eV For Xenon Research Articles

Aromatic hydrocarbons as Molecular Propellants for Electric Propulsion Thrusters

The aromatic hydrocarbons (AHs) fluorobenzene, naphthalene, and 1-fluoronaphthalene are introduced as promising alternatives to xenon as propellant for in-space electric propulsion (EP). These storable molecules have similar mass, lower cost, and lower ionization energies compared to xenon, as well as the critical advantage of low post-ionization fragmentation compared to other molecular propellant candidates. The ionization characteristics of AHs are compared with those of xenon and the diamondoid adamantane, previously evaluated as a molecular propellant for EP. Quantum chemical calculations and BEB theory together with 25 eV electron ionization mass spectrometry (EI-MS) measurements have been used to predict the fragmentation of the AHs and adamantane when ionized in a plasma with an electron temperature of 7 eV (a typical electron temperature in EP plasmas). A high fraction (81 − 86%) of the detected AH ions originate from intact molecules, compared to 34% for adamantane, indicating extraordinarily low fragmentation for the selected AHs. The ionization potential of the AHs is similar to that of adamantane but lower compared to xenon (8.14–9.2 eV for the AHs, 9.25 for adamantane and 12.13 eV for xenon). BEB calculations have also been used to predict total ionization cross sections. The calculated ionization cross section of the AHs is comparable to that of adamantane but 3–5 times higher than that of xenon, which together with the low ionization potential can contribute to more efficient ionization. The AHs may have the potential to perform better than xenon, despite the absence of fragmentation in xenon.

Efficient XUV-light out-coupling of intra-cavity high harmonics by a coated grazing-incidence plate.

We experimentally demonstrate an efficient and broadband extreme-ultraviolet light (XUV) out-coupling mechanism of intra-cavity generated high harmonics. The mechanism is based on a coated grazing-incidence plate (GIP), which utilizes the enhanced reflectivity of s-polarized light in comparison to p-polarized light for large angles of incidence (AoI). We design and produce a 60°-AoI coated GIP, tailored specifically for the high demands inside a sub-50-fs Kerr-lens mode-locked Yb:YAG thin-disk laser oscillator in which high harmonic generation (HHG) is driven at ∼450 MW peak power and 17 MHz repetition rate. The coated GIP features an XUV out-coupling efficiency of >25% for photon energies ranging from 10 eV to 60 eV while being anti-reflective for the driving laser field. The XUV spectra reach up to 52 eV in argon and 30 eV in xenon. In a single harmonic, we out-couple 1.3 µW of XUV average power at 37 eV in argon and 5.4 µW at 25 eV in xenon. The combination of an improved HHG driving laser performance and the out-coupling via the coated GIP enabled us to increase the out-coupled XUV average power in a single harmonic by a factor of 20 compared to previous HHG inside ultrafast laser oscillators. Our source approaches the state-of-the-art out-coupled XUV power levels per harmonic of femtosecond enhancement cavities operating at comparable photon energies.

Negative Electron Transfer Dissociation of Deprotonated Phosphopeptide Anions: Choice of Radical Cation Reagent and Competition between Electron and Proton Transfer

Despite significant developments in mass spectrometry technology in recent years, no routine proteomics sequencing tool is currently available for peptide anions. The use of a molecular open-shell cation is presented here as a possible reaction partner to induce electron transfer dissociation with deprotonated peptide anions. In this negative electron transfer dissociation (NETD) scheme, an electron is abstracted from the peptide anion and transferred to the radical cation. This is demonstrated for the example of the fluoranthene cation, C(16)H(10)(+*), which is reacted with deprotonated phosphorylated peptides in a 3-D ion trap mass spectrometer. Selective backbone cleavage at the C(alpha)-C bond is observed to yield a and x fragments, similarly to electron detachment dissociation (EDD) of peptide anions. Crucially, the phosphorylation site is left intact in the dissociation process, allowing an identification and localization of the post-translational modification (PTM) site. In contrast, NETD using Xe(+*) as the reagent cation results in sequential neutral losses (CO(2) and H(3)PO(4)) from a/x fragments, which complicate the interpretation of the mass spectra. This difference in dissociation behavior can be understood in the framework of the reduced recombination energy of the electron transfer process for fluoranthene, which is estimated at 2.5-4.5 eV, compared to 6.7-8.7 eV for xenon. Similarly to ETD, proton transfer is found to compete with electron transfer processes in NETD. Isotope fitting of the charge-reduced species shows that in the case of fluoranthene-mediated NETD, proton transfer only accounts for <20%, whereas this process highly abundant for Xe(+*) (43 and 82%). Since proton abstraction from Xe(+*) is not possible, this suggests that Xe(+*) ionizes other transient species in the ion trap, which then engage in proton transfer reactions with the peptide anions.

Characteristics of correlation satellites below 25 eV in xenon probed by pulsed-field-ionization–zero-kinetic-energy photoelectron spectroscopy

The technique of pulsed-field-ionization--zero-kinetic-energy photoelectron spectroscopy, typically applied to the investigation of ionic states of atoms and molecules resulting from single electron excitation, has been used to probe all known correlation ionic states of xenon up to 25 eV. The high-resolution (1 meV) spectra that result show the formation of satellite states clearly resolved from their neighbors. Their intensities are comparable to that of the ${5s5p}^{6}{}^{2}{S}_{1/2}$ main line ionic state and for a given multiplet are relatively independent of the total angular momentum of the ionic state formed. The Rydberg states converging onto different ionic limits demonstrate effective lifetimes that are related to the excitation of the associated ionic core. These spectra allow the determination of relative intensities, and from them, the partial cross sections for the formation of each state at threshold. This supplements the existing intensity studies from the near-threshold to the x-ray region and increases our understanding of the dominant correlation processes within this atom as a function of photon energy.

Partial cross sections and photoelectron angular distributions in the region of the 4s-->5p and 5s-->6p resonances in krypton and xenon.

Using synchrotron radiation and photoelectron spectrometry, we have examined the 4s4${\mathit{p}}^{6}$5p resonance region in krypton and the 5s5${\mathit{p}}^{6}$6p resonance region in xenon. We have obtained partial and total cross sections, intensity ratios, and photoelectron angular distribution parameters for the energy regions 20.6--21.5 eV in xenon and 24.6--25.3 eV in krypton. We also report Shore parameters for all cross-section data taken. In addition to the anticipated single-electron transition, we clearly resolve features attributable to two-electron transitions in both species. Characteristics of these autoionizing states may differ considerably in the two available exit channels.

Probabilistic description of particle transport. II. Analysis of low-energy electron transmission through thin solid Xe and N2 films.

The probabilistic description of quasielastic particle transport given in a previous paper is used to analyze the results of low-energy electron transmission experiments on thin solid xenon and molecular nitrogen films deposited on a metal substrate. Values of the entrance probabilities of the incident electrons at the vacuum-film interface and of the electron scattering mean free paths in the films are extracted in the electron energy range 1.6--7.9 eV for xenon, and 2.4--7.4 eV for molecular nitrogen. The effects of anisotropy in the surface scattering and in the reflections at the two interfaces of the films are also discussed.

Momentum transfer cross sections for low-energy electrons in krypton and xenon from characteristic energies

Characteristic energies of low-energy electrons in krypton and xenon gases have been measured at room temperature by the Townsend method as a function of reduced electric field. For both gases, observed characteristic energies were twice or even three times as large as the early estimates by Frost and Phelps (1964) based on the momentum transfer cross sections for electrons in these gases derived from drift velocity data. The cross sections were determined from the present experimental data over an energy range from 0.01 to 6.0 eV for krypton and from 0.01 to 5.0 eV for xenon. The minimum values and positions of Ramsauer minima in the derived cross sections were 1.0*10-17 cm2 at 0.5 eV for krypton and 4.0*10-17 cm2 at 0.6 eV for xenon. These minimum values are both definitely smaller than the previous experimental results of Frost and Phelps, while being rather close to, but still somewhat smaller than, those of Hoffmann and Skarsgard (1969). Theoretical cross sections calculated for krypton and xenon by Sin Fai Lam (1982) and by McEachran and Stauffer (1984) and those for krypton by Fon et al. (1984) all agree with the present experimental ones within +or-30% for energies below about 5 eV in both gases.

Initial Ionization Rates in Shock-Heated Argon, Krypton, and Xenon

The rate of ionization behind strong shock waves in argon, krypton, and xenon, is observed by a transverse microwave probe, over a range of electron densities low enough that atom-atom inelastic collisions are the rate-determining mechanism. Shocks of Mach number 7.0 to 10.0 propagate down a 2-in. sq. aluminum shock tube into ambient gases at pressures of 3.0 to 17.0 mm. Hg., heating them abruptly to atomic temperatures of 5500°K to 9600°K. The subsequent relaxation toward ionization equilibrium is examined in its early stages by the reflection, transmission, and phase shifts of a 24.0 Gc/sec (1.25 cm) transverse microwave beam propagating between two rectangular horns abreast a glass test section. The data yield effective activation energies of 11.9 ± 0.5 eV for argon, 10.4 ± 0.5 eV for krypton, and 8.6 ± 0.5 eV for xenon. These coincide, within experimental error, with the first excitation potentials, rather than the ionization potentials of the gases, indicating that in this range ionization proceeds via a two-step process involving the first excited electronic states of which the excitation step is rate controlling.