Resonant two photon ionization of phenanthrene via its transientS 2 state

Abstract

Abstract. We report the vibrationally resolved photoelectron spectrum of phenanthrene obtained by two photon ionization via the $2 electronic state. The experiments were performed with picosecond laser pulses with a bandwidth sufficiently large to span a significant fraction of the intermediate resonance state. Therefore the photoelectron spectrum is dominated by signal corresponding to the unrelaxed intermediate resonance, in spite of this state's 420 fs lifetime. PACS: 33.60.Cv; 33.80.Rv Photoelectron spectroscopy in conjunction with resonance enhanced multiphoton ionization (REMPI) is uniquely suited to solve difficult problems in the study of molecular properties. A Franck-Condon analysis of photoelectron spectra obtained by ionizing a molecule from its ground state via a vibronic intermediate state selected by the laser frequency yields a relation between the potential energy surface of the intermediate electronic state and the final state of the ion: if one of them is known from separate studies, such as fluorescence emission, then the photoelectron spectrum characterizes the other electronic surface [1][2][3][4][5][6]. A particularly interesting twist of resonant two-photon ionization photoelectron spectroscopy is the possibility to introduce a time delay between the two photons [7][8][9]. In this mode the analysis of the time dependent Franck-Condon factors reveals the time evolution of the wavepacket in the intermediate resonance state.Many medium sized and large molecules feature absorption spectra to high-lying electronic states with a significant homogeneous broadening. In many examples the absorption widths point to subpicosecond lifetimes of the corresponding zero order states. Such extraordinarily short lifetimes may preclude the application of the recently developed photoelectron techniques if laser pulses of nanosecond durations are used: if there is a large mismatch between the molecular lifetime and the laser pulse duration, or equivalently between the molecular absorption width and the laser bandwidth, then the laser pulse does not coherently span all molecular eigenstates that make up the zero order singlet state. As demonstrated by the photoelectron spectrum of azulene via the S 3 intermediate state [ 10], this leads to a photoelectron spectrum showing exclusively the hot ions obtained from the vibrationally hot intermediate resonance. The implication of this reasoning is that sharp photoelectron spectra corresponding to ionization out of the vibrationally cold intermediate resonance could be obtained for situations where the laser pulse duration comes closer to the molecular lifetime. We illustrate this idea by choosing a differnt molecule, phenanthrene, of which photoelectron spectra have been obtained via a molecular resonance with a 420 fs lifetime. This lifetime is much closer than the one of azulene (~30 fs) to the pulse durations obtainable with our picosecond laser system.A wide variety of apparatus are currently being employed to perform REMPI-photoelectron spectroscopy. The lase...

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Spectroscopy and femtosecond dynamics of the ring opening reaction of 1,3-cyclohexadiene

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Fundamental Limits on Spatial Resolution in Ultrafast X-ray Diffraction

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