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The latter was observed also for the radiatively inactive H + 3 ion. Recent
experimental results suggest in addition some influence of collisions with
residual gas on the molecular rotations.
[1] L. Lammich et al., Phys. Rev. Lett. 91 (2003), 143201
MO 12.26 Di 14:00 Schellingstr. 3
Combination of Supersonic Molecular Beam and Low Temperature
22-pol Ion Trap. Association of CH + 3 + CO. — •Gheorghe
Borodi 1 , Alfonz Luca 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
A low temperature radio-frequency 22-pole ion trap has been combined
with a molecular beam for studying ion-neutral association processes at
temperatures where an effusive gas inlet cannot be used due to condensation.
CH + 3 ions have been stored in 22-pole trap at 10 K and He
buffer gas with number densities ranging from 3 × 10 14 cm −3 down to
5 × 10 10 cm −3 . The pulsed supersonic beam of CO has been characterized
via the N + + CO charge transfer reaction. Within a width of 160 µs, the
momentary CO number density in the trap was 1.7 × 10 11 cm −3 . Assuming
ideal CO expansion the collision energy in the central of mass frame
is 310 meV. For calculation of an effective temperature the remaining
internal excitation of both reactants has to be assumed additionally. The
collision complexes have been stabilized by the cold He buffer gas, reaching
a temperature as low as 10 K. The ternary rate coefficient has been
determined to be 8 × 10 −25 cm 6 s −1 . In order to further reduce the translational
temperature in the beam experiment, the method of loading the
beam with heavy rare gases is used.
MO 12.27 Di 14:00 Schellingstr. 3
Influence of the Bead–Bead Interaction on the Rotational Dynamics
of Nonrigid Macromolecules in Solution. — •Alexander
Uvarov and Stephan Fritzsche — Universität Kassel, D–34132 Kassel,
Germany
To understand the relaxation mechanisms of macromolecules, i.e. their
relaxation from a nonequilibrium into their equilibrium state, remains one
of the central problems in studying proteins and DNA in solution. During
the last years, therefore, a large number of experiments and molecular
dynamic simulations [1] have been carried out in order to analyze the
statical and relaxational properties of macromolecules. An alternative
description of biomolecules is offered by statistical methods if they are
treated in terms of their molecular subsystems, the so–called beads of the
macromolecule.
In this contribution, we present and discuss the Internal Diffusion equation
which describes the time evolution of the conformational phase–
space distribution function of a nonrigid macromolecule. To investigate
the effects of the bead–bead interaction potential on the orientation and
relaxation of the macromolecules, the end–to–end autocorrelation function
of the macromolecule, immobilized on a surface, has been calculated
for a hookean, fene and dna bead–bead potentials. Moreover, the rotational
diffusion coefficient of the macromolecule has been computed
and compare with theories which start from a rigid–rod model [2].
[1] B. Liu and B. Dünweg, J. Chem. Phys. 118, (2003), 8061
[2] M. Doi, S. F. Edwards, The Theor. of Polym. Dynam. (Oxf. Univ.
Press, 1986); R. Pecora, J. M. Deutch, J. Chem. Phys. 83, (1985), 4823
MO 13 Spektroskopie von Molekülen in He-Clustern
Zeit: Dienstag 16:30–17:00 Raum: HS 355
Fachvortrag MO 13.1 Di 16:30 HS 355
Spectroscopy of Organic Molecules in Helium Nanodroplets —
•Wolfgang E Ernst 1 , Frank Ruediger Bruehl 2 und Shuo Jiang
3 — 1 Institut fuer Experimentalphysik TU Graz — 2 Physikalisch-
Technische Bundesanstalt Berlin — 3 Tsinghua University Beijing PRC
Helium nanodroplets of 0.4 K internal temperature are generated in
a molecular beam experiment. They provide an almost non-interacting
matrix for the study of molecular spectra at such low temperature. We
doped large helium clusters with organic molecules such as pentacene
to investigate the influence of helium on the spectral line shapes. The
inhomogeneous contribution to the linewidth of about 12 GHz is due to
the size distribution of helium clusters in the beam. Various porphyrin
type molecules were embedded in helium and their relaxation after electronic
excitation was measured. Regardless of the excitation wavelength,
we observe emission emission only from the ground vibrational level of
the S1 state. This demonstrates that the vibrational relaxation in the
droplet is complete within the fluorescence lifetime. For zinc tetrabenzoporphine,
four sets of excitation and emission spectra shifted by 30 to 50
cm-1 with respect to one another, seem to indicate the presence of four
different conformers of this molecule in helium. In the spectra of free base
tetraphenylporphine, the low frequency torsional modes are strongly
broadened in helium.
MO 14 Spektroskopie von Molekülen in He-Clustern
Zeit: Dienstag 17:00–18:30 Raum: HS 355
MO 14.1 Di 17:00 HS 355
Spektroskopie von CO Molekülen in Heliumtröpfchen — •Klaus
von Haeften 1 , Carsten Krause 1 , Anja Metzelthin 1 , Stephan
Rudolph 1 , Andreas Rüdiger 1 , Iaroslav Simanovski 2 und Martina
Havenith 1 — 1 Lehrstuhl für Physikalische Chemie II, Ruhr-
Universität Bochum, D-44780 Bochum — 2 General Physics Institute
RAS, ul. Vavilova 38, 119991 Moscow, Russia
Die Linienbreiten der Infrarotspektren schwerer Moleküle wie SF6 oder
OCS in Heliumtröpfchen sind deutlich schmaler, als die des leichten HF
und deuten auf die Rotationsrelaxation als Ursache hin. Während für
langsam rotierende Moleküle die Kopplung an das superflüssige Helium
schwach ist, können die leichteren, schnell rotierenden Moleküle unter
Umständen Rotonen in den Heliumtröpfchen anregen und so ihre Relaxationsrate
erhöhen. Die Isotopomere zweiatomiger Moleküle sind aufgrund
ihrer jeweiligen B-Konstanten ideale Kandidaten, um diesen Effekt
in Heliumtröpfchen zu untersuchen. Wir präsentieren erste Spektren der
Isotopomere von CO. Die beobachteten Linienbreiten zeigen eine klare
Abhängigkeit von der jeweiligen B-Konstante. Der Trend ist in qualitativer
Übereinstimmung mit dem Modell des resonanten Energietransfers
auf Rotonenzustände des supraflüssigen Heliums.
61
MO 14.2 Di 17:15 HS 355
Relaxationsdynamik der Solvatationshülle von Molekülen in
superflüssigen Heliumtropfen — •Rudolf Lehnig und Alkwin
Slenczka — Institut für Physikalische und Theoretische Chemie, Universität
Regensburg, D-93040 Regensburg
Die Emissionsspektren von Phthalocyanin (Pc) in Heliumtröpfchen zeigen
eine Verdoppelung aller Linien [1]. Aus der spektralen Charakteristik
dieser Verdoppelung folgt ein Vier-Niveau-Schema des Systems aus Pc
und seiner Heliumumgebung. Die Ausbildung dieser Zustände wird mit
einer Relaxation der Heliumumgebung des eingebetteten Moleküls nach
dessen elektronischer Anregung erklärt. Mit diesem Relaxationsmodell
wurden bereits pump-probe-Spektren von Tetracen in Heliumtröpfchen
interpretiert [2]. Die Emissionsspektren von Tetracen in Heliumtröpfchen
zeigen die im pump-probe Spektrum gemessenen stimulierten Emissionen
in spontaner Emission. Das Auftreten dieser Signale in den Emissionsspektren
kann mit einem Drei-Niveau-Schema beschrieben werden. Dies
gilt ebenso für die pump-probe-Spektren. Den Beobachtungen in den
Emissionsspektren des Pc und des Tetracens in Heliumtröpfchen werden
zwei unterschiedliche Mechanismen der Dynamik der Solvatationshülle
zugrunde gelegt.
[1] R. Lehnig, A. Slenczka, J. Chem. Phys., 2003, 118, 8256.
[2] A. Lindinger, J. P. Toennies, A. F. Vilesov, Phys. Chem. Chem. Phys.,
2001, 3, 2581.