A recently introduced approach to photoacoustic detection of trace gases utilizing a quartz tuning fork ͑TF͒ as a resonant acoustic transducer is described in detail. Advantages of the technique called quartz-enhanced photoacoustic spectroscopy ͑QEPAS͒ compared to conventional resonant photoacoustic spectroscopy include QEPAS sensor immunity to environmental acoustic noise, a simple absorption detection module design, and its capability to analyze gas samples ϳ1 mm 3 in volume. Noise sources and the TF properties as a function of the sampled gas pressure, temperature and chemical composition are analyzed. Previously published results for QEPAS based chemical gas sensing are summarized. The achieved sensitivity of 5.4ϫ 10 −9 cm −1 W/ ͱ Hz is compared to recent published results of photoacoustic gas sensing by other research groups. An experimental study of the long-term stability of a QEPAS-based ammonia sensor is presented. The results of this study indicate that the sensor exhibits very low drift, which allows data averaging over Ͼ3 h of continuous concentration measurements. Architecture and practical implementation of autonomous QEPAS-sensor controller electronics is described. Future developments of QEPAS technique are outlined.

A spectroscopic gas sensor for nitric oxide (NO) detection based on a cavity ringdown technique was designed and evaluated. A cw quantum-cascade distributed-feedback laser operating at 5.2 mum was used as a tunable single-frequency light source. Both laser-frequency tuning and abrupt interruptions of the laser radiation were performed through manipulation of the laser current. A single ringdown event sensitivity to absorption of 2.2 x 10(-8) cm(-1) was achieved. Measurements of parts per billion (ppb) NO concentrations in N(2) with a 0.7-ppb standard error for a data collection time of 8 s have been performed. Future improvements are discussed that would allow quantification of NO in human breath.

The detailed studies of spontaneous Raman spectra are performed for highly excited SF6 molecules in the vicinity of the ν1 mode. Interpretation of the measured spectra is given within the framework of theoretical model, which assumes the dominating role of the statistical inhomogeneous broadening in the spectrum formation. A good agreement is achieved between the experimental and theoretical spectra throughout the whole temperature range used (T vib = 850−1660 K). A conclusion is drawn that the homogeneous broadening due to intramolecular vibrational relaxation gives a relatively small contribution to the observed spectra.

Intramolecular vibrational redistribution is a fundamental phenomenon observed in polyatomic molecules when sufficiently excited vibrationally. In this paper, results mostly from the last two decades of research on this subject are summarized, obtained either from infrared spectroscopy with a resolution of as high as 10 À4 cm ÀI or, in a different approach, by using various pump-probe schemes with a temporal resolution from dozens of picoseconds to subpicoseconds.A A Makarov, A L Malinovsky, E A Ryabov

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