Rotationally resolved absorption cross sections of formaldehyde in the 28100-28500 cm ^-1 (351-356 nm) spectral region: Implications for in situ LIF measurements

The rotationally resolved ultraviolet absorption cross sections for the 2 ₀ ⁰4 ₀ ¹ vibrational band of the A ¹A ₂-X ¹A ₁ 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 cm ² molecule ^-1 was observed at high resolution-almost an order of magnitude greater than any previously reported data at low resolution.