RF-irradiation, which reflects an improved efficiency of the polarization transfer process. In contrast to the wide range of ONP experiments employing electron spins of stable paramagnetic systems the pulsed version of the ONP experiment permits direct time resolution of the electron nuclear cross-polarization process. In order to explain the results a two-step process is proposed and tested against the experimental data. In the primary step, the RF changes the spin order in the highly polarized electronic non-Zeeman reservoir in a similar way to hole-burning in a wide EPR line. This leads to a state which is far off equilibrium. In a second step, on a slower time-scale, the non-Zeeman reservoir re-equilibrates, mediated by electron nuclear spin-coupling causing the nuclear spins to be polarized. The process is compared with the thermal mixing concept as developed for dynamic nuclear polarization in solids with inhomogeneously broadened EPR lines.

Item Type: Article
Erschienen: 1991
Creators: Buntkowsky, G. and Stehlik, D. and Vieth, H. M. and Salikhov, K. M.
Title: Nanosecond Time Resolution of Electron-Nuclear Cross Polarization within the Optical Nuclear-Polarization (ONP) Process
Language: English
Abstract:

The spin dynamics of the generation of high nuclear spin polarization as a result of optical excitation (ONP) is studied with time resolution extending into the nanosecond range with the help of synchronized light and RF pulses. The well-characterized system - acridine doped into a crystalline fluorene matrix - has been used for the present dynamics study. With a short laser pulse, a selective sublevel population of the lowest acridine triplet state is generated. Resonant RF pulses of variable length initiate transfer of the electronic to nuclear spin polarization; the ONP-amplitude as a function of the RF pulse-length shows oscillating behaviour closely related to nutations of the electronic spins due to the resonant RF field. The ONP amplitude at the oscillation maximum is more than twice as high as the ONP level obtainable by CW RF-irradiation, which reflects an improved efficiency of the polarization transfer process. In contrast to the wide range of ONP experiments employing electron spins of stable paramagnetic systems the pulsed version of the ONP experiment permits direct time resolution of the electron nuclear cross-polarization process. In order to explain the results a two-step process is proposed and tested against the experimental data. In the primary step, the RF changes the spin order in the highly polarized electronic non-Zeeman reservoir in a similar way to hole-burning in a wide EPR line. This leads to a state which is far off equilibrium. In a second step, on a slower time-scale, the non-Zeeman reservoir re-equilibrates, mediated by electron nuclear spin-coupling causing the nuclear spins to be polarized. The process is compared with the thermal mixing concept as developed for dynamic nuclear polarization in solids with inhomogeneously broadened EPR lines.

Journal or Publication Title: Journal of Physics-Condensed Matter
Volume: 3
Number: 32
Uncontrolled Keywords: doped fluorene crystals double-resonance rotating-frame induction spectroscopy hydrogen abstraction solids dynamics nmr oscillations relaxation
Divisions: 07 Department of Chemistry
07 Department of Chemistry > Physical Chemistry
Date Deposited: 27 Oct 2014 20:38
Official URL: http://apps.webofknowledge.com/full_record.do?product=WOS&se...
Additional Information:

Gb593 Times Cited:8 Cited References Count:65

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