Pulsed-laser excitation of acoustic modes in open high-Q photoacoustic resonators for trace gas monitoring: results for C_2H_4

The pulsed excitation of acoustic resonances was studied with a continuously monitoring photoacoustic detector system. Acoustic waves were generated in C_2H_4/N_2 gas mixtures by light absorption of the pulses from a transversely excited atmospheric CO_2 laser. The photoacoustic part consisted of high-Q cylindrical resonators (Q 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 frequency spectrum of high resolution. For the first radial mode a Lorentzian profile was fitted to the measured data. The outside noise suppression and the signal-to-noise ratio were investigated in a normal laboratory environment in the flow-through mode. The acoustic and electric filter system combined with the averaging of the photoacoustic signal in the time domain suppressed the outside noise by a factor of 4500 (73 dB). The detection limit for trace gas analysis of ethylene in pure N_2 was 2.0 parts in 10^9 by volume (ppbV) (minimal absorption coefficient α_min = 6.1 × 10^−8 cm^−1, pulse energy 20 mJ, 1-bar N_2), and in environmental air, in which the absorption of other gas components produces a high background signal, we can detect C_2H_4 to ~180 ppbV. In addition, an alternative experimental technique, in which the maximum signal of the second azimuthal mode was monitored, was tested. To synchronize the sampling rate at the resonance frequency, a resonance tracking system was applied. The detection limit for ethylene measurements was α_min = 9.1 × 10^−8 cm^−1 for this system.