1. Evidence of Radiational Transitions in the Triplet Manifold of Large Molecules Haifeng Xu, Philip Johnson Stony Brook University Trevor Sears Brookhaven National Laboratory 61 st OSU International Symposium on Molecular Spectroscopy June 19-23, 2006, Columbus, Ohio

2. Outline Neutral Phenylacetylene and Benzonitrile Pump-Probe delayed Ionization Long-lived excited states lasting tens of microseconds Radiative transitions between triplet states

3. Pump-Probe ionization Excited state dynamics can be studied by time- delayed pump-probe ionization T1T1 IP S1S1 S0S0 Pump Probe ISC A. Sur and P. M. Johnson J. CHEM. PHYS. 84 1206-1209 (1986)

4. Decay of Phenylacetylene Fast decay Singlet S 1 state, ~50ns lifetime Slow decay anomalously long lifetime Pump: S 1  S 0 0 0 0 Probe: 193nm

5. Ionization threshold of long- lived phenylacetylene species 30 μs pump-probe delay Long-lived component being from ionization of lowest triplet state T 1 ~46500cm -1 T 1 ~3.05eV (this work) 3.12eV (Phosphorescence 1 ) 3.15eV (solid phase 2 ) 3.04eV (calculation 3 ) 1, Zhuravleva, et al, Opt. Spectroc. 22, 489 (1967). 2, Swiderek, et al, Chem. Phys. Lett. 285, 246 (1998) 3, Serrano-Andres,et al, J. Chem. Phys. 119, 4294 (2003)

6. Why such long lifetime? S 1 -T 1 gap ~1.4eV, triplet states formed initially by ISC are expected to be very highly vibrational excited, and should not have such long lifetimes. Lifetime of Cold triplet state can be very long. From dissociation of the clusters ( Hillenbrand et al JCP 95 2237(1991))? If cold triplet, how to make up the ~1.4eV S 1 -T 1 energy gap in collision free conditions?

7. Excitation scan of Phenylacetylene 20 ns delay 3 μs delay Simulation Not from Clusters But Monomers! 33

8. Mechanism Discussion Long-lived component is from ionization of the cold triplet T 1 state. Rotational resolved structure ensures it is from the monomers, not clusters. Only viable mechanism to relax isolated monomers from S 1 energy to cold vibrational levels of T 1 is by radiative emission.

9. Possible electronic triplet states for radiative emission S 0 ( 1 B 2 ) 0 S 1 ( 1 B 2 ) 4.45eV T 1 ( 3 A 1 ) 3.05eV T 2 ( 3 B 2 )/ T 3 ( 3 A 1 ) 4.26eV * Radiative Transition ISC Mechanism Discussion (cont.) * Swiderek, et al, Chem. Phys. Lett. 285, 246 (1998)

10. Benzonitrile

11. Conclusion Pump-probe delayed ionization of phenylacetylene and benzonitrile Evidence of triplet-triplet emission under supersonic molecular beam condition Analysis of emission should give information about radiationless transitions.

12. Acknowledgment Prof. Dr. Philip Johnson Prof. Dr. Trevor Sears Chemistry Department Stony Brook University Supported by U.S. Department of Energy

13. Experimental arrangement for decay measurements Pump Laser (scanned in time) Probe Laser Pulsed Valve Valve and probe are synchronized in time Sample