Hi Luboš,

It is the evanescent field that interacts with, and is absorbed by, the sample in an ATR set-up. It would therefore be possible to have a small air gap between the sample and substrate, so long as the sample is still within this field. In other words, the sample would need to be within the "skin depth" of the evanescent wave, which is usually in range of tens to a few hundred nanometers from the surface (depending on the materials and wavelengths involved).

Even though it is technically possible, the evanescent field exponentially decays from the surface, meaning that the absorption from the sample will be small. Also, the vibrational modes of the air molecules will also contribute to absorption, and the air-sample interface would scatter some of the field. If you're trying to do non-contact sensing, there may be a better approach for your application.

I am in agreement with Robert Hunter . The evanescence wave has a limited depth of penetration through a sample. Having direct sample to crystal contact is usually required to get good, reliable data with FTIR. Bruker provided a good explanation that can be explored more here: https://www.bruker.com/en/products-and-solutions/infrared-and-raman/ft-ir-routine-spectrometer/what-is-ft-ir-spectroscopy.html

Hello, thanks for answers. A few hundred nanometers at best could be enough for me.

Actually, for IR ATR the relevant penetration depth is more a few microns rather than a few tens or hundred nanometers...

Thomas Mayerhöfer is correct - as a rule of thumb, the extent of the evanescent field is of the order of the wavelength.... and also dependent on the wavelength.

The extent of the evanescent field depends actually on a number of parameters like the refractive index of the ATR crystal, the angle of incidence, the wavenumber (or wavelength) and the polarization state. If you are interested, have a look at Preprint Wave optics in Infrared Spectroscopy