aktualisiertes pdf - DPG-Tagungen

MO 9.7 Mo 18:00 HS 332
Dreiteilchen-Fragmentation von H3 und D3 — •Martin Jungen
— Institut für Physikalische Chemie, Universität Basel
Es wurde beobachtet, dass bereits tiefliegende Rydbergzustände von
H3 und D3 nach H + H + H prädissoziieren können, freilich mit sehr geringer
Rate, z.B. der Zustand 3pe ′ : 2 E ′ ab einer innern Energie, die ca.
3 eV über der Dissoziationsgrenze liegt (s. den vorhergehenden Vortrag).
Mit einer quantenchemische Studie der 6 tiefsten Elektronenzustände
von H3 lässt sich ein möglicher Weg dieses Prozesses erkennen. Der 3pe ′
Rydbergzustand m¨sste sich aus seinem D3h Gleichgewicht über oder
durch eine niedrige Barriere zu einer linearen Geometrie verformen; er
bliebe dort in einer lokalen Potentialmulde gefangen bis zu seinem durch
nichtadiabatische Kopplung bewirkten Übergang auf die dissoziative Potentialfläche,
welche in D∞h-Symmetrie zum Dreiteilchenaufbruch führt.
MO 9.8 Mo 18:15 HS 332
Photodissociation studies of state-selected molecular ions by
velocity map ion imaging — •N. Hendrik Nahler, Olivier P.J.
Vieuxmaire, Josephine A. Jones und Michael N.R. Ashfold —
University of Bristol, School of Chemistry, Bristol BS8 1TS, UK
High resolution ion imaging methods have been used to study the photodissociation
of ground state Br + 2 ( 2 Πg) and BrCl + ( 2 Π) cations, with
spin-orbit state specification, monitoring 79 Br + and 35 Cl + fragments as
appropriate. The state-selected molecular ions are prepared by a 2+1
REMPI scheme and than dissociated by absorption of a single photon
from a second tunable dye laser. Image analysis allows precise determination
of the dissociation energies of Br + 2 ( 2 Π3/2,g), Br + 2 ( 2 Π1/2,g), BrCl +
( 2 Π3/2), and BrCl + ( 2 Π1/2) and values for the respective spin-orbit splitting
constants. The values so derived for BrCl + are in excellent accord
with those obtained from complementary photoelectron imagin studies.
Photoexcitation of Br + 2 at wavelengths just above the first dissociation
threshold yields ground state Br + and Br fragments, the recoil anisotropy
of which is found to be sensitively wavelength dependent. Careful measurements
of the angular anisotropy (β-parameter) versus the dissociation
wavelength reveals the presence of high frequency structure, in the region
up to the first excited [Br( 2 P3/2) + Br + ( 3 P1)] dissociation limit; this we
attribute to predissociating vibrational levels in an adiabatic potential
converging to this excited limit. The dissociation channels from BrCl +
show predominantly parallel character. Branching ratios into the various
Br + ( 3 PJ) and Cl ( 2 PJ) channels will be presented and discussed in the
context of the various participating excited state potentials.
MO 9.9 Mo 18:30 HS 332
Dissoziative Photoionisation von Kohlenwasserstoffradikalen —
•Thomas Schüßler 1 , Ingo Fischer 1 , Christian Alcaraz 2 und
Wolfgang Roth 1 — 1 Institut für Physikalische Chemie, Am Hubland,
97074 Würzburg — 2 LURE, Bât. 209, BP 34, Centre Universitaire
Paris-Sud, 91898 Orsay, France
Es werden Ionisierungsenergien und der H- bzw. H2-Verlust verschiedener
kleiner organischer Kohlenwasserstoffsysteme vorgestellt, die mittels
einer Synchrotronstrahlungseinrichtung gemessen wurden.
Im Propargyl-Kation wurde etwa 3.5 eV oberhalb der Ionisierungsenergie
ein H-Verlust beobachtet, nicht jedoch, wie vorhergesagt, der von
H2.
Das Ethyl-Kation verliert ein H2-Molekül 3.5 eV oberhalb seiner Ionisierungsenergie,
was jedoch 1 eV oberhalb der theoretischen Aussagen
liegt.
Das Allyl-Kation verliert H2 etwa 2 eV oberhalb der Ionisierungsschwelle.
MO 10 Femtosekundenspektroskopie: Schwingungsdynamik
Zeit: Montag 16:30–18:00 Raum: HS 315
MO 10.1 Mo 16:30 HS 315
IR versus Raman Spectroscopy: Monitoring Vibrational Cooling
after Electronic Excitation — •Arne Sieg, Tobias Schrader,
Florian Koller, Wolfgang Schreier, Qingrui An, and Peter
Gilch — Sektion Physik, Ludwig-Maximilians-Universität, Oettingenstr.
67, D-80538 München, Germany
Molecules undergoing ultrafast (∼ 100 fs) internal conversion processes
(IC) are very suitable model systems for the examination of the vibrational
dynamics related with chemical reactions. The detection of these
dynamics can be performed by time resolved IR and Raman spectroscopy.
In a combined study on the vibrational cooling of para-nitroaniline (pNA)
both methods are compared. Only Raman spectroscopy gives access to
the excitation of a stretch mode, which is shown to decay in ∼ 3 ps [1]. IR
spectroscopy is better suited to monitor the frequency shift of this resonance.
This shift to lower wavenumbers relaxes with time constants of 1
– 11 ps. It stems from anharmonic coupling to other vibrational modes.
Therefore, this shift is an useful sensor for the population and dynamics
of these low frequency modes. [1] Q. An, P. Gilch, Chem. Phys. Lett. 363
(2002) 397
MO 10.2 Mo 16:45 HS 315
Solvent Effects on theVibrational Relaxation of para-
Nitroaniline — •Wolfgang Schreier, Tobias Schrader, Arne
Sieg, Florian Koller, Wolfgang Zinth, and Peter Gilch
— Sektion Physik, Ludwig-Maximilians-Universität, Oettingenstr. 67,
D-80538 München, Germany
The re-distribution of excess energy released in photo-chemical processes
in solution is crucial for the stabilization of the reaction products.
Since these processes occur on the picosecond time scale their examination
calls for advanced time resolved techniques and suitable model systems.
Para-Nitroaniline (pNA) can serve as such a system. After photoexcitation
pNA rapidly reforms its ground state and thereby releases
25000 cm −1 of excess energy. Its transfer to the solvent surroundings has
been studied in dependence of the solvent nature by optical pump-probe
spectroscopy [1]. This yields a global measure for dynamics of the vibrational
excitation. Here, time-resolved IR experiments on pNA yielding
mode specific informations are presented. The temporal evolution of an
IR line shape is evaluated and discussed in relation to solvent properties.
[1] S.A. Kovalenko et al. J. Chem. Phys. 115 (2001) 3256
56
MO 10.3 Mo 17:00 HS 315
Vibrational multi-level quantum coherences due to anharmonic
couplings in intermolecular hydrogen bonds — •Nils Huse,
Karsten Heyne, Jens Dreyer, Erik T.J Nibbering, and Thomas
Elsässer — Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie,
Max-Born-Str. 2a, D-12489 Berlin
Femtosecond 3-pulse photon echo studies of acetic acid dimers in solution
reveal multi-level coherences of O-H stretching excitations caused
by the anharmonic coupling between the high-frequency stretching and
low-frequency hydrogen bond motions. We demonstrate for the first time
that such multi-level coherences determine the nonexponential decay of
the macroscopic O-H stretching polarization, whereas spectral diffusion
processes play a minor role as echo peak shift measurements and dynamic
hole burning show [1,2]. The dephasing time of individual vibrational
transitions contributing to the overall polarization is approximately 200
fs, much longer than the apparent rapid polarization decay.
[1] N. Huse, K. Heyne, J. Dreyer, E.T.J. Nibbering, Th. Elsaesser,
Phys. Rev. Lett. (91), 197401 (2003)
[2] K. Heyne, N. Huse, J. Dreyer, E.T.J. Nibbering, Th. Elsaesser,
Chem. Phys. Lett. (369), 591 (2003)
MO 10.4 Mo 17:15 HS 315
Femtosecond Vibrational Spectroscopy based on Stimulated
Raman Scattering — •Stefan Laimgruber und Peter Gilch —
Sektion Physik, Ludwig-Maximilians-Universität, Oettingenstr. 67, D-
80538 München, Germany
The urge to follow structural dynamics of reacting molecules by femtosecond
techniques has led to the extension of vibrational spectroscopy
(IR and Raman) to that time regime. Raman techniques often suffer from
extremely low scattering cross sections. This drawback can be diminished
be reverting to stimulated Raman scattering. In a novel implementation
[1] of this technique, a femtosecond white light pulse and a spectrally
narrow laser pulse are overlapped in the sample. This allows for the acquisition
of complete Raman spectra within very short times (≤ 1 s).
If a third femtosecond laser pulse excites the sample, Raman spectra of
transient species can be recorded. Here, a novel set-up for this Raman
spectroscopy is introduced and first results on model compounds are
presented. Particular emphasis is laid on the amplification of the solute
signal by a resonance enhancement. [1] M. Yoshizawa, and M. Kurosawa,