The first observation of vibration-rotation transitions in HC${\mathrm{O}}^{+}$ is reported. The ${\nu }_1$ absorption band was measured with a color-center laser by modulating the drift velocity of the ion in an ac discharge and detecting the Doppler-shifted absorptions with lock-in techniques. The ${\nu }_1$ frequency is determined as 3088.727±0.003 ${\mathrm{cm}}^{-1\mathrm{}}$.

• Received 19 January 1983

DOI:https://doi.org/10.1103/PhysRevLett.50.727