I am in the process of building a new setup and have a discussion with my colleague on the subject.
We will measure several monolayers on a flat reflective surface inside a vacuum chamber with ~1 wavenumber resolution. The two 38mm windows will be placed at ~300 mm distance from the surface. The angle of incidence is 75 degrees. The window ports are normal to the IR beam axis. The beam is focused at the sample surface with a parabollic mirror placed before the entrance window.
As far as I understand, when the interference fringes are highly reproducible, background subtraction deals with them just fine. In my case of measurements in vacuum, the only irregularities I can think of are thermal expansion due to lab T variations, a slight misalignment of the reflective sample surface, and T variations at a liquid nitrogen cooled MCT detector assembly depending on the Dewar fill level, I guess. The room T variations can affect the distance between the KBr windows, and between the exit window and the external MCT detector. But these distances are so large that interferences do not survive at 1 cm-1 resolution. The fringes created via multiple reflections inside individual KBr windows will be highly reproducible, in my view. I cannot do much to improve the detector assembly.
Do I miss something and wedge/tilt in KBr windows could be useful?
If you want to avoid fringes - the cross-sectionof your film should have a wedge shape
if you wish to bypass or evade the fringes then you should have wedge shaped film.
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