Abstract.The results of a submillimetre wavelength spectral line survey between 455.1-507.4 GHz of the Orion-KL hot cloud core are reported. A total of 254 lines were detected to a main beam brightness temperature sensitivity T mb ∼ 1-3 K. The detected lines are identified as being associated with 30 different molecular species or their isotopomeric variants. The strongest line detected was the J = 4-3 transition of the CO molecule. Apart from abundant diatomic rotors such as CO and CS, the spectrum is dominated by SO, SO 2 and CH 3 OH and large organic molecules such as (CH 3 ) 2 O, CH 3 CN, C 2 H 3 CN, C 2 H 5 CN and HCOOCH 3 which make up ∼72% of the total number of lines; unidentified lines ∼13%; and other lines the remaining ∼15% of the total. Rotational temperatures and column densities derived using standard rotation diagram analysis techniques were found to range from 70-300 K, and 10 14 -10 17 cm 2 respectively.

Benzonitrile and its van der Waals (vdW) complexes with argon (benzonitrile-Ar and benzonitrile-Ar 2 ) have been studied in supersonic jets by two-color zero-kinetic-energy (ZEKE) photoelectron spectroscopy for the first time. From the (1+1′) ZEKE photoelectron spectra obtained via the first electronic excited state (S 1 ), we have determined adiabatic ionization potentials as I a (benzonitrile) ) 78 490 ( 2 cm -1 (9.7315 ( 0.0002 eV), I a (benzonitrile-Ar) ) 78 241 ( 4 cm -1 (9.7007 ( 0.0005 eV), and I a (benzonitrile-Ar 2 ) ) 78 007 ( 4 cm -1 (9.6716 ( 0.0005 eV). These values indicate that the decrease in ionization potential due to the complex formation (∆I a ) -249 and -483 cm -1 , respectively) is very large compared with other monosubstituted benzenes. Eleven low-frequency vibrational modes of the bare benzonitrile cation below 1300 cm -1 have been identified in the ZEKE photoelectron spectra. The in-plane CsCtN bending mode (33 + ) and the out-of-plane C-C-C bending mode (22 + ) have been clearly identified from the ZEKE photoelectron spectra obtained via the S 1 vibrational levels 33 1 and 22 2 , respectively. Furthermore, in the benzonitrile-Ar and -Ar 2 spectra, we have observed well-resolved vibrational progressions with very low frequencies of 12 and 9 cm -1 , respectively, which may be assigned to the vdW bending mode (b x + ) and the vdW symmetric bending mode (b xs + ) along the substituent axis, respectively.

A high-resolution gas-phase spectrum of a molecular absorption band around 604 nm is assigned as due to an electronic transition of a nonlinear C 6 H 4 ϩ planar species starting from its 2 AЉ electronic ground state. The spectrum is observed in direct absorption by cavity ringdown spectroscopy through a supersonic planar discharge through a mixture of acetylene in helium. The spectrum has a clear rotational and K-type structure. This allows an accurate determination of the B and C rotational constants and an estimate for the A rotational constant in ground and electronically excited states. The resolved spectrum of the fully deuterated species C 6 D 4 ϩ has been obtained as well. The results are compared both to the outcome of ab initio geometry optimizations and low-resolution absorption spectra in 6 K neon matrices obtained after mass-selective deposition.

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