Although coherence is a property inherent to the quantum evolution of molecular electronic states, the high dephasing rates commonly found in these systems has traditionally prevented the manifestation of coherent electronic effects. The development of femtosecond sources has changed this situation, by providing the necessary access to the early stages of electronic evolution that allows the observation of coherent evolution. As remarkable examples, recent research has demonstrated the influence of electronic coherence on controlling the energy flow in two relevant aspects of the natural photosynthesis process: the coherent evolution of the electronic excitation in a multicromophore [1] light-harvesting protein and the intracromophore coherence between electronic excited states of carotenoid pigments.[2] Both cases show how electronic coherence drives the initial behaviour of the system in situations where the memory loss associated to dephasing was thought to dominate the evolution of the system.Herein we report on new aspects of the coherent evolution of the electronic excitation and relaxation of a prototype cromophoric system, naphthalene. The electronic spectroscopy of this aromatic molecule has been extensively studied, yielding a detailed picture of its electronic structure. [3][4][5][6][7] The absorption of the molecule on the low-energy portion of the spectrum is dominated by two pp* excitations that give raise to the L b (S 1 ) and L a (S 2 ) states. The weak S 0 -L b transition is polarized along the long axis of the molecule, while the S 0 -L a with much higher oscillator strength is mainly polarized along the short axis.[3] Despite the relatively large energy gap between both states (~4000 cm À1 ), they are strongly coupled, as demonstrated by the ultrafast internal conversion (t = 30 fs) that populates the lower L b after pumping the S 0 -L a transition. [6] The L a /L b manifold of naphthalene appears to be a good model system to study the intramolecular coherent evolution of ultrafast electronic excitation. It mimics the conditions often found for this phenomenon, where the process is triggered by the absorption of a bright state that is strongly non-adiabatically coupled to a dark one.[8] Herein, the evolution of isolated naphthalene molecules seeded in a supersonic expansion was tracked at femtosecond resolution by delayed ionization, after excitation at different wavelengths in the range of the L a and L b absorptions. Figure 1 shows the transients corresponding to the early evolution of the molecule, collected by excitation at energies resonant with the absorption of the L b state (except Figure 1 d), and delayed ionization with three 800 nm photons. The decays are composed of a prominent peak centred at Dt = 0 that reproduces the ethene nonresonant signal, followed by a constant background extending towards longer delay times. No other dynamic features are found in the signals. Although the Gaussian-like peak at Dt = 0 could be interpreted as a nonresonant contribution, the dependence of its ...
