Quantum chemistry based inversion of experimental pump–probe spectra: Model simulations for CpMn(CO) 3

Abstract

Experimental pump–probe spectra may be used to refine a quantum chemistry based model Hamiltonian for quantum simulations of photodissociation and -ionization. The design of the model Hamiltonion involves several steps which are demonstrated, exemplarily for the organo-metallic model system, cyclo-pentadienyl-manganese-tricarbonyl CpMn(CO) 3 (cymanthrene). First, we consider the specific experimental scenario e.g. single photon transitions, zero electron kinetic energy (ZEKE) of the resulting free electron, and investigations of just the short time evolution (few 100 fs). This suggests to describe the molecular dynamics in reduced dimensionality of just the most important degree(s) of freedom, e.g. the bond distance between the metal atom Mn and the photodissociated ligand CO, assuming C s symmetry. Next, the relevant adiabatic potential energy surfaces (PES) are evaluated by means of ab initio quantum chemistry techniques, together with the transition dipole and kinetic couplings of the neutral or ionic states. Non-Condon transition dipole couplings between excited neutral and ionic states are approximated in terms of the coefficients for configuration interaction, depending on electron correlation. Exemplarily, we consider the effects of five plus three coupled adiabatic PES of the neutral and ionic systems, respectively. The resulting laser driven reaction dynamics is then simulated in terms of representative time-dependent wave packets moving on the coupled PES, together with corresponding pump–probe spectrum of the parent ion, CpMn(CO) 3 + . Few parameters of the quantum chemistry based model are then adjusted, within the accuracy of the quantum method, in order to achieve near quantitative agreement of the experimental and theoretical pump–probe spectra. In the present application, the relevant excitation energies, kinetic couplings, and the relative displacement of two potential wells in the electronic ground and excited states, are scaled and adjusted. The refined quantum chemistry based model Hamiltonian is then used to calculate the pump–probe spectrum of the daughter ion, CpMn(CO) 2 + . Good agreement with the experimental spectrum supports the validity of the model, allowing extended applications, e.g. wave packet dynamics simulations of optimal laser pulse control, and prediction of the sensitivity of the pump–probe spectra to electron correlation in the excited state.