Imaging and controlling coherent phonon wave packets in single graphene nanoribbons

The motion of atoms is at the heart of any chemical or structural transformation in molecules and materials. Upon activation of this motion by an external source, several (usually many) vibrational modes can be coherently coupled, thus facilitating the chemical or structural phase transformation. These coherent dynamics occur on the ultrafast timescale, as revealed, e.g., by nonlocal ultrafast vibrational spectroscopic measurements in bulk molecular ensembles and solids. Tracking and controlling vibrational coherences locally at the atomic and molecular scales is, however, much more challenging and in fact has remained elusive so far. Here, we demonstrate that the vibrational coherences induced by broadband laser pulses on a single graphene nanoribbon (GNR) can be probed by femtosecond coherent anti-Stokes Raman spectroscopy (CARS) when performed in a scanning tunnelling microscope (STM). In addition to determining dephasing (~440 fs) and population decay times (~1.8 ps) of the generated phonon wave packets, we are able to track and control the corresponding quantum coherences, which we show to evolve on time scales as short as ~70 fs. We demonstrate that a two-dimensional frequency correlation spectrum unequivocally reveals the quantum couplings between different phonon modes in the GNR.

From a one-mode to a multi-mode understanding of conical intersection mediated ultrafast organic photochemical reactions

This review discusses how ultrafast organic photochemical reactions are controlled by conical intersections, highlighting that decay to the ground-state at multiple points of the intersection space results in their multi-mode character.

Broadband 2DES detection of vibrational coherence in the Sx state of canthaxanthin

The nonadiabatic mechanism that mediates nonradiative decay of the bright S₂ state to the dark S₁ state of carotenoids involves population of a bridging intermediate state, S_x, in several examples. The nature of S_x remains to be determined definitively, but it has been recently suggested that S_x corresponds to conformationally distorted molecules evolving along out-of-plane coordinates of the isoprenoid backbone near a low barrier between planar and distorted conformations on the S₂ potential surface. In this study, the electronic and vibrational dynamics accompanying the formation of S_x in toluene solutions of the ketocarotenoid canthaxanthin (CAN) are characterized with broadband two-dimensional electronic spectroscopy (2DES) with 7.8 fs excitation pulses and detection of the linear polarization components of the third-order nonlinear optical signal. A stimulated-emission cross peak in the 2DES spectrum accompanies the formation of S_x in <20 fs following excitation of the main absorption band. S_x is prepared instantaneously, however, with excitation of hot-band transitions associated with distorted conformations of CAN's isoprenoid backbone in the low frequency onset of the main absorption band. Vibrational coherence oscillation maps and modulated anisotropy transients show that S_x undergoes displacements from the Franck-Condon S₂ state along out-of-plane coordinates as it passes to the S₁ state. The results are consistent with the conclusion that CAN's carbonyl-substituted β-ionone rings impart an intramolecular charge-transfer character that frictionally slows the passage from S_x to S₁ compared to carotenoids lacking carbonyl substitution. Despite the longer lifetime, the S₁ state of CAN is formed with retention of vibrational coherence after passing through a conical intersection seam with the S_x state.

Tracking Ultrafast Structural Dynamics by Time-Domain Raman Spectroscopy

In traditional Raman spectroscopy, narrow-band light is irradiated on a sample, and its inelastic scattering, i.e., Raman scattering, is detected. The energy difference between the Raman scattering and the incident light corresponds to the vibrational energy of the molecule, providing the Raman spectrum that contains rich information about the molecular-level properties of the materials. On the other hand, by using ultrashort optical pulses, it is possible to induce Raman-active coherent nuclear motion of the molecule and to observe the molecular vibration in real time. Moreover, this time-domain Raman measurement can be combined with femtosecond photoexcitation, triggering chemical changes, which enables tracking ultrafast structural dynamics in a form of “time-resolved” time-domain Raman spectroscopy, also known as time-resolved impulsive stimulated Raman spectroscopy. With the advent of stable, ultrashort laser pulse sources, time-resolved impulsive stimulated Raman spectroscopy now realizes high sensitivity and a wide detection frequency window from THz to 3000 cm^–1, and has seen success in unveiling the molecular mechanisms underlying the efficient functions of complex molecular systems. In this Perspective, we overview the present status of time-domain Raman spectroscopy, particularly focusing on its application to the study of femtosecond structural dynamics. We first explain the principle and a brief history of time-domain Raman spectroscopy and then describe the apparatus and recent applications to the femtosecond dynamics of complex molecular systems, including proteins, molecular assemblies, and functional materials. We also discuss future directions for time-domain Raman spectroscopy, which has reached a status allowing a wide range of applications.

An exact solution in the theory of fluorescence resonance energy transfer with vibrational relaxation

The elegant expression of Förster that predicts the well-known 1/R⁶ distance (R) dependence of the rate of energy transfer, although widely used, was derived using several approximations. Notable among them is the neglect of the vibrational relaxation in the reactant (donor) and product (acceptor) manifolds. Vibrational relaxation can play an important role when the energy transfer rate is faster than the vibrational relaxation rate. Under such conditions, donor to acceptor energy transfer can occur from the excited vibrational states. This phenomenon is not captured by the usual formulation based on the overlap of donor emission and acceptor absorption spectra. Here, we develop a Green's function-based generalized formalism and obtain an exact solution for the excited state population relaxation and the rate of energy transfer in the presence of vibrational relaxation. We find that the application of the well-known Förster's expression might lead to overestimation of R.

Susceptibility of two-dimensional resonance Raman spectroscopies to cascades involving solute and solvent molecules

Two-dimensional resonance Raman (2DRR) spectroscopies have been used to investigate the structural heterogeneity of ensembles and chemical reaction mechanisms in recent years. Our previous work suggests that the intensities of artifacts may be comparable to the desired 2DRR response for some chemical systems and experimental approaches. In a type of artifact known as a "cascade," the four-wave mixing signal field radiated by one molecule induces a four-wave mixing process in a second molecule. We consider the susceptibility of 2DRR spectroscopy to various types of signal cascades in the present work. Calculations are conducted using empirical parameters obtained for a molecule with an intramolecular charge-transfer transition in acetonitrile. For a fully impulsive pulse sequence, it is shown that "parallel" cascades involving two solute molecules are generally more intense than that of the desired 2DRR response when the solute's mode displacements are 1.0 or less. In addition, we find that the magnitudes of parallel cascades involving both solute and solvent molecules (i.e., a solute-solvent cascade) may exceed that of the 2DRR response when the solute possesses small mode displacements. It is tempting to assume that solute-solvent cascades possess negligible intensities because the off-resonant Raman cross sections of solvents are usually 4-6 orders of magnitude smaller than that of the electronically resonant solute; however, the present calculations show that the difference in solute and solvent concentrations can fully compensate for the difference in Raman cross sections under common experimental conditions. Implications for control experiments and alternate approaches for 2DRR spectroscopy are discussed.

Transient Two-Dimensional Electronic Spectroscopy: Coherent Dynamics at Arbitrary Times along the Reaction Coordinate

Recent advances in two-dimensional electronic spectroscopy (2DES) have enabled identification of fragile quantum coherences in condensed-phase systems near the equilibrium molecular geometry. In general, traditional 2DES cannot measure such coherences associated with photophysical processes that occur at times significantly after the initially prepared state has dephased, such as the evolution of the initial excited state into a charge transfer state. We demonstrate the use of transient two-dimensional electronic spectroscopy (t-2DES) to probe coherences in an electron donor-acceptor dyad consisting of a perylenediimide (PDI) acceptor and a perylene (Per) donor. An actinic pump pulse prepares the lowest excited singlet state of PDI followed by formation of the PDI^•--Per^•+ ion pair, which is probed at different times following the actinic pulse using 2DES. Analysis of the observed coherences provides information about electronic, vibronic, and vibrational interactions at any time along the reaction coordinate for ion pair formation.

Effect of point mutations on the ultrafast photo-isomerization of Anabaena sensory rhodopsin

Anabaena sensory rhodopsin (ASR) is a particular microbial retinal protein for which light-adaptation leads to the ability to bind both the all-trans, 15-anti (AT) and the 13-cis, 15-syn (13C) isomers of the protonated Schiff base of retinal (PSBR). In the context of obtaining insight into the mechanisms by which retinal proteins catalyse the PSBR photo-isomerization reaction, ASR is a model system allowing to study, within the same protein, the protein-PSBR interactions for two different PSBR conformers at the same time. A detailed analysis of the vibrational spectra of AT and 13C, and their photo-products in wild-type ASR obtained through femtosecond (pump-) four-wave-mixing is reported for the first time, and compared to bacterio- and channelrhodopsin. As part of an extensive study of ASR mutants with blue-shifted absorption spectra, we present here a detailed computational analysis of the origin of the mutation-induced blue-shift of the absorption spectra, and identify electrostatic interactions as dominating steric effects that would entail a red-shift. The excited state lifetimes and isomerization reaction times (IRT) for the three mutants V112N, W76F, and L83Q are studied experimentally by femtosecond broadband transient absorption spectroscopy. Interestingly, in all three mutants, isomerization is accelerated for AT with respect to wild-type ASR, and this the more, the shorter the wavelength of maximum absorption. On the contrary, the 13C photo-reaction is slightly slowed down, leading to an inversion of the ESLs of AT and 13C, with respect to wt-ASR, in the blue-most absorbing mutant L83Q. Possible mechanisms for these mutation effects, and their steric and electrostatic origins are discussed.

Fifth-order time-domain Raman spectroscopy of photoactive yellow protein for visualizing vibrational coupling in its excited state

We report fifth-order time-domain Raman spectroscopy of photoactive yellow protein (PYP), with the aim to visualize vibrational coupling in its excited state. After the ultrashort actinic pump pulse prepared the vibrational coherence and population in the excited state, the evolving vibrational structure was tracked by time-resolved impulsive stimulated Raman spectroscopy using sub-7-fs pulses. The obtained fifth-order time-domain Raman data were translated to a two-dimensional (2D) frequency-frequency correlation map, which visualizes the correlation between low- and high-frequency vibrational modes of the excited state. The 2D map of PYP reveals a cross peak, indicating the coupling between the phenolic C─O stretch mode of the chromophore and the low-frequency modes (~160 cm^-1), assignable to the intermolecular motions involving the surrounding hydrogen-bonded amino acids. The unveiled coupling suggests the importance of the low-frequency vibrational motion in the primary photoreaction of PYP, highlighting the unique capability of this spectroscopic approach for studying ultrafast reaction dynamics.

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. Pump-DFWM results indicate three major features for ATR and RSB. (i) Excited state vibrational spectra of ATR and RSB consist predominately of low-frequency modes in the energetic range 100-500 cm^-1. (ii) Excited state vibrational spectra show distinct differences for excitation in specific regions of electronic transitions of excited state absorption and emission. (iii) Low-frequency modes in ATR and RSB are inducible during the entire lifetime of the excited electronic states. This latter effect points to a transient molecular structure that, following initial relaxation between different excited electronic states, does not change anymore over the lifetime of the finally populated excited electronic state.

Mapping the ultrafast vibrational dynamics of all-trans and 13-cis retinal isomerization in Anabaena Sensory Rhodopsin

Discrepancies in the isomerization dynamics and quantum yields of the trans and cis retinal protonated Schiff base is a well-known issue in the context of retinal photochemistry. Anabaena Sensory Rhodopsin (ASR) is a microbial retinal protein that comprises a retinal chromophore in two ground state (GS) conformations: all-trans, 15-anti (AT) and 13-cis, 15-syn (13C). In this study, we applied impulsive vibrational spectroscopic techniques (DFWM, pump-DFWM and pump-IVS) to ASR to shed more light on how the structural changes take place in the excited state within the same protein environment. Our findings point to distinct features in the ground state structural conformations as well as to drastically different evolutions in the excited state manifold. The ground state vibrational spectra show stronger Raman activity of the C14-H out-of-plane wag (at about 805 cm-1) for the 13C isomer than that for the AT isomer, which hints at a pre-distortion of 13C in the ground state. Evolution of the Raman frequency after interaction with the actinic pulse shows a blue-shift for the C[double bond, length as m-dash]C stretching and CH3 rocking mode for both isomers. For AT, however, the blue-shift is not instantaneous as observed for the 13C isomer, rather it takes more than 200 fs to reach the maximum frequency shift. This frequency blue-shift is rationalized by a decrease in the effective conjugation length during the isomerization reaction, which further confirms a slower formation of the twisted state for the AT isomer and corroborates the presence of a barrier in the excited state trajectory previously predicted by quantum chemical calculations.

Understanding/unravelling carotenoid excited singlet states

Carotenoids are essential light-harvesting pigments in natural photosynthesis. They absorb in the blue-green region of the solar spectrum and transfer the absorbed energy to (bacterio-)chlorophylls, and thus expand the wavelength range of light that is able to drive photosynthesis. This process is an example of singlet-singlet excitation energy transfer, and carotenoids serve to enhance the overall efficiency of photosynthetic light reactions. The photochemistry and photophysics of carotenoids have often been interpreted by referring to those of simple polyene molecules that do not possess any functional groups. However, this may not always be wise because carotenoids usually have a number of functional groups that induce the variety of photochemical behaviours in them. These differences can also make the interpretation of the singlet excited states of carotenoids very complicated. In this article, we review the properties of the singlet excited states of carotenoids with the aim of producing as coherent a picture as possible of what is currently known and what needs to be learned.

Vibrational Spectroscopy on Photoexcited Dye-Sensitized Films via Pump-Degenerate Four-Wave Mixing

Molecular sensitization of semiconductor films is an important technology for energy and environmental applications including solar energy conversion, photocatalytic hydrogen production, and water purification. Dye-sensitized films are also scientifically complex and interesting systems with a long history of research. In most applications, photoinduced heterogeneous electron transfer (HET) at the molecule/semiconductor interface is of critical importance, and while great progress has been made in understanding HET, many open questions remain. Of particular interest is the role of combined electronic and vibrational effects and coherence of the dye during HET. The ultrafast nature of the process, the rapid intramolecular vibrational energy redistribution, and vibrational cooling present complications in the study of vibronic coupling in HET. We present the application of a time domain vibrational spectroscopy-pump-degenerate four-wave mixing (pump-DFWM)-to dye-sensitized solid-state semiconductor films. Pump-DFWM can measure Raman-active vibrational modes that are triggered by excitation of the sample with an actinic pump pulse. Modifications to the instrument for solid-state samples and its application to an anatase TiO₂ film sensitized by a Zn-porphyrin dye are discussed. We show an effective combination of experimental techniques to overcome typical challenges in measuring solid-state samples with laser spectroscopy and observe molecular vibrations following HET in a picosecond time window. The cation spectrum of the dye shows modes that can be assigned to the linker group and a mode that is localized on the Zn-phorphyrin chromophore and that is connected to photoexcitation.

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. This includes the identification of vibrational coupling in molecules, characterization of spectral diffusion dynamics, chemical exchange of chemical bond formation and breaking, as well as dynamics of intra- and intermolecular energy transfer for molecules in bulk solution and thin films. In the second part, several important, recently developed variants and new applications of 2D IR spectroscopy are introduced. These methods focus on (i) applications to molecules under two- and three-dimensional confinement, (ii) the combination of 2D IR with electrochemistry, (iii) ultrafast 2D IR in conjunction with diffraction-limited microscopy, (iv) several variants of non-equilibrium 2D IR spectroscopy such as transient 2D IR and 3D IR, and (v) extensions of the pump and probe spectral regions for multi-dimensional vibrational spectroscopy towards mixed vibrational-electronic spectroscopies. In light of these examples, the important open scientific and conceptual questions with regard to intra- and intermolecular dynamics are highlighted. Such questions can be tackled with the existing arsenal of experimental variants of 2D IR spectroscopy to promote the understanding of fundamentally new aspects in chemistry, biology and materials science. The final part of the chapter introduces several concepts of currently performed technical developments, which aim at exploiting 2D IR spectroscopy as an analytical tool. Such developments embrace the combination of 2D IR spectroscopy and plasmonic spectroscopy for ultrasensitive analytics, merging 2D IR spectroscopy with ultra-high-resolution microscopy (nanoscopy), future variants of transient 2D IR methods, or 2D IR in conjunction with microfluidics. It is expected that these techniques will allow for groundbreaking research in many new areas of natural sciences.

Probing the early stages of photoreception in photoactive yellow protein with ultrafast time-domain Raman spectroscopy

Unveiling the nuclear motions of photoreceptor proteins in action is a crucial goal in protein science in order to understand their elaborate mechanisms and how they achieve optimal selectivity and efficiency. Previous studies have provided detailed information on the structures of intermediates that appear during the later stages (>ns) of such photoreception cycles, yet the initial events immediately after photoabsorption remain unclear because of experimental challenges in monitoring nuclear rearrangements on ultrafast timescales, including protein-specific low-frequency motions. Using time-domain Raman probing with sub-7-fs pulses, we obtain snapshot vibrational spectra of photoactive yellow protein and a mutant with high sensitivity, providing insights into the key responses that drive photoreception. Our data show a drastic intensity drop of the excited-state marker band at 135 cm^-1 within a few hundred femtoseconds, suggesting a rapid weakening of the hydrogen bond that anchors the chromophore. We also track formation of the first ground-state intermediate over the first few picoseconds and fully characterize its vibrational structure, revealing a substantially-twisted cis conformation.

Broadband UV-Vis vibrational coherence spectrometer based on a hollow fiber compressor

We describe a broadband transient absorption (TA) spectrometer devised to excite and probe, in the blue to UV range, vibrational coherence dynamics in organic molecules in condensed phase. A 800-nm Ti:Sa amplifier and a hollow fiber compressor are used to generate a 6-fs short pulse at 1 kHz. Broadband sum frequency generation with the fundamental pulse is implemented to produce a 400-nm, 8-fs Fourier limited short pulse. A UV-Vis white-light supercontinuum is implemented as a probe with intensity self-referencing to achieve a shot-noise-limited sensitivity. Rapid scanning of the pump-probe delay is shown very efficient in suppressing the noise resulting from low-frequency pump intensity fluctuations. Using either of the 800-nm or 400-nm broadband pulses as the pump for TA spectroscopy of organic molecules in solution, we resolve oscillatory signals down to the 320 nm probing wavelength with a 3200 cm^-1 FWHM bandwidth. Their Fourier transformation reveals the corresponding molecular vibrational spectra. Finally, we demonstrate the use of this setup as a vibrational coherence spectrometer for the investigation of the vibrational dynamics accompanying the sub-ps C=C photoisomerization of a retinal-like molecular switch through a conical intersection.

A Unified Picture of S* in Carotenoids

In π-conjugated chain molecules such as carotenoids, coupling between electronic and vibrational degrees of freedom is of central importance. It governs both dynamic and static properties, such as the time scales of excited state relaxation as well as absorption spectra. In this work, we treat vibronic dynamics in carotenoids on four electronic states (|S0⟩, |S1⟩, |S2⟩, and |Sn⟩) in a physically rigorous framework. This model explains all features previously associated with the intensely debated S* state. Besides successfully fitting transient absorption data of a zeaxanthin homologue, this model also accounts for previous results from global target analysis and chain length-dependent studies. Additionally, we are able to incorporate findings from pump-deplete-probe experiments, which were incompatible to any pre-existing model. Thus, we present the first comprehensive and unified interpretation of S*-related features, explaining them by vibronic transitions on either S1, S0, or both, depending on the chain length of the investigated carotenoid.

Femtosecond time-resolved impulsive stimulated Raman spectroscopy using sub-7-fs pulses: Apparatus and applications

We describe details of the setup for time-resolved impulsive stimulated Raman spectroscopy (TR-ISRS). In this method, snapshot molecular vibrational spectra of the photoreaction transients are captured via time-domain Raman probing using ultrashort pulses. Our instrument features transform-limited sub-7-fs pulses to impulsively excite and probe coherent nuclear wavepacket motions, allowing us to observe vibrational fingerprints of transient species from the terahertz to 3000-cm(-1) region with high sensitivity. Key optical components for the best spectroscopic performance are discussed. The TR-ISRS measurements for the excited states of diphenylacetylene in cyclohexane are demonstrated, highlighting the capability of our setup to track femtosecond dynamics of all the Raman-active fundamental molecular vibrations.

Role of Coherent Low-Frequency Motion in Excited-State Proton Transfer of Green Fluorescent Protein Studied by Time-Resolved Impulsive Stimulated Raman Spectroscopy

Green fluorescent protein (GFP) from jellyfish Aequorea victoria, an essential bioimaging tool, luminesces via excited-state proton transfer (ESPT) in which the phenolic proton of the p-hydroxybenzylideneimidazolinone chromophore is transferred to Glu222 through a hydrogen-bond network. In this process, the ESPT mediated by the low-frequency motion of the chromophore has been proposed. We address this issue using femtosecond time-resolved impulsive stimulated Raman spectroscopy. After coherently exciting low-frequency modes (<300 cm(-1)) in the excited state of GFP, we examined the excited-state structural evolution and the ESPT dynamics within the dephasing time of the low-frequency vibration. A clear anharmonic vibrational coupling is found between one high-frequency mode of the chromophore (phenolic CH bend) and a low-frequency mode at ∼104 cm(-1). However, the data show that this low-frequency motion does not substantially affect the ESPT dynamics.

Vibronic coupling in the excited-states of carotenoids

The ultrafast femtochemistry of carotenoids is governed by the interaction between electronic excited states, which has been explained by the relaxation dynamics within a few hundred femtoseconds from the lowest optically allowed excited state S₂to the optically dark state S₁.

Mode-selective vibrational modulation of charge transport in organic electronic devices

The soft character of organic materials leads to strong coupling between molecular, nuclear and electronic dynamics. This coupling opens the way to influence charge transport in organic electronic devices by exciting molecular vibrational motions. However, despite encouraging theoretical predictions, experimental realization of such approach has remained elusive. Here we demonstrate experimentally that photoconductivity in a model organic optoelectronic device can be modulated by the selective excitation of molecular vibrations. Using an ultrafast infrared laser source to create a coherent superposition of vibrational motions in a pentacene/C₆₀ photoresistor, we observe that excitation of certain modes in the 1,500–1,700 cm⁻¹ region leads to photocurrent enhancement. Excited vibrations affect predominantly trapped carriers. The effect depends on the nature of the vibration and its mode-specific character can be well described by the vibrational modulation of intermolecular electronic couplings. This presents a new tool for studying electron–phonon coupling and charge dynamics in (bio)molecular materials.

The electronic properties of organic molecules are sensitive to structural dynamics, but device control through this phenomenon has not been attained. Bakulin et al. show that the photoconductivity can be modulated by selective excitation of molecular vibrations in an organic optoelectronic device.

Resonance Femtosecond-Stimulated Raman Spectroscopy without Actinic Excitation Showing Low-Frequency Vibrational Activity in the S2 State of All-Trans β-Carotene

Raman scattering with stimulating femtosecond probe pulses (FSR) was used to observe vibrational activity of all-trans β-carotene in n-hexane. The short-lived excited electronic state S2 was accessed in two ways: (i) by transient FSR after an actinic pulse to populate the S2 state, exploiting resonance from an Sx ← S2 transition, and (ii) by FSR without actinic excitation, using S2 ↔ S0 resonance exclusively and narrow-band Raman/broad-band femtosecond probe pulses only. The two approaches have nonlinear optical susceptibilities χ((5)) and χ((3)), respectively. Both methods show low-frequency bands of the S2 state at 200, 400, and ∼600 cm(-1), which are reported for the first time. With (ii) the intensities of low-frequency vibrational resonances in S2 are larger compared to those in S0, implying strong anharmonicities/mode mixing in the excited state. In principle, for short-lived electronic states, the χ((3)) method should allow the best characterization of low-frequency modes.

Observation of structural relaxation during exciton self-trapping via excited-state resonant impulsive stimulated Raman spectroscopy

We detect the change in vibrational frequency associated with the transition from a delocalized to a localized electronic state using femtosecond vibrational wavepacket techniques. The experiments are carried out in the mixed-valence linear chain material [Pt(en)2][Pt(en)2Cl2]⋅(ClO4)4 (en = ethylenediamine, C2H8N2), a quasi-one-dimensional system with strong electron-phonon coupling. Vibrational spectroscopy of the equilibrated self-trapped exciton is carried out using a multiple pulse excitation technique: an initial pump pulse creates a population of delocalized excitons that self-trap and equilibrate, and a time-delayed second pump pulse tuned to the red-shifted absorption band of the self-trapped exciton impulsively excites vibrational wavepacket oscillations at the characteristic vibrational frequencies of the equilibrated self-trapped exciton state by the resonant impulsive stimulated Raman mechanism, acting on the excited state. The measurements yield oscillations at a frequency of 160 cm(-1) corresponding to a Raman-active mode of the equilibrated self-trapped exciton with Pt-Cl stretching character. The 160 cm(-1) frequency is shifted from the previously observed wavepacket frequency of 185 cm(-1) associated with the initially generated exciton and from the 312 cm(-1) Raman-active symmetric stretching mode of the ground electronic state. We relate the frequency shifts to the changes in charge distribution and local structure that create the potential that stabilizes the self-trapped state.

Three-pulse femtosecond spectroscopy of PbSe nanocrystals: 1S bleach nonlinearity and sub-band-edge excited-state absorption assignment

Above band-edge photoexcitation of PbSe nanocrystals induces strong below band gap absorption as well as a multiphased buildup of bleaching in the 1Se1Sh transition. The amplitudes and kinetics of these features deviate from expectations based on biexciton shifts and state filling, which are the mechanisms usually evoked to explain them. To clarify these discrepancies, the same transitions are investigated here by double-pump-probe spectroscopy. Re-exciting in the below band gap induced absorption characteristic of hot excitons is shown to produce additional excitons with high probability. In addition, pump-probe experiments on a sample saturated with single relaxed excitons prove that the resulting 1Se1Sh bleach is not linear with the number of excitons per nanocrystal. This finding holds for two samples differing significantly in size, demonstrating its generality. Analysis of the results suggests that below band edge induced absorption in hot exciton states is due to excited-state absorption and not to shifted absorption of cold carriers and that 1Se1Sh bleach signals are not an accurate counter of sample excitons when their distribution includes multiexciton states.

Stochastic Liouville equations for femtosecond stimulated Raman spectroscopy

Electron and vibrational dynamics of molecules are commonly studied by subjecting them to two interactions with a fast actinic pulse that prepares them in a nonstationary state and after a variable delay period T, probing them with a Raman process induced by a combination of a broadband and a narrowband pulse. This technique, known as femtosecond stimulated Raman spectroscopy (FSRS), can effectively probe time resolved vibrational resonances. We show how FSRS signals can be modeled and interpreted using the stochastic Liouville equations (SLE), originally developed for NMR lineshapes. The SLE provide a convenient simulation protocol that can describe complex dynamics caused by coupling to collective bath coordinates at much lower cost than a full dynamical simulation. The origin of the dispersive features that appear when there is no separation of timescales between vibrational variations and the dephasing time is clarified.

100 fs photo-isomerization with vibrational coherences but low quantum yield in Anabaena Sensory Rhodopsin

Counter-intuitive photochemistry: in Anabaena Sensory Rhodopsin, the retinal 13-cis isomer isomerizes much faster than all-trans ASR, but with a 3-times lower quantum yield.

Vibronic energy relaxation approach highlighting deactivation pathways in carotenoids

Energy relaxation between two electronic states of a molecule is mediated by a set of relevant vibrational states.

Population-controlled impulsive vibrational spectroscopy: background- and baseline-free Raman spectroscopy of excited electronic states

We have developed the technique of population-controlled impulsive vibrational spectroscopy (PC-IVS) aimed at providing high-quality, background-free Raman spectra of excited electronic states and their dynamics. Our approach consists of a modified transient absorption experiment using an ultrashort (<10 fs) pump pulse with additional electronic excitation and control pulses. The latter allows for the experimental isolation of excited-state vibrational coherence and, hence, vibrational spectra. We illustrate the capabilities of PC-IVS by reporting the Raman spectra of well-established molecular systems such as the carotenoid astaxanthin and trans-stilbene and present the first excited-state Raman spectra of the retinal protonated Schiff base chromophore in solution. Our approach, illustrated here with impulsive vibrational spectroscopy, is equally applicable to transient and even multidimensional infrared and electronic spectroscopies to experimentally isolate spectroscopic signatures of interest.

Cycloreversion dynamics of a photochromic molecular switch via one-photon and sequential two-photon excitation

Ultrafast pump-probe (PP) and pump-repump-probe (PReP) measurements examine the ring-opening reaction of a photochromic molecular switch following excitation to the first and higher excited states. Sequential two-photon excitation is a sensitive probe of the excited-state dynamics, because the secondary excitation maps the progress along the S1 reaction coordinate onto the higher excited states of the molecule. In this contribution, secondary excitation at 800 nm accesses more reactive regions of the excited-state potential energy surfaces than are accessible with direct vertical excitation in the visible or UV. The quantum yield for cycloreversion increases by a factor of 3.5 ± 0.9 compared with one-photon excitation when the delay between the 500 nm pump and 800 nm repump laser pulses increases beyond ~100 fs, in contrast with a slower ~3 ps increase that was previously observed for one-color sequential excitation at 500 nm. The evolution of an excited-state absorption band reveals the dynamics of the higher-lying excited state for both one-photon excitation in the UV and sequential two-photon excitation. The spectroscopy and dynamics of the higher-lying excited state are similar for both excitation pathways, including a lifetime of ~100 fs. The complementary PP and PReP measurements provide a detailed picture of the ultrafast excited-state dynamics, including new insight on the role of excited states above S1 in controlling the photochemical cycloreversion reaction.

The photocycle and ultrafast vibrational dynamics of bacteriorhodopsin in lipid nanodiscs

The photocycle and vibrational dynamics of bacteriorhodopsin in a lipid nanodisc microenvironment have been studied by steady-state and time-resolved spectroscopies. Linear absorption and circular dichroism indicate that the nanodiscs do not perturb the structure of the retinal binding pocket, while transient absorption and flash photolysis measurements show that the photocycle which underlies proton pumping is unchanged from that in the native purple membranes. Vibrational dynamics during the initial photointermediate formation are subsequently studied by ultrafast broadband transient absorption spectroscopy, where the low scattering afforded by the lipid nanodisc microenvironment allows for unambiguous assignment of ground and excited state nuclear dynamics through Fourier filtering of frequency regions of interest and subsequent time domain analysis of the retrieved vibrational dynamics. Canonical ground state oscillations corresponding to high frequency ethylenic and C-C stretches, methyl rocks, and hydrogen out-of-plane wags are retrieved, while large amplitude, short dephasing time vibrations are recovered predominantly in the frequency region associated with out-of-plane dynamics and low frequency torsional modes implicated in isomerization.

Vibrationally coherent crossing and coupling of electronic states during internal conversion in β-carotene

Coupling of nuclear and electronic degrees of freedom mediates energy flow in molecules after optical excitation. The associated coherent dynamics in polyatomic systems, however, remain experimentally unexplored. Here, we combined transient absorption spectroscopy with electronic population control to reveal nuclear wave packet dynamics during the S2 → S1 internal conversion in β-carotene. We show that passage through a conical intersection is vibrationally coherent and thereby provides direct feedback on the role of different vibrational coordinates in the breakdown of the Born-Oppenheimer approximation.

Time-resolved broadband Raman spectroscopies: a unified six-wave-mixing representation

Excited-state vibrational dynamics in molecules can be studied by an electronically off-resonant Raman process induced by a probe pulse with variable delay with respect to an actinic pulse. We establish the connection between several variants of the technique that involve either spontaneous or stimulated Raman detection and different pulse configurations. By using loop diagrams in the frequency domain, we show that all signals can be described as six wave mixing which depend on the same four point molecular correlation functions involving two transition dipoles and two polarizabilities and accompanied by a different gating. Simulations for the stochastic two-state-jump model illustrate the origin of the absorptive and dispersive features observed experimentally.

Probing the Conical Intersection Dynamics of the RNA Base Uracil by UV-Pump Stimulated-Raman-Probe Signals; Ab Initio Simulations

Nonadiabatic electron and nuclear dynamics of photoexcited molecules involving conical intersections is of fundamental importance in many reactions such as the self-protection mechanism of DNA and RNA bases against UV irradiation. Nonlinear vibrational spectroscopy can provide an ultrafast sensitive probe for these processes. We employ a simulation protocol that combines nonadiabatic on-the-fly molecular dynamics with a mode-tracking algorithm for the simulation of femtosecond stimulated Raman spectroscopy (SRS) signals of the high frequency C–H- and N–H-stretch vibrations of the photoexcited RNA base uracil. The simulations rely on a microscopically derived expression that takes into account the path integral of the excited state evolution and the pulse shapes. Analysis of the joint time/frequency resolution of the technique reveals a matter chirp contribution that limits the inherent temporal resolution. Characteristic signatures of relaxation dynamics mediated in the vicinity of conical intersection are predicted. The C–H and N–H spectator modes provide high sensitivity to their local environment and act as local probes with submolecular and high temporal resolution.