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[1] M. Walther et al., Chem. Phys. 288 261 (2003) [2] B. Fischer et al., Phys. Med. Biol. 47 3807 (2002)
MO 18 Stossprozesse, Energietransfer
Zeit: Donnerstag 16:30–18:15 Raum: HS 355
MO 18.1 Do 16:30 HS 355
Donor-Akzeptor-Aggregation untersucht in ns-transienter
Spektroskopie — •Melanie Mucke, Christian Spitz, David von
Seggern und Ralf Menzel — Universität Potsdam, Institut für
Physik, Lehrstuhl Photonik, Am Neuen Palais 10, 14469 Potsdam
Pump-&-Probe-Spektroskopie mit Verzögerungszeiten im ns-Bereich
erlaubt es, Übergangsraten in Donor-Akzeptor-Systemen quantitativ
zu bestimmen. Dazu wurde die Fluoreszenz von Pyren mit einer
Cetylpyridiniumchlorid(CPC)-Lösung in systematischer Konzentrationsvariation
gelöscht. Die konzentrationsabhängigen Raten folgen nicht dem
Mott-Schottky-Gesetz. Vielmehr weist das biexponentielle Abklingen
der angeregten Zustände des Pyrens mit konstanten Relaxationszeiten
auf eine statische Aggregation zwischen Donor- und Akzeptormolekül
hin. Diese Interpretation deckt sich mit den Meßergebnissen zu den
zeitabhängigen Besetzungen der ionischen Pyrenzustände.
MO 18.2 Do 16:45 HS 355
FRET-based Scanning Near-Field Optical Microscopy — •Felix
Müller, Stephan Götzinger, and Oliver Benson — Nano-Optik,
Institut für Physik, Humboldt-Universität zu Berlin, D-10117 Berlin
We investigate Förster Resonant Energy Transfer (FRET) by means
of Scanning Near-Field Optical Microscopy (SNOM). Our setup allows
to actively control the distance between the acceptor and donor and thus
to investigate the strong distance dependence of the energy transfer efficiency.
As donor and acceptor we use CdTe nanocrystals 1 and organic
dye molecules (Alexa Fluor 633). The spectral parameters of the donoracceptor
pair are shown to be appropriate for FRET.
We produce an active SNOM probe by attaching a single 500nm sized
colloid covered with a layer of nanocrystals. This donor probe can be
positioned at will above a cover slip coated with a thin layer of the acceptor
molecules. Within a tip-sample distance of a few nanometers, a
sudden increase in the acceptor’s optical signal is observed. This effect
gives strong evidence for FRET.
Accordingly, we propose a novel FRET-SNOM geometry. We discuss
the improved reliability, stability and resolution of this system.
[1] provided by N. Gaponik, H. Weller, Univ. Hamburg
MO 18.3 Do 17:00 HS 355
The dependence of the differential cross section for
Li+HF→LiF+H on the rotational state of HF — •Rolf
Bobbenkamp 1 , Alessandra Paladini 2 , Andrea Russo 2 , and
Hansjürgen Loesch 2 — 1 Dipartimento di Chimica , Universita’ di
Perugia , 06123 - PERUGIA , Italy — 2 Fakultä für Physik, Universit¨t
Bielefeld, Universitätsstrasse 25, D-33615 Bielefeld
Quantum and quasi classical studies have shown that the j-specific differential
cross section of the title process depends strongly on the rotational
state (j) of HF with respect to both shape and magnitude. Based
on these results we simulated the experimentally accessible j-averaged
cross sections for various j-distributions of the HF molecule and found
significant effects. In a crossed molecular beam experiment we have now
examined these predictions.
We measured angular distributions of the reaction product LiF of the
title reaction at several collision energies ranging from 130 to 350 meV
for various rotational temperatures of the HF nozzle beam. We observe
both a significant increase of the product intensity with the temperature
and a change of the shape. Calculations are being performed to allow
a quantitative comparison of the experimental results with theoretical
predictions based on various recent ab-initio potential energy surfaces.
MO 18.4 Do 17:15 HS 355
Experimental Investigations of Radiative and Ternary Association
for CH + 3 + CO: Interstellar Implications. — •Alfonz
Luca 1 , Gheorghe Borodi 1 , Radek Plaˇsil 2 , and Dieter Gerlich 1
— 1 Fakultät für Naturwissenschaften, Technische Universität Chemnitz,
09126 Chemnitz — 2 Department of Electronics and Vacuum Physics,
Mathematics and Physics Faculty, Charles University, 180 00 Prague
Formation of certain molecules in interstellar clouds must be ex-
68
plained either by surface processes or by complex reaction schemes
including radiative association followed by ion-electron recombination.
A low temperature radio-frequency 22-pole ion trap has been used
to study association of CH + 3 + CO at low number of densities
and temperatures from 80 K to 280 K. In pure CO gas the temperature
dependence of the ternary rate coefficient has been determined
as k3(T) = 1.8 × 10 −25 cm 6 s −1 (T / 300 K) −2.2 . If helium is
used for stabilizing the collision complexes a rate coefficient k3(T) =
1.7 × 10 −25 cm 6 s −1 (T / 300 K) −1.5 has been obtained. At CO number
densities below 10 11 cm −3 radiative stabilization becomes dominant.
The radiative rate coefficient has been determined at 80 K, kr =
6.9 × 10 −13 cm 3 s −1 . In order to study this reaction under conditions prevailing
in dense interstellar clouds a loaded supersonic molecular beam
has to be utilized and the trapped CH + 3 ions have to be cooled not only
in He but also in p-H2.
MO 18.5 Do 17:30 HS 355
Dissoziative Rekombination von 3 He 4 He + — •H. Buhr 1 , H.
B. Pedersen 1 , L. Lammich 1 , H. Kreckel 1 , S. Altevogt 1 , V.
Andrianarijaona 1 , D. Strasser 2 , E. M. Staicu-Casagrande 3 ,
N. De Ruette 3 , D. Schwalm 1 , X. Urbain 3 , A. Wolf 1 und
D. Zajfman 1,2 — 1 Max-Planck-Institut für Kernphysik, D-69117
Heidelberg — 2 Weizmann Institute of Science, Rehovot, 76100, Israel —
3 Département de Physique, Université Catholique de Louvain, B1348,
Louvain-la-Neuve, Belgien
Die Dissoziative Rekombination von 3 He 4 He + wurde am Schwerionen
Testspeicherring (TSR) in Heidelberg im Bereich von 0-45 eV Relativenergie
untersucht. Bei einer Relativenergie von 0 eV wurde eine Rate
für die dissoziative Rekombination des Vibrationsgrundzustandes von
αv=0(0eV ) ≤ 2.0·10 −9 cm 3 /s gefunden. Der Prozess führt dann vor allem
zu den atomaren Endzuständen 1s2p 3 P (∼ 62%), 1s2s 1 S (∼ 35%) und
1s2s 3 S (∼ 3%). Für v ≥ 3 wurde eine Rate für die dissoziative Rekombination
≥ 1 · 10 −7 cm 3 /s gefunden. In diesen Fällen werden vor allem
atomare Endzustände mit n ≥ 3 erreicht.
Für niedrige Relativenergien zeigt der Wirkungsquerschnitt für die dissoziative
Rekombination eine Struktur bei 0.2 eV. Bei höheren Energien
findet man einen glatten Peak bei etwa 7.3 eV und einen Bereich zwischen
15 und 45 eV, der reich an Strukturen ist.
MO 18.6 Do 17:45 HS 355
Chemical probing spectroscopy of H + 3 in a 22-pole ion trap —
•J. Mikosch 1,2 , H. Kreckel 1 , R. Wester 2 , J. Glosík 3 , R. Plaˇsil 3 ,
D. Gerlich 4 , D. Schwalm 1 , and A. Wolf 1 — 1 Max-Planck-Institut
für Kernphysik, 69117 Heidelberg — 2 Physikalisches Institut, Universität
Freiburg, 79104 Freiburg — 3 Charles University Prague, Czech Republic
— 4 Institut für Physik, TU Chemnitz, 09107 Chemnitz
The triatomic hydrogen molecular ion H + 3 is the dominant ion in hydrogen
plasmas at moderate temperatures. Consequently it plays important
roles in many fields from interstallar chemistry and planetary
atmospheres to cooling processes in fusion plasmas and model calculations
for polyatomic molecular ions. The recombination rate of H + 3 with
electrons is one of the crucial parameters for the modelling of interstellar
clouds and it is still a controversial issue.
In the last years a cryogenic 22-pole trap project was launched at the
MPI für Kernphysik, to create a temperature-variable source of H + 3 for
electron recombination experiments at the TSR storage ring. Here we
report a new method for H + 3 spectroscopy that operates at low number
densities and relies on the reaction H + 3 +Ar−→ArH + +H2 which is endoergic
by 0.57eV and thus proceeds only for H + 3 states with v ≥ 2. In the
experiment about 500 H + 3 molecules are stored in the ion trap together
with Ar buffer gas; a tunable diode laser excites H + 3 from its lowest rotational
states to v = 3 from where immediately ArH + is formed, which
is detected by a mass spectrometer ion detector. Relative line strengths
and Doppler widths are used as a thermometer of the ion temperature
in the 22-pole trap.