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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Probing C–I bond fission in the UV photochemistry of 2-iodothiophene with core-to-valence transient absorption spectroscopy

The UV photochemistry of small heteroaromatic molecules serves as a testbed for understanding fundamental photo-induced chemical transformations in moderately complex compounds, including isomerization, ring-opening, and molecular dissociation. Here, a combined experimental-theoretical study of 268 nm UV light-induced dynamics in 2-iodothiophene (C 4 H 3 IS) is performed. The dynamics are experimentally monitored with a femtosecond extreme ultraviolet (XUV) probe that measures iodine N-edge 4d core-to-valence transitions. Experiments are complemented by density functional theory calculations of both the pump-pulse induced valence excitations and the XUV probe-induced core-to-valence transitions. Possible intramolecular relaxation dynamics are investigated by ab initio molecular dynamics simulations. Furthermore, gradual absorption changes up to ~0.5 to 1 ps after excitation are observed for both the parent molecular species and emerging iodine fragments, with the latter appearing with a characteristic rise time of 160 ± 30 fs. Comparison of spectral intensities and energies with the calculations identifies an iodine dissociation pathway initiated by a predominant π → π* excitation. In contrast, initial excitation to a nearby n ⟂ → σ* state appears unlikely based on a significantly smaller oscillator strength and the absence of any corresponding XUV absorption signatures. Excitation to the π → π* state is followed by contraction of the C–I bond, enabling a nonadiabatic transition to a dissociative π→σ C-I * state. For the subsequent fragmentation, a relatively narrow bond-length region along the C–I stretch coordinate between 230 and 280 pm is identified, where the transition between the parent molecule and the thienyl radical + iodine atom products becomes prominent in the XUV spectrum due to rapid localization of two singly occupied molecular orbitals on the two fragments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Electronic Absorption Spectroscopy and Photochemistry of Criegee Intermediates

Interest in Criegee intermediates (CIs), often termed carbonyl oxides, and their role in tropospheric chemistry has grown massively since the demonstration of laboratory-based routes to their formation and characterization in the gas phase. This article reviews current knowledge regarding the electronic spectroscopy of atmospherically relevant CIs like CH 2 OO, CH 3 CHOO, (CH 3 ) 2 COO and larger CIs like methyl vinyl ketone oxide and methacrolein oxide that are formed in the ozonolysis of isoprene, and of selected conjugated carbene-derived CIs of interest in the synthetic chemistry community. Of the aforementioned atmospherically relevant CIs, all except CH 2 OO and (CH 3 ) 2 COO exist in different conformers which, under tropospheric conditions, can display strikingly different thermal loss rates via unimolecular and bimolecular processes. Calculated photolysis rates based on their absorption properties suggest that solar photolysis will rarely be a significant contributor to the total loss rate for any CI under tropospheric conditions. Nonetheless, there is ever-growing interest in the absorption cross sections and primary photochemistry of CIs following excitation to the strongly absorbing 1 ππ* state, and how this varies with CI, with conformer and with excitation wavelength. Furthermore, the later part of this review surveys the photochemical data reported to date, including a range of studies that demonstrate prompt photo-induced fission of the terminal O–O bond, and speculates about possible alternate decay processes that could occur following non-adiabatic coupling to, and dissociation from, highly internally excited levels of the electronic ground state of a CI.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

The emergent photophysics and photochemistry of molecular polaritons: a theoretical and computational investigation (Final Technical Report)

When molecules are placed between closely spaced mirrors, they interact strongly with the photons that are trapped between them, generating new quantum states which are no longer exclusively material nor photonic alone, but rather, coherent superpositions of both. These hybrid states are known as molecular polaritons, given that they arise from the strong interaction between the electric field of light and the electrical polarization of the molecules. Recently, experimental advances in nano‐ and microfabrication of molecular polariton architectures have successfully demonstrated their feasibility to control the rate and outcome of a certain class of chemical reactions in condensed phases. Importantly, these reactions proceed in strongly dissipative environments such as liquid solvents and lossy mirrors that allow for photons to escape from their confinement. The purpose of this research is to formulate quantum mechanical theories and computational tools that can elucidate the origin of these intriguing phenomena and simultaneously predict capabilities that this new generation of molecular materials affords. Attention is placed on harnessing polaritons to carry out photophysics and photochemistry that challenge currently existing paradigms, such as the optimization of energy conversion processes in organic solar cell or light‐emitting devices, or unconventional phenomena such as long‐range excitation energy transfer, remote control of chemical reactions, and a new quantum mechanical regime of chemical reactivity due to wavefunction overlaps amongst a large number of molecular polaritons (Bose condensation). This research explores a frontier of Chemistry and Physics where electrons, vibrations, and photons interact strongly with each other to generate emergent behavior that can be creatively exploited to address contemporary challenges in Basic Energy Sciences.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Imaging the photochemistry of cyclobutanone using ultrafast electron diffraction: Experimental results

We investigated the ultrafast structural dynamics of cyclobutanone following photoexcitation at λ = 200 nm using gas-phase megaelectronvolt ultrafast electron diffraction. Our investigation complements the simulation studies of the same process within this special issue. It provides information about both electronic state population and structural dynamics through well-separable inelastic and elastic electron scattering signatures. We observe the depopulation of the photoexcited S 2 state of cyclobutanone with n3s Rydberg character through its inelastic electron scattering signature with a time constant of (0.29 ± 0.2) ps toward the S 1 state. The S 1 state population undergoes ring-opening via a Norrish Type-I reaction, likely while passing through a conical intersection with S 0 . The corresponding structural changes can be tracked by elastic electron scattering signatures. These changes appear with a delay of (0.14 ± 0.05) ps with respect to the initial photoexcitation, which is less than the S 2 depopulation time constant. This behavior provides evidence for the ballistic nature of the ring-opening once the S 1 state is reached. The resulting biradical species react further within (1.2 ± 0.2) ps via two rival fragmentation channels yielding ketene and ethylene, or propene and carbon monoxide. Furthermore, our study showcases the value of both gas-phase ultrafast diffraction studies as an experimental benchmark for nonadiabatic dynamics simulation methods and the limits in the interpretation of such experimental data without comparison with such simulations.

Carbon monoxide↗
DOE OSTI2023

Influence of Linker Identity on the Photochemistry of Uranyl-Organic Frameworks

While uranyl-based metal–organic frameworks (MOFs) boast impressive photocatalytic abilities, significant questions remain regarding their excitation pathways and methods to fine-tune their performance due to the lack of information regarding heterogeneous uranyl catalysis. Herein, we investigated how linker identity and photoexcitation impact uranyl photocatalysis when the uranyl coordination environment remains constant. Toward this end, we prepared three uranyl-based MOFs (NU-1301, NU-1307, and ZnTCPP-U2) and then examined the structural and photochemical properties of each through X-ray diffraction, X-ray absorption, and photoluminescence. We then correlated our observations to the photocatalytic performance for fluorination of cyclooctane. The excitation profile from NU-1301 and NU-1307 exhibited spin-forbidden linker transitions and uranyl vibronic progressions, with uranyl excitation and emission being most dominant in NU-1301. Consequently, NU-1301 was a more active photocatalyst than NU-1307. In contrast, the excitation profile from ZnTCPP-U2 contained transitions associated with the porphyrin linker exclusively. Photocatalytic activity from ZnTCPP-U2 significantly underperformed in comparison to that of the other two MOFs. Furthermore, these data suggest that linkers’ photophysical properties can be used to predict the photocatalytic behavior of uranyl-containing MOFs.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Challenges in the study of chemistry and photochemistry at air–water interfaces: Toward in situ monitoring of reaction kinetics with spectroscopic techniques

Air–water interfaces, including those on droplet surfaces, have been the subject of many recent experiments due to their propensity for unique chemistry. Here, in this study, an overview of some recent advancements in understanding interfacial reaction kinetics is provided, highlighting non-surface-specific methods—such as mass spectrometry—compared with the advantages of surface-specific techniques—such as reflection–absorption spectroscopy, sum frequency generation, and photoelectron spectroscopy. This Perspective discusses the information depth and time constraints of common surface-specific spectroscopic methods that need to be addressed to monitor interfacial chemistry in situ and uses a few key examples from the literature as case studies. It concludes by advocating for the continued development of advanced spectroscopic methods to further investigate interfacial chemistry, underscoring the need for interdisciplinary collaboration to bridge the gap between molecular-level insights and macroscopic observations in future research.

Air-water interface↗
DOE OSTI2020

Origin of the “odd” behavior in the ultraviolet photochemistry of ozone

Significance This study solves a 30-y mystery about the origin of dramatic oscillations in the quantum product states of 16 O 3 upon photodissociation. Ozone is a key component of the atmosphere and undergoes complex cycles of formation and destruction by photodissociation and reaction. These dynamics are a rich proving ground for theories of quantum reaction dynamics as well as subtleties such as nuclear spin-statistics and symmetry. A previous theory constructed to explain the observed oscillations, although plausible, was not fully consistent with experimental measurements. Our quantum model and calculations show conclusively the origin of the effect, and match new more detailed experiments.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Understanding and controlling the fundamental photochemistry of protonic solar energy conversion (Final Scientific/Technical Report)

The selectivity and activity of most fuel-forming reactions are tunable by the concentration of electrons, holes, protons, and/or hydroxides. That is one reason why traditional solar energy conversion processes are used to drive these reactions, where light directly increases the concentration of electrons and holes. We, instead, took a different approach, using light to increase the concentration of protons and/or hydroxides. For this, we leveraged photophysical and solid-state physics theories, techniques, and design strategies previously developed for the study of traditional solar energy conversion processes. We designed and fabricated several materials platforms based on molecular photoacid and photobase dyes coupled with ion-exchange polymer membranes and assessed their fundamental photoelectrochemistry. We demonstrated control over built-in electric potentials, photovoltages, proton-transfer kinetics, and efficiency limits to our approach. Understanding the basic science of these dye-sensitized protonic membranes and their photochemical mechanisms is of use to the DOE-relevant processes of solar photochemical fuel formation, solar photodialytic desalination, and solar cells. Moreover, functional materials that we developed from this project may be of use to several broad-reaching fields and may provide the foundation for a completely new, inexpensive, and robust solar energy conversion technology.

14 SOLAR ENERGY↗
DOE OSTI2025

Photochemistry of Hypervalent Iodoazide Derivatives

The photochemical properties and reaction mechanisms of a series of hypervalent iodoazide compounds (R–IN3) were investigated, with substituents (−CH 3 , −H, −CF 3 ) tuning the electronic density of the phenyl ring. With the help of UV irradiation, ultrafast time-resolved spectroscopy, and density functional theory (DFT) calculations, we elucidated the mechanisms of azide radical (N 3 • ) release and its subsequent reactivity. UV–vis spectroscopy reveals that the photoconversion rates follow the trend CH3 > H > CF3, aligning with DFT-calculated ΔG values for ring-opening transitions. Homolytic cleavage of the I–N bond is identified as the dominant pathway for N3• generation upon UV irradiation, occurring within 400 fs. The released azide radicals react with the solvent molecules, while the iodo radical R–I C • fragments undergo a thermodynamically uphill lactone ring-opening (RO) reaction and subsequent hydrogen atom abstraction from the solvent to form carboxylic acids R–I–COOH, as validated by NMR and IR spectroscopy. The study also highlights the role of substituents in influencing reaction kinetics and intermediate stability, with electron-donating groups accelerating the release of the N 3 • species. This work bridges experimental observations with computational predictions, offering a foundation for future advancements in azide-based reactions and materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

The Photochemistry of Amino Acids Produced on the Polar Cryovolcanic Regions of Titan

The cryovolcanic regions of Titan offer transient opportunities for prebiotic molecules to exist in water–ammonia solutions on the surface of the Saturnian moon. The upcoming NASA’s Dragonfly mission will search for high nitrogen concentrations and amino acids on Titan’s equatorial terrains. Cryovolcanic features, however, are most common on the polar regions. To mitigate the distance, bubble bursting may encapsulate the prebiotic molecules into aerosols, which Titan’s Hadley circulation would subsequently transport to the equator. Here we investigate whether alanine and glycine survive this meridional journey. Despite the unconstrained meridional wind velocities, our results suggest that the amino acids can survive the transport through the mesosphere. Dragonfly may find cryo-volcanogenic amino acids on Titan’s equator. Further, the interaction between the two amino acids increased 10-fold the photodegradation rate of glycine. We justify it based on changes in the environment polarity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Singly Reduced Iridium Chromophores: Synthesis, Characterization, and Photochemistry

One-electron reduced photosensitizers have been invoked as crucial intermediates in photoredox catalysis, including multiphoton excitation and electrophotocatalytic processes. However, such reduced chromophores have been less investigated, limiting mechanistic studies of their associated electron transfer processes. Here, we report a total of 11 different examples of isolable singly reduced iridium chromophores. Chemical reduction of a cyclometalated iridium complex with potassium graphite affords a 19-electron species. Structural and spectroscopic characterizations reveal a ligand-centered reduction product. The reduced chromophore absorbs a wide range of light from ultraviolet to near-infrared and exhibits photoinduced bimolecular electron transfer reactivity. Furthermore, these studies shed light on elusive reduced iridium chromophores in both ground and excited states, providing opportunities to investigate a commonly invoked intermediate in photoredox catalysis.

14 SOLAR ENERGY↗
DOE OSTI2024

Bond Selective Photochemistry at Metal Nanoparticle Surfaces: CO Desorption from Pt and Pd

The use of visible photon fluxes to influence catalytic reactions on metal nanoparticle surfaces has attracted attention based on observations of reaction mechanisms and selectivity not observed under equilibrium heating. These observations suggest that photon fluxes can selectively impact the rates of certain elementary steps, creating nonequilibrium energy distributions among various reaction pathways. However, quantitative studies validating these hypotheses on metal nanoparticle surfaces are lacking. Here, we examine the influence of continuous wave visible photon fluxes on the CO desorption rates from 1 to 2 nm diameter Pt and Pd nanoparticle surfaces supported on γ-Al 2 O 3 . Temperature-programmed desorption measurements quantified via diffuse reflectance infrared Fourier transform spectroscopy demonstrate that visible photon fluxes significantly enhanced the rate of CO desorption from Pt nanoparticles in a wavelength-dependent manner. 440 nm photons most efficiently promoted CO desorption from Pt nanoparticle surfaces, aligning with the excitation energy for the interfacial electronic transition within the Pt–CO bond. Conversely, visible photon fluxes had no measurable influence on CO desorption rates from Pd nanoparticle surfaces after accounting for photon-induced heating. Density functional theory calculations demonstrate that the Pt–CO bond exhibits a narrower LUMO resonance, stronger coupling between the photoexcitation and forces induced on the metal–C bond, and vibrational energy dissipation that more effectively couples to desorption as compared to Pd–CO. These results demonstrate the specificity photons provide in facilitating chemical reactions on metal nanoparticle surfaces and substantiate the idea that photon fluxes can steer processes and outcomes of catalytic reactions in ways not achievable by equilibrium heating.

desorption↗