Slow photoelectron velocity-map imaging spectroscopy of cold negative ions

Anion slow photoelectron velocity-map imaging (SEVI) spectroscopy is a high-resolution variant of photoelectron spectroscopy used to study the electronic and geometric structure of atoms, molecules, and clusters. To benefit from the high resolution of SEVI when it is applied to molecular species, it is essential to reduce the internal temperature of the ions as much as possible. Here, we describe an experimental setup that combines a radio-frequency ion trap to store and cool ions with the highresolution SEVI spectrometer. For C 5 -, we demonstrate ion temperatures down to 10 ± 2 K after extraction from the trap, as measured by the relative populations of the two anion spin-orbit states. Vibrational hot bands and sequence bands are completely suppressed, and peak widths as narrow as 4 cm −1 are seen due to cooling of the rotational degrees of freedom

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

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

The Speed of Sound in Methane under Conditions of the Thermal Boundary Layer of Uranus

We present the first direct observations of acoustic waves in warm dense matter. We analyze wavenumber- and energy-resolved X-ray spectra taken from warm dense methane created by laser-heating a cryogenic liquid jet. X-ray diffraction and inelastic free electron scattering yield sample conditions of 0.3$\pm$0.1 eV and 0.8$\pm$0.1 g/cm$^3$, corresponding to a pressure of $\sim$13 GPa and matching the conditions predicted in the thermal boundary layer between the inner and outer envelope of Uranus. Inelastic X-ray scattering was used to observe the collective oscillations of the ions. With a highly improved energy resolution of $\sim$50 meV, we could clearly distinguish the Brillouin peaks from the quasi-elastic Rayleigh feature. Data at different wavenumbers were used to obtain a sound speed of 5.9$\pm$0.5 km/s, which enabled us to validate the use of Birch's law in this new parameter regime.Comment: 7 pages, 4 figures with supplementary informatio

Structures, Energetics, And Vibrations Of Small Transition Metal Oxide Clusters By High-resolution Anion Photoelectron Spectroscopy

Anion photoelectron spectroscopy has been a major tool in understanding the vibronic structure of metal oxide clusters, due to its universality and sensitivity. However, high ion temperatures and modest photoelectron energy resolutions have hampered the observation of vibrational structure. We have recently coupled our high-resolution slow photoelectron velocity-map imaging (SEVI) spectrometer to a cryogenic ion trap and a laser ablation ion source, allowing for the acquisition of photoelectron spectra of vibrationally cold metal oxide anions with a resolution down to $\sim$4~cm$^{-1}$, limited by unresolved rotational structure. A test study of the simple $d^0$ group 4 MO$_2$ triatomic metal oxides yielded fully vibrationally-resolved spectra, allowing for reassignments of electron affinities, new measurements of vibrational fundamentals, and estimates of the anion geometries based on the observed FC structure. Studies of the corresponding Ti$_2$O$_4$ and Zr$_2$O$_4$ systems revealed vibrational progressions that allows for an unambiguous assignment of the anion isomers; previous photoelectron spectra could not distinguish the isomers based on detachment energies alone. Spectra of the VO$_2^-$ anion identified the first three electronic states of the $d^1$ neutral as well as $\nu_1$ and $\nu_2$ vibrations in each state

Slow Photoelectron Velocity-Map Imaging Spectroscopy of the <i>ortho</i>-Hydroxyphenoxide Anion

We report high-resolution photodetachment spectra of cryogenically cooled <i>ortho-</i>hydroxyphenoxide anions (<i>o-</i>HOC₆H₄O^–) using slow photoelectron velocity-map imaging spectroscopy (cryo-SEVI). We observe transitions to the three lowest-lying electronic states of the <i>ortho-</i>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 <i>X̃</i>²<i>A</i>″ 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 <i>Ã</i>²<i>A</i>′ and <i>B̃</i>²<i>A</i>″ excited states respectively, representing their first resolution and clear assignment. Photodetachment threshold effects are considered to explain the structure of these bands

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 &lt;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

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 &lt;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

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 &lt;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