Atmospheric pressure air plasmas are often thought to be in local thermodynamic equilibrium owing to fast interspecies collisional exchange at high pressure. This assumption cannot be relied upon, particularly with respect to optical diagnostics. Velocity gradients in flowing plasmas and/or elevated electron temperatures created by electrical discharges can result in large departures from chemical and thermal equilibrium. This paper reviews diagnostic techniques based on optical emission spectroscopy and cavity ring-down spectroscopy that we have found useful for making temperature and concentration measurements in atmospheric pressure plasmas under conditions ranging from thermal and chemical equilibrium to thermochemical nonequilibrium.

We have constructed a cavity ring-down spectrometer employing a near-IR external cavity diode laser capable of measuring 13C/12C isotopic ratios in CO2 in human breath. The system, which has a demonstrated minimum detectable absorption loss of 3.2 x 10(-11) cm(-1) Hz(-1/2), determines the isotopic ratio of 13C16O16O/12C16O16O by measuring the intensities of rotationally resolved absorption features of each species. As in isotope ratio mass spectrometry (IRMS), the isotopic ratio of a sample is compared to that of a standard CO2 sample calibrated to the Pee Dee Belemnite scale and reported as the sample's delta13C value. Measurements of eight replicate CO2 samples standardized by IRMS and consisting of 5% CO2 in N2 at atmospheric pressure demonstrated a precision of 0.22/1000 for the technique. Delta13C values were also obtained for breath samples from individuals testing positive and negative for the presence of Helicobacter pylori, the leading cause of peptic ulcers in humans. This study demonstrates the ability of the instrument to obtain delta13C values in breath samples with sufficient precision to serve as a useful medical diagnostic.

We have performed cavity ring-down spectroscopy by locking a high-finesse resonator to the probe laser. We have obtained combination overtone spectra of water vapor in the ambient environment with a baseline noise of 5ϫ10 Ϫ9 cm Ϫ1 for decay constants ͑Rϭ99.93% reflectors͒ of 1 s. This cavity-locked approach ensures single transverse mode excitation, reduces shot-to-shot fluctuations in the decay constant to 4ϫ10 Ϫ3 , and eliminates oscillations in spectral backgrounds. This approach also allows ring-down decay acquisition rates limited only by the ring-down and buildup constants of the resonator, and holds the promise of offering truly shot-noise-limited cavity ring-down spectroscopy measurements.

A system is described that employs a diode-pumped Nd:YAG continuous-wave laser source servolocked to a three-mirror optical cavity and an analog detection circuit that extracts the ring-down rate from the exponentially decaying ring-down waveform. This scheme improves on traditional cavity ring-down spectroscopy setups by increasing signal acquisition rates to tens of kilohertz and reducing measurement noise sources. For example, an absorption spectrum of a weak CO 2 transition at 1064 nm is obtained in less than 10 s at a spectral resolution of 75 kHz employing a cavity with an empty-cavity ring-down decay lifetime of 2.8 s and a total roundtrip path length of 42 cm. The analog detection system enables laser frequency scan rates greater than 500 MHz/s. The long-term sensitivity of this system is 8.8ϫ10 Ϫ12 cm Ϫ1 Hz Ϫ1/2 and the short-term sensitivity is 1.0ϫ10 Ϫ12 cm Ϫ1 Hz Ϫ1/2 .

Cavity ringdown spectra of ammonia at 10 parts in 10(9) by volume (ppbv) and higher concentrations were recorded by use of a 16-mW continuous-wave quantum-casacde distributed-feedback laser at 8.5 mum whose wavelength was continuously temperature tuned over 15 nm. A sensitivity (noise-equivalent absorbance) of 3.4x10(-9) cm(-1) Hz(-1/2) was achieved for ammonia in nitrogen at standard temperature and pressure, which corresponds to a detection limit of 0.25 ppbv.