Quartz-enhanced photoacoustic spectroscopy exploiting low-frequency tuning forks as a tool to measure the vibrational relaxation rate in gas species

We demonstrated that quartz-enhanced photoacoustic spectroscopy (QEPAS) is an efficient tool to measure the vibrational relaxation rate of gas species, employing quartz tuning forks (QTFs) as sound detectors. Based on the dependence of the QTF resonance frequency on the resonator geometry, a wide range of acoustic frequencies with narrow detection bandwidth was probed. By measuring the QEPAS signal of the target analyte as well as the resonance properties of different QTFs as a function of the gas pressure, the relaxation time can be retrieved. This approach has been tested in the near infrared range by measuring the CH(4) (nν(4)) vibrational relaxation rate in a mixture of 1% CH(4), 0.15 % H(2)O in N(2), and the H(2)O (ν(1)) relaxation rate in a mixture of 0.5 % H(2)O in N(2). Relaxation times of 3.2 ms Torr and 0.25 ms Torr were estimated for CH(4) and H(2)O, respectively, in excellent agreement with values reported in literature.