Classification of dynamic vibronic couplings in vibrational real-time spectra of a thiophene derivative by few-cycle pulses

Theoretical model of femtosecond coherence spectroscopy of vibronic excitons in molecular aggregates

When used as pump pulses in transient absorption spectroscopy measurements, femtosecond laser pulses can produce oscillatory signals known as quantum beats. The quantum beats arise from coherent superpositions of the states of the sample and are best studied in the Fourier domain using Femtosecond Coherence Spectroscopy (FCS), which consists of one-dimensional amplitude and phase plots of a specified oscillation frequency as a function of the detection frequency. Prior works have shown ubiquitous amplitude nodes and π phase shifts in FCS from excited-state vibrational wavepackets in monomer samples. However, the FCS arising from vibronic-exciton states in molecular aggregates have not been studied theoretically. Here, we use a model of vibronic-exciton states in molecular dimers based on displaced harmonic oscillators to simulate FCS for dimers in two important cases. Simulations reveal distinct spectral signatures of excited-state vibronic-exciton coherences in molecular dimers that may be used to distinguish them from monomer vibrational coherences. A salient result is that, for certain relative orientations of the transition dipoles, the key resonance condition between the electronic coupling and the frequency of the vibrational mode may yield strong enhancement of the quantum-beat amplitude and, perhaps, also cause a significant decrease of the oscillation frequency to a value far lower than the vibrational frequency. Future studies using these results will lead to new insights into the excited-state coherences generated in photosynthetic pigment-protein complexes.

Characterizing Mode Anharmonicity and Huang-Rhys Factors Using Models of Femtosecond Coherence Spectra

Femtosecond laser pulses readily produce coherent quantum beats in transient-absorption spectra. These oscillatory signals often arise from molecular vibrations and therefore may contain information about the excited-state potential energy surface near the Franck-Condon region. Here, by fitting the measured spectra of two laser dyes to microscopic models of femtosecond coherence spectra (FCS) arising from molecular vibrations, we classify coherent quantum-beat signals as fundamentals or overtones and quantify their Huang-Rhys factors and anharmonicity values. We discuss the extracted Huang-Rhys factors in the context of quantum-chemical computations. This work solidifies the use of FCS for analysis of coherent quantum beats arising from molecular vibrations, which will aid studies of molecular aggregates and photosynthetic proteins.

Signatures of Vibrational and Electronic Quantum Beats in Femtosecond Coherence Spectra

Femtosecond laser pulses can produce oscillatory signals in transient-absorption spectroscopy measurements. The quantum beats are often studied using femtosecond coherence spectra (FCS), the Fourier domain amplitude, and phase profiles at individual oscillation frequencies. In principle, one can identify the mechanism that gives rise to each quantum-beat signal by comparing its measured FCS to those arising from microscopic models. To date, however, most measured FCS deviate from the ubiquitous harmonic oscillator model. Here, we expand the inventory of models to which the measured spectra can be compared. We develop quantum-mechanical models of the fundamental, overtone, and combination-band FCS arising from harmonic potentials, the FCS of anharmonic potentials, and the FCS of a purely electronic dimer. This work solidifies the use of FCS for identifying electronic coherences that can arise in measurements of molecular aggregates including photosynthetic proteins. Furthermore, future studies can use the derived expressions to fit the measured FCS and thereby extract microscopic parameters of molecular potential-energy surfaces.

Finite-Temperature, Anharmonicity, and Duschinsky Effects on the Two-Dimensional Electronic Spectra from Ab Initio Thermo-Field Gaussian Wavepacket Dynamics

Accurate description of finite-temperature vibrational dynamics is indispensable in the computation of two-dimensional electronic spectra. Such simulations are often based on the density matrix evolution, statistical averaging of initial vibrational states, or approximate classical or semiclassical limits. While many practical approaches exist, they are often of limited accuracy and difficult to interpret. Here, we use the concept of thermo-field dynamics to derive an exact finite-temperature expression that lends itself to an intuitive wavepacket-based interpretation. Furthermore, an efficient method for computing finite-temperature two-dimensional spectra is obtained by combining the exact thermo-field dynamics approach with the thawed Gaussian approximation for the wavepacket dynamics, which is exact for any displaced, distorted, and Duschinsky-rotated harmonic potential but also accounts partially for anharmonicity effects in general potentials. Using this new method, we directly relate a symmetry breaking of the two-dimensional signal to the deviation from the conventional Brownian oscillator picture.

Primary Electronic and Vibrational Dynamics of Cytochrome c Observed by Sub-10 fs NUV Laser Pulses

The primary reaction mechanism of cytochrome c (Cyt c) was elucidated for two redox forms of ferric (oxidized) and ferrous (reduced) Cyt c by measuring their transient absorption (TA) spectra using a homemade sub-10 fs broadband NUV laser pulses system. The TA traces measured in the broad probe wavelength region were analyzed by the global analysis method to study the electronic dynamics. The difference of relaxation dynamics dependent on the excitation bandwidth enabled us to elucidate that the 2.5 ps component in ferrous Cyt c can be assigned to intramolecular vibration energy redistribution and not to vibrational cooling, which was not clear until this work. The temporal resolution of 10 fs observes TA signal modulation caused by the molecular vibration in the time domain, which can be used to calculate the instantaneous frequency of the molecular vibration mode. The observed vibrational dynamics has visualized that the heme structure changes in 0.8 ps for ferric Cyt c and in >1.0 ps for ferrous Cyt c. These estimated lifetimes of vibrational dynamics reflect vibrational relaxation in the ground state of ferric Cyt c and electronic transition from the S2 state to the S1 state in ferrous Cyt c, respectively.

Surface-Enhanced Impulsive Coherent Vibrational Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) has attracted a lot of attention in molecular sensing because of the remarkable ability of plasmonic metal nanostructures to enhance the weak Raman scattering process. On the other hand, coherent vibrational spectroscopy triggered by impulsive excitation using ultrafast laser pulses provides complete information about the temporal evolution of molecular vibrations, allowing dynamical processes in molecular systems to be followed in "real time". Here, we combine these two concepts and demonstrate surface-enhanced impulsive vibrational spectroscopy. The vibrational modes of the ground and excited states of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), spin-coated on a substrate covered with monodisperse silver nanoparticles, are impulsively excited with a sub-10 fs pump pulse and characterized with a delayed broad-band probe pulse. The maximum enhancement in the spectrally and temporally resolved vibrational signatures averaged over the whole sample is about 4.6, while the real-time information about the instantaneous vibrational amplitude together with the initial vibrational phase is preserved. The phase is essential to determine the vibrational contributions from the ground and excited states.

Excited-state molecular vibration observed for a probe pulse preceding the pump pulse by real-time optical spectroscopy

It is shown experimentally that the absorbance change observed in the "negative" time range, where probe pulse precedes pump pulse in real-time vibrational spectroscopy is induced only by the excited-state wave-packet motion as theoretically expected. Coherent molecular vibration of a polymer in the excited state was observed in the real-time trace without the effect of wave-packet motion in the ground state, which usually makes it difficult to ascribe the signal either to the ground state or to the excited state.