Thermal detection of ESR on spin-polarized hydrogen: Study of surface recombination

Abstract

We introduce a new method for the detection of electron-spin resonance in spin-polarized atomic hydrogen gas (H↓). Instead of observing the microwave power absorbed in the ESR transition, we monitor the recombination heat deposited by ESR-induced spin-up atoms (H↑) onto a liquid-helium coated carbon bolometer. The signal from this sensor reproduces well the ESR absorption lineshape registered by our 128 GHz homodyne spectrometer. Using the recombination detection we have achieved a density detection limit of n=3·1010 atoms/cm3 for 0.2 nW microwave power incident on the resonant cavity at the temperature T=150 mK. We have studied the decay rate of recombination heat absorbed by the bolometer after an ESR excitation pulse and the dependence of this rate on T, n and nuclear polarization of the H↓ sample. The bolometer signal is found to be related mainly to second-order H↑ + H↓ recombination to ortho-H2 on the surfaces of the sample cell. From the signals we have determined the rate constant K bc c =3.2(5)·10−5√T cm2/s·K−1/2 in the interval from T=250 to 425 mK in a field of 4.5 T.