Visible absorption spectrum of liquid ethylene

Abstract

The visible pbsorption spectrum of liquid ethylene at -108 K from 5500 A to 7200 A was measured by using a pulsed tunable dye laser, immersed-transducer, gated-detection opto-acoustic spectroscopy technique. The absorption features show the strongest band with an absorption coefficient of m2 x 10-2 cm-' and the weakest band with an absorption coefficient of _I x 10-4 cM-. Proposed assignments of the observed absorption peaks involve combinations of overtones of local and normal modes of vibration of ethylene.The measurement of visible absorption spectra of the simple hydrocarbons methane, ethane, acetylene, and ethylene is of growing interest in the interpretation of planetary spectra and as a test of the theory of localized modes of vibration in highly vibrationally excited molecules. These gases dominate the visible and near infrared spectra of the major planets (1), with abundances decreasing in approximately the order listed (2-4). As early as 1930, photographic laboratory spectra of high pressure gaseous methane, ethane, and ethylene were measured by Adel and Slipher (5) for comparison to planetary spectra in the determination of atmospheric compositions, pressures, and temperatures. Subsequently there have been improved measurements of the visible absorption spectra of methane (1, 6-11), ethane (6), and acetylene (12); but to our knowledge no further measurement of the visible spectrum of ethylene has been made in spite of its identification in infrared planetary spectra (13) and its importance in the interpretation of the photochemistry of Jupiter's and Titan's atmospheres (4,14,15). This is in part due to the small abundances of ethylene believed to exist in these planetary atmospheres (4, 16). Because of the dense spacing of vibrational-rotational lines within the absorption bands of methane (6, 9, 17) the effects of pressure broadening are minimized (18), and the liquid absorption spectrum of methane has been shown to follow the high pressure gas absorption spectrum with small systematic shifts in band frequencies and asymmetries in band shapes (10,11). This may allow the laboratory spectrum of liquid ethylene reported here to be used as an approximation for the planetary spectrum.We recently have shown that pulsed opto-acoustic (OA) spectroscopy is a superior technique for the measurement of the weak visible absorption bands of liquid methane (11), and we have used the same technique here to measure what we believe to be the first visible absorption spectrum of liquid ethylene. The spectrum is strikingly similar to the liquid methane spectrum, continuing a correlation that has been observed between the visible methane and ethane spectra (6). Adel and Slipher (5) could report only a single gas phase ethylene absorption band within the spectral region of our measurements at an abundance of 1.8 km-amagat. At an equivalent abundance of 10-2 km-amagat we have observed eight well-resolved bands.The simplicity of vibrational overtone spectra of benzene led Henry and Siebrand (19) to propose that th...

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