The electron and nuclear dynamics of C60 fullerenesirradiated with femtosecond laser pulses are investigated withphotoelectron and photoion spectroscopy. The focus of this work isthe detailed exploration of the population mechanism of Rydberglevels within the excitation process of neutral C60. The effectof excitation wavelength, intensity, chirp, and polarization on thekinetic energy distribution of photoelectrons in single-pulseexperiments gives first insight into the underlying processes. Incombination with time-resolved two-color pump-probe spectroscopydepending on either pump, or probe pulse intensity, a more completepicture of the interaction can be drawn. The results point towards avery interesting but nevertheless complex behavior including foursteps: (i) non-adiabatic multielectron excitation of the HOMO(hu) → LUMO+1 (t1g) transition; (ii)thermalization within the hot electron cloud on a time scale below100 fs, followed by a coupling of energy to vibrational modes ofthe molecule via doorway state(s); (iii) population ofelectronically excited Rydberg states by multiphoton absorption, and(iv) single photon ionization from the excited Rydberg states. Thisexcitation process results in a characteristic sequence ofphotoelectron lines in the photoemission spectra. The comparison ofthe experimental results with recent theoretical work givesconvincing evidence that non-adiabatic multielectron dynamics (NMED)plays a key role for the understanding of the response of C60to short-pulse laser radiation.

Double ionization of Ne by 25 fs, 1.0 PW cm−2 laser pulses has been explored in a kinematically complete experiment using a 'reaction microscope'. Electrons are found to be emitted into a narrow cone along the laser polarization (ε), much more confined than for single ionization, with a broad maximum in their energy distribution along ε. Correlated momentum spectra show both electrons being ejected into the same hemisphere, in sharp contrast to predictions based on field-free (e, 2e) recollision dynamics, but in overall agreement with recent semiclassical calculations for He.

Rydberg series of C60 are reported for the first time. The Rydberg states are seen in photoelectron spectra using ultrashort pulsed-laser excitation, where the excited states formed are ionized with one further photon from the same laser pulse. The structure is observed for pulse durations as short as 100 fs with indications of residual structure for even shorter pulse excitation. The production mechanism is discussed and the Rydberg states are modeled by analytically solving the Schrödinger equation with a simple jelliumlike potential for C60.

: The timescale for the coupling of electronic and vibrational excitation in isolated fullerenes is determined by recording positive ion time-of-flight mass spectra on excitation with ultrashort laser pulses at 790 nm of the same fluence but different pulse durations. The coupling leads to the onset of a delayed ionisation “tail” on the parent fullerene ion peak. This occurs for a pulse duration of 500-1000 fs, depending on laser fluence.

The photoionisation of fullerenes has been studied using 800 nm laser pulses of different pulse durations as a function of the laser intensity, Three distinct ionisation regimes are observed: for the shortest pulses direct multiphoton ionisation, for intermediate pulses (50 fs - 500 fs) statistical electron emission from the electronic manifold of states and for longer pulses (> 500 fs) statistical electron emission on the mus time scale after coupling of the energy to vibrational degrees of freedom.

Multiply charged C60q+, q<6, is observed in the multiphoton ionization of C60 with 790 nm, 25 fs pulses. Determination of the laser intensity dependence of the ion intensities indicates that the multiple ionization occurs predominantly sequentially. Coupling of the electronic excitation energy to vibrational degrees of freedom occurs in competition with further ionization.

Gas-phase C60 photoionization and photofragmentation experiments were performed using a sub-50 fs Ti Sapphire laser system and reflectron time-of-flight (RETOF) mass spectrometer. The dependence of the C60+ and C602+ signals on the laser intensity for the fundamental (795 nm) and second harmonic (ca. 400 nm) has been determined. For low laser intensities, before the onset of fragmentation, single ionization is a direct multiphoton process. Double ionization is a sequential process in which C602+ originates from already singly ionized fullerenes. At laser intensities beyond the onset of C602+ there is considerable metastable fragmentation indicating a strong coupling of electronic excitation energy into vibrational degrees of freedom that appears to be in competition with multiple ionization.

The photoelectron spectra of C60 ionized using a 790 nm laser with pulse durations varying from 25 fs to 5 ps have been determined. For 25 fs pulses, in the absence of fragmentation, the ionization mechanism is direct multiphoton ionization with clear observation of above threshold ionization. As the pulse duration is increased, this becomes dominated by a statistical ionization due to equilibration among the electronic degrees of freedom. For pulse durations on the order of a ps coupling to the vibrational degrees of freedom occurs and the well-known phenomenon of delayed (&mgr;s) ionization is observed.

COLTRIMS (COLd Target Recoil-Ion Momentum Spectroscopy) was used to measure the vector momentum distribution of Nen+(n = 1, 2, 3) ions formed in ultrashort (30 fsec) high-intensity (approximate to 10(15) W/cm(2)) laser pulses with center wavelength at 795 nm. To a high degree of accuracy the length of the Ne2+ ion momentum vector is equal to the length of the total momentum vector of the n photoelectrons released, with both vectors pointing into opposite directions. At a light intensity where non-sequential ionization of the atom dominates the Ne2+ and Ne3+ momentum distributions show distinct maxima at 4.0 a.u. and 7.5 a.u. along the polarization axis of the linearly polarized light beam. First, this is a clear signature of non-sequential multiple ioniza tion. Second, it indicates that instantaneous emission of two (or more) electrons at electric field strength maxima of the light wave can be ruled out as main mechanism of non-sequential strong-field multiple ionization. In contrast, this experimental result is in accordance with the kinematical constraints of the "rescattering model".

Vector momentum distributions of Ne(n+) (n = 1,2,3) ions created by 30 fs, approximately 1 PW/cm(2) laser pulses at 795 nm have been measured using recoil-ion momentum spectroscopy. Distinct maxima along the light polarization axis are observed at 4.0 and 7.5 a.u. for Ne2+ and Ne3+ production, respectively. Hence, mechanisms based on an instantaneous release of two (or more) electrons can be ruled out as a dominant contribution to nonsequential strong-field multiple ionization. The positions of the maxima are in accord with kinematical constraints set by the classical "rescattering model."