Photofragmentation experiments on molecules and clusters often involve multiple photon absorption. The distributions of the absorbed number of photons are frequently approximated by Poisson distributions. For realistic laser beam profiles, this approximation fails seriously due to the spatial variation of the mean number of absorbed photons across the laser beam. We calculate the distribution of absorbed energy for various laser and molecular-beam parameters. For a Gaussian laser beam, the spatially averaged distributions have a power-law behavior for low energy with a cutoff at an energy which is proportional to fluence. The power varies between -1 for an almost parallel laser beam and -5/2 for a divergent beam (on the scale of the molecular beam). We show that the experimental abundance spectra of fullerenes and small carbon clusters can be used to reconstruct the distribution of internal energy in the excited C60 molecule prior to fragmentation and find good agreement with the calculated curves.

The fragmentation behavior of the endohedral metallofullerene La@C82 was studied using gas phase time-of-flight mass spectrometry. The results were compared with the fragmentation of C60. When the metallofullerene was excited by a 337 nm ns laser, small lanthanum–carbide fragments LaCn+, n=0–6, were detected. A simple statistical maximum entropy model was used to simulate the excitation energy dependence of the loss of the LaCn+ molecules as well as C2 evaporation to form smaller metallofullerenes. By comparing experimental ion intensities with the model, the appearance energies of LaCn+, n=0–6, were found to lie above 65 eV. The lanthanum–carbide fragments ejected from La@C82 decrease in size with increasing internal energy. The ejection of La+ and LaC2+ is preferred at the expense of larger fragments such as LaC4+ and metallofullerenes at the higher excitation.

Articles you may be interested inPhotodissociation of (SO2XH) Van der Waals complexes and clusters (XH = C2H2, C2H4, C2H6) excited at 32 040-32090 cm−1 with formation of HSO2 and X J. Chem. Phys. 140, 054304 (2014); 10.1063/1.4863445Mass spectrometry study of the fragmentation of valence and core-shell ( Cl 2p) excited CHCl 3 and CDCl 3 molecules J. Chem. Phys. 120, 9547 (2004); 10.1063/1.1701658 Formation of small lanthanum-carbide ions from laser induced fragmentation of La @ C 82We report the first measurements of delayed ionization of the metallofullerene La@C 82 . When highly excited in laser desorption, La@C 82 was found to ionize over several microseconds. The rate of delayed ionization of La@C 82 was compared to C 60 under the same extraction conditions, and found to be significantly different. A theoretical model was used for the fitting of the ion signal of the two fullerenes. The metallofullerene results can be fitted well over a time window Ͻ30 s and are consistent with a low ionization potential ͑6.3 eV͒ and a high radiative cooling rate. The C 60 data cannot be fitted over the same time window, indicative of an additional time-dependent competing decay channel.

The lifetime of the ground triplet state of vibrationally excited C60 has been determined in a two-color pump–probe experiment. An intense pump pulse (337 nm, 4 ns) was used to excite the neutral molecules. The triplet state was probed via single photon ionization with a weak probe pulse (193 nm). The lifetime was found to be on the order of 1 μs. This is rather low to explain recent delayed ionization results but higher than expected from an extrapolation of earlier measurements at lower temperatures.