An Axial Molecular Beam Mid-Infrared Tunable Diode Laser Spectrometer.
H. D. Osthoff,1 J. Walls,2 W. A. van Wijngaarden,2 and W. Jäger1
2Department of Physics & Astronomy, York University, Toronto ON M3J 1P3, Canada
A novel molecular beam spectrometer for the purpose of trace gas sensing is described. Sensitivity is greatly enhanced and absorption interference by atmospheric H2O and CO2 is greatly reduced by using a molecular expansion of helium gas seeded with the analyte gas. The adiabatic expansion results in rotational cooling and population enhancement of low-lying energy levels. The instrument employs a tunable lead salt diode, which is operated in single mode near the R(0) transition of the asymmetric stretch of CO2. We have constructed a specially adapted 60 m Herriott multipass cell. The sample gas is injected axially through a coupling hole in one of the spherical mirrors, resulting in the observation of a Doppler pair. Axial injection increases the residence time of the molecular beam in the sampling region. Pulsed operation of the nozzle allows background subtracted spectra to be acquired. The laser is either rapidly scanned over the absorption feature of interest, or locked to the centre frequency and modulated at a frequency of 50 kHz. 2f demodulation and careful tuning of the modulation amplitude allow the simultaneous sampling of both Doppler components.
Sample spectra of CO2 and of several CO2 containing van der Waals complexes are presented. The instrument’s stability was assessed in long-term environmental monitoring trials. Key problem areas are pulse-to-pulse reproducibility, diode laser stabilization and gas sampling issues. In addition, several post-data acquisition digital filtering techniques, including Wiener and Kalman filters, were evaluated.