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1. Fragmentation Dynamics of Fluorene Explored Using Ultrafast XUV-Vis Pump-Probe Spectroscopy

   https://doi.org/10.3389/fphy.2022.880793  

   Garg, D.; Lee, J. W.; Tikhonov, D. S.; Chopra, P.; Steber, A. L.; Lemmens, A. K.; Erk, B.; Allum, F.; Boll, R.; Cheng, X.; et al (May 2022, Frontiers in Physics)

   We report on the use of extreme ultraviolet (XUV, 30.3 nm) radiation from the Free-electron LASer in Hamburg (FLASH) and visible (Vis, 405 nm) photons from an optical laser to investigate the relaxation and fragmentation dynamics of fluorene ions. The ultrashort laser pulses allow to resolve the molecular processes occurring on the femtosecond timescales. Fluorene is a prototypical small polycyclic aromatic hydrocarbon (PAH). Through their infrared emission signature, PAHs have been shown to be ubiquitous in the universe, and they are assumed to play an important role in the chemistry of the interstellar medium. Our experiments track the ionization and dissociative ionization products of fluorene through time-of-flight mass spectrometry and velocity-map imaging. Multiple processes involved in the formation of each of the fragment ions are disentangled through analysis of the ion images. The relaxation lifetimes of the excited fluorene monocation and dication obtained through the fragment formation channels are reported to be in the range of a few tens of femtoseconds to a few picoseconds. 

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2. Disentangling sequential and concerted fragmentations of molecular polycations with covariant native frame analysis

   https://doi.org/10.1039/d2cp03029b  

   McManus, Joseph W.; Walmsley, Tiffany; Nagaya, Kiyonobu; Harries, James R.; Kumagai, Yoshiaki; Iwayama, Hiroshi; Ashfold, Michael N.R.; Britton, Mathew; Bucksbaum, Philip H.; Downes-Ward, Briony; et al (September 2022, Physical Chemistry Chemical Physics)

   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. 

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3. Structural dynamics during the three-ion Coulomb explosion of highly charged CS2 in intense soft x-ray pulses

   https://doi.org/10.1063/5.0346118  

   Lam, Chow-Shing; Allum, Felix; Erk, Benjamin; Bromberger, Hubertus; Bucksbaum, Philip H; Britton, Mathew; Burt, Michael; Ekanayake, Nagitha; Gabalski, Ian; Garg, Diksha; et al (September 2026, The Journal of Chemical Physics)

   Using intense femtosecond soft x-ray pulses above the sulfur 2p edge produced by the Free-electron LASer in Hamburg (FLASH), we investigate the multiple ionization and fragmentation dynamics of CS2. We focus on the three-ion fragmentation pathways of CS2 polycations with total charges of +4 to +10, which can only be reached through absorption of multiple x-ray photons. By coupling three-dimensional velocity map imaging with covariance analysis, we determine the relative momentum distribution of all ions produced in each fragmentation channel at high ion count rates per shot. Deviations between these measured relative ion momentum distributions and those predicted by classical Coulomb explosion simulations, assuming an instantaneous charge buildup, grow with increasing total charge state, indicative of nuclear motion during the multiple ionizations that occur within the x-ray pulse duration—as confirmed by comparison with the main fragmentation channel of the trication, which can be formed following single-photon interaction. These dynamics can be modeled using a gradual charge buildup picture of sequential single-photon single-ionization events occurring during the pulse, following the initial core ionization. Our study underscores the importance of complete and channel-resolved measurements, the advances enabled by covariance analysis in high-count-rate experiments, and the critical role of charge buildup and nuclear dynamics during the x-ray free-electron laser pulse in Coulomb explosion imaging. The impacts of this nuclear motion can be controlled and minimized by utilizing shorter x-ray pulses, as will be of great importance in using time-resolved x-ray Coulomb explosion to probe the fastest dynamics in molecular photochemistry. 

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4. Photodissociation Pathways of Methanol: Dissociation, Roaming, and Migration

   https://doi.org/10.1021/acs.jpclett.5c02294  

   Frese, Noah; Mishra, Debadarshini; Bray, Clark; Brady, Cameron; LaForge, Aaron C; Rossi, Angelo R; Gascón, José A; Berrah, Nora (October 2025, The Journal of Physical Chemistry Letters)

   NotIn this joint experimental and theoretical study, we examine the two-body photodissociation pathways induced in deuterated methanol by both single and double laser pulses. The measurements were conducted using IR pump−IR probe spectroscopy paired with the coincident momentum imaging technique. Our work unambiguously shows the roaming behavior of a D2 neutral resulting in D2 +, which is a strong pathway after direct excitation to a dication. Two pulse measurements reveal that despite the low formation probability, the migration pathways occur fastest of any photodissociation relaxation route. To elucidate the mechanistic origin of the observed fragmentation channels, we performed ab initio molecular dynamics simulations based on DFT for ground-state reactivity and state-averaged CASSCF for excited-state dynamics, enabling direct comparison with experimental yields. Available 

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5. Tandem mass spectrometry using continuous‐wave infrared multiphoton dissociation in an electrostatic linear ion trap

   https://doi.org/10.1002/rcm.9698  

   Carrick, Ian J; Fabijanczuk, Kimberly C; Rong, Jiayue; McLuckey, Scott A (March 2024, Rapid Communications in Mass Spectrometry)

   RationaleThe electrostatic linear ion trap (ELIT) can be operated as a multi‐reflection time‐of‐flight (MR‐TOF) or Fourier transform (FT) mass analyzer. It has been shown to be capable of performing high‐resolution mass analysis and high‐resolution ion isolations. Although it has been used in charge‐detection mass spectrometry (CDMS), it has not been widely used as a conventional mass spectrometer for ensemble measurements of ions, or for tandem mass spectrometer. The advantages of tandem mass spectrometer with high‐resolution ion isolations in the ELIT have thus not been fully exploited. MethodsA homebuilt ELIT was modified with BaF₂viewports to facilitate transmission of a laser beam at the turnaround point of the second ion mirror in the ELIT. Fragmentation that occurs at the turnaround point of these ion mirrors should result in minimal energy partitioning due to the low kinetic energy of ions at these points. The laser was allowed to irradiate ions for a period of many oscillations in the ELIT. ResultsDue to the low energy absorption of gas‐phase ions during each oscillation in the ELIT, fragmentation was found to occur over a range of oscillations in the ELIT generating a homogeneous ion beam. A mirror‐switching pulse is shown to create time‐varying perturbations in this beam that oscillate at the fragment ion characteristic frequencies and generate a time‐domain signal. This was found to recover FT signal for protonated pYGGFL and pSGGFL precursor ions. ConclusionsFragmentation at the turnaround point of an ELIT by continuous‐wave infrared multiphoton dissociation (cw‐IRMPD) is demonstrated. In cases where laser power absorption is low and fragmentation occurs over many laps, a mirror‐switching pulse may be used to recover varying time‐domain signal. The combination of laser activation at the turnaround points and mirror‐switching isolation allows for tandem MS in the ELIT. 

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