Multipass absorption cell designed for high temperature UHV operation

This paper describes the design and performance of a tunable diode laser system, incorporating a multipass absorption cell to allow determination of water concentration below 10 ppb and carbon dioxide concentration below 1 ppb in nitrogen semiconductor gas. The cell is used with a tunable Pb-salt diode laser spectrometer frequency locked to a first derivative error signal from an intense vibration-rotation molecular absorption line. Sample concentrations are monitored in the second derivative mode, and the system automatically compensates for laser intensity fluctuations. A flowing gas method is used to minimize adsorption/desorption effects from the sample cell walls.