Infrared multiphoton excitation, dissociation and ionization of C60

Probing the binding and activation of small molecules by gas-phase transition metal clusters via IR spectroscopy

Isolated transition metal clusters have been established as useful models for extended metal surfaces or deposited metal particles, to improve the understanding of their surface chemistry and of catalytic reactions. For this objective, an important milestone has been the development of experimental methods for the size-specific structural characterization of clusters and cluster complexes in the gas phase. This review focusses on the characterization of molecular ligands, their binding and activation by small transition metal clusters, using cluster-size specific infrared action spectroscopy. A comprehensive overview and a critical discussion of the experimental data available to date is provided, reaching from the initial results obtained using line-tuneable CO₂ lasers to present-day studies applying infrared free electron lasers as well as other intense and broadly tuneable IR laser sources.

Intensity-resolved IR multiple photon ionization and fragmentation of C60

The sequential absorption of multiple infrared (IR) photons by isolated gas-phase species can lead to their dissociation and/or ionization. Using the newly constructed "Free-Electron Laser for IntraCavity Experiments" (FELICE) beam line at the FELIX facility, neutral C(60) molecules have been exposed to an extremely high number (approximately 10(23)) of photons/cm(2) for a total time duration of up to 5 micros. At wavelengths around 20 microm, resonant with allowed IR transitions of C(60), ionization and extensive fragmentation of the fullerenes are observed. The resulting photofragment distributions are attributed to absorption in fragmentation products formed once C(60) is excited to internal energies at which fragmentation or ionization takes place within the duration of the laser pulse. The high IR intensities available combined with the large interaction volume permit spatially resolved detection of the ions inside the laser beam, thereby disentangling the contributions from different IR intensities. The use of spatial imaging reveals intensity dependent mass distributions that are substantially narrower than what has been observed previously, indicating rather narrow energy distributions. A simple rate-equation modeling of the excitation process supports the experimental observations.

Infrared multiphoton ionization of superhot C60: experiment and model calculations

We address, both experimentally and theoretically, the issue of infrared (IR) resonance enhanced multiphoton ionization (IR-REMPI) and thermally induced redshifts of IR absorption lines in a very large and highly vibrationally excited molecular system. Isolated superhot C60 molecules with well defined and variable average vibrational energy in the range of 9-19 eV, effusing out of a constant flux thermal source, are excited and ionized after the absorption of multiple (500-800) infrared photons in the 450-1800 cm(-1) spectral energy range. Recording the mass-selected ion signal as a function of IR wavelength gives well resolved IR-REMPI spectra, with zero off-resonance background signal. An enhancement of the ion signal of about a factor of 10 is observed when the temperature is increased from 1200 to 1800 K under otherwise identical conditions. A pronounced temperature dependent redshift of some of the IR absorption lines is observed. The observations are found to be in good agreement with a model which is based on the sequential absorption of single photons, always followed by instantaneous vibrational energy redistribution. The mass spectra (C60(+) fragmentation pattern) are found to be strongly excitation wavelength dependent. Extensive fragmentation down to C32(+) is observed following the absorption of 1350-1400 cm(-1) as well as 1500-1530 cm(-1) photons while negligible fragmentation is observed when exciting around 520 cm(-1).

Delayed ionization and fragmentation en route to thermionic emission: statistics and dynamics

Thermionic emission is discussed as a long time (microseconds) decay mode of energy-rich large molecules, metallic and metcar clusters, and fullerenes. We review what is known and consider the many experiments, systems, and theoretical and computational studies that still need to be done. We conclude with a wish list for future work. Particular attention is given to the experimental signatures, such as the dependence on the mode of energy acquisition, and theoretical indications of a not-quite-statistical delayed ionization and to the competition of electron emission with other decay modes, such as fragmentation or radiative cooling. Coupling of the electronic and nuclear modes can be a bottleneck and quite long time-delayed ionization can be observed, as in the decay of high Rydberg states probed by ZEKE spectroscopy, before the onset of complete energy partitioning.