Characterization of Regolith And Trace Economic Resources (CRATER): An Orbitrap-based laser desorption mass spectrometry instrument for in situ exploration of the Moon.

RATIONALE: Characterization of Regolith And Trace Economic Resources (CRATER), an Orbitrap™-based laser desorption mass spectrometry instrument designed to conduct high-precision, spatially resolved analyses of planetary materials, is capable of answering outstanding science questions about the Moon's formation and the subsequent processes that have modified its (sub)surface. METHODS: Here, we describe the baseline design of the CRATER flight model, which requires <20 000 cm3  volume, <10 kg mass, and <60 W peak power. The analytical capabilities and performance metrics of a prototype that meets the full functionality of the flight model are demonstrated. RESULTS: The instrument comprises a high-power, solid-state, pulsed ultraviolet (213 nm) laser source to ablate the surface of the lunar sample, a custom ion optical interface to accelerate and collimate the ions produced at the ablation site, and an Orbitrap mass analyzer capable of discriminating competing isobars via ultrahigh mass resolution and high mass accuracy. The CRATER instrument can measure elemental and isotopic abundances and characterize the organic content of lunar surface samples, as well as identify economically valuable resources for future exploration. CONCLUSION: An engineering test unit of the flight model is currently in development to serve as a pathfinder for near-term mission opportunities.

Linear Ion Trap Mass Spectrometer (LITMS) Instrument Field and Laboratory Tests as Part of the ARADS Field Campaigns.

The highly compact Linear Ion Trap Mass Spectrometer (LITMS), developed at NASA Goddard Space Flight Center, combines Mars-ambient laser desorption-mass spectrometry (LD-MS) and pyrolysis-gas chromatography-mass spectrometry (GC-MS) through a single, miniaturized linear ion trap mass analyzer. The LITMS instrument is based on the Mars Organic Molecule Analyser (MOMA) investigation developed for the European Space Agency's ExoMars Rover Mission with further enhanced analytical features such as dual polarity ion detection and a dual frequency RF (radio frequency) power supply allowing for an increased mass range. The LITMS brassboard prototype underwent an extensive repackaging effort to produce a highly compact system for terrestrial field testing, allowing for molecular sample analysis in rugged planetary analog environments outside the laboratory. The LITMS instrument was successfully field tested in the Mars analog environment of the Atacama Desert in 2019 as part of the Atacama Rover Astrobiology Drilling Studies (ARADS) project, providing the first in situ planetary analog analysis for a high-fidelity, flight-like ion trap mass spectrometer. LITMS continued to serve as a laboratory tool for continued analysis of natural Atacama samples provided by the subsequent 2019 ARADS final field campaign.

Metabolic Processes Preserved as Biosignatures in Iron-Oxidizing Microorganisms: Implications for Biosignature Detection on Mars.

Iron-oxidizing bacteria occupy a distinct environmental niche. These chemolithoautotrophic organisms require very little oxygen (when neutrophilic) or outcompete oxygen for access to Fe(II) (when acidophilic). The utilization of Fe(II) as an electron donor makes them strong analog organisms for any potential life that could be found on Mars. Despite their importance to the elucidation of early life on, and potentially beyond, Earth, many details of their metabolism remain unknown. By using on-line thermochemolysis and gas chromatography-mass spectrometry (GC-MS), a distinct signal for a low-molecular-weight molecule was discovered in multiple iron-oxidizing isolates as well as several iron-dominated environmental samples, from freshwater and marine environments and in both modern and older iron rock samples. This GC-MS signal was neither detected in organisms that did not use Fe(II) as an electron donor nor present in iron mats in which organic carbon was destroyed by heating. Mass spectral analysis indicates that the molecule bears the hallmarks of a pterin-bearing molecule. Genomic analysis has previously identified a molybdopterin that could be part of the electron transport chain in a number of lithotrophic iron-oxidizing bacteria, suggesting one possible source for this signal is the pterin component of this protein. The rock samples indicate the possibility that the molecule can be preserved within lithified sedimentary rocks. The specificity of the signal to organisms requiring iron in their metabolism makes this a novel biosignature with which to investigate both the evolution of life on ancient Earth and potential life on Mars.

The Mars Organic Molecule Analyzer (MOMA) Instrument: Characterization of Organic Material in Martian Sediments.

The Mars Organic Molecule Analyzer (MOMA) instrument onboard the ESA/Roscosmos ExoMars rover (to launch in July, 2020) will analyze volatile and refractory organic compounds in martian surface and subsurface sediments. In this study, we describe the design, current status of development, and analytical capabilities of the instrument. Data acquired on preliminary MOMA flight-like hardware and experimental setups are also presented, illustrating their contribution to the overall science return of the mission. Key Words: Mars-Mass spectrometry-Life detection-Planetary instrumentation. Astrobiology 17, 655-685.

Mars Organic Molecule Analyzer (MOMA) laser desorption/ionization source design and performance characterization.

The Mars Organic Molecule Analyzer (MOMA), a dual-source, ion trap-based instrument capable of both pyrolysis-gas chromatography mass spectrometry (pyr/GC-MS) and laser desorption/ionization mass spectrometry (LDI-MS), is the core astrobiology investigation on the ExoMars rover. The MOMA instrument will be the first spaceflight mass analyzer to exploit the LDI technique to detect refractory organic compounds and characterize host mineralogy; this mode of analysis will be conducted at Mars ambient conditions. In order to achieve high performance in the Martian environment while keeping the instrument compact and low power, a number of innovative designs and components have been implemented for MOMA. These include a miniaturized linear ion trap (LIT), a fast actuating aperture valve with ion inlet tube. and a Microelectromechanical System (MEMS) Pirani sensor. Advanced analytical capabilities like Stored Waveform Inverse Fourier Transform (SWIFT) for selected ion ejection and tandem mass spectrometry (MS/MS) are realized in LDI-MS mode, and enable the isolation and enhancement of specific mass ranges and structural analysis, respectively. We report here the technical details of these instrument components as well as system-level analytical capabilities, and we review the applications of this technology to Mars and other high-priority targets of planetary exploration.

Aluminum Zintl anion moieties within sodium aluminum clusters.

Through a synergetic combination of anion photoelectron spectroscopy and density functional theory based calculations, we have established that aluminum moieties within selected sodium-aluminum clusters are Zintl anions. Sodium-aluminum cluster anions, Na(m)Al(n)(-), were generated in a pulsed arc discharge source. After mass selection, their photoelectron spectra were measured by a magnetic bottle, electron energy analyzer. Calculations on a select sub-set of stoichiometries provided geometric structures and full charge analyses for both cluster anions and their neutral cluster counterparts, as well as photodetachment transition energies (stick spectra), and fragment molecular orbital based correlation diagrams.

Multiphoton photoelectron emission microscopy of single Au nanorods: combined experimental and theoretical study of rod morphology and dielectric environment on localized surface plasmon resonances.

Multiphoton photoelectron emission from individual Au nanorods deposited on indium tin oxide (ITO) substrates is studied via scanning photoionization microscopy, based on femtosecond laser excitation at frequencies near the rod longitudinal surface plasmon resonance (LSPR). The observed resonances in photoemission correlate strongly with plasmon resonances measured in dark field microscopy (DFM), thus establishing a novel scheme for wavelength-resolved study of plasmons in isolated metallic nanoparticles based on highly sensitive electron counting methods. In this work, we explore experimental and theoretical effects of (i) morphology and (ii) aspect ratio (AR) for longitudinal plasmon resonance behavior in Au nanorods. A quasilinear dependence between LSPR and aspect ratio (AR) is experimentally determined [Δλ≈ +100(10) nm/AR unit] for Au nanorods on ITO, in excellent agreement with the first principles value from finite element computer modeling [Δλ = +108(5) nm/AR unit]. Interestingly, however, LSPR values for larger vs. smaller diameter rods (w≈ 20 nm and 10 nm) are systematically red-shifted [ΔE≈-0.03(1) eV; Δλ≈ +15(5) nm at λ≈ 800 nm], indicating that electromagnetic retardation effects must also be considered for highest accuracy in LSPR position. To augment these results, the influence of the dielectric environment on the rod LSPR has been explored both experimentally and numerically. In particular, detailed finite-element simulations for ITO supported Au nanorods are found to yield plasmon resonances in near quantitative agreement (ΔE≈±0.04 eV) with experiment, with residual differences arising from uncertainty in the refractive index of the ITO thin film. Furthermore, the results indicate that plasmon resonance predictions based on infinitely thick ITO substrates are reliable to a few meV for film thicknesses larger than approximately twice the rod width.

Coherent multiphoton photoelectron emission from single au nanorods: the critical role of plasmonic electric near-field enhancement.

Electron emission from individual Au nanorods deposited on indium-tin-oxide (ITO) following excitation with femtosecond laser pulses near the rod longitudinal plasmon resonance is studied via scanning photoionization microscopy. The measured electron signal is observed to strongly depend on the excitation laser polarization and wavelength. Correlated secondary electron microscopy (SEM) and dark-field microscopy (DFM) studies of the same nanorods unambiguously confirm that maximum electron emission results from (i) laser polarization aligned with the rod long axis and (ii) laser wavelength resonant with the localized surface plasmon resonance. The experimental results are in good agreement with quantitative predictions for a coherent multiphoton photoelectric effect, which is identified as the predominant electron emission mechanism for metal nanoparticles under employed excitation conditions. According to this mechanism, the multiphoton photoemission rate is increased by over 10 orders of magnitude in the vicinity of a localized surface plasmon resonance, due to enhancement of the incident electromagnetic field in the particle near-field. These findings identify multiphoton photoemission as an extremely sensitive metric of local electric fields (i.e., "hot spots") in plasmonic nanoparticles/structures that can potentially be exploited for direct quantitation of local electric field enhancement factors.

Plasmonic near-electric field enhancement effects in ultrafast photoelectron emission: correlated spatial and laser polarization microscopy studies of individual Ag nanocubes.

Electron emission from single, supported Ag nanocubes excited with ultrafast laser pulses (λ = 800 nm) is studied via spatial and polarization correlated (i) dark field scattering microscopy (DFM), (ii) scanning photoionization microscopy (SPIM), and (iii) high-resolution transmission electron microscopy (HRTEM). Laser-induced electron emission is found to peak for laser polarization aligned with cube diagonals, suggesting the critical influence of plasmonic near-field enhancement of the incident electric field on the overall electron yield. For laser pulses with photon energy below the metal work function, coherent multiphoton photoelectron emission (MPPE) is identified as the most probable mechanism responsible for electron emission from Ag nanocubes and likely metal nanoparticles/surfaces in general.

Photoelectron spectroscopic and computational studies of the Pt@Pb10⁻¹ and Pt@Pb12¹â»/²â» anions.

A combination of anion photoelectron spectroscopy and density functional theory calculations has elucidated the geometric and electronic structure of gas-phase endohedral Pt/Pb cage cluster anions. The anions, Pt@Pb10⁻¹ and Pt@Pb12¹â» were prepared from "preassembled" clusters generated from crystalline samples of [K(2,2,2-crypt)]2[Pt@Pb12] that were brought into the gas phase using a unique infrared desorption/photoemission anion source. The use of crystalline [K(2,2,2-crypt)]2[Pt@Pb12] also provided access to K[Pt@Pb(n)](-) anions in the gas phase (i.e., the K⁺ salts of the Pt@Pb(n)²â» anions). Anion photoelectron spectra of Pt@Pb10⁻¹, Pt@Pb12¹â», and K[Pt@Pb12]¹â» are presented. Extensive density functional theory calculations on Pt@Pb10³â»/²â»/¹â»/° and Pt@Pb12²â»/¹â» provided candidate structures and anion photoelectron spectra for Pt@Pb10⁻¹ and Pt@Pb12¹â». Together, the calculated and measured photoelectron spectra show that Pt@Pb10⁻¹ and Pt@Pb12²â»/¹â» endohedral complexes maintain their respective D(4d) and slightly distorted I(h) symmetries in the gas phase even for the charge states with open shell character. Aside from the fullerenes, the Pt@Pb12²â» endohedral complex is the only bare cluster that has been structurally characterized in the solid state, solution, and the gas phase.

Polarization-dependent scanning photoionization microscopy: ultrafast plasmon-mediated electron ejection dynamics in single Au nanorods.

This work investigates plasmon-enhanced multiphoton scanning photoelectron emission microscopy (SPIM) of single gold nanorods under vacuum conditions. Striking differences in their photoemission properties are observed for nanorods deposited either on 2 nm thick Pt films or 10 nm thick indium tin oxide (ITO) films. On a Pt support, the Au nanorods display fourth-order photoionization when excited at 800 nm, a wavelength corresponding to their plasmon resonance in aqueous solution. A cos(8)(θ) dependence of the photoelectron flux on laser polarization implies photoemission mediated by the dipolar plasmon; however, no plasmon resonance signature is exhibited over the 750-880 nm range. Electromagnetic simulations confirm that the resonance is severely broadened compared to aqueous solution, indicative of strong interactions between the Au nanorod and propagating surface plasmon modes in the Pt substrate. On ITO substrates, by way of contrast, sharp plasmon resonances in the photoemission from individual Au nanorods are observed, with widths limited only by fundamental internal electron collision processes. Furthermore, the ensemble-averaged plasmon resonance for Au nanorods on ITO is almost unshifted compared to its frequency in solution. Both findings suggest that plasmonic particle-substrate interactions are suppressed in the Au/ITO system. However, Au nanorods on ITO exhibit a surprising third-order photoemission (observed neither in Au nor ITO by itself), indicating that electrostatic interactions introduce a substantial shift in the work function for this fundamental nanoparticle-substrate system.

Electronic structure and properties of isoelectronic magic clusters: Al13X (X=H,Au,Li,Na,K,Rb,Cs).

The equilibrium structure, stability, and electronic properties of the Al(13)X (X=H,Au,Li,Na,K,Rb,Cs) clusters have been studied using a combination of photoelectron spectroscopy experiment and density functional theory. All these clusters constitute 40 electron systems with 39 electrons contributed by the 13 Al atoms and 1 electron contributed by each of the X (X=H,Au,Li,Na,K,Rb,Cs) atom. A systematic study allows us to investigate whether all electrons contributed by the X atoms are alike and whether the structure, stability, and properties of all the magic clusters are similar. Furthermore, quantitative agreement between the calculated and the measured electron affinities and vertical detachment energies enable us to identify the ground state geometries of these clusters both in neutral and anionic configurations.

Communications: Chain and double-ring polymeric structures: Observation of Al(n)H(3n+1) (-) (n=4-8) and Al(4)H(14) (-).

A pulsed arc discharge source was used to prepare gas-phase, aluminum hydride cluster anions, Al(n)H(m) (-), exhibiting enhanced hydrogen content. The maximum number of hydrogen atoms in Al(n)H(m) (-) species was m=3n+1 for n=5-8, i.e., Al(n)H(3n+1) (-), and m=3n+2 for n=4, i.e., Al(4)H(14) (-), as observed in their mass spectra. These are the most hydrogen-rich aluminum hydrides to be observed thus far, transcending the 3:1 hydrogen-to-aluminum ratio in alane. Even more striking, ion intensities for Al(n)H(m) (-) species with m=3n+1 and m=3n+2 hydrogen atoms were significantly higher than those of nearby Al(n)H(m) (-) mass peaks for which m<3n+1, i.e., the ion intensities for Al(n)H(3n+1) (-) and for Al(4)H(14) (-) deviated from the roughly bell-shaped ion intensity patterns seen for most Al(n)H(m) (-) species, in which m ranges from 1 to 3n. Calculations based on density functional theory showed that Al(n)H(3n+1) (-) clusters have chain and/or double-ring polymeric structures.

Reactivity of aluminum cluster anions with ammonia: selective etching of Al11(-) and Al12(-).

Reactivity of aluminum cluster anions toward ammonia was studied via mass spectrometry. Highly selective etching of Al(11)(-) and Al(12)(-) was observed at low concentrations of ammonia. However, at sufficiently high concentrations of ammonia, all other sizes of aluminum cluster anions, except for Al(13)(-), were also observed to deplete. The disappearance of Al(11)(-) and Al(12)(-) was accompanied by concurrent production of Al(11)NH(3)(-) and Al(12)NH(3)(-) species, respectively. Theoretical simulations of the photoelectron spectrum of Al(11)NH(3)(-) showed conclusively that its ammonia moiety is chemisorbed without dissociation, although in the case of Al(12)NH(3)(-), dissociation of the ammonia moiety could not be excluded. Moreover, since differences in calculated Al(n)(-) + NH(3) (n=9-12) reaction energies were not able to explain the observed selective etching of Al(11)(-) and Al(12)(-), we concluded that thermodynamics plays only a minor role in determining the observed reactivity pattern, and that kinetics is the more influential factor. In particular, the conversion from the physisorbed Al(n)(-)(NH(3)) to chemisorbed Al(n)NH(3)(-) species is proposed as the likely rate-limiting step.

Photoelectron spectroscopy of lanthanide-silicon cluster anions LnSi(n)(-) (3

Photoelectron spectroscopy was utilized to study a variety of LnSi(n)(-) cluster anions (Ln = Yb, Eu, Sm, Gd, Ho, Pr; 3 or= 10, adopt a nominal +2 oxidation state while Ho, Pr, Gd, and in case of Sm, sizes n

Heteroborane analogs of silicon clusters: experimental and theoretical studies on Bi2Si5 and Bi2Si5(-).

We have investigated the electronic structure of anionic and neutral Bi(2)Si(5) by means of anion photoelectron spectroscopy and density functional calculations. Both the experiments and calculations reveal that the Bi(2)Si(5)(-) anion prefers to adopt a distorted trigonal-bipyramidal structure with Bi(2) bridges. Following the isolobal analogy between divalent Si and B-H group, we show that both neutral Bi(2)Si(5) and neutral Bi(2)B(5)H(5) adopt similar pentagonal-bipyrmidal geometries and have analogous orbital energy patterns.

Photoelectron spectroscopy of europium-silicon cluster anions, EuSin- (3

We report the photoelectron spectra of EuSi(n) (-) cluster anions (3

Photoelectron spectroscopy of the parent anions of the nucleotides, adenosine-5'-monophosphate and 2'deoxyadenosine-5'-monophosphate.

The parent anions of the nucleotides, adenosine-5(')-monophosphate (AMPH) and 2(')deoxyadenosine-5(')-monophosphate (dAMPH) were generated in a novel source and their photoelectron spectra recorded with 3.49 eV photons. Vertical detachment energy (VDE) and the adiabatic electron affinity (EA(a)) values were extracted from each of the two spectra. Concurrently, Kobylecka et al. [J. Chem. Phys. 128, 044315 (2008)] conducted calculations which explored electron attachment to dAMPH. Based on the agreement between their calculated and our measured VDE and EA(a) values, we conclude that the dAMPH(-) anions studied in these experiments were formed by electron-induced, intramolecular, (barrier-free) proton-transfer as predicted by the calculations. Given the similarities between the photoelectron spectra of dAMPH(-) and AMPH(-), it is likely that AMPH(-) can be described in the same manner.

Intrinsic electrophilic properties of nucleosides: photoelectron spectroscopy of their parent anions.

The nucleoside parent anions 2(')-deoxythymidine(-), 2(')-deoxycytidine(-), 2(')-deoxyadenosine(-), uridine(-), cytidine(-), adenosine(-), and guanosine(-) were generated in a novel source, employing a combination of infrared desorption, electron photoemission, and a gas jet expansion. Once mass selected, the anion photoelectron spectrum of each of these was recorded. In the three cases in which comparisons were possible, the vertical detachment energies and likely adiabatic electron affinities extracted from these spectra agreed well with the values calculated both by Richardson et al. [J. Am. Chem. Soc. 126, 4404 (2004)] and by Li et al. [Radiat. Res. 165, 721 (2006)]. Through the combination of our experimental results and their theoretical calculations, several implications emerge. (1) With the possible exception of dG(-), the parent anions of nucleosides exist, and they are stable. (2) These nucleoside anions are valence anions, and in most cases the negative charge is closely associated with the nucleobase moiety. (3) The nucleoside parent anions we have generated and studied are the negative ions of canonical, neutral nucleosides, similar to those found in DNA.

Ground state structures and photoelectron spectroscopy of [Com(coronene)]- complexes.

A synergistic approach involving theory and experiment has been used to study the structure and properties of neutral and negatively charged cobalt-coronene [Com(coronene)] complexes. The calculations are based on density functional theory with generalized gradient approximation for exchange and correlation potential, while the experiments are carried out using photoelectron spectroscopy of mass selected anions. The authors show that the geometries of neutral and anionic Co(coronene) and Co2(coronene) are different from those of the corresponding iron-coronene complexes and that both the Co atom and the dimer prefer to occupy eta2-bridge binding sites. However, the magnetic coupling between the Co atoms remains ferromagnetic as it is between iron atoms supported on a coronene molecule. The accuracy of the theoretical results is established by comparing the calculated vertical detachment energies, and adiabatic electron affinities with their experimental data.