High-field ESR measurements of a bond alternating Heisenberg chain system Ni-2(mu-N-3)(3)(dpt)(2)(ClO4)

High-field electron spin resonance has been measured on a Ni-2(mu -N-3)(3)( dpt)(2)(ClO4) single crystal, which is an S = 1 Heisenberg antiferromagnet with bond alternation, in the frequency region from 50 to 370 GHz using pul sed magnetic fields up to 16 T. The g-values for H perpendicular and parall el to the single-crystal plane were determined to be 2.19 and 2.13, respect ively. By analysing our results on frequency dependence at 20 K by the simp le dimer model considering the singlet and triplet states, we were able to determine the single-ion anisotropy of the system as /D/ = 10.6 K and /E/ = 3.1 K for the first time. The spin gap was estimated to be 93 K from the a nalysis of the temperature dependence of the integrated intensity. These re sults will be discussed in connection with other S = 1 Heisenberg antiferro magnets with bond alternation. (C) 2001 Elsevier Science B.V. All rights re served.