The people involved: W. Werncke, V. Kozich, J. Dreyer, E.T.J. Nibbering, T. Elsaesser
Former team members: S. Wachsmann-Hogiu, A. Kummrow, M. Pfeiffer

International collaboration: V. Orlovich,^† A. Vodschitz^†
†: Institute of Physics, Academy of Sciences Belarus, Minsk, Belarussia

Intramolecular vibrational excitations play a central role in understanding the microscopic mechanisms and dynamics of photoreactions. Photoexcitation to electronically excited states results in elongations of the vibrational modes which couple to the electronic transition and determine the initial time evolution of the system on the potential energy surface of the excited state. Anharmonic coupling of such modes to other intramolecular modes and/or degrees of freedom of the environment lead to a dephasing of coherent vibrational motions, relaxation of vibrational populations, and deactivation of electronically excited states through radiationless processes like internal conversion. Processes involving changes of vibrational populations underlie vibrational energy redistribution (IVR) and, in condensed phase systems, the transfer of vibrational excess energy to the environment (vibrational cooling). Internal conversion (IC) between electronic singlet states frequently occurs through conical intersections of potential energy surfaces. After photoexcitation, the system moves along the so-called tuning modes to the conical intersection where the two electronic states are coupled by vibrational modes of appropriate symmetry. Vibrational excess energy is released into a third type of vibrations, the accepting modes of the lower electronic state. Accepting modes are characterized by non-vanishing Franck-Condon vibrational overlap integrals between the coupled electronic states and - thus - can be monitored in resonance Raman measurements. Time resolved vibrational spectroscopy provides the most direct experimental access to observe vibrational dynamics and transient vibrational populations during and after internal conversion. We study vibrational dynamics after photochemical reactions, e.g. electron transfer, by stationary and time-resolved resonance Raman spectroscopy. These methods allow to gain new insights into the action of vibrational modes during and after elementary photo reactions:

1) Stationary resonance Raman scattering (together with density functional theory (DFT) calculations) provides information about vibrational modes strongly coupled to the reaction and about corresponding geometrical changes.
2) Time-resolved anti-Stokes Raman spectroscopy enables to monitor the kinetics of the vibrational populations of the vibrational modes after excitation in real time.
 

1. We have invetsigated 4-nitroaniline (PNA), a p-conjugated push-pull molecule with pronounced electron transfer after excitation and - therefore - interesting nonlinear optical properties. Ultrafast vibrational excitation and energy redistribution in the electronic ground state after internal conversion is investigated by anti-Stokes resonance Raman spectroscopy with a time resolution of 1 ps.

2. For assignment of vibrational modes these studies are accompanied by stationary vibrational spectroscopy and density functional theory calculations. We observe Raman lines of overtones and/or combination bands of out-of-plane vibrations displaying pronounced excess populations with rise times close to the decay time of the electronically excited state. Compared to such fast dynamics, the strongly Raman active totally-symmetric modes show a considerably slower picosecond rise time. This can be seen for PNA.

3. Our results demonstrate: (i) excitation of overtones of out-of-plane vibrations by internal conversion (g(NO2-wagging) at 754cm-1 and g(ph-4) at 700cm-1), i.e. these modes represent primary accepting modes. (ii) excitation of strongly Raman active vibrations (ns(NO2) at 1310cm-1 and n(ph-1), n(ph-NO2) at 860cm-1)  mainly by redistribution of the vibrational energy, i.e. these modes represent secondary accepting modes). (iii) a thermal equilibrium distribution of the modes does not exist earlier than 6 ps after excitation. (iv) vibrational populations are sensitive to isotopic substitution.

g(NO2-wagging) at 754 cm-1

g(ph-4) at 700cm-1

ns(NO2) at 1310cm-1

n(ph-1), n(ph-NO2) at 860cm-1