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1. Isotope-selective strong-field ionization of semiheavy water

   https://doi.org/10.1103/PhysRevResearch.7.L012062  

   Howard, A J; Britton, M; Streeter, Z L; Cheng, C; Lucchese, R R; McCurdy, C W; Bucksbaum, P H (March 2025, Physical Review Research)

   Semiheavy water (HOD) is one of the simplest molecules in which the bonds are labeled by isotope. We demonstrate that a pair of intense few-femtosecond infrared laser pulses can be used to selectively tunnel ionize along one of the two bonds. The first pulse doubly ionizes HOD, inducing rapid bond stretching and unbending. Femtoseconds later, the second pulse arrives and further ionization is selectively enhanced along the OH bond. These conclusions arise from 3D time-resolved measurements of ${\mathrm{H}}^{+}, {\mathrm{D}}^{+}$, and ${\mathrm{O}}^{+}$momenta following triple ionization. Published by the American Physical Society2025 

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2. Two- and three-body fragmentation of multiply charged tribromomethane by ultrafast laser pulses

   https://doi.org/10.1039/d2cp03089f  

   Bhattacharyya, Surjendu; Borne, Kurtis; Ziaee, Farzaneh; Pathak, Shashank; Wang, Enliang; Venkatachalam, Anbu Selvam; Marshall, Nathan; Carnes, Kevin D.; Fehrenbach, Charles W.; Severt, Travis; et al (November 2022, Physical Chemistry Chemical Physics)

   We investigate the two- and three-body fragmentation of tribromomethane (bromoform, CHBr 3 ) resulting from multiple ionization by 28-femtosecond near-infrared laser pulses with a peak intensity of 6 × 10 14 W cm −2 . The analysis focuses on channels consisting exclusively of ionic fragments, which are measured by coincidence momentum imaging. The dominant two-body fragmentation channel is found to be Br + + CHBr 2 + . Weaker HBr + + CBr 2 + , CHBr + + Br 2 + , CHBr 2+ + Br 2 + , and Br + + CHBr 2 2+ channels, some of which require bond rearrangement prior to or during the fragmentation, are also observed. The dominant three-body fragmentation channel is found to be Br + + Br + + CHBr + . This channel includes both concerted and sequential fragmentation pathways, which we identify using the native frames analysis method. We compare the measured kinetic energy release and momentum correlations with the results of classical Coulomb explosion simulations and discuss the possible isomerization of CHBr 3 to BrCHBr–Br (iso-CHBr 3 ) prior to the fragmentation. 

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3. Perspective on the Recent Measurements of Reduced Nitrogen Compounds in the Atmosphere

   https://doi.org/10.3389/fenvs.2022.868534  

   Lee, S.-H. (April 2022, Frontiers in Environmental Science)

   Reduced-nitrogen compounds (RNC), such as ammonia and amines, play important roles in atmospheric aerosol nucleation, secondary organic aerosol (SOA), and cloud formation processes. Fast measurements of ammonia and amines are made with a chemical ionization mass spectrometer (CIMS). Clusters containing RNC are measured with an atmospheric pressure interface time of flight mass spectrometer (APi-TOF) or chemical ionization APi-TOF (CI-APi-TOF). Aerosol-phase amines can be detected with a single particle mass spectrometer at real-time, or with offline chemical analytical methods using filter samples. However, the application of these instruments in real atmospheric measurements is still very limited. This perspective article highlights recent measurements of RNC in the atmosphere and discusses their implications in new particle formation (NPF). 

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4. Tracking ultrafast non-adiabatic dissociation dynamics of the deuterated water dication molecule

   https://doi.org/10.1063/5.0219029  

   Iskandar, W; Rescigno, T N; Orel, A E; Larsen, K A; Severt, T; Streeter, Z L; Jochim, B; Griffin, B; Call, D; Davis, V; et al (July 2024, The Journal of Chemical Physics)

   We applied reaction microscopy to elucidate fast non-adiabatic dissociation dynamics of deuterated water molecules after direct photo-double ionization at 61 eV with synchrotron radiation. For the very rare D+ + O+ + D breakup channel, the particle momenta, angular, and energy distributions of electrons and ions, measured in coincidence, reveal distinct electronic dication states and their dissociation pathways via spin–orbit coupling and charge transfer at crossings and seams on the potential energy surfaces. Notably, we could distinguish between direct and fast sequential dissociation scenarios. For the latter case, our measurements reveal the geometry and orientation of the deuterated water molecule with respect to the polarization vector that leads to this rare 3-body molecular breakup channel. Aided by multi-reference configuration-interaction calculations, the dissociation dynamics could be traced on the relevant potential energy surfaces and particularly their crossings and seams. This approach also unraveled the ultrafast time scales governing these processes. 

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5. Machine learning analysis of dynamic‐dependent bond formation in trajectories with consecutive transition states

   https://doi.org/10.1002/poc.4405  

   Melville, Jesse; Hargis, Cal; Davenport, Michael T.; Hamilton, R. Spencer; Ess, Daniel H. (November 2022, Journal of Physical Organic Chemistry)

   Abstract Dynamic motion often controls selectivity in reactions featuring two consecutive potential‐energy transition states. Here we report density functional theory (DFT)‐based direct dynamics trajectories and machine learning classification analysis for cyclopentadienone dimerization and a N 2 extrusion reaction leading to semibullvalene. These reactions have consecutive transition states, and there is dynamic selectivity that determines which of two possible C‐C bonds is formed after the first transition state. For cyclopentadienone dimerization with a bispericyclic first transition state, machine learning analysis using transition‐state based features provided >90% trajectory classification accuracy, but only using AdaBoost and random forest algorithms. Many other relatively sophisticated machine learning algorithms showed poor accuracy despite the obvious motion responsible for selectivity. Feature importance analysis confirmed that the sigmatropic rearrangement vibrational motion in the bispericyclic transition state provides prediction of which of the second C‐C bonds is dynamically formed. For the reaction leading to semibullvalene, machine learning analysis provides solid accuracy for classifying trajectories and predicting which C‐C bond is formed and which C‐C bond is broken immediately after N 2 ejection. Like the cyclopentadienone dimerization reaction, machine learning feature importance analysis showed that the sigmatropic rearrangement vibrational motion in the N 2 extrusion transition state determines which C‐C bond is formed and which is broken. Surprisingly, machine learning struggles to predict which trajectories undergo a subsequent [3,3] sigmatropic rearrangement process, which isomerizes equivalent forms of semibullvalene. 

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