Dissipation and Coherence: Halogens in Rare Gas Solids

1. Dissipation and Coherence:Halogens in Rare Gas Solids • Signatures of Dissipation in Pump-Probe Spectra • Dissipation of Energy in Excited Halogens • Dispersion and „Decoherence“: Classical vs. Quantum Effects • New Experiments with Phase-Locked Pulses M. Bargheer, M. Gühr, P. Dietrich, M. Fushitani, T. Kiljunen and N. Schwentner Institut für Experimentalphysik

2. strong coupling to „solvent“ • some similarities to gas phase Diatomics in Solid Rare Gas • I2 in Kr • Fcc lattice, closest packing • Adiabatic dynamics including dissipation are well described in classical simulations

3. Energy loss from oscillations Bargheer et al., PCCP 4, 75 (2002)

4. Energy loss from signal envelope

5. Vibrational relaxation ofI2/Kr

6. Vibrational relaxation ofI2/Kr, ClF/Ar and Cl2/Ar

7. well defined timining of collision • well defined scattering geometry! Collisions Cause Coherence? Collision of I2 with surrounding Kr • Width of wavepacket: 500 cm-1 • Energy loss in collision: 1500 cm -1 • Collision populates new vibrational levels coherently!

8. DE Mechanisms of „Dephasing“ • Decoherence due to collisions with solvent (pure dephasing T2´) • Population decay by vibrational relaxation (and non-adiabatic couplings) (relaxation time T1) • Dispersion due to anharmonicity (dispersion time Tdisp) Energy R

9. Dispersion: Classical and Quantum Effects • Compensation of dispersion by negative chirp of excitation pulse • Classical! • Rephasing of wavepacket after dispersion • Rephasing time Trep = 1/wexe (after dispersion of the packet) • discrete vibrational levels needed

10. Dispersion-time:(wave packet width DT > 1/2Tmorse) T1 DT = n(T1 - T2) Energy DE T2 Dispersion in Morse-Potential (Classical) • Morse-potential: • Frequency: R

11. Dispersion of I2-Wave-Packets If N = number of excited vibrational levels: Tdisp= 2 ps Tdisp= 5 ps

12. Dispersion of ClF-Wave-Packets Tdisp< 1 ps

13. Experiments with Phase-Locked Pulses Constructive Interference Destructive Interference Generation of Pulse-Pairs: Piezo Piezo tunes phase by moving distance l/2 Scherer et al., J. Chem. Phys. 95, 1487 (1991)

14. Observed signal: Fluorescence, i.e. integrate from -∞ to +∞ • Vibrational states act as a monochromator => interference constructivedestructive no interference Explanation in Frequency Domain • Pulse-Pairs in frequency domain yield spectral interferences, if pulses overlap. • Frequency resolution of monochromator broadens pulses.

15. Excitation of phonon sidebands => dissipative dynamics • Coherent control of dissipative vs. free wave packet motion Phase-Locked Pulses in the Presence of Dissipation: Proposed Experiment: Cl2 in Ar • Phonon side-bands increase for higher vibrational levels • Excitation of zero-phonon lines => oscillation of free molecule? Cl2 / Ar constructivedestructive no interference

16. 1.0 10 states 0.8 4 states simulated signal 0.6 0.4 0.2 0.0 0 1 2 3 4 5 t / ps Summary • Signatures of relaxation • Energy loss of halogens in Rg • Collisions cause coherence • Dispersion in anharmonic potentials • Experiments with phase-locked pulse pairs