PULSED-LASER EXCITATION OF ACOUSTIC MODES IN OPEN HIGH-Q PHOTOACOUSTIC RESONATORS FOR TRACE GAS MONITORING - RESULTS FOR C2H4

The pulsed excitation of acoustic resonances was studied with a contin uously monitoring photoacoustic detector system. Acoustic waves were g enerated in C2H4/N-2 gas mixtures by light absorption of the pulses fr om a transversely excited atmospheric CO2 laser. The photoacoustic par t consisted of high-and cylindrical resonators (and factor 820 for the first radial mode in N-2) and two adjoining variable acoustic filter systems. The time-resolved signal was Fourier transformed to a frequen cy spectrum of high resolution. For the first radial mode a Lorentzian profile was fitted to the measured data. The outside noise suppressio n and the signal-to-noise ratio were investigated in a normal laborato ry environment in the flow-through mode. The acoustic and electric fil ter system combined with the averaging of the photoacoustic signal in the time domain suppressed the outside noise by a factor of 4500 (73 d B). The detection limit for trade gas analysis of ethylene in pure N-2 was 2.0 parts in 10(9) by volume (ppbV) (minimal absorption coefficie nt alpha(min) = 6.1 x 10(-8) cm(-1), pulse energy 20 mJ, 1-bar N-2), a nd in environmental air, in which the absorption of other gas componen ts produces a high background signal, we can detect C2H4 to similar to 180 ppbV. In addition, an alternative experimental technique, in whic h the maximum signal of the second azimuthal mode was monitored, was t ested. To synchronize the sampling rate at the resonance frequency, a resonance tracking system was applied. The detection limit for ethylen e measurements was alpha(min) = 9.1 x 10(-8) cm(-1) for this system.