Low temperature cell for cavity ring down absorption studies

Phase shift cavity ring down is a technique that due to its long optical path length is an ideal method to detect weak absorptions. Coupling the method to a custom fitted cryostat allows gas phase molecules to be studied at cryogenic temperatures in a thermally isolated vacuum chamber. A novel design is described to construct the complete instrument. With optical cavities of length 10⩽ℓ⩽43cm, optical path lengths between 200m and 6km have been achieved. High vibrational overtones C–H (Δυ=5) are measured at 130K (methane), 150K (ethylene), and 155K (ethane). Oscillator strengths of each molecule calculated at different temperatures are in excellent agreement. The experimental setup can be used to study kinetics and spectroscopy of atmospheric molecules, planetary atmospheres, and molecular complexes in the gas phase. Low temperatures can be obtained using liquid He or liquid N2 as cryogens.