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)(0)4(1)(0) vibrational band of the A(1)A(2)-X(1)A(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 x 10(-4) cm(-1) Torr(-1) for several isolated lines is reported. Gaseous HCHO was quantitatively introduced into a flow cell by evaporating micron-sized 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 (approximately 1 cm(-1)) obtained with our calibration technique are in excellent agreement with previous measurements. A maximum differential cross section of 5.7 x 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.