Knowledge Bank

The Superfluid He droplet is an ideal matrix for making large sized molecular clusters, because of its extremely low temperature and very weak interaction with molecules. In this study, we observed vibrational spectra for the CH stretching band of the HCN-$(H2/D2)n$ cluster in the He droplets under a strong electric field (30 kV/cm). For the $HCN-(H2)n$ system, HCN-$(o-H2)n$ (n = 1-5), $HCN-(p-H2)n$ (n = 1-3), and $HCN-(o-H2)1$ $(p-H2)1$ have been observed. Since $o-H2$ (j = 1) has a quadrupole moment, it is supposed that the interaction between $o-H2$ and HCN is stronger than that between $p-H2$ (j = 0) and HCN. Nevertheless, the observed red-shift of the CH-stretching band for $HCN-(o-H2)n$ is much smaller than that of $HCN-(p-H2)n$. This phenomenon strongly suggests that the geometrical structure for $HCN-(o-H2)n$, and that for $-(p-HCN)n$, are distinctly different. That is, in $HCN-(o-H2)n$, the $H2$ molecules are bonding to the N site of HCN. Contrastedly in $HCN-(p-H2)n$, the $H2$ molecules are bonding to the H site of HCN, and affect strongly to the CH-stretching vibration. For the $HCN-(D2)n$ system, we obtained the same conclusion. In addition to the vibrational spectra under the strong electric field, we mention vibrational-rotational spectra for $HCN-(H2/D2)1$ clusters under the field-free condition.