Zeeman spectroscopy of CaH molecules in a magnetic trap

Abstract

In a recent experiment [Weinstein et al., Nature 395, 148 (1998)] we magnetically trapped 108108ground-statecalcium monohydride molecules, CaH(X2Σ,v″=0,J″=0).CaH(X 2Σ,v″=0, J″=0). The molecules were prepared by laser ablation of a solid sample of CaH2CaH2 and loaded via thermalization with a cold (<1(<1 K) 3He3He buffer gas. The magnetic trap was formed by superconducting coils arranged in the anti-Helmholtz configuration. The detection was done by laser fluorescence spectroscopy excited at 635635 nm (in the B2Σ,v′=0−X2Σ,v″=0B 2Σ,v′=0−X 2Σ,v″=0 band) and detected at 692692 nm (within the B,v′=0−X,v″=1B,v′=0−X,v″=1 band). Both a photomultiplier tube and a CCD camera were used. Due to the thermalization of molecular rotation, only a transition from the lowest rotational state could be detected at zero field, N′=1,J′=3/2←N″=0,J″=1/2.N′=1, J′=3/2←N″=0, J″=1/2. In the magnetic field this rotational transition splits into two features, one shifted towards lower and one towards higher frequencies. The measured shifts are linear in field strength and indicate a small difference (0.02 μB)μB) in the magnetic moments between the ground and excited states. Here we present a theoretical analysis of the observed magnetic shifts. These are identified as arising from a rotational perturbation of the B2Σ,v′=0B 2Σ,v′=0 state by a close-lying A2Π,v′=1A 2Π,v′=1 state that lends the B state some of its A character. We find that the Hamiltonian can be well approximated by a 3×33×3 matrix built out of elements that connect states from within the Σ-doublet and the 2Π3/22Π3/2 manifolds. The interaction parameter describing the Σ−Π coupling in the Zeeman Hamiltonian is determined from the observed shifts and the field-free molecular parameters of CaH given by Berg and Klyning [Phys. Scr. 10, 331 (1974)] and by Martin [J. Mol. Spectrosc 108, 66 (1984)].