Initial state preparation internal conversion

Figure 1. Quantum-mechanical (thick lines) and mean-field-trajectory (thin lines) calculations obtained for Model 1 describing the S2 — Si internal-conversion process in pyrazine. Shown are the time-dependent population probabilities Pf t) and Pf (t) of the initially prepared adiabatic and diabatic electronic state, respectively, as well as the mean momenta pi (t) and P2 t) of the two totally symmetric modes Vi and V( of the model.

Figure 46. Diabatic population (a) and modulus of the autocorrelation function (b) of the initially prepared state for a four-mode model describing the S2 Si internal-conversion process in pyrazine. The full line is the quantum result, the dashed-dotted line is the result of the semiclassical spin-coherent state propagator, and the dashed line depicts the result of Suzuki s propagator. The semiclassical data have been normalized. Panel (c) shows the norm of the semiclassical wave functions.

The aforementioned E — D nonadiabatic interactions also lead to efficient, i.e., femtosecond internal conversion processes which we now briefly address. The presence of such processes is indicated indirectly by experimental studies, where Bz+ has been prepared initially in the E state, but fragmentation been found to occur via lower (even the X) electronic states [45,46]. To describe this non-radiative decay... 

Figure 26 Potential energy diagram for D2CO. An initially prepared Si rovibronic state can undergo both radiative (rad) and non-radiative (nr) decay. Non-radiative decay consists of internal conversion to highly excited So levels followed by unimolecular (uni) dissociation into products D2 and CO. Redrawn from Ref. 10.

Unimolecular reactants are energized by a variety of experimental techniques including collisional and chemical activation, internal conversion and intersystem crossing transitions between electronic states, and different photo-activation techniques, which include excitation of isolated resonance states for reactants with a low density of states (see also Sect. 3). Trajectory simulations usually begin with the preparation of an ensemble of trajectories, whose initial coordinates and momenta resemble — as close as possible — those realized in a particular experiment [20,329]. 

The So Si internal conversion step excites So nonrandomly. A microcanonical ensemble of states is not prepared, although So may relax to this ensemble after efficient and complete IVR. Thus, to accurately simulate the intramolecular and unimolecular dynamics of the excited So molecule, it is necessary to choose correct initial conditions for So- The specific vibrational excitations on So have probabilities proportional to Ajj, where i is the initial vibrational level on Si and j is the vibrational level on So. " The term includes a Franck-Condon factor so that only certain types of So mode excitations have high probabilities and therefore the excitation of So may be highly... 

---