28 research outputs found
Struktur und Dynamik von MolekĂĽlen und molekularen Clustern in der Gasphase untersucht mit Femtosekundenspektroskopie
Large amplitude intramolecular motions in non-rigid molecules are a fundamental issue in chemistry and biology. The conventional approaches for study these motions by far-infrared and microwave spectroscopy are not applicable when the molecule is non-polar. Therefore, in the current thesis an alternative approach for the investigation of large amplitude intramolecular motions was developed and tested. This new method is based on femtosecond rotational degenerate four-wave mixing spectroscopy (fs DFWM), which is a particular implementation of rotational coherence spectroscopy. The method was successfully applied for the investigation of pseudorotation in pyrrolidine and the ring-puckering vibration in cyclopentene. Another important subject is the photophysics of molecules and molecular clusters which have an ultrashort lifetime of their electronically excited state (photoreactivity). These ultrashort lifetimes often represent a protective mechanism causing photostability. The photoreactivity is usually the manifestation either of an “elementary” reaction, such as proton or electron transfer, which occurs in the excited state or of a fast non-radiative deactivation processes, such as internal conversion via conical intersection of the electronically excited and ground state. Due to a short-lived excited state, the conventional vibrational spectroscopic methods, such as IR depletion detected by resonance two-photon ionization spectroscopy (IR/R2PI), are not applicable for the structural investigation of these systems. Therefore, new approach, termed IR depletion detected by multiphoton ionization with femtosecond laser pulses (IR/fsMPI), was developed for studying the structure of photoreactive microsolvated molecules. The IR/fsMPI technique was applied for investigating the clusters of 1H-pyrrolo[3,2-h]quinoline with water/methanol as well as adenine- and 9-methyl-adenine-hydrates. In addition, the excited state dynamics of bifunctional azaaromatic molecule 7-(2'-pyridyl)indole (7PyIn) was studied by femtosecond pump-probe resonance excitation multiphoton ionization technique (fs REMPI). Under electronic excitation of this molecule a fast proton transfer (phototautomerization) takes place, which is followed by radiationless excited state deactivation process. The fs REMPI spectra lead to the conclusion that the phototautomerization in 7PyIn is coupled with a twisting of the molecule, and that the twisting provides an efficient channel for ultrafast radiationless excited state deactivation. This pattern of excited-state tautomerization/deactivation might be quite general.Ultraschnelle Prozesse sind oft Bestandteil der molekularen Dynamik in Molekülen und molekaren Aggregaten und in der Regel mit Reaktionen verbunden. Die vorliegende Arbeit befasst sich mit zwei Grundtypen solcher Prozessen: 1. Schwingungen mit großer Amplidude im elektronischen Grundzustand bestimmter Moleküle. Diese sind oft Minimumsenergiepfade für konformationelle Änderungen. Als Beispiel wurde die Pseudorotation in Pyrrolidin, einem aliphatischen, fünfgliedrigen Ring untersucht, welcher in wichtigen molekularen Bausteinen der Biochemie (Ribose) eine wichtige Rolle spielt. Die Dynamik wurde mit der Femtosekunden-Vierwellenmischmethode untersucht. Mit ihr werden Rotationskohärenzeffekte zeitaufgelöst gemessen und durch modellbasierte Simulationen analysiert. Damit lassen sich die Energie und Struktur entlang solcher Schwingungen genauestens untersuchen. Ein zweites Beispiel war die Puckerbewegung in Cyclopenten, bei der ein Teil des Rings aus der Ebene schwingt. Hier ergab ein Vergleich mit sehr genauen Mikrowellendaten eine hervorragende Übereinstimmung der Rotationskonstanten. 2. Relaxationsprozesse in elektronisch angeregten Zuständen von isolierten und mikrosolvatierten Molekülen. Diese laufen oft im Subpikosekundenbereich ab. Beispiele sind Protonen- und Elektronentransfer, schnelle internal conversion, Kurvenüberkreuzungen (S1 oder Sn mit S0) etc. Oft spielt das Lösungsmittel eine entscheidende Rolle. Um diese ultrakurzlebigen Zustände zu charakterisieren und die dabei ablaufenden Prozesse aus molekularer Sicht aufzuklären, wurden ultrakalte Modellmoleküle bzw. Aggregate mit einem in dieser Arbeit erstmals angewandten Doppelresonanzverfahren (IR/fsMPI) untersucht. Mit ihr können die O-H, und N-H Steckschwingungen der Molekülen selbst bei extrem kleinen Lebensdauern der angeregten Zustände analysiert und daraus die Struktur der Moleküle bzw. der intermolekularen Aggregate abgeleitet werden. Gleichzeitig kann mit der so genannten Pump-Probe Spektroskopie die intramolekulare Dynamik in Echtzeit abgefragt werden. Modellsysteme waren bifunktionale Azoaromaten isoliert bzw. mikrosolvatisiert, die nach Anregung eine ultraschnelle Tautomeriereaktion zeigen. Diese schnellen Relaxationsprozesse werden als Hauptgrund für die hohe Photostabilität von DNA diskutiert
Dynamical characteristics of Rydberg electrons released by a weak electric field
The dynamics of ultra-slow electrons in the combined potential of an ionic
core and a static electric field is discussed. With state-of-the-art detection
it is possible to create such electrons through strong intense-field
photo-absorption and to detect them via high-resolution time-of-flight
spectroscopy despite their very low kinetic energy. The characteristic feature
of their momentum spectrum, which emerges at the same position for different
laser orientations, is derived and could be revealed experimentally with an
energy resolution of the order of 1meV.Comment: 5 pages, 5 figure
Streaking temporal double slit interference by an orthogonal two-color laser field
We investigate electron momentum distributions from single ionization of Ar
by two orthogonally polarized laser pulses of different color. The two-color
scheme is used to experimentally control the interference between electron wave
packets released at different times within one laser cycle. This intracycle
interference pattern is typically hard to resolve in an experiment. With the
two-color control scheme these features become the dominant contribution to the
electron momentum distribution. Furthermore the second color can be used for
streaking of the otherwise interfering wave packets establishing a which-way
marker. Our investigation shows that the visibility of the interference fringes
depends on the degree of the which-way information determined by the
controllable phase between the two pulses.Comment: submitted to PR
Observation of the Efimov state of the helium trimer
Quantum theory dictates that upon weakening the two-body interaction in a
three-body system, an infinite number of three-body bound states of a huge
spatial extent emerge just before these three-body states become unbound. Three
helium atoms have been predicted to form a molecular system that manifests this
peculiarity under natural conditions without artificial tuning of the
attraction between particles by an external field. Here we report experimental
observation of this long predicted but experimentally elusive Efimov state of
He by means of Coulomb explosion imaging. We show spatial images of
an Efimov state, confirming the predicted size and a typical structure where
two atoms are close to each other while the third is far away
Holographic detection of parity in atomic and molecular orbitals
We introduce a novel and concise methodology to detect the parity of atomic
and molecular orbitals based on photoelectron holography, which is more general
than the existing schemes. It fully accounts for the Coulomb distortions of
electron trajectories, does not require sculpted fields to retrieve phase
information and, in principle, is applicable to a broad range of electron
momenta. By comparatively measuring the differential photoelectron spectra from
strong-field ionization of N molecules and their companion atoms of Ar,
some photoelectron holography patterns are found to be dephased for both
targets. This is well reproduced by the full-dimensional time-dependent
Schr\"{o}dinger equation and the Coulomb quantum-orbit strong-field
approximation (CQSFA) simulation. Using the CQSFA, we trace back our
observations to different parities of the 3 orbital of Ar and the
highest-occupied molecular orbital of N via interfering Coulomb-distorted
quantum orbits carrying different initial phases. This method could in
principle be used to extract bound-state phases from any holographic structure,
with a wide range of potential applications in recollision physics and
spectroscopy
Angular dependence of the Wigner time delay upon tunnel ionization of
More than 100 years after its discovery and its explanation in the energy
domain, the duration of the photoelectric effect is still heavily studied. The
emission time of a photoelectron can be quantified by the Wigner time delay.
Experiments addressing this time delay for single-photon ionization became
feasible during the last 10 years. A missing piece, which has not been studied,
so far, is the Wigner time delay for strong-field ionization of molecules. Here
we show experimental data on the Wigner time delay for tunnel ionization of
molecules and demonstrate its dependence on the emission direction of
the electron with respect to the molecular axis. We find, that the observed
changes in the Wigner time delay can be quantitatively explained by
elongated/shortened travel paths of the electrons that are due to spatial
shifts of the electron's birth position after tunneling. This introduces an
intuitive perspective towards the Wigner time delay in strong-field ionization.Comment: 17 pages, 6 figure
Subcycle interference upon tunnel ionization by counter-rotating two-color fields
We report on three-dimensional (3D) electron momentum distributions from single ionization of helium by a laser pulse consisting of two counter-rotating circularly polarized fields (390 and 780 nm). A pronounced 3D low-energy structure and subcycle interferences are observed experimentally and reproduced numerically using a trajectory-based semiclassical simulation. The orientation of the low-energy structure in the polarization plane is verified by numerical simulations solving the time-dependent Schrödinger equation.This Rapid Communication was supported by the DFG
Priority Programme “Quantum Dynamics in Tailored Intense
Fields” of the German Research Foundation (Project No.
DO 604/29-1). A.H. and K.H. acknowledge support from the
German Merit Foundation. A.K. acknowledges support from
the Wilhelm and Else Heraeus Foundation
Direct Determination of Absolute Molecular Stereochemistry in Gas Phase by Coulomb Explosion Imaging
Bijvoet’s method, which makes use of anomalous x-ray diffraction or dispersion, is the standard means of directly determining the absolute (stereochemical) configuration of molecules, but it requires crystalline samples and often proves challenging in structures exclusively comprising light atoms. Herein, we demonstrate a mass spectrometry approach that directly images the absolute configuration of individual molecules in the gas phase by cold target recoil ion momentum spectroscopy after laser ionization–induced Coulomb explosion. This technique is applied to the prototypical chiral molecule bromochlorofluoromethane and the isotopically chiral methane derivative bromodichloromethane