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1. Photofragment imaging differentiates between one- and two-photon dissociation pathways in MgI⁺

   https://doi.org/10.1063/5.0134668  

   Lockwood, Schuyler P; Metz, Ricardo B (February 2023, The Journal of Chemical Physics)

   The bond strength and photodissociation dynamics of MgI+ are determined by a combination of theory, photodissociation spectroscopy, and photofragment velocity map imaging. From 17 000 to 21 500 cm−1, the photodissociation spectrum of MgI+ is broad and unstructured; photofragment images in this region show perpendicular anisotropy, which is consistent with absorption to the repulsive wall of the (1) Ω = 1 or (2) Ω = 1 states followed by direct dissociation to ground state products Mg+ (2S) + I (2P3/2). Analysis of photofragment images taken at photon energies near the threshold gives a bond dissociation energy D0(Mg+-I) = 203.0 ± 1.8 kJ/mol (2.10 ± 0.02 eV; 17 000 ± 150 cm−1). At photon energies of 33 000–41 000 cm−1, exclusively I+ fragments are formed. Over most of this region, the formation of I+ is not energetically allowed via one-photon absorption from the ground state of MgI+. Images show the observed product is due to resonance enhanced two-photon dissociation. The photodissociation spectrum from 33 000 to 38 500 cm−1 shows vibrational structure, giving an average excited state vibrational spacing of 227 cm−1. This is consistent with absorption to the (3) Ω = 0+ state from ν = 0, 1 of the (1) Ω = 0+ ground state; from the (3) Ω = 0+ state, absorption of a second photon results in dissociation to Mg* (3P°J) + I+ (3PJ). From 38 500 to 41 000 cm−1, the spectrum is broad and unstructured. We attribute this region of the spectrum to one-photon dissociation of vibrationally hot MgI+ at low energy and ground state MgI+ at higher energy to form Mg (1S) + I+ (3PJ) products. 

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2. Electric fields imbue enzyme reactivity by aligning active site fragment orbitals

   https://doi.org/10.1073/pnas.2411976121  

   Eberhart, Mark E; Wilson, Timothy R; Jones, Travis E; Alexandrova, Anastassia N (October 2024, Proceedings of the National Academy of Sciences)

   It is broadly recognized that intramolecular electric fields, produced by the protein scaffold and acting on the active site, facilitate enzymatic catalysis. This field effect can be described by several theoretical models, each of which is intuitive to varying degrees. In this contribution, we show that a fundamental effect of electric fields is to generate electrostatic potentials that facilitate the energetic alignment of reactant frontier orbitals. We apply this model to demystify the impact of electric fields on high-valent iron–oxo heme proteins: catalases, peroxidases, and peroxygenases/monooxygenases. Specifically, we show that this model easily accounts for the observed field-induced changes to the spin distribution within peroxidase active sites and explains the transition between epoxidation and hydroxylation pathways seen in Cytochrome P450 active site models. Thus, for the intuitive interpretation of the chemical effect of the field, the strategy involves analyzing the response of the orbitals of active site fragments, and their energetic alignment. We note that the energy difference between fragment orbitals involved in charge redistribution acts as a measure for the chemical hardness/softness of the reactive complex. This measure, and its sensitivity to electric fields, offers a single parameter model from which to quantitatively assess the effects of electric fields on reactivity and selectivity. Thus, the model provides an additional perspective to describe electrostatic preorganization and offers ways for its manipulation. 

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3. UV Photodissociation Dynamics of the Acetone Oxide Criegee Intermediate: Experiment and Theory

   https://doi.org/10.1039/D3CP00207A  

   Wang, Guanghan; Liu, Tianlin; Zou, Meijun; Karsili, Tolga; Lester, Marsha I (February 2023, Physical Chemistry Chemical Physics)

   The photodissociation dynamics of the dimethyl-substituted acetone oxide Criegee intermediate [(CH 3 ) 2 COO] is characterized following electronic excitation on the π*←π transition, which leads to O ( 1 D) + acetone [(CH 3 ) 2 CO, S0] products. The UV action spectrum of (CH 3 ) 2 COO recorded with O ( 1 D) detection under jet-cooled conditions is broad, unstructured, and essentially unchanged from the corresponding electronic absorption spectrum obtained using a UV-induced depletion method. This indicates that UV excitation of (CH 3 ) 2 COO leads predominantly to the O ( 1 D) product channel. A higher energy O ( 3 P) + (CH3)2CO (T1) product channel is not observed, although it is energetically accessible. This is attributed to the relatively weak absorption cross section at UV excitation energies above the threshold. In addition, complementary MS-CASPT2 trajectory surface-hopping (TSH) simulations indicate minimal population leading to the O ( 3 P) channel and non-unity overall probability for dissociation (within 100 fs). Velocity map imaging of the O ( 1 D) products is utilized to reveal the total kinetic energy release (TKER) distribution upon photodissociation of (CH 3 ) 2 COO at various UV excitation energies. Simulation of the TKER distributions is performed using a hybrid model that combines an impulsive model with a statistical component, the latter reflecting the longer-lived (> 100 fs) trajectories identified in the TSH calculations. The impulsive model accounts for vibrational activation of (CH 3 ) 2 CO arising from geometrical changes between the Criegee intermediate and the carbonyl product, indicating the importance of CO stretch, CCO bend, and CC stretch along with activation of hindered rotation and rock of the methyl groups in the (CH 3 ) 2 CO product. Detailed comparison is also made with the TKER distribution arising from photodissociation dynamics of CH 2 OO upon UV excitation. 

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4. Photoelectron spectroscopy with entangled photons; enhanced spectrotemporal resolution

   https://doi.org/10.1073/pnas.2300541120  

   Gu, Bing; Sun, Shichao; Chen, Feng; Mukamel, Shaul (May 2023, Proceedings of the National Academy of Sciences)

   In this theoretical study, we show how photoelectron signals generated by time-energy entangled photon pairs can monitor ultrafast excited state dynamics of molecules with high joint spectral and temporal resolutions, not limited by the Fourier uncertainty of classical light. This technique scales linearly, rather than quadratically, with the pump intensity, allowing the study of fragile biological samples with low photon fluxes. Since the spectral resolution is achieved by electron detection and the temporal resolution by a variable phase delay, this technique does not require scanning the pump frequency and the entanglement times, which significantly simplifies the experimental setup, making it feasible with current instrumentation. Application is made to the photodissociation dynamics of pyrrole calculated by exact nonadiabatic wave packet simulations in a reduced two nuclear coordinate space. This study demonstrates the unique advantages of ultrafast quantum light spectroscopy. 

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5. Universal Nonthermal Power-law Distribution Functions from the Self-consistent Evolution of Collisionless Electrostatic Plasmas

   https://doi.org/10.3847/1538-4357/ad91a1  

   Banik, Uddipan; Bhattacharjee, Amitava; Sengupta, Wrick (December 2024, The Astrophysical Journal)

   Abstract Collisionless systems often exhibit nonthermal power-law tails in their distribution functions. Interestingly, collisionless plasmas in various physical scenarios (e.g., the ion population of the solar wind) feature av⁻⁵tail in their velocity (v) distribution, whose origin has been a long-standing puzzle. We show this power-law tail to be a natural outcome of the collisionless relaxation of driven electrostatic plasmas. Using a quasi-linear analysis of the perturbed Vlasov–Poisson equations, we show that the coarse-grained mean distribution function (DF),f₀, follows a quasi-linear diffusion equation with a diffusion coefficientD(v) that depends onvthrough the plasma dielectric constant. If the plasma is isotropically forced on scales larger than the Debye length with a white-noise-like electric field,D(v) ∼v⁴forσ<v<ω_P/k, withσthe thermal velocity,ω_Pthe plasma frequency, andkthe characteristic wavenumber of the perturbation; the corresponding quasi-steady-statef₀develops av^{−(d+ 2)}tail inddimensions (v⁻⁵tail in 3D), while the energy (E) distribution develops anE⁻²tail independent of dimensionality. Any redness of the noise only alters the scaling in the highvend. Nonresonant particles moving slower than the phase velocity of the plasma waves (ω_P/k) experience a Debye-screened electric field, and significantly less (power-law suppressed) acceleration than the near-resonant particles. Thus, a Maxwellian DF develops a power-law tail, while its core (v<σ) eventually also heats up but over a much longer timescale. We definitively show that self-consistency (ignored in test-particle treatments) is crucial for the emergence of the universalv⁻⁵tail. 

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