We investigated the significant differences in the angular distribution of carbon ions ejected from dimethylacetylenes (CH 3-C≡C-CH 3 and CH 3-C≡C-C≡C-CH 3) and diiodoacetylenes (I-C≡C-I and I-C≡C-C≡C-I) induced by a Coulomb explosion. The longest linear chain molecule ever reported was exposed to intense femtosecond laser fields (40 fs, < 5×10 14 Wcm-2) in this study. In the cases of the diiodoacetylenes, the angular distributions of the carbon ions were orthogonal with respect to the laser polarization direction and became narrower as the length of the molecules became longer. This is in sharp contrast to the dimethylacetylenes, in which the angular distributions of the carbon ions were parallel with respect to the laser polarization direction and became broader as the length of the molecules became longer. The specific angular distributions of carbon ions ejected from the diiodoacetylenes are explained in terms of the frozen molecular rotation, the blocking of carbon ion emission, and the bending of carbon chains at highly charged states due to the presence of iodines.

Strong electric fields that are larger than those of valence electrons in molecules shake out many electrons from the molecules.[1] The generation of highly charged molecular ions by femtosecond laser fields followed by a Coulomb explosion [2] prepares charged atoms in close proximity. The ion dynamics have been investigated by covariance mapping, [3] momentum imaging techniques, [4] and simple consideration of kinetic energy releases. The interaction between ionic species is usually solved by a classical equation of motion under point-charge approximation. Supposing that two charges with different masses exist in close proximity, the light ion flies away while the heavy counterion remains near the original location to conserve momentum. If the heavy ion obstacles disturb the direction in which the precursor ion move, the light ions would undergo structural deformation that enables the light ions to fly away. Linear molecules exposed to intense laser fields would provide interesting information on this issue, especially when the terminal heavy atoms block the movement of lighter ions or ion clusters in the linear molecule. We demonstrate the mass effects on the Coulomb explosion processes of acetylene derivatives of similar geometry but with different electronic orbitals and weights of terminal atoms.We compare the Coulomb explosion dynamics between acetylene and diiodoacetylene (DIA, IÀCCÀI). Multiple ionization was carried out with a 40 fs pulse centered at 800 nm, and the ions were detected by a time-of-flight mass spectrometer. Figure 1 shows the time-of-flight mass spectrum of DIA. The principal ions were C 2 I 2 w + (w = 1À4), C 2

Several features distinguish intact multiply charged molecular cations (MMCs) from other species such as monocations and polycations: high potential energy, high electron affinity, a high density of electronic states with various spin multiplicities, and charge-dependent reactions. However, repulsive Coulombic interactions make MMCs quite unstable, and hence small organic MMCs are currently not readily available. Herein, we report that the isolated four-atom molecule diiodoacetylene survives after the removal of four electrons via tunneling. We show that the tetracation remains metastable towards dissociation because of the localization (91-95 %) of the positive charges on the terminal iodine atoms, ensuring minimum Coulomb repulsion between adjacent atoms as well as maximum charge-induced attractive dipole interactions between iodine and carbon. Our approach making use of iodines as the positively charged sites enables small organic MMCs to remain intact.

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