We report here the first measurement of the electric dipole moment of theAtilde;thinsp;1Alsquo;state of HCN. Fluorescence excitation spectra of theAtilde;larr;Xtilde; 210K10band were recorded using narrow band (0.1 cmminus;1) vacuum ultraviolet (VUV) produced by fourhyphen;wave sum mixing in an improved strontium heat pipe. Accurate (plusmn;0.007 cmminus;1) term values for this band are reported. Surprisingly, we found that the asymmetry doubling of the upper state does not extrapolate to zero atJ=0. We suggest that the most plausible explanation for this anomalous asymmetry doubling is a spinndash;orbit perturbation by a nearly degenerate level of the 1thinsp;3Arsquo;state. Theacomponent of the dipole moment has been determined by the Stark splitting of theQ(1) transition as a function of electric field in a novel Stark cell capable of sustaining high electric fields (41 kV/cm) at pressures of 50 mTorr. The observed value of the dipole moment mgr;a=0.99(10) D is in agreement with simple molecular orbital expectations and a semiempirical study.
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