Coulomb Explosion of Dichloroethene Geometric Isomers at 1 PW cm–2

Two-Dimensional Projected-Momentum Covariance Mapping for Coulomb Explosion Imaging

We introduce projected-momentum covariance mapping, an extension of recoil-frame covariance mapping for 2D ion imaging studies. By considering the two-dimensional projection of the ion momenta as recorded by the detector, one opens the door to a complex suite of analysis tools adapted from three-dimensional momentum imaging studies. This includes the use of different frames of reference to unravel the dynamics of fragmentation and the application of fragment momentum constraints to isolate specific fragmentation channels. The technique is demonstrated on data from a two-dimensional ion imaging study of the Coulomb explosion of the cis and trans isomers of 1,2-dichloroethene, following strong-field ionization by an intense near-infrared femtosecond laser pulse. Classical simulations are used to guide the interpretation of projected-momentum covariance maps. The results offer a detailed insight into the distinct Coulomb explosion dynamics for this pair of isomers and lay the groundwork for future time-resolved studies of photoisomerization dynamics in this molecular system.

Two-Body Metastable Dissociation of n-Pentane and n-Hexane Triplet Dications in Intense Femtosecond-Laser Fields

Mass spectra of n-pentane and n-hexane ionized through femtosecond-laser pulses were measured using a time-of-flight mass spectrometer. Fragment ions ejected with large kinetic energies were identified as side peaks in which a two-body dissociation pathway, C₅H₁₂⁺⁺ → C₂H₅⁺ + C₃H₇⁺, was identified for n-pentane, and two for n-hexane, C₆H₁₄⁺⁺ → C₂H₅⁺ + C₄H₉⁺ and C₃H₇⁺ + C₃H₇⁺, based on momentum matching of the fragments. The two-body dissociation pathways were observed when the polarization direction of the linearly polarized laser light was perpendicular to the molecular axis. However, when the polarization direction was parallel to the molecular axis or the laser light was circularly polarized, these signals were weak or difficult to identify. These results suggest that the two-body dissociation pathways are caused by nonsequential double ionization (NSDI), which begins with ionization from the π-type second highest occupied molecular orbital (HOMO-1) via the laser electric field perpendicular to the molecular axis rather than bonding the σ-type HOMO. Quantum chemical calculations show that the dication has a triplet metastable state with the same formula as the neutral state (i.e., ³[CH₃-(CH₂)_n-CH₃]⁺⁺). Therefore, the relevant two-body dissociation channels open through transition states with the (HOMO)¹(HOMO-1)¹ electron configuration and the estimated kinetic energy release values correlate with those observed.

Coulomb Explosion of Multi-charged Atomic Alkaline Metal Clusters

In the present work, a computational study of the Coulomb explosions of atomic metal clusters of the type X₈²⁺ was carried out, X = (Li-Cs). The work was done within the Kohn-Sham methodology using the Born-Oppenheimer molecular dynamics approximation. The dominant fission channels were established and the electron bonding patterns were analyzed with the help of the Electron Localization Function (ELF). A simple theoretical model was developed to understand and describe, in a qualitatively way, the main physical mechanism involved in the fission of these multicharged clusters. It has been found that the most possible fragments after explosion are the same considering the dynamics or the thermodynamics results. The bonds breaking and formation are well depicted by the ELF, and the main physical effects are well described by the developed model.

Dissociation of Singly and Multiply Charged Nitromethane Cations: Femtosecond Laser Mass Spectrometry and Theoretical Modeling

Dissociation pathways of singly- and multiply charged gas-phase nitromethane cations were investigated with strong-field laser photoionization mass spectrometry and density functional theory computations. There are multiple isomers of the singly charged nitromethane radical cation, several of which can be accessed by rearrangement of the parent CH₃-NO₂ structure with low energy barriers. While direct cleavage of the C-N bond from the parent nitromethane cation produces NO₂⁺ and CH₃⁺, rearrangement prior to dissociation accounts for fragmentation products including NO⁺, CH₂OH⁺, and CH₂NO⁺. Extensive Coulomb explosion in fragment ions observed at high laser intensity indicates that rapid dissociation of multiply charged nitromethane cations produces additional species such as CH₂⁺, H⁺, and NO₂²⁺.  On the basis of analysis of Coulomb explosion in the mass spectral signals and pathway calculations, sufficiently intense laser fields can remove four or more electrons from nitromethane.

Identification of absolute geometries of cis and trans molecular isomers by Coulomb Explosion Imaging

An experimental route to identify and separate geometric isomers by means of coincident Coulomb explosion imaging is presented, allowing isomer-resolved photoionization studies on isomerically mixed samples. We demonstrate the technique on cis/trans 1,2-dibromoethene (C₂H₂Br₂). The momentum correlation between the bromine ions in a three-body fragmentation process induced by bromine 3d inner-shell photoionization is used to identify the cis and trans structures of the isomers. The experimentally determined momentum correlations and the isomer-resolved fragment-ion kinetic energies are matched closely by a classical Coulomb explosion model.

Electronic Spectroscopy of a C7H4+ Isomer in a Neon Matrix: Methyltriacetylene Cation

Absorptions commencing at 602.6 nm are detected following deposition of mass-selected C7H4+ in a 6 K neon matrix produced from a 1 : 1 mixture of diacetylene and propyne in an ion source. The 602.6 nm system, and a weaker one near 421.1 nm, are assigned to the A 2E ← X 2E and B 2E ← X 2E electronic transitions of methyltriacetylene cation (C3V symmetry), based on mass-selection, spectroscopic analysis of the vibrational structure, and the excitation energies calculated with CASPT2. Structured fluorescence is detected in the 600–760 nm range upon laser excitation at wavelengths of the CH3C6H+ absorptions. The vibrational bands observed in the absorption and fluorescence spectra are assigned with the aid of calculated frequencies of the totally symmetric (a1) vibrations of methyltriacetylene cation.