Jennifer MannIndiana University

Caroline Jarrold Group67th International Symposium on Molecular Spectroscopy

June 18, 2012

Resonant two-photon detachment of WO2

−

Introduction

[1] M. Tulej, D. A. Kirkwood, G. Maccaferri, O. Dopfer, and J. P. Maier, Chem. Phys. 228 (1998) 293.; A. E. Bragg, J. R. R. Verlet, A. Kammrath, and D. M. Neumark, J. Chem. Phys. 121 (2004) 3515.; W. C. Lineberger and T. Patterson, Chem. Phys. Lett. 13 (1972) 40. ; M. Tulej, T. Pino, M. Pachkov, and J. P. Maier, Mol. Phys. 108 (2010) 865. ; T. Pino, M. Tulej, F. Guthe, M. Pachkov, and J. P. Maier, J. Chem. Phys. 116 (2002) 6126.[2] G. B. Griffin, A. Kammrath, O. T. Ehrler, R. M. Young, O. Cheshnovsky, and D. M. Neumark, Chem. Phys. 350 (2008) 69.[3] N. I. Hammer, R. N. Compton, L. Adamowicz, and S. G. Stepanian, Phys. Rev. Lett. 94 (2005) 153004.; C. Desfrançois, B. Baillon, J. P. Schermann, S. T. Arnold, J. H. Hendricks, and K. H. Bowen, Phys. Rev. Lett. 72 (1994) 48.[4] T. Andersen, K. R. Lykke, D. M. Neumark, and W. C. Lineberger, J. Chem. Phys. 86 (1987) 1858.; K. K. Murray, K. R. Lykke, and W. C. Lineberger, Phys. Rev. A 36 (1987) 699.

• Anion PE spectroscopy commonly used to characterize ground electronic states of anions• Far fewer studies on excited states of anions

• Typically not bound relative to neutral + e− continuum• Delocalized nature of high-lying electrons makes excited states difficult to characterize

computationally• Classes of anions where bound excited states are common

• Cumulenic carbon chains: Cn− , CnHm

−, and C2nH − [1]• Metal clusters: Hgn

− [2]• Molecules where neutral has a > 2 Debye dipole moment

• Dipole Bound State • Bound by less than 100 cm−1 to the neutral + e− continuum• e− bound in a delocalized orbital, does not perturb core• Acetone, water-ammonia dimer [3]• Transition-metal containing diatomics: PtN− FeO− [4]

• Current Study• WO2

− open shell, near degenerate d-like orbitals• High electron affinity• WO2 3.5 Debye dipole moment

Resonant Two-Photon Detachment

Anion Ground State

Anion Excited State

NeutralGround State

Detachment Continuum

Photon EnergyEl

ectr

on c

ount

s

EAEA

Ener

gy

Qsym

Experimental• Ions - laser ablation (532 nm) of W rod in pulse (30Hz) of UHP He• Mass selected by time-of-flight

• WO2– intersected by a Nd:YAG pumped OPO (410 – 709 nm, 5 cm-1 resolution)

• Photodetached e− extracted by weak field (2 Vcm−1) up to dual MCP detector• Ion signal, electron signal and laser power are sent to three gated integrators (SRS SR250)• Absolute line positions within 28 cm-1 accuracy, relative spacings 7 cm-1

Computational Methods• Density Functional Theory (DFT) Calculations

• Gaussian 09 Suite [1]• B3LYP• SDD pseudopotentials• Triple-ζ Level

• aug-cc-PVTZ • Time-dependent DFT (TDDFT)

• Electronic excitations predicted compared to experiment• Excitations to high-lying orbitals not suitably treated by exchange

functional• CAM-B3LYP[2] - Coulomb-attenuated method

• combines hybrid B3LYP with long range correction

• Method, Basis Set advice provided by Raghavachari Group

[1] M. J. T. Frisch, et. Al, Gaussian 09, Revision A.1 (Gaussian, Inc., Wallingford, CT, 2009).[2] T. Yanai, D. P. Tew, and N. C. Handy, Chem. Phys. Lett. 393 (2004) 51.

R2PD and PE Spectra

R2PDSpectrum

PESpectrum

EA

• PE spectrum• Anion: 2B1

• Neutral: 1A1

• EA not definitively assigned• ≤ 1.998 (10) eV• 3B1 ⟵ 2B1, 320 cm-1 progression

• Significant portion of R2PD spectrum above detachment continuum

• R2PD bears resemblance to PE Spectrum

G. E. Davico, R. L. Schwartz, T. M. Ramond, and W. C. Lineberger, J. Phys. Chem. A 103 (1999) 6167.

1.8 1.9 2.0 2.1 2.2 2.3 2.4Photon Energy (eV)

Rela

tive

Elec

tron

Sig

nal

R2PD Spectrum

220 cm-1

890 cm-1

220 cm-1

944 cm-1

40 cm-1

56 cm-1

65 cm-1

B

A

Band A : • Single peaks• Transitions are doublet ⟵ 2B1

Band B :• Doublets

• Spin-forbidden quartet ⟵ 2B1

• W ζ(5d) = 2085 cm-1

• Anion ground state E1/2

• Splitting is too small – not expected in doublet states• Quartet states resolve into two E1/2 sub-states• Similar splitting (47 cm-1) seen in R2PI of Bi3 [1]

• Assigned as transition from 4Aʺ(E3/2, E5/2) ⟵ 2Eʺ (E1/2)

ν1 symmetric stretchν2 bend

* *

*

1072 cm-1

823 cm-1

[1] C. A. Arrington and M. D. Morse, J. Phys. Chem. B 112 (2008) 16182.

x10

Simulations of Bands A and B

∠OWO(°) W−O (Å) ν1 (cm-1) ν2 (cm-1) ν3 (cm-1) Anion

2B1 116.5 1.735 965 264 8984B1 130.0 1.768 918 222 836

Neutral1A1 104.5 1.685 1072 374 10143B1 113.4 1.712 1017 314 945

ν1 (cm-1) ν2 (cm-1)

Band A 855 255

Band B 945 229

Band B

Band A

s-wave

1.831

2.0762.081

• Anion structure• Bond lengths and angles larger than neutral• Lower vibrational frequenciesExp.

Sim.

• Band A and B• Frequencies are more consistent with anion• Valence bound state

TDDFT Results• Which electrons are involved in the excitation?• Excitations below 3.7 eV involve HOMO and HOMO-1• Same orbitals as in the photoelectron spectrum• Transitions to W-local unoccupied orbitals• Multiconfigurational in nature• Orbital description and transitions energies vary for method used• Uncorrected B3LYP

• 26 states between 709 nm – 410 nm• CAM-B3LYP (Coulomb attenuating method) [1]

• 11 states between 709 nm – 410 nm

[1] T. Yanai, D. P. Tew, and N. C. Handy, Chem. Phys. Lett. 393 (2004) 51.

Photoelectron Spectrum

R2PDSpectrum

PESpectrum

EA

Unoccupiedorbitals

15a1

14b2

Anion Ground State, 2B1Neutral Ground State, 1A1

16b1

Neutral Excited State, 1B1Neutral Excited State, 3B1

ConclusionsR2PD of WO2

–

• At least two distinct electronic states observed• Assigned to valence bound states

• Bending and stretching frequencies similar to anion frequencies• Doublet fine structure in Band B• Spin forbidden quartet ⟵ 2B1 transition• Large spin-orbit splitting of W atom• Quartet state resolves to two E1/2 sub-states

• Diverging peaks in band B• Mimics frequencies of neutral• Coupling to a DBS?

• Unidentified peaks likely due to more than one additional electronic state• TDDFT

• Need a more sophisticated level of theory?• CASSCF, MRSDCI, FOCI, SOCI levels• Generated potential curves and spectroscopic constants • Transition metal containing carbides, WC [1]

[1] K. Balasubramanian, J. Chem. Phys. 112 (2000) 7425.

Acknowledgements

Prof. Caroline JarroldSarah Waller

Raghavachari Group

Frequency anharm ΔQ

Band A225 3 0.75

855 2 0.40

Band B229 3 0.38

945 3 0.10

Peak

Energy Spacing (cm-1)

Assignment

Spin-Orbit Splitting

(cm-1)

a0 0 0

a1 226

a2 218

a3 234

a4 0 895

a5 169

a6 210

a7 218

a8, bʹ 0 823

a9 210

a10 210

a11 202

b0 0 0 40

b1 226 40

b2 202 56

b3 0 944 65

b4 339 65

Term symbo

l

Relative energy

(eV)2B1 0.04B1 0.22

6Aʺ 4.211A1 2.043B1 2.185A2 4.62

Computational Results

Anion

Neutral

Current Study• Resonant two-photon detachment• Characterize anion bound states of transition metal containing oxides• WO2

−

• Neutral dipole moment: 3.53 Debye • PE spectrum previously assigned by Lineberger[1]• Anion: 2B1

• Neutral: 1A1

• C2v symmetry• Anion is open shell, many near degenerate 5d-like orbitals

• Observation of at least two different excited electronic states of WO2−

• Valence bound state• Possibly some DBS character• Spin allowed doublet doublet transition⟵• Spin-forbidden quartet doublet transition⟵

[1] G. E. Davico, R. L. Schwartz, T. M. Ramond, and W. C. Lineberger, J. Phys. Chem. A 103 (1999) 6167.

Doublets in Band B

• W ζ(5d) = 2085 cm-1

• Extended point group for selection rules• For a S = 1/2 ground state, B1 E1/2 = E1/2

• WO2– ground state is doubly degenerate 2E1/2

• Degeneracy broken by magnetic field or rotation (too small for our resolution!)

• Band B• Doublet Peaks• Quartet states (S = 3/2) resolve to two E1/2 sub-states• Arrington and Morse [1]

• Similar splitting (47 cm-1) seen in photon ionization of Bi3

• Assigned as transition from 4Aʺ(E3/2, E5/2) ⟵ 2Eʺ (E1/2)

16b1

15a1

14b2

17a1

BA

[1] C. A. Arrington and M. D. Morse, J. Phys. Chem. B 112 (2008) 16182.

Introduction

[1] P. J. Silva, R. A. Carlin, and K. A. Prather, Atmos. Environ. 34 (2000) 1811.[2] L. E. Hatch, J. M. Creamean, A. P. Ault, J. D. Surratt, M. N. Chan, J. H. Seinfeld, E. S. Edgerton, Y. X. Su, and K. A. Prather, Environ. Sci. Technol. 45 (2011) 5105.[3] M. Agundez, et al., Astron. Astrophys. 517 (2010) L2.

• Spectroscopic and computational studies characterizing cations far exceed that of anions

• Importance of negative ions increasingly evident• Atmospheric: Silicates, phosphates,[1] organosulfates (ROSO3H)[2]• Interstellar medium: C6H−, C4H−, C8H−, C3N−, C5N− and CN− [3]

• Negative ion photoelectron spectroscopy commonly used to characterize ground electronic states of anions

• Far fewer studies of excited states of anions• Excited states typically not bound relative to neutral + e− continuum

• Delocalized nature of high-lying electrons makes excited electronic states of anions difficult to characterize computationally

Anion doublet

Anion quartet

17

Term symbo

l

Relative energy

(eV)2B1 0.04B1 0.22

6Aʺ 4.211A1 2.043B1 2.185A2 4.62

Computational Results

Anion

Neutral