In situ FTIR and UV/VIS Spectroscopy

Picture of mirror mountings for the various White cell arrangements at the chamber's interior walls at level two.

Infrared spectra of trace gases as well as aerosol and cloud particles are recorded in situ in the AIDA chamber by coupling a FTIR (Fourier transform infrared) spectrometer (type IFS66v, Bruker) to a White-type multiple reflection cell with a base length of 3.8 m. The field mirror and its opposite double mirrors are mounted at the inside of two flanges which are enclosed in the chamber walls (see picture above). All mirrors are gold-coated and underlain with heating foils to prevent them from icing when performing experiments at low temperatures. The adjustment of the White cell mirrors is computer-controlled, using stepping motors especially designed for operation at the cold temperatures inside the isolating housing of the chamber. Alignment is done by switching to the halogen light source of the spectrometer. Small video cameras are used to observe the spots on the mirrors for controlling the optical path length. Typically, optical paths ranging from 23 m (6 traversals) to 257 m (68 traversals) are used, depending on the mass concentration of the investigated species. The infrared beam enters and exits the in situ White cell through flanges equipped with heated BaF₂ cell windows, their transmittance allowing for measurements from typically 6000 to 800 wavenumbers. 

The infrared measurements are supplemented by spectra recordings at UV-VIS wavelengths. For this purpose, the light from a halogen lamp (emission from 360 to 1700 nm, type HL-2000, Mikropack) is coupled to another in situ White-type multiple reflection cell with silver-coated mirrors. The light transmitted through AIDA is detected by individual 2-channel UV-VIS and NIR diode array spectrometers (tec5, types MCS UV-NIR, 360 - 1000 nm, and MMS NIR 1.7tc, 900 - 1700 nm).

For further information please contact Dr. Harald Saathoff or Dr. Robert Wagner.