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Three electronic absorption systems for C5 at 511, 445, and 232 nm and one for C6, C8, and C9 centered at 228, 259, and 288 nm have been observed in the gas phase. The C5 chain was produced in both discharge and ablation sources and detected using resonant two-color two-photon ionization spectroscopy involving 10.5 eV photons. The decay of the excited singlet electronic states indicates fast intramolecular processes on a subpicosecond time scale. The internal energy is assumed to be trapped in a triplet state for at least 15 μs. Hole-burning experiments on the 2 ² Σ u-← X ³ Σg- transition of C6, C8, and ¹Σu+←X ¹Σg+ of C9 confirm the predissociative nature of the excited electronic states. [ABSTRACT FROM AUTHOR]

Three electronic absorption systems for C5 at 511, 445, and 232 nm and one for C6, C8, and C9 centered at 228, 259, and 288 nm have been observed in the gas phase. The C5 chain was produced in both discharge and ablation sources and detected using resonant two-color two-photon ionization spectroscopy involving 10.5 eV photons. The decay of the excited singlet electronic states indicates fast intramolecular processes on a subpicosecond time scale. The internal energy is assumed to be trapped in a triplet state for at least 15 μs. Hole-burning experiments on the 2 ² Σ u-← X ³ Σg- transition of C6, C8, and ¹Σu+←X ¹Σg+ of C9 confirm the predissociative nature of the excited electronic states. [ABSTRACT FROM AUTHOR]

The B [SUP3]Σ[SUP-,SUBg] ← X[SUP3]Σ[SUP-,SUBg] transitions of HC[SUB13]H and HC[SUB19]H have been measured in the gas phase, exhibiting broad, Lorentzian shaped bands. More extensive A [SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] spectra have been observed for HC[SUB2n+1]H (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 [SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg], B[SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg], and C [SUP3]Γ[SUBu] ← X[SUP3]Σ[SUP-,SUBg], located, for the smaller members of the series, in the visible, UV and VUV range, respectively. The A [SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] system is found to be of medium intensity and the B[SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] 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[SUP3]Σ[SUP-,SUBu] ← X[SUP3]Σ[SUP-,SUBg] system increases monotonically with size. The C [SUP3]Γ[SUBu] 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[SUP13] cm[SUP-2], based on calculated oscillator strengths. [ABSTRACT FROM AUTHOR]

A three-dimensional potential energy function (PEF) of the [sup 2]A[sub 1] electronic ground state of C[sub 2]B has been generated by electronic structure calculations. The PEF possesses a minimum in an isosceles triangular structure which lies 2204 cm-1 below two equivalent minima having linear equilibrium geometry. The barrier height between the minima relative to the triangular structure has been calculated to the 2383 cm-1. The nuclear motion problem has been solved variationally in Jacobi coordinates for J=0 and 1. Ten vibrational states of A[sub 1] and nine of B[sub 2] symmetry are calculated to lie below the linear minima. The permutational splitting between the (000)[sup +] and (000)[sup -] states in the linear [sup 12]C[sub 2] [sup 11]B has been calculated to be 0.064 cm-1, in [sup 12]C[sup 13]C[sup 11]B this is 0.530 cm-1. Above the energy of the barrier to linearity there are large amplitude vibrations with triangular structure character. In the dense stack of such states vibrational modes of the linear structure are discernible, including their permutational splittings. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Theelectronic spectrum of C17H12N+,phenanthrene with a side chain, was measured in the gas phase ata vibrational and rotational temperature of ∼15 K in an iontrap using a resonant multiphoton dissociation technique. The C17H12N+structure was produced in a chemicalionization source and identified by a comparison with theoreticalcalculations of stable structures and excitation energies. The (3), (2), (1) 1A ← X 1A electronic transitions of this nitrogen-containing aromatic specieswith 30 atoms have origin band maxima at 23 586 ± 1 cm–1, 16 120 ± 50 cm–1,and 14 519 ± 30 cm–1. Distinct vibrational structure in the (3) 1A state is observed, and assignments are made. Astronomical aspectsare considered. [ABSTRACT FROM AUTHOR]