Rotation-tunneling spectrum of the deuterated ammonia dimer

The millimeter and submillimeter-wave molecular-beam spectrum of the perdeu terated ammonia dimer (ND3)(2) has been measured between approximately 50 a nd 400 GHz using an electric-resonance optothermal spectrometer (EROS). As in the case of the (NH3)(2), the spectrum is complicated by the threefold i nternal rotation of the ND3 subunits, the interchange tunneling of the two subunits, and the inversion of the subunits through their respective center s of masses. These tunneling motions split the rigid-molecule energy levels into 22 components, which all have nonzero statistical weights in the case of the deuterated dimer. Transitions have been assigned for rotation-tunne ling states correlating to A-A (ortho-ortho) combinations of the ND3 monome r states, where A designates the rovibronic symmetries of the ND3 subunits. One K=1<--1, one K=1<--0, one K=0<--1, and two K=0<--0 progressions have b een assigned. The data have been fit to 0.28 MHz using linear molecule-type energy-level expressions to determine rotational constants, band origins, l/K-type double constants, and centrifugal distortion constants. The two K= 0<--0 subbands, with origins near 264 GHz, are split by 64 MHz due to monom er inversion, as observed previously in the NH3 dimer. The 264 GHz, K=0 spl itting arises predominantly from monomer interchange tunneling and is nearl y a factor of 2 less than the 483 GHz value for the NH3 dimer. The separati on is also approximately 25% smaller than predicted by Olthof et al. [E. H. T. Olthof, A. van der Avoird, and P. E. S. Wormer, J. Chem. Phys. 101, 843 0 (1994)] from dynamical calculations on a model potential energy surface a djusted to fit the observed far-infrared rotation-tunneling spectrum of the NH3 dimer. (C) 1999 American Institute of Physics. [S0021-9606(99)01419-1] .