Technique: Optical Spectroscopy |
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The optical laboratory is mainly designed to measure triplet state solvation dynamics in a time window from 10 μs to 1 s by recording T1 → S0 (0-0) emission spectra as a function of time. We also measure optical depolarization of triplet states and solvation by time-integrated fluorescence spectroscopy. |
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Schematic diagram of the optical setup used to measure solvation dynamics and probe
rotation: excimer laser (XeCl), UV-pass filter (F1), beam size limiter (BL),
Glan-Taylor polarizer (P1), λ/2 quartz retardation plate (RP), cryostat
(CS), sample cell (SC), beam dump (BD), Glan-Thompson polarizer (P2), UV
block filter, fibre optics (FO), and detector system (OMA) for solvation
measurements or monochromator with photomultiplier (PMT) and digital oscilloscope
to record probe rotation. [105] |
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Our experimental setup is equipped with an excimer laser (308 nm) and a frequency-tripled Q-switched Nd:YAG laser (355 nm), both with pulse energies of 100 mJ. Polarizers are used to measure solvation at the magic angle or to detect the rotational behavior of chromophores. Emission spectra are recorded with a gated micro-channel-plate intensified diode-array detector. Sample temperatures between 25 K and 325 K are provided by a closed-cycle He-refrigerator cryostat. Solvation dynamics experiments probe dielectric [34, 45, 143] and/or mechanical [82] relaxation phenomena on molecular spatial scales (2-3 molecular diameters resolution), because the energy level of a chromophore depends upon the solvent state [96]. The information regarding structural relaxation is derived from the time-resolved average emission energy. Information regarding the spatial variation of solvent response times is obtained from the time dependent line width [70, 93]. |
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Amplitude map of the T1 → S0(0-0) emission
intensity for quinoxaline in MTHF at T = 93.9 K as a function of wavenumber
ν and time t. The plot includes the limiting values of the
average emission energy, ‹ν(0)› and ‹ν(∞)›, and the resulting normalized
C(t) decay pattern which traces the peak energy ‹ν(t)›.
Dashed lines indicate the Gaussian width σ(t), which is time dependent. [96] |
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With this technique, we investigate local molecular dynamics and its relation to macroscopic properties by comparing with dielectric relaxation results. We study the spatial variation of relaxation times (dynamic heterogeneity) [70, 93, 100, 101, 102, 106, 114], how the dynamics of guest molecules differ from that of the host [105, 119], the dynamics of ionic liquids [139, 148], the dynamics of alcohols [119, 205], and the effects of geometrical confinement in nanoporous media [59, 65, 86, 94, 113, 116, 146, 170, 187] as well as the behavior of liquids within a few nanometers of the liquid-solid interface [108, 126, 144, 187]. Typical samples are organic supercooled liquids, ionic liquids, polymeric systems, and glasses. |
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Reference numbers refer to the list of publications |
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Experimental techniques:
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Selected projects:
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