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The journal of chemical physics 158, 174305 (2023). doi:10.1063/5.0147456, Naphthalene and azulene are isomeric polycyclic aromatic hydrocarbons (PAHs) and are topical in the context of astrochemistry due to the recent discovery of substituted naphthalenes in the Taurus Molecular Cloud-1 (TMC-1). Here, the thermal- and photo-induced isomerization, dissociation, and radiative cooling dynamics of energized (vibrationally hot) naphthalene (Np$^+$) and azulene (Az$^+$) radical cations, occurring over the microsecond to seconds timescale, are investigated using a cryogenic electrostatic ion storage ring, affording “molecular cloud in a box” conditions. Measurement of the cooling dynamics and kinetic energy release distributions for neutrals formed through dissociation, until several seconds after hot ion formation, are consistent with the establishment of a rapid (sub-microsecond) Np$^+ ⇌$ Az$^+$ quasi-equilibrium. Consequently, dissociation by C$_2$H$_2$-elimination proceeds predominantly through common Az$^+$ decomposition pathways. Simulation of the isomerization, dissociation, recurrent fluorescence, and infrared cooling dynamics using a coupled master equation combined with high-level potential energy surface calculations [CCSD(T)/cc-pVTZ], reproduce the trends in the measurements. The data show that radiative cooling via recurrent fluorescence, predominately through the Np$^+$ D$_0$ ← D$_2$ transition, efficiently quenches dissociation for vibrational energies up to ≈1 eV above dissociation thresholds. Our measurements support the suggestion that small cations, such as naphthalene, may be more abundant in space than previously thought. The strategy presented in this work could be extended to fingerprint the cooling dynamics of other PAH ions for which isomerization is predicted to precede dissociation., Published by American Institute of Physics, Melville, NY

Multi-mass velocity map imaging studies of charged fragments formed by near infrared strong field ionization together with covariance map image analysis offer a new window through which to explore the dissociation dynamics of several different highly charged parent cations, simultaneously - as demonstrated here for the case of CF

Physical chemistry, chemical physics 24, 23096 - 23105 (2022). doi:10.1039/D2CP02252D special issue: "Ions, electrons, coincidences and dynamics: Festschrift for John H.D. Eland", We investigated the dissociation of dications and trications of three polycyclic aromatic hydrocarbons (PAHs), fluorene, phenanthrene, and pyrene. PAHs are a family of molecules ubiquitous in space and involved in much of the chemistry of the interstellar medium. Ions are formed by ionization with 30.3 nm extreme ultraviolet (XUV) photons, and their velocity map images are recorded using a PImMS multi-mass imaging sensor. Application of recoil-frame covariance analysis allows the total kinetic energy release (TKER) associated with multiple fragmentation channels to be determined to high precision. Experimental measurements are supported by Born-Oppenheimer molecular dynamics (BOMD) molecular dynamics simulations., Published by RSC Publ., Cambridge

Journal of physics / B 55(1), 014001 (2022). doi:10.1088/1361-6455/ac489d, The ultraviolet (UV)-induced dissociation and photofragmentation of gas-phase CH$_2$BrI molecules induced by intense femtosecond extreme ultraviolet (XUV) pulses at three different photon energies are studied by multi-mass ion imaging. Using a UV-pump���XUV-probe scheme, charge transfer between highly charged iodine ions and neutral CH$_2$Br radicals produced by C���I bond cleavage is investigated. In earlier charge-transfer studies, the center of mass of the molecules was located along the axis of the bond cleaved by the pump pulse. In the present case of CH$_2$BrI, this is not the case, thus inducing a rotation of the fragment. We discuss the influence of the rotation on the charge transfer process using a classical over-the-barrier model. Our modeling suggests that, despite the fact that the dissociation is slower due to the rotational excitation, the critical interatomic distance for charge transfer is reached faster. Furthermore, we suggest that charge transfer during molecular fragmentation may be modulated in a complex way., Published by IOP Publ., Bristol

Physical chemistry, chemical physics 24(37), 22699 - 22709 (2022). doi:10.1039/D2CP03029B, We present results from an experimental ion imaging study into the fragmentation dynamics of 1-iodopropane and 2-iodopropane following interaction with extreme ultraviolet intense femtosecond laser pulses with a photon energy of 95 eV. Using covariance imaging analysis, a range of observed fragmentation pathways of the resulting polycations can be isolated and interrogated in detail at relatively high ion count rates (∼12 ions shot$^{−1}$). By incorporating the recently developed native frames analysis approach into the three-dimensional covariance imaging procedure, contributions from three-body concerted and sequential fragmentation mechanisms can be isolated. The angular distribution of the fragment ions is much more complex than in previously reported studies for triatomic polycations, and differs substantially between the two isomeric species. With support of simple simulations of the dissociation channels of interest, detailed physical insights into the fragmentation dynamics are obtained, including how the initial dissociation step in a sequential mechanism influences rovibrational dynamics in the metastable intermediate ion and how signatures of this nuclear motion manifest in the measured signals., Published by RSC Publ., Cambridge

Correction for ‘Multi-mass velocity map imaging study of the 805 nm strong field ionization of CF3I’ by Stuart W. Crane et al., Phys. Chem. Chem. Phys., 2022, DOI: https://doi.org/10.1039/d2cp02449g.

We present a comprehensive experimental study into the dissociative electron ionization dynamics of CF3I at energies ranging from 20 to 100 eV. A beam-gas instrument has been used to measure the absolute total ionization cross-section for the molecule over the energy range from 0 to 300 eV. Coupled with data from an electron-molecule crossed beam velocity-map imaging instrument, this allows absolute partial ionization cross-sections to be determined for formation of each ionic fragment. These reveal a number of fragmentation channels involving both C-I and C-F bond cleavage, in some cases followed by further fragmentation of the resulting molecular ion. Velocity-map images have been recorded for the I+ and CF3+ products of C-I bond cleavage and the CF2I+ products of C-F bond cleavage. Analysis of fragment kinetic energy distributions extracted from the images reveals that CF3+ product of C-I bond cleavage appears to be formed via a statistical mechanism occurring over long timescales, while the CF2I+ products of C-F cleavage are formed via a much faster, more direct dissociation mechanism involving one or more repulsive states of the parent molecular ion. The I+ fragments arising from C-I bond cleavage display behaviour intermediate between the two extremes. For all fragments, the images show little or no dependence on the energy of the incident electron, implying that the initially excited ion state or states undergo rapid relaxation to the dissociative state(s) in all cases. Only a very small fraction of the incident electron's kinetic energy is released into kinetic energy of the recoiling atomic and molecular fragments, implying that most of the available energy remains with the two departing electrons. The kinetic energy distributions obtained for the various fragments of dissociative electron ionization are compared with the corresponding distributions reported from photoionization studies in order to gain insight into the electronic states involved.

The photodissociation dynamics of CH

This account introduces a new variant of time-of-flight mass spectrometry, termed velocity-map imaging mass spectrometry (VMImMS). While the ion abundances recorded in conventional ToF-MS measurements are highly useful for molecular quantification and structure determination, the final parent and fragment ion yields are Largely blind to the dynamics of the processes in which the ions were formed inside the mass spectrometer. By recording the velocity distribution of each ion in tandem with the mass spectrum, not only can the details of the dissociative ionisation dynamics be unravelled, but the extra dimensions of information can be used for enhanced molecular fingerprinting, separating contributions from ions with identical mass-to-charge ratio and resolving components within mixtures, to name but a few examples. Measuring ion-velocity distributions within a mass spectrometry measurement is not new, but incorporating imaging techniques developed within the reaction dynamics community provides vastly improved velocity resolution for all ions simultaneously in a single-stage instrument. This account provides an introduction to VMImMS, outlines the fundamental instrumentation and detector requirements and the challenges associated with developing the method further, and details proof-of-concept work from our Laboratory on a number of potential applications of the technique.