Cross sections in the 2ν5 band of formaldehyde studied by cavity enhanced absorption spectroscopy near 1.76 μm

Abstract

The 2ν₅ C–H stretching band of formaldehyde has been studied by cavity enhanced absorption spectroscopy near 1.76 μm using the output of an external cavity diode laser. Strong individual transitions have been spectroscopically identified, and the absorption cross section and air broadening parameter measured for the ^QR₁(10) line at 5676.21 cm⁻¹, a wavenumber at which interference from absorption by atmospheric CH₄ and other constituents is minimal. The integrated cross section is found to be 5.7 ± 0.6 × 10⁻²² cm² cm⁻¹, and the pressure broadening coefficient for air at 295 K is 3.93 MHz Torr⁻¹ (half width at half maximum of the Lorentzian component). The integrated intensity of the 2ν₅ absorption band is found to be a factor of 108 smaller than that of the ν₅ fundamental. Implications of these values for the measurement of atmospheric formaldehyde by absorption near 1.76 μm are discussed.