MODERATE RESOLUTION JET COOLED CAVITY RINGDOWN SPECTROSCOPY OF THE A STATE OF NO3 RADICAL

Terrance J. Codd, Ming-Wei Chen, Mourad Roudjane and Terry A. Miller

The Ohio State University

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Introduction

• NO3 is the primary oxidant in the night-time troposphere

• Has three low lying electronic states: X2A2, A2E and

B2E

• Significant Jahn-Teller coupling is expected in both excited electronic states1

• A well resolved spectrum of the A state vibronic structure would provide information to determine the strength of this coupling

1. W. Eisfeld, K. Morokuma, J. Chem. Phys. 114 (2001) M. Okumura, J. F. Stanton, A. Deev, J. Sommar, Physica Scripta 73 (2006)

NO3 A State: Previous Experiments

• Ambient CRDS: Deev et al.1 • Made several assignments and estimated origin• Broad rotational contours and hot bands complicated the analysis

• Ne matrix: Jacox and Thompson2

• Several additional assignments• Studied several isotopes• Satellite bands from matrix interactions were present

1. A. Deev, J. Sommar, M. Okumura J. Chem. Phys. 122, 2243051 (2005)2. M. Jacox, W. Thompson J. Phys. Chem. A 114, 4712-4718 (2010)

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NO3: Vibronic Structure

• Red arrows show vibronically allowed transitions

• Vibronic structure here assumes linear and quadratic JT coupling are present

Mod-Res Jet Cooled Apparatus

Nd:YAG

20 Hz

Sirah

Dye Laser

H2 Raman CellFilters

1st or 2nd Stokes2-10 mJ

650 - 700 mJ 75 - 115 mJ

20 m Fiber Optic

~2 GHz FWHM

N2O5 in Neon

NO2 + NO3

PD

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Spectrum

• Shown is our jet cooled spectrum from 7550 cm-1 to 9750 cm-1.

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• Shown in the red is ambient data from Okumura’s lab

Spectrum

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• We used the Deev1

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• Additional assignments can be made following Jacox’s2 work

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1. A. Deev, J. Sommar, M. Okumura J. Chem. Phys. 122, 2243051 (2005)2. M. Jacox, W. Thompson J. Phys. Chem. A 114, 4712-4718 (2010)

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Analysis: Harmonic

• Using these assignments and the origin frequency from Deev it is possible to predict transition frequencies of unassigned bands

• Assignments can be made and then band frequencies and anharmonic constants can be fit

• As a first approximation, use harominic oscillator energy expressions with lowest order anharmonic terms

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Analysis: Harmonic

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Analysis: Harmonic

• The unassigned band lies at 8755.7 cm-1 (1691.7 cm-1 from origin) and the other at 9271.9 cm-1 (516.2 cm-1 to the blue of this band)

• The 3 mode has been predicted to be 1602 cm-1 using state-averaged CASSCF1

• The higher frequency band is consistent with a combination band with 4.

1. Eisfeld, W. Morokuma, K, J. Chem. Phys. 144, 9430, (2001)

Analysis: Simple Harmonic

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All units are in wavenumbersFitted

FrequenciesAnharmonic Constants

v1 772.73 v1 -4.603v2 713.59 v2 -10.268v3 1688.12 v3 0v4 511.20 v4 4.785

RMS Error: 6.42 cm-1

Analysis: Including Jahn-Teller

• A very simple analysis does a surprisingly good job of describing our spectra

• However, ab initio calculations indicate that the JT coupling in the A state should be strong

• A new version of SOCJT has been written which can include non-degenerate modes and bilinear coupling

• , exe, D, K, and bilinear coupling (b) can be fit

• NO3 has only 4 modes so it is possible to do a global fit

Analysis: Including Jahn-Teller

• The initial fit results are shown below• The RMS error is 11.86 cm-1.• If we disregard two bands involving ν1 and ν2 the RMS error

drops to 3.89 cm-1.• JT constants are effectively 0.

Mode Frequency Anharmonic D K

1 756.42 0.156-- --

2 682.24 0.126-- --

4 533.27 0.005 0 -0.00062

1-4 Bilinear Coupling Constant

 -0.00377

14 Combination Bands

• Combination bands of 1 and 4 have a distinct ‘split-parallel’ band contour

• Based on symmetry considerations they should have a parallel band type.

• The splitting is as large as 26 cm-1 but is smaller for most bands

• This is unique to the 14 combination bands and is seen in all of them.

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Spectrum

• Shown is our jet cooled spectrum from 7550 cm-1 to 9750 cm-1.

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Conclusions

• Little or no evidence of strong JT coupling in the 4 mode

• Spectrum is well described by even a very simple analysis

• Fitting our spectrum including JT terms does not significantly improve our fit and

• More work needs to be done to understand the nature of the splitting observed in the 14 combination bands

Acknowledgements

• Terry Miller• Miller Group

• Neal Kline• Rabi Chhantyal-Pun• Mourad Roudjane• Takashige Fujiwara• Dianping Sun• Ming-Wei Chen

• Mitchio Okumura for allowing the use of his data

• NSF - $$$

• You for your attention!Currently at University of Illinois Urbana-Champaign