ABSTRACT9 I' A method is described for the measurement of peak intensity ratios of gas phase microwave absorption lines.Large errors associated with multiple reflections in the waveguide have been eliminated by careful design and the use of ferrite isolators.Detector nonlinearities have been neutralized by using a precisely ___ • calibrated step attenuator allowing both lines involved in a ratio to be displayed with approximately the same amplitude.Means of minimizing add tional sources of error are discussed. CnApplication of the method to a number of problems indicates that ratios accurate to a few percent can be obtained.

Accurate microwave relative intensity measurements have been made on rotational transitions of ketene and its deutero derivatives in the ground and first vibrational states. The results allow the three lowest fundamental frequencies of these molecules to be positively determined. For CH2CO these modes are found to be 443, 527, and 617 cm−1. The lowest frequency is nearly 100 cm−1 lower than any fundamental previously cited from infrared data for ketene. To substantiate the microwave results solid-state infrared spectra of the molecules at −192° have been investigated in the region 300–4000 cm−1. The low-frequency spectra are in good agreement with the microwave data, and a complete analysis allows the nine fundamentals of CH2CO, CHDCO, and CD2CO to be evaluated. The three lowest fundamentals of CH2CO in the solid are found at 433, 513, and 613 cm−1. The thermodynamic properties of ketene have been recalculated.