Journal of Physical Chemistry A, Vol.113, No.24, 6689-6696, 2009
High-Resolution Absorption Studies of the (A)over-tilde(1)A(2)-(X)over-tilde(1)A(1) 2(0)(2)4(0)(1) Band of Formaldehyde
Absolute peak absorption cross sections and pressure broadening coefficients have been recorded with sub-Doppler limited instrumental resolution for selected rotational lines in the 2(0)(2)4(0)(1) vibronic band of the formaldehyde (A) over tilde (1)A(2)-(X) over tilde (1)A(1) electronic transition. The measured absorption cross sections range between (0.18 +/- 0.01) and (10.1 +/- 0.08) x 10-(19) cm(2) molecule(-1) and are considerably larger than values from the literature recorded using apparatus where instrumental broadening was significant. However, comparisons with spectral simulations with equivalent resolution from Smith et al. (J. Phys. Chem. A 2006, 110, 11645-11653) are in excellent agreement. Pressure broadening was studied for the collision partners CH2O, CO2, N-2, O-2, Ne, Kr, Ar, and He, and the resulting broadening coefficients were found to be reduced in comparison to equivalent values measured in infrared regions, consistent with the reduced dipole moment of the upper state probed in this work. Cavity-enhanced absorption spectroscopy (CEAS) measurements were undertaken using calibrated low concentration (2.9-4.6 ppmv) samples from a permeation source and demonstrate a noise equivalent absorption of 1.2 x 10(-6) cm(-1) Hz(-1/2). This implies a minimum detectable formaldehyde concentration with the current system in atmospheric air of 172 ppbv Hz(-1/2).