Laser Excitation of Stationary States in Molecules: The Coupling of Real Eigenstates to Phonons and Photons

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Abstract
When a molecule is excited by a light source one considers the following optical cycle; absorption to a singlet state, internal conversion, intersystem crossing, and finally a phosphorescence from the triplet state (Jablonski process) to the ground state. On the other hand, one knows from quantum mechanics that a singlet state isoenergetic with other singlets or triplets interacts with these levels thus producing new states which diagonalize the exact molecular Hamiltonian. The question of course; which state do we excite and under what conditions does the intermolecular interaction (say with a phonon bath) influence the pathways for the excitation process? In this communication we will present for the first time experimental evidence for the laser excitation of true eigenstates in molecules. We will also discuss the theoretical treatment and the experimental findings on thermal broadening of electronic resonances which result in the loss of the “identity” of molecular states. Finally, it will be shown that vibrational relaxation and magnetic field induced couplings depend crucially on the preparation of the state.