Research interests

Ultrafast dynamics of large molecules in the condensed phase is induced by optical excitation and monitored by    Fs Transient Absorption   over the entire UV/Vis/NIR range, and by    Fs Fluorescence Spectroscopy   using broadband upconversion. Both techniques give transient spectra of photometric quality. We compare transient absorption and fluorescence quantitatively and seek patterns by looking at a number of related molecules. Our strategy may be called "Femtosecond Screening of Structure-Activity Relationships". In this way we gain insight into

• electronic change upon Charge-Transfer, Proton-Transfer,
• intramolecular structural reorganisation such as Isomerisation,
• Solvent Reorganisation.

Theoretical developments are directed towards understanding transient optical spectra at earliest time when Electronic Coherence is still present.

Applications are presently focussed to obtain a Molecular Infrared Spectrometer (see Figure). The dynamic Stokes shift of fluorescence (yellow-red band) images the relaxation of the surrounding solvent in the S₁ state of the solute. Solvent coordinate Q represents the configurational state of the environment and is treated classically. High-frequency motion along internal coordinate q for bond lengths and angles is quantized in vibrational states. Before femtosecond excitation (A) the solute is in the equilibrated ground-state S₀ and the (blue) absorption band extends over a Franck-Condon progression for upward optical transitions. Immediately after excitation (B) the emission overlaps the absorption band at the electronic absorption origin 00. Partially relaxed E₁(Q) (C) corresponds to solvent configurations which have raised E₀(Q). The fluorescence therefore changes from yellow to red as solvation proceeds. After several picoseconds (D) a new equilibrium is reached for the S₁ state. A point dipole in a spherical polarizable cavity represents the solute while a continuum with dielectric dispersion ε (ω) represents the liquid. By following the Stokes-shift in time, we find ε (ω) and thus ωε "(ω), which is the (local) infrared spectrum. This idea is being applied to the Polar Dynamics of DNA Duplex Oligomers.

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Femtosecond Transient Absorption for UV/Vis/NIR broadband coverage

We focus on best performance in the near-UV with routine capability for screening many molecular systems. This combination enables fs optical spectroscopy of bioploymers and corresponding applications in molecular physiology. The specs are:

• simultaneous coverage 250-600 nm or 400-1000 nm by the supercontinuum probe pulse
• highest time and spectral resolution combined (fwhm δt &asymp 20 fs for probe wavelengths λ_probe > 400 nm, ≈ 60 fs for 400 > λ_probe> 300 nm)
• pump pulse energy ∼0.6 μJ at 250 Hz, i.e. 0.15 mW average pump power
• probe spot diameter 50 μm, pump spot diameter 100 μm
• sample volume as small as 250 μl
• spot size and repetition rate optimized for sample replenishment
• absorbance noise of 10_-4 rms upon averaging 100 shots

The supercontinuum is generated in a 1 mm CaF₂ plate by ∼20 μJ pulses at 400 nm or longer wavelengths. It is evened with a filter and imaged onto a 50 μm pinhole which is positioned just before a beam splitter. We use separate spectrographs for the sample and reference beams. Reflective optics are optimized for minimal astigmatism and coma. Thus, the 50 μm probe spot is relayed undistorted from the source through the sample onto the detector (Hamamatsu PDA, 512 pixels, 16 bit A/D at ≤1 kHz, for each of two parallel channels). A measurement cycle contains four spectra: sample and reference with pump, and same without pump. The induced spectrum is calculated for every cycle and then averaged. Microscopes can be swivelled to view the input plane of the spectrographs and the overlap region of the sample. Note that time resolution in the near-UV is usually limited by the sample thickness, hence by the solubility. Two identical TA setups are operated in our laboratory. They are driven by

• CLARK MXR 2001 → 2-stage NOPA for pump, 1-stage NOPA for probe
• FEMTOLASER Pro → frequency-doubled or TOPAS (LIGHT CONVERSION) for pump, frequency-doubled for probe.

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Femtosecond Fluorescence Spectroscopy by broadband upconversion

Upconversion is a well-established method for tracing emission at selected wavelengths, and transient spectra are usually reconstructed afterwards. But it is much better to record a gated emission spectrum in its entirety before changing the delay time. By using gate pulses in the near-infrared and a thin crystal we up-convert fluorescence at all wavelengths simultaneously. The key is a noncollinear geometry with tilted gate pulses which allows to measure background-free spectra. In this way we obtain transient fluorescence spectra of photometric quality. The characteristics of our approach are:

• simultaneous detection at all emission frequencies
• time resolution Δt≤80 fs by using tilted gate pulses
• pump pulse energy ≤ 0.6 μJ at 1 kHz (0.6 mW)
• S/N > 500, at the peak of typical fluorescent dyes, with 1 s integration time.

The setup is driven by FEMTOLASER Pro → TOPAS for 1300 nm gate and 450 nm excitation pulses. 400 nm excitation pulses are obtained directly by frequency doubling.

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