1. 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

2. 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

3. Time Resolved PhotoElectron PhotoIon COincidence (PEPICO) Spectroscopy
electron kinetic energy
ion masses

4. Time Resolved PEPICO: Experiment
Time-of-Flight
SHG / THG
SHG
magnetic bottle HV

5. Toy molecule acetone: electronic structureD0 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, , 2016 Diau, EW., Kotting C., Solling T., Zewail, AH.Chem. Phy. Chem. 3, 57-78, 2002

6. Activation energy for fragmentationD0 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, , 2017

7. 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

8. 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

9. 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, , 2016

10. Motivation and OutlookSn : 3p
directly observe population transfer to lower states: 3s and π- π* 3s
π- π*
work in progress: access states directly: OPA

11. 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

12. 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:

13. Thank you for your attention!

14. 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, , 2017

15. 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, , 2017