A new spectroscopic technique applicable to the detection of ultra-weak and for- bidden molecular transitions is presented. The method is based on o -axis integrated- cavity-output spectroscopy (ICOS) and has been applied to the detection of singlet (a1 g) oxygen in the (1,0) band of the electric-quadrupole allowed Noxon system (b1 + g a1 g) of oxygen. The details of the method as well as spectroscopic data con rming the absolute line positions of twenty-one lines in the (1,0) band of the b1 + g a1 g have been completed to within 0:004 cm 1. A Boltzmann analysis veri ed previously calculated and unproven integrated cross sections for each of the lines. Ten lines were also pressure broadened from 25 Torr to 100 Torr, and pres- sure broadening coe cients were determined from -p = 3:61 10 6 cm 1=torr to 5:81 10 6 cm 1=torr. These coe cients illustrate that rotationally-inelastic colli- sions are the dominant mechanism in molecular pressure broadening. The singlet oxy- gen was generated in a microwave plasma and the afterglow passed through an o -axis ICOS measurement system consisting of an 82 cm long, high- nesse (F 248000) optical cavity. The mirror re-ectivity, R, was determined by performing a cavity- ringdown measurement and observing ringdown times of 220 s in a range from 1494 nm to 1512 nm.

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