Ionization Thresholds of Small Carbon Clusters Tunable VUV
Ionization Thresholds of Small Carbon Clusters: Tunable VUV Experiments and Theory 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 30602 -2556 2 Chemical 3 Departments Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720 of Chemistry and Physics and Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720 http: //www. arches. uga. edu/~maduncan The Advanced Light Source (ALS) at Lawerence Berkeley National Laboratory Berkeley, California Tunable VUV radiation from 8 – 30 e. V
Background: Why Study Carbon Clusters? • 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 (Cn, 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)
Chemical Dynamics Beamline at ALS: Laser Ablation Endstation Problem: ALS output is quasicontinuous (500 MHz); cluster sources are pulsed at low repetition rate (usually 10 Hz). Solution: Use higher rate source (50 -100 Hz). Operate without synchronization. Either 0. 2 e. V or 0. 05 e. V 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 e. V (NIST) or 12. 15 e. V (H&H) Implications for fullerene growth mechanism
C 6 and C 6+ cc-p. VTZ CCSD(T) optimized structures and focal-point energetics S. E. Wheeler, W. D. Allen, and H. F. Schaefer, University of Georgia D∞h, 3Σg– +15. 3 kcal/mol D∞h, 2Π +8. 5 kcal/mol IPv = 10. 0 ± 0. 2 e. V D 3 h, 1 A 1´ +0. 0 kcal/mol C 2 v, 2 A 1 +0. 0 kcal/mol IPv = 10. 6 ± 0. 2 e. V Photoionization Threshold = 9. 45 ± 0. 1 e. V Charge Transfer Expt. IP = 9. 7 ± 0. 2 e. Va a. Bach, S. B. H. ; Eyler, J. R. , "Determination of Carbon Cluster Ionization Potentials Via Charge-Transfer Reactions, " J. Chem. Phys. 1990, 92, 358 -363.
C 7 and C 7+ cc-p. VTZ CCSD(T) optimized structures and focal-point energetics S. E. Wheeler, W. D. Allen, and H. F. Schaefer, University of Georgia D∞h, 1 Sg+ +0. 0 kcal/mol D∞h, 2 P +35. 4 kcal/mol IPv = 10. 4 ± 0. 2 e. V C 2 v, 1 A 1 +9. 6 kcal/mol C 2 v, 2 B 2 +0. 0 kcal/mol IPv = 9. 1 ± 0. 2 e. V Photoionization Threshold = 10. 1 ± 0. 1 e. V Charge Transfer Expt. IP = 8. 09 ± 0. 1 e. Va a. Bach, S. B. H. ; Eyler, J. R. , "Determination of Carbon Cluster Ionization Potentials Via Charge-Transfer Reactions, " J. Chem. Phys. 1990, 92, 358 -363.
C 9 and C 9+ cc-p. VTZ CCSD(T) optimized structures and focal-point energetics S. E. Wheeler, W. D. Allen, and H. F. Schaefer, University of Georgia D∞h, 1 Sg+ +0. 0 kcal/mol D∞h, 2 P +25. 7 kcal/mol IPv = 9. 6 ± 0. 2 e. V C 2, 1 A +3. 4 kcal/mol C 2 v, 2 B 1 +0. 0 kcal/mol IPv = 8. 8 ± 0. 2 e. V Photoionization Threshold = 9. 4 ± 0. 1 e. V Charge Transfer Expt. IP = 8. 76 ± 0. 1 e. Va a. Bach, S. B. H. ; Eyler, J. R. , "Determination of Carbon Cluster Ionization Potentials Via Charge-Transfer Reactions, " J. Chem. Phys. 1990, 92, 358 -363.
C 10 and C 10+ cc-p. VTZ CCSD(T) optimized structures and focal-point energetics S. E. Wheeler, W. D. Allen, and H. F. Schaefer, University of Georgia D∞h, 3 Sg– +70. 6 kcal/mol D 5 h, 1 A 1´ +0. 0 kcal/mol D∞h, 3 P +60. 0 kcal/mol IPv = 8. 8 ± 0. 2 e. V D 5 h, 2 Ag +0. 0 kcal/mol IPv = 9. 5 ± 0. 2 e. V Photoionization Threshold = 9. 2 ± 0. 1 e. V Charge Transfer Expt. IP = 9. 08 ± 0. 1 e. Va a. Bach, S. B. H. ; Eyler, J. R. , "Determination of Carbon Cluster Ionization Potentials Via Charge-Transfer Reactions, " J. Chem. Phys. 1990, 92, 358 -363.
Measured Ionization Thresholds, Theory, and Previous Experiments Cluster Size Expt. Threshold (e. V) Focal Pointa IPa/IPv 3 4 11. 6 ± 0. 2 --------- 12. 97 ± 0. 1 10. 35 ± 0. 1 10. 9/11. 3 (cyclic) ± 0. 2 11. 0/11. 1 (linear) ± 0. 2 12. 54 ± 0. 35 5 9. 9 ± 0. 1 11. 4/11. 4 (linear) ± 0. 3 10. 4/10. 8 (cyclic)± 0. 3 12. 26 ± 0. 1 11. 4 ± 0. 5 6 9. 45 ± 0. 1 10. 2/10. 6 (cyclic) ± 0. 2 9. 9/10. 0 (linear) ± 0. 2 9. 7 ± 0. 2 7 10. 1 ± 0. 1 10. 4/10. 4 (linear) ± 0. 2 8. 4/9. 1 (cyclic) ± 0. 2 8. 09 ± 0. 1 8 9. 15 ± 0. 1 8. 8/9. 0 (cyclic) ± 0. 2 9. 2/9. 3 (linear) ± 0. 2 8. 76 ± 0. 1 9 9. 4 ± 0. 1 9. 6/9. 6 (linear) ± 0. 2 8. 4/8. 8 (cyclic) ± 0. 2 8. 76 ± 0. 1 10 9. 2 ± 0. 1 9. 2/9. 5 (cyclic) ± 0. 2 8. 8/8. 8 (linear) ± 0. 2 9. 08 ± 0. 1 11 9. 4 ± 0. 2 7. 45 ± 0. 1 12 8. 4 ± 0. 2 8. 50 ± 0. 1 13 14 9. 3 ± 0. 2 8. 09 ± 0. 1 8. 7 ± 0. 2 8. 52 ± 0. 1 15 8. 9 ± 0. 2 7. 2 ± 0. 3 Charge Transfer. Electron Impact Expt IPb Expt. IPc a. This work. Focal point extrapolated values, computed at cc-p. VTZ CCSD(T) optimized geometries, except for C 11. 9 ± 0. 5 9 and C 10, which were computed at cc-p. VDZ 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, 358 -363. (2) Ramanathan, R. ; Zimmerman, J. A. ; Eyler, J. R. , "Ionization-Potentials of Small Carbon Clusters, " J. Chem. Phys. 1993, 98, 7838 -7845. 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 Cn (n = odd) and cyclic for Cn (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 e. V
Known energetics for carbon clusters species C 2 C 3 C 4+ C 5+ C 6+ C 7+ C 8+ Binding Energy per atom in e. V 3. 11 (Herzberg), 2. 9 (Raghavachari) 4. 23 (Raghavachari), 4. 58 (Curtiss) 4. 33 (Raghavachari) Measured Dissociation Energies in e. V 4. 7 (CID Anderson) 6. 0 (CID Anderson) 5. 2 (CID Anderson) 6. 3 (CID Anderson) 5. 3 (CID Anderson)
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