L36. Dissociation Dynamics of Hydrochloride Dimer Radical Cation
Q. Zhong, L. Poth, J. V. Ford, A. W. Castleman, Jr.
The Pennsylvania State University
University Park, Pennsylvania 16802
The four dissociation reactions of hydrogen chloride dimer radical cation, (HCl)2+ ® H2Cl+ + Cl, (HCl)2+ ® HCl+ + HCl, (HCl)2+ ® Cl2H+ + H, and (HCl)2+ ® Cl2+ + H2 are studied by a molecular beam–multiphoton ionization technique in the femtosecond time domain, and in an ab initio framework. In the reflectron time–of–flight mass spectra, besides the protonated and unprotonated dimer ions, dehydrogenated ions, HCl2+ and Cl2+, are also observed under high laser fluence. Intermolecular potential energy levels for the dimer ion is computed at the level of second order Møller–Plesset approximations (MP2), using a 6–31G(d,p) basis set. Both the calculations and the experimental results indicate that: (1) the equilibrium dimer ion formed by femtosecond ionization has two stable structures, (HClH)+ Cl, with a disproportionated structure, and (HCl)+ HCl, with a hydrogen–peroxide–like structure. The (HCl)+ HCl ion is found out to be 0.33 eV more stable than the (HClH)+ Cl ion. The conversion from (HClH)+ Cl to (HCl)+ HCl involves a barrier of 0.4 eV. (2) the dissociation pathways of the dimer ion depend on the internal energies, with the HClH+ + Cl channel dominating at low energies. At higher internal excitations, dehydrogenated fragments appear. The dissociation from (HCl)+ HCl into Cl2H+ involves a barrier of 3.08 eV; whereas the dissociation into Cl2+, with a concerted H2 loss, involves a barrier of 3.2 eV.