Orientation effects on nitric acid dihydrate films

Area of Science:

• Solid-state chemistry
• Spectroscopy
• Materials science

Background:

• Nitric acid dihydrate (NAD) films are crucial in various applications, including remote sensing.
• Understanding the optical properties of NAD films requires knowledge of their molecular orientation.
• Previous studies have not fully addressed orientation effects in crystalline NAD films.

Purpose of the Study:

• To investigate orientation effects in crystalline alpha-NAD films.
• To evaluate the accuracy of optical indices for NAD using infrared spectroscopy.
• To correlate spectral features with crystallographic orientation.

Main Methods:

• Preparation of crystalline alpha-NAD films via vapor deposition at 175 K on Ge and Al substrates.
• Systematic study of transmission and reflection-absorption infrared (RAIR) spectra.
• Recording spectra at normal and grazing incidence (75 degrees) with polarized radiation.

Main Results:

• Satisfactory agreement between experimental data and Fresnel model for transmission and s-polarized RAIR spectra.
• Discrepancies observed for p-polarized RAIR spectra, indicating orientation effects.
• Identification of a preferential alignment of the (10-1) crystallographic plane parallel to the substrate.

Conclusions:

• Preferential orientation of alpha-NAD films significantly influences their infrared spectra, particularly in p-polarized RAIR measurements.
• The observed orientation affects the accuracy of optical indices derived from spectral data.
• This finding is critical for accurate interpretation of remote sensing data involving NAD.