The polarized optical spectrum of ErH in calcium tungstate at 4.2 and 77°K is reported. Radial integrals and spin-orbit coupling parameter derived from fitting the position of the centers of gravity of the J manifolds in intermediate coupling are F2=423.89 em-I, F4=67.995 cm-I , F6=6.667 cm-I , 1=2377 cm-I . A first-order crystal field calculation based on the intermediate coupling states of the free ion is shown to account very satisfactorily for the optical spectrum, paramagnetic resonance, and magnetic hyperfine structure when using (r-a > and the nuclear moment measured by atomic beam triple magnetic resonance. The agreement between the experimental and calculated crystalline quantum numbers is encouraging since they were not forced to fit in the calculation. Also, the fact that the (r-a ) values obtained for a truly free erbium atom can be used for the case of the ion in a crystalline host indicates that the 41 wavefunctions are little affected by the surroundings. The values of the crystal field parameters obtained from fitting the optical spectrum are: C 2 0",,231±7 cm-I , C.4=±(852±18) cm-I , C 4 o= -90±2 cm-I , C 6 o= -0.6±0.5 cm-I , C 6 4=±(396±5) cm-I , B s -4=±(75±12) cm-I • The above parameters predict the Stark splittings of the 53 states up to 25000 cm-I with an rms deviation of 4.4 cm-I . The calculated g factors are gl (ZI) = 1.21, gJ.c(ZI) =8.45, gllc(Z2) =3.65, gJ. c (Z2) = 7.03; the corresponding measured values are 1.247, 8.38, 3.42, and6.98,respectively. The calculated and measured values of the hyperfine structure constants are: A. ale IS7 = 41.61 X 10-4 cm-I , A meas l67 = (42.5±0.5) X 10-4 cm-I , B eale I67 =290.60X 10-4 cm-I, B meas I67 = (293±3) X 10-4 cm-I •This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to