Cavity Ringdown Spectroscopy of the A 2A2 - B 2B2 Vibronically Mixed Excited

States of the Benzyl Radical and the 1 2A2 ← X 2B1 Transition of the Phenoxy

Radical in a Supersonic Expansion

Michael N. Sullivan*, Keith Freel, J. Park, M.C. Lin, and Michael C. Heaven

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Soot Formation and PAH’s

D’Anna, A.; Violi, A.; D’Alessio, A. Combust. Flame 121, 418 (2000).

Benzyl and Phenoxy Radicals

• Benzyl (C6H5CH2)– Combustion (PAH formation)

• Phenoxy (C6H5O)– Combustion (PAH formation)– Atmospheric & biological

Previous Studies

• Phenoxy– First obs. by Porter and Wright1

– Numerous studies (see ref. 2 )– Conflicting results – Recent work

• Matrix isolation studies in the UV and Vis regions by Radziszewski et al.2

• Cavity ringdown studies in the UV by Tonokura et al.3

• Theoretical study of the first excited state by Witek et al.4

• Benzyl– First obs. by Schüler et al.5

– LIF studies in a supersonic jet by Miller et al.6,7

– A 2A2 - B 2B2 vibronically mixed excited states studied previously by Widen and Weisshaar8

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1Porter, G. and Wright, F.J., Trans. Faraday Soc. 51, 1469 (1955).2Radziszewski, J. G.; Gil, M.; Gorski, A.; Spanget-Larsen, J.; Waluk, J.; Mroz, B. J., J. Chem. Phys. 115, 9733 (2001).3Tonokura, K.; Ogura, T.; Koshi, M., J. Phys. Chem. A 108, 7801 (2004).4Cheng, C.; Lee, Y.; Witek, H., J. Phys. Chem. A 112, 2648 (2008).5Schüler, H.; Reinebeck, L.; Kaberle, A.R., Z. Naturforsch. 79, 421 (1952).6Heaven, M; DiMauro, L.; Miller, T.A., Chem. Phys. Lett. 95, 347 (1983).7Lin, T.; Tan, X.; Cerny, T.; Williamson, J.; Cullin, D.; Miller, T.A., Chem. Phys. 167, 203 (1992).8Eiden, G. and Weisshaar, J., J. Chem. Phys. 104, 8896 (1996).

Cavity Ring-Down SpectroscopyLoss = (2αL)(tc/2L) Total loss = [(1-R)+ αL] (tc/L)

PMTR R

PMTRR

Absorbing Sample Added

Empty Cavity

R at 620 nm, 99.9985%. Pathlength of 6 km.

I(t) = I0 exp{-[(1-R) + αL](tc/L)]

α = absorption coefficient t = timeL = length of cavityR = mirror reflectivityI = intensity

Excimer Pumped Dye Laser

Mirror Curtains

Valves/Discharge

PMT

Computer

Cavity MirrorCavity Mirror

Vacuum Chamber

Three Pulsed Solenoid Valves

1

1. Ground Plate2. Phenolic Insulator3. High Voltage Jaw

2

3

Experimental Setup• Radical Production

– Electrical Discharge– Jet Expansion Cooling

• Radical Detection– Cavity Ringdown Spectroscopy

H. Linnartz, O. Vaizert, P. Cias, L. Gruter, and J. Maier, Chem. Phys. Lett. 345, 89 (2001).L. Biennier, F. Salama, L. J. Allamandola, and J. J. Scherer, J. Chem. Phys. 118, 7863 (2003).W. Shenghai, P. Dupre, P. Rupper, and T. A. Miller, J. Chem. Phys. 127, 224305 (2007).

Experimental Conditions

• Precursor – benzyl chloride (C6H5CH2Cl)

• 1% precursor in 1 atm back pressure Ar (at 60 °C)

• 3 pinhole pulsed expansion– 900V– 100 mA– Slit width: 0.5 mm– Detection distance: 8 mm

Benzyl Radical

445.7 445.8 445.9 446.03.00E-009

4.00E-009

5.00E-009

6.00E-009

7.00E-009

Wavelength (nm)

446.6 446.7 446.8 446.9

3.00E-009

6.00E-009

9.00E-009

1.20E-008

Wavelength (nm)

447.8 447.9 448.0 448.1

6.00E-009

1.20E-008

1.80E-008

2.40E-008

Wavelength (nm)

Benzyl RadicalRot. Temp ≈ 20K

Experimental Conditions

• Precursor – anisole (C6H5OCH3)

• 1% precursor in .6 atm back pressure Ar (at 24°C)

• 3 pinhole pulsed expansion– 1200V– 100 mA– Slit width: 0.8 mm– Detection distance: 8 mm

Phenoxy Radical

Ar

C2

ν11 1

0

ν11 20

ν11 30

Optimized Geometry

C2C1

C3

C4C5

C6

Level B3LYP B3LYP TDDFT

Basis cc-pVDZ cc-pVDZ cc-pVDZ

State X A B X-A X-B

rCO 1.256 1.323 1.244 -0.067 0.012

rC1C2 1.455 1.413 1.451 0.041 0.004

rC2C3 1.380 1.394 1.439 -0.014 -0.059

rC3C4 1.412 1.397 1.394 0.015 0.018

rC2H1 1.092 1.090 1.093 0.002 -0.001

rC3H2 1.093 1.093 1.094 0.000 -0.002

rC4H3 1.092 1.091 1.089 0.001 0.004

aC6C1C2 117.0 119.9 111.7 -2.917 5.312

aC1C2C3 121.0 119.2 123.6 1.733 -2.602

aC2C3C4 120.2 121.4 122.4 -1.165 -2.158

aC1C2H1 116.9 119.1 117.3 -2.263 -0.460

aC4C3H2 119.5 120.2 119.7 -0.702 -0.233

Values of r = ÅValues of a = degrees

H1

H2

H3

H4

H5

(Values in cm-1 ) B3LYP TDDFT

Basis cc-pVDZ cc-pVDZ

E. state X x0.967 B x0.967 X-BA1 528.31 511 515 498 -13A1 809.11 782 814 787 5A1 978.37 946 968 936 -10A1 1011.62 978 1040 1005 27A1 1151.92 1114 1166 1127 13A1 1413.92 1367 1413 1367 -1A1 1485.36 1436 1581 1529 93A1 1594.58 1542 1630 1576 34A1 3174.85 3070 3172 3067 -3A1 3196.61 3091 3194 3089 -3A1 3207.80 3102 3230 3123 21

A2 381.19 369 335 324 -44A2 810.79 784 836 809 25A2 990.27 958 979 947 -11

B1 194.71 188 109 106 -83B1 488.67 473 413 400 -73B1 660.97 639 530 512 -127B1 807.17 781 765 739 -41B1 933.46 903 863 834 -68B1 1007.24 974 984 952 -22

B2 443.38 429 430 416 -13B2 595.50 576 593 574 -2B2 1082.63 1047 1019 985 -61B2 1155.69 1118 1165 1126 9B2 1272.08 1230 1245 1204 -27B2 1344.49 1300 1356 1312 12B2 1443.05 1395 1429 1382 -14B2 1549.28 1498 1620 1567 69B2 3181.75 3077 3175 3070 -6B2 3204.41 3099 3193 3088 -11

Theoretical Results

Ground StateA: 0.1832835 B: 0.0923975 C: 0.0614295

Excited StateA: 0.1938119 B: 0.0865032 C: 0.0598089

Rot. Temp ≈ 45KRot. Temp ≈ 20K

Excited state lifetime ≈ 88 +/- 10 fs

Lorenztian ≈ 60 cm-1 FWHM

Rot. Temp ≈ 45KRot. Temp ≈ 20K

Excited state lifetime ≈ 88 +/- 10 fs

Lorenztian ≈ 60 cm-1 FWHM

Summary

• Detection of discharge generated benzyl and phenoxy radicals by CRDS in a jet expansion

• Determined excited state lifetime for the phenoxy radical to be 88 +/- 10 fs

• Vibronic spectrum simulated using TDDFT calculations match well with experimental data

• Future work on this project will focus on the phenyl peroxy (C6H5O2) radical and is currently in progress

Acknowledgements

• Group Members and Colleagues:Dr. Jiande Han, Ivan Antonov, Kyle Mascaritolo, Joshua Barlett, Chien-Lin Tseng

• Cody Anderson at the Emory Machine Shop

• Dept. of Energy

• Thank you for listening!

References• D’Anna, A.; Violi, A.; D’Alessio, A. Combust. Flame 121, 418 (2000).• Porter, G. and Wright, F.J., Trans. Faraday Soc. 51, 1469 (1955).• Radziszewski, J. G.; Gil, M.; Gorski, A.; Spanget-Larsen, J.; Waluk, J.; Mroz,

B. J., J. Chem. Phys. 115, 9733 (2001).• Tonokura, K.; Ogura, T.; Koshi, M., J. Phys. Chem. A 108, 7801 (2004).• Cheng, C.; Lee, Y.; Witek, H., J. Phys. Chem. A 112, 2648 (2008).• Schüler, H.; Reinebeck, L.; Kaberle, A.R., Z. Naturforsch. 79, 421 (1952).• Heaven, M; DiMauro, L.; Miller, T.A., Chem. Phys. Lett. 95, 347 (1983).• Lin, T.; Tan, X.; Cerny, T.; Williamson, J.; Cullin, D.; Miller, T.A., Chem. Phys.

167, 203 (1992).• Eiden, G. and Weisshaar, J., J. Chem. Phys. 104, 8896 (1996).• H. Linnartz, O. Vaizert, P. Cias, L. Gruter, and J. Maier, Chem. Phys. Lett.

345, 89 (2001).• L. Biennier, F. Salama, L. J. Allamandola, and J. J. Scherer, J. Chem. Phys.

118, 7863 (2003).• W. Shenghai, P. Dupre, P. Rupper, and T. A. Miller, J. Chem. Phys. 127,

224305 (2007).