The Pure Rotational Spectrum of FeO+ (X6S+) DeWayne T. Halfen and Lucy M. Ziurys Steward Observatory Arizona Radio Observatory University of Arizona June 18, 2007
Metal-Containing Ions Studied by Rotational Spectroscopy Why FeO+ ? Few metal-containing molecular ions studied by pure rotational spectroscopy Four studied in last 2 years (Halfen & Ziurys 2005, 2006, 2007a, 2007b) Powerful gas-phase oxidizing catalyst Capable of activating C-H bonds Oxidizes methane into methanol FeO+ + CH4 Fe+ + CH3OH Model for the active site in Methane monooxygenase Present in the mesosphere from reaction of Fe+ and O3 Astrophysically interesting FeO tentatively detected in ISM (Walmsley et al. 2002) Fe should be ionized in the ISM due to radiation field, so FeO+ could be present Metal-Containing Ions Studied by Rotational Spectroscopy TiCl+ TiF+ FeCO+ VCl+
Past Work on FeO+ Several ab initio calculations performed of FeO+ (Fiedler et al. 1993, 1994; Gutsev et al. 2003) Predicted a 6S+ ground state Husband et al. (1999) measured 6S+-X6S+ (v = 0-0, 1-0, & 1-1) transition using photofragment spectroscopy Confirmed the ground state as 6S+ Determined rotational constants for ground and excited state, as well as the spin-spin constant for excited state Aguirre et al. (2003) measured 6P7/2-X6S+ (v = 8-0 & 9-0) transition using resonance-enhanced photodissociation spectroscopy Determined rotational and fine structure constants for the ground and excited state Performed TD-DFT calculations on excited states of FeO+
Velocity Modulation Spectrometer
Detector Reactant Gas Cell Radiation Source
Gas-Phase Production of FeO+ Add Fe(CO)5 Pressure: 3 mTorr 1 mTorr of N2O 30 mTorr Ar gas also added AC discharge 200 W at 600 W Pink glow
Rotational Spectrum of FeO+ (X6S+) Source Modulation Initially searched over 50 GHz Used predictions based on Aguirre et al. study Multiple strong FeO lines observed up to v = 4 Found 6 sets of harmonically related lines with expected B value Much weaker than FeO Due to Fe(CO)5 and N2O Also detected in velocity modulation mode Halfen & Ziurys 2007 Halfen & Ziurys 2007 Velocity Modulation Low S/N Quartet coincidental
Transition Frequencies of FeO+ (X6S+) Measured 9 transitions of FeO+ 53 lines measured Fine structure resolved for all transitions For each rotational transition, fine structure components spread out over range of 4 GHz N N J J nobs nobs - ncalc 9 10 11.5 12.5 299815.073 -0.644 10.5 300932.520 -0.011 9.5 301477.780 -0.098 8.5 301717.387 0.363 7.5 302521.535 -0.229 6.5 303683.858 -0.587 11 13.5 329944.227 0.361 331027.048 0.163 331634.396 -0.180 331973.158 0.162 332764.915 -0.106 333868.134 -0.335 12 14.5 360072.603 0.263 361129.440 -0.094 361782.925 0.185 362196.854 -0.014 362981.934 0.004 364037.261 0.891 13 15.5 390198.984 0.150 391236.081 0.022 391923.691 0.093 392396.710 -0.092 393178.240 0.107 394196.530 0.430
Spectroscopic Analysis of FeO+ Constants agree well with past work B, D, l are in excellent agreement g smaller than previously determined, but within the error Accurate enough for astronomical observations at 1mm Lower frequency predictions not as accurate Measurements with FTMW spectrometer planned Parameter a Millimeter-wave Optical b B 15092.815(63) 15050(36) D 0.02147(26) 0.0210 c H -0.00000024(37) g -829.47(91) -989(180) gD -0.00549(83) l -3768(19) -3777(540) lD 0.2969(98) lH 0.000020(16) gs 1.60(12) gsD -0.00009(12) q -487.3(2.2) qD 0.0067(44) qH -0.0000058(82) rms 0.257 a In MHz. b Aguirre et al. 2003. c Held fixed.
Structure of FeO+ and FeO Bond lengths of FeO+ and FeO very similar r0(FeO+) = 1.641 Å vs. r0 (FeO) = 1.619 Å Only small difference (0.022 Å) FeO+: (core) d2p2s1 FeO: (core) d3p2s1 Different by one non-bonding d electron Structure agrees well with theoretical calculations (Gutsev et al. 2003)
Future Work Conduct measurements using FTMW spectrometer ARO SMT Deperturbation analysis may be needed Astronomical searches for FeO+ at 1mm TiO+ – X2Dr CrO+ – X4S- CoO+ – X5Di ARO SMT