Magnetic moments of small hydrogen clusters

Molecular beams of small normal hydrogen (nH2) clusters (N≈20) produced in a low temperature free jet expansion are investigated for their magnetic properties by deflecting them in an inhomogeneous Stern-Gerlach type magnetic field. The mass distributions of the clusters are measured by diffraction from a nano-structured transmission grating. With their masses known the Gaussian shaped symmetric deflection patterns of the pure nH2 clusters can be simulated by a random distribution of both the nuclear and rotational magnetic moments of the 75% orthoJ=1 component. Upon cooling the nH2 clusters to below 1 K by expanding a mixture of 5% nH2 in 95% 4He gas the deflection pattern is split into three central peaks. This observation is consistent with a partial orientational ordering of the ortho H2 molecules leading to a reduction of the total rotational magnetic moments of the clusters.