High-resolution infrared spectra of vibrationally excited HC4H in a supersonic hydrocarbon plasma

High-resolution infrared spectra of diacetylene (HC4H) are recorded in the 3 mu m C-H stretch region using continuous wave cavity ring-down spectroscopy (cw-CRDS). The HC4H molecules are generated in a pulsed and supersonic planar plasma expansion by discharging a C2H2/He/Ar gas mixture. The jet-cooling realizes a low rotational temperature of T-rot similar to 17 K. Vibrational temperatures are found to be relatively high, allowing the detection of vibrationally excited HC4H in direct absorption with energies up to 1800 cm(-1). In total, sixteen ro-vibrational bands are identified, where nine of them were not reported before. Detailed and systematic rotational analyses are presented, yielding spectroscopic parameters for a series of vibrational levels over energy regions of 0-1800 and 3300-5100 cm(-1). Using the observed infrared band intensities, two vibrational excitation temperatures of HC4H in the plasma jet,similar to 570(50) K for the v(6), v(7) and v(8) bending vibrations, and similar to 125(10) K for the v(g) bending vibration, are deduced. (C) 2013 Elsevier Inc. All rights reserved