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Andrew Mills, Brian Siller, Michael Porambo, Manori Perera, Holger Kreckel, Ben McCall

PROGRESS & RESULTS IN THE DEVELOPMENTS OF THE SENSITIVE, COOLED, RESOLVED ION BEAM SPECTROMETER (SCRIBES). Andrew Mills, Brian Siller, Michael Porambo, Manori Perera, Holger Kreckel, Ben McCall. University of Illinois @ Urbana/Champaign. Ion Beam NICE-OHMS. Motivation Ion beam setup

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Andrew Mills, Brian Siller, Michael Porambo, Manori Perera, Holger Kreckel, Ben McCall

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  1. PROGRESS & RESULTS IN THE DEVELOPMENTS OF THE SENSITIVE, COOLED, RESOLVED ION BEAM SPECTROMETER (SCRIBES) Andrew Mills, Brian Siller, Michael Porambo, Manori Perera, Holger Kreckel, Ben McCall University of Illinois @ Urbana/Champaign

  2. Ion Beam NICE-OHMS • Motivation • Ion beam setup • Line shape • N2+ signal • Sensitivity • Spectroscopy characteristics • Future plans Measured rotational temperature {maybe}

  3. Motivations for studying molecular ions Fundamental: Structure of molecular ions Astrochemical Systems: Drive chemistry in interstellar medium (ISM) Need spectra to locate in ISM Challenges to studying ions Reactive, transient species: Production under harsh conditions (discharge) Discharges often rotationally and vibrationally excited ions Weak signal from dilute analyte Large background of neutrals and other excited species

  4. Direct Absorption Ion Spec. Techniques       Ion Beam Velocity Modulation Supersonic Expansion CE Velocity Modulation Hollow Cathode     High ion density      Ion-neutral discrimination   Low rotational temperature       Narrow linewidth     Cavity-enhanced spectroscopy     Mass Spectrum     Mass ID of Spectral Line Setup

  5. S _ R I Be S Sensitive Cooled Resolved Ion BEam Spectroscopy EOM Laser Laser retractable Faraday cup Brewster window Einzel lens 2 TOF beam modulation electrodes electrostatic deflector 2 wire beam profile monitors electron multiplier TOF detector drift tube (overlap) variable apertures Ion source – Currently uncooled electrostatic deflector 1 Ion optics steerers ion source Current measurements Co-linearity with laser Mass spectrometer Laser coupling Concentration / velocity modulation Einzel lens 1 Cavity Modes Sideband spacing Faraday cup Brewster window Coe et al., JCP 90, 3893 (1989) Mass ID

  6. Mass ID of Spectroscopic Lines Red “Transition” Blue “Transition” Rest “Transition” 10853.45 10853.50 10859.344 10865.20 10865.25 Ion Beam Laser Line Shape

  7. ABSORPTION CM Line shape DISPERSION n0 nfm Overall line shape

  8. ABSORPTION VM Line shape DISPERSION Concentration Modulation Line shape 10 V nvm ABSORPTION DISPERSION Velocity Modulation

  9. Example Spectra DISPERSION t = 1 s Concentration Modulation VBeam ~ 3865 V qQ22(14.5) N2+ t = 30 s Velocity Modulation

  10. Sensitivity Expected fractional signal strength: Cavity Finesse Path length Line Strength Ion Density Heterodyne Loss Linewidth Equivalent Fractional Absorption Observed Expected Factor off N2+ VM 2.71E-07 5.25E-07 1.94 N2+ CM 1.48E-07 4.62E-07 3.11 HN2+ VM 2.4E-06 Mid IR implications: HN2+ without any rotational cooling Spectroscopy of larger carbocations (like CH5+ and C3H3+) will require rotational cooling

  11. S_RIBES Characteristics Sensitive Cooled Resolved Ion BEam Spectroscopy          Ion density 6x106 ion/cm3 Ion neutral discrimination Complete spatial, and modulation discrimination from excited neutrals. Rotational temperature ~ 600 K. Surprisingly low temperature. Supersonic cooling available. Linewidth ~ 120 MHz in the NIR. ~33 MHz in midIR. Mass spectrometry of ions Confirms species probed… Optimize plasma conditions. Mass ID of spectral lines    Compare with OKA Saykally VM of positive column. Hirota, Amano, Hollow Cathode Supersonic Expansion Maier, Nesbit

  12. NICE-OHMS VMS DFG Wavemeter Frequency comb Function generator Ti:Sapph YAG PPLN BD LP Piezo

  13. Preliminary Optimization 20 V 10 V Increased finesse and refined laser locking HF+ t = 30 s t = 10 s Coe et al., JCP 90, 3893 (1989)

  14. Conclusions • Using NICE-OHMS, an N2+ equivalent absorption signal has been obtained from our ion beam. • The NICE-OHMS-S_RIBES technique: • Yields narrow linewidth spectral lines • Yields mass ID for every spectral line • Yields complete ion/neutral discrimination • Is compatible with supersonic cooling • Is sensitive enough to compensate for low ion density • The signal strength matches up with expected values. • Construction of a mid-IR DFG NICE-OHMS setup will soon begin.

  15. Acknowledgments Air Force Young Investigator Award Dreyfus New Faculty, Teacher-Scholar Awards NASA Laboratory Astrophysics NSF Chemistry, Physics, Astronomy Packard Fellowship Cottrell Scholarship Visit us at: http://bjm.scs.uiuc.edu Sloan Fellowship

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