Double resonance experiments in low magnetic field: dynamic polarization of protons by (14)N and measurement of low NQR frequencies. 
The possibilities of dynamically polarizing proton spin system via the quadrupole (14)N spin system in low magnetic field are analyzed.  The increase of the proton magnetization is calculated.  The polarization rate of the proton spin system is related to the transition probabilities per unit time between the (14)N quadrupole energy levels and proton energy levels.  The experiments performed in 1,3,5-triazine confirm the results of the theoretical analysis.  A new double resonance technique is proposed for the measurement of nuclear quadrupole resonance frequencies nu(Q) of the order of 100kHz and lower.  The technique is based on magnetic field cycling between a high and a low static magnetic field and observation of the proton NMR signal in the high magnetic field.  In the low magnetic field the quadrupole nuclei and protons resonantly interact at the proton Larmor frequency nu(H)=nu(Q)/2.  The quadrupole nuclei are simultaneously excited by a resonant rf magnetic field oriented along the direction of the low static magnetic field.  The experimental procedure is described and the sensitivity of the new technique is estimated.  Some examples of the measurement of low (14)N and (2)H nuclear quadrupole resonance frequencies are presented.