Laser induced excitation spectroscopy of copper trimer in various stages of supersonic expansion: Observation of fluorescence from dissociative levels

Abstract

Gas phase spectroscopic studies of copper trimer are reported. Using laser induced excitation spectroscopy and a trimer source producing concentrations some three to six orders of magnitude in excess of those available from alternative devices, we obtain the visible spectrum of the trimer. This spectrum has been associated with a 2E″–2E′ transition of a D3h molecule in which both electronic states undergo Jahn–Teller distortion. Spectra obtained under a wide variety of experimental conditions encompass the wavelength range 6050–5180 Å corresponding to a range of effective temperatures. Data obtained in the present study are correlated with the recent studies of Morse et al. (laser vaporization, R2PI, and depletion spectroscopies) and Moskovits and co-workers (resonance Raman-matrix isolation). The observed spectroscopic features are in one-to-one correspondence with the detailed excited state vibronic calculations of Thompson et al. and in some respects may agree with the ‘‘first order’’ ground state vibronic calculations of Moskovits. The generated concentration of trimer molecules is such as to allow the characterization of strongly predissociative levels lost to the R2PI technique. The extent of the observed spectra allow the separation of level structure associated on the one hand with the excited state symmetric stretch mode and on the other with the vibronic levels resulting from the coupling of the asymmetric stretch and bending modes. Significant changes in those features which dominate observed excitation spectra as a function of effective temperature are interpreted in terms of a change in the ground state population distribution and hence an alteration in the configuration space connecting populated ground state and accessed excited state levels. These temperature dependent effects and observed hot band structure imply that the pseudorotation barrier in the ground electronic state of the trimer is considerably smaller than previously suggested.