Persistence of Iodines and Deformation of Molecular Structure in Highly Charged Diiodoacetylene: Anisotropic Carbon Ion Emission

Abstract

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

Anisotropic Coulomb explosion of acetylene and diacetylene derivatives

“…Carbon ions originated from diiodoacetylene are emitted in an orthogonal direction, whereas those from acetylene are emitted in a parallel direction with respect to the laser polarization direction. We have explained the distinct carbon ion emissions from diiodoacetylene in terms of the persistence of a molecular frame at a high charge state, and the deformation within the Coulomb field formed by the two adjacent iodine ions [24]. This finding was confirmed by the coincidence study for the Coulomb explosion of diiodoacetylene induced by the collision with high-energy projectiles (Ar 8+ ) [25].…”

“…In a previous paper, we explained that the different fragment ion distributions between acetylene and DIA are due to the elongation of the chemical bond during higher-order ionization [24]. The nuclei have time to move apart from each other during the high-order ionization by femtosecond laser pulses, and terminal protons are expelled before reaching a higher charge state in the case of acetylene, while the timing of the C−I bond elongation should be slower than that of the C−H bond due to the large mass.…”

“…We have demonstrated the mass effects on the angular distribution of carbon ions ejected from acetylene and diiodoacetylene, which have similar geometry but different terminal atom masses [24].…”

Multiple ionization and Coulomb explosion of molecules, molecular complexes, clusters and solid surfaces

“…The liberation of hydrogen atoms by tunneling as well as ejection of protons by Coulomb explosion is expected to take place easily in organic MMCs. In Figure 5 are shown organic molecular tetracations that have been produced by several methods such as EI [121,122], collision with high energy projectiles [123] and femtosecond laser ionization [42,55,124,125]. The organic molecular tetracation of ovalene (C32H14), a large polycyclic aromatic hydrocarbon that has a structure containing 10 fused benzene rings that enable delocalization of multiple charges, was detected for the first time using EI (80 eV) [121].…”

“…If the heavy ion disturbs the direction in which light ions move, the light ions undergo structural deformation that enables them to move away. For example, heavy iodine atoms, which have a mass that is 11-times that of carbon, block the movement of lighter carbon ions in the Coulomb explosion of diiodoacetylene at 3.6×10 14 W cm −2 (800 nm, 45 fs) [42]. As a result, the angular distributions of carbon and hydrogen ions closely coincide in the case of acetylene, while carbon ions are ejected perpendicularly with respect to the iodine ones.…”