ULTRASENSITIVE ABSORPTION MEASUREMENTS OF MET AST ABLE SPECIES: PRESSURE BROADENING OF THE b1?g - a1Ag (1,0) BAND IN OXYGEN USING OFF-AXIS INTEGRATED-CAVITY OUTPUT SPECTROSCOPY

摘要

A novel laser-based technique applicable to metastable species detection is discussed. Off-axis integrated-cavity-output spectroscopy (ICOS) has been applied to the study of singlet oxygen. Singlet oxygen, O2(a), was generated in a microwave plasma, and the afterglow passed through an off-axis ICOS measurement system consisting of an 82-cm long, high-finesse optical cavity bounded by two highly reflective mirrors. The mirror reflectivity was determined by performing cavity-ringdown measurements and observing ringdown times of 219-253 us in the 1494 nm to 1512 nm range. A diode laser was current tuned, and light exiting the cavity was focused onto an InGaAs detector. The cavity transmission was recorded as a function of laser frequency. Details of the method are presented as well as the spectroscopic characterization of selected transitions of the (1,0) band of the b1Xg+- a1Ag Noxon system of oxygen (radiative lifetime 160 minutes). Pressure broadening of the electric quadrupole allowed Noxon system (a1Ag-"b1Sg) of oxygen by helium has been examined using the ultra-sensitive off-axis ICOS technique. The absorption lineshapes of ten rotational lines covering J = 4 -16 within the (1,0) band have been measured with a spectral resolution of 0.004 cm'1. Pressure broadening at low pressure (25 - 100 Torr) yields broadening coefficients (HWHM) in the range of y= 3.61 -5.81 x 10"6 cm"1/Torr with uncertainties of 5 - 20%. These rates are similar to those for b1Zg -^sX3Eg" and a1Ag -^sX3?g" transitions. The rates systematically decrease with rotational state by about 35%, consistent with inelastic collisions. The ICOS technique enabled the detection on this very weak transition with single pass absorptions of less than 5 x 108 and produced an O2(a) detection limit of 2 x 1012 molecules/cm3 per quantum state.