The people involved: Valeri Kozich, Jens Dreyer, Wolfgang Werncke, Thomas Elsaesser
Former team members: Sebastian Wachsmann-Hogiu, Andreas Kummrow, Michael Pfeiffer, Satoshi Ashihara

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 Stokes resonance Raman spectroscopy allows to monitor the formation of new products and to derive structural information.
3) Time-resolved anti-Stokes Raman spectroscopy enables to monitor the kinetics of the vibrational populations of the vibrational modes after photochemical excitation in real time.
4) Information about IVR and vibrational cooling after excitation of a distinct mode in the electronic ground state is obtained by time-resolved infrared - anti-Stokes Raman pump- probe spectroscopy.
5) Coupling of the vibrational modes is calculated using ab initio quantum chemical methods.
 

1.  Our earlier work has been devoted to 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, follow this link.

3. Our recent work deals with the energy flow in heterocyclic aromatic molecules showing internal hydrogen transfer. We investigate vibrational excitation, intramolecular vibrational energy redistribution and vibrational cooling after excited state hydrogen transfer (ESIHT) in the excited keto and enol electronic ground states.

4. Kinetics of vibrational excitation of the S1 state of the keto-tautomer of 2-(2'-hydroxyphenyl)benzoxazole (HBO) formed after ESIHT. The most intense modes of HBO observed after ESIHT show characteristic NH bending and CO stretching displacements, respectively.

5. These high-frequency modes are not populated by the ESIHT process. Instead thermal equilibrium between the low-frequency modes in the excited electronic state being excited by ESIHT is already reached within 2ps.

6. Vibrational excitation and IVR after a completed hydrogen transfer cycle of TINUVIN. We demonstrate that for large excess energies vibrational kinetics as well as product formation kinetics cannot be derived directly from time-dependent resonance Raman intensities. Instead the amount of excitation of a distinct mode is derived from the ratios of time-dependent anti-Stokes and Stokes resonance Raman intensities. They show predominant excitation of a low-frequency mode which is known to be coherently excited during and after the hydrogen transfer on a subpicosecond time scale.

Publications on vibrational kinetics after elementary photochemical reactions

VOW07 C1-P-2008.03 A. I. Vodchits, V. A. Orlovich, W. Werncke and V. P. Kozich Influence of gas circulation on stimulated Raman scattering and amplification of ultrashort laser pulses in methane Opt. Commun. 281 (2008) 3190-3195 Download PDF/PS-File or URL: C1-P-2008.03   DKW07 C1-P-2007.06 J. Dreyer, V. Kozich and W. Werncke Tuning intramolecular anharmonic vibrational coupling in 4-nitroaniline by solvent-solute interaction J. Chem. Phys. 127 (2007) 234505 Download PDF/PS-File or URL: C1-P-2007.06   KWe07 C1-P-2007.02 V. Kozich and W. Werncke. Non-degenerate second-order scattering and difference combination bands in infrared-pump/anti-Stokes resonance Raman-probe experiments J. Raman Spectrosc. 38 (2007) 1180-1185 Download PDF/PS-File or URL: C1-P-2007.02   KDV05 C1-P-2006.15 W. Werncke, V. Kozich, J. Dreyer, S. Ashihara and T. Elsaesser Mode-selective O-H stretching relaxation in a hydrogen bond studied by ultrafast vibrational spectroscopy in: Ultrafast Phenomena XV, P. Corkum, D. Jonas, R. J. D. Miller, A. M. Weiner eds. (Springer Berlin 2007) pp. 448-450 Download PDF/PS-File or URL: C1-P-2006.15   KDA06 C1-P-2006.07 V. Kozich, J. Dreyer, S. Ashihara, W. Werncke and T. Elsaesser Mode-selective O-H stretching relaxation in a hydrogen bond studied by ultrafast vibrational spectroscopy J. Chem. Phys. 125 (2006) 074504 Download PDF/PS-File or URL: C1-P-2006.07   WKV06 C1-P-2003.20 W. Werncke, V. Kozich, A. I. Vodchits and J. Dreyer Ultrafast excitation of out-of-plane vibrations and vibrational energy redistribution after internal conversion of 4-nitroaniline in: Time Resolved Vibrational Spectroscopy XI, S. Califano, P. Foggi, R. Righini eds. (Citta di Castello, Italy 2006) pp. 281-286   VKW06 C1-P-2005.07 A. I. Vodchits, V. P. Kozich, W. Werncke and V. A. Orlovich Efficient multi-frequency generation of ultrashort light pulses using stimulated Raman scattering and optical parametric amplification Opt. Commun. 260 (2006) 307-310 Download PDF/PS-File or URL: C1-P-2005.07   KDV05 C1-P-2005.06 V. Kozich, J. Dreyer, A. Vodchits and W. Werncke Time-resolved resonance Raman scattering of the excited singlet state of 2-(2'-hydroxyphenyl)benzoxazole after excited state intramolecular proton transfer Chem. Phys. Lett. 415 (2005) 121-125 Download PDF/PS-File or URL: C1-P-2005.06   WKD05 C1-P-2004.14  W. Werncke, V. Kozich and J. Dreyer. Vibrational excitation and energy redistribution after ultrafast intramolecular proton transfer of TINUVIN in: Ultrafast Phenomena XIV, T. Kobayashi, T. Okada, T. Kobayashi, K. A. Nelson, S. D. Silvestri eds. (Springer Verlag Berlin 2005) pp. 485-487 Download PDF/PS-File or URL: C1-P-2004.14   KWe05 C1-P-2004.14  V. Kozich and W. Werncke. Influence of vibrational cooling on the time-dependence of stokes and anti-stokes resonance raman scattering J. Mol. Struct. 735-736 (2005) 145-151 Download PDF/PS-File or URL: C1-P-2004.14   VSO05 C1-P-2004.08  A. I. Vodchits, A. G. Shvedko, V. A. Orlovich, V. P. Kozich and W. Werncke. Stimulated Raman amplification of ultrashort seed pulses in compressed methane J. Opt. Soc. Am. B 22 (2005) 453-458 Download PDF/PS-File or URL: C1-P-2004.08   KDW04 C1-P-2004.09  V. Kozich, J. Dreyer and W. Werncke. Vibrational excitation after ultrafast Intramolecular proton transfer of TINUVIN: a time-resolved resonance Raman study Chem. Phys. Lett. 399 (2004) 484-489 Download PDF/PS-File or URL: C1-P-2004.09   WKD03 C1-P-2003.18  W. Werncke, V. Kozich, J. Dreyer, M. Rini, A. Kummrow and T. Elsaesser. Vibrational excitation and energy redistribution due to back-electron transfer in para-nitroaniline. in: Recent advances in ultrafast spectroscopy; Proceedings of the 'XII UPS Conference', S. Califano, P. Foggi, R. Righini eds. (Leo S. Olschki Florence, Italy 2003) pp. 397-403.   KWV03 C1-P-2002.09  V. Kozich, W. Werncke, A. I. Vodchits and J. Dreyer.  Ultrafast excitation of out-of-plane vibrations and vibrational energy redistribution after internal conversion of 4-nitroaniline J. Chem. Phys 118 (2003) 1808-1814 Download PDF/PS-File or URL: C1-P-2002.09   KWD02 C1-P-2002.01  V. Kozich, W. Werncke, J. Dreyer, K.-W. Brzezinka, M. Rini, A. Kummrow and T. Elsaesser.  Vibrational excitation and energy redistribution after ultrafast internal conversion in 4-nitroaniline J. Chem. Phys. 117 (2002) 719-726 Download PDF/PS-File or URL: C1-P-2002.01   WWD02 C1-P-2001.15  W. Werncke, S. Wachsmann-Hogiu, J. Dreyer, A. Vodtchits and T. Elsaesser.  Ultrafast intramolecular electron transfer studied by picosecond and stationary Raman spectroscopy Bull. Chem. Soc. Jpn. 75 (2002) 1049-1055 Download PDF/PS-File or URL: C1-P-2001.15   VWH01 C1-P-2001.13  A. I. Vodchits, W. Werncke, S. Hogiu, V. A. Orlovich and Valentin . Stimulated Raman scattering in compressed gases by short laser pulses SPIE Proc., A. A. Andreev and V. E. Yashin, Eds.,  4352 (2001) 52-58 Download PDF/PS-File or URL: C1-P-2001.13   WWD01 C1-P-2000.22 S. Wachsmann-Hogiu, W. Werncke, J. Dreyer, A. I. Vodchits, K.-W. Brzezinka and T. Elsaesser. Ultrafast intramolecular electron transfer studied by stationary vibrational and time-resolved resonance Raman spectroscopy combined with ab initio calculations in Recent Research Developments in Chemical Physics 2, (Transworld Research Network, Trivandrum, India, 2001) pp. 61-84   HDP00 C1-P-2000.03  S. Hogiu, J. Dreyer, M. Pfeiffer, K.-W. Brzezinka and W. Werncke.  Vibrational analysis and excited state geometric changes of betaine-30 derived from Raman and infrared spectra combined with ab initio calculations J. Raman Spectrosc. 31 (2000) 797-803 Download PDF/PS-File or URL: C1-P-2000.03   HWP00 C1-P-2000.02 S. Hogiu, W. Werncke, M. Pfeiffer, J. Dreyer and T. Elsaesser.  Mode specific vibrational excitation and energy redistribution after ultrafast intramolecular electron transfer J. Chem. Phys. 113 (2000) 1587-94 Download PDF/PS-File or URL: C1-P-2000.02   HWP99a C1-P-1999.03  S. Hogiu, W. Werncke, M. Pfeiffer and T. Elsaesser.  Mode specific vibrational kinetics after intramolecular electron transfer studied by picosecond anti-Stokes Raman spectroscopy Chem. Phys. Lett. 312 (1999) 407-414 Download PDF/PS-File or URL: C1-P-1999.03

Publications on excited-state properties of cyanines and polyenes

WHP00  C1-P-1999.09 W. Werncke, S. Hogiu, M. Pfeiffer, A. Lau and A. Kummrow.  Strong S1-S2 vibronic coupling and enhanced third order hyperpolarizability in the first excited singlet state of diphenylhexatriene studied by time-resolved CARS J. Phys. Chem. A 104 (2000) 4211-4217 Download PDF/PS-File or URL: C1-P-1999.09   HWP99b C1-P-1998.01 S. Hogiu, W. Werncke, M. Pfeiffer and A. Lau.  Evidence of strong vibronic coupling in the first excited singlet state of diphenylhexatriene studied by picosecond CARS spectroscopy Chem. Phys. Lett. 303 (1999) 218-222 Download PDF/PS-File or URL: C1-P-1998.01   PWH98 C1-P-1998.03  M. Pfeiffer, W. Werncke, S. Hogiu, A. Kummrow and A. Lau.  Strong vibronic coupling in the first excited singlet state of diphenylhexatriene by an asymmetric low frequency mode Chem. Phys. Lett. 295 (1998) 56-62 Download PDF/PS-File or URL: C1-P-1998.03   HWP98 C1-P-1997.03  S. Hogiu, W. Werncke, Pfeiffer, A. Lau and T. Steinke.  Picosecond time-resolved CARS spectroscopy of a mixed excited singlet state of diphenylhexatriene Chem. Phys. Lett. 287 (1998) 8-16 Download PDF/PS-File or URL: C1-P-1997.03   WPL98 C1-P-1997.01  W. Werncke, M. Pfeiffer, A. Lau, W. Grahn, H.-H. Johannes and L. Dähne.  Alternation and saturation effects in the length dependences of the third order hyperpolarisabilities of cyanines J. Opt. Soc. Am. B 15 (1998) 863-870 Download PDF/PS-File or URL: C1-P-1997.01   TJW97 C1-P-1996.10  M. Tamm, T. Jentzsch and W. Werncke.  Complexes with (2,4,6-Cyclohepatrien-1-ylidene)ethenylidene ligands: Strongly polarized ruthenium-(II)-allenylidene complexes Organometallics 16 (1997) 1418-24 Download PDF/PS-File or URL: C1-P-1996.10   WPJ97 C1-P-1996.07 W. Werncke, M. Pfeiffer, T. Johr, A. Lau, W. Grahn, H.-H. Johannes and L. Dähne.  Increase and saturation of the third-order hyperpolarizabilities in a homologous series of symmetric cyanines Chem. Phys. 216 (1997) 337-47 Download PDF/PS-File or URL: C1-P-1996.07   WPL97 C1-P-1996.03  W. Werncke, M. Pfeiffer, A. Lau, W. Grahn and H.-H. Johannes.  Third order hyperpolarizabilities of a homologous series of meso-nitrogen substituted thiacyanines Chem. Phys. Lett. 266 (1997) 99-106 Download PDF/PS-File or URL: C1-P-1996.03