Chirp Dependence of Wave Packet Motion in Oxazine 1

Chirped Laser Pulse Control of Vibronic Wavepackets and Energy Transfer in Phycocyanin 645

Photosynthetic organisms use light-harvesting complexes to increase the spectrum of light that they absorb from solar photons. Recent ultrafast spectroscopic studies have revealed that efficient (sub-ps) energy transfer is mediated by vibronic coherence in the phycobiliprotein phycocyanin 645 (PC645). Here, we report studies that employ broadband pump-probe spectroscopy with linearly chirped excitation pulses to further investigate the relationship between vibronic state preparation and energy transfer dynamics in PC645. Negatively chirped pulse excitation is found to enhance wavepackets of a high-frequency mode (1580 cm-1) and increase the rate of downhill energy transfer, while on the other hand, positively chirped pulses suppress these oscillatory features and decrease this rate. Model calculations incorporating the influence of the chirped pump pulse are used to understand its effect on initial state preparation. These results provide mechanistic insight into how the overall nonequilibrium rate of energy transfer is influenced by initial state preparation.

Vibrational coherences in half-broadband 2D electronic spectroscopy: Spectral filtering to identify excited state displacements

Vibrational coherences in ultrafast pump-probe (PP) and 2D electronic spectroscopy (2DES) provide insights into the excited state dynamics of molecules. Femtosecond coherence spectra and 2D beat maps yield information about displacements of excited state surfaces for key vibrational modes. Half-broadband 2DES uses a PP configuration with a white light continuum probe to extend the detection range and resolve vibrational coherences in the excited state absorption (ESA). However, the interpretation of these spectra is difficult as they are strongly dependent on the spectrum of the pump laser and the relative displacement of the excited states along the vibrational coordinates. We demonstrate the impact of these convoluting factors for a model based upon cresyl violet. A careful consideration of the position of the pump spectrum can be a powerful tool in resolving the ESA coherences to gain insights into excited state displacements. This paper also highlights the need for caution in considering the spectral window of the pulse when interpreting these spectra.

Application of density matrix Wigner transforms for ultrafast macromolecular and chemical x-ray crystallography

Time-Resolved Serial Femtosecond Crystallography (TR-SFX) conducted at X-ray Free Electron Lasers (XFELs) has become a powerful tool for capturing macromolecular structural movies of light-initiated processes. As the capabilities of XFELs advance, we anticipate that a new range of coherent control and structural Raman measurements will become achievable. Shorter optical and x-ray pulse durations and increasingly more exotic pulse regimes are becoming available at free electron lasers. Moreover, with high repetition enabled by the superconducting technology of European XFEL (EuXFEL) and Linac Coherent Light Source (LCLS-II) , it will be possible to improve the signal-to-noise ratio of the light-induced differences, allowing for the observation of vibronic motion on the sub-Angstrom level. To predict and assign this coherent motion, which is measurable with a structural technique, new theoretical approaches must be developed. In this paper, we present a theoretical density matrix approach to model the various population and coherent dynamics of a system, which considers molecular system parameters and excitation conditions. We emphasize the use of the Wigner transform of the time-dependent density matrix, which provides a phase space representation that can be directly compared to the experimental positional displacements measured in a TR-SFX experiment. Here, we extend the results from simple models to include more realistic schemes that include large relaxation terms. We explore a variety of pulse schemes using multiple model systems using realistic parameters. An open-source software package is provided to perform the density matrix simulation and Wigner transformations. The open-source software allows us to define any arbitrary level schemes as well as any arbitrary electric field in the interaction Hamiltonian.

Spin-vibronic coherence drives singlet-triplet conversion

Design-specific control over the transitions between excited electronic states with different spin multiplicities is of the utmost importance in molecular and materials chemistry1-3. Previous studies have indicated that the combination of spin-orbit and vibronic effects, collectively termed the spin-vibronic effect, can accelerate quantum-mechanically forbidden transitions at non-adiabatic crossings4,5. However, it has been difficult to identify precise experimental manifestations of the spin-vibronic mechanism. Here we present coherence spectroscopy experiments that reveal the interplay between the spin, electronic and vibrational degrees of freedom that drive efficient singlet-triplet conversion in four structurally related dinuclear Pt(II) metal-metal-to-ligand charge-transfer (MMLCT) complexes. Photoexcitation activates the formation of a Pt-Pt bond, launching a stretching vibrational wavepacket. The molecular-structure-dependent decoherence and recoherence dynamics of this wavepacket resolve the spin-vibronic mechanism. We find that vectorial motion along the Pt-Pt stretching coordinates tunes the singlet and intermediate-state energy gap irreversibly towards the conical intersection and subsequently drives formation of the lowest stable triplet state in a ratcheting fashion. This work demonstrates the viability of using vibronic coherences as probes6-9 to clarify the interplay among spin, electronic and nuclear dynamics in spin-conversion processes, and this could inspire new modular designs to tailor the properties of excited states.

A Revisit on Impulsive Stimulated Raman Spectroscopy: Importance of Spectral Dispersion of Chirped Broadband Probe

The usefulness of a chirped broadband probe and spectral dispersion to obtain Raman spectra under nonresonant/resonant impulsive excitation is revisited. A general methodology is presented that inherently takes care of phasing the time-domain low-frequency oscillations without probe pulse compression and retrieves the absolute phase of the oscillations. As test beds, neat solvents (CCl₄, CHCl₃, and CH₂Cl₂) are used. Observation of periodic intensity modulation along detection wavelengths for particular modes is explained using a simple electric field interaction picture. This method is extended to diatomic molecule (iodine) and polyatomic molecules (Nile blue and methylene blue) to assign vibrational frequencies in ground/excited electronic state that are supported by density functional theory calculations. A comparison between frequency-domain and time-domain counterparts, i.e., stimulated Raman scattering and impulsive stimulated Raman scattering using degenerate pump-probe pairs is presented, and most importantly, it is shown how impulsive stimulated Raman scattering using chirped broadband probe retains unique advantages offered by both.

Broadband visible two-dimensional spectroscopy of molecular dyes

Two-dimensional Fourier transform spectroscopy is a promising technique to study ultrafast molecular dynamics. Similar to transient absorption spectroscopy, a more complete picture of the dynamics requires broadband laser pulses to observe transient changes over a large enough bandwidth, exceeding the inhomogeneous width of electronic transitions, as well as the separation between the electronic or vibronic transitions of interest. Here, we present visible broadband 2D spectra of a series of dye molecules and report vibrational coherences with frequencies up to ∼1400 cm^-1 that were obtained after improvements to our existing two-dimensional Fourier transform setup [Al Haddad et al., Opt. Lett. 40, 312-315 (2015)]. The experiment uses white light from a hollow core fiber, allowing us to acquire 2D spectra with a bandwidth of 200 nm, in a range between 500 and 800 nm, and with a temporal resolution of 10-15 fs. 2D spectra of nile blue, rhodamine 800, terylene diimide, and pinacyanol iodide show vibronic spectral features with at least one vibrational mode and reveal information about structural motion via coherent oscillations of the 2D signals during the population time. For the case of pinacyanol iodide, these observations are complemented by its Raman spectrum, as well as the calculated Raman activity at the ground- and excited-state geometry.

Nonresonant Raman Effects on Femtosecond Pump-Probe with Chirped White Light: Challenges and Opportunities

Impulsive Raman excitation in neat organic liquids far from resonance is followed using chirped broad-band supercontinuum probe pulses. Spectral modulations due to impulsively induced coherent vibrations vary in intensity 10-fold as a function of the probe's linear chirp. Simulations clarify why the vibrational signature is maximized for a group delay dispersion (GDD) in reduced units of ν_vib^-2 = 0.5 while a probe GDD of twice that quenches the same spectral modulations. Accordingly, recent claims that chirped white-light probe pulses provide equivalent information on material response to their compressed analogues must be taken with caution. In particular, interactions that induce spectral shifts in the probe depend crucially on the arrival chronology of the continuum colors. On one hand, this presents limitations to application of chirped continuum radiation as-is in pump-probe experiments. It also presents the opportunity for using this dependence to control the relative amplitude of nonresonant interactions in pump-probe signals such as that of solvent vibrations.

Slow Intramolecular Vibrational Relaxation Leads to Long-Lived Excited-State Wavepackets

Broadband optical pump and compressed white light continuum probe were used to measure the transient excited-state absorption, ground-state bleach, and stimulated emission signals of cresyl violet solution in methanol. Amplitude oscillations caused by wavepacket motion in the ground and excited electronic states were analyzed. It was found that vibrational coherences in the excited state persist for more than the experimental waiting time window of 6 ps, and the strongest mode had a dephasing time constant of 2.4 ps. We hypothesize the dephasing of the wavepacket in the excited state is predominantly caused by intramolecular vibrational relaxation (IVR). Slow IVR indicates weak mode-mode coupling and therefore weak anharmonicity of the potential of this vibration. Thus, the initially prepared vibrational wavepacket in the excited state is not significantly perturbed by nonadiabatic coupling to other electronic states, and hence the diabatic and adiabatic representations of the system are essentially identical within the Born-Oppenheimer approximation. The wavepacket therefore evolves with time in an almost harmonic potential, slowly dephased by IVR and the pure vibrational decoherence. The consistency in the position of node (phase change in the wavepacket) in the excited-state absorption and stimulated emission signals without undergoing any frequency shift until the wavepacket is completely dephased conforms to the absence of any reactive internal conversion.

Multidimensional Electronic Spectroscopy of Photochemical Reactions

Coherent multidimensional electronic spectroscopy can be employed to unravel various channels in molecular chemical reactions. This approach is thus not limited to analysis of energy transfer or charge transfer (i.e. processes from photophysics), but can also be employed in situations where the investigated system undergoes permanent structural changes (i.e. in photochemistry). Photochemical model reactions are discussed by using the example of merocyanine/spiropyran-based molecular switches, which show a rich variety of reaction channels, in particular ring opening and ring closing, cis-trans isomerization, coherent vibrational wave-packet motion, radical ion formation, and population relaxation. Using pump-probe, pump-repump-probe, coherent two-dimensional and three-dimensional, triggered-exchange 2D, and quantum-control spectroscopy, we gain intuitive pictures on which product emerges from which reactant and which reactive molecular modes are associated.

Ultrafast coherent oscillations reveal a reactive mode in the ring-opening reaction of fulgides

Ultrafast coherent dynamics reveals a low frequency vibrational mode governing the photochromic ring-opening reaction in indolylfulgide molecular switches.

Coherent longitudinal-optical ground-state phonon in CdSe quantum dots triggered by ultrafast charge migration

We observe the CdSe longitudinal-optical ground-state phonon in the electron transfer system composed of CdSe quantum dots and methylviologen directly by femtosecond absorption spectroscopy. A significant phase shift indicates that the coherent oscillations are triggered by an ultrafast charge migration, which is the consequence of an electron transfer from the photoexcited quantum dot to the molecular acceptor methylviologen. In contrast, the observed coherent phonons in isolated quantum dots stem from the frequency modulation of the quantum dot excited-state spectrum. From the probe wavelength dependence of the longitudinal-optical phonons in the electronic ground state and excited state it is possible to determine a biexciton binding energy of 35 meV.

Ring-opening reaction of a trifluorinated indolylfulgide: mode-specific photochemistry after pre-excitation

The ring-opening reaction of a trifluorinated indolylfulgide has been studied as a function of temperature and optical pre-excitation where it was found that reaction times decreased as temperature increased from 10.3 ps at 12 degrees C to 7.6 ps at 60 degrees C. Simultaneously, the quantum yields for the ring-opening reaction grew from 3.1% (12 degrees C) to 5.0% (60 degrees C). When the reaction was started from a non-equilibrium state generated by a directly preceding ring-closure process, the ring-opening reaction became faster and the quantum efficiency increased by more than a factor of three. Analysis of the experimental results points to mode-specific photochemistry in that the promoting, photochemically active modes of the photoreaction are efficiently excited by the directly preceding ring-closure reaction.

Pump-Degenerate Four Wave Mixing as a Technique for Analyzing Structural and Electronic Evolution: Multidimensional Time-Resolved Dynamics near a Conical Intersection

Pump-degenerate four wave mixing (pump-DFWM) is used to simultaneously study the early events in structural and electronic population dynamics of the non-adiabatic passage between two excited electronic states. After the precursor state S2 is populated by an initial pump beam, a DFWM sequence is set resonant with the S1 --> Sn transition on the successor state S1. The information obtained by pump-DFWM is two-fold: by scanning the delay between the initial pump and the DFWM sequence, the evolution of the individual excited-state modes is observed with a temporal resolution of 20 fs and a spectral resolution of 10 cm-1. Additionally, pump-DFWM yields information on electronic population dynamics, resulting in a comprehensive description of the S2 --> S1 internal conversion. As a system in which the interplay between structural and electronic evolution is of great interest, all-trans-beta-carotene in solution was chosen. The pump-DFWM signal is analyzed for different detection wavelengths, yielding results on the ultrafast dynamics between 1Bu+ (S2) and 2Ag- (S1). The process of vibrational cooling on S1 is discussed in detail. Furthermore, a low-lying vibrationally hot state is excited and characterized in its spectroscopic properties. The combination of highly resolved vibrational dynamics and simultaneously detected ultrafast electronic state spectroscopy gives a complete picture of the dynamics near a conical intersection. Because pump-DFWM is a pure time domain technique, it offers the prospect of coherent control of excited-state dynamics on an ultrafast time scale.

Optical control of excited-state vibrational coherences of a molecule in solution: The influence of the excitation pulse spectrum and phase in LD690

Spectral and phase shaping of femtosecond laser pulses is used to selectively excite vibrational wave packets on the ground (S0) and excited (S1) electronic states in the laser dye LD690. The transient absorption signals observed following excitation near the peak of the ground-state absorption spectrum are characterized by a dominant 586 cm(-1) vibrational mode. This vibration is assigned to a wave packet on the S0 potential energy surface. When the excitation pulse is tuned to the blue wing of the absorption spectrum, a lower frequency 568 cm(-1) vibration dominates the response. This lower frequency mode is assigned to a vibrational wave packet on the S1 electronic state. The spectrum and phase of the excitation pulse also influence both the dephasing of the vibrational wave packet and the amplitude profiles of the oscillations as a function of probe wavelength. Excitation by blue-tuned, positively chirped pulses slows the apparent dephasing of the vibrational coherences compared with a transform-limited pulse having the same spectrum. Blue-tuned negatively chirped excitation pulses suppress the observation of coherent oscillations in the ground state.

Rapid motion capture of mode-specific quantum wave packets selectively generated by phase-controlled optical pulses

Rapid motion capture of phase-controlled wave packets was realized using a sensitive wave-packet spectrometer, which was previously developed by the present authors. Two-dimensional Fourier-transformed spectrograms obtained by the wave-packet spectrometer provide us full information about the wave-packet motion on both excited- and ground-state potential surfaces. Vibrational wave packet associated with a twisting mode in a DTTCI molecule was observed to be dependent on the pulse chirp, and was generated in the excited state preferably with negatively chirped excitation. The result indicates that the excited-state wave packet can be driven along a favorable configuration coordinate by using phase-controlled femtosecond pulses. The present method is essential to adaptive coherent-control application.

Appearance of intramolecular high-frequency vibrations in two-dimensional, time-integrated three-pulse photon echo data

An alternative experimental outline to measure homodyne detected three-pulse photon-echo data is presented. The novel experimental approach allowing for online monitoring and correction of experimental timing and stability is discussed in detail using the paradigm system of Nile blue in alcohol solution. It is shown that excellent signal-to-noise ratios together with high reproducibility of the data can be routinely achieved. We report in detail on the appearance of high-frequency intramolecular vibrations in the two-dimensional three-pulse photon-echo data and suggest that besides the conventionally discussed three-pulse photon-echo peak-shift the width of the integrated echo signal as a function of population time contains identical and easily accessible information on high-frequency intramolecular vibrations. A comparison of experimental data with theoretical modeling is performed showing that the observed echo-width oscillations are in line with predictions of the Brownian oscillator model.

Femtosecond quantum control of molecular dynamics in the condensed phase

We review the progress in controlling quantum dynamical processes in the condensed phase with femtosecond laser pulses. Due to its high particle density the condensed phase has both high relevance and appeal for chemical synthesis. Thus, in recent years different methods have been developed to manipulate the dynamics of condensed-phase systems by changing one or multiple laser pulse parameters. Single-parameter control is often achieved by variation of the excitation pulse's wavelength, its linear chirp or its temporal subpulse separation in case of pulse sequences. Multiparameter control schemes are more flexible and provide a much larger parameter space for an optimal solution. This is realized in adaptive femtosecond quantum control, in which the optimal solution is iteratively obtained through the combination of an experimental feedback signal and an automated learning algorithm. Several experiments are presented that illustrate the different control concepts and highlight their broad applicability. These fascinating achievements show the continuous progress on the way towards the control of complex quantum reactions in the condensed phase.

Following Photoinduced Dynamics in Bacteriorhodopsin with 7-fs Impulsive Vibrational Spectroscopy

Sub-10-fs laser pulses are used to impulsively photoexcite bacteriorhodopsin (BR) suspensions and probe the evolution of the resulting vibrational wave packets. Fourier analysis of the spectral modulations induced by transform-limited as well as linearly chirped excitation pulses allows the delineation of excited- and ground-state contributions to the data. On the basis of amplitude and phase variations of the modulations as a function of the dispersed probe wavelength, periodic modulations in absorption above 540 nm are assigned to ground-state vibrational coherences induced by resonance impulsive Raman spectral activity (RISRS). Probing at wavelengths below 540 nm-the red edge of the intense excited-state absorption band-uncovers new vibrational features which are accordingly assigned to wave packet motions along bound coordinates on the short-lived reactive electronic surface. They consist of high- and low-frequency shoulders adjacent to the strong C=C stretching and methyl rock modes, respectively, which have ground-state frequencies of 1008 and 1530 cm-1. Brief activity centered at approximately 900 cm-1, which is characteristic of ground-state HOOP modes, and strong modulations in the torsional frequency range appear as well. Possible assignments of the bands and their implication to photoinduced reaction dynamics in BR are discussed. Reasons for the absence of similar signatures in the pump-probe spectral modulations at longer probing wavelengths are considered as well.

Enhancement of molecular modes by electronically resonant multipulse excitation: Further progress towards mode selective chemistry

We show that molecular vibrations induced by resonant excitation pulses can be enhanced by pulse trains, compared to Fourier-limited pulses of equal pulse energy. As a proof-of-principle, a low frequency mode of Nile Blue at 600 cm(-1) is observed and amplified in a pump and probe experiment. In addition to previous experiments in our group, an increased population transfer to the excited electronic state is identified as an important element of the underlying physical mechanism. These results suggest an enhancement on the level of individual molecules rather than a macroscopic effect.

Analysis of Wave Packet Motion in Frequency and Time Domain: Oxazine 1

Wave packet motion in the laser dye oxazine 1 in methanol is investigated by spectrally resolved transient absorption spectroscopy. The spectral range of 600-690 nm was accessible by amplified broadband probe pulses covering the overlap region of ground-state bleach and stimulated emission signal. The influence of vibrational wave packets on the optical signal is analyzed in the frequency domain and the time domain. For the analysis in the frequency domain an algorithm is presented that accounts for interference effects of neighbored vibrational modes. By this method amplitude, phase and decay time of vibrational modes are retrieved as a function of probe wavelength and distortions due to neighbored modes are reduced. The analysis of the data in the time domain yields complementary information on the intensity, central wavelength, and spectral width of the optical bleach spectrum due to wave packet motion.