Zeeman spectroscopy of CaH molecules in a magnetic trap

In a recent experiment [Weinstein et al., Nature 395, 148 (1998)] we magnet ically trapped 10(8) ground-state calcium monohydride molecules, CaH(X(2)Si gma,upsilon" = 0,J" = 0). The molecules were prepared by laser ablation of a solid sample of CaH2 and loaded via thermalization with a cold (< 1 K) He -3 buffer gas. The magnetic trap was formed by superconducting coils arrang ed in the anti-Helmholtz configuration. The detection was done by laser flu orescence spectroscopy excited at 635 nm (in the B (2)Sigma, upsilon' = 0 - X (2)Sigma, upsilon" = 0 band) and detected at 692 nm (within the B, upsil on' = 0 - X,upsilon" = 1 band ). Both a photomultiplier tube and a CCD came ra were used. Due to the thermalization of molecular rotation, only a trans ition from the lowest rotational state could be detected at zero field, N' = 1,J' = 3/2 <--N" = 0,J" = 1/2. In the magnetic field this rotational tran sition splits into two features, one shifted towards lower and one towards higher frequencies. The measured shifts are linear in field strength and in dicate a small difference (0.02 mu(B)) in the magnetic moments between the ground and excited states. Here we present a theoretical analysis of the ob served magnetic shifts. These are identified as arising from a rotational p erturbation of the B (2)Sigma,upsilon' = 0 state by a close-lying A (2)Pi,u psilon' = 1 state that lends the B state some of its A character. We find t hat the Hamiltonian can be well approximated by a 3 x 3 matrix built out of elements that connect states from within the Sigma-doublet and the (2)Pi(3 /2) manifolds. The interaction parameter describing the Sigma-Pi coupling i n the Zeeman Hamiltonian is determined from the observed shifts and the fie ld-free molecular parameters of CaH given by Berg and Klyning [Phys. Scr. 1 0, 331 (1974)] and by Martin [J. Mol. Spectrosc 108, 66 (1984)]. (C) 1999 A merican Institute of Physics. [S0021-9606(99)00805-3].