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Atomic and Molecular Physics
Atomic and molecular physics researchers are engaged in a number of different programs that explore new phenomena and provide basic collision data relevant to high electron-temperature plasmas (solar and stellar atmospheres), to cometary atmospheres, and to the interstellar medium.
Atomic and molecular physics (AMP) research includes the story of collision-physics phenomena occurring in solar, stellar, interstellar, and planetary environs; and developing miniature mass spectrometer systems for planetary exploration.
Researchers at JPL are engaged in the measurement of absolute excitation cross sections, single- and multiple charge exchange cross sections, and metastable lifetimes of highly-charged ions that are important collision partners in solar, stellar, interstellar, and interplanetary regions. Additional efforts include research in the area of fast hydrogen and oxygen atom collisions with grain-adsorbed molecules, leading to polyatomic formation. Also, there is work under way in the development-to-flight of miniature mass spectrometer systems, with emphasis on theoretical understanding of the Paul trap, and miniaturization of radio frequency electronics.
Selected Current Research Projects
Metastable Lifetimes and Absolute Excitation Cross Sections
View of the JPL Merged-Beams Excitation and Charge-Exchange Beam Lines for HCIs.
There has been increasing emphasis in solar and astrophysics missions to observe the X-ray region of the electromagnetic spectrum. It is in this region that emissions from highly-charged ions (HCIs) predominate. The primary excitation mechanism is by electrons, and hence absolute cross sections for excitation of the electron density- and temperature-sensitive HCI transitions are critical for modeling the emissions from Chandra, Newton, SOHO Hinode, etc. missions. The Atomic and Molecualr Physics Group at JPL was the first group to demonstrate the electron energy-loss approach to the study of ion excitation: that one could scatter an electron from a positive ion, and detect the inelastically-scattered electron. This has led to a broad range of applications wherein one can measure cross sections for dipole-allowed, as well as spin- and symmetry-forbidden transitions. Moreover, there is no “wavelength barrier” to the measurements. One can measure cross sections for transitions occurring in the infrared to the hard X-ray regions. In addition to the excitation cross sections one is also able to measure excited-state metastable lifetimes in the range 0.5-100 ms to obtain branching fractions and emission rates for understanding plasmas in coronal equilibrium.
Cometary X-ray Emission Spectra and Absolute Charge-Exchange cross Sections
The surprising observation that comets emit X-rays as they approach the Sun was successfully explained by the process of charge exchange between HCIs in the solar wind, and comet neutral species that are evaporated from the comet surface. The project involves simulating the X-ray emissions in the laboratory using a low-resolution (100 eV) germanium detector, and then a grazing-incidence spectrometer with a Charge-Coupled Device (CCD) detector (0.4-6.8 eV). Also reported were absolute single, double, and triple charge-exchange cross sections for the highly charged ion (HCI)-comet neutral interaction. These cross sections have been used successfully to model emission spectra observed by the ROSAT, Chandra, and Newton X-ray spectrometers.
Molecular Formation in Fast-Atom Collisions with Analogues of Interstellar Dust
The JPL fast atom source. Components are the photodetaching laser (1), electron attachment region (2), pumps (3,8), superconducting magnet to confine electrons and negative ions (4,5), target gas system (6), atom-surface collision region (7), mass spectrometer for TPD (9), and retractable cold head (10).
A large array of molecular species has been detected in the Interstellar medium (ISM) by microwave spectrometer of the National Radio Astronomy Observatory (NRAO) and infrared spectrometer of the Spitzer Space Telescope. In many cases, the large abundances cannot be explained by gas-phase reactions, but are successfully modeled in terms of gas-grain catalyzed reactions. Laboratory study of the formation of polyatomic species on simulants of ISM dust grains is important to understanding the formation of the building blocks of life. While considerable work has been carried out using thermal-energy (0.001-0.02 eV) atomic species, there has been no work on superthermal species (>0.1 eV). Researcher have developed a fast-atom facility that produces large fluxes of ground-state H(2S) and O(3P) atoms, at any energy between 0.2 and 100 eV. The atoms are produced by first creating H- or O- ions by dissociative electron attachment. The negative ion is accelerated to the desired final energy, and the electron is laser-detached to create the ground neutral state. The neutral beam is free of metastable states, and has a small energy width and angular spread. The target molecules are selected for their large abundance in the ISM. They are frozen on a dust-grain simulant at 4.8K and exposed to the fast-atom beam. Recent JPL studies using fast atoms and temperature-programmed desorption (TPD) have shown ample CO2 production from the reaction of 5 eV O(3P) atoms with surface-adsorbed CO(s) molecules; formation of H2CO from the reaction H+CO(s); and formation of H2CO, CH3OH, and CH3CH2OH from the reaction O+CH4(s). These are first examples of polyatomic molecule formation using superthermal atomic beams.
Left Image: The Miniature Paul Ion Trap for the ISS Flight Vehicle Cabin Atmosphere Monitor (VCAM). Right Image: Miniature, board-level RF electronics for driving the VCAM Paul trap mass spectrometer. Board reduction to chip-level dimensions is in progress.
Miniature Mass Spectrometry and Flight Missions
Gas chromatograph/mass spectrometers have been workhorse instruments on practically every NASA planetary mission. The mass spectrum of a species is a unique fingerprint that identifies one species amongst all others. Missions to the outer planets impose a high premium on system mass, volume, and power. Research has focused on providing a miniature system that provides critical capabilities such as mass range, mass resolution, and mass crosstalk. Also, while reduction in mass spectrometer size is important, it must be accompanied by miniature electronics. This requires an understanding of how mass-resolving properties are affected by miniaturization. To this end, considerable study is given to modeling the Paul ion trap, and to investigating ion space-charge and its effect on trapped-ion trajectories. One goal is to improve the measurement of isotope ratios from the present 1.5-3.0% level to the range 0.5-1.0%. Finally, a miniature GC/MS system is presently undergoing verification and validation for flight to the ISS. The system includes a new-design digitally synthesized radiofrequency generator board that was developed from Technology Readiness Level (TRL) 2 to TRL 8 during the Vehicle Cabin Atmosphere Monitor (VCAM) build process. A separate Trace Gas Analyzer is also being tested for re-flight to the International Space Station (ISS). This older system uses a miniature quadrupole mass spectrometer array, and conventional air-core transformer technology to detect leaks of ammonia coolant in lines outside the ISS hull.