Excited State Wavepacket Research Articles

Excited-State Vibrational Coherence in Perylene Bisimide Probed by Femtosecond Broadband Pump-Probe Spectroscopy.

Broadband laser pulses with ultrashort duration are capable of triggering impulsive excitation of the superposition of vibrational eigenstates, giving rise to quantum beating signals originating from coherent wave packet motions along the potential energy surface. In this work, coherent vibrational wave packet dynamics of an N,N'-bis(2,6-dimethylphenyl)perylene bisimide (DMP-PBI) were investigated by femtosecond broadband pump-probe spectroscopy which features fast and balanced data acquisition with a wide spectral coverage of >200 nm. Clear modulations were observed in the envelope of the stimulated emission decay profiles of DMP-PBI with the oscillation frequencies of 140 and 275 cm(-1). Fast Fourier transform analysis of each oscillatory mode revealed characteristic phase jumps near the maxima of the steady-state fluorescence, indicating that the observed vibrational coherence originates from an excited-state wave packet motion. Quantum calculations of the normal modes at the low-frequency region suggest that low-frequency C-C (C═C) stretching motions accompanied by deformation of the dimethylphenyl substituents are responsible for the manifestation of such coherent wave packet dynamics.

Excited-state wavepacket and potential reconstruction by coherent anti-Stokes Raman scattering

Among the major challenges in the chemical sciences is controlling chemical reactions and deciphering their mechanisms. Since much of chemistry occurs in excited electronic states, in the last three decades scientists have employed a wide variety of experimental techniques and theoretical methods to recover excited-state potential energy surfaces and the wavepackets that evolve on them. These methods have been partially successful but generally do not provide a complete reconstruction of either the excited state wavepacket or potential. We have recently proposed a methodology for reconstructing excited-state molecular wavepackets and the corresponding potential energy surface [Avisar and Tannor, Phys. Rev. Lett., 2011, 106, 170405]. In our approach, the wavepacket is represented as a superposition of the set of vibrational eigenfunctions of the molecular ground-state Hamiltonian. We assume that the multidimensional ground-state potential surface is known, and therefore these vibrational eigenfunctions are known as well. The time-dependent coefficients of the basis functions are obtained by experimental measurement of the resonant coherent anti-Stokes Raman scattering (CARS) signal. Our reconstruction strategy has several significant advantages: (1) the methodology requires no a priori knowledge of any excited-state potential. (2) It applies to dissociative as well as to bound excited-state potentials. (3) It is general for polyatomics. (4) The excited-state potential surface is reconstructed simultaneously with the wavepacket. Apart from making a general contribution to the field of excited-state spectroscopy, our method provides the information on the excited-state wavepacket and potential necessary to design laser pulse sequences to control photochemical reactions.

Characterization of a conical intersection in a charge-transfer dimer with two-dimensional time-resolved stimulated Raman spectroscopy.

Photochemical reactions are mediated by conical intersections (CI), which are difficult to directly probe and characterize. To gain insight into CIs, two-dimensional femtosecond stimulated Raman spectroscopy (2D-FSRS) is used to examine a model excited-state charge-transfer (CT) complex consisting of an electron donor, tetramethylbenzene (TMB), and an acceptor, tetracyanoquinodimethane (TCNQ). Following impulsive excitation, the excited-state transient absorption reveals large-amplitude excited-state wave packet motion along low-frequency modes, in particular TCNQ's totally symmetric 323 cm(-1) CCN bend, which persist for ∼5 ps. These low-frequency coherences modulate the intensity and peak frequencies of the excited-state FSRS vibrational spectra. In particular, large-magnitude oscillations at 323 cm(-1) are observed in the peak frequency (Δω = 2 cm(-1)) and intensity (ΔOD = 1.5 mOD) of the nontotally symmetric 1271 cm(-1) C═C rocking mode. The magnitude of these oscillations is analyzed to determine the first-order anharmonic coupling between the high- and low-frequency degrees of freedom in the excited state. The anharmonic coupling between the totally symmetric 323 cm(-1) and the nontotally symmetric 1271 cm(-1) modes is estimated to be in excess of 50 cm(-1), strongly suggesting that they are the tuning and coupling modes, respectively, for the CI that connects the CT excited state to the neutral ground state and controls charge recombination internal conversion.

Excited state dynamics in SO2. I. Bound state relaxation studied by time-resolved photoelectron-photoion coincidence spectroscopy.

The excited state dynamics of isolated sulfur dioxide molecules have been investigated using the time-resolved photoelectron spectroscopy and time-resolved photoelectron-photoion coincidence techniques. Excited state wavepackets were prepared in the spectroscopically complex, electronically mixed (B̃)(1)B1/(Ã)(1)A2, Clements manifold following broadband excitation at a range of photon energies between 4.03 eV and 4.28 eV (308 nm and 290 nm, respectively). The resulting wavepacket dynamics were monitored using a multiphoton ionisation probe. The extensive literature associated with the Clements bands has been summarised and a detailed time domain description of the ultrafast relaxation pathways occurring from the optically bright (B̃)(1)B1 diabatic state is presented. Signatures of the oscillatory motion on the (B̃)(1)B1/(Ã)(1)A2 lower adiabatic surface responsible for the Clements band structure were observed. The recorded spectra also indicate that a component of the excited state wavepacket undergoes intersystem crossing from the Clements manifold to the underlying triplet states on a sub-picosecond time scale. Photoelectron signal growth time constants have been predominantly associated with intersystem crossing to the (c̃)(3)B2 state and were measured to vary between 750 and 150 fs over the implemented pump photon energy range. Additionally, pump beam intensity studies were performed. These experiments highlighted parallel relaxation processes that occurred at the one- and two-pump-photon levels of excitation on similar time scales, obscuring the Clements band dynamics when high pump beam intensities were implemented. Hence, the Clements band dynamics may be difficult to disentangle from higher order processes when ultrashort laser pulses and less-differential probe techniques are implemented.

Coherent wavepacket motion in an ultrafast electron transfer system monitored by femtosecond degenerate four-wave-mixing and pump–probe spectroscopy

Coherent nuclear wavepacket motions were monitored by three types of femtosecond time-resolved spectroscopy, namely, transient absorption measurement utilizing white-light supercontinuum (WC-TA), degenerate four-wave-mixing (DFWM), and pump–probe (PP) measurements, for an ultrafast intermolecular electron transfer (ET) system with a dye molecule, oxazine 1 (Ox1), dissolved in an electron donating solvent, N,N-dimethylaniline (DMA). Vibrational frequencies of the wavepacket motion in the excited and in the ground states were 560–562 and 567–569cm−1, respectively, with only a few frequency difference of 5–9cm−1, which were clearly distinguishable by the highly accurate measurements. In DMA, the excited state wavepacket motion declined with time constant of 160–240fs which is somewhat longer than that of the ultrafast ET; 60–80fs.

Real-Time Spectroscopy of Single-Walled Carbon Nanotubes for Negative Time Delays by Using a Few-Cycle Pulse Laser

Several investigations using the femtosecond pump−probe technique have been made for coherent phonon dynamics in single-walled carbon nanotubes (SWNTs), which are promising candidates as devices for nanoelectronics and photonics due to their unique properties. However, the most relevant spectroscopic analyses by far have been performed in the positive time range, with the pump pulse coming before the probe. We studied the real-time coherent phonon in SWNTs using 7.1-fs pump−probe measurements for negative time delays in which vibronic coherence between the ground state and exciton state can be observed. Coherent vibrations corresponding to the radial breathing mode caused by the wave packet generated in the exciton state were detected, in contrast to the ground-state wave packet dynamics for positive delays previously reported. The present results show that excited-state wave packets contribute greatly to radial breathing modes in four, (6,5), (6,4), (7,5), and (8,3), chiral systems, but very little to G modes, even though the oscillation period of the latter is 20 fs, which is much shorter than the 100 fs of the exciton life. We discussed the differences in dephasing times in the four different chiral systems.

Carrier envelope phase effects induced by weak multicycle pulses: Localized quantum dynamics in double well potentials

We study the laser-driven quantum dynamics in a double well potential. Within a two-pulse excitation scheme the first pulse prepares a linear combination of excited states with ungerade symmetry. The interaction with a second uncorrelated pulse of low intensity results in coherent superpositions of states with gerade and ungerade symmetry. This is associated with a localization of excited state wave packets. It is shown that the carrier envelope phase of the second laser pulse critically influences this time-dependent localization.

Nonlinear optical signatures of ultraviolet light-induced ring opening in α-terpinene

Photoinduced electrocyclic ring opening reactions in conjugated cylcoalkenes are among the most elementary processes in organic chemistry. One prototypical ring opening reaction transforms cyclohexadiene into hexatriene. It is known that a sequence of sub-100 fs internal conversion transitions precedes bond breaking in cyclohexadiene and some of its derivatives. However, these excited state dynamics have never been directly monitored in solution because of insufficient time resolution. Here we aim to uncover the extraordinary photophysics behind related ultrafast internal conversion processes in a derivative of cyclohexadiene, α-terpinene (α-TP), solvated in cyclohexane. Transient absorption anisotropy experiments conducted with 20 fs laser pulses at 267 nm expose non-exponential depopulation kinetics for the ππ* electronic state of α-TP. Our data show that population transfer rapidly accelerates within the first 100 fs after photoexcitation. In addition, recurrences in two-dimensional photon echo (2DPE) line shapes reveal strong vibronic coupling in a normal mode near 523 cm−1, which involves torsions of the C=C bonds and hydrogen out-of-plane (HOOP) wagging on a vinyl group. With the support of several experiments, we hypothesize that the excited state wavepacket inα-TP undergoes several recurrences in the C=C stretching coordinate before displacement along the C=C torsion/vinyl HOOP coordinate finally sets it free from the Franck–Condon region of the potential energy surface. The unconfined wavepacket departs the ππ* electronic state by way of a conical intersection with a lower energy excited state. The present observations are made possible by recent improvements to both the time resolution and detection sensitivity of our experimental setup. This work demonstrates that it is now possible to acquire 2DPE signals in the deep ultraviolet, which are comparable with high-quality measurements in the visible spectral region. These technical developments open the door to studies of many beautiful models for elementary chemical dynamics.

Optimized production of ultracold ground-state molecules: Stabilization employing potentials with ion-pair character and strong spin-orbit coupling

We discuss the production of ultracold molecules in their electronic ground state by photoassociation employing electronically excited states with ion-pair character and strong spin-orbit interaction. A short photoassociation laser pulse drives a non-resonant three-photon transition for alkali atoms colliding in their lowest triplet state. The excited state wave packet is transferred to the ground electronic state by a second laser pulse, driving a resonant two-photon transition. After analyzing the transition matrix elements governing the stabilization step, we discuss the efficiency of population transfer using transform-limited and linearly chirped laser pulses. Finally, we employ optimal control theory to find the most efficient stabilization pathways. We find that the stabilization efficiency can be increased by one and two orders of magnitude for linearly chirped and optimally shaped laser pulses, respectively.

Multi-dimensional wavepacket and potential reconstruction by resonant coherent anti-Stokes Raman scattering: Application to H2O and HOD

We have recently proposed a methodology for reconstructing excited-state (ExS) molecular wavepackets, and the corresponding potential energy surface, from three-pulse resonant coherent anti-Stokes Raman scattering and knowledge of the ground-state potential [Avisar and Tannor, Phys. Rev. Lett. 106, 170405 (2011)]. The methodology is general for polyatomics and applies to any form of ExS potential--bound or dissociative. In our previous work we demonstrated the method on diatomics. Here, we demonstrate the method on the triatomics H(2)O and HOD, reconstructing the ExS wavepacket and potential in the two bond-stretching coordinates.

Validity of the Bersohn–Zewail model beyond justification

The absorption of probe pulses in ultrafast pump–probe experiments can be determined from the Bersohn–Zewail (BZ) model. The model relies on classical mechanics to describe the dynamics of the nuclei in the excited electronic state prepared by the ultrashort pump pulse. The BZ model provides excellent agreement between the classical trajectory and the average position of the excited state wave packet. By investigating the approximations connecting the nuclear dynamics described by quantum mechanics and the BZ model, we conclude that this agreement goes far beyond the validity of the individual approximations.

System-Dependent Signatures of Electronic and Vibrational Coherences in Electronic Two-Dimensional Spectra.

In this work, we examine vibrational coherence in a molecular monomer, where time evolution of a nuclear wavepacket gives rise to oscillating diagonal- and off-diagonal peaks in two-dimensional electronic spectra. We find that the peaks oscillate out-of-phase, resulting in a cancellation in the corresponding pump-probe spectra. Our results confirm the unique disposition of two-dimensional electronic spectroscopy (2D-ES) for the study of coherences. The oscillation pattern is in excellent agreement with the diagrammatic analysis of the third-order nonlinear response. We show how 2D-ES can be used to distinguish between ground- and excited-state wavepackets. On the basis of our results, we discuss coherences in coupled molecular aggregates involving both electronic and nuclear degrees of freedom. We conclude that a general distinguishing criterion based on the experimental data alone cannot be devised.

Theoretical study on ultrafast intersystem crossing of chromium(III) acetylacetonate

In the relaxation process from the 4T2g state of chromium(III) acetylacetonate, CrIII(acac)3, ultrafast intersystem crossing (ISC) competes with vibrational relaxation (VR). This contradicts the conventional cascade model, where ISC rates are slower than VR ones. We hence investigate the relaxation process with quantum chemical calculations and excited-state wavepacket simulations to obtain clues about the origins of the ultrafast ISC. It is found that a potential energy curve of the 4T2g state crosses those of the 2T1g states near the Franck–Condon region and their spin–orbit couplings are strong. Consequently, ultrafast ISC between these states is observed in the wavepacket simulation.

Quantum beat oscillations in the two-color-photoionization continuum of neon and their dependence on the intensity of the ionizing laser pulse

We investigate quantum beat oscillations in the photoionization continuum of Ne atoms that are photoionized by absorption of two photons via a group of excited bound states using ultrashort extreme ultraviolet and infrared laser pulses. The extreme ultraviolet pulse starts an excited-state wave packet that is photoionized by a high-intensity infrared pulse after a variable time delay. We analyze the continuum quantum beats from this two-step photoionization process and their dependence on the photoelectron kinetic energy. We find a pronounced dependence of the quantum beat amplitudes on the photoelectron kinetic energy. The dependence changes significantly with the applied infrared laser-pulse intensity. The experimental results are in good qualitative agreement with a model calculation that is adapted to the experimental situation. It accounts for the intensity dependence of the quantum beat structure through the coupling of the excited-state wave packet to other bound Ne states induced by the high-intensity infrared laser pulse.

Efficient and robust strong-field control of population transfer in sensitizer dyes with designed femtosecond laser pulses

We demonstrate control of electronic population transfer in molecules with the help of appropriately shaped femtosecond laser pulses. To this end we investigate two photosensitizer dyes in solution being prepared in the triplet ground state. Excitation within the triplet system is followed by intersystem crossing and the corresponding singlet fluorescence is monitored as a measure of population transfer in the triplet system. We record control landscapes with respect to the fluorescence intensity on both dyes by a systematic variation of laser pulse shapes combining second order and third order dispersion. In the strong-field regime we find highly structured topologies with large areas of maximum or minimum population transfer being insensitive over a certain range of applied laser intensities thus demonstrating robustness. We then compare our experimental results with simulations on generic molecular potentials by solving the time-dependent Schrödinger equation for excitation with shaped pulses. Control landscapes with respect to population transfer confirm the general trends from experiments. An analysis of regions with maximum or minimum population transfer indicates that coherent processes are responsible for the outcome of our excitation process. The physical mechanisms of joint motion of ground and excited state wave packets or population of a vibrational eigenstate in the excited state permit us to discuss the molecular dynamics in an atom-like picture.

Wavepacket and potential reconstruction by four-wave mixing spectroscopy: preliminary application to polyatomic molecules

We have recently shown how the excited-state wavepacket of a polyatomic molecule can be completely reconstructed from resonant coherent anti-Stokes Raman spectroscopy [Avisar and Tannor, Phys. Rev. Lett., 2011, 106, 170405]. The method assumes knowledge of the ground-state potential but not of any excited-state potential, however the latter can be computed once the excited-state wavepacket is known. The formulation applies to dissociative as well as bound excited potentials. We demonstrate the method on the Li2 molecule with its bound first excited-state as well as with a model dissociative excited state potential. Preliminary results are shown for a model two-dimensional molecular system. The calculations assume constant transition dipole moment (Condon approximation), delta-pulse excitation and a single excited-state potential, but we discuss the implications of removing these assumptions.

Long-lived coherence in carotenoids

We use two-colour vibronic coherence spectroscopy to observe long-lived vibrational coherences in the ground electronic state of carotenoid molecules, with decoherence times in excess of 1 ps. Lycopene and spheroidene were studied isolated in solution, and within the LH2 light-harvesting complex extracted from purple bacteria. The vibrational coherence time is shown to increase significantly for the carotenoid in the complex, providing further support to previous assertions that long-lived electronic coherences in light-harvesting complexes are facilitated by in-phase motion of the chromophores and surrounding proteins. Using this technique, we are also able to follow the evolution of excited state coherences and find that for carotenoids in the light-harvesting complex the ⟨S2|S0⟩ superposition remains coherent for more than 70 fs. In addition to the implications of this long electronic decoherence time, the extended coherence allows us to observe the evolution of the excited state wavepacket. These experiments reveal an enhancement of the vibronic coupling to the first vibrational level of the C–C stretching mode and/or methyl-rocking mode in the ground electronic state 70 fs after the initial excitation. These observations open the door to future experiments and modelling that may be able to resolve the relaxation dynamics of carotenoids in solution and in natural light-harvesting systems.

Theoretical Study of the Influence of Femtosecond Laser Wavelength on the Evolution of a Double-Minimum Electronic Excited State Wave Packet for NaRb

Employing the two-state model and the time-dependent wave packet method, the influence of femtosecond laser wavelength on the evolution of the double-minimum electronic excited state wave packet is numerically investigated. For different laser wavelengths, evolutions of the double-minimum electronic excited state wave packet with time and internuclear distance are different. One can control the evolution of the wave packet by varying the laser wavelength appropriately, which will benefit the light manipulation of atomic and molecular processes. Furthermore, study of the dynamics of the NaRb molecule may yield clues to creating an ultracold molecule.

Electronic and Vibrational Coherence Dynamics in a Cyanine Dye Studied Using a Few‐Cycle Pulsed Laser

We report the relaxation times of electronic and vibrational coherence in the cyanine dye 1,1',3,3,3',3'-hexamethyl-4,4',5,5'-dibenzo-2,2'-indotricarbocyanine, measured using a 7.1 fs pulsed laser. The vibrational phase relaxation times are found to be between 380 and 680 fs in the ground and lowest excited singlet states. The vibrational dephasing times of the 294, 446, and 736 cm(-1) modes are relatively long among the six modes associated with excited-state wave packets. The slower relaxations are explained in terms of a coupled triplet of vibrational modes, which preserves coherence by forming a tightly bound group to satisfy the condition of circa conservation of vibrational energy. Using data from the negative-time range (i.e., when the probe pulse precedes the pump pulse), the electronic phase relaxation time is found to be 31+/-1 fs. The dynamic vibrational mode in the excited state (1171 cm(-1)), detected in the positive-time range, is also studied from the negative-time traces under the same experimental conditions.

Dynamics of vibrational and electronic coherences in the electronic excited state studied in a negative-time range

Femtosecond pump–probe spectroscopy signals in a negative-time range (i.e., when the probe pulse precedes pump pulse) were analyzed to study the dynamics of electronic and vibrational coherences in the photodissociation of oxyhemoglobin. The decay time of the electronic coherence between the ground and Q-band electronic states is found to be 45 ± 5 fs. The dephasing times of the vibrational modes of several frequencies in the ground state have been determined. Those in the excited state are found to be ∼26 fs. The absence or presence of the modes in the negative-time range due to the excited state wavepacket is explained in terms of the mode coupling strength.