Low temperature cavity ring down spectroscopy with off-axis alignment: application to the A- and γ-bands of O2 in the visible at 90 K

Abstract

The cavity ring down (CRD) technique presented here involves an optical cavity attached to a cryostat. The static cell and mirrors of the optical cavity are all inside a vacuum chamber at the same temperature of the cryostat. The temperature of the cell can be changed between 77 K and 298 K. An off-axis alignment of the laser beam into the cavity is used to increase the number of resonant modes inside the cavity and improve the signal to noise ratio of the absorption bands. To demonstrate the capabilities of the low temperature CRD cell, the absorption spectra of O 2 are recorded at 90 K for the A (υ = 0 ← υ = 0) and γ (υ = 2 ← υ = 0) bands of the b 1 + g ← X 3 − g transition using cavity ring down spectroscopy. The optical cavity performance was tested using two variations of the CRD technique. The A-band is measured using the phase-shift cavity ring down method and the γ -band using the pulsedlaser exponential-decay method. A comparison between experimental and simulated spectra of the O 2 bands at 90 K confirms the molecular temperature measured by a sensor localized in the cell. Quantitative measurements of the individual rotational line intensities are made for the oxygen γ -band to confirm the temperature of the cell and calculate the vibrational band intensity. The application of this technique for laboratory studies of planetary atmospheres and the spectroscopy of molecular complexes is emphasized.

Low-Temperature Cavity-Enhanced Spectroscopy with a Diode Laser: Vibration–Rotation Absorption of CO (3ν) at 80 K and CH3C≡C–H (2ν1) at 165 K

“…We obtained oxygen absorption (γ and A bands) spectra to determine temperatures between 90 and 296 K in a cryogenic cell. 18 We have used the same technique flowing air into heated open tubes to obtain air temperatures from O 2 spectra. 19,20…”

“…We obtained oxygen absorption (γ and A bands) spectra to determine temperatures between 90 and 296 K in a cryogenic cell. 18 We have used the same technique flowing air into heated open tubes to obtain air temperatures from O 2 spectra. 19,20 To illustrate the use of the CEAS technique at low temperatures, the vibrational-rotational overtone transitions (υ = 0→3) of carbon monoxide (CO) at 295 and 80 K and the acetylenic C-H (υ = 0→2) bond of CH 3 C≡C-H at 293 and 165 K are obtained.…”