Vibrational Relaxation in Carbon Monoxide by the Spectrophone Frequency-Response Method

Although the spectrophone has been employed earlier to measure vibrational relaxation times by a phase-shift method, the use of a frequency-response (amplitude) method does not seem to have been previously reported. The limitations of microphone response to audio frequencies precludes the investigation of most gases by this procedure, but the expected long relaxation time in carbon monoxide suggested that a response loss might be observable for this molecule. After some difficulties due to acoustic resonances in the spectrophone cell were overcome, a relaxation loss (relative to CO2) was found at room temperature for carbon monoxide, which corresponds to a relaxation time of approximately 2 msec at 1 atm. The result is compared with values found at much higher temperature by shock-tube experiments. While the result for τ appears to be somewhat low in comparison with the shock-tube results, which may be due to traces of H2O or of H2 in the sample, the method appears to be useful in extending the range of measurements beyond that previously available by other techniques.