Spectroscopy of the predissociated C state of Na₃ Available to Purchase

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 D_3h 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 1S²1P_x,y to 1S²1D_xz,yz).