Cavity-enhanced absorption: detection of nitrogen dioxide and iodine monoxide using a violet laser diode

We present an application of cavity-enhanced absorption spectroscopy with an off-axis alignment of the cavity formed by two spherical mirrors and with time integration of the cavity-output intensity for detection of nitrogen dioxide (NO2) and iodine monoxide (IO) radicals using a violet laser diode at *l=404.278 nm. A noise-equivalent (1*s= root-mean-square variation of the signal) fractional absorption for one optical pass of 4.5X10 was demonstrated with a mirror reflectivity of ~0.99925, a cavity length of 0.22 m and a lock-in-amplifier time constant of 3 s. Noise-equivalent detection sensitivities towards nitrogen dioxide of 1.8X10 molecule cm and towards the IO radical of 3.3X10 molecule cm were achieved in flow tubes with an inner diameter of 4 cm for a lock-in-amplifier time constant of 3 s. Alkyl peroxy radicals were detected using chemical titration with excess nitric oxide (RO2+NO->RO+NO2). Measurement of oxygen-atom concentrations was accomplished by determining the depletion of NO2 in the reaction NO2+O->NO+O2. Noise-equivalent concentrations of alkyl peroxy radicals and oxygen atoms were 3X10 molecule cm in the discharge-flow-tube experiments.