The Advanced Light Source (ALS) at Lawerence Berkeley National Laboratory Berkeley, California Tunable VUV radiation from 8 – 30 eV Brian W. Ticknor 1, Leonid Belau 2, Steven E. Wheeler 1, Musahid Ahmed 2, Stephen R.Leone 2,3, Wesley D. Allen 1, Henry F. Schaefer III 1, Michael A. Duncan 1 1 Department of Chemistry, The University of Georgia Athens, Georgia Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California Departments of Chemistry and Physics and Lawrence Berkeley National Laboratory, University of California, Berkeley, California Ionization Thresholds of Small Carbon Clusters: Tunable VUV Experiments and Theory

Isomers: linear chains, rings, and fullerenes Important in combustion, astrophysics, and as precursors for materials IP values vs. cluster size may provide insight into structures and bonding of neutral clusters Previous experimental measurements of carbon clusters IPs (C n, n > 4) limited to charge transfer bracketing technique employed by Eyler (C 3 – C 24 ) and electron impact study by Benedikt and co-workers (C 1 – C 5 ) Background: Why Study Carbon Clusters?

Chemical Dynamics Beamline at ALS: Laser Ablation Endstation Problem: ALS output is quasi- continuous (500 MHz); cluster sources are pulsed at low repetition rate (usually 10 Hz). Solution: Use higher rate source ( Hz). Operate without synchronization. Either 0.2 eV or 0.05 eV step size for synchrotron light, averaging 8000 ablation laser shots per step

C 3 (not C 2 ) is the most abundant species present IP(C 2 ) = 11.4 eV (NIST) or eV (H&H)  Implications for fullerene growth mechanism

D ∞h, 3 Σ g – kcal/mol D ∞h, 2 Π +8.5 kcal/mol IP v = 10.0 ± 0.2 eV D 3h, 1 A 1 ´ +0.0 kcal/mol C 2v, 2 A kcal/mol IP v = 10.6 ± 0.2 eV Photoionization Threshold = 9.45 ± 0.1 eV Charge Transfer Expt. IP = 9.7 ± 0.2 eV a C 6 and C 6 + cc-pVTZ CCSD(T) optimized structures and focal-point energetics S.E. Wheeler, W.D. Allen, and H.F. Schaefer, University of Georgia a Bach, S. B. H.; Eyler, J. R., "Determination of Carbon Cluster Ionization Potentials Via Charge-Transfer Reactions," J. Chem. Phys. 1990, 92,

D ∞h, 1  g kcal/mol C 2v, 1 A kcal/mol D ∞h, 2  kcal/mol IP v = 10.4 ± 0.2 eV C 2v, 2 B kcal/mol IP v = 9.1 ± 0.2 eV Photoionization Threshold = 10.1 ± 0.1 eV Charge Transfer Expt. IP = 8.09 ± 0.1 eV a C 7 and C 7 + cc-pVTZ CCSD(T) optimized structures and focal-point energetics S.E. Wheeler, W.D. Allen, and H.F. Schaefer, University of Georgia a Bach, S. B. H.; Eyler, J. R., "Determination of Carbon Cluster Ionization Potentials Via Charge-Transfer Reactions," J. Chem. Phys. 1990, 92,

D ∞h, 1  g kcal/mol C 2, 1 A +3.4 kcal/mol D ∞h, 2  kcal/mol IP v = 9.6 ± 0.2 eV C 2v, 2 B kcal/mol IP v = 8.8 ± 0.2 eV Photoionization Threshold = 9.4 ± 0.1 eV Charge Transfer Expt. IP = 8.76 ± 0.1 eV a C 9 and C 9 + cc-pVTZ CCSD(T) optimized structures and focal-point energetics S.E. Wheeler, W.D. Allen, and H.F. Schaefer, University of Georgia a Bach, S. B. H.; Eyler, J. R., "Determination of Carbon Cluster Ionization Potentials Via Charge-Transfer Reactions," J. Chem. Phys. 1990, 92,

D ∞h, 3  g – kcal/mol D 5h, 1 A 1 ´ +0.0 kcal/mol D ∞h, 3  kcal/mol IP v = 8.8 ± 0.2 eV D 5h, 2 A g +0.0 kcal/mol IP v = 9.5 ± 0.2 eV Photoionization Threshold = 9.2 ± 0.1 eV Charge Transfer Expt. IP = 9.08 ± 0.1 eV a C 10 and C 10 + cc-pVTZ CCSD(T) optimized structures and focal-point energetics S.E. Wheeler, W.D. Allen, and H.F. Schaefer, University of Georgia a Bach, S. B. H.; Eyler, J. R., "Determination of Carbon Cluster Ionization Potentials Via Charge-Transfer Reactions," J. Chem. Phys. 1990, 92,

Measured Ionization Thresholds, Theory, and Previous Experiments Cluster Size Expt. Threshold (eV)Focal Point a Charge Transfer. Electron Impact IP a /IP v Expt IP b Expt. IP c ± ± ± /11.3 (cyclic) ± ± ± /11.1 (linear) ± ± /11.4 (linear) ± ± ± /10.8 (cyclic)± ± /10.6 (cyclic) ± ± /10.0 (linear) ± ± /10.4 (linear) ± ± /9.1 (cyclic) ± ± /9.0 (cyclic) ± ± /9.3 (linear) ± ± /9.6 (linear) ± ± /8.8 (cyclic) ± ± /9.5 (cyclic) ± ± /8.8 (linear) ± ± ± ± ± ± ± ± ± ± ± 0.3 a This work. Focal point extrapolated values, computed at cc-pVTZ CCSD(T) optimized geometries, except for C 9 and C 10, which were computed at cc-pVDZ CCSD(T) optimized geometries. b (1) Bach, S. B. H.; Eyler, J. R., "Determination of Carbon Cluster Ionization Potentials Via Charge-Transfer Reactions," J. Chem. Phys. 1990, 92, (2) Ramanathan, R.; Zimmerman, J. A.; Eyler, J. R., "Ionization-Potentials of Small Carbon Clusters," J. Chem. Phys. 1993, 98, c Benedikt, J.; Agarwal, S.; Eijkman, D.; Vandamme, W.; Creatore, M.; van de Sanden, M.C.M., J. Vac. Sci. Tech. A 2005, 23, 1400.

Blue/red circle/ rectangle symbols indicate the vertical IPs calculated for the more/less stable ring/chain structures. General downward trend in IP with cluster size is expected Most stable structure has highest IP (exception C 8 )

Conclusions First study of photoionization thresholds of carbon clusters Successful coupling of quasi-continuous ALS to pulsed experiment C 3 + dominates mass spectrum at high photon energy, indicating C 3 neutral has high abundance in laser generated carbon plasmas New theoretical treatment shows the lowest energy isomers for neutrals are linear for C n (n = odd) and cyclic for C n (n = even) For C 4 -C 6, onset of ionization occurs below calculated values of either isomer due to population of low lying electronic excited states. Comparison to theory suggests the measured IP’s for the larger odd numbered clusters (n = 7, 9, 11, 13) correspond to the linear structure C 10 and larger even numbered species more clearly favor cyclic structure Acknowledgements: Air Force Office of Scientific Research Department of Energy

IP=11.6 eV

speciesBinding Energy per atom in eV C (Herzberg), 2.9 (Raghavachari) C (Raghavachari), 4.58 (Curtiss) C (Raghavachari) Measured Dissociation Energies in eV C (CID Anderson) C (CID Anderson) C (CID Anderson) C (CID Anderson) C (CID Anderson) Known energetics for carbon clusters