C. Ferrante

Structural rearrangement accompanying the ultrafast electrocyclization reaction of a photochromic molecular switch

Probing the structural rearrangement of a model photochromic molecular switch provides a window on the fundamental dynamics of electrocyclization reactions. Taking advantage of resonance-enhanced femtosecond stimulated Raman scattering (FSRS) with a broadly tunable Raman excitation wavelength, we selectively probe the competing dynamics of both the reactive and nonreactive conformers of a diarylethene (DAE) derivative that are simultaneously present in solution. Measurements that preferentially probe the electrocyclization (ring-closing) reaction of the reactive species reveal an unexpectedly slow nuclear rearrangement, stretching to tens of picoseconds, in striking contrast with the prompt electronic dynamics in the formation of the closed-ring isomer. The different results from transient electronic and vibrational spectroscopies reflect the different aspects of the reaction that are probed by each technique, depending on whether one considers the electronic state of the molecule or the structural rearrangement of the nuclei. Using a different Raman excitation wavelength selectively probes the picosecond-scale intersystem crossing dynamics of the nonreactive conformer, revealing the vibrational spectra of the singlet and triplet excited states for the first time. The present study paves the way to a more complete understanding of the structural mechanisms accompanying the reversible photochromic switching process.

Acoustic dynamics of network-forming glasses at mesoscopic wavelengths

The lack of long-range structural order in amorphous solids induces well known thermodynamic anomalies, which are the manifestation of distinct peculiarities in the vibrational spectrum. Although the impact of such anomalies vanishes in the long wavelength, elastic continuum limit, it dominates at length scales comparable to interatomic distances, implying an intermediate transition regime still poorly understood. Here we report a study of such mesoscopic domains by means of a broadband version of picosecond photo-acoustics, developed to coherently generate and detect hypersonic sound waves in the sub-THz region with unprecedented sampling efficiency. We identify a temperature-dependent fractal v3/2 frequency behaviour of the sound attenuation, pointing to the presence of marginally stable regions and a transition between the two above mentioned limits. The essential features of this behaviour are captured by a theoretical approach based on random spatial variation of the shear modulus, including anharmonic interactions.

Spectrally tailored narrowband pulses for femtosecond stimulated Raman spectroscopy in the range 330-750 nm

Spectral compression of femtosecond pulses by second harmonic generation in the presence of substantial group velocity dispersion provides a convenient source of narrowband Raman pump pulses for femtosecond stimulated Raman spectroscopy (FSRS). We discuss here a simple and efficient modification that dramatically increases the versatility of the second harmonic spectral compression technique. Adding a spectral filter following second harmonic generation produces narrowband pulses with a superior temporal profile. This simple modification i) increases the Raman gain for a given pulse energy, ii) improves the spectral resolution, iii) suppresses coherent oscillations associated with slowly dephasing vibrations, and iv) extends the useful tunable range to at least 330-750 nm.

Electronic resonances in broadband stimulated Raman spectroscopy

Spontaneous Raman spectroscopy is a formidable tool to probe molecular vibrations. Under electronic resonance conditions, the cross section can be selectively enhanced enabling structural sensitivity to specific chromophores and reaction centers. The addition of an ultrashort, broadband femtosecond pulse to the excitation field allows for coherent stimulation of diverse molecular vibrations. Within such a scheme, vibrational spectra are engraved onto a highly directional field, and can be heterodyne detected overwhelming fluorescence and other incoherent signals. At variance with spontaneous resonance Raman, however, interpreting the spectral information is not straightforward, due to the manifold of field interactions concurring to the third order nonlinear response. Taking as an example vibrational spectra of heme proteins excited in the Soret band, we introduce a general approach to extract the stimulated Raman excitation profiles from complex spectral lineshapes. Specifically, by a quantum treatment of the matter through density matrix description of the third order nonlinear polarization, we identify the contributions which generate the Raman bands, by taking into account for the cross section of each process.

Probing equilibrium glass flow up to exapoise viscosities

Significance “Does the glass cease to flow at some finite temperature?” Answering this question––of pivotal importance for glass formation theories––would require ridiculously long observation times. We circumvent this infeasibility relating the (directly inaccessible) ultraviscous flow of a liquid to the elastic properties of the corresponding glass, which we measure as a function of its age. The older the glass, the lower the temperature at which viscosity can be determined. Taking advantage of physical vapor deposition, we rapidly obtain a wide spectrum of ages rivaling those of millenary ambers, enabling viscosity determinations at values as large as those pertaining to the asthenosphere. Our result ultimately rules out the finite-temperature divergence of the molecular diffusion timescale in a glass. Glasses are out-of-equilibrium systems aging under the crystallization threat. During ordinary glass formation, the atomic diffusion slows down, rendering its experimental investigation impractically long, to the extent that a timescale divergence is taken for granted by many. We circumvent these limitations here, taking advantage of a wide family of glasses rapidly obtained by physical vapor deposition directly into the solid state, endowed with different “ages” rivaling those reached by standard cooling and waiting for millennia. Isothermally probing the mechanical response of each of these glasses, we infer a correspondence with viscosity along the equilibrium line, up to exapoise values. We find a dependence of the elastic modulus on the glass age, which, traced back to the temperature steepness index of the viscosity, tears down one of the cornerstones of several glass transition theories: the dynamical divergence. Critically, our results suggest that the conventional wisdom picture of a glass ceasing to flow at finite temperature could be wrong.

Direct observation of subpicosecond vibrational dynamics in photoexcited myoglobin

Determining the initial pathway for ultrafast energy redistribution within biomolecules is a challenge, and haem proteins, for which energy can be deposited locally in the haem moiety using short light pulses, are suitable model systems to address this issue. However, data acquired using existing experimental techniques that fail to combine sufficient structural sensitivity with adequate time resolution have resulted in alternative hypotheses concerning the interplay between energy flow among highly excited vibrational levels and potential concomitant electronic processes. By developing a femtosecond-stimulated Raman set-up, endowed with the necessary tunability to take advantage of different resonance conditions, here we visualize the temporal evolution of energy redistribution over different vibrational modes in myoglobin. We establish that the vibrational energy initially stored in the highly excited Franck-Condon manifold is transferred with different timescales into low- and high-frequency modes, prior to slow dissipation through the protein. These findings demonstrate that a newly proposed mechanism involving the population dynamics of specific vibrational modes settles the controversy on the existence of transient electronic intermediates.

Manipulating Impulsive Stimulated Raman Spectroscopy with a Chirped Probe Pulse

Photophysical and photochemical processes are often dominated by molecular vibrations in various electronic states. Dissecting the corresponding, often overlapping, spectroscopic signals from different electronic states is a challenge hampering their interpretation. Here we address impulsive stimulated Raman spectroscopy (ISRS), a powerful technique able to coherently stimulate and record Raman-active modes using broadband pulses. Using a quantum-mechanical treatment of the ISRS process, we show the mode-specific way the various spectral components of the broadband probe contribute to the signal generated at a given wavelength. We experimentally demonstrate how to manipulate the signal by varying the probe chirp and the phase-matching across the sample, thereby affecting the relative phase between the various contributions to the signal. These novel control knobs allow us to selectively enhance desired vibrational features and distinguish spectral components arising from different excited states.

Visualizing Excited-State Dynamics of a Diaryl Thiophene: Femtosecond Stimulated Raman Scattering as a Probe of Conjugated Molecules

Conjugated organic polymers based on substituted thiophene units are versatile building blocks of many photoactive materials, such as photochromic molecular switches or solar energy conversion devices. Unraveling the different processes underlying their photochemistry, such as the evolution on different electronic states and multidimensional structural relaxation, is a challenge critical to defining their function. Using femtosecond stimulated Raman scattering (FSRS) supported by quantum chemical calculations, we visualize the reaction pathway upon photoexcitation of the model compound 2-methyl-5-phenylthiophene. Specifically, we find that the initial wavepacket dynamics of the reaction coordinates occurs within the first ≈1.5 ps, followed by a ≈10 ps thermalization. Subsequent slow opening of the thiophene ring through a cleavage of the carbon-sulfur bond triggers an intersystem crossing to the triplet excited state. Our work demonstrates how a detailed mapping of the excited-state dynamics can be obtained, combining simultaneous structural sensitivity and ultrafast temporal resolution of FSRS with the chemical information provided by time-dependent density functional theory calculations.

Response to “Comment on ‘Visualizing coherent phonon propagation in the 100 GHz range: A broadband picosecond acoustic approach’” [Appl. Phys. Lett.98, 246101 (2011)]

in the 100 GHz range: A broadband picosecond acoustic approach’” †Appl. Phys. Lett. 98, 246101 „2011...‡ E. Pontecorvo, C. Ferrante, M. Ferretti, M. Ortolani, D. Polli, G. Ruocco, G. Cerullo, and T. Scopigno Dipartimento di Fisica, Universita’ Roma “Sapienza,” P.le Aldo Moro 2, 00185 Roma, Italy IFN–CNR, Via Cineto Romano 42, 00156 Rome, Italy IFN–CNR and Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

Probing femtosecond lattice displacement upon photo-carrier generation in lead halide perovskite

Electronic properties and lattice vibrations are expected to be strongly correlated in metal-halide perovskites, due to the soft fluctuating nature of their crystal lattice. Thus, unveiling electron–phonon coupling dynamics upon ultrafast photoexcitation is necessary for understanding the optoelectronic behavior of the semiconductor. Here, we use impulsive vibrational spectroscopy to reveal vibrational modes of methylammonium lead-bromide perovskite under electronically resonant and non-resonant conditions. We identify two excited state coherent phonons at 89 and 106 cm−1, whose phases reveal a shift of the potential energy minimum upon ultrafast photocarrier generation. This indicates the transition to a new geometry, reached after approximately 90 fs, and fully equilibrated within the phonons lifetime of about 1 ps. Our results unambiguously prove that these modes drive the crystalline distortion occurring upon photo-excitation, demonstrating the presence of polaronic effects.The electron–phonon coupling is the key to understand optoelectronic properties in lead halide perovskites but it is difficult to probe. Here Batignani et al. observe two new phonon modes with impulsive vibrational spectroscopy providing the evidence of the polaronic nature of the photo-excitation.