Anisotropic S=1 magnetic chain in an external field: Low lying energy levels and ESR transition rates

We investigate the spectrum of elementary excitations for S = 1 Heisenberg chains with single ion anisotropy in an external magnetic field and their s ignatures in ESR transition rates. We find that the order of levels as foun d previously for wavevector q - pi (Haldane gap) changes drastically at q n ot equal pi, leading to an accidental degeneracy at q approximate to 0.75 p i. We calculate the dependence on magnetic field of ESR transition rates fo r transitions inside the group of lowest excited states tone magnon states) as well as for ground state transitions to some low-energy two magnon stat es. Characteristic variations of intensity allow to identify the participat ing levels. Numerical calculations for S = 1 chains up to 14 spins confirm the approximations used in obtaining the analytical results.