Reactive Radical Etching of Quartz by Microwave Activated CH/H Plasmas Promotes Gas Phase Nanoparticle Formation.

An attenuation of visible probe radiation identified in earlier absorption studies of microwave plasma-activated CH/H/Ar gas mixtures is shown to arise from nanoparticles in under-pumped regions on opposing sides of a reactor used for diamond chemical vapor deposition. The present modeling studies highlight (i) ejection of Si-containing species into the gas phase by reactive radical etching of the quartz window through which the microwave radiation enters the reactor, enabled by suitably high window temperatures () and the synergistic action of near-window H atoms and CH radicals; (ii) subsequent processing of the ejected material, some of which are transported to and accumulate in stagnation regions in the entrance to the reactor side arms; and (iii) the importance of Si in facilitating homogeneous gas phase nucleation, clustering, and nanoparticle growth in these regions. The observed attenuation, its probe wavelength dependence, and its variations with changes in process conditions can all be rationalized by a combination of absorption and scattering contributions from Si/C/H containing nanoparticles with diameters in the range of 50-100 nm.

Quantum State-Dependent Fragmentation Dynamics of DS Molecules Following Excitation at Wavelengths ∼ 129.1 and ∼ 139.1 nm.

The first high-resolution translational spectroscopy studies of D atom photoproducts following excitation to the Rydberg states of DS are reported. Excitation at wavelengths λ ∼ 139.1 nm reveals an unusual 'inverse' isotope effect; the B(3←2) Rydberg state of DS predissociates much faster than its counterpart in HS.

The Role of Momentum Partitioning in Covariance Ion Imaging Analysis.

We present results from a covariance ion imaging study, which employs extensive filtering, on the relationship between fragment momenta to gain deeper insight into photofragmentation dynamics. A new data analysis approach is introduced that considers the momentum partitioning between the fragments of the breakup of a molecular polycation to disentangle concurrent fragmentation channels, which yield the same ion species. We exploit this approach to examine the momentum exchange relationship between the products, which provides direct insight into the dynamics of molecular fragmentation.

Unraveling the Rich Fragmentation Dynamics Associated with S-H Bond Fission Following Photoexcitation of HS at Wavelengths ∼129.1 nm.

HS is being detected in the atmospheres of ever more interstellar bodies, and photolysis is an important mechanism by which it is processed. Here, we report H Rydberg atom time-of-flight measurements following the excitation of HS molecules to selected rotational (') levels of the B Rydberg state associated with the strong absorption feature at wavelengths of λ ∼ 129.1 nm.

Exploring the ultrafast and isomer-dependent photodissociation of iodothiophenes site-selective ionization.

C-I bond extension and fission following ultraviolet (UV, 262 nm) photoexcitation of 2- and 3-iodothiophene is studied using ultrafast time-resolved extreme ultraviolet (XUV) ionization in conjunction with velocity map ion imaging. The photoexcited molecules and eventual I atom products are probed by site-selective ionization at the I 4d edge using intense XUV pulses, which induce multiple charges initially localized to the iodine atom. At C-I separations below the critical distance for charge transfer (CT), charge can redistribute around the molecule leading to Coulomb explosion and charged fragments with high kinetic energy.

Valence shell electronically excited states of norbornadiene and quadricyclane.

The absolute photoabsorption cross sections of norbornadiene (NBD) and quadricyclane (QC), two isomers with chemical formula C7H8 that are attracting much interest for solar energy storage applications, have been measured from threshold up to 10.8 eV using the Fourier transform spectrometer at the SOLEIL synchrotron radiation facility. The absorption spectrum of NBD exhibits some sharp structure associated with transitions into Rydberg states, superimposed on several broad bands attributable to valence excitations.

Monitoring the Evolution of Relative Product Populations at Early Times during a Photochemical Reaction.

Identifying multiple rival reaction products and transient species formed during ultrafast photochemical reactions and determining their time-evolving relative populations are key steps toward understanding and predicting photochemical outcomes. Yet, most contemporary ultrafast studies struggle with clearly identifying and quantifying competing molecular structures/species among the emerging reaction products. Here, we show that mega-electronvolt ultrafast electron diffraction in combination with molecular dynamics calculations offer a powerful route to determining populations of the various isomeric products formed after UV (266 nm) excitation of the five-membered heterocyclic molecule 2(5)-thiophenone.

Ultrafast electronic relaxation pathways of the molecular photoswitch quadricyclane.

The light-induced ultrafast switching between molecular isomers norbornadiene and quadricyclane can reversibly store and release a substantial amount of chemical energy. Prior work observed signatures of ultrafast molecular dynamics in both isomers upon ultraviolet excitation but could not follow the electronic relaxation all the way back to the ground state experimentally. Here we study the electronic relaxation of quadricyclane after exciting in the ultraviolet (201 nanometres) using time-resolved gas-phase extreme ultraviolet photoelectron spectroscopy combined with non-adiabatic molecular dynamics simulations.

Exploring the vacuum ultraviolet photochemistry of astrochemically important triatomic molecules.

The recently constructed vacuum ultraviolet (VUV) free electron laser (FEL) at the Dalian Coherent Light Source (DCLS) is yielding a wealth of new and exquisitely detailed information about the photofragmentation dynamics of many small gas-phase molecules. This Review focuses particular attention on five triatomic molecules-HO, HS, CO, OCS and CS. Each shows excitation wavelength-dependent dissociation dynamics, yielding photofragments that populate a range of electronic and (in the case of diatomic fragments) vibrational and rotational quantum states, which can be characterized by different translational spectroscopy methods.

Vacuum ultraviolet photodissociation of sulfur dioxide and its implications for oxygen production in the early Earth's atmosphere.

The emergence of molecular oxygen (O) in the Earth's primitive atmosphere is an issue of major interest. Although the biological processes leading to its accumulation in the Earth's atmosphere are well understood, its abiotic source is still not fully established. Here, we report a new direct dissociation channel yielding S(D) + O(aΔ/XΣ) products from vacuum ultraviolet (VUV) photodissociation of SO in the wavelength range between 120 and 160 nm.

Molecular photodissociation dynamics revealed by Coulomb explosion imaging.

Coulomb explosion imaging (CEI) methods are finding ever-growing use as a means of exploring and distinguishing the static stereo-configurations of small quantum systems (molecules, clusters, ). CEI experiments initiated by ultrafast (femtosecond-duration) laser pulses also allow opportunities to track the time-evolution of molecular structures, and thereby advance understanding of molecular fragmentation processes. This Perspective illustrates two emerging families of dynamical studies.

Agro-climatic sensitivity analysis for sustainable crop diversification; the case of Proso millet (Panicum miliaceum L.).

Current agricultural production depends on very limited species grown as monocultures that are highly vulnerable to climate change, presenting a threat to the sustainability of agri-food systems. However, many hundreds of neglected crop species have the potential to cater to the challenges of climate change by means of resilience to adverse climate conditions. Proso millet (Panicum miliaceum L.

The vibronic state dependent predissociation of HS: determination of all fragmentation processes.

Photochemistry plays a significant role in shaping the chemical reaction network in the solar nebula and interstellar clouds. However, even in a simple triatomic molecule photodissociation, determination of all fragmentation processes is yet to be achieved. In this work, we present a comprehensive study of the photochemistry of HS, derived from cutting-edge translational spectroscopy measurements of the H, S(D) and S(S) atom products formed by photolysis at wavelengths across the range 155-120 nm.

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

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). 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.

Comparing the Excited State Dynamics of CHOO, the Simplest Criegee Intermediate, Following Vertical versus Adiabatic Excitation.

molecular dynamics studies of CHOO molecules following excitation to the minimum-energy geometry of the strongly absorbing S (ππ*) state reveal a much richer range of behaviors than just the prompt O-O bond fission, with unity quantum yield and retention of overall planarity, identified in previous vertical excitation studies from the ground (S) state. Trajectories propagated for 100 fs from the minimum-energy region of the S state show a high surface hopping (nonadiabatic coupling) probability between the near-degenerate S and S (nπ*) states at geometries close to the S minimum, which enables population transfer to the optically dark S state. Greater than 80% of the excited population undergoes O-O bond fission on the S or S potential energy surfaces (PESs) within the analysis period, mostly from nonplanar geometries wherein the CH moiety is twisted relative to the COO plane.

Large-scale reforestation can increase water yield and reduce drought risk for water-insecure regions in the Asia-Pacific.

Large-scale reforestation can potentially bring both benefits and risks to the water cycle, which needs to be better quantified under future climates to inform reforestation decisions. We identified 477 water-insecure basins worldwide accounting for 44.6% (380.

Probing the electronic structure and photophysics of thiophene-diketopyrrolopyrrole derivatives in solution.

Diketopyrrolopyrroles are a popular class of electron-withdrawing unit in optoelectronic materials. When combined with electron donating side-chain functional groups such as thiophenes, they form a very broad class of donor-acceptor molecules: thiophene-diketopyrrolopyrroles (TDPPs). Despite their widescale use in biosensors and photovoltaic materials, studies have yet to establish the important link between the electronic structure of the specific TDPP and the critical optical properties.

Correction: Multi-mass velocity map imaging study of the 805 nm strong field ionization of CFI.

Correction for 'Multi-mass velocity map imaging study of the 805 nm strong field ionization of CFI' by Stuart W. Crane , , 2022, DOI: https://doi.org/10.

Multi-mass velocity map imaging study of the 805 nm strong field ionization of CFI.

Multi-mass velocity map imaging studies of charged fragments formed by near infrared strong field ionization together with covariance map image analysis offer a new window through which to explore the dissociation dynamics of several different highly charged parent cations, simultaneously - as demonstrated here for the case of CFI cations with charges ranging from 1 to at least 5. Previous reports that dissociative ionization of CFI cations yields CF, I and CFI fragment ions are confirmed, and some of the CF fragments are deduced to undergo secondary loss of one or more neutral F atoms. Covariance map imaging confirms the dominance of CF + I products in the photodissociation of CFI cations and, again, that some of the primary CF photofragments can shed one or more F atoms.

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 OO, CH CHOO, (CH ) 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 OO and (CH ) COO exist in different conformers which, under tropospheric conditions, can display strikingly different thermal loss rates via unimolecular and bimolecular processes.

Non-Born-Oppenheimer effects in molecular photochemistry: an experimental perspective.

Non-adiabatic couplings between Born-Oppenheimer (BO)-derived potential energy surfaces are now recognized as pivotal in describing the non-radiative decay of electronically excited molecules following photon absorption. This opinion piece illustrates how non-BO effects provide photostability to many biomolecules when exposed to ultraviolet radiation, yet in many other cases are key to facilitating 'reactive' outcomes like isomerization and bond fission. The examples are presented in order of decreasing molecular complexity, spanning studies of organic sunscreen molecules in solution, through two families of heteroatom containing aromatic molecules and culminating with studies of isolated gas phase HO molecules that afford some of the most detailed insights yet available into the cascade of non-adiabatic couplings that enable the evolution from photoexcited molecule to eventual products.

Towards developing novel and sustainable molecular light-to-heat converters.

Light-to-heat conversion materials generate great interest due to their widespread applications, notable exemplars being solar energy harvesting and photoprotection. Another more recently identified potential application for such materials is in molecular heaters for agriculture, whose function is to protect crops from extreme cold weather and extend both the growing season and the geographic areas capable of supporting growth, all of which could help reduce food security challenges. To address this demand, a new series of phenolic-based barbituric absorbers of ultraviolet (UV) radiation has been designed and synthesised in a sustainable manner.

Rotational and nuclear-spin level dependent photodissociation dynamics of HS.

The detailed features of molecular photochemistry are key to understanding chemical processes enabled by non-adiabatic transitions between potential energy surfaces. But even in a small molecule like hydrogen sulphide (HS), the influence of non-adiabatic transitions is not yet well understood. Here we report high resolution translational spectroscopy measurements of the H and S(D) photoproducts formed following excitation of HS to selected quantum levels of a Rydberg state with B electronic symmetry at wavelengths λ ~ 139.