Magnetic resonance study of H atoms in thin films ofH2at temperatures below 1 K

Abstract

We present an experimental study of H atoms embedded in thin films of solid ${\text{H}}_{2}$ at temperatures below 1 K. ${\text{H}}_{2}$ films up to 50 nm thick were first grown as a result of slow recombination of atomic hydrogen gas on the sample cell walls. If the recombination occurred in three-body atomic collisions in the gas phase, small concentrations of atoms could be captured inside the films during the film deposition. As a second method of generating atomic populations inside the ${\text{H}}_{2}$ films, we used a direct dissociation by a low power rf discharge in the sample cell. With this latter method, we achieved record high atomic concentrations exceeding $2\ifmmode\times\else\texttimes\fi{}{10}^{19}\text{ }{\text{cm}}^{\ensuremath{-}3}$. The samples were characterized by means of magnetic resonance: electron spin resonance (ESR) and electron-nuclear double resonance (ENDOR) in a magnetic field of 4.6 T. We observed density-dependent broadening and shifts of the ESR lines due to the dipolar interactions, and resolved these effects for like and unlike atoms. Relaxation of the relative hyperfine populations was measured as a function of temperature for H in ${\text{H}}_{2}$ films grown on different substrates. For ${\text{H}}_{2}$ films on Mylar substrates, the relative equilibrium populations of the two lowest hyperfine states of H were found to deviate substantially from the prediction of Boltzmann statistics. We also found two narrow lines in the ENDOR spectra of H in ${\text{H}}_{2}$ films shifted to the red from the position for free atoms. This indicates two possible substitutional positions of the atoms in ${\text{H}}_{2}$ matrices, both characterized by very homogeneous crystalline fields.