Using ATR to Correlate the Polarizer Indicator

Commercial infrared polarizers usually contain an indicator that shows the orientation of the polarizing element in its mount. Generally, the polarizer manuals relate the direction of the electric field vector to the indicator. This aspect can then be correlated to the direction of the sample plane to establish which polarization, perpendicular (s) or parallel (p), is impinged on the sample.

This relationship is simple for transmission. An accessory is generally used for reflectance to direct the infrared radiation to and from the sample through mirrors and is capable of rotating or even scrambling the direction of the electric field.

Although the desired polarization can be theoretically determined, it is not necessarily an easy process. If the optical layout of the accessory or polarizer manual is not available, it becomes complicated to rely solely on theoretical considerations.

Figure 1. The Horizon ATR Accessory.

On the other hand, the orientation of the polarizer can also be determined experimentally. In the case of ATR, the effecting thickness for p-polarized light is almost twice that for s-polarized light at a 45° incident angle.¹ This factor of two in the effective thickness is directly reflected in the spectral band intensities.

This article describes a quick test to experimentally differentiate the two perpendicular positions; usually marked ‘0’ and ‘90’ on the polarizer mount, using ATR spectroscopy.

Experiment

All spectra were collected using the Horizon, a fixed 45^o incident angle multiple reflection ATR accessory, and a wire grid polarizer (KRS-5 substrate) installed in a commercial FT-IR spectrometer. The FT-IR spectrometer was then calibrated for 16 scans at 8 cm^-1 resolution.

After setting the polarizer indicator to ‘90’, the background spectrum was collected. The sample spectrum was then calculated after filling the Horizon trough with water. Finally, the polarizer was rotated 90° to the ‘0’ setting to deliver the other polarization, and the data collection process was again repeated.

Results and Discussion

Figure 2. ATR Spectra of Water Recorded with the Polarizer Set to ‘90’ (red) and ‘0’ (blue).

Figure 2 shows the resulting spectra. One of the spectra has band intensities about twice that of the other, as expected from the relative effective thicknesses at a 45° incident angle. Therefore, the ‘0’ setting on this polarizer offers p-polarized radiation to the sample.

This technique can be used with almost any type of sample and any type of ATR accessory to verify or determine the polarization. The only exception is for those accessories that utilize a rod-style ATR crystal, as these tend to scramble the incident polarization.

References

¹ N.J. Harrick, Internal Reflection Spectroscopy (Wiley, NY, 1967).

This information has been sourced, reviewed and adapted from materials provided by Harrick Scientific Products, Inc.

For more information on this source, please visit Harrick Scientific Products, Inc.