international symposium on molecular spectroscopy rh: cold /ultra-cold /physics 6/19/14 6/19/14 paul...
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International Symposium on Molecular SpectroscopyRH: Cold /Ultra-Cold /Physics
6/19/14
Paul L. Raston, Tao Liang, and Gary E. Douberly
Infrared Laser Stark Spectroscopy and Ab Initio Computations of the OHCO Complex
Department of Chemistry, University of Georgia Athens, Georgia, USA
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Acknowledgments
Post-Doc: Paul Raston Graduate Student: Tao Liang
Support: ACS-Petroleum Research Fund U.S. National Science Foundation (CAREER) U.S. Department of Energy, Office of Science (BES-GPCP)
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Motivation
• OH+CO [HOCO]* H+CO2
• Oxidative conversion of CO to CO2 in combustion environments
• Poster child for non-Arrhenius behavior
+ MHOCO (cis / trans)
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Notable Previous Spectroscopy
• FIR-LMR (trans-HOCO) T. Sears• Pure rotational spectroscopy (cis/trans-HOCO) Y.
Endo • Transient IR Absorption (trans-HOCO) C. B.
Moore • Matrix Isolation (trans/cis-HOCO) M. Jacox• Anion Photoelectron spectroscopy (trans/cis-HOCO) R.
Continetti • Sub-Doppler IR absorption (trans-HOCO) D.
Nesbitt
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J. Phys. Chem. A 2013, 117, 13255-13264.
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Overtone action spectroscopy of the linear OHCO complex
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10000 He atoms can dissipate 6 eV (140 kcal/mol)
Cooling timescale < 1 s, pick-up timescale 10 s
Spectroscopic study of the outcome of both reactive and non-reactive “cold” collisions between picked-up reactants
pick-up cells
T=0.4 K
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X
Droplet beam
∙OH + (CH3)2CO + CH∙ 3
Rough Pump
Gate Valve
Air VacuumO-ring seal
Water cooled copper electrodes
Ta filament / Quartz tube
Hydroxyl Radical Productionvia Flash Vacuum Pyrolysis of TBHP
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Elaser
EStark
Elaser
EStark
or
M = 0 M = ±1
Droplet Beam
cw-OPO (idler 3m)
OH CO Detect laser-induced depletionof ionization cross-section in mass channel m/z=17
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Agrees with predicted redshift from OHat CCSD(T)/aug-cc-pVTZ
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OHCO
23/2
5B
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Elaser
EStark
M = ±1
Droplet Beam
cw-OPO (idler 3m)
Stark Spectroscopy
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Spectroscopic Analysis
Parity conserving Hund’s case (a) basis
Couples and electronic states independent of J
Constant contribution absorbed into electronic origin
Spin –uncoupling term couples J levels in different || manifolds parity doubling (1 MHz)
Couples and electronic states -doubling (10-100 kHz)
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Spectroscopic Analysis
Parity conserving Hund’s case (a) basis
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Spectroscopic Analysis
Parity conserving Hund’s case (a) basis
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Spectroscopic Analysis
Parity conserving Hund’s case (a) basis
Parallel polarization Perpendicular polarization
Random Polarization
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Perpendicular Polarization: M = 1
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CCSD(T)/aug-cc-pVQZ
CCSD(T)/Def2-TZVPD
Debye
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for OHCO
𝜇𝐻𝑒=𝜇𝐶𝐶𝑆𝐷(𝑇 )−0.33D
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Vibrationally Averaged Permanent Dipole Moment
O
OH-CO OH-OC
Planar, R = 4.0 Å surface
Red: Dipole momentContours = 0.1 Debye
Black: Binding EnergyContours = 50 cm-1
CCSD(T)/Def2-TZVPD
Strategy: DMC on a 4D PES to get gs
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Summary
• Sequential addition of OH and CO to He droplets leads exclusively to the formation of the linear OHCO entrance channel complex.
• OHOC formation preclude perhaps by long-range electrostatic effects
• Stark Spectra are indicative of large-amplitude motion in entrance-channel well.
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q
Quantization axis
Orientational Anisotropy {P2cos}
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Random Polarization: M = 0, 1
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