Absorption cross-sections and pressure broadening of rotational lines in the ν5 + ν9 band of ethene measured by diode laser cavity ring down spectroscopy

Abstract

Absorption cross sections are reported for several rotationally resolved lines in the ^QQ-branch of the ν₅ + ν₉ combination vibrational band of ethene (ethylene) at wavelengths around 1.625 μm. The measurements were made using cavity ring-down spectroscopy with a near infra-red diode laser. In the strongest region of the band, a line-integrated absorption cross section for the ^QQ₄(4) feature of 5.08 ± 0.38 × 10⁻²² cm² molecule⁻¹ cm⁻¹ is obtained, with a peak cross section at line centre of 3.23 ± 0.03 × 10⁻²⁰ cm² molecule⁻¹. The effects of addition of pressures of up to 200 Torr of air on spectral linewidths are analysed to obtain an air broadening coefficient (HWHM) of γ = 0.1060 ± 0.0011 cm⁻¹ atm⁻¹. In 1 atm of air, the line centre absorption cross sections in the ^QQ₄ branch are reduced by a factor of ∼8 because of line broadening. The consequences of these data for direct monitoring of ethene in the atmosphere using sensors based on diode laser cavity ring-down spectroscopy are discussed.