Some transitions of the 2ngr;2larr;ngr;2hot band of H2CO near 5.8 mgr;m have been observed by the laser Stark technique using a 20 cm long intracavity absorption cell with about 4 mtorr total pressure at room temperature. The detection of this weak hot band (partial pressure in thev2=1 state quest;8times;10minus;7torr) illustrates the high sensitivity of intracavity laser absorption spectroscipy. From the measured resonant fields of the transitions, the dipole moment of H2CO in thev2=2 state has been determined. The use of phase sensitive detection at twice the Stark modulation frequency is illustrated; this 2fdetection effectively measures the strength of the transient nutation signal in the absorbing gas, and in some cases results in a better signal to noise ratio than does conventional 1fdetection.
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