1. 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

2. 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+

3. 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+

4. Velocity Modulation Spectrometer

5. Detector
Reactant
Gas Cell
Radiation
Source

6. 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

7. 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

8. 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
-0.644
10.5
-0.011
9.5
-0.098
8.5
0.363
7.5
-0.229
6.5
-0.587 11
13.5
0.361
0.163
-0.180
0.162
-0.106
-0.335 12
14.5
0.263
-0.094
0.185
-0.014
0.004
0.891 13
15.5
0.150
0.022
0.093
-0.092
0.107
0.430

9. 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
(63)
15050(36) D
(26) c H
(37) g
(91)
-989(180) gD
(83) l
-3768(19)
-3777(540) lD
0.2969(98) lH
(16) gs
1.60(12)
gsD
(12) q
-487.3(2.2) qD
0.0067(44) qH
(82)
rms
0.257
a In MHz. b Aguirre et al c Held fixed.

10. Structure of FeO+ and FeO
Bond lengths of FeO+ and FeO very similar
r0(FeO+) = Å vs. r0 (FeO) = 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)

11. 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