Making and Breaking of Chemical Bonds:

Dynamics of Elementary Reactions From Gas Phase to Condensed Phase

Abstract

The present thesis is concerned with the dynamics of elementary chemical reactions. In particular, the processes of bond formation (association) and of bond cleavage (dissociation) are studied. Both photo-induced and solvent-induced reaction mechanisms are elucidated. By embedding simple diatomic model systems in rare gas clusters and matrices, the transition of the dynamics of making and breaking of chemical bonds from the gas phase to the condensed phase is systematically investigated.

Methodology

The methodologies employed here depend on the size of the systems as well as on the quantities of interest. They range from purely classical trajectory simulations or quantum-classical hybrid techniques for larger systems up to quantum mechanical (exact and perturbative) wavepacket propagations for the smallest (diatomic) systems. In the latter case also the interaction with external, time-dependent electromagnetic fields of pulsed lasers is accounted for. Fully three-dimensional quantal treatments of continuum wavefunctions and wavepackets have been adapted from scattering theory and further developed. In particular, in the present work rotational effects on photoassociation processes such as shape resonances and rotational coherence have been considered for the first time. For photodissociation dynamics in clusters and matrices, a highly efficient technique for symmetry adaption of wavepackets has been developed.

Photo-induced stabilization of collision pairs

In this chapter of the present thesis photoassociative collisions of two colliding atoms are treated. By stimulated emission of light, a collision pair can be stabilized to form a ground state molecule. The work in [17] and in [20] represents the first studies of ground state photoassociation by infrared ps light pulses. It is shown that by simultaneous optimization of the incoming wavepacket and of the laser pulse, photoassociation can be achieved with a high efficiency of >80% for a rotation-less model of an O + H collision pair. At the same time, an extremely high vibrational state selectivity close to 100% can be achieved if there is no coincidence with higher-order transitions to lower vibrational states. An extension of these models to include rotational degrees of freedom is presented in [21]. Taking advantage of the high scattering cross section of shape resonance states we have shown that the efficiency of photoassociation reactions can be considerably enhanced. At the same time, preparation of molecules in specific ro-vibrational states is possible because the condition for a shape resonance is only met for one specific partial wave. In principle, the process of ground state photoassociation by stimulated emission competes with photoacceleration induced by absorption of photons. In these inelastic free-to-free transitions, the collision pair is accelerated in the field of the laser pulse. In analogy to the phenomena of above-threshold-ionisation and above-threshold-dissociation, the increase of kinetic energy corresponds to the energy of one or more photons and leads to sharp peaks in the energy distribution of the scattered particles. In [20] and [21] this process is investigated and the ranges of suitable laser parameters are explored. Also the question of electronic state-selectivity has been addressed in the present work. This is especially interesting for molecules where low-lying bound electronic states exist and where ground and excited state photoassociation are eligible in a similar frequency regime. It is demonstrated in [26] for the example of the OH radical that even in these cases various control mechanisms exist. Apart from the different polarization of permanent and transition dipole moments, effective control can be exerted through variation of the scattering energy of a collision pair. When going to even shorter pulses in the fs regime, coherent superpositions of ro-vibrational states can be excited. In modern pump-probe experiments the time evolution of wavepackets be oberserved in real time. This has been illustrated in [19] for the example of the exciplex formation of the mercury dimer. On the borderline of classical and quantum mechanics, (fractional) revivals in the wavepacket dynamics are predicted. The fast and the slow timescales of the pump-probe experiments of Marvet and Dantus are interpreted in terms of quantum beats of different vibrational and rotational states, respectively, which have been coherently excited in the photoassociation process. Note that these spectral features persist even for relatively high temperatures. Moreover, by excluding the possibility that the observed signals originate from van der Waals precursors, our results provide evidence for the bimolecular nature of this experiment thus rendering it one of the first of its kind.

Solvent-induced stabilization of collision pairs

In this chapter the (radiation-less) stabilization of collision pairs in the presence of a solvent is studied. In particular, we investigated solvent effects on hydrogen halide association reactions by attaching a rare gas ``microsolvation'' to the halide reagent, see [9]. The following two cases serve as models: For the H + Cl...Ar system the existence and the extent of the third-body or ``chaperon'' effect is explored. Already a single solvent atom can stabilize a collision pair by removing some energy from it such as to keep it from re-dissociating. Due to kinematic constraints for the energy transfer from the H-Cl mode to the Cl-Ar mode, we find ro-vibrationally hot products while only a very small fraction of the available energy is found in product translation. Among the most interesting predictions is the discovery of very long-lived orbiting resonances with lifetimes in the order of picoseconds. To explore the transition from gas to condensed phase dynamics, the cage effect on association reactions was studied in [8] for the H + Cl...Ar12 system where a first complete solvation shell shields the Cl reactant. We found a pronounced structural transition between T=40 K and T=45 K in MD simulations of ClAr12. The reactivity to form HCl molecules in collisions with a H atom can be extremely sensitive to the melting-like transition. Our novel approach can serve to elucidate more on the much debated question of ``phase transitions'' in finite systems. In this way, reactive collisions could provide an alternative to the spectroscopy of chromophores embedded in solvent clusters. Finally, the studies of association reactions are extended towards larger systems in order to model nucleation from the gas phase towards the formation of bulk matter. In particular, associative collisions of alkali halide molecules and clusters are studied in [13] as a model for cluster growth from the gas phase. The collision complexes investigated here are large enough to redistribute the excess energy among their abundant internal degrees of freedom for a relatively long time before they finally are stabilized by evaporation. The cluster sizes considered here (up to the tetramer) are on the border of the validity of statistical RRKM models. While general trends can be explained in terms of these models, a detailed microscopic study is necessary to understand the quantitative effects.

Photodissociation of molecules in clusters and matrices

The process of light-induced bond breaking is investigated in the following. Here our special emphasis is on the cage effect delaying or hindering the photofragments from separation and on possible cage exit mechanisms. In analogy to our treatment of solvent-induced association reactions, hydrogen halide molecules in the environment of rare gas clusters and matrices are chosen as model systems. As a presupposition for the understanding of the photodissociation dynamics, the vibrational and rotational spectroscopy of these guest-host systems is investigated first. Vibrational frequency shifts of the guest molecule serve to characterize the structure and size of the rare gas host systems. Using vibrationally adiabatic atom-molecule potentials the following two effects are found in [15]: (1) Icosahedral and octahedral symmetry give rise to distinctly different frequency shifts and (2) between three and five complete solvation shells are required in order to reach convergence of the shift towards the value for bulk matter. The rotational spectroscopy is characterized by nearly free rotations of the hydrogen halide molecules with respect to the rare gas cluster [27] or matrix [18] because the anisotropy of the atom-molecule interaction cancels to a large extent for systems with closed solvation shells. Apart from small but characteristic splittings of degenerate energy levels, the main influence of the solvation is that it imposes its symmetry on the rotational wavefunctions. Based on this knowlegde of the initial state, the excited state dynamics is investigated. The hydrogen atom wavepacket motion shows distinctly quantal features such as bifurcations and interferences. The initial rotational wavefunctions have important implications on this dynamics i. e. the cage exit probability sensitively depends on its initial rotational state. Furthermore, the quantum yield of a photodissociation reaction also depends on the vibrational state of the guest molecule. As a consequence, we suggest a novel extension from vibrationally to rotationally, or ro-vibrationally mediated chemistry for isolated molecules in clusters [27] or matrices [18] which proceeds via (far-)infrared pre-excitation of the molecules to be photolyzed. A further development of this idea is the recently proposed librational control of the reactivity of photofragments in small hydrogen containing clusters [25]. Finally, first steps towards a full treatment of the photodissociation dynamics of molecules in clusters and matrices have been made. Here quantum-classical or other approximate quantum-mechanical methods have to be applied. For clusters with one complete solvation shell the different energy transfer of H versus D atoms leads to vibrational excitation or complete decomposition of the solvent cage, respectively. Another consequence of the cage dynamics is the quenching of resonance structures in the absorption spectrum.

HabilitationThesis.pdf

Burkard Schmidt, Habilitation thesis
Freie Universität Berlin, July 1998