Refining the thermochemical properties of CF, SiF, and their cations by combining photoelectron spectroscopy, quantum chemical calculations, and the Active Thermochemical Tables approach.

Fluorinated species have a pivotal role in semiconductor material chemistry and some of them have been detected beyond the Earth's atmosphere. Achieving good energy accuracy on fluorinated species using quantum chemical calculations has long been a challenge. In addition, obtaining direct experimental thermochemical quantities has also proved difficult. Here, we report the threshold photoelectron and photoion yield spectra of SiF and CF radicals generated with a fluorine reactor. The spectra were analysed with the support of ab initio calculations, resulting in new experimental values for the adiabatic ionisation energies of both CF (9.128 ± 0.006 eV) and SiF (7.379 ± 0.009 eV). Using these values, the underlying thermochemical network of Active Thermochemical Tables was updated, providing further refined enthalpies of formation and dissociation energies of CF, SiF, and their cationic counterparts.

Solvated dielectrons from optical excitation: An effective source of low-energy electrons.

Low-energy electrons dissolved in liquid ammonia or aqueous media are powerful reducing agents that promote challenging reduction reactions but can also cause radiation damage to biological tissue. Knowledge of the underlying mechanistic processes remains incomplete, particularly with respect to the details and energetics of the electron transfer steps. In this work, we show how ultraviolet (UV) photoexcitation of metal-ammonia clusters could be used to generate tunable low-energy electrons in situ. Specifically, we identified UV light-induced generation of spin-paired solvated dielectrons and their subsequent relaxation by an unconventional electron transfer-mediated decay as an efficient, low-energy electron source. The process is robust and straightforward to induce with the prospect of improving our understanding of radiation damage and fostering mechanistic studies of solvated electron reduction reactions.

Photoionization Dynamics and Proton Transfer within the Adenine-Thymine Nucleobase Pair.

Studying the stability of hydrogen-bonded nucleobase pairs, at the heart of the genetic code, is of utmost importance for an in-depth understanding of basic mechanisms of life and biomolecular evolution. We present here a VUV single photon ionization dynamic study of the nucleobase pair adenine-thymine (AT), revealing its ionization and dissociative ionization thresholds via double imaging electron/ion coincidence spectroscopy. The experimental data, consisting of cluster mass-resolved threshold photoelectron spectra and photon energy-dependent ion kinetic energy release distributions, allow the unambiguous distinction of the dissociation of AT into protonated adenine AH+ and a dehydrogenated thymine radical T(-H) from dissociative ionization processes of other nucleobase clusters. Comparison to high-level ab initio calculations indicates that our experimental observations can be explained by a single hydrogen-bonded conformer present in our molecular beam and allows the estimation of an upper limit of the barrier of the proton transfer in the ionized AT pair.

Ring-Opening Dynamics of the Cyclopropyl Radical and Cation: the Transition State Nature of the Cyclopropyl Cation.

We provide compelling experimental and theoretical evidence for the transition state nature of the cyclopropyl cation. Synchrotron photoionization spectroscopy employing coincidence techniques together with a novel simulation based on high-accuracy ab initio calculations reveal that the cation is unstable via its allowed disrotatory ring-opening path. The ring strains of the cation and the radical are similar, but both ring opening paths for the radical are forbidden when the full electronic symmetries are considered. These findings are discussed in light of the early predictions by Longuet-Higgins alongside Woodward and Hoffman; we also propose a simple phase space explanation for the appearance of the cyclopropyl photoionization spectrum. The results of this work allow the refinement of the cyclopropane C-H bond dissociation energy, in addition to the cyclopropyl radical and cation cyclization energies, via the Active Thermochemical Tables approach.

Photoionization spectroscopy of the SiH free radical in the vacuum-ultraviolet range.

The first measurement of the photoelectron spectrum of the silylidyne free radical, SiH, is reported between 7 and 10.5 eV. Two main photoionizing transitions involving the neutral ground state, X+1Σ+ ← X2Π and a+3Π â† X2Π, are assigned by using ab initio calculations. The corresponding adiabatic ionization energies are derived, IEad(X+1Σ+) = 7.934(5) eV and IEad(a+3Π) = 10.205(5) eV, in good agreement with our calculated values and the previous determination by Berkowitz et al. [J. Chem. Phys. 86, 1235 (1987)] from a photoionization mass spectrometric study. The photoion yield of SiH recorded in this work exhibits a dense autoionization landscape similar to that observed in the case of the CH free radical [Gans et al., J. Chem. Phys. 144, 204307 (2016)].

Photoelectron spectroscopy of low valent organophosphorus compounds, P-CH3, H-PCH2 and PCH2.

We report the mass-selected slow photoelectron spectra of three reactive organophosphorus species, PCH2, and the two isomers, methylenephosphine or phosphaethylene, HPCH2 and methylphosphinidine, P-CH3. All spectra were recorded by double imaging photoelectron-photoion coincidence spectroscopy (i2PEPICO) using synchrotron radiation and all species were generated in a flow reactor by the reaction of trimethyl phosphine with fluorine atoms. Adiabatic ionisation energies of 8.80 ± 0.02 eV (PCH2), 10.07 ± 0.03 eV (H-PCH2) and 8.91 ± 0.04 eV (P-CH3) were determined and the vibronic structure was simulated by calculating Franck-Condon factors from optimised structures based on quantum chemical methods. Observation of biradicalic P-CH3 isomer with its triplet ground state is surprising because it is less stable than H-PCH2.

Photoelectron Circular Dichroism as a Signature of Subtle Conformational Changes: The Case of Ring Inversion in 1-Indanol.

Chirality plays a fundamental role in the molecular recognition processes. Molecular flexibility is also crucial in molecular recognition, allowing the interacting molecules to adjust their structures and hence optimize the interaction. Methods probing simultaneously chirality and molecular conformation are therefore crucially needed. Taking advantage of a possible control in the gas phase of the conformational distribution between the equatorial and axial conformers resulting from a ring inversion in jet-cooled 1-indanol, we demonstrate here the sensitivity of valence-shell photoelectron circular dichroism (PECD) to both chirality and subtle conformational changes, in a case where the photoelectron spectra of the two conformers are identical. For the highest occupied orbital, we observe a dramatic inversion of the PECD-induced photoelectron asymmetries, while the photoionization cross-section and usual anisotropy (ß) parameter are completely insensitive to conformational isomerism. Such a sensitivity is a major asset for the ongoing developments of PECD-based techniques as a sensitive chiral (bio)chemical analytical tool in the gas phase.

Vacuum ultraviolet photochemistry of sulfuric acid vapor: a combined experimental and theoretical study.

We present a vacuum ultraviolet (VUV) photoionization study of the gas-phase sulfuric acid (H2SO4) molecule in the 11-14 eV energy range by using the method of synchrotron radiation-based double imaging photoelectron photoion coincidence (i2PEPICO) spectroscopy complemented with accurate theoretical calculations. The slow photoelectron spectrum (SPES) of H2SO4 has been acquired and the three electronic states of H2SO4+, X2A, A2A and B2A have been populated and assigned. The adiabatic ionization energy of the H2SO4 molecule towards the X2A cationic ground state is measured at 11.684 ± 0.006 eV, in accordance with high-level calculated findings. With increasing photon energies, the H2SO4+ cation dissociates into HSO3+ and OH fragments and their adiabatic appearance energy is measured at 13.498 ± 0.007 eV. Then, the enthalpies of formation for the species involved in the photoionization and dissociative photoionization have been determined through a thermochemical cycle.

Photoelectron Spectroscopy of the Water Dimer Reveals Unpredicted Vibrational Structure.

Hydrogen bonds and proton transfer reactions can be considered as being at the very heart of aqueous chemistry and of utmost importance for many processes of biological relevance. Nevertheless, these processes are not yet well understood, even in seemingly simple model systems like small water clusters. We present a study of the photoelectron spectrum of the water dimer, revealing previously unresolved vibrational structure with 10-30 meV (80-242 cm-1) typical splitting, in disagreement with a previous theoretical photoionization study predicting an apparent main vibrational progression with an ∼130 meV spacing [Kamarchik et al.; J. Chem. Phys. 2010, 132, 194311]. The observed vibrational structure and its deviation from the theoretical prediction is discussed in terms of known difficulties with calculations of strongly coupled anharmonic systems involving large amplitude motions. Potential contributions of the nonzero vibrational energy of the neutral water dimer at a finite experimental internal temperature are addressed. The internal temperature is estimated from the breakdown diagram associated with the dissociative ionization of the water dimer to be around to 130 K. This analysis also provides two additional, independently measured values for the 0 K appearance energy of the hydronium ion (H3O+) from dissociative ionization of the water dimer.

Low-energy constraints on photoelectron spectra measured from liquid water and aqueous solutions.

We report on the effects of electron collision and indirect ionization processes, occurring at photoexcitation and electron kinetic energies well below 30 eV, on the photoemission spectra of liquid water. We show that the nascent photoelectron spectrum and, hence, the inferred electron binding energy can only be accurately determined if electron energies are large enough that cross sections for quasi-elastic scattering processes, such as vibrational excitation, are negligible. Otherwise, quasi-elastic scattering leads to strong, down-to-few-meV kinetic energy scattering losses from the direct photoelectron features, which manifest in severely distorted intrinsic photoelectron peak shapes. The associated cross-over point from predominant (known) electronically inelastic to quasi-elastic scattering seems to arise at surprisingly large electron kinetic energies, of approximately 10-14 eV. Concomitantly, we present evidence for the onset of indirect, autoionization phenomena (occurring via superexcited states) within a few eV of the primary and secondary ionization thresholds. These processes are inferred to compete with the direct ionization channels and primarily produce low-energy photoelectrons at photon and electron impact excitation energies below ∼15 eV. Our results highlight that vibrational inelastic electron scattering processes and neutral photoexcitation and autoionization channels become increasingly important when photon and electron kinetic energies are decreased towards the ionization threshold. Correspondingly, we show that for neat water and aqueous solutions, great care must be taken when quantitatively analyzing photoelectron spectra measured too close to the ionization threshold. Such care is essential for the accurate determination of solvent and solute ionization energies as well as photoelectron branching ratios and peak magnitudes.

Condensation Effects on Electron Chiral Asymmetries in the Photoionization of Serine: From Free Molecules to Nanoparticles.

Structural changes at the molecular level, occurring at the onset of condensation, can be probed by angle-resolved valence photoelectron spectroscopy, which is inherently sensitive to the electronic structure. For larger condensed systems like aerosol particles, the observation of intrinsic angular anisotropies in photoemission (ß parameters) is challenging due to the strong reduction of their magnitude by electron transport effects. Here, we use a less common, more sensitive observable in the form of the chiral asymmetry parameter to perform a comparative study of the VUV photoelectron spectroscopy and photoelectron circular dichroism (PECD) between pure gas phase enantiomers of the amino acid serine and their corresponding homochiral nanoparticles. We observe a relatively large (1%) and strongly kinetic energy-dependent asymmetry, discussed in terms of the emergence of local order and conformational changes potentially counterbalancing the loss of angular information due to electron transport scattering. This demonstrates the potential of PECD as a sensitive probe of the condensation effects from the gas phase to bulk-like chiral aerosol particles surpassing the potential of conventional photoemission observables such as ß parameters.

VUV photoionization of the CH2NC radical: adiabatic ionization energy and cationic vibrational mode wavenumber determinations.

The photoelectron spectroscopy of CH2NC (isocyanomethyl) radical species is investigated for the first time between 9.3 and 11.2 eV in the vicinity of the first photoionizing transition X+1A1← X 2B1. The experiment combines a microwave discharge flow-tube reactor to produce the radicals through the CH3NC + F → CH2NC + HF reaction, a VUV synchrotron radiation excitation, and a double imaging electron/ion coincidence spectrometer which allows the recording of mass-selected threshold photoelectron spectra. Assignment of the observed vibrational structure of the CH2NC+ cation is guided by ab initio calculations and Franck-Condon simulations. From the experimental spectrum, the first adiabatic ionization energy of the CH2NC radical is measured as 9.439(6) eV. Fundamental wavenumbers are determined for several vibrational modes of the cation: [small nu, Greek, tilde]1+(CH2 symmetric stretch) = 2999(80) cm-1, [small nu, Greek, tilde]2+(NC stretch) = 1925(40) cm-1, [small nu, Greek, tilde]4+(H2C-N stretch) = 1193(40) cm-1, [small nu, Greek, tilde]6+(CNC out-of-plane bend) = 237(50) cm-1, and [small nu, Greek, tilde]8+(CH2 rock) = 1185(60) cm-1.

Relaxation Dynamics and Genuine Properties of the Solvated Electron in Neutral Water Clusters.

We have investigated the solvation dynamics and the genuine binding energy and photoemission anisotropy of the solvated electron in neutral water clusters with a combination of time-resolved photoelectron velocity map imaging and electron scattering simulations. The dynamics was probed with a UV probe pulse following above-band-gap excitation by an EUV pump pulse. The solvation dynamics is completed within about 2 ps. Only a single band is observed in the spectra, with no indication for isomers with distinct binding energies. Data analysis with an electron scattering model reveals a genuine binding energy in the range of 3.55-3.85 eV and a genuine anisotropy parameter in the range of 0.51-0.66 for the ground-state hydrated electron. All of these observations coincide with those for liquid bulk, which is rather unexpected for an average cluster size of 300 molecules.

Electron scattering in large water clusters from photoelectron imaging with high harmonic radiation.

Low-energy electron scattering in water clusters (H2O)n with average cluster sizes of n < 700 is investigated by angle-resolved photoelectron spectroscopy using high harmonic radiation at photon energies of 14.0, 20.3, and 26.5 eV for ionization from the three outermost valence orbitals. The measurements probe the evolution of the photoelectron anisotropy parameter ß as a function of cluster size. A remarkably steep decrease of ß with increasing cluster size is observed, which for the largest clusters reaches liquid bulk values. Detailed electron scattering calculations reveal that neither gas nor condensed phase scattering can explain the cluster data. Qualitative agreement between experiment and simulations is obtained with scattering calculations that treat cluster scattering as an intermediate case between gas and condensed phase scattering.

Size-Resolved Photoelectron Anisotropy of Gas Phase Water Clusters and Predictions for Liquid Water.

We report the first measurements of size-resolved photoelectron angular distributions for the valence orbitals of neutral water clusters with up to 20 molecules. A systematic decrease of the photoelectron anisotropy is found for clusters with up to 5-6 molecules, and most remarkably, convergence of the anisotropy for larger clusters. We suggest the latter to be the result of a local short-range scattering potential that is fully described by a unit of 5-6 molecules. The cluster data and a detailed electron scattering model are used to predict the anisotropy of slow photoelectrons in liquid water. Reasonable agreement with experimental liquid jet data is found.

Metal Transition in Sodium-Ammonia Nanodroplets.

The famous nonmetal-to-metal transition in Na-ammonia solutions is investigated in nanoscale solution droplets by photoelectron spectroscopy. In agreement with the bulk solutions, a strong indication for a transition to the metallic state is found at an average metal concentration of 8.8±2.2 mole%. The smallest entity for the phase transition to be observed consists of approximately 100-200 solvent molecules. The quantification of this critical entity size is a stepping stone toward a deeper understanding of these quantum-classical solutions through direct modeling at the molecular level.

Stability of aerosol droplets in Bessel beam optical traps under constant and pulsed external forces.

We report on the dynamics of aerosol droplets in optical traps under the influence of additional constant and pulsed external forces. Experimental results are compared with simulations of the three-dimensional droplet dynamics for two types of optical traps, the counter-propagating Bessel beam (CPBB) trap and the quadruple Bessel beam (QBB) trap. Under the influence of a constant gas flow (constant external force), the QBB trap is found to be more stable compared with the CPBB trap. By contrast, under pulsed laser excitation with laser pulse durations of nanoseconds (pulsed external force), the type of trap is of minor importance for the droplet stability. It typically needs pulsed laser forces that are several orders of magnitude higher than the optical forces to induce escape of the droplet from the trap. If the droplet strongly absorbs the pulsed laser light, these escape forces can be strongly reduced. The lower stability of absorbing droplets is a result of secondary thermal processes that cause droplet escape.