Threshold ion-pair production spectroscopy of HCN

The spectroscopic technique of threshold ion-pair production spectroscopy (TIPPS) has been applied to the triatomic molecule HCN. We have recorded the total ion-pair yield and TIPP spectra for the HCN→H++CN− process using coherent vacuum ultraviolet excitation. From the simulation of our high-resolution TIPP spectrum we have precisely measured the HCN ion-pair threshold EIP0 to be 122244±4cm−1. This value could be used to determine the bond dissociation energy D0(H–CN) to unprecedented accuracy. Our fitting result also showed that rotationally excited instead of cold CN− fragment is favored as the ion-pair dissociation product in the threshold region.