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

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)

Motivation

• OH+CO [HOCO]* H+CO2

• Oxidative conversion of CO to CO2 in combustion environments

• Poster child for non-Arrhenius behavior

+ MHOCO (cis / trans)

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

J. Phys. Chem. A 2013, 117, 13255-13264.

Overtone action spectroscopy of the linear OHCO complex

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

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

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

Agrees with predicted redshift from OHat CCSD(T)/aug-cc-pVTZ

OHCO

23/2

5B

Elaser

EStark

M = ±1

Droplet Beam

cw-OPO (idler 3m)

Stark Spectroscopy

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)

Spectroscopic Analysis

Parity conserving Hund’s case (a) basis

Spectroscopic Analysis

Parity conserving Hund’s case (a) basis

Spectroscopic Analysis

Parity conserving Hund’s case (a) basis

Parallel polarization Perpendicular polarization

Random Polarization

Perpendicular Polarization: M = 1

CCSD(T)/aug-cc-pVQZ

CCSD(T)/Def2-TZVPD

Debye

for OHCO

𝜇𝐻𝑒=𝜇𝐶𝐶𝑆𝐷(𝑇 )−0.33D

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

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.

q

Quantization axis

Orientational Anisotropy {P2cos}

Random Polarization: M = 0, 1