Fragmentation Coulomb explosion

In large clusters do you observe new fragmentation pathways due to Coulomb explosion as observed in the nice experiments of Castleman s group ... 

In the present chapter, we describe how molecules respond to an ultra-short intense laser held by referring to our recent series of studies. In Sect. 1.2, we describe the experimental techniques - such as mass-resolved momentum imaging (MRMI) and coincidence momentum imaging (CMI) - that have been developed in order to measure the momentum distributions of fragment ions generated by the Coulomb explosions of molecules in intense laser helds. In Sect. 1.3, we show how the dynamics induced within CS2 by ultra-short intense laser helds is elucidated by the CMI method from momentum... 

Fig. 1.1. Schematic view of the Coulomb explosion imaging of nuclear dynamics. Molecules exposed to an intense laser field undergo structural deformation in response to the formation of light-dressed potential energy surfaces, and decompose into fragment ions after multiple ionization. Since the momentum vectors of fragment ions sensitively reflect the geometrical structure just before the Coulomb explosion, the ultrafast nuclear dynamics of a molecule in an intense laser field can be elucidated through measurements of the momenta of fragment ions...

In order to correlate the different ion species ejected from the same Coulomb explosion pathway, the covariance mapping technique [17] was introduced, in which correlations among the fragment ions are extracted from the fluctuation in the ion signals of a large number of TOF mass spectra recorded at respective laser shots. When the fragment ions are produced through the same explosion pathway, their covariance becomes positive. 

For a three-body Coulomb explosion event, the total number of momentum components determined is nine (three for each fragment ion) in the laboratory frame. However, the number of independent momentum parameters required to describe the Coulomb explosion event in the molecular frame is reduced to three under conditions of conserved momentum. This is because three degrees of freedom in the momentum vector space are reserved to describe the translational momentum vector of the center of mass, and another three are used for the overall rotation of the system that describes the conversion from the laboratory frame to the molecular frame. In other words, the nuclear dynamics of a single Coulomb explosion event of CS, CS —> S+ + C+ + S+ in the molecular frame can be fully described in the three-dimensional momentum space specified by a set of three independent momentum parameters. There... 

This correlation can be described in terms of the geometrical structure of CS3+ just before the Coulomb explosion, which is defined as the geometrical structure that reproduces the observed momentum vectors of the three fragment ions when the internuclear potentials are assumed to be purely Coulombic. The geometrical structures of CS determined from the distribution peaks (marked by a cross) in the three Ap12 — 012 plots in Fig. 1.8b-d show that the S-C-S bond angle becomes smaller (dropping from 162° to 140°) as the two C-S bonds stretch simultaneously from 1.8 A to 3.0 A [23]. 

Fig. 1.11. Angular distributions of the fragment ions for the two-body Coulomb explosion of CH3CN2+ through the an = 0, bn = l,cn = 2 pathways derived from a thin slice (5pz = 16 x 103amum/s) of the momentum vector distribution at pz 0. The results from the least-squares fit [32] in which the effect of molecular rotation is taken into account are shown for comparison (solid lines). The fit was improved for the n = 2 pathways (c) when the effect of fragment ejection along the tilted direction with respect to the molecular axis is included (dashed lines)...

Mechanisms of molecular ion formation and fragmentation applicable to organic molecules is shown in Fig. 2.7. a) Molecular-ion formation will be dominant, provided the excitation wavelength is non-resonant with the electronic absorption of the molecular cation. Multiply charged ions are produced, and eventually lead to a Coulomb explosion even with an intensity level of 1014 Wcm 2. On the other way (Fig. 2.7b), fragmentation is vigor-... 

M+ and M2+ molecular ions are 100%. Fragments appeared at a higher intensity therefore, the ratios decreased with an increase in the intensity. The important observation is that there is no distinct difference in the ion yields with linearly and circularly polarized light excitations. It can be concluded that there is no enhancement of fragmentation by linearly polarized light, i.e., by electron recollision for this molecule. Fragments observed with small intensities were composed of acetylene+ and (naphthalene minus acetylene)2+. These ions are ascribed to the products of the Coulomb explosion of triply charged molecular ion. 

To emphasize this point, the Coulomb explo sion mechanism is defined in the following terms. The Coulomb explosion mechanism for fragmentation... 

In cases where the yield of molecular ions is higher than 10% and where the fragmentation pattern depends upon the atomic site of the core hole, the dissociation processes clearly depend upon the electronic structure of the molecule and the details of the electronic relaxation, i.e. not all pathways produce essentially the same result. The mechanism then may involve vibrational dissociation or electronic or vibrational predissociation as well as direct dissociation. Even in these cases, some of the electronic relaxation channels may rupture all the bonds in a molecule and high-kinetic-energy fragments can be produced. Such channels sometimes are labeled a Coulomb explosion, but this terminology should not be confused with the more specific use of the term that is proposed above. 

The fragmentation of nitrous oxide following site-selective excitation of Is electrons of ah three atoms to the lowest unoccupied molecular orbital clearly shows a dependence on the atomic site of excitation as shown in Fig. 6 and Table V. The fragmentation patterns are not consistent with the idea of a Coulomb explosion as defined in the preceding section, or with the concept of localized bond rupture around the atomic site of excitation. The... 

While fragmentation is the dominant chemical reaction induced by core electron excitation of molecules in the gas phase, other reactions such as rearrangements can be expected, and this possibility needs to be investigated. The extent of a Coulomb explosion in a large molecule is not known, and the role of Coulomb localization in the chemistry of isolated molecules needs to be examined further. Electron-multiple ion coincidence experiments are essential in the study of the chemistry because it is necessary to relate specific electronic decay channels to particular fragmentation patterns as identified by the several ions that are produced. 

Multiphoton effects in molecules are, as one might expect, similar to those in atoms but even richer. As for atoms, more than the minimum number of photons required to produce ionisation can be absorbed, so that ATI is observed [499, 501]. Many other effects are possible, for example, Coulomb explosions [502] which arise when two charged fragments fly apart. 

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