Generally, we have observed thin film interference patterns (maxima and minima) in FTIR spectra, which are measured in T mode for sufficiently thick film (few micrometers) on silicon. However, is it possible to get an interference pattern in IR spectra for very thin film (~100 nm) in RAS mode?
You can use Attenuated Total Reflectance (ATR) technique which can be used in conjunction with infrared spectroscopy. This enables samples to be examined directly in the solid or liquid state without further preparation.
The penetration depth into the sample is typically between ~100-500nm, and the exact value can be determined.
Dear Prof. Khalil,
Thank you sir for giving me such important information. I have also this type of measurement mode (ATR) in FTIR set up beside RAS mode. But, my quire is different. I am elaborating my quire.
In generally, for interference, the path difference between two rays should be comparable to wavelength of em wave. The range of IR wave length is 2.5 to 25 micrometer. So, if the thin film thickness is in the order of few micrometer, we can observed the interference pattern in spectrum in T mode. But, if the film thickness is in the order of 100nm and if we measured the FTIR in RAS mode, then is it possible to get an interference pattern?
The optical path difference between the reflected rays (in a geometric optics simplification) is minimal compared to the wavelength so negligible interference is seen. A very high refactive index increases the path difference, but it wold need to be very high.
Yes, i am agreed with your opinion for the case of Transmission mode. But in case of RAS ( Reflection Absorption Spectroscopy ) mode, where light can suffers multiple reflections which enhanced the optical path difference in comparable to wavelength. Is it possible? Thanks.
In reflection, the intensity within the film after each internal reflection is reduced, depending on the reflectivity of the surfaces. Usually, unless you specifically design a Fabry-Perot cavity, the buildup is minimal. To estimate this you can use a treatment like this - “Electric Field Standing Wave Effects in FT-IR Transflection Spectra of biological tissue sections: simulated models of experimental variability”,
Tomasz P. Wrobel, Barbara Wajnchold, Hugh J. Byrne, Malgorzata Baranska,
Vibrational Spectroscopy, 69, 84-92 (2013)
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