PUMPPROBE DELAYED IONIZATION STUDY OF PHENYLACETYLENE Gary V
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 65 th OSU ISMS 1
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 S 0 levels and intersystem crossing (ISC) takes place between the first singlet excited state (S 1) and the lowest triplet state (T 1) 06/21/10 65 th OSU ISMS 2
Typical behavior-Pyrazine IP Probe/Ionization S 1 ISC Pump T 1 S 0 • Excited state dynamics can be studied by timedelayed pump-probe ionization 06/21/10 Figure 1*: Decay of the fluorescence intensity of pyrazine-d 4 and pyrazine-h 4 following nanosecond excitation of the Q branch of 000 band of the 1 B 3 u ← 1 Ag transition in a He supersonic jet. *Figure taken from Kommandeur et al. , Ann. Rev. Phys. Chem. , 1987, 38, 433 65 th OSU ISMS 3
Recent Experiments-Phenylacetylene (PA) Fast decay Singlet S 1 state due to fluorescence ~75 ns 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 S 1←S 0 transition, while an excimer laser was used to ionize the molecules. The fast initial decay is due to the decay of S 1. *Figure taken from Hofstein et al. , J. Phys. Chem. A, 2008, 112, 1195 06/21/10 65 th OSU ISMS 4
Goal: try to identify the long-lived species formed after the photoionization of the PA S 1 state Strategy: Experimentally: look at the decay curves for the strong bands in the ionization of the S 1<--S 0 transition Theoretically: do IP calculations of PA isomers and compare them with the energy used to ionized the S 1 state (probe/ionization laser) 06/21/10 65 th OSU ISMS 5
Experimentally: Pump: S 1 S 0 Probe: 193 nm Figure 3*: 1+1 REMPI of Phenylacetylene (S 1 state) *Figure taken from Chang et al. , out for publication 06/21/10 65 th OSU ISMS 6
Experimentally: Results Band (S 1) / (Long-Lived Species) SD* Origin 3. 03 0. 38 15 1. 48 0. 33 6 a 1. 75 0. 19 bcc 3. 11 0. 36 1 2. 77 0. 35 *SD: Standard Deviation 06/21/10 65 th OSU ISMS 7
Vibrational Modes PA S 1 n 6 a Ring Breathing PA S 1 n 15 Phenyl CCH bend 06/21/10 65 th OSU ISMS 8
Possible Isomers considering PA S 1 n 15 PHENYL-VINYLEDENE (PV) 06/21/10 BENZOCYCLOBUTADIENE (BCBD) 65 th OSU ISMS CYCLOOCTA-1, 3, 5 -TRIEN-7 -YNE (COTY) 9
Theoretically: Calculate Ionization Potentials (IP) for the possible PA isomers and compare them with the IP for PA (S 1) and experiment Density functional theory (B 3 LYP: Becke, Lee, Yang and Parr) was used at 6 -31+G(d, p) basis set in Gaussian 03 W. IP for Singlet and triplet states were calculated. 06/21/10 65 th OSU ISMS 10
Theoretically: Results IP (e. V) State PV BCBD COTY singlet 8. 6 7. 4 7. 3 triplet 6. 9 6. 1 6. 9 BENZOCYCLOBUTADIENE (BCBD) Probe laser: 193 nm (6. 4 e. V) BCBD triplet state is a good candidate to be isomer formed after S 1←S 0 excitation of PA in pump-probe delay photoionization experiments. 06/21/10 65 th OSU ISMS 11
Conclusions Long-Lived species (PA isomers and low IP) are formed after the photoionization of the PA S 1 state Experimental evidence indicates that mode 15 and 6 a could be the promoters Theoretical calculations indicate that BCBD triplet state is a good candidate to be the longlived species since it is a PA isomer with low IP 06/21/10 65 th OSU ISMS 12
Acknowledgments: Sears' Group: Dr. Philip Johnson Dr. Trevor Sears Dr. Chih-hsuan Chang Matthew Cich Funding: 06/21/10 DOE 65 th OSU ISMS 13
Thank you!!! 06/21/10 65 th OSU ISMS 14
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