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.