Coherent Excitation and Relaxation of the Coupled S₁/S₂ Electronic States of Naphthalene

Abstract

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 ...

Ultrafast dynamics of aniline in the 294-234 nm excitation range: The role of the πσ* state

“…This kind of behavior has been reported for naphthalene, where part of the population is permanently transferred to the dark S 1 during the adiabatic evolution of the bright S 2 state, due to the loss coherence induced by the ultrafast coupling between both states. 42 A similar situation can take place in the aniline molecule, considering that the S 3 (see below) decays in 21 fs to the S 1 . We cannot discard either that in this process part of the population is transferred to the S 2 πσ * .…”

“…IV). 41,42 Similar measurements have been carried out probing with 400 nm laser pulses. The results collected in Fig.…”

“…As it has been demonstrated recently for the naphthalene molecule, higher electronic excited states can be adiabatically excited even when the detuning is several times the laser bandwidth. 42 In the case of aniline, several electronic states, mainly of ππ * character have enough oscillator strength to contribute to the CPR signal. As the contribution of each state can be estimated with the expression: S CPR = f / 2 , where f is the oscillator strength and is the detuning.…”

“…Other phenomena, as the adiabatic excitation by the ultrashort pulses of higher electronic excited states are also observed, and their influence discussed. [39][40][41][42] …”

Revisiting the relaxation dynamics of isolated pyrrole

“…A clear difference is observable between the data , were originally attributed to dynamics on the 1 A 2 πσ * surface reached by direct excitation, or by internal conversion from the upper 1 B 2 ππ * state, being the latter coherently out-of-resonance excited during the pump pulse. 37,38 In order to model this behavior a coherent treatment was applied, which could nicely fit all the transient signal collected. 22 However, on the light of some findings in other molecules as phenol [Unpublished results], the validity of this interpretation was questioned, and the influence over the signals of the multi-photon probe used in the ionization step carefully examined.…”