Struktur und Dynamik von Molekülen 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

Multiorbital tunneling ionization of the CO molecule

We coincidently measure the molecular frame photoelectron angular distribution and the ion sum-momentum distribution of single and double ionization of CO molecules by using circularly and elliptically polarized femtosecond laser pulses, respectively. The orientation dependent ionization rates for various kinetic energy releases allow us to individually identify the ionizations of multiple orbitals, ranging from the highest occupied to the next two lower-lying molecular orbitals for various channels observed in our experiments. Not only the emission of a single electron, but also the sequential tunneling dynamics of two electrons from multiple orbitals are traced step by step. Our results confirm that the shape of the ionizing orbitals determine the strong laser field tunneling ionization in the CO molecule, whereas the linear Stark effect plays a minor role.Comment: This paper has been accepted for publication by Physical Review Letter

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 $^{4}$He$_{3}$ 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