| People involved: | Tõnu Pullerits |
| Former members: | Niklas Christensson |
| Involved facilities: | Fluorescence line narrowing setup |
This technique has the following projects (and possibly other techniques) related to it:
The goal of molecular spectroscopy is twofold. First, the frequency at which a specific compound absorbs energy gives information about the energy level structure of the investigated compound. For a molecule placed in a non-interacting environment the resulting absorption spectra would consist of narrow lines (stick spectra). For a molecule in solution the spectra looks strikingly different where the absorption spectra are, in general, broad and featureless. The broadening occurs as a result of the time dependent interaction of the surrounding molecules with the transition dipole moment (by electrostatic interaction) of the molecule. This time dependent interaction results in a modulation of the transition frequency in time, which directly translates into a broadening of the line shapes in the frequency domain. The understanding and analysis of spectral line shapes can thus provide valuable information about the interaction of molecules with their surroundings. In the simplest model of optical line shapes one assumes that the modulations of the transition frequency is either infinitely fast or infinitely slow. This means that the absorption lines can be considered either homogenous or inhomogenously broadened, respectively. Inhomogenous broadening means that different molecules have a (static) frequency different from the other molecules in the ensemble. This type of broadening limits the possibilities to analyse the line shape in terms of a time dependent modulation of the transition frequency. Moreover, the fluctuations of the transition frequency occurs on many timescales ranging from 10s of fs to in principle infinitely slow processes. It is not possible to disentangle all the dynamical information from a single absorption spectra and more elaborate techniques are needed.
One of the techniques that allows for the elimination of inhomogenous broadening is so called fluorescence line narrowing. This technique is based on the selective elimination of inhomogeneous broadening by selecting to probe the dynamical evolution of only a small portion of the molecules that make up the overwhelmingly inhomogenously broadened profile. Picture 1 depicts the line shapes of the individual chromophores in a matrix at low temperature. The absorption line is to a large extent inhomogenously broadened, i.e every chromophore has its own transition frequency. At low temperature the individual line shape of a single site consists of a zero-phonon line (ZPL) and a phonon wing (Stokes process).
The ZPL can be understood as a direct excitation of the chromophore while the phonon wing is the result of the excitation of the chromophore and a simultaneous excitation of a phonon in the surrounding lattice. For higher temperatures processes involving the annihilation of a phonon also becomes possible since the thermally excited phonons are readily available (anti-Stokes process). At sufficiently low temperature the thermal occupation of phonons is negligible and only the Stokes processes contribute to the signal. If a narrow band laser is tuned to the red side of the absorption spectra then the laser will mostly interact with molecules trough the ZPL, i.e via direct excitation with the involvement of phonons. Since the laser has a narrow bandwidth the interactions also takes place with molecules of a defined range of transition frequencies and this eliminates the inhomogenous broadening. After the molecules have absorbed the incoming laser radiation they relaxes and emit fluorescence with a line shape that is determined by the homogenous fluctuations of the transition frequency.
Fluorescence line narrowing has a number of interesting features that makes it attractive for spectroscopic analysis. As a frequency domain technique it offers the advantage of a large dynamical range and is well suited for studying the fastest interactions between the molecule and its environment. It can display the strength and frequency of the intermolecular vibrations of the molecule under study (the so called Huang-Rhys factors). The drawback of the method is that it has to be performed at cryogenic temperatures, i.e less then 10K, to ensure that the mean phonon number in the matrix is sufficiently low so that absorption doesnt take place through the phonon wing.
An illustration of a Fluorescence Line narrowing spectra are shown in figure 2. The spectra clearly display distinct vibronic features. These features were analysed by a model and from which the Huang-Rhys factors and the vibrational frequencies could be determined.
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