Transient population gratings

The polarization selection was used for the differentiation of the direction of the transition dipole moment. This technique was also demonstrated to be very useful to separate spectrally overlapped transient signals from different chromophores in a complex reaction. A careful analysis of the population grating data was carried out to determine rotational anisotropy, two-photon absorption state symmetry, and the relative orientation of the donor and acceptor. 

Fig. 5.33 Measured signal from high-harmonic transient grating spectroscopy of NO2 (left) and computed diabatic state population (right). Reprinted form [221. (Reprinted with permission from P. M. Kraus et al., Phys. Rev. A 85, 043409 (2012)).

In Refs. [466] and [221], the transient grating interferometry signal from the conical intersection dynamics in NO2 (see Sec. 5.4.1.2) was measured for a 401 nm (3.09 eV), FWHM 40 fs pump pulse. The left panel of Fig. 5.33 shows the time-dependence of Im= , t) for the 13th harmonic, which re-hects the time evolution of the diabatic excited state population (Fig. 5.33, right). In the case of ionization out of the same orbital and spatially spread vibrational waveftmction as in the present case, the dependence of di Q,t) on the molecular geometry (vibrational dynamics) was found to be small... 

This setup allows one to obtain two types of transient data (i) at fixed tu, the monochromator grating is scanned or (ii) at fixed monochromator frequency, the time delay is changed. Figure 4.13 shows the transient absorption spectra of CO molecules adsorbed on a Pf(lll) surface and obtained for differ-enf fime delays. A distinct shift to lower frequencies and a spectral broadening are observed at small fp as well as the rapid return of the spectrum to the equilibrium lineshape. Such temporal behavior can be described as originating from a one-phonon band of fhe adlayer which shiffs with the increasing excited-state population. 

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