Time-resolved spectroscopies polarization

Cross A J, Waldeck D H and Fleming G R 1983 Time resolved polarization spectroscopy level kinetics and rotational diffusion J. Chem. Phys. 78 6455-67... 

Doppler-Free Time-Resolved Polarization Spectroscopy of Large Molecules Measurement of Excited State Rotational Constants, J. S. Baskin, P. M. Felker, and A. H. Zewail, J. Chem. Phys. 84, 4708 (1986). 

Lange, W. and Mlynek, J. (1978). Quantum beats in transmission by time-resolved polarization spectroscopy, Phys. Rev. Lett., 40, 1373-1375. 

Quantum beats can be observed not only in emission but also in the transmitted intensity of a laser beam passing through a coherently prepared absorbing sample. This has first been demonstrated by Lange et al. [872, 873]. The method is based on time-resolved polarization spectroscopy (Sect. 2.4) and uses the pump-and-probe technique discussed in Sect. 6.4. A polarized pump pulse orientates atoms in a cell placed between two crossed polarizers (Fig. 7.12) and generates a coherent superposition of levels involved in the pump transition. This results in an oscillatory time dependence of the transition dipole moment with an oscillation period AF = 1/Av... 

The applied method is an extension of a previously described technique [I] of time-resolved polarization spectroscopy into the femtosecond range. It relies on the creation of a coherent superposition of adjacent states or substates by an optical pulse, which is short compared to the reciprocal of the frequency splitting of the respective states. Such an atomic coherence causes an optical anisotropy in the sample, oscillating exactly with the splitting frequency of the coherently excited states. 

In siammary, the preliminary results presented in this contribution already demonstrate that time resolving polarization spectroscopy offers a number of favourable and new features for direct observation of fast evolving events on a femtosecond time scale and detection of oscillations up to the THz-range. The described technique can be applied to free atoms, liquids and solids to measure coherent transients in groimd and excited states. Since the observed beats result from an atomic interference effect, narrow structures which may be hidden by inhomogeneous broadening mechanisms can still be resolved. 

Fig. 12.1 la,b. Quantum-beat spectroscopy of atomic or molecular ground states measured by time-resolved polarization spectroscopy (a) experimental arrangement and (b) Zee-man quantum beat signal of the Na 3 Si/2 ground state recorded by a transient digitizer with a time resolution of 100 ns. (Single pump pulse, time scale 1 rs/div, magnetic field 1.63 X 10-4 T) [12.40]... 

The experimental arrangement which is similar to that of time-resolved polarization spectroscopy (Fig. 12.11) is depicted in Fig. 12.16. The pulse train is provided by a synchroneously pumped, mode-locked CW dye laser. A fraction of each pulse is split by the beam splitter BS and passes... 

In addition to fluorescence intensity and polarization, fluorescence spectroscopy also includes measurement of the lifetime of the excited state. Recent improvements in the design of fluorescence instrumentation for measuring fluorescence lifetime have permitted additional applications of fluorescence techniques to immunoassays. Fluorescence lifetime measurement can be performed by either phase-resolved or time-resolved fluorescence spectroscopy. 

The 3.5- and 8-ntn nanoparticles show well-resolved peaks at 362 and 473 nm, respectively, as well as other features at higher energies. The 4.5-nm particles show a well-resolved peak at 400 nm and a shoulder at 450 nm. It is tempting to assume that in each case, the lowest energy absorption corresponds to the lowest allowed transition (the A exciton) in bulk M0S2. Polarization spectroscopy can be used to determine if this is the case. The lowest allowed transitions in bulk material, the A and B excitons, are polarized perpendicular to the crystallographic c axis. If the lowest allowed transition correlates to the A exciton, then it would be expected to also be a planar (xy polarized) oscillator. However, tire results of polarization studies reveal that the actual situation is more complicated. A combination of time-resolved polarized emission and one-color time-resolved polarized absorption (transient bleach) studies facillitate assignment of the polarizations of the observed nanoparticle transitions. The 3.5-nm particles are emissive and the polarization of the several of the lowest transitions may be determined... 

Time-resolved fluorescence spectroscopy of polar fluorescent probes that have a dipole moment that depends upon electronic state has recently been used extensively to study microscopic solvation dynamics of a broad range of solvents. Section II of this paper deals with the subject in detail. The basic concept is outlined in Figure 1, which shows the dependence of the nonequilibrium free energies (Fg and Fe) of solvated ground state and electronically excited probes, respecitvely, as a function of a generalized solvent coordinate. Optical excitation (vertical) of an equilibrated ground state probe produces a nonequilibrium configuration of the solvent about the excited state of the probe. Subsequent relaxation is accompanied by a time-dependent fluorescence spectral shift toward lower frequencies, which can be monitored and analyzed to quantify the dynamics of solvation via the empirical solvation dynamics function C(t), which is defined by Eq. (1). 

Basic principles and applications of time-resolved fluorescence spectroscopy have been outlined in a very illustrative way by Valeur [16]. Although punctiform spectroscopy is still the best way to get a detailed knowledge of all the important parameters that characterize fluorescence emission (exact spectral properties, decay time behavior, polarization), imaging is always preferred whenever the localization of the distribution of any biomolecule of interest is required or a great number of samples have to be analyzed [22]. 

Most of the time-resolved emission spectroscopy setups are home made in the sense that they are built from individual devices (laser, detection system,. ..) hence they are not of a plug and press type, so that their exact characteristics may vary from one installation to the other. Some of these differences have no impact on the overall capabilities of the system but some have a drastic influence on the way the collected data are processed and analysed. This aspect will be detailed in the next section, while this section deals with a general description of the apparatus. The most basic type of apparatus will be described, with no reference to sophisticated techniques such as Time Correlated Single Photon Counting or Circularly Polarized Luminescence devices. 

The pump pulse in time-resolved pump-probe absorption spectroscopy is often linearly polarized, so photoexcitation generally creates an anisotropic distribution of excited molecules. In essence, the polarized light photoselects those molecules whose transition moments are nominally aligned with respect to the pump polarization vector (12,13). If the anisotropy generated by the pump pulse is probed on a time scale that is fast compared to the rotational motion of the probed transition, the measured anisotropy can be used to determine the angle between the pumped and probed transitions. Therefore, time-resolved polarized absorption spectroscopy can be used to acquire information related to molecular structure and structural dynamics. 

Normally w-amino-a-arylalkanes can adopt a conformation which produces exciplex fluorescence by a process involving C—C-bond rotation which is very fast. Both 3-(4-dimethylaminophenyl)-l-(9-anthracenyl)propane and 3-(4-dimethylaminophenyl)-l-(l-pyrenyl)propane form fluorescent exciplexes, which by means of picosecond time-resolved fluorescence spectroscopy have been shown to take a few nanoseconds to be formed (Migita et al., 1980, 1981). The rate of intramolecular fluorescent exciplex formation has also been shown to be dependent upon the length of the linking chain, the polarity of the solvent (the build up time decreases as solvent polarity is increased)... 

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