PUMP-PROBE DELAYED IONIZATION STUDY OF PHENYLACETYLENE Gary V. Lopez*, Chih-hsuan Chang†, Trevor Sears*† and Philip Johnson* *Stony Brook University †Brookhaven National Laboratory 06/21/10 65th OSU ISMS

Motivation: Background radiationless transitions play an important role in the decay of excited states in aromatic molecules radiationless transition: a change in the electronic state of the molecule that occurs when no radiation has been absorbed or emitted these transitions are associated with internal conversion (IC) to vibrationally excited S0 levels and intersystem crossing (ISC) intersystem crossing (ISC) takes place between the first singlet excited state (S1) and the lowest triplet state (T1) 06/21/10 65th OSU ISMS

Typical behavior-Pyrazine IP S1 S0 Pump Probe/Ionization ISC Excited state dynamics can be studied by time- delayed pump-probe ionization Figure 1*: Decay of the fluorescence intensity of pyrazine-d4 and pyrazine-h4 following nanosecond excitation of the Q branch of 000 band of the 1B3u ← 1Ag transition in a He supersonic jet. *Figure taken from Kommandeur et al., Ann. Rev. Phys. Chem., 1987, 38, 433 06/21/10 65th OSU ISMS

Recent Experiments-Phenylacetylene (PA) Fast decay  Singlet S1 state due to fluorescence ~75ns lifetime Slow decay : Long-lived species ~100 ms lifetime PA isomers with low IP? No Identity determination Figure 2*: Decay Curve of PA ions as a function of delay time between pump and probe lasers. The wavelength of the pump laser corresponds to the origin of S1←S0 transition, while an excimer laser was used to ionize the molecules. The fast initial decay is due to the decay of S1. *Figure taken from Hofstein et al., J. Phys. Chem. A, 2008, 112, 1195 06/21/10 65th OSU ISMS

Goal: try to identify the long-lived species formed after the photoionization of the PA S1 state Strategy: Experimentally: look at the decay curves for the strong bands in the ionization of the S1<--S0 transition Theoretically: do IP calculations of PA isomers and compare them with the energy used to ionized the S1 state (probe/ionization laser) 06/21/10 65th OSU ISMS

Experimentally: Pump: S1S0 Probe: 193nm Figure 3*: 1+1 REMPI of Phenylacetylene (S1 state) *Figure taken from Chang et al., out for publication 06/21/10 65th OSU ISMS

(S1) / (Long-Lived Species) Experimentally: Results Band (S1) / (Long-Lived Species) SD* Origin 3.03 0.38 15 1.48 0.33 6a 1.75 0.19 bcc 3.11 0.36 1 2.77 0.35 *SD: Standard Deviation 06/21/10 65th OSU ISMS

Vibrational Modes PA S1 n6a PA S1 n15 Ring Breathing Phenyl CCH bend 06/21/10 65th OSU ISMS

CYCLOOCTA-1,3,5-TRIEN-7-YNE (COTY) Possible Isomers considering PA S1 n15 Here we can see some possible PA isomers that could be formed considering the PA S1 mode 15 PHENYL-VINYLEDENE (PV) BENZOCYCLOBUTADIENE (BCBD) CYCLOOCTA-1,3,5-TRIEN-7-YNE (COTY) 06/21/10 65th OSU ISMS

Theoretically: Calculate Ionization Potentials (IP) for the possible PA isomers and compare them with the IP for PA (S1) and experiment Density functional theory (B3LYP: Becke, Lee, Yang and Parr) was used at 6-31+G(d,p) basis set in Gaussian03W. IP for Singlet and triplet states were calculated. 06/21/10 65th OSU ISMS

Theoretically: Results Probe laser: 193nm (6.4 eV) IP (eV) State PV BCBD COTY singlet 8.6 7.4 7.3 triplet 6.9 6.1 BENZOCYCLOBUTADIENE (BCBD) BCBD triplet state is a good candidate to be isomer formed after S1←S0 excitation of PA in pump-probe delay photoionization experiments. 06/21/10 65th OSU ISMS

Conclusions Long-Lived species (PA isomers and low IP) are formed after the photoionization of the PA S1 state Experimental evidence indicates that mode 15 and 6a could be the promoters Theoretical calculations indicate that BCBD triplet state is a good candidate to be the long- lived species since it is a PA isomer with low IP 06/21/10 65th OSU ISMS

Acknowledgments: Sears' Group: Funding: Dr. Philip Johnson Dr. Trevor Sears Dr. Chih-hsuan Chang Matthew Cich Funding: DOE 06/21/10 65th OSU ISMS

Thank you!!! 06/21/10 65th OSU ISMS