Molecular relaxation effects in hydrogen chloride photoacoustic detection

Abstract

A photoacoustic (PA) sensor has been developed to monitor hydrogen chloride at sub-ppm level in the 1740-nm region. The system was designed to control the process in the novel low-water-peak optical fiber manufacturing process. Relaxation effects in hydrogen chloride PA detection in oxygenhelium and nitrogen-helium gas mixtures are presented, showing that the generation of the PA signal is strongly affected by the ratio of these substances. In addition, the role of water vapor in the PA signal is investigated. Photoacoustic spectroscopy (PAS) is a widely recognized technique for its high performance in the detection of trace gases in various applications [1][2][3]. The main advantages of this technique are its intrinsically zero background nature and its achromaticity, which result in a very sensitive and simple setup arrangement suitable in a broad spectral region ranging from the ultraviolet to the mid infrared.The development of photoacoustic (PA) sensors operating in the near-infrared (NIR) range has strongly increased in recent years due to the excellent properties of the available diode lasers. Such laser sources provide single-mode emission (for instance distributed feedback (DFB) lasers) with narrow line width (typically a few MHz), an output power ranging from a few mW to several tens of mW, modulation capabilities and thousands of hours of operation. Moreover, their compact size and their fiber-coupled output make them easy to align with the PA cell.Despite its above-mentioned advantages, PAS is an indirect method since the optical energy absorbed by the molecules is detected through an acoustic wave generated in the gas due to thermal expansion resulting from the vibrationto-translation (V-T) relaxation of the excited molecules. Consequently, the conversion from optical to thermal energy depends on the physico-thermal properties of the gas, so that a calibration of the sensor is required. Whereas molecular re-

Broadening and shifting coefficients of rotation–vibrational lines in the fundamental and first overtone bands of HCl and HBr induced by oxygen and air

“…The only literature value of the air-induced overtone coefficient γ of R 2 (4) [15] is in good agreement with our data. The deviation is about 3.5%.…”

“…Experimental inaccuracies are estimated to be 4%. (7) 11.4 (5) 11.9 (5) 15 7.2 (3) 6.9 (6) 9.9 (7) 10.1 (6) 16 10 (1) 6.6 (6) absorbers shows a small gradual loss within the rising amount of the perturbers. The contribution of the selfbroadening and shifting (with corrected pressures of the hydrogen halides) was accounted for, but found to to be much smaller than the stated unaccuracies of γ(m) and δ(m).…”

“…A high resolution tunable difference-frequency laser work by Pine and Looney reports N 2 and air broadening and shift effects for the (1←0) HCl band [13]. Airinduced shifts were measured in the first overtone long ago by Kimel et al [14], the broadening of the R 2 (4) line was obtained with a photo-acoustic method at atmospheric condition almost five decades later [15].…”

Performance evaluation of a near infrared QEPAS based ethylene sensor

“…For 100 ppmv C 2 H 4 , no difference in the QEPAS signal obtained in both dry and humid gas is observed. This demonstrates that a fast vibration-to-translation relaxation occurs from the C 2 H 4 excited state even in dry N 2 , so that relaxation effects that have been reported for some conditions in PAS [15][16][17] and which are also relevant …”

Standard photoacoustic spectrometer: Model and validation using O2 A-band spectra

“…However, the conversion of optical-to-acoustic energy can make the acoustic signal vary nonlinearly with absorber and buffer-gas concentration. [6][7][8] This and additional complications of the acoustic field often prevent robust prediction of absolute PAS system response over a wide range of gas composition, pressure, and temperature. Typically, the PAS system response is calibrated in terms of a reference sample of known absorption coefficient.…”