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MO 12.5 Di 14:00 Schellingstr. 3
Photoisomerization dynamics of azobenzene analyzed by timeresolved
photoelectron spectroscopy — •Thomas Schultz 1,2 ,
Jason Quenneville 3 , Todd J. Martinez 3 , Susanne Ullrich 1 ,
Marek Zgierski 1 , and Albert Stolow 1 — 1 Steacie Institute for
Molecular Sciences, National Research Council, Ottawa, Canada —
2 Now at: Max-Born-Institut Berlin, Max-Born Str. 2a, 12489 Berlin —
3 Department of Chemistry, University of Illinois, Urbana, USA
The excited state dynamics of trans-azobenzene were investigated by
time-resolved photoelectron spectroscopy. Two excited ππ ∗ -states with
distinct dynamics were identified in the wavelength region 285-350 nm.
Based on the experimental results and ab initio molecular-dynamics calculations,
we present a new model to explain the photoisomerization
mechanism of azobenzene: (1) Two excited electronic states S2 and S3
are near-degenerate. (2) Both, S2 and S3 absorb with similar absorption
cross sections. (3) S2 undergoes direct and very efficient internal conversion
to the ground state, violating Kasha’s rule, while S3 relaxes to the
first excited state S1 and thus obeys Kasha’s rule. The presented results
can resolve a long-standing dispute about the isomerization mechanism
in azobenzene.
MO 12.6 Di 14:00 Schellingstr. 3
Polarisationsabhängige Pump-Probe Dynamik in K2 —
•Christian Horn 1 , Matthias Wollenhaupt 1 , Oksana Graefe 1 ,
Dirk Liese 1 , Thomas Baumert 1 , Tobias Brixner 2 , Gerhard
Krampert 2 , Reimer Selle 2 und Gustav Gerber 2 — 1 University of
Kassel, Institute of Physics and Center for Interdisciplinary Nanostructure
Science and Technology (CINSaT), Heinrich-Plett-Str. 40, D-34132
Kassel, Germany — 2 Physikalisches Institut, Universität Würzburg, Am
Hubland, 97074 Würzburg, Germany
In der Vergangenheit wurde die kohärente Kontrolle molekularer Dynamik
durch verschiedene Kontrollmechanismen erreicht, die darauf basieren,
die zeitliche Struktur des elektrischen Feldes eines Laserpulses
anzupassen. Eine Erweiterung der Möglichkeiten ergibt sich, wenn
man zusätzlich auch den zeitlichen Verlauf des Polarisationszustandes
verändert. Wir untersuchen die Multi-Photon Ionisation des K2 Moleküls
in einem Pump-Probe Experiment, in dem die Polarisationrichtung des
Pump und des Probe Pulses unterschiedlich sind. Die Übergangsmomente
A 1 Σ + u ← X 1 Σ + g und 2 1 Πg ← A 1 Σ + u stehen senkrecht aufeinander, so dass
unterschiedliche Polarisationen des Probe-Pulses zu einer unterschiedlichen
Dynamik führen. Ergebnisse aus Experimenten mit verschiedenen
Polarisationen des Probe-Pulses werden vorgestellt.
MO 12.7 Di 14:00 Schellingstr. 3
Femtosecond Spectroscopy of Chlorine Molecules embedded in
Solid Argon — •MIZUHO FUSHITANI, MATIAS BARGHEER,
MARKUS GÜHR, and NIKOLAUS SCHWENTNER — Institut
für Experimentalphysik, FU Berlin, Arnimallee 14, 14195, Germany
Ultrafast dynamics of chlorine molecules(Cl2) embedded in solid Argon(Ar)
is studied by using fs pump-probe spectroscopy. A wavepacket of
Cl2 was created on the inner limb of the B( 3 Π 0 + u) and/or C( 1 Π1u) electronic
excited potentials by a pump pulse and the subsequent relaxation
was monitored by a probe pulse which further excites the wavepacket to
the ion-pair manifold. Due to the anharmonicity of the molecular potential,
the wavepacket gets dispersed with time. The dispersion, however,
can be precompensated by an appropriate chirped pulse. In this study,
negatively chirped pulses were used to examine the wavepacket focusing
of Cl2. The energy relaxation rate of a wavepacket at different vibrational
energy levels in the B state was obtained by changing the wavelength of
pump and probe pulses. The rate shows very different behaviors above
or below the dissociation limit, indicating the different energy loss in the
collision with the cage. We also carried out a phase-locked pulse pair experiment
where the electronic coherence of molecules can be investigated.
A pump pulse pair with a 0(π) phase shift excites more(less) number of
molecules than a phase-unlocked pulse pair does, as long as molecules
preserve the electronic coherence after each vibrational period. We observed
the vibrational recurrence of Cl2 in solid Ar, which indicates that
Cl2 keeps its electronic coherence for 250fs even if it is embedded in the
solid where many-body electronic interactions take place.
MO 12.8 Di 14:00 Schellingstr. 3
Control of molecular photodynamics by strong-field alignment
in solids — •Toni Kiljunen 1 , Burkhard Schmidt 2 , and Nikolaus
Schwentner 1 — 1 Institut für Experimentalphysik, FU-Berlin,
Arnimallee 14, 14195 Berlin — 2 Institut für Mathematik II, FU-Berlin,
Arnimallee 2–6, 14195 Berlin
Molecular response to intense nonresonant laser fields is studied theoretically,
in terms of alignment of a molecule with field polarization. The
oscillating field induces anisotropic polarization and the energy gain in
the field leads to formation of pendular states, i.e. directional hybridizations
of free molecule rotations. Two qualitatively different time scales
are interrogated in the process: adiabatic regime, where the population
on excited molecular rotational states follows the laser pulse envelope,
and ultrafast regime, where post-pulse alignment occurs. The effect of
condensed phase surrounding, such as a solid rare gas hindering the induced
molecular rotation, is assessed by incorporation of highly symmetric
model potentials in the time-dependent Schrödinger equation. The
main goal of the research is to attain optical control on molecular photophysics
and -chemistry by affecting product yields of photoprocesses,
such as dissociation and recombination, via suitable preorientation in
condensed phase conditions.
MO 12.9 Di 14:00 Schellingstr. 3
Br2 in Ar-Kristallen: Ultraschnelle Dynamik in kondensierter
Umgebung — •Markus Gühr, Mizuho Fushitani, Heide Ibrahim,
Toni Kiljunen und Nikolaus Schwentner — Institut für Experimentalphysik,
FU Berlin, Arnimallee 14, 14195 Berlin
Wir diskutieren am Beispiel von in Argonkristallen eingebautem Br2
verschiedene ultraschnelle Prozesse in der kondensierten Phase. Die Experimente
werden nach dem Schema der fs-Pump-Probe Spektroskopie
mit durchstimmbaren Laserpulsen (280-650 nm) und einer Zeitauflösung
von teilweise unter 50 fs durchgeführt. Im elektronischen A Zustand ( 3 Π1)
als auch im B Zustand ( 3 Π0) wird eine ausgeprägte Schwingungswellenpaketdynamik
beobachtet. Mittlere Trajektorien der Schwingungswellenpakete
wurden bis über das Gasphasendissoziationslimit hinaus vermessen.
Daraus wird das Wechselwirkungspotenzial mit dem Käfig bestimmt.
Eine impulsive Deformation des Käfigs führt zur Erzeugung von
Kristallphononen. Sie modulieren die Pump-Probe Spektren unter bestimmten
Nachweisbedingungen. Weiterhin lassen sich die Schwingungswellenpakete
durch spektrale Formung der Anregungspulse ( ” chirp“) im
anharmonischen Molekülpotential räumlich fokussieren. Durch die Variation
der relativen Polarisation von Pump- und Probepuls können wir
in den Ultrakurzzeitspektren nichtadiabatische Übergänge zwischen verschiedenen
elektronischen Zuständen isolieren. Schliesslich erläutern wir
Szenarien zur kohärenten Kontrolle der nichtadiabatischen Übergänge
von Br2 in kondensierten Umgebungen.
MO 12.10 Di 14:00 Schellingstr. 3
Molecular orientation via a dynamically induced pulse–train:
wave–packet dynamics of NaI in a static electric field — •Philipp
Marquetand 1 , Arnulf Materny 2 , Niels E. Henriksen 3 und Volker
Engel 1 — 1 Institut für Physikalische Chemie, Am Hubland, 97074
Würzburg, Germany — 2 School of Engineering and Science, International
University Bremen, Campus Ring 8, D-28759 Bremen, Germany —
3 Department of Chemistry, Technical University of Denmark, DTU 207,
DK-2800 Lyngby, Denmark
We regard the ro–vibrational wave–packet dynamics of NaI molecules
in a static electric field after femtosecond excitation to its first electronically
excited state. The following quasi–bound nuclear wave–packet
motion is accompanied by a bonding situation changing from covalent to
ionic. At times when the charge separation is present. i.e. when the bondlength
is large, a strong dipole moment exists and rotational excitation
takes place. Upon bond contraction, the then covalently bound molecule
does not experience the external field. This scenario repeats itself
periodically. Thus, the vibrational dynamics causes a situation which is
comparable to the interaction of the molecule with a train of pulses where
the pulse separation is determined by the vibrational period.
MO 12.11 Di 14:00 Schellingstr. 3
Indirect versus direct photoionization with ultrashort pulses:
interferences and time–resolved bond-length changes —
•Volker Engel and Stefanie Graefe — Institut für Physikalische
Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg,
Germany
The photoionization of NaI molecules with femtosecond laser pulses
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