Ultra-short pulse laser acceleration of protons to 80 MeV from cryogenic hydrogen jets tailored to near-critical density.

Laser plasma-based particle accelerators attract great interest in fields where conventional accelerators reach limits based on size, cost or beam parameters. Despite the fact that particle in cell simulations have predicted several advantageous ion acceleration schemes, laser accelerators have not yet reached their full potential in producing simultaneous high-radiation doses at high particle energies. The most stringent limitation is the lack of a suitable high-repetition rate target that also provides a high degree of control of the plasma conditions required to access these advanced regimes. Here, we demonstrate that the interaction of petawatt-class laser pulses with a pre-formed micrometer-sized cryogenic hydrogen jet plasma overcomes these limitations enabling tailored density scans from the solid to the underdense regime. Our proof-of-concept experiment demonstrates that the near-critical plasma density profile produces proton energies of up to 80 MeV. Based on hydrodynamic and three-dimensional particle in cell simulations, transition between different acceleration schemes are shown, suggesting enhanced proton acceleration at the relativistic transparency front for the optimal case.

Observation of resonances in the transition state region of the F + NH3 reaction using anion photoelectron spectroscopy.

The transition state of a chemical reaction is a dividing surface on the reaction potential energy surface (PES) between reactants and products and is thus of fundamental interest in understanding chemical reactivity. The transient nature of the transition state presents challenges to its experimental characterization. Transition-state spectroscopy experiments based on negative-ion photodetachment can provide a direct probe of this region of the PES, revealing the detailed vibrational structure associated with the transition state. Here we study the F + NH3 → HF + NH2 reaction using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled FNH3- anions. Reduced-dimensionality quantum dynamical simulations performed on a global PES show excellent agreement with the experimental results, enabling the assignment of spectral structure. Our combined experimental-theoretical study reveals a manifold of vibrational Feshbach resonances in the product well of the F + NH3 PES. At higher energies, the spectra identify features attributed to resonances localized across the transition state and into the reactant complex that may impact the bimolecular reaction dynamics.

Photoelectron spectroscopy of cryogenically cooled NiO2-via slow photoelectron velocity-map imaging.

High-resolution anion photoelectron spectra of cryogenically cooled NiO2- anions, obtained using slow photoelectron velocity-map imaging (cryo-SEVI), are presented in tandem with coupled cluster electronic structure calculations including relativistic effects. The experimental spectra encompass the X̃1Σg+ ← X̃2Πg, ã3Πg ← X̃2Πg, and Ã1Πg ← X̃2Πg photodetachment transitions of linear ONiO0/-, revealing previously unobserved vibrational structure in all three electronic bands. The high-resolution afforded by cryo-SEVI allows for the extraction of vibrational frequencies for each state, consistent with those previously measured in the ground state and in good agreement with scalar-relativistic coupled-cluster calculations. Previously unobserved vibrational structure is observed in the ã3Πg and Ã1Πg states and is tentatively assigned. Further, a refined electron affinity of 3.0464(7) eV for NiO2 is obtained as well as precise term energies for the ã and à states of NiO2 of 0.3982(7) and 0.7422(10) eV, respectively. Numerous Franck-Condon forbidden transitions involving the doubly degenerate ν2 bending mode are observed and ascribed to Herzberg-Teller coupling to an excited electronic state.

Cryogenic Liquid Jets for High Repetition Rate Discovery Science.

This protocol presents a detailed procedure for the operation of continuous, micron-sized cryogenic cylindrical and planar liquid jets. When operated as described here, the jet exhibits high laminarity and stability for centimeters. Successful operation of a cryogenic liquid jet in the Rayleigh regime requires a basic understanding of fluid dynamics and thermodynamics at cryogenic temperatures. Theoretical calculations and typical empirical values are provided as a guide to design a comparable system. This report identifies the importance of both cleanliness during cryogenic source assembly and stability of the cryogenic source temperature once liquefied. The system can be used for high repetition rate laser-driven proton acceleration, with an envisioned application in proton therapy. Other applications include laboratory astrophysics, materials science, and next-generation particle accelerators.

High-Pressure Melt Curve and Phase Diagram of Lithium.

We investigate the phase diagram of lithium at temperatures of 200 to 400 K, to pressures over 100 GPa using x-ray diffraction in diamond anvil cells, covering the region in which the melting curve is disputed. To overcome degradation of the diamond anvils by dense lithium we utilize a rapid compression scheme taking advantage of the high flux available at modern synchrotrons. Our results show the hR1 and cI16 phases to be stable to higher temperature than previously reported. The melting minima of lithium is found to be close to room temperature between 40 and 60 GPa, below which the solid is crystalline. Analysis of the stability fields of the cI16 and oC88 phases suggest the existence of a triple point between these and an undetermined solid phase at 60 GPa between 220 and 255 K.

Author Correction: Generation and characterization of ultrathin free-flowing liquid sheets.

The original version of this Article contained an error in Eq. (1). This has been corrected in both the PDF and HTML versions of the Article.

All-optical structuring of laser-driven proton beam profiles.

Extreme field gradients intrinsic to relativistic laser-interactions with thin solid targets enable compact MeV proton accelerators with unique bunch characteristics. Yet, direct control of the proton beam profile is usually not possible. Here we present a readily applicable all-optical approach to imprint detailed spatial information from the driving laser pulse onto the proton bunch. In a series of experiments, counter-intuitively, the spatial profile of the energetic proton bunch was found to exhibit identical structures as the fraction of the laser pulse passing around a target of limited size. Such information transfer between the laser pulse and the naturally delayed proton bunch is attributed to the formation of quasi-static electric fields in the beam path by ionization of residual gas. Essentially acting as a programmable memory, these fields provide access to a higher level of proton beam manipulation.

Development and characterization of liquid argon and methane microjets for high-rep-rate laser-plasma experiments.

A cryogenic microjet system has been used for delivering micron-scale continuous liquid hydrogen targets for laser-plasma experiments. This technique has been extended to higher-Z, higher boiling-point gases, including argon and methane. High-resolution shadowgraphy has been used to characterize the jet's morphology and pointing stability. A split and delay illumination source has also been developed for direct measurement of jet speeds without relying on assumptions of fluid flow conditions. Under typical conditions, the argon jets freeze solid due to evaporative cooling, but the methane jets remain liquid and break up to a droplet stream. A piezo driver is used to ensure the droplets are of uniform size. This jet has enabled the investigation of methane in planetary core conditions with high-rep-rate laser heating and x-ray laser probing.

Author Correction: Generation and characterization of ultrathin free-flowing liquid sheets.

The original version of this article omitted the following from the Acknowledgements:'P.B. was funded by the ELI Extreme Light Infrastructure Phase 2 (CZ.02.1.01/0.0/0.0/15008/0000162) from the European Regional Development Fund and the EUCALL project funded from the EU Horizon 2020 research and innovation programme under grant agreement No 654220,' which replaces the previous 'P.B. was funded by the ELI Extreme Light Infrastructure Phase 2 (CZ.02.1.01/0.0/0.0/15008/0000162) from the European Regional Development Fund.'This has been corrected in both the PDF and HTML versions of the article.

Generation and characterization of ultrathin free-flowing liquid sheets.

The physics and chemistry of liquid solutions play a central role in science, and our understanding of life on Earth. Unfortunately, key tools for interrogating aqueous systems, such as infrared and soft X-ray spectroscopy, cannot readily be applied because of strong absorption in water. Here we use gas-dynamic forces to generate free-flowing, sub-micron, liquid sheets which are two orders of magnitude thinner than anything previously reported. Optical, infrared, and X-ray spectroscopies are used to characterize the sheets, which are found to be tunable in thickness from over 1 µm  down to less than 20 nm, which corresponds to fewer than 100 water molecules thick. At this thickness, aqueous sheets can readily transmit photons across the spectrum, leading to potentially transformative applications in infrared, X-ray, electron spectroscopies and beyond. The ultrathin sheets are stable for days in vacuum, and we demonstrate their use at free-electron laser and synchrotron light sources.

Low-lying vibronic level structure of the ground state of the methoxy radical: Slow electron velocity-map imaging (SEVI) spectra and Köppel-Domcke-Cederbaum (KDC) vibronic Hamiltonian calculations.

A joint experimental and theoretical study is reported on the low-lying vibronic level structure of the ground state of the methoxy radical using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled, mass-selected anions (cryo-SEVI) and Köppel-Domcke-Cederbaum (KDC) vibronic Hamiltonian calculations. The KDC vibronic model Hamiltonian in the present study was parametrized using high-level quantum chemistry, allowing the assignment of the cryo-SEVI spectra for vibronic levels of CH3O up to 2000 cm-1 and of CD3O up to 1500 cm-1 above the vibrational origin, using calculated vibronic wave functions. The adiabatic electron affinities of CH3O and CD3O are determined from the cryo-SEVI spectra to be 1.5689 ± 0.0007 eV and 1.5548 ± 0.0007 eV, respectively, demonstrating improved precision compared to previous work. Experimental peak splittings of <10 cm-1 are resolved between the e1/2 and e3/2 components of the 61 and 51 vibronic levels. A pair of spin-vibronic levels at 1638 and 1677 cm-1 were predicted in the calculation as the e1/2 and e3/2 components of 62 levels and experimentally resolved for the first time. The strong variation of the spin-orbit splittings with a vibrational quantum number is in excellent agreement between theory and experiment. The observation of signals from nominally forbidden a1 vibronic levels in the cryo-SEVI spectra also provides direct evidence of vibronic coupling between ground and electronically excited states of methoxy.

Efficient laser-driven proton acceleration from cylindrical and planar cryogenic hydrogen jets.

We report on recent experimental results deploying a continuous cryogenic hydrogen jet as a debris-free, renewable laser-driven source of pure proton beams generated at the 150 TW ultrashort pulse laser Draco. Efficient proton acceleration reaching cut-off energies of up to 20 MeV with particle numbers exceeding 109 particles per MeV per steradian is demonstrated, showing for the first time that the acceleration performance is comparable to solid foil targets with thicknesses in the micrometer range. Two different target geometries are presented and their proton beam deliverance characterized: cylindrical (∅ 5 µm) and planar (20 µm × 2 µm). In both cases typical Target Normal Sheath Acceleration emission patterns with exponential proton energy spectra are detected. Significantly higher proton numbers in laser-forward direction are observed when deploying the planar jet as compared to the cylindrical jet case. This is confirmed by two-dimensional Particle-in-Cell (2D3V PIC) simulations, which demonstrate that the planar jet proves favorable as its geometry leads to more optimized acceleration conditions.

Isomer-specific vibronic structure of the 9-, 1-, and 2-anthracenyl radicals via slow photoelectron velocity-map imaging.

Polycyclic aromatic hydrocarbons, in various charge and protonation states, are key compounds relevant to combustion chemistry and astrochemistry. Here, we probe the vibrational and electronic spectroscopy of gas-phase 9-, 1-, and 2-anthracenyl radicals (C14H9) by photodetachment of the corresponding cryogenically cooled anions via slow photoelectron velocity-map imaging (cryo-SEVI). The use of a newly designed velocity-map imaging lens in combination with ion cooling yields photoelectron spectra with <2 cm(-1) resolution. Isomer selection of the anions is achieved using gas-phase synthesis techniques, resulting in observation and interpretation of detailed vibronic structure of the ground and lowest excited states for the three anthracenyl radical isomers. The ground-state bands yield electron affinities and vibrational frequencies for several Franck-Condon active modes of the 9-, 1-, and 2-anthracenyl radicals; term energies of the first excited states of these species are also measured. Spectra are interpreted through comparison with ab initio quantum chemistry calculations, Franck-Condon simulations, and calculations of threshold photodetachment cross sections and anisotropies. Experimental measures of the subtle differences in energetics and relative stabilities of these radical isomers are of interest from the perspective of fundamental physical organic chemistry and aid in understanding their behavior and reactivity in interstellar and combustion environments. Additionally, spectroscopic characterization of these species in the laboratory is essential for their potential identification in astrochemical data.

REACTION DYNAMICS. Spectroscopic observation of resonances in the F + H2 reaction.

Photodetachment spectroscopy of the FH2(-) and FD2(-) anions allows for the direct observation of reactive resonances in the benchmark reaction F + H2 → HF + H. Using cooled anion precursors and a high-resolution electron spectrometer, we observe several narrow peaks not seen in previous experiments. Theoretical calculations, based on a highly accurate F + H2 potential energy surface, convincingly assign these peaks to resonances associated with quasibound states in the HF + H and DF + D product arrangements and with a quasibound state in the transition state region of the F + H2 reaction. The calculations also reveal quasibound states in the reactant arrangement, which have yet to be resolved experimentally.

Slow Photoelectron Velocity-Map Imaging Spectroscopy of the ortho-Hydroxyphenoxide Anion.

We report high-resolution photodetachment spectra of cryogenically cooled ortho-hydroxyphenoxide anions (o-HOC6H4O(-)) using slow photoelectron velocity-map imaging spectroscopy (cryo-SEVI). We observe transitions to the three lowest-lying electronic states of the ortho-hydroxyphenoxy radical, and resolve detailed vibrational features. Comparison to Franck-Condon simulations allows for clear assignment of vibronic structure. We find an electron affinity of 2.3292(4) eV for the neutral X̃(2)A″ ground state, improving upon the accuracy of previous experiments. We measure term energies of 1.4574(7) eV and 1.5922(48) eV for the Ã(2)A' and B̃(2)A″ excited states respectively, representing their first resolution and clear assignment. Photodetachment threshold effects are considered to explain the structure of these bands.

Vibrational and electronic structure of the α- and ß-naphthyl radicals via slow photoelectron velocity-map imaging.

Slow photoelectron velocity-map imaging (SEVI) spectroscopy has been used to study the vibronic structure of gas-phase α- and ß-naphthyl radicals (C(10)H(7)). SEVI of cryogenically cooled anions yields spectra with <4 cm(-1) resolution, allowing for the observation and interpretation of congested vibrational structure. Isomer-specific photoelectron spectra of detachment to the radical ground electronic states show detailed structure, allowing assignment of vibrational fundamental frequencies. Transitions to the first excited states of both radical isomers are also observed; vibronic coupling and photodetachment threshold effects are considered to explain the structure of the excited bands.

Slow photoelectron velocity-map imaging spectroscopy of the Fe3O⁻ and Co3O⁻ anions.

We report high-resolution photoelectron spectra of the transition metal suboxide clusters Fe3O(-) and Co3O(-). The combination of slow electron velocity-map imaging and cryogenic cooling yields vibrationally well-resolved spectra, from which we obtain precise values of 1.4408(3) and 1.3951(4) eV for the electron affinities of Fe3O and Co3O. Several vibrational frequencies of the neutral ground state Fe3O and Co3O clusters are assigned for the first time, and a low-lying excited state of Fe3O is observed. The experimental results are compared with density functional electronic structure calculations and Franck-Condon spectral simulations, enabling identification of the structural isomer and electronic states. As has been found in photoelectron spectra of other trimetal oxo species, Fe3O(0/-) and Co3O(0/-) are assigned to a µ2-oxo isomer with planar C(2v) symmetry. We identify the ground states of Fe3O(-) and Co3O(-) as (12)A1 and (9)B2 states, respectively. From these states we observe photodetachment to the (11)B2 ground and (13)A1 excited states of Fe3O, as well as to the (8)A1 ground state of Co3O.

Structural isomers of Ti2O4 and Zr2O4 anions identified by slow photoelectron velocity-map imaging spectroscopy.

High-resolution anion photoelectron spectra are reported for the group 4 metal dioxide clusters Ti2O4(-) and Zr2O4(-). Slow photoelectron velocity-map imaging (SEVI) spectroscopy of cryogenically cooled, mass-selected anions yields photoelectron spectra with submillielectronvolt resolution, revealing extensive and well-resolved vibrational progressions. By comparison of the spectra with Franck-Condon simulations, we have identified the C(2v) and C(3v) isomers as the ground states of Ti2O4(-) and Zr2O4(-) anions, respectively. Minor contributions from the C(2h) isomer of Ti2O4(-) and the C(2v) isomer of Zr2O4(-) are also seen. The SEVI spectra yield upper bounds for the adiabatic detachment energies, as well as vibrational frequencies for various modes of the neutral Ti2O4 and Zr2O4 species.

Rovibronic structure in slow photoelectron velocity-map imaging spectroscopy of CH2CN⁻ and CD2CN⁻.

We report high-resolution anion photoelectron spectra of the cryogenically cooled cyanomethide anion, CH2CN(-), and its isotopologue, CD2CN(-), using slow photoelectron velocity-map imaging (SEVI) spectroscopy. Electron affinities of 12 468(2) cm(-1) for CH2CN and 12 402(2) cm(-1) for CD2CN are obtained, demonstrating greater precision than previous experiments. New vibrational structure is resolved for both neutral species, especially activity of the ν5 hydrogen umbrella modes. The ν6 out-of-plane bending mode fundamental frequency is measured for the first time in both systems and found to be 420(10) cm(-1) for CH2CN and 389(8) cm(-1) for CD2CN. Some rotational structure is resolved, allowing for accurate extraction of vibrational frequencies. Temperature-dependent SEVI spectra show marked effects ascribed to controlled population of low-lying anion vibrational levels. We directly measure the inversion splitting between the first two vibrational levels of the anion ν5 umbrella mode in both species, finding a splitting of 130(20) cm(-1) for CH2CN(-) and 81(20) cm(-1) for CD2CN(-). Franck-Condon forbidden activity is observed and attributed to mode-specific vibrational autodetachment from the CH2CN(-) and CD2CN(-) dipole bound excited states. We also refine the binding energy of the anion dipole bound states to 39 and 42 cm(-1), respectively, for CH2CN(-) and CD2CN(-).

Vibronic structure of VO2 probed by slow photoelectron velocity-map imaging spectroscopy.

We report high-resolution anion photoelectron spectra of vanadium dioxide (VO2 (-)) obtained by slow electron velocity-map imaging of trapped and cryogenically cooled anions. Vibrationally resolved spectra are obtained for photodetachment to the first three neutral electronic states, giving an electron affinity of 1.8357(5) eV for the X̃A12 ground state and term energies of 0.1845(8) eV and 0.8130(5) eV for the ÃB12 and B̃A12 excited states, respectively. The vibrational fundamentals ν1 and ν2 are obtained for all three states. Experimental assignments are confirmed by energies from electronic structure calculations and Franck-Condon spectral simulations. These simulations support assigning the anion ground state as the X̃B13 state. With this assignment, photodetachment to the B̃A12 state involves a nominally forbidden two-electron transition, suggesting extensive configuration interaction in neutral VO2.