Bimolecular reaction dynamics and spectroscopy of weakly bound complexes

A large number of experimental studies revealing the dynamics of unimolecular reactions at femtosecond time resolution have been carried out in the last thirty to fourty years and are supported by a substantial amount of theoretical work. However, the study of time-resolved bimolecular reactions is limited to a few studies from the late 1980’s and early 1990’s and, in this context, no theoretical study using a full quantum mechanical approach exists to date. Here, bimolecular reactions are initiated by preforming a weakly bound complex (A – B)· · · C of two molecules A – B and C and then dissociating the A – B bond with a ultrashort laser pulse, so as to have the B fragment colliding against the C moiety. The reaction starting time and the relative orientations of the reactants are thus well-defined. The work presented in this thesis aims at reviving the interest in this type of studies. In particular, we explore the dynamics of the exchange reactions

H + H'OD −→ (HOD + H' HOH' + D

initiated by vertical photoexcitation and subsequent dissociation of HCl in the (HCl)· · ·(HOD) hydrogen-bound complex. This reaction is studied using quantum mechanical wavepacket propagations with the MCTDH method on state-of-the art potential energy surfaces. When initiated in these conditions, the collision of H with HOD does not give rise to new products most of the time, i.e. it mostly leads to the scattering of the hydrogen atom against the HOD molecule. The cause lies in the fact that the photodissociation of HCl yields a large collision energy (about 3 eV) between H and HOD. This makes the collision too fast to allow a redistribution of the relative kinetic energy of the reactants over the bonds of HOD. However, to a minor extent, some exchange products are obtained, whereas abstraction products (i.e. HD + OH or H₂ + OD) are not. Overall, the typical reaction time in these exchange processes is of the order of 10 fs. Moreover, the natural selectivity of the reaction, i.e. the preference in producing HOD + H' over HOH' + D, can be tuned by vibrational pre-excitation of the HOD molecule within the complex. The presence of the chlorine atom is not expected to be more than a negligible perturbation to the reaction and thus acts as a spectator atom.

On the other hand, this Ph.D. project features an experimental component aiming at characterizing weakly bound complexes via Fourier Transform Infrared spectroscopy. To this end, we describe a new design which has been constructed in our laboratory for the realization of matrix isolation spectroscopy of these complexes. In particular, this setup has been used to deposit HCl, H₂O and deuterated isotopologues in solid neon matrices and measure their mid-infrared spectrum. This lead to the observation of spectral bands associated to the (HCl)· · ·(H₂O) complex and its analogous isotopologues. In view of carrying out matrix isolation spectroscopy experiments in the challenging far-infrared spectral region, the development of a new para-hydrogen enrichment setup is also presented. The designed instrumentation and software allow for routine production of this matrix host material with purity larger than 99.95 %. In the future, this will be applied for a number of studies, including that of the (HCl)· · ·(H₂O) complex.
para-hydrogen enrichment setup is also presented. The designed instrumentation and
software allow for routine production of this matrix host material with purity larger
than 99.95 %. In the future, this will be applied for a number of studies, including
that of the (HCl)· · ·(H2O) complex.