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CsI Cesium Iodide Technical Data

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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

WAVELENGTH (Microns) INDEX
0.50 1.80635
1.00 1.75721
2.00 1.74661
3.00 1.74400
4.00 1.74305
5.00 1.74239
6.00 1.74181
7.00 1.74122
8.00 1.74059
9.00 1.73991
10.0 1.73916
11.0 1.73835
12.0 1.73746
13.0 1.73650
14.0 1.73547
15.0 1.73436
16.0 1.73317
17.0 1.73190
18.0 1.73056
19.0 1.72913
20.0 1.72762
21.0 1.72602
22.0 1.72435
23.0 1.72258
24.0 1.72073
WAVELENGTH (Microns) INDEX
25.0 1.71880
26.0 1.71677
27.0 1.71465
28.0 1.71244
29.0 1.71014
30.0 1.70774
31.0 1.70525
32.0 1.70266
33.0 1.69996
34.0 1.69717
35.0 1.69427
36.0 1.69127
37.0 1.68815
38.0 1.68493
39.0 1.68159
40.0 1.67814
41.0 1.67457
42.0 1.67088
43.0 1.66707
44.0 1.66312
45.0 1.65905
46.0 1.65485
47.0 1.65051
48.0 1.64602
49.0 1.64139
50.0 1.63662

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.

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