NASA - National Aeronautics and Space Administration

The rotationally resolved ultraviolet absorption cross sections for the 200410 vibrational band of the A1A2X1A1 electronic transition of formaldehyde (HCHO) at an apodized resolution of 0.027 cm-1 (approximately 0.0003 nm at 352 nm) over the spectral range 28100-28500 cm-1 (351-356 nm) at 298 and 220 K, using Fourier transform spectroscopy, are first reported here. Accurate rotationally resolved cross sections are important for the development of in situ HCHO laser-induced fluorescence (LIF) instruments and for atmospheric monitoring. Pressure dependence of the cross sections between 75 and 400 Torr at 298 K was explored, and an average pressure broadening coefficient in dry air of 1.8 × 10-4 cm-1 Torr-1 for several isolated lines is reported. Gaseous HCHO was quantitatively introduced into a flow cell by evaporating micronsized droplets of HCHO solution, using a novel microinjector technique. The condensed-phase concentrations of HCHO were determined by iodometric titrations to an accuracy of <1%. Accuracy of the measured absorption cross sections is estimated to be better than (5%. Integrated and differential cross sections over the entire band at low resolution (∼1 cm-1) obtained with our calibration technique are in excellent agreement with previous measurements. A maximum differential cross section of 5.7 × 10-19 cm2 molecule-1 was observed at high resolutionsalmost an order of magnitude greater than any previously reported data at low resolution.