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 N 2 + 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-neutral discrimination Low rotational temperature Narrow linewidth Cavity-enhanced spectroscopy CE Velocity Modulation Supersonic Expansion Hollow Cathode  High ion density  Ion Beam Velocity Modulation  Mass Spectrum Mass ID of Spectral Line       Setup

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

6. Mass ID of Spectroscopic Lines Ion Beam Laser 10859.34410865.2010865.2510853.4510853.50 Rest “Transition” Blue “Transition” Red “Transition” Line Shape

7. CM Line shape ABSORPTIONDISPERSION Overall line shape 0 fm

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

9. Example Spectra DISPERSION Velocity Modulation Concentration Modulation V Beam ~ 3865 V q Q 22 (14.5) N 2 +  = 30 s  = 1 s

10. Sensitivity Expected fractional signal strength: Cavity Finesse Path length Line Strength Ion Density LinewidthHeterodyne Loss Mid IR implications: HN 2 + without any rotational cooling Spectroscopy of larger carbocations (like CH 5 + and C 3 H 3 + ) will require rotational cooling ObservedExpectedFactor off 2.71E-07 1.48E-07 5.25E-07 4.62E-07 1.94 3.11 VM CM VM N2+N2+ N2+N2+ HN 2 + 2.4E-06 Equivalent Fractional Absorption

11. S_RIBES Characteristics S ensitive C ooled R esolved I on BE am S pectroscopy Ion density 6x10 6 ion/cm 3 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 Piezo Function generator Ti:Sapph PPLN BD LP Frequency comb Wavemeter YAG

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

14. Conclusions Using NICE-OHMS, an N 2 + 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 Visit us at: http://bjm.scs.uiuc.edu NASA Laboratory Astrophysics NSF Chemistry, Physics, Astronomy Dreyfus New Faculty, Teacher- Scholar Awards Packard Fellowship Cottrell Scholarship Sloan Fellowship