Photo-orientation equations

This system of equations shows, through even orders, that polarized light irradiation creates anisotropy and photo-orientation by photoisomerization. A solution to the time evolution of the cis and trans expansion parameters cannot be found without approximations this is when physics comes into play. Approximate numerical simulations are possible. 1 will show that for detailed and precise comparison of experimental data with the photo-orientation theory, it is not necessary to have a solution for the dynamics, even in the most general case where there is not enough room for approximations, i.e., that of push-pull azo dyes, such as DRl, because of the strong overlap of the linear absorption spectra of the cis and trans isomers of such chromophores. Rigorous analytical expressions of the steady-state behavior and the early time evolution provide the necessary tool for a full characterization of photo-orientation by photoisomerization. 

For photo-orientation analysis of actual data. Equation 3.10 must be combined with Equations 3.1 and 3.2. The time-evolution simulation (see... 

Equation was derived without approximations. It is noteworthy that these solutions do not couple tensorial components of different orders and that they confirm that rotational diffusion and cis—>trans thermal isomerization are isotropic processes that do not favor any spatial direction. In Section 3.4, I discuss, through the example of azobenzene, how Equation 3.11 can be used to study reorientation processes during cis—>trans thermal isomerization after the end of irradiation. The next subsection gives analytical expressions at the early-time evolution and steady-state of photo-orientation, for the full quantification of coupled photo-orientation and photoisomerization in A<- B photoisomerizable systems where B is unknown. 

I will go on to show how Equations 3.12 through 3.15 can be used to determine and 2 (cos from A—>B photo-orientation experi-... 

Equations 3.16 through 3.18 were derived by a method similar to that used in deriving Equations 3.12 through 3.15. They allow for the measurement of and 2 (cos cob) from B—>A photo-orientation experiments. 

During the steady state of photo-orientation, the expansion parameters and are constants, i.e., dC /dt = dCB /dt = 0. If the first equation of the system of Equation 3.9 is multiplied by and added to the second equation of that system, the following relation is obtained after rearrangement. 

Equation 3.23 is derived without truncation above any order by assuming that the geometrical order parameters, A2, of the orientational distribution of the A and B isomers are equal at the photostationary state of irradiation. Although this assumption physically mirrors a uniform molecular orientational distribution, it does simplify considerably the expression of the photostationary-state orientational order and provides a simple law for steady-state photo-orientation characterization. Equation 3.23 holds when analysis is performed at the irradiation wavelength, and fits by Equations 3.22 and 3.23 allow for the measurement of 2 (cos [Pg.78]

When the analysis of photo-orientation is performed at a wavelength different from the irradiation wavelength, the symmetry of the molecular transitions in both A and B isomers can be found. Indeed, setting n=0 in Equation 3.9, yields the following relation for the photostationary state of irradiation ... 

F G. 4.25 ln(k/ko) versus the applied pressure, k and kg are the rates given by the slope of the early time evolution of photo-orientation at a given pressure and at I MPa, respectively. The markers are experimental data points and the line is a theoretical fit by Equation 4.2. The number in parentheses corresponds to the activation volume of the trans->cis photo-orientation. After reference 48, redrawn by permission of OSA. 

The contribution of photo-induced nonpolar orientation through even order parameters (see Equation 8A.9 in the appendix, page 286) to the effects observed in the EFISH decrease is negligible. The same decrease is observed for a TE- or TM-polarized probe regardless of the pump polarization (not shown). The photochemically induced molecular shape change of the NLO dye blows out the strong optical field driven anharmonic movement of the... 

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