The specific mass shift in the electron affinity between 35 Cl and 37 Cl has been determined by tunable laser photodetachment spectroscopy to be −0.51(14) GHz. The isotope shift was observed as a difference in the onset of the photodetachment process for the two isotopes. In addition, the electron affinity of Cl was found to be 29 138.59(22) cm −1 , giving a factor of 2 improvement in the accuracy over earlier measurements. Many-body calculations including lowest-order correlation effects demonstrates the sensitivity of the specific mass shift and show that the inclusion of higherorder correlation effects would be necessary for a quantitative description.

International audienceWe present experimental evidence for the dominance of prompt single-atom knockout in fragmenting collisions between large polycyclic aromatic hydrocarbon cations and He atoms at center-of-mass energies close to 100 eV. Such nonstatistical processes are shown to give highly reactive fragments. We argue that nonstatistical fragmentation is dominant for any sufficiently large molecular system under similar conditions

Time-resolved laser spectroscopy was used to measure radiative lifetimes of eight excited states of neutral sulfur in a laser-produced plasma. Excitation from the atomic ground state was performed with pulsed vacuumultraviolet ͑126-140 nm͒ radiation generated by resonant sum-difference four-wave mixing in krypton gas. The lifetimes of the 3 p 3 ns 3 S o ͑nϭ5-7͒ and 3p 3 nd 3 D o ͑nϭ4,5͒ states were measured and are presented together with data available from literature. Relativistic Hartree-Fock calculations of radiative lifetimes were also performed for states where experimental data were available. ͓S1050-2947͑97͒04403-X͔ PACS number͑s͒: 32.70. Cs, 42.62.Fi, 42.65.Ky

We report experimental total, absolute, fragmentation cross sections for anthracene C14H10, acridine C13H9N, and phenazine C12H8N2 ions colliding with He at center-of-mass energies close to 100 eV. In addition, we report results for the same ions colliding with Ne, Ar, and Xe at higher energies. The total fragmentation cross sections for these three ions are the same within error bars for a given target. The measured fragment mass distributions reveal significant contributions from both delayed (≫10(-12) s) statistical fragmentation processes as well as non-statistical, prompt (∼10(-15) s), single atom knockout processes. The latter dominate and are often followed by secondary statistical fragmentation. Classical Molecular Dynamics (MD) simulations yield separate cross sections for prompt and delayed fragmentation which are consistent with the experimental results. The intensity of the single C/N-loss peak, the signature of non-statistical fragmentation, decreases with the number of N atoms in the parent ion. The fragment intensity distributions for losses of more than one C or N atom are rather similar for C14H10 and C13H9N but differ strongly for C12H8N2 where weak C-N bonds often remain in the fragments after the first fragmentation step. This greatly increases their probability to fragment further. Distributions of internal energy remaining in the fragments after knockout are obtained from the MD simulations.