Tunneling motion in (HCl)₂ hydrogen-bonded dimer probed by electrostatic hexapole and doppler-selected TOF measurement for the internal energy distribution of [ClHCl]

The tunneling motion in (HCl)₂ hydrogen bonded dimer and its deuterate was probed by a 2 m long electrostatic hexapole field. The focusing curves of the dimers confirmed the existence of homo and heterodimers in the cluster beam. The homodimer, either H³⁵Cl-H³⁵Cl or H³⁷Cl-H³⁷Cl, undergoes a fast tunneling motion for the two hydrogen atoms in the dimer. The heterodimer, namely H³⁵Cl-H³⁷Cl, on the other hand, does not show such fast tunneling motion in the time scale of the experiment. The electric dipole moments for both (DCl)₂ isotopomers were determined to be 1.5 ± 0.2 D, which is the same value for (HCl)₂. The observed ratio of homo to heterodimer was estimated to be 30 ± 10, and this value differs largely from the natural abundance for the chlorine isotope. An experimental scheme to discern homo and heterodimers is proposed here. By looking at fragments in the (HCl)₂ dimer photodissociation using a Doppler-selected time-of-flight (TOF) technique, internal energy distribution of the [ClHCl] fragment was measured in 121.6 nm photodissociation. The TOF spectrum consists of fast and slow velocity components for the dissociated H atoms. It is found that the slow H component that arises from the hydrogen escapes after many collisions. The fast H component that arises from the direct H escape without any collision, thus this component reflects an internal and/or electronic state of the counter part fragment, i.e. [ClHCl]. The vibrational structure of [ClHCl] was observed for the fast H component of the TOF spectrum. (C) 2000 Elsevier Science B.V.