Optimal Angle of Incidence for ATR-SEIRAS

The enhancement factor for ATR-SEIRAS depends on both the polarization and the angle of incidence (AOI) of the light through the internal reflection element (IRE). Furthermore, the AOI providing maximum enhancement, AOImax, depends on the morphology of the metal film. For films of contiguous metal islands, such as those typically formed from both the electroless and physical vapour deposition methods cited in the literature, AOImax is close to 70-80° (all angles measured from the surface normal). For films composed of more isolated metal islands, AOImax is closer to the critical angle for total internal reflection. Having the JF cell mounted on a variable angle instrument, such as PIKE’s VeeMAX 3, is very convenient for determining AOImax for a particular configuration. Many of the additional considerations for selecting the optimal angle of incidence are described by Sigrist et al (Applied Spectroscopy. 2019, 73, 1394-1402).

When selecting the AOI on the ATR accessory, one must also consider the effects of refraction at the air/IRE interfaces. In other words, the AOI setting on the accessory does not necessarily equate to the effective AOI (AOIeff) through the internal reflection element. When using a curved surface IRE, such as a Si or ZnSe hemisphere, the value of AOI remains unperturbed by the the air/IRE interface as infrared light should be normally incident on the crystal surface. However, in the case of face angle crystals, or IRUBIS wafers, refraction should be considered.

Below is a simple widget that will allow the calculation of AOIeff through a crystal of known refractive index and face angle.