Hole burning spectroscopy excited state

Kovalenko S A, Ernsting N P and Ruthmann J 1996 Femtosecond hole-burning spectroscopy of the dye DCM in solution the transition from the locally excited to a charge-transfer state Chem. Phys. Lett. 258 445-54... 

Pairwise EET rates cannot be directly measured in antenna systems. The closest approach to direct determination is offered on the one hand by time resolved picosecond and sub-picosecond absorption and fluorescence measurements and on the other hand by hole burning spectroscopies. Time resolved techniques do not detect transfer between isoenergetic sites. A somewhat more indirect approach to determining pairwise rates is that of analysing excited state lifetime data in terms of a particular antenna and an EET model. 

The very first fluorescence spectra of jet-cooled exciplexes indicated the existence of two types of ground-state van der Waals adducts. For instance, the anthracene-dimethylaniline system displayed two types of cluster bands in the fluorescence excitation spectrum broad ( 150 cm ) and structureless, leading to typical ex-ciplex emission, and narrow (1 cm ), leading to resonance-type emission [10, 20]. It was assumed that they are due to different 1 1 adducts, distinguished by different geometries. Recently, laser-based techniques were developed that allow the discrimination of different species. One is hole-burning spectroscopy and another— mass-selected photoionization. 

In hole-burning spectroscopy, on the other hand, the bum laser frequency is fixed while the probe laser frequency is tuned. The pump laser depletes the ground state of a single conformation, so that the probe laser records an excitation spectmm of all conformers except the one selected by the bum laser. It should be noted that in both cases the hot bands are not affected, because only the ground state is depopulated. In addition, for both methods to be effective, the bum laser pulse energy should be sufficient to induce a substantial ground state depletion. 

Relaxations of solvent-chromophore interactions can be studied experimentally by hole-burning spectroscopy, time-resolved pump-probe measurements, and photon-echo techniques that we discuss in the next chapter. If the temperature is low enough to freeze out pure dephasing, and a spectrally narrow laser is used to bum a hole in the absorption spectmm (Sect. 4.11), the zero-phonon hole should have the Lorentzian lineshape determined by the homogeneous lifetime of the excited state. The hole width increases with increasing temperature as the pure dephasing associated with tP comes into play [36, 37]. 

Schmitt, M Muller, H., Henrichs, U., Gerhards, M., Perl, W., Deusen, C., and Kleinermanns, K Structure and vibrations of phenol-CH(CDjOD) in the electronic ground and excited state, revealed by spectral hole burning and dispersed fluorescence spectroscopy, J. Chem. Phys. 103, 584-594 (1995). 

In addition to the relation of hole burning techniques to dynamic saturation methods, it is also important to stress the close relation to another rather different technique, namely, Mossbauer spectroscopy. The hole burning technique has been called the optical analog of the Mossbauer effect. The relation is very close, indeed. On the one hand, both techniques work at the ultimate limit of resolution given by the natural line width. It is not the lasers which limit the resolution in hole burning it is the fast excited state lifetimes which set the resolution limit. For organic dye molecules these lifetimes are in the nanosecond time regime. Conse-... 

Fig. 3 a) mass selected one-color REMPI spectrum, b) IR-UV ion dip spectra, and c) IR-UV hole-bum spectra of the capped dlpeptide PheCys adapted from 8] The REMPI spectmm shows the vibrational progression of the electronic excited state of multiple conformations. The contributions of these conformers are visualized using the conformer selective IR-UV hole-burning method. The structure of the individual conformers can be probed by IR-UV ion-dip spectroscopy, yielding conformer specific ground-state IR spectra. 

In recent years, there have been several successful attempts to use lasers for re-exciting ions after they have been cooled down. Since the ion cloud consists of a limited number, various strategies are possible for deriving information on state specific collision dynamics. The methods range from a dedicated perturbation of a stationary low temperature equilibrium via burning a hole into the state population to two colour pump — probe experiments. Most of such activities are used for spectroscopy or for understanding the low temperature populations of trapped hydrogen ions. They are discussed in Chapter 6. 

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