This paper reported an excellent detail and considerations on polarization of nuclear spin by Dynamic Nuclear Polarization (DNP) method. the polarizing sample is naphthalene with 0.018mol% of pentacene. Pentacene has a paramagnetic triplet state with population independence of external magnetic field and temperature. This paramagnetic triplet state is very suitable on DNP, since the paramagnetic state like a switch to turn on the nuclear spin relaxation, due to the state will decay to diamagnetic state.
There are 5 conditions for sample:
- Highly polarization of electron triplet state ( since it is the source of polarization )
- High concentration of guest molecule ( another aspect for polarization source ) ( but too high, guest molecule cluster forms and this reduce the polarization )
- High Inter-System-Crossing rate, which is the rate from higher excited state to the triplet state, so faster the polarization build up time.
- Long nuclear spin-lattice relaxation time ( the nuclear spin lost rate )
- Suitable triplet state life time and electron spin-lattice relaxation time. long enough for transfer the electron spin to nuclear spin, but not so long that nuclear spin can use the paramagnetic triplet state as a channel for spin relaxation.
- the pulse width should longer than the lifetime from excited state to the triplet state, but shorter than the triplet lifetime.
- the intensity should be very high, so, the triplet state excitation depth increase. but it is not so high to increase the stimulated emission.
- high intensity may melt the sample.
- the time interval should be long and not excess the adiabatic limit.
- the sweeping range should be cover the ESR line width.
- the field sweep should be within the lifetime of the triplet state.