OBSERVATION OF THE LOWEST (1)DELTA(U) RYDBERG STATE OF ACETYLENE BY MULTIPHOTON IONIZATION SPECTROSCOPY

The 1 Delta(u), 3d pi component of the lowest ungerade 3d + 4s Rydberg supercomplex of acetylene has been observed by (3 + 1) photon ionizat ion spectroscopy of C2H2 and C2D2. In both isotopic species the vibrat ionless level of this new electronic state is sufficiently long lived to support rotational structure. From the observed isotopic shift and from the rotational and vibrational parameters determined in the prese nt analyses, this state could be unambiguously identified as the lowes t Rydberg state of (1) Delta(u) symmetry, belonging to the 3d complex of acetylene. The weak intensity of this three-photon (1) Delta(u)-(1) Sigma(g)(+) electronic transition is discussed in comparison with the other Rydberg transitions within the same supercomplex. Excited vibro nic bands in the same energy region allowed to determine the nu 1 and nu(2) frequencies for some components of this Rydberg supercomplex. Th e absence of the nearby (H) over tilde (1) Pi(u), 3d delta Rydberg sta te in the MPI spectra is discussed in terms of predissociation. (C) 19 96 American Institute of Physics.