The peak at 860 cm-1 is the only spectral feature that is at exactly the same
frequency in both ATR – IR and IRRAS measurements. This fact together with the
unusual shape of this peak supports the interpretation of this peak as a mechanical
vibration that is detected in the IR spectrometer. The vibration most likely
comes from one of the mechanical pumps connected to an ultra high vacuum
chamber. The FTIR-spectrometer is connected to this ultra high vacuum chamber, as the FTIR is also used to make IRRAS measurements under ultra high vacuum conditions [13] using an external detector.