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In Trace Gas Measurement, Bigger Is Not Better

Photonics Spectra
Nov 1999
Chemists at the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., have developed a spectroscopy chamber for measuring trace gases that is 100 times more sensitive than previous models. Surprisingly, the new device is also one-tenth the size.

Cavity ringdown spectroscopy detects trace gas contamination for industrial applications such as the fabrication of semiconductor wafers. Gas is pumped into the test cavity, and an infrared laser that is tuned to the absorption bands of the impurity illuminates the sample. The rate of loss of the beam's intensity indicates the presence and concentration of a contaminant.

The NIST researchers discovered that a 10-cm-long test chamber is far more accurate than meter-long ones. Longer chambers leak light more slowly, which should ease the detection of changes in intensity. The team found, however, that longer cavities produce wave interactions that complicate the signals. The short chambers and a pulsed beam generate simpler wave patterns.

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