Alignment-orientation transition excited state

An independent verification of this procedure was possible for lb [82]. The four n-propyl substituents ensure that this molecule can be oriented to a high degree in stretched polymer sheets. Linear dichroism (LD) measurements on the aligned samples resulted in the determination of transition moment directions. A pattern similar to that of parent 1 was obtained. Both LD and emission anisotropy procedures yielded similar values, which, additionally, confirmed that the assignment of fluorescence depolarization to excited state tautomerization was correct. 

Anistropy measurements are based on the photose-lective excitation of fluorophores by plane-polarized light. In an isotropic medium, the fluorophores are randomly oriented. Upon excitation with polarized light, those fluorophores whose absorption transition dipole is aligned parallel to the electric vector of the excitation, will be preferentially excited. If the molecule rotates and tumbles out of this plane during the excited state, light is emitted in a different plane from the excitation light. The intensity of the emitted light can be monitored in vertical and horizontal planes and thus, fluorescence anisotropy (r) and polarization ( ) are defined by ... 

A transition will be characterized by a helical displacement of the electron if the magnetic and electric transition dipoles are aligned. This is reflected in the Rosen-feld equation for the CD intensity or rotatory strength, TZa- j, for a transition from a ground state n to an excited state j in a collection of randomly-oriented molecules ... 

Dichroic ultraviolet absorption measurements performed on the carbazole absorption band for homogeneously aligned samples showed, surprisingly, that the long axis of the carbazole ring plane is perpendicular to the nematic director, i.e. negative order parameters, as shown schematically in Fig. 2(a). The transition to the first excited state is known to be short-axis polarized for carbazole. This fact is disturbing in as much as probe techniques have been used previously to determine both transition moment directions as well as order parameters.This perpendicular orientation has been... 

The system of equations obtained, (5.22) and (5.23), in broad line approximation in many cases allows us to carry out the analysis of non-linear optical pumping of both atoms and molecules in an external magnetic field. Some examples will be considered in Section 5.5, among them the comparatively unexplored problem of transition from alignment to orientation under the influence of the dynamic Stark effect. But before that we will return to the weak excitation and present, as examples, some cases of the simultaneous application of density matrix equations (5.7) and expansion over state multipoles (5.20). 

Finally, two theoretical papers should be mentioned. The first [141] concerns orientation and alignment relaxation and transfer in resonant transitions 2S1/2-2P1/2, 2P3/2, and the second [142] deals with non-resonant excitation transfer for spinless S-P states. All results obtained refer to the simple dipole-dipole interaction, proportional to R3. 

Thus after a period of tf the intensity has dropped to 37% of h, that is 63% of the molecules return to the ground state before tf. In many cases the above expression needs to be modified into more complex expressions. First of all it is assumed that the instrument yields an infinite (or very) short light pulse at time zero. In cases where tf is small Ig must be replaced by a function, which describes the lamp profile of the instrument. Also, more than one lifetime parameter is often needed to describe the decay profile, which is l(t) must be expressed as a sum of exponentials. Finally the concept of anisotropy should be mentioned. Anisotropy is based on selectively exciting molecules with their absorption transition moments aligned parallel to the electric vector of polarized light. By looking at the polarization of the emission the orientation of the fluorophore can be measured. The anisotropy of the system is defined as (Equation 6) (Rendell, 1987 Lakowicz, 2006) ... 

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