MEASUREMENT OF THE HYPERFINE STRUCTURE OF $(DCCD)_{2}$ DIMER

Last year, in a theoretical study of the hyperfine structure of $(DCCD)_{2}$, the effects of nonrigidity of this dimer on the form of the quadrupole hyperfine Hamitonian were described. It was shown that, for nondegcnerate A or B-type tunneling sublevels, the hyperfine Hamiltonian is the sum of four single deuterium Hamiltonians with the same effective coupling constant; while, for doubly degenerate E-type tunneling sublevels, the hyperfine Hamiltonian is less symmetrical and two effective coupling constants arise. This year we present measurements of the hyperfine structure of several transitions of $(DCCD)_{2}$ in the radio frequency region. One of the three components of the $1_{10} - 1_{11}$ rotational transition, the $B_{2}\\leftarrow B_{2}$ has already been measured with instrumental linewidth of 1.5 kHz. Using the theoretical formalism, it was possible to analyze the frequency of the 11 transitions of the hyperfine pattern with an R.M.S. deviation smaller than 0.1 kHz. and to determine the effective coupling constants of the upper and the lower tunneling states. The results of this analysis will be discussed and an interpretation of these constants in terms of the components of the effective quadrupole tensor will be given. Measurements are in progress for the $1_{10} A_{2}^{+} - 1_{11} A_{2}^{-}$ and the $1_{01} E^{-} 0_{00} E^{\\prime}$ transitions. The first of these two transitions should be a test for the theorctical formalism since the effective coupling constants should be the same as for the transition already analyzed. The second of these two transitions, involving doubly degenerate sublevels, is of special interest since it will allow us to determine the change in quadrupole coupling for the deuterium atom involved in the hydrogen bond.