Gas phase electronic spectra of the linear carbon chains HC2n+1H (n=3-6,9)

The B (3)Sigma(u)(-)<--X (3)Sigma(-)(g) transitions of HC13H and HC19H have been measured in the gas phase, exhibiting broad, Lorentzian shaped bands. More extensive A (3)Sigma(u)(-)<--X (3)Sigma(-)(g) spectra have been observed for HC2n+1H (n=3-6) than before with many new vibronic bands identified. The spectra were obtained by means of a mass selective resonant two-color two-photon ionization technique coupled to a supersonic plasma source. The electronic structures of this series of molecules (n=2-9) in both the ground and excited states have been investigated using DFT, MP2, and state-averaged CASSCF theories. The three lowest dipole allowed electronic transition systems are A (3)Sigma(u)(-)<--X (3)Sigma(-)(g), B (3)Sigma(u)(-)<--X (3)Sigma(-)(g), and C (3)Pi(u)<--X (3)Sigma(-)(g), located, for the smaller members of the series, in the visible, UV and VUV range, respectively. The A (3)Sigma(u)(-)<--X (3)Sigma(-)(g) system is found to be of medium intensity and the B (3)Sigma(u)(-)<--X (3)Sigma(-)(g) transition is predicted to be very strong. This is a result of configuration mixing in the excited states. The oscillator strength of the lowest energy transition is not strongly dependent on the length of the chain, but that of the B (3)Sigma(u)(-)<--X (3)Sigma(-)(g) system increases monotonically with size. The C (3)Pi(u) state is Rydberg in character. The astrophysical implications are considered and an upper limit of the column densities of these carbon chains in diffuse clouds has been estimated as 10(13) cm(-2), based on calculated oscillator strengths. (C) 2003 American Institute of Physics.