CsI is a relatively low refractive index (1.74 @ 12 microns) material that is used as an extended spectral range replacement for KBr. It is soft and very hygroscopic and must be protected from humidity at all times. It is a difficult material to polish flat and handle while assembling for any application. CsI will fog over time in a humid environment. This fogging may not effect the IR beam but the laser reference beam used in most FT-IR systems is scattered and thus unusable. In addition, for FT-IR beamsplitter, virgin powered CsI containing no scrap CsI should be used to grow the original crystal ingot from which the beamsplitter is manufactured.

Advantages

1. Long wavelength transmission
2. Low refractive index

Disadvantages

1. Expensive
2. Very soft and difficult to polish
3. Very hygroscopic, fogs easily
4. Scatters visible radiation

Physical Data

Melting Point: 621 °C
Density: 4.51 g/cm³
Solubility in H₂O: 44 gm/100 gm water @ 50 °C
Hardness: 1 kg/mm²
Appearance: Clear, crystalline. Fogs quickly upon exposure to moisture.
Crystal Structure: Cubic
Molecular Weight: 259.83

Refractive Index¹

Specific index listed. Generic: 1.73 @ 10 microns

Spectral Range

Short Wavelength Limit: 42,000 cm^-1 (1 mm)
Long Wavelength Limit: 172 cm^-1 (4 mm)

Coatings

CsI is usually coated with a thin protective coating to inhibit fogging. Broad band beamsplitter coatings have been developed for CsI.

Typical Uses

• Extended range (to 50 microns) beamsplitters for commercial FT-IR instruments
• Room temperature infrared detector windows
• Liquid cell windows for use with anhydrous non-polar solvents
• Scintillation detectors

Notes

Short and Long Wavelength Ranges for which transmissivity is greater than 50% of stated crystal thickness.

1. W.S. Rodney; Journal of the Optical Society of America, Vol 45, p. 987–992, 1955.