TUNABLE VACUUM ULTRAVIOLET PHOTOIONIZATION MEASUREMENTS OF CARBON CLUSTERS

TUNABLE VACUUM ULTRAVIOLET PHOTOIONIZATION MEASUREMENTS OF CARBON CLUSTERS E. Dinesh Pillai 1, Brian W. Ticknor 1, Musahid Ahmed 2, Leonid Belau 2, Stephen Leone 2, Steven E. Wheeler 1, Henry F. Schaefer III 1, Michael A. Duncan 1 1 Department of Chemistry, The University of Georgia Athens, Georgia 30602 -2556 2 Lawerence Berkeley National Laboratory, Berkeley, California http: //www. arches. uga. edu/~maduncan The Advanced Light Source (ALS) at Lawerence Berkeley National Laboratory Berkeley, California Tunable VUV radiation from 5 – 30 e. V

Chemical Dynamics Beamline at ALS: Laser Ablation Endstation Problem: ALS output is quasicontinuous; cluster sources are pulsed at low repetition rate (usually 10 Hz). Solution: Use higher rate source (50 -100 Hz). Operate without synchronization. 0. 2 e. V step size averaging 5 minutes 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 nanotube growth mechanism.

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)

IP=11. 6 e. V

Calculated structures for C 6 neutral and cation clusters at cc-p. VTZ ROCCSD(T) optimized geometries S. E. Wheeler and H. F. Schaefer, University of Georgia +15. 4 kcal/mol IP = 10. 0 e. V ± 0. 1 0. 0 kcal/mol IP = 10. 6 e. V ± 0. 1 Experimental IP = 9. 9 e. V ± 0. 2

cyclic linear

Calculated structures for C 7 neutral and cation clusters at cc-p. VTZ ROCCSD(T) optimized geometries S. E. Wheeler and H. F. Schaefer, University of Georgia 0. 0 kcal/mole IP = 10. 4 e. V ± 0. 1 +14. 7 kcal/mol IP = 8. 6 e. V ± 0. 1 Experimental IP = 10. 1 e. V ± 0. 2

linear cyclic

Cluster Size 3 ALS Expt. IP (e. V) Charge Transfer Expt. IP 11. 6 ± 0. 2 12. 97 ± 0. 1 a 10. 8 ± 0. 2 12. 54 ± 0. 35 a 10. 5 ± 0. 2 12. 26 ± 0. 1 a 6 9. 9 ± 0. 2 9. 7 ± 0. 2 a 7 10. 1 ± 0. 2 8. 09 ± 0. 1 b 8 10. 0 ± 0. 2 8. 76± 0. 1 b 9 9. 4 ± 0. 2 8. 76 ± 0. 1 b 10 8. 9 ± 0. 2 9. 08 ± 0. 1 b 11 9. 3 ± 0. 2 7. 45 ± 0. 1 b 12 8. 4 ± 0. 2 8. 50 ± 0. 1 b 13 8. 9 ± 0. 2 8. 09 ± 0. 1 b 14 8. 8 ± 0. 2 8. 50 ± 0. 1 b 15 9. 0 ± 0. 2 7. 2 ± 0. 3 b 4 5 a R. Ramanathan, J. A. Zimmerman, and J. R. Eyler, J. Chem. Phys. , 98 (1993) 7838; b. S. B. H. Bach and J. R. Eyler, J. Chem. Phys. , 92 (1990), 358.

Carbon Clusters: IP Versus Cluster Size

Conclusions • The first direct measurements of the ionization potentials of small carbon clusters have been made • There is a general downward trend in IP with increasing cluster size • The clusters exhibit some alteration in IP corresponding to clusters with an even or odd number of carbon atoms • Previous attempts to bracket IPs by measuring charge transfer reactions seem to overestimate our measured value at small sizes and underestimate it for larger sized clusters

Acknowledgements • Dr. Mike Duncan • ALS Chemical Dynamics Beamline Director Prof. Stephen Leone • Dr. Musa Ahmed LBNL Staff Scientist • Dr. Leonid Belau LBNL Postdoctoral Fellow • Stephen Wheeler and the Schaefer group at UGA • Air Force Office of Scientific Research and the National Science Foundation

Symmetry Electronic State Relative Energy C 4 D 2 h 1 A g – 1. 5 C 4+ C 2 v 2 A 1 – 5. 2 C 5 C 2 v 1 A 1 53. 0 C 5+ C 2 v 2 A 1 30. 4 C 6 D 3 h 1 A 1' – 15. 4 C 6+ C 2 v 2 A 1 – 8. 5 C 7 C 2 v 1 A 1 14. 7 C 7+ C 2 v 2 B 2 – 35. 5 C 8 C 4 h 1 A g – 10. 6 C 8+ C 4 h 2 A u – 20. 4 C 9 C 2 1 A 3. 4 C 9+ C 2 v 2 B 1 – 25. 7 C 10 D 5 h 1 A 1' – 70. 6 C 10+ D 5 h 2 A 2' – 56. 0 Table XX. Point group symmetries, electronic states, and energies relative to corresponding linear isomers (kcal mol– 1) of cyclic structures. a A negative value indicates that the cyclic isomer lies lower than the linear isomer.

Cluster Size Expt. IP (e. V) Charge Transfer Expt. IP CCSD(T)/cc-p. VDZ (most stable isomer)c extrapolated CCSD(T) computed at cc-p. VTZ ROCCSD(T) optimized geometriesd 11. 6 ± 0. 2 12. 97 ± 0. 1 a ----------------- 10. 8 ± 0. 2 12. 54 ± 0. 35 a 10. 3 (cyclic) 9. 4 (linear) 11. 3 (cyclic) ± 0. 1 11. 1 (linear) ± 0. 1 5 10. 5 ± 0. 2 12. 26 ± 0. 1 a 10. 5 (linear) 10. 0 (cyclic) 11. 4 (linear) ± 0. 2 10. 8 (cyclic)± 0. 2 6 9. 9 ± 0. 2 9. 7 ± 0. 2 a 8. 2 (linear) 9. 9 (cyclic) 10. 0 (linear) ± 0. 1 10. 6 (cyclic) ± 0. 1 7 10. 1 ± 0. 2 8. 09 ± 0. 1 b 8. 2 (cyclic) 9. 7 (linear) 8. 6 (cyclic) ± 0. 1 10. 4 (linear) ± 0. 1 8 10. 0 ± 0. 2 8. 76± 0. 1 b 8. 5 (cyclic) 7. 6 (linear) 9. 0 (cyclic) ± 0. 1 9. 3 (linear) ± 0. 1 9 9. 4 ± 0. 2 8. 76 ± 0. 1 b 8. 2 (cyclic) 9. 1 (linear) 9. 6 (linear) ± 0. 1 10 8. 9 ± 0. 2 9. 08 ± 0. 1 b 9. 0 (cyclic) 8. 2 (linear) 9. 5 (cyclic) ± 0. 1 8. 8 (linear) ± 0. 1 11 9. 3 ± 0. 2 7. 45 ± 0. 1 b 7. 6 (cyclic) 3 4 8. 6 (linear)

linear