Molecular Double Ionization Using Strong Field Few-Cycle Laser Pulses

We study strong field double ionization of a series of organic molecules by making use of coincidence detection of fragment ions. We measure the double ionization yield as a function of pulse duration, intensity, polarization, and molecular conjugation. For conjugated molecules we find strong enhancement in the double ionization rate over what one would expect on the basis of tunneling or multiphoton ionization rates. Calculations reveal a correlation between the electronic structure of the different molecules and the observed double ionization yields, highlighting the removal of electrons from inner orbitals.

Transient X-ray fragmentation: probing a prototypical photoinduced ring opening

We report the first study of UV-induced photoisomerization probed via core ionization by an x-ray laser. We investigated x-ray ionization and fragmentation of the cyclohexadiene-hexatriene system at 850 eV during the ring opening. We find that the ion-fragmentation patterns evolve over a picosecond, reflecting a change in the state of excitation and the molecular geometry: the average kinetic energy per ion fragment and H(+)-ion count increase as the ring opens and the molecule elongates. We discuss new opportunities for molecular photophysics created by optical pump x-ray probe experiments.

Theoretical study of unimolecular decomposition of allene cations

Ab initio coupled clusters and multireference perturbation theory calculations with geometry optimization at the density functional or complete active space self-consistent-field levels have been carried out to compute ionization energies and to unravel the dissociation mechanism of allene and propyne cations, C(3)H(4)(n+) (n=1-3). The results indicate that the dominant decomposition channel of the monocation is c-C(3)H(3)(+) + H, endothermic by 37.9 kcal/mol and occurring via a barrier of 43.1 kcal/mol, with possible minor contributions from H(2)CCCH(+) + H and HCCCH(+) + H(2). For the dication, the competing reaction channels are predicted to be c-C(3)H(3)(+) + H(+), H(2)CCCH(+) + H(+), and CCCH(+) + H(3)(+), with dissociation energies of -20.5, 8.5, and 3.0 kcal/mol, respectively. The calculations reveal a H(2)-roaming mechanism for the H(3)(+) loss, where a neutral H(2) fragment is formed first, then roams around and abstracts a proton from the remaining molecular fragment before leaving the dication. According to Rice-Ramsperger-Kassel-Marcus calculations of energy-dependent rate constants for individual reaction steps, relative product yields vary with the available internal energy, with c-C(3)H(3)(+) + H(+) being the major product just above the dissociation threshold of 69.6 kcal/mol, in the energy range of 70-75 kcal/mol, and CCCH(+) + H(3)(+) taking over at higher energies. The C(3)H(4)(3+) trication is found to be not very stable, with dissociation thresholds of 18.5 and 3.7 kcal/mol for allene and propyne, respectively. Various products of Coulomb explosion of C(3)H(4)(3+), H(2)CCCH(2+) + H(+), CHCHCH(2+) + H(+), C(2)H(2)(2+) + CH(2)(+), and CCH(2)(2+) + CH(2)(+) are highly exothermic (by 98-185 kcal/mol). The tetracation of C(3)H(4) is concluded to be unstable and therefore no more than three electrons can be removed from this molecule before it falls apart. The theoretical results are compared to experimental observations of Coulomb explosions of allene and propyne.