DOI: 10.1143/JPSJ.73.2313

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:

  1. Highly polarization of electron triplet state  ( since it is the source of polarization )
  2. High concentration of guest molecule ( another aspect for polarization source ) ( but too high, guest molecule cluster forms and this reduce the polarization )
  3. High Inter-System-Crossing rate, which is the rate from higher excited state to the triplet state, so faster the polarization build up time.
  4. Long nuclear spin-lattice relaxation time ( the nuclear spin lost rate )
  5. 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.
On the laser:
  1. the pulse width should longer than the lifetime from excited state to the triplet state, but shorter than the triplet lifetime.
  2. the intensity should be very high, so, the triplet state excitation depth increase. but it is not so high to increase the stimulated emission.
  3. high intensity may melt the sample.
about the microwave and field sweep:
  1. the time interval should be  long and not excess the adiabatic limit.
  2. the sweeping range should be cover the ESR line width.
  3. the field sweep should be within the lifetime of the triplet state.
they used water as a reference for the polarization measurement. the enhancement Q is defined to be:
Q = \left( \frac { T E}{N g} \right) / \left( \frac{ T_w E_w}{ N_w g_w } \right)
where T is temperature, N is number of proton spin, g is receiver gain and  E is the recorded signal amplitude.
at the end of this paper, it talks about some applications.