THERMAL DETECTION OF ESR ON SPIN-POLARIZED HYDROGEN - STUDY OF SURFACE RECOMBINATION

We introduce a new method for the detection of electron-spin resonance in spin-polarized atomic hydrogen gas (H down arrow).Instead of observing the microwave power absorbed in the ESR transition, we monitor the recombination heat deposited by ESR-induced spin-up atoms (H up arrow) 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 . 10(10) atoms/cm(3) 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 down arrow sample. The bolometer signal is found to be related mainly to second-order H up arrow + H down arrow recombination to ortho-H-2 on the surfaces of the sample cell. From the signals we have determined mined the rate constant K-bc(s) = 3.2(5). 10(-5) root T cm(2)/s . K--1/2 in the interval from T = 250 to 425 mK in a field of 4.5 T.