1. 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

2. 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

3. Typical behavior-PyrazineIP 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

4. 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

5. 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

6. 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

7. (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
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65th OSU ISMS

8. Vibrational Modes PA S1 n6a PA S1 n15 Ring Breathing Phenyl CCH bend
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65th OSU ISMS

9. 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

10. 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

11. 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

12. 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

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

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