Modulating Carrier Dynamics in PdSe2: The Role of Pressure in Electronic and Phononic Interactions.

PdSe2 is a puckered transition metal dichalcogenide that has been reported to undergo a two-dimensional to three-dimensional structural transition under pressure. Here, we investigated the electronic and phononic evolution of PdSe2 under high pressure using pump-probe spectroscopy. We observed the electronic intraband and interband transitions occurring in the d orbitals of Pd, revealing the disappearance of the Jahn-Teller effect under high pressure. Furthermore, we found that the decay rates of interband recombination and intraband relaxation lifetimes change at 3 and 7 GPa, respectively. First-principles calculations suggest that the bandgap closure slows the decay rate of interband recombination after 3 GPa, while the saturation of phonon-phonon scattering is the main reason for the relatively constant intraband relaxation lifetime. Our work provides a novel perspective for understanding the evolution of the electron and modulation of the carrier dynamics by phonons under pressure.

Time-Resolved Ultraviolet Photodissociation Mass Spectrometry Probes the Mutation-Induced Alterations in Protein Stability and Unfolding Dynamics.

How mutations impact protein stability and structure dynamics is crucial for understanding the pathological process and rational drug design. Herein, we establish a time-resolved native mass spectrometry (TR-nMS) platform via a rapid-mixing capillary apparatus for monitoring the acid-initiated protein unfolding process. The molecular details in protein structure unfolding are further profiled by a 193 nm ultraviolet photodissociation (UVPD) analysis of the structure-informative photofragments. Compared with the wild-type dihydrofolate reductase (WT-DHFR), the M42T/H114R mutant (MT-DHFR) exhibits a significant stability decrease in TR-nMS characterization. UVPD comparisons of the unfolding intermediates and original DHFR forms indicate the special stabilization effect of cofactor NADPH on DHFR structure, and the M42T/H114R mutations lead to a significant decrease in NADPH-DHFR interactions, thus promoting the structure unfolding. Our study paves the way for probing the mutation-induced subtle changes in the stability and structure dynamics of drug targets.

Unveiling the Intrinsic Photophysics in Quasi-Two-Dimensional Perovskites.

The two-dimensional (2D) perovskites have drawn intensive attention due to their unique stability and outstanding optoelectronic properties. However, the debate surrounding the spatial phase distribution and band alignment among different 2D phases in the quasi-2D perovskite has created complexities in understanding the carrier dynamics, hindering material and device development. In this study, we employed highly sensitive transient absorption spectroscopy to investigate the carrier dynamics of (BA)2(MA)n-1PbnI3n+1 quasi-2D Ruddlesden-Popper perovskite thin films, nominally prepared as n = 4. We observed the carrier-density-dependent electron and hole transfer dynamics between the 2D and three-dimensional (3D) phases. Under a low carrier density within the linear response range, we successfully resolved three ultrafast processes of both electron and hole transfers, spanning from hundreds of femtoseconds to several picoseconds, tens to hundreds of picoseconds, and hundreds of picoseconds to several nanoseconds, which can be attributed to lateral-epitaxial, partial-epitaxial, and disordered-interface heterostructures between 2D and 3D phases. By considering the interplay among the phase structure, band alignment, and carrier dynamics, we have proposed material synthesis strategies aimed at enhancing the carrier transport. Our results not only provide deep insights into an accurate intrinsic photophysics of quasi-2D perovskites but also inspire advancements in the practical application of these materials.

Spiers Memorial Lecture: New directions in molecular scattering.

The field of molecular scattering is reviewed as it pertains to gas-gas as well as gas-surface chemical reaction dynamics. We emphasize the importance of collaboration of experiment and theory, from which new directions of research are being pursued on increasingly complex problems. We review both experimental and theoretical advances that provide the modern toolbox available to molecular-scattering studies. We distinguish between two classes of work. The first involves simple systems and uses experiment to validate theory so that from the validated theory, one may learn far more than could ever be measured in the laboratory. The second class involves problems of great complexity that would be difficult or impossible to understand without a partnership of experiment and theory. Key topics covered in this review include crossed-beams reactive scattering and scattering at extremely low energies, where quantum effects dominate. They also include scattering from surfaces, reactive scattering and kinetics at surfaces, and scattering work done at liquid surfaces. The review closes with thoughts on future promising directions of research.

Insights into ultrafast decay dynamics of electronically excited pyridine-N-oxide.

The ultrafast decay dynamics of pyridine-N-oxide upon excitation in the near-ultraviolet range of 340.2-217.6 nm is investigated using the femtosecond time-resolved photoelectron imaging technique. The time-resolved photoelectron spectra and photoelectron angular distributions at all pump wavelengths are carefully analyzed and the following view is derived: at the longest pump wavelengths (340.2 and 325.6 nm), pyridine-N-oxide is excited to the S1(1ππ*) state with different vibrational levels. The depopulation rate of the S1 state shows a marked dependence on vibrational energy and mode, and the lifetime is in the range of 1.4-160 ps. At 289.8 and 280.5 nm, both the second 1ππ* state and the S1 state are initially prepared. The former has an extremely short lifetime of ∼60 fs, which indicates that the ultrafast deactivation pathway such as a rapid internal conversion to one close-lying state is its dominant decay channel, while the latter is at high levels of vibrational excitation and decays within the range of 380-520 fs. At the shortest pump wavelengths (227.3 and 217.6 nm), another excited state of Rydberg character is mostly excited. We assign this state to the 3s Rydberg state which has a lifetime of 0.55-2.2 ps. This study provides a comprehensive picture of the ultrafast excited-state decay dynamics of the photoexcited pyridine-N-oxide molecule.

Vibration to Vibration: Product Energy Distribution of F + HD Crossed Molecular Beam Experiments.

We investigated the F + HD(v = 1, j = 0) → HF + D reaction using the crossed molecular beam technique combined with the D atom Rydberg tagging time-of-flight spectroscopy. By detecting the products at various scattering angles for different collision energies in the range of 0.8-1.2 kcal/mol, we observed the forward-scattering products of HF(v' = 4) and determined the threshold energy for the opening of this reaction channel. Similar experiments were conducted for the F + HD(v = 0, j = 0) → HF + D reaction within the range of 1.1-1.6 kcal/mol, where forward-scattering products of HF(v' = 3) were observed, and the threshold energy for this reaction channel was determined as well. Furthermore, we measured the differential cross-sections for the F + HD → HF + D reaction in both the vibrational ground state and the excited state of HD and analyzed the vibrational quantum-state distribution of the HF products. It was found that the population of vibrational quantum states of the HF products increases synchronously with the excitation of the reactant HD vibrationally.

Ultrafast Decay Dynamics of the 21ππ* Electronic State of N-Methyl-2-pyridone.

The ultrafast decay dynamics of N-methyl-2-pyridone upon excitation in the near-ultraviolet range of 261.5-227.9 nm is investigated using the femtosecond time-resolved photoelectron spectroscopy method. Irradiation at 261.5 nm prepares N-methyl-2-pyridone molecules with high vibrational levels in the 11ππ* state. The radiation-less decay to the ground state via internal conversion is suggested to be the dominant channel for the 11ππ* state with large vibrational excess energy, which is revealed by a lifetime of 1.6 ± 0.2 ps. As the pump wavelength decreases, we found that irradiation at 238.5 and 227.9 nm results in the population of the 21ππ* state. This is in agreement with the assignment of the vapor-phase UV absorption bands of N-methyl-2-pyridone. On the basis of the detailed analysis of our measured time-resolved photoelectron spectra at all pump wavelengths, we conclude that the 21ππ* state has an ultrashort lifetime of 50 ± 10 fs. In addition, the S1(11ππ*) state is subsequently populated via internal conversion and decays over a lifetime of 680-620 fs. The most probable whole deactivation pathway of the 21ππ* state is discussed. This experimental study provides new insights into the excitation energy-dependent decay dynamics of electronically excited N-methyl-2-pyridone.

Unraveling the Photodissociation Branching and Pathways of Methane at 118 nm by Imaging the CH3, CH2, and CH Fragments.

Under irradiation of a vacuum ultraviolet (VUV) photon, methane dissociates and yields multiple fragments. This photochemical behavior is not only of fundamental importance but also with wide-ranging implications in several branches of science. Despite that and numerous previous investigations, the product channel branching is still under debate, and the underlying dissociation mechanisms remain elusive. In this study, the photofragment imaging technique was exploited for the first time to map out the momentum and anisotropy parameter distributions of the CH3, CH2, and CH fragments at the 118 nm photolysis wavelength (10.48 eV photon energy). In conjunction with previously reported results of the H atom fragment at 121.6 nm (10.2 eV), a complete set of product channel branching in both two-body and three-body fragmentations is accurately determined. We concluded that extensive nonadiabatic transitions partake in the processes with two-body fragmentations accounting for more than 90% of overall photodissociation, for which the channel branching values for CH2 + H2 and CH3 + H are about 0.66 and 0.25, respectively. Careful kinematic analysis enables us to untangle the intertwined triple fragmentations into the CH2(X̃ 3B1 and ã 1A1) + H + H and CH(X2Π) + H + H2 channels and to evidence their underlying sequential (or stepwise) mechanisms. With the aid of electronic correlation and prior theoretical calculations of the potential energy surfaces, the most probable or dominant dissociation pathways are elucidated. Comparisons with fragmentary reports in the literature on various photochemical aspects are also documented, and discrepancies are clarified. This comprehensive study benchmarks the VUV photochemistry of methane and advances our understanding of this important process.

Reaction Kinetics of CH2OO and syn-CH3CHOO Criegee Intermediates with Acetaldehyde.

Criegee intermediates exert a crucial influence on atmospheric chemistry, functioning as powerful oxidants that facilitate the degradation of pollutants, and understanding their reaction kinetics is essential for accurate atmospheric modeling. In this study, the kinetics of CH2OO and syn-CH3CHOO reactions with acetaldehyde (CH3CHO) were investigated using a flash photolysis reaction tube coupled with the OH laser-induced fluorescence (LIF) method. The experimental results indicate that the reaction of syn-CH3CHOO with CH3CHO is independent of pressure in the range of 5-50 Torr when using Ar as the bath gas. However, the rate coefficient for the reaction between CH2OO and CH3CHO at 5.5 Torr was found to be lower compared to the near-constant values observed between 10 and 100 Torr. Furthermore, the reaction of syn-CH3CHOO with CH3CHO demonstrated positive temperature dependence from 283 to 330 K, with a rate coefficient of (2.11 ± 0.45) × 10-13 cm3 molecule-1 s-1 at 298 K. The activation energy and pre-exponential factor derived from the Arrhenius plot for this reaction were determined to be 2.32 ± 0.49 kcal mol-1 and (1.66 ± 0.61) × 10-11 cm3 molecule-1 s-1, respectively. In comparison, the reaction of CH2OO with CH3CHO exhibited negative temperature dependence, with a rate coefficient of (2.16 ± 0.39) × 10-12 cm3 molecule-1 s-1 at 100 Torr and 298 K and an activation energy and a pre-exponential factor of -1.73 ± 0.31 kcal mol-1 and (1.15 ± 0.21) × 10-13 cm3 molecule-1 s-1, respectively, over the temperature range of 280-333 K.

Vibronic coupling of Rhodamine 6G molecules studied by doubly resonant sum frequency generation spectroscopy with narrowband infrared and broadband visible.

Doubly resonant sum frequency generation (DR-SFG) serves as a potent characteristic technique for probing the electronic spectra and vibronic coupling of molecules on surfaces. In this study, we successfully developed a novel infrared (IR)-white light (WL) DR-SFG spectroscopy based on narrowband IR and tunable broadband WL. This novel method was employed to explore the excitation spectrum and vibronic couplings of sub-monolayer Rhodamine 6G molecules. Our findings elucidate that the xanthene skeleton vibrational modes exhibit strong coupling with the S0-S1 electronic transition. Notably, we observed not only the 0-0 transition of the S0-S1 electronic continuum but also the 0-1 transition, a first time observation in the realm of DR-SFG spectroscopy. This advanced DR-SFG spectroscopy methodology facilitates a more sensitive examination of electronic spectra and the coupling between electronic transitions and vibrational modes, heralding a significant advancement in the understanding of molecular interactions on surfaces.

Photodissociation of CO2 via the 1Πg state: Wavelength-dependent imaging studies of O(1D2) photoproducts.

Photodissociation of CO2 via the 1Πg state is investigated using a time-sliced velocity-mapped ion imaging apparatus combined with a tunable vacuum ultraviolet photolysis source. The main O(1D2) + CO(X1Σ+) channel is directly observed from the measured images of O(1D2) photoproducts at 129.08-134.76 nm. The total kinetic energy release spectra determined based on these images show that the energetic thresholds for the O(1D2) + CO(X1Σ+) photoproducts correspond to the thermochemical thresholds for the photodissociation of CO2(v2 = 0) and CO2(v2 = 1). One significant difference among the CO(X1Σ+, v) vibrational distributions for the predominant CO2(v2 = 0) dissociation is that the population of CO(v = 0) becomes favorable at 130.23-133.45 nm compared to the Boltzmann-like component (v > 0) that always exists at 129.08-134.76 nm. The wavelength dependences of the overall ß are found to follow the variation trend of the CO(v = 0) abnormal intensity. The vibrational state-specific ß values present a roughly decreasing trend with an increase in v, whereas ß(v = 0) appears to be significantly larger than ß(v = 1) at 130.23-133.45 nm compared to 134.76 and 129.08 nm. The non-statistical CO(v = 0) with larger ß values at 130.23-133.45 nm implies that an additional pathway may open through the conical intersection coupling to the dissociative 21A' state, except for the ever-existing pathway that yields the Boltzmann-like component. In contrast, at 129.08 nm, the restoration of the statistical equilibrium in the CO(X1Σ+, v) vibrational distribution may be caused by the emergence of novel dissociation pathways arising from the participation of the 31A″ state.

Photodissociation dynamics of SO2 via the G̃1B1 state: The O(1D2) and O(1S0) product channels.

Produced by both nature and human activities, sulfur dioxide (SO2) is an important species in the earth's atmosphere. SO2 has also been found in the atmospheres of other planets and satellites in the solar system. The photoabsorption cross sections and photodissociation of SO2 have been studied for several decades. In this paper, we reported the experimental results for photodissociation dynamics of SO2 via the G̃1B1 state. By analyzing the images from the time-sliced velocity map ion imaging method, the vibrational state population distributions and anisotropy parameters were obtained for the O(1D2) + SO(X3Σ-, a1Δ, b1Σ+) and O(1S0) + SO(X3Σ-) channels, and the branching ratios for the channels O(1D2) + SO(X3Σ-), O(1D2) + SO(a1Δ), and O(1D2) + SO(b1Σ+) were determined to be ∼0.3, ∼0.6, and ∼0.1, respectively. The SO products were dominant in electronically and rovibrationally excited states, which may have yet unrecognized roles in the upper planetary atmosphere.

Vibrational state-specific nonadiabatic photodissociation dynamics of OCS+ via A2Π1/2 (ν1 0 ν3) states.

The identification and analysis of quantum state-specific effects can significantly deepen our understanding of detailed photodissociation dynamics. Here, we report an experimental investigation on the vibrational state-mediated photodissociation of the OCS+ cation via the A2Π1/2 (ν1 0 ν3) states by using the velocity map ion imaging technique over the photolysis wavelength range of 263-294 nm. It was found that the electronically excited S+ product channel S+(2Du) + CO (X1Σ+) was significantly enhanced when the ν1 and ν3 vibrational modes were excited. Clear deviations in the branching ratios of the electronically excited S+ channel were observed when the vibrational modes ν1 and ν3 were selectively excited. The results reveal that vibrationally excited states play a vital role in influencing the nonadiabatic couplings in the photodissociation process.

Formation of Supernarrow Borophene Nanoribbons.

Borophenes have sparked considerable interest owing to their fascinating physical characteristics and diverse polymorphism. However, borophene nanoribbons (BNRs) with widths less than 2 nm have not been achieved. Herein, we report the experimental realization of supernarrow BNRs. Combining scanning tunneling microscopy imaging with density functional theory modeling and ab initio molecular dynamics simulations, we demonstrate that, under the applied growth conditions, boron atoms can penetrate the outermost layer of Au(111) and form BNRs composed of a pair of zigzag (2,2) boron rows. The BNRs have a width self-contained to ∼1 nm and dipoles at the edges to keep them separated. They are embedded in the outermost Au layer and shielded on top by the evacuated Au atoms, free of the need for post-passivation. Scanning tunneling spectroscopy reveals distinct edge states, primarily attributed to the localized spin at the BNRs' zigzag edges. This work adds a new member to the boron material family and introduces a new physical feature to borophenes.

Ultraviolet Photodissociation Reveals the Molecular Mechanism of Crown Ether Microsolvation Effect on the Gas-Phase Native-like Protein Structure.

Maintaining the protein high-order structures and interactions during the transition from aqueous solution to gas phase is essential to the structural analysis of native mass spectrometry (nMS). Herein, we systematically interrogate the effects of charge state and crown ether (CE) complexation on the gas-phase native-like protein structure by integrating nMS with 193 nm ultraviolet photodissociation (UVPD). The alterations of photofragmentation yields of protein residues and the charge site distribution of fragment ions reveal the specific sites and sequence regions where charge and CE take effect. Our results exhibit the CE complexation on protonated residues can largely alleviate the structure disruption induced by the intramolecular solvation of charged side chains. The influences of CE complexation and positive charge on gas-phase protein structure exhibit generally opposite trends because the CE microsolvation avoids the hydrogen-bonding formation between the charged side chains with backbone carbonyls. Thus, CE complexation leads to a more stable and native-like protein structure in the gas phase.

Structural Mass Spectrometry Probes the Inhibitor-Induced Allosteric Activation of CDK12/CDK13-Cyclin K Dissociation.

The rational design and development of effective inhibitors for cyclin-dependent kinases 12 and 13 (CDK12 and CDK13) are largely dependent on the understanding of the dynamic inhibition conformations but are difficult to be achieved by conventional characterization tools. Herein, we integrate the structural mass spectrometry (MS) methods of lysine reactivity profiling (LRP) and native MS (nMS) to systematically interrogate both the dynamic molecular interactions and overall protein assembly of CDK12/CDK13-cyclin K (CycK) complexes under the modulation of small molecule inhibitors. The essential structure insights, including inhibitor binding pocket, binding strength, interfacial molecular details, and dynamic conformation changes, can be derived from the complementary results of LRP and nMS. We find the inhibitor SR-4835 binding can greatly destabilize the CDK12/CDK13-CycK interactions in an unusual allosteric activation way, thereby providing a novel alternative for the kinase activity inhibition. Our results underscore the great potential of LRP combination with nMS for the evaluation and rational design of effective kinase inhibitors at the molecular level.

Molecular Structure Characterization of Micro/Nanoplastics by 193 nm Ultraviolet Photodissociation Mass Spectrometry.

The degradation of macroplastics results in micro/nanoplastics (MNPs) in the natural environment, inducing high health risks worldwide. It remains challenging to characterize the accurate molecular structures of MNPs. Herein, we integrate 193 nm ultraviolet photodissociation (UVPD) with mass spectrometry to interrogate the molecular structures of poly(ethylene glycol) terephthalate and polyamide (PA) MNPs. The backbones of the MNP polymer can be efficiently dissociated by UVPD, producing rich types of fragment ions. Compared to high-energy collision dissociation (HCD), the structural informative fragment ions and corresponding sequence coverages obtained by UVPD were all improved 2.3 times on average, resulting in almost complete sequence coverage and precise structural interrogation of MNPs. We successfully determine the backbone connectivity differences of MNP analogues PA6, PA66, and PA610 by improving the average sequence coverage from 26.8% by HCD to 89.4% by UVPD. Our results highlight the potential of UVPD in characterizing and discriminating backbone connectivity and chain end structures of different types of MNPs.

The simplest Criegee intermediate CH2OO reaction with dimethylamine and trimethylamine: kinetics and atmospheric implications.

We have used the OH laser-induced fluorescence (LIF) method to measure the kinetics of the simplest Criegee intermediate (CH2OO) reacting with two abundant amines in the atmosphere: dimethylamine ((CH3)2NH) and trimethylamine ((CH3)3N). Our experiments were conducted under pseudo-first-order approximation conditions. The rate coefficients we report are (2.15 ± 0.28) × 10-11 cm3 molecule-1 s-1 for (CH3)2NH at 298 K and 10 Torr, and (1.56 ± 0.23) × 10-12 cm3 molecule-1 s-1 for (CH3)3N at 298 K and 25 Torr with Ar as the bath gas. Both reactions exhibit a negative temperature dependence. The activation energy and pre-exponential factors derived from the Arrhenius equation were (-2.03 ± 0.26) kcal mol-1 and (6.89 ± 0.90) × 10-13 cm3 molecule-1 s-1 for (CH3)2NH, and (-1.60 ± 0.24) kcal mol-1 and (1.06 ± 0.16) × 10-13 cm3 molecule-1 s-1 for (CH3)3N. We propose that the electronegativity of the atom in the co-reactant attached to the C atom of CH2OO, in addition to the dissociation energy of the fragile covalent bonds with H atoms (H-X bond), plays an important role in the 1,2-insertion reactions. Under certain circumstances, the title reactions can contribute to the sink of amines and Criegee intermediates and to the formation of secondary organic aerosol (SOA).

Excitation wavelength dependent S1-state decay dynamics of 2-aminopyridine and 3-aminopyridine.

The decay dynamics of 2-aminopyridine and 3-aminopyridine excited to the S1 state is investigated using femtosecond time-resolved photoelectron imaging. The lifetime of the S1 state for both molecules shows a rapid decrease with the increase of the vibrational energy. It is shown that, besides intersystem crossing to the lower-lying triplet state of T1, the decay to the ground state (S0) via internal conversion through a conical intersection plays an increasingly important role and becomes dominant for vibrational states well above the S1 state origin. The comparison between 2-aminopyridine and 3-aminopyridine suggests that the intramolecular hydrogen bonding between a hydrogen atom of the NH2 group and the heterocyclic nitrogen atom in 2-aminopyridine effectively hinders the ring deformation at lower vibrational states which is required for the wavepacket to reach the S1/S0 conical intersection, and therefore slows down the S1 to S0 internal conversion.

Slice imaging study of NO2 photodissociation via the 12B2 and 22B2 states: the NO(X2Π) + O(3PJ) product channel.

The state-resolved photodissociation of NO2via the 12B2 and 22B2 excited states has been investigated by using time-sliced velocity-mapped ion imaging technique. The images of the O(3PJ=2,1,0) products at a series of excitation wavelengths are measured by employing a 1 + 1' photoionization scheme. The total kinetic energy release (TKER) spectra, NO vibrational state distributions and anisotropy parameters (ß) are derived from the O(3PJ=2,1,0) images. For the 12B2 state photodissociation of NO2, the TKER spectra mainly present a non-statistical vibrational state distribution of the NO co-products, and the profiles of most vibrational peaks display a bimodal structure. The ß values show a gradual decrease with the photolysis wavelength increasing except for a sudden increase at 357.38 nm. The results suggest that the NO2 photodissociation via the 12B2 state proceeds via the non-adiabatic transition between the 12B2 and X̃2A1 states, leading to the NO(X2Π) + O(3PJ) products with wavelength-dependent rovibrational distributions. As for photodissociation of NO2via the 22B2 state, the NO vibrational state distribution is relatively narrow with the main peak shifting from v = 1, 2 at 235.43-249.22 nm to v = 6 at 212.56 nm. The ß values exhibit two distinctly different angular distributions, i.e., near isotropic at 249.22 and 246.09 nm and anisotropic at the rest of the excitation wavelengths. These results are consistent with the fact that the 22B2 state potential energy surface has a barrier, and the dissociation process is fast when the initial populated level is above this barrier. A bimodal vibrational state distribution is clearly observed at 212.56 nm, in which the main distribution (peaking at v = 6) is ascribed to dissociation via an avoided crossing with the higher electronically excited state while the subsidiary distribution (peaking at v = 11) likely arises due to dissociation via the internal conversion to the 12B2 state or to the X̃ ground state.