Spectroscopy of the predissociated C state of Na3

Abstract

The C state of Na3 has been explored in detail by the methods of resonant two-photon ionization spectroscopy and photofragmentation spectroscopy of a supercooled cluster beam. The photofragmentation spectrum, recorded by depletion of Na3 from the beam, reveals a long progression of bands that had been invisible to the two-photon ionization method, and it appears that ultrafast fragmentation occurs for all levels more than 400 cm−1 above the zero-point level. This is consistent with earlier observations of Na D-line emission following Na3 excitation in this region, assuming the responsible channel is Na2 2X and Na 2P production. The vibronic fine structure of the C band system is complex at lower energies, and yields to a detailed explanation only through consideration of the dynamical Jahn–Teller effect. This analysis demonstrates that the C state has electronic symmetry E″, and is subject to a symmetry-lowering deformation of 180 cm−1, or 1.4 times the frequency of the e′ vibrational mode of D3h Na3. The corresponding minimum energy structure is an obtuse isosceles triangle (65° apex angle), but the barrier to pseudorotation (estimated to be 40 cm−1) is small. Furthermore, the computed vibronic wave functions imply that only the lowest few states are well described within the adiabatic Born–Oppenheimer approximation; at intermediate energies the spectrum is correspondingly very irregular, while at higher energies a near harmonic regularity returns by virtue of the relatively small magnitude of the Jahn–Teller distortion. These quantitative conclusions are intermediate among those pertaining to the other known states of Na3, and, in combination with them, permit extensive comparison with the results of high quality electronic structure computations. The tentative assignment is to the 12E to 22E″ electronic transition (united atom 1S21Px,y to 1S21Dxz,yz).