The radiationless electronic relaxation and α -CC bond fission dynamics of jet-cooled acetone in the S (nπ) state and in high-lying 3p and 3d Rydberg states have been investigated by femtosecond time-resolved mass spectrometry and photoelectron imaging. The S state was accessed by absorption of a UV pump photon at selected wavelengths between λ = 320 and 250 nm. The observed acetone mass signals and the S photoelectron band decayed on sub-picosecond time scales, consistent with a recently proposed ultrafast structural relaxation of the molecules in the S state away from the Franck-Condon probe window. No direct signatures could be observed by the experiments for CC dissociation on the S potential energy hypersurface in up to 1 ns. The observed acetyl mass signals at all pump wavelengths turned out to be associated with absorption by the molecules of one or more additional pump and/or probe photons. In particular, absorption of a second UV pump photon by the S (nπ) state was found to populate a series of high-lying states belonging to the n = 3 Rydberg manifold. The respective transitions are favored by much larger cross sections compared to the S ←  S transition. The characteristic energies revealed by the photoelectron images allowed for assignments to the 3p and 3d states. At two-photon excitation energies higher than 8.1 eV, an ultrafast reaction pathway for breaking the α -CC bond in 50-90 fs via the 3d Rydberg state and the elusive ππ state was observed, explaining the formation of acetyl radicals after femtosecond laser excitation of acetone at these wavelengths.

The dynamics of pentafluorobenzene after femtosecond laser excitation to the optically bright ππ(*) first excited electronic state have been investigated by femtosecond time-resolved time-of-flight mass spectrometry and femtosecond time-resolved photoelectron imaging spectroscopy. The observed temporal profiles exhibit a bi-exponential decay behavior with a superimposed, long-lived, large-amplitude oscillation with a frequency of νosc = 78-74 cm(-1) and a damping time of τD = 5-2 ps. On the basis of electronic structure and quantum dynamics calculations, the oscillations have been shown to arise due to vibronic coupling between the optically bright ππ(*) state and the energetically close-lying optically dark πσ(*) state. The coupling leads to a pronounced double-well character of the lowest excited adiabatic potential energy surface along several out-of-plane modes of b1 symmetry. The optical electronic excitation initiates periodic wavepacket motion along these modes. In the out-of-plane distorted molecular configuration, the excited state acquires substantial πσ(*) character, thus modulating the ionization probability. The photoelectron spectra and the anisotropy of their angular distribution confirm the periodically changing electronic character. The ionizing probe laser pulse directly maps the coupled electron-nuclear motion into the observed signal oscillations.

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Femtosecond photoelectron imaging spectroscopy is a powerful technique for following state-resolved molecular transformations in complex coupled potential energy landscapes. To avoid unwanted nonlinear side-effects, the employed laser pulse energies are usually reduced to minimal values. However, the energy calibration of the photoelectron imaging detector is ideally performed using multi-photon above-threshold ionization of suitable atomic species, for which rather high laser intensities are required. In this work, we show that the calibration spectra of xenon obtained with high laser pulse energies cannot be directly used for the evaluation of molecular photoelectron spectra recorded using low-energy laser pulses. The reason is the intensity-dependent AC Stark shift of the atomic ionization energies to larger values, which in turn leads to a corresponding decrease of the photoelectron kinetic energies. We present a simple procedure to quantify this so-called ponderomotive shift and calculate the theoretically expected un-shifted photoelectron energies.

The ultrafast dynamics of pentafluoropyridine in the 1 B (ππ*) electronic state excited at λ = 255 nm is investigated by femtosecond time-resolved time-of-flight mass spectrometry and photoelectron imaging spectroscopy. A pronounced, long-lived, and complex periodic modulation of the transient ion yield signal with contributions by four distinct frequency components, 72 cm, 144 cm, 251 cm, and 281 cm, is observed for up to 9 ps. The recorded photoelectron images display a spectral band from the excited 1 B (ππ*) state only in the oscillation maxima; the signal is strongly reduced in the oscillation minima. Supported by electronic structure calculations at the RI-SCS-CC2 and XMCQDPT2 levels of theory, the oscillating components of the signal are identified as frequencies of b symmetry coupling modes in a vibronic coherence of the 1 B (ππ*) and 1 A (πσ*) electronic states. The optical excitation initiates regular and periodic wavepacket motion along those out-of-plane modes. In the distorted molecular structure, the initially excited state acquires substantial πσ* character that modulates the transition dipole moment for ionization and results in the observed oscillations.