Laser-induced nuclear motions in the Coulomb explosion of C2H2+ ions

A theoretical study of Ar8+-acetylene collisions at 1.2 MeV: Ionization and dissociation dynamics

We theoretically study Ar⁸⁺-induced dissociation of C₂H₂ molecule at 1.2 MeV using the time-dependent density-functional theory non-adiabatically coupled to nuclear dynamics. We find that molecular dissociation depends strongly on the ionization at the initial stage and the collision configuration. A detailed analysis shows a correspondence between the charge state of [C₂H₂]^q+ and the final fragments. A remarkable impact parameter effect provides deep insights of bond breakup and electronic transport. We analyze two typical sequential dissociation channels reported in experiments by tracking structural and electronic dynamics in real time. Our results provide better understanding of experiments. Moreover, the comparison between various exchange-correlation functionals reveals that electrons' correlation and self-interaction do not significantly impact the initial ionization and fragment distribution in the present study.

Visualization of bond rearrangements in acetylene using near single-cycle laser pulses

The migration of hydrogen atoms resulting in the isomerization of hydrocarbons is an important process which can occur on ultrafast timescales. Here, we visualize the light-induced hydrogen migration of acetylene to vinylidene in an ionic state using two synchronized 4 fs intense laser pulses. The first pulse induces hydrogen migration, and the second is used for monitoring transient structural changes via Coulomb explosion imaging. Varying the time delay between the pulses reveals the migration dynamics with a time constant of 54 ± 4 fs as observed in the H⁺ + H⁺ + CC⁺ channel. Due to the high temporal resolution, vibrational wave-packet motions along the CC- and CH-bonds are observed. Even though a maximum in isomerization yield for kinetic energy releases above 16 eV is measured, we find no indication for a backwards isomerization — in contrast to previous measurements. Here, we propose an alternative explanation for the maximum in isomerization yield, namely the surpassing of the transition state to the vinylidene configuration within the excited dication state.

Ab Initio/RRKM Study of Dissociation Mechanism of Benzene Trication

Density functional B3LYP /6-31 G (d,p) calculations have been performed in order to investigate isomerization and dissociation of [Formula: see text] in the ground electronic state, which are relevant to the Coulomb explosion mechanism of benzene. The results demonstrate that the benzene-like isomer of [Formula: see text], 1, can decompose through various pathways leading to distinct fragmentation products. The most kinetically favorable channel involves ring opening in 1 accompanied with 1,2-shifts of two hydrogen atoms followed by rotation around the middle C–C bond and dissociation to H 2 CCCH 2+ + H 2 CCCH + with exothermicity and the highest barrier of 100.2 and 38.4 kcal/mol, respectively. Several other product channels, including [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text], share the same highest barrier for the initial ring opening step with the pathway producing H 2 CCCH 2+ + H 2 CCCH + but exhibit higher second highest barriers. Elimination of [Formula: see text] to form [Formula: see text] is 137.9 kcal/mol exothermic but the highest barrier on the pathway leading to these products is 53.0 kcal/mol. A simple proton elimination to produce [Formula: see text] is computed to be 82–84 kcal/mol exothermic and to depict a 64–65 kcal/mol barrier. RRKM calculations of rate constants for individual reaction steps assuming that the initial internal energy of 1 is 110 kcal/mol and solving kinetic master equations to obtain relative branching ratios show that H 2 CCCH 2+ + H 2 CCCH + are the dominant products (81.5%) followed by [Formula: see text] (13.2%) and the other minor products include [Formula: see text] (2.6%), [Formula: see text] (1.1%), [Formula: see text] (0.55%), [Formula: see text] (0.49%), [Formula: see text] (0.20%), and [Formula: see text] (0.14%). The fragments are expected to be produced with high translational energy due to high Coulomb repulsion energy barriers.

High energy proton ejection from hydrocarbon molecules driven by highly efficient field ionization

We investigated the ejection of energetic protons from a series of polyatomic hydrocarbon molecules exposed to 790 nm 27 fs laser pulses. Using multiparticle coincidence imaging we were able to decompose the observed proton energy spectra into the contributions of individual fragmentation channels. It is shown that the molecules can completely fragment already at relatively low peak intensities of a few 10(14)  W/cm(2), and that the protons are ejected in a concerted Coulomb explosion from unexpectedly high charge states. The observations are in agreement with enhanced ionization taking place at many C-H bonds in parallel.

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.

Butterfly structure: Signature of vibrational flopping in dissociative acetylene

We report experimental evidence for molecular deformation due to a vibrationally active transition state of multiply charged acetylene molecules under the impact of low energy Ar8+ projectiles. "Butterflylike" structures are observed in the experimental coincidence spectra between hydrogen and carbon ionic fragments. Such structures can be generated by numerical simulations and are found to originate from the bending motion of the dissociating molecule. Angular distributions for dissociation products from triply charged C2H2 ion are reported.

Dissociation of Benzene Dication [C6H6]2+: Exploring the Potential Energy Surface

The singlet potential energy surface for the dissociation of benzene dication has been explored, and its three major dissociation channels have been studied: C6H6(2+) --> C3H3(+) + C3H3(+), C4H3(+) + C2H3(+), and C5H3(+) + CH3(+). The calculated energetics suggest that the products will be formed with considerable translational energy because of the Coulomb repulsion between the charged fragments. The calculated energy release in the three channels shows a qualitative agreement with the experimentally observed kinetic energy release. The formation of certain intermediates is found to be common to the three dissociation channels.

Concerted and sequential Coulomb explosion processes of N2O in intense laser fields by coincidence momentum imaging

The Coulomb explosion dynamics of N2O in intense laser fields (800 nm, 60 fs, approximately 0.16 PWcm2) is studied by the coincidence momentum imaging method. From the momentum correlation maps obtained for the three-body fragmentation pathway, N2O3+-->N++N++O+, the ultrafast structural deformation dynamics of N2O prior to the Coulomb explosion is extracted. It is revealed that the internuclear N-N and N-O distances stretch simultaneously as the bond angle less than approximately N-N-O decreases. In addition, two curved thin distributions are identified in the momentum correlation maps, and are interpreted well as those originating from the sequential dissociation pathway, N2O3+-->N++NO2+-->N++N++O+.

The onset of coulomb explosions in polyatomic molecules

With the development of high intensity femtosecond lasers, the ionisation and dissociation dynamics of molecules has become an area of considerable interest. Using the technique of femtosecond laser mass spectrometry (FLMS), the molecules carbon disulphide, pyrimidine, toluene, cyclohexanone and benzaldehyde are studied with pulse widths of 50 fs in the near infrared (IR) wavelength region (790 nm). Results are presented and contrasted for laser beam intensities around 10(15) and 10(16) W cm(-2). For the lower intensities, the mass spectra yield dominant singly charged parent ions. Additionally, the appearance of doubly charged parent ions is evident for carbon disulphide, toluene and benzaldehyde with envelopes of doubly charged satellite species existing in these local regions. Carbon disulphide also reveals a small triply charged component. Such atomic-like features are thought to be a strong fingerprint of FLMS at these intensities. However, upon increasing the laser intensity to approximately 10(16) W cm(-2), parent ion dominance decreases and the appearance of multiply charged atomic species occurs, particularly carbon. This phenomenon has been attributed to Coulomb explosions in which the fast absorption of many photons may produce transient highly ionised parent species which can subsequently blow apart. Copyright 1999 John Wiley & Sons, Ltd.