Work Group Prof. Dr. F. Temps

Femtosecond Photophysical and Photochemical Dynamics

Femtosecond Photophysical and Photochemical Dynamics of DNA Building Blocks

DNA helix

The DNA bases adenine (A), cytosine (C), guanine (G), and thymine (T) stand out for their extraordinary photochemical stabilities which arise owing to ultrafast electronic deactivation processes. After absorption of a UV photon, the molecules are thereby returned to their ground electronic states before chemical reactions in the excited states can cause profound damage. We are studying the underlying ultrafast dynamics of the different building blocks of DNA by femtosecond fluorescence and absorption spectroscopies.

 

 

 

 

Ultrafast Reaction Dynamics of Photochromic Molecular Switches

Fulgides

Photochromic molecules can be reversibly interconverted, i.e., switched, between two or more isomeric forms with distinctive physical and chemical properties by absorption of light at different wavelengths. The light-driven transformations are of great interest for the development of optimal memory and information storage devices, for applications as molecular switches, or for designing molecular machines. Towards these ends, however, one needs detailed information on the underlying ultrafast photochemical reaction dynamics. In this project within the new Collaborative Research Centre SFB 667 "Function by Switching", we use femtosecond spectroscopy as the method of choice to directly monitor the absorption and emission changes of the molecules during their light-induced transformations with time resolutions of 50 fs (5x10-14 s) or even better.

 

 

  

Femtosecond Time-Resolved Mass Spectrometry and Photoelectron Imaging

Hexafluorobenzene

Femtosecond time-resolved mass spectrometry and photoelectron imaging of electronically excited polyatomic molecules allow us to monitor the excited state populations of the molecules as function of time and obtain very detailed pictures of their electronic dynamics. Of special interest are halogenated aromatics and N-containing heteroaromatic molecules which are important building blocks of many biological molecules.

Vibration-Rotation Quantum State-Resolved Unimolecular Reaction Dynamics and Photofragment Imaging

Velocity Mapped Photofragment Ion Imaging

Ion image from HBr photodissociation

 

 

 

 

 

 

 

 

Vibration-Rotation Quantum State-Resolved Unimolecular Dynamics by Stimulated Emission Pumping

Stimulated emission pumping

 

 

 

 

 

 

New Theoretical Approaches to Unimolecular Dynamics

Wavepacket propagation with an effective Hamiltonian

While statistical unimolecular theory has been immensely successful, it cannot describe the true dynamics of molecules in specific quantum state. On the other hand, exact quantum dynamics calculations on ab initio potential energy hypersurfaces can only be carried out for small (3- or 4-atom) molecules. We employed an effective Hamiltonian, obtained by fitting to SEP spectra and complemented with an imaginary decay term, to model intramolecular vibrational energy redistribution (IVR) and the unimolecular decay kinetics of DCO (X 2A') by simple wavepacket propagation methods. The model can be extended to larger molecules.

Kinetics of Radical-Molecule and Radical-Radical Reactions in the Gas Phase

Kinetics of Elementary Reactions by Time-Resolved Mass Spectrometry

Laser photolysis reactor with time-resolved mass spectrometer

Time-resolved studies of the kinetics of elementary chemical reactions are required for developing complex reaction mechanisms and modeling practically important chemical processes, such as combustion, atmospheric chemistry, chemical vapor deposition, and exhaust gas cleaning technologies. We have developed a very versatile experimental setup by coupling excimer laser photolysis for producing specific radicals with time-resolved mass spectrometric detection. A recently studied reaction is HO2 + C2H5 = OH +  C2H5O, an important chain branching step leading to engine knock.

  

 

Shock Tube Studies of High Temperature Reactions

Shock tube (with cw dye laser in front)

The shock tube technique is a very powerful method for investigating gas phase reactions at high temperatures. A shock wave propagates along the shock tube at supersonic speed and heats and compresses the test gas within less than 1 μs (incident shock wave). The shock wave is reflected at the end wall and passes through the test gas once more (reflected shock wave). Our shock tube is designed for investigating the elementary reactions of small radicals (e.g., NH2, HCO, SiH2, 1CH2) at temperatures of 700 K < T < 3500 K and pressures of 0.75 bar < p < 3.5 bar. Radicals are detected by FM and UV absorption spectroscopies.

 

  

NOx-Reburning

Kinetics of the reaction CH2+NO

"NOx-Reburning" is a technically important process for the reduction of NO in power plants and engines. Trace amounts of hydrocarbons are injected into the exhaust gases. The NO thus reacts with small hydrocarbon radicals and is recycled into the oxidation chain. Two of the main reburn reactions of NO are those with CH2 and with HCCO.  We have studied these reactions using Laser Magnetic Resonance (LMR) and Fourier-Transform Infrared (FTIR) spectroscopies, ab initio quantum chemistry and density functional theory, and unimolecular rate theory.

 

 

 

Cavity Ringdown Spectroscopy

Cavity ringdown spectrometer

Cavity ringdown spectroscopy (CRDS) is an ultra-sensitive laser based absorption technique. The high detection sensitivity is primarily based on the long absorption length (>10 km) arising from a multiple reflection of a laser pulse in the optical cavity formed by two highly reflective mirrors. Moreover, CRDS is inherently immune to intensity fluctuations of the laser pulses, and enables one to determine absolut concentrations. The molecular absorption is directly related to the lifetime of the exponential "ringdown" of the signal that is transmitted through the cavity. We employ CRDS in the visible and the near IR (around 1.6 μm) for measuring free radical reactions and for surface studies.

 

  

Kinetics of Si Containing Radicals

Kinetics of Si containing radicals

 

 

 

 

   

Radical-Radical Kinetics by ab initio Quantum Chemistry and Unimolecular Rate Theory

Kinetics of the reaction CH2+NO

Spectroscopy of Highly Reactive Free Radicals in the Gas Phase

Molecular Beam Fourier-Transform Microwave Spectroscopy of Free Radicals and van der Waals Complexes

Fourier transform microwave spectroscopy

 

 

 

 

 

 

 

 

other work...

● Laser Induced Fluorescence

● Stimulated Emission Pumping

● Spectroscopy and Dynamics of Jahn-Teller active Radicals

● Cavity Ringdown Spectroscopy

● Laser Magnetic Resonance

● Vibration-Rotation Energy Transfer in Highly Excited Molecules

● Dynamics of Collision-Induced Inter-System Crossing Processes (CIISC)