Strong combination-band IR emission from highly vibrationally excited acetylene Matthew Nikow, a Michael J. Wilhelm, a Jonathan M. Smith b and Hai-Lung Dai* b Received 9th September 2009, Accepted 23rd December 2009 First published as an Advance Article on the web 2nd February 2010 DOI: 10.1039/b918211j The n 4 + n 5 combination band, which appears relatively weak in the IR absorption spectrum, has been identified with exceptionally high intensity in the IR emission spectra from highly vibrationally excited acetylene, which is produced with B71 kcal mol 1 of vibrational energy from the 193 nm photolysis of vinyl bromide. The ‘fundamental’ transition of this combination band, from the (0,0,0,1 1 ,1 1 ) level to the zero point, occurs at 1328 cm 1 . The intensity and frequency of this band as well as the n 3 and n 5 bands, IR active but with lower emission intensity, as a function of the acetylene energy can be modeled accurately using the normal mode harmonic oscillator model with frequency anharmonicity corrections. Good fitting results are achieved even though the normal mode quantum numbers are no longer good for levels in the high energy region and the combination band is forbidden in the harmonic oscillator model. The identification of this intense combination band in emission, compared to its weak intensity in the absorption spectrum, highlights the necessity to include in consideration the combination bands for assignment of emission spectra in general and in particular emission from vibrationally hot acetylene which is ample in combustion, atmospheric, and interstellar environments. 1. Introduction Acetylene is one of the most important and subsequently well studied hydrocarbons. 1 Its importance ranges from the inter- stellar, where it was recently discovered by high-resolution IR in large abundance in the comets Hyakutake and Hale–Bopp, 2 to the planetary where its significance is evident in origins of life studies, 3,4 to the environmental, 5,6 and to numerous other energy and chemical applications. In spectroscopic studies, acetylene has in general been well characterized. Recent studies have focused more on the highly vibrationally excited levels in the electronic ground state. 7–13 Emission from acetylene has been found in large quantities in the atmosphere of Jupiter 14 and the carbon asymptotic giant branch star IRC +10216. 15 Acetylene is thought to exist primarily in the upper atmosphere of Jupiter where many highly excited molecules and radicals are produced through photochemical processes. Experimental evidence for most of these processes proposed to occur in the planetary atmosphere is in the form of emission spectra. Among the several bands in the emission from the branch star detected by the NASA Infrared Telescope Facility in the range of 720–864 cm 1 include bands assignable to acetylene and hydrogen cyanide. However, there are still many other unassigned bands which may arise from higher vibrational excitation. An accurate account of emission from these excited species would be useful for the understanding of the recorded emission spectra. Similarly, the knowledge on IR emission from highly excited species such as acetylene would also benefit the characterization of the large plume of hot gases erupted from the atmosphere which was observed through NASA’s Infrared Telescope Facility when Jupiter was recently struck by a passing object. 16,17 There has been a substantial amount of spectroscopic works using the techniques of double resonance 13,18–20 (IR-UV and Raman-UV), stimulated emission pumping, 8 laser induced fluorescence 18,21 and dispersed fluorescence 12,22 to probe vibration- ally excited acetylene. In addition to a wealth of information obtained on inter- and intra-molecular vibrational relaxation, 1 the vibrational manifold has been well documented to near 20 000 cm 1 . 23,24 The rovibrational manifold has been found to rapidly become complex even at low quanta vibrational states. At high vibrational energies, perturbations to the normal mode coordinates become pronounced and disrupt the normal mode progression. 1 The anharmonic resonances, Coriolis couplings, and l-type resonances eventually destroy the 7 vibrational quantum numbers in the normal mode progression. Instead, a three quantum number {N s , N r , k} poly-ad system emerges to provide a better zero-order description. Though the normal mode quantum numbers begin to become discernibly less effective starting at B7000 cm 1 , a significant number of eigenstates still retain their original normal mode character. 25 Whether the normal mode, poly-ad, or local mode descriptions is more suitable in describing the vibrationally excited levels depends on the experimental method used to observe the vibrationally excited molecules. IR emission from vibration- ally excited molecules is particularly effective in representing the normal mode characters of the vibrational levels since the strongest emission is through Dn = 1 transitions of IR active normal modes. In contrast, local modes are more useful in single (visible) photon transition to high overtones. 26 The IR a Department of Chemistry, University of Pennsylvania, Philadelphia, PA, USA b Department of Chemistry, Temple University, Philadelphia, PA, USA. E-mail: hldai@temple.edu This journal is  c the Owner Societies 2010 Phys. Chem. Chem. Phys., 2010, 12, 2915–2922 | 2915 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics