COMMENTS ON PHOTOACOUSTIC AND PHOTOTHERMAL SPECTROSCOPY OF GASES COMPARED TO OPTICAL METHODS*

Résumé- On analyse l'influence de quelques paramètres physiques (moment de transi-tion, temps de relaxation, pression...) qui conditionnent la sensibilité ultime des détections photoacoustique et photothermique (PA/PT) que l'on compare aux méthodes de détection optique. On montre que c'est seulement à haute pression (faible résolu-tion) et/ou pour des transitions faiblement autorisées que les méthodes PA et PT sont avantageuses en spectroscopie. Dans le domaine sous-Doppler on aura intérêt à utiliser une détection optique pour les transitions fortes et une détection bolomé-trique pour les transitions faibles. Abstract- We discuss the influence of some critical parameters, especially the pressure, on the sensitivity of photoacoustic and photothermal methods for high resolution spectroscopy. We show that these methods lose their sensitivity in the sub-Doppler regime where they are advantageously replaced by optical detection (for which the shot noise limit can be reached with high frequency modulation techniques) or by detection of the internal excitation energy carried by the molecules onto a bolometer. When one wishes to detect a weak absorption of light by a molecular gas, one can either measure the change in light intensity (transmission spectroscopy) or the change induced in the gas itself by molecular excitation (e.g. the pressure in pho-toacoustic spectroscopy). The sensitivity of the first technique may appear to be poor because it is difficult to detect a small change on top of a large signal. On the contrary, photoacoustic (PA) or photothermal (PT) detection as "dark background methods " have been broadly used in gas spectroscopy because they seem to exhibit a tremendous sensitivity especially when coupled with powerful laser:sources. One knows, for instance, that atmospheric pollutants have been detected by this, me-thod at concentrations as low as a fraction of PPB [ 1], Nevertheless one may wonder Informal contribution to the Panel Discussion on Weakly Absorbing Systems. Article published online by EDP Sciences and available a