MILLIMETER-WAVE SPECTROSCOPY OF THE $HCN-H_{2}$ CLUSTER

Millimeter-wave absorption spectroscopy combined with a pulsed-jet expansion technique was applied to the measurement of the rotational and rovibrational transitions of the $HCN-H_{2}$ cluster in the frequency region of 75 - 150 GHz. So far, four rotational lines for the ground $\\Sigma_{0}$ state of the $HCN-(o-H_{2})$ cluster split into hyperfine structure due to the nitrogen nucleus were observed. Rotational constant $B_{0}=12899.718(20)$ MHz and centrifugal distortion constant $D_{0} = 12.2470(16)$ MHz were derived together with its higher constants. The hyperfine constants determined $eqQ =-2.830(33) MHz$ which is smaller than that of HCN molecule means a large amplitude motion of HCN of $<\\theta>= 31.1$ degree in the ground linear form. The bond length between HCN and $H_{2}$ parts is derived to be $3.90 {\\AA}$. Some lines belonging to the $\\Sigma_{1}-\\Sigma_{0}$ van der Waals bending band of $HCN-(o-H_{2})$ were also observed. The $\\Sigma_{1}-\\Sigma_{0}$ van der Waals bending frequency 136.831 GHz of $HCN-(o-H_{2})$ is larger than that of He-HCN 98.70 GHz, but much smaller than that of Ar-HCN 164.89 GHz. The hyperfine constant in the $\\Sigma_{0}$ state indicate the cluster has T-shape in the excited state of the vdW bending mode. A search of the rotational lines of the ground $\\Sigma_{0}$ state of $HCN -(p-H_{2})$ and the $\\Pi_{0}$ state of $HCN-(o-H_{2})$ are now in progress as well as the vdW mode rovibrational lines for both $HCN-(o-H_{2})$ and $-(p-H_{2})$ clusters.