Ultrafast molecular photodissociation dynamics studied by femtosecond photoelectron-photoion coincidence spectroscopy Bernhard Thaler Graz University of Technology, Institute of Experimental Physics International Symposium on Molecular Spectroscopy 2017
Motivation: time-resolved coincidence measurements in the gas phase population transfer energy conversion => fragmentation photoionization => fragmentation in neutral or ion? relaxation always to lowest states (Kashas rule) => Energy conversion leads to fragmentation
Time Resolved PhotoElectron PhotoIon COincidence (PEPICO) Spectroscopy electron kinetic energy ion masses
Time Resolved PEPICO: Experiment Time-of-Flight SHG / THG SHG magnetic bottle HV
Toy molecule acetone: electronic structure D0 Sn Rydberg series : 3p acetone IC mention Acetone as the standard ketone, π- π* S0 Shastri, A. Singh, PJ., J. Quant. Spectr. Rad. Trans., V.273, 92-105, 2016 Diau, EW., Kotting C., Solling T., Zewail, AH.Chem. Phy. Chem. 3, 57-78, 2002
Activation energy for fragmentation D0 Sn : 3p ΔE π- π* S0 Energy conversion threshold: (0.79 ±0.03) eV Koch M., Heim P., Thaler B., Kitzler. M., Ernst, WE., J. Phys. B: At. Mol. Opt. Phys. 50, 125102, 2017
Population transfer τ1 = 320 fs τ1 τ2 τ2 = 80 – 130 fs D0 Sn : 3p π- π* τ2 τ2 = 80 – 130 fs S0 Koch M., Thaler B., Heim P., Ernst W.E., submitted, 2017
Population transfer τ1 = 230 fs τ2 τ2 = 80 – 130 fs D0 Sn : 3p π- π* Koch M., Thaler B., Heim P., Ernst W.E., submitted, 2017
Fragmentation in the ion AND the neutral Δt = 20 ps Sn ? Norrish Type 1 dissociation : 3p 3p, 3d,… Sn π- π* excited acetyl 3p S0 Maierhofer P., Bainschab M., Thaler B., Heim P., Ernst W.E., Koch M., J. Phys. Chem. A 120, 6418-23, 2016
Motivation and Outlook Sn : 3p directly observe population transfer to lower states: 3s and π- π* 3s π- π* work in progress: access states directly: OPA
Conclusions couplings of Rydberg series with valence states (π- π*) provide very efficient relaxation pathways we observe direct population transfer between Rydberg states the energy conversion causes fragmentation of the molecule PEPICO spectroscopy allows to distinguish fragmentation in the ion or in the neutral
Acknowledgements Femtosecond Dynamics group: Sascha Ranftl Stefan Cesnik Ass. Prof. Dr. Markus Koch Bernhard Thaler Pascal Heim Leonhard Treiber Institute of Experimental Physics Financial support:
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Ultrafast photodissociation dynamics in gas phase acetone vary pulse length rise of electron band of lower Rydberg states for longer pulses => population transfer within pulse duration strong increase of acetyl signal for negatively chirped pulses => different relaxation channels are activated Koch M., Heim P., Thaler B. et. al., J. Phys. B: At. Mol. Opt. Phys. 50, 125102, 2017
Ultrafast photodissociation dynamics in gas phase acetone time resolved study population decrease in Sn plateau for the 3p/3d/… states => direct observation of population transfer Koch M., Heim P., Thaler B. et. al., J. Phys. B: At. Mol. Opt. Phys. 50, 125102, 2017