Coherent Wave Packet Motion Research Articles

Electronic transient spectroscopy from the deep UV to the NIR: unambiguous disentanglement of complex processes

Complex multi-stage relaxation and reaction pathways after the optical excitation of molecules makes the disentanglement of the underlying mechanisms challenging. We present four examples that a new transient spectrometer with excitation fully tunable from the deep UV to the IR and 225 to 1700 nm probing allows for an analysis with greatly reduced ambiguity. The temporal resolution of about 50 fs allows us to resolve all relevant processes. For each example there is a new twist in the sequence of relaxation steps that had previously been overlooked. In malachite green it appears that the importance of the phenyl twisting has been overemphasized and rather a charge transfer state should be considered. In TINUVIN-P the predicted twisting as the driving motion for the ultrafast IC is confirmed and leads to a resolution of the earlier puzzle that the sub-5 ps regime shows kinetics deviating from a pure cooling process despite the sub-ps proton transfer cycle. For the bond cleavage of Ph2CH-Cl and Ph2CH-Br the degree of electron transfer within the radical pair can now be determined quantitatively and leads to a profound understanding of the long-term cation yield. For the first time coherent wavepacket motion in the photoproducts is reported. Last but not least the measurement of the GSB recovery in the deep UV allows for the surprising result, that even after S2 excitation of cyclopentenones the triplet states are reached with near unity probability within a few picoseconds.

Coherent Nuclear Wave Packets Generated by Ultrafast Intramolecular Charge-Transfer Reaction

Intramolecular charge-transfer (ICT) dynamics, including reaction coordinates, structural changes, and reaction rate, has been noted experimentally and theoretically. Here we report the ICT dynamics of laurdan investigated by time-resolved fluorescence at extreme time resolution of 30 fs. A single high-frequency coherent nuclear wave-packet motion on the product potential surface is observed through the modulation of the fluorescence intensity in time. Theory and experiment show that this vibrational mode involves large displacement of the carbon atoms in the naphthalene backbone, which indicates that the naphthalene backbone coordinates are strongly coupled to the ICT reaction of laurdan, not the twisting or planarization of the dimethylamino group.

Ultrafast insulator-metal phase transition in VO2studied by multiterahertz spectroscopy

The ultrafast photoinduced insulator-metal transition in VO${}_{2}$ is studied at different temperatures and excitation fluences using multi-THz probe pulses. The spectrally resolved midinfrared response allows us to trace separately the dynamics of lattice and electronic degrees of freedom with a time resolution of 40 fs. The critical fluence of the optical pump pulse, which drives the system into a long-lived metallic state, is found to increase with decreasing temperature. Under all measurement conditions, we observe a modulation of the eigenfrequencies of the optical phonon modes induced by their anharmonic coupling to the coherent wave-packet motion of V-V dimers at 6.1 THz. Furthermore, we find a weak quadratic coupling of the electronic response to the coherent dimer oscillation resulting in a modulation of the electronic conductivity at twice the frequency of the wave-packet motion. The findings are discussed in the framework of a qualitative model based on an approximation of local photoexcitation of the vanadium dimers from the insulating state.

Two-dimensional vibronic spectroscopy of coherent wave-packet motion

We theoretically study two-dimensional (2D) spectroscopic signals obtained from femtosecond pulse interactions with diatomic molecules. The vibrational wave-packet dynamics is monitored in the signals. During the motion in anharmonic potentials the wave packets exhibit vibrational revivals and fractional revivals which are associated with particular quantum phases. The time-dependent phase changes are identified by inspection of the complex-valued 2D spectra. We use the Na(2) molecule as a numerical example and discuss various pulse sequences which yield information about vibrational level structure and phase relationships in different electronic states.

Conical intersection dynamics of the primary photoisomerization event in vision

Ever since the conversion of the 11-cis retinal chromophore to its all-trans form in rhodopsin was identified as the primary photochemical event in vision, experimentalists and theoreticians have tried to unravel the molecular details of this process. The high quantum yield of 0.65 (ref. 2), the production of the primary ground-state rhodopsin photoproduct within a mere 200 fs (refs 3-7), and the storage of considerable energy in the first stable bathorhodopsin intermediate all suggest an unusually fast and efficient photoactivated one-way reaction. Rhodopsin's unique reactivity is generally attributed to a conical intersection between the potential energy surfaces of the ground and excited electronic states enabling the efficient and ultrafast conversion of photon energy into chemical energy. But obtaining direct experimental evidence for the involvement of a conical intersection is challenging: the energy gap between the electronic states of the reacting molecule changes significantly over an ultrashort timescale, which calls for observational methods that combine high temporal resolution with a broad spectral observation window. Here we show that ultrafast optical spectroscopy with sub-20-fs time resolution and spectral coverage from the visible to the near-infrared allows us to follow the dynamics leading to the conical intersection in rhodopsin isomerization. We track coherent wave-packet motion from the photoexcited Franck-Condon region to the photoproduct by monitoring the loss of reactant emission and the subsequent appearance of photoproduct absorption, and find excellent agreement between the experimental observations and molecular dynamics calculations that involve a true electronic state crossing. Taken together, these findings constitute the most compelling evidence to date for the existence and importance of conical intersections in visual photochemistry.

Femtosecond stage of electron transfer in reaction centers of the triple mutant SL178K/GM203D/LM214H of Rhodobacter sphaeroides

Coherent processes in an initial phase of charge transfer in reaction centers (RCs) of the triple mutant S(L178)K/G(M203)D/L(M214)H of Rhodobacter sphaeroides were investigated by difference (light - dark) absorption spectroscopy with 18 fsec time resolution. Electron transfer in the B cofactor branch is activated in this mutant, while the A-branch electron transfer is slowed in comparison with native RCs of Rba. sphaeroides. A bulk of absorption difference spectra was analyzed in the 940-1060 nm range (stimulated emission of excited bacteriochlorophyll dimer P* and absorption of bacteriochlorophyll anions B(A)(-) and beta(-), where beta is a bacteriochlorophyll substituting the native bacteriopheophytin H(A)) and in the 735-775 nm range (bleaching of the absorption band of the bacteriopheophytin H(B) in the B-branch) in the -0.1 to 4 psec range of delays with respect to the moment of photoexcitation of P at 870 nm. Spectra were measured at 293 and 90 K. The kinetics of P* stimulated emission at 940 nm shows its decay with a time constant of approximately 14 psec at 90 K and approximately 18 psec at 293 K, which is accompanied by oscillations with a frequency of approximately 150 cm(-1). A weak absorption band is found at 1018 nm that is formed approximately 100 fsec after excitation of P and reflects the electron transfer from P* to beta and/or B(A) with accumulation of the P(+)beta(-) and/or P(+)B(A)(-) states. The kinetics of DeltaA at 1018 nm contains the oscillations at approximately 150 cm(-1) and distinct low-frequency oscillations at 20-100 cm(-1); also, the amplitude of the oscillations at 150 cm(-1) is much smaller at 293 than at 90 K. The oscillations in the kinetics of the 1018 nm band do not contain a 32 cm(-1) mode that is characteristic for native Rba. sphaeroides RCs having water molecule HOH55 in their structure. The DeltaA kinetics at 751 nm reflects the electron transfer to H(B) with formation of the P(+)H(B)(-) state. The oscillatory part of this kinetics has the form of a single peak with a maximum at ~50 fsec completely decaying at ~200 fsec, which might reflect a reversible electron transfer to the B-branch. The results are analyzed in terms of coherent nuclear wave packet motion induced in the P* excited state by femtosecond light pulses, of an influence of the incorporated mutations on the mutual position of the energy levels of charge separated states, and of the role of water HOH55 in the dynamics of the initial electron transfer.

Vibrational Coherence in Polar Solutions of ZnII tetrakis(N-methylpyridyl)porphyrin with Soret-Band Excitation: Rapidly Damped Intermolecular Modes with Clustered Solvent Molecules and Slowly Damped Intramolecular Modes from the Porphyrin Macrocycle

Ground-state coherent wavepacket motions arising from intermolecular modes with clustered, first-shell solvent molecules were observed using the femtosecond dynamic absorption technique in polar solutions of Zn(II) meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP) with excitation in the Soret absorption band. As was observed previously in bacteriochlorophyll a solution, the pump-probe transients in ZnTMPyP solutions are weakly modulated by slowly damped (effective damping time gamma > 1 ps) features that are assigned to intramolecular modes, the skeletal normal modes of vibration of the porphyrin. The 40 cm(-1) and 215 cm(-1) modes from the metal-doming and metal-solvent-ligand modes, respectively, are members of this set of modulation components. A slowly damped 2-4 cm(-1) component is assigned to the internal rotation of the N-methylpyridyl rings with respect to the porphyrin macrocycle; this mode obtains strong resonance Raman intensity enhancement from an extensive delocalization of pi-electron density from the porphyrin in the ground state onto the rings in the pi* excited states. The dominant features observed in the pump-probe transients are a pair of rapidly damped (gamma < 250 fs) modulation components arising from intermolecular modes with solvent molecules. This structural assignment is supported by an isotope-dependent shift of the average mode frequencies in methanol and perdeuterated methanol. The solvent dependence of the mean intermolecular mode frequency is consistent with a van der Waals intermolecular potential that has significant contributions only from the London dispersion and induction interactions; ion-dipole or ion-induced-dipole terms do not make large contributions because the pi-electron density is not extensively delocalized onto the N-methylpyridyl rings. The modulation depth associated with the intermolecular modes exhibits a marked dependence on the electronic structure of the solvent that is probably related to the degree of covalency; the strongest modulations are observed in acetonitrile and dimethylsulfoxide. The results strongly support a structural assignment of the low-frequency modes that are coupled to the primary and secondary electron-transfer reactions in photosynthetic reaction centers to intermolecular modes between the redox-active chromophores and first-solvation shell groups from the surrounding protein, and an important additional function of the intermolecular modes in the stabilization of charged intermediates is suggested.

Resonant uv pump-probe spectroscopy of dipicolinic acid via impulsive excitation

We present experimental evidence of coherent wave packet motion in dipicolinic acid $({\mathrm{C}}_{7}{\mathrm{H}}_{5}\mathrm{N}{\mathrm{O}}_{4})$ which is an important marker molecule for bacterial spores. Resonant impulsive excitation is achieved by applying a uv pump pulse ($267\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $16\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$) which has a duration that is shorter than the vibrational period of the molecules. The resulting dynamics is then probed with a weaker pulse of the same width and frequency. Evidence of the important ``fingerprint'' region for this molecule (between $1000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ and $1500\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$) is found in the transient absorption of the probe. We present simulations of the pump-probe experiment, based on the Liouville equation for the density matrix, and predict the optimal pulse width and detuning.

Coherent Structural Dynamics and Electronic Correlations during an Ultrafast Insulator-to-Metal Phase Transition inVO2

We directly trace the multi-THz conductivity of VO2 during an insulator-metal transition triggered by a 12-fs light pulse. The femtosecond dynamics of lattice and electronic degrees of freedom are spectrally discriminated. A coherent wave packet motion of V-V dimers at 6 THz modulates the lattice polarizability for approximately 1 ps. In contrast, the electronic conductivity settles to a constant value already after one V-V oscillation cycle. Based on our findings, we propose a qualitative model for the nonthermal phase transition.

Intermolecular Vibrational Coherence in Bacteriochlorophyllawith Clustered Polar Solvent Molecules

We show that resonant impulsive excitation of the Qy absorption band of bacteriochlorophyll a (BChl) launches a rapidly damped (gamma < 200 fs) ground-state coherent wave-packet motion that arises from intermolecular modes with clustered solvent molecules. Femtosecond pump-probe, dynamic-absorption signals were obtained at room temperature with BChl solutions in pyridine, acetone, and 1-propanol. The vibrational coherence observed in the 0-800-fs regime is modeled in the time domain by two (or three, in the case of 1-propanol) modulation components with asymmetric, inhomogeneously broadened line shapes and frequencies in the 100-200-cm(-1) range. The mean frequency of the vibrational coherence exhibits at least a quadratic dependence on the dipole moment of the solvent molecules and a y-intercept in the 100-cm(-1) regime. This trend is modeled by an expression for the natural frequency of a "6-12" potential composed of attractive terms from van der Waals forces and a repulsive term from the exchange (Pauli exclusion) force. The model suggests that comparable contributions to the potential are provided by the dipole-dipole and London dispersion interactions. These results support the hypothesis that the low-frequency vibrational modes in the 100-cm(-1) regime that are coupled to the light-driven charge-separation reactions in the reaction center from purple bacteria are derived from intermolecular vibrational modes between the chromophores and the surrounding protein medium.

Observation of Nuclear Wavepacket Motion of Reacting Excited States in Solution

AbstractThree fundamental ultrafast photochemical reactions in solution, i.e., photoisomerization of cis‐stilbene, photodissociation of diphenylcyclopropenone and excited‐state intramolecular proton transfer of 10‐hydroxybenzoquinoline, were studied by two color pump‐probe spectroscopy with time‐resolution as high as 30 −; 70 fs. The coherent nuclear wavepacket motions of the excited‐states that undergo ultrafast reactions were observed, and their relevance to the reaction was discussed. It was concluded that these reactions are classified to three different types for the relation between the initial nuclear wavepacket motion and the reaction coordinate.

A 40-fs time-resolved absorption study on cis-stilbene in solution: observation of wavepacket motion on the reactive excited state

Time-resolved absorption spectroscopy with ∼40 fs time resolution was applied to the S 1 state of cis-stilbene in solution. An oscillatory signal caused by ∼220 cm −1 wavepacket motion was clearly observed. Its damping rate was much faster than the isomerization rate and insensitive to the change of the solvent. The obtained ultrafast pump–probe data showed that the dephasing of the initial coherent wavepacket motion first takes place rapidly, followed by the ∼1 ps isomerization reaction. The significance of the ∼220 cm −1 mode, intramolecular vibrational energy redistribution (IVR) process, and high anharmonicity of the S 1 potential surface in the initial stage of cis-stilbene photochemistry is discussed.

Quantum dynamical simulations of ultrafast photoinduced electron-transfer processes

Multilevel Redfield theory is employed for the modeling of ultrafast electron-transfer (ET) reactions in polyatomic systems, exhibiting coherent wave-packet motion in excited and/or ground electronic states. Two-electronic-state models are constructed for excited-state ET driven by coherent vibrational motion in the initially excited donor state and characteristic features of ET dynamics in normal and inverted regions are outlined. Three-electronic-state models focus on the mechanisms of coherent repopulation of the electronic ground state, which may accompany ultrafast photoinduced ET. It is shown that wave-packet motion in the electronic ground state may be initialized not only by the laser excitation, but also by the reaction itself. Employing the numerically exact so-called self-consistent hybrid method, the validity of Redfield theory is critically examined.

Coherent low-frequency motions of hydrogen bonded acetic acid dimers in the liquid phase.

Ultrafast vibrational dynamics of cyclic hydrogen bonded dimers and the underlying microscopic interactions are studied in temporally and spectrally resolved pump-probe experiments with 100 fs time resolution. Femtosecond excitation of the O-H and/or O-D stretching mode gives rise to pronounced changes of the O-H/O-D stretching absorption displaying both rate-like kinetic and oscillatory components. A lifetime of 200 fs is measured for the v=1 state of the O-H stretching oscillator. The strong oscillatory absorption changes are due to impulsively driven coherent wave packet motions along several low-frequency modes of the dimer between 50 and 170 cm(-1). Such wave packets generated via coherent excitation of the high-frequency O-H/O-D stretching oscillators represent a clear manifestation of the anharmonic coupling of low- and high-frequency modes. The underdamped low-frequency motions dephase on a time scale of 1-2 ps. Calculations of the vibrational potential energy surface based on density functional theory give the frequencies, anharmonic couplings, and microscopic elongations of the low-frequency modes, among them intermolecular hydrogen bond vibrations. Oscillations due to the excitonic coupling between the two O-H or O-D stretching oscillators are absent as is independently confirmed by experiments on mixed dimers with uncoupled O-H and O-D stretching oscillators.

Dynamic-Absorption Spectral Contours: Vibrational Phase-Dependent Resolution of Low-Frequency Coherent Wave-Packet Motion of IR144 on the Ground-State and Excited-State π → π* Surfaces

We have characterized the femtosecond dynamic-absorption spectrum from the tricarbocyanine dye IR144 with the aid of a new approach that uses time-probe-wavelength contour lines to project the phase relationships between the coherent wave-packet motions on the ground-state and excited-state potential-energy surfaces. The distinct phase of the waveform carried by a dynamic-absorption contour line reports the motion of a single wave packet. The spectrum from IR144 exhibits four regions of alternating contour phase; simulations show that this interference pattern results from the antiphase motion and partial spectral overlap of the ground-state depletion and stimulated-emission spectra and the narrowing of the spectra that occurs when the excited-state and ground-state wave packets reach their turning points. The Fourier-magnitude spectra of contour lines and intensity transients observed in the spectral region, which are assigned to the turning point of the ground-state wave packet, have been compared to those obtained in the long-wavelength-limit spectral region that is associated with the excited-state wave packet. The intensities and frequencies of eleven modes observed over the 15-634 cm - 1 region suggest that the excited-state wave packet rapidly moves from the Franck-Condon geometry to a region of the potential-energy surface that is relatively flat with respect to the global normal coordinates that bend and twist the conjugated polyene backbone of IR144. These coordinates are evidently anharmonically coupled to high-frequency modes that are not impulsively excited by the 12-fs pulses used in the dynamic-absorption experiments.

Ultrafast cis-trans photoswitching: A model study

A quantum-mechanical model description of a molecular photoswitch is developed. It takes into account (i) the electronic curve crossing arising from the cis-trans twisting of a double bond, resulting in an ultrafast internal-conversion process of the system and (ii) the coupling of the initially excited chromophore (the “system”) to the remaining degrees of freedom (the “bath”), affecting a vibrational cooling of the hot photoproducts. The latter mechanism is responsible for the localization of the molecule in the cis and trans configuration, respectively, thus determining the quantum yield of the photoreaction. Following a discussion of the validity and the numerical implementation of the Redfield formulation employed, detailed numerical studies of the time-dependent dissipative photoisomerization dynamics are presented. While the short-time dynamics (≲1 ps) is dominated by the coherent wave-packet motion of the system, the time evolution at larger times mainly reflects the interaction between system and bath. The quantum yield of the cis-trans forward reaction (Yc→t) and the trans-cis backward reaction (Yt→c) is shown to depend on the energy storage of the photoreaction and, in particular, on the form of the system–bath coupling. On the other hand, it is found that Yt→c=1−Yc→t, that is the population probabilities of the cis and trans configuration at long times do not depend on the initial preparation of the system.

Ultrafast coherent electron transport in GaAs/AlGaAs quantum cascade structures

We report on the first direct time-resolved study of coherent electron transport in an electrically driven quantum cascade laser structure. The ultrafast quantum transport of electrons from the injector into the upper laser state is investigated by means of femtosecond mid-infrared pump–probe experiments directly monitoring the ultrafast saturation and subsequent recovery of electrically induced optical gain. For forward bias and spectral positions around the gain maximum we observe pronounced gain oscillations upon excitation giving direct evidence for a coherent wave packet motion from the injector into the upper laser state.

Modeling of ultrafast electron-transfer dynamics: multi-level Redfield theory and validity of approximations

The short-time electron-transfer (ET) dynamics following short-pulse optical excitation is investigated for representative models by numerical reduced density-matrix calculations. The multi-level Redfield-theory equations are solved for two-site models with up to three strongly coupled reaction modes, which are weakly coupled to a dissipative environment. The so-called normal and inverted regimes of ET and stationary as well as nonstationary initial-state preparation are considered. The simulations illustrate the importance of electronic backflow in the case of strong electronic coupling and the effect of coherent vibrational wave-packet motion on the ET process. Three approximations, which have widely been used in ET modeling, are tested against the Redfield-theory results: the golden rule (GR) formula for nonadiabatic electron transfer, the secular approximation to the Redfield tensor and the diabatic-damping approximation (DDA) (neglect of the electronic interstate coupling in the construction of the Redfield tensor). The results illustrate the breakdown of the GR formula with increasing electronic coupling strength and the failure of the secular approximation for coherently driven ET. It is found that the DDA can provide a surprisingly accurate description of ultrafast ET processes when the zero-order vibrational levels are nearly in resonance. It is demonstrated by a benchmark calculation for a three-mode ET model that the near-resonance condition is generally fulfilled for multi-mode models in the inverted regime.

Ultrafast optical dynamics of excitons in J-aggregates

Time evolution of the coherence properties of excitons in J-aggregates of an organic molecule is studied by femtosecond transient absorption and three-pulse stimulated photon echo peak shifts. The exciton energy correlation function decays to zero by 120 fs single exponential, manifesting an ultrafast rupture of the coherent wave packet motion and the absence of inhomogeneity beyond the 100 fs time scale. Insensitivity of the correlation function to the isotopic substitution of the solvent suggests that the phonon modes of the molecular chain are responsible for the ultrafast dynamics.

Ultrafast dynamics of light-induced electron injection from a molecular donor into the wide conduction band of a semiconductor as acceptor

Coherent vibrational wave packet motion is shown here to survive the heterogeneous light-induced electron transfer reaction and to continue even in the product state M +. Thus, the reaction does not start from a thermally equilibrated occupation of the vibrational modes in the donor molecule M*, in contrast to the hitherto standard model applied in photoelectrochemistry. Whereas the rise of the transient absorption signals of the product states are probing the forward electron injection reaction their decay does not simply probe the recombination reaction to the ground state dye and is complicated due to several additional effects. It is postulated here that the injection of hot electrons is in general faster than the capture of hot electrons by an adsorbed molecule.