Excited State Vibrational Spectra of All- trans Retinal Derivatives in Solution Revealed By Pump-DFWM Experiments.

The ultrafast structural changes during the photoinduced isomerization of the retinal-protonated Schiff base (RPSB) is still a poorly understood aspect in the retinal's photochemistry. In this work, we apply pump-degenerate four-wave mixing (pump-DFWM) to all- trans retinal (ATR) and retinal Schiff bases (RSB) to resolve coherent high- and low-frequency vibrational signatures from excited electronic states. We show that the vibrational spectra of excited singlet states in these samples exhibit pronounced differences compared to the relaxed ground state.

Introduction to State-of-the-Art Multidimensional Time-Resolved Spectroscopy Methods.

The field of multidimensional laser spectroscopy comprises a variety of highly developed state-of-the-art methods, which exhibit broad prospects for applications in several areas of natural, material, and even medical sciences. This collection summarizes the main achievements from this area and gives basic introductory insight into what is currently possible with such methods. In the present introductory contribution, we briefly outline the general concept behind multidimensional laser spectroscopy, for instance by highlighting the often-employed analogy between multidimensional laser spectroscopy and NMR methods.

Plasmonic Substrates Do Not Promote Vibrational Energy Transfer at Solid-Liquid Interfaces.

Intermolecular vibrational energy transfer in monolayers of isotopically mixed rhenium carbonyl complexes at solid-liquid interfaces is investigated with the help of ultrafast 2D Attenuated Total Reflectance Infrared (2D ATR IR) spectroscopy in dependence of plasmonic surface enhancement effects. Dielectric and plasmonic materials are used to demonstrate that plasmonic effects have no impact on the vibrational energy transfer rate in a regime of moderate IR surface enhancement (enhancement factors up to ca. 30).

Ultrafast structural molecular dynamics investigated with 2D infrared spectroscopy methods.

Ultrafast, multi-dimensional infrared (IR) spectroscopy has been advanced in recent years to a versatile analytical tool with a broad range of applications to elucidate molecular structure on ultrafast timescales, and it can be used for samples in a many different environments. Following a short and general introduction on the benefits of 2D IR spectroscopy, the first part of this chapter contains a brief discussion on basic descriptions and conceptual considerations of 2D IR spectroscopy. Outstanding classical applications of 2D IR are used afterwards to highlight the strengths and basic applicability of the method.

Surface-Sensitive and Surface-Specific Ultrafast Two-Dimensional Vibrational Spectroscopy.

Ultrafast two-dimensional infrared spectroscopy (2D IR) has been advanced in recent years toward measuring signals from only a monolayer of sample molecules at solid-liquid and solid-gas interfaces. A series of experimental methods has been introduced, which in the chronological order of development are 2D sum-frequency-generation (2D SFG), transmission 2D IR, and reflection 2D IR, the latter in either internal, attenuated total reflection (ATR), or external reflection configuration. The different variants of 2D vibrational spectroscopy are based on either the even-order or the odd-order nonlinear susceptibility, and all allow resolving similar molecular temporal and spectral information.

Ultrafast Vibrational Energy Transfer in Catalytic Monolayers at Solid-Liquid Interfaces.

We investigate the ultrafast vibrational dynamics of monolayers from adsorbed rhenium-carbonyl CO-reduction catalysts on a semiconductor surface (indium-tin-oxide (ITO)) with ultrafast two-dimensional attenuated total reflection infrared (2D ATR IR) spectroscopy. The complexes are partially equipped with isotope-labeled (C) carbonyl ligands to generate two spectroscopically distinguishable forms of the molecules. Ultrafast vibrational energy transfer between the molecules is observed via the temporal evolution of cross-peaks between their symmetric carbonyl stretching vibrations.

Molecule-specific interactions of diatomic adsorbates at metal-liquid interfaces.

Ultrafast vibrational dynamics of small molecules on platinum (Pt) layers in water are investigated using 2D attenuated total reflectance IR spectroscopy. Isotope combinations of carbon monoxide and cyanide are used to elucidate inter-adsorbate and substrate-adsorbate interactions. Despite observed cross-peaks in the CO spectra, we conclude that the molecules are not vibrationally coupled.

Vibrational ladder-climbing in surface-enhanced, ultrafast infrared spectroscopy.

In a recent work (J. Phys. Chem.

Ultrafast, Multidimensional Attenuated Total Reflectance Spectroscopy of Adsorbates at Metal Surfaces.

Ultrafast dynamics of molecules at solid-liquid interfaces are of outstanding importance in chemistry and physics due to their involvement in processes of heterogeneous catalysis. We present a new spectroscopic approach to resolve coherent, time-resolved, 2D vibrational spectra as well as ultrafast vibrational relaxation dynamics of molecules adsorbed on metallic thin films in contact with liquids. The setup is based on the technique of attenuated total reflectance (ATR) spectroscopy, which is used at interfaces between materials that exhibit different refractive indices.

2D attenuated total reflectance infrared spectroscopy reveals ultrafast vibrational dynamics of organic monolayers at metal-liquid interfaces.

We present two-dimensional infrared (2D IR) spectra of organic monolayers immobilized on thin metallic films at the solid liquid interface. The experiments are acquired under Attenuated Total Reflectance (ATR) conditions which allow a surface-sensitive measurement of spectral diffusion, sample inhomogeneity, and vibrational relaxation of the monolayers. Terminal azide functional groups are used as local probes of the environment and structural dynamics of the samples.

Mapping multidimensional excited state dynamics using pump-impulsive-vibrational-spectroscopy and pump-degenerate-four-wave-mixing.

Pump-impulsive vibrational spectroscopy (pump-IVS) is used to record excited state vibrational dynamics following photoexcitation of two carotenoids, β-carotene and lycopene, with <30 fs temporal resolution, and covering the full vibrational spectrum of the investigated chromophores. The results record the course of S2-S1 internal conversion, followed by vibrational relaxation and decay to the electronic ground state. This interpretation is corroborated by comparison with pump-degenerate-four-wave-mixing (pump-DFWM) experiments on the same systems.

Coherent High-Frequency Vibrational Dynamics in the Excited Electronic State of All-Trans Retinal Derivatives.

Coherent vibrational dynamics of retinal in excited electronic states are of primary importance in the understanding of photobiology. Using pump-DFWM, we demonstrate for the first time the existence of coherent double-bond high-frequency modulations (>1300 cm(-1)) in the excited electronic state of different retinal derivatives. All-trans retinal as well as retinal Schiff bases exhibit a partial frequency downshift of the C═C double-bond mode from ∼1580 cm(-1) in the ground state to 1510 cm(-1) in the excited state.

Evidence for the Two-State-Two-Mode model in retinal protonated Schiff-bases from pump degenerate four-wave-mixing experiments.

We apply spectrally-resolved pump degenerate four-wave-mixing for the characterization of excited state low-frequency vibrational coherences during the initial events in excited state double-bond isomerization of retinal protonated Schiff-bases. A set of low-frequency coherences in the energetic range of 100-350 cm(-1) appears in the dynamics already for very early delays after initial excitation (<100 fs). The modulations are rapidly damped (<800 fs) and detectable only in a certain time window after initial excitation (<0.

Selective nonlinear response preparation using femtosecond spectrally resolved four-wave-mixing.

A novel method is presented to assist the assignment of vibrational coherence in the homodyne degenerate four-wave-mixing technique. The dependence of vibrational coherence dynamics on the interaction sequence of chirped pump and Stokes excitation pulses is exploited to distinguish quantum beating from polarization interference. Moreover, by combining chirped excitation and variable delays between pump and Stokes pulses, it is possible to achieve a controlled excitation of response pathways from a single electronic state and separation of population dynamics and vibrational coherence dynamics within a single response pathway.

Vibrational analysis of excited and ground electronic states of all-trans retinal protonated Schiff-bases.

We report on vibrational coherence dynamics in excited and ground electronic states of all-trans retinal protonated Schiff-bases (RPSB), investigated by time-resolved Degenerate Four-Wave-Mixing (DFWM). The results show that wave packet dynamics in the excited state of RPSB consist of only low-frequency (<800 cm(-1)) modes. Such low-frequency wave packet motion is observed over a broad range of detection wavelengths ranging from excited state absorption (∼500 nm) to stimulated emission (>600 nm).

Ground- and excited-state vibrational coherence dynamics in Bacteriorhodopsin probed with degenerate four-wave-mixing experiments.

Vibrational coherence dynamics in the all-trans retinal chromophore in Bacteriorhodopsin (BR) are investigated by means of temporally and spectrally resolved degenerate four-wave-mixing experiments. The coherence dynamics depend on the excitation wavelength when BR samples are excited at different wavelengths in a spectral range between 520 nm to above 620 nm. The trends in the dynamics observed by tuning of the excitation wavelength allow an assignment of the wave packet dynamics to ground- and excited-state potential energy surfaces.