Femtosecond stimulated Raman spectrometer in the 320-520nm range

Efficient generation of narrowband picosecond pulses from a femtosecond laser

In some applications of broadband ultrafast spectroscopy, such as surface sum frequency generation vibrational spectroscopy, femtosecond stimulated Raman spectroscopy (SRS), and coherent anti-Stokes Raman spectroscopy, a narrowband picosecond pulse is required to obtain a high spectral resolution. Here, we present a method to generate narrowband picosecond second harmonic (SH) and fundamental frequency (FF) pulses with high-conversion efficiency from a Ti:sapphire femtosecond laser amplifier. The narrowband picosecond SH pulse was generated based on the group velocity mismatch between the SH and FF pulses in a nonlinear crystal of β-barium borate (BBO). The small SH nonlinear optical coefficient was optimized by changing the azimuth angle of a thick BBO crystal, successfully avoiding the saturation effect in the SH generation process. The SH pulse was then used to pump an optical parametric amplifier to efficiently amplify the narrowband FF seed pulse, which was obtained with an etalon by spectrally filtering the output from the femtosecond laser amplifier. Dual-wavelength output, which could be very useful in femtosecond SRS, was also realized.

Improved spectral resolution of the femtosecond stimulated Raman spectroscopy achieved by the use of the 2nd-order diffraction method

Prolongation of the picosecond Raman pump laser pulse in the femtosecond stimulated Raman spectroscopy (FSRS) setup is essential for achieving the high spectral resolution of the time-resolved vibrational Raman spectra. In this work, the 2nd-order diffraction has been firstly employed in the double-pass grating filter technique for realizing the FSRS setup with the sub-5 cm^-1 spectral resolution. It has been experimentally demonstrated that our new FSRS setup gives rise to a highly-resolved Raman spectrum of the excited trans-stilbene, which is much improved from those reported in the literatures. The spectral resolution of the present FSRS system has been estimated to be the lowest value ever reported to date, giving Δν = 2.5 cm^-1.

Ultrafast Dynamics and Vibrational Relaxation in Six-Coordinate Heme Proteins Revealed by Femtosecond Stimulated Raman Spectroscopy

Identifying the structural rearrangements during photoinduced reactions is a fundamental challenge for understanding from a microscopic perspective the dynamics underlying the functional mechanisms of heme proteins. Here, femtosecond stimulated Raman spectroscopy is applied to follow the ultrafast evolution of two different proteins, each bearing a six-coordinate heme with two amino acid axial ligands. By exploiting the sensitivity of Raman spectra to the structural configuration, we investigate the effects of photolysis and the binding of amino acid residues in cytochrome c and neuroglobin. By comparing the system response for different time delays and Raman pump resonances, we show how detailed properties of atomic motions and energy redistribution can be unveiled. In particular, we demonstrate substantially faster energy flow from the dissociated heme to the protein moiety in cytochrome c, which we assign to the presence of covalent heme–protein bonds.

The Potential of EuPRAXIA@SPARC_LAB for Radiation Based Techniques

A proposal for building a Free Electron Laser, EuPRAXIA@SPARC_LAB, at the Laboratori Nazionali di Frascati, is at present under consideration. This FEL facility will provide a unique combination of a high brightness GeV-range electron beam generated in a X-band RF linac, a 0.5 PW-class laser system and the first FEL source driven by a plasma accelerator. The FEL will produce ultra-bright pulses, with up to 10 12 photons/pulse, femtosecond timescale and wavelength down to 3 nm, which lies in the so called “water window”. The experimental activity will be focused on the realization of a plasma driven short wavelength FEL able to provide high-quality photons for a user beamline. In this paper, we describe the main classes of experiments that will be performed at the facility, including coherent diffraction imaging, soft X-ray absorption spectroscopy, Raman spectroscopy, Resonant Inelastic X-ray Scattering and photofragmentation measurements. These techniques will allow studying a variety of samples, both biological and inorganic, providing information about their structure and dynamical behavior. In this context, the possibility of inducing changes in samples via pump pulses leading to the stimulation of chemical reactions or the generation of coherent excitations would tremendously benefit from pulses in the soft X-ray region. High power synchronized optical lasers and a TeraHertz radiation source will indeed be made available for THz and pump–probe experiments and a split-and-delay station will allow performing XUV-XUV pump–probe experiments.

Probing Dynamics in Higher-Lying Electronic States with Resonance-Enhanced Femtosecond Stimulated Raman Spectroscopy

Femtosecond stimulated Raman scattering (FSRS) measurements typically probe the structural dynamics of a molecule in the first electronically excited state, S₁. While these measurements often rely on an electronic resonance condition to increase signal strength or enhance species selectivity, the effects of the resonance condition are usually neglected. However, mode-specific enhancements of the vibrational transitions in an FSRS spectrum contain detailed information about the resonant (upper) electronic state. Analogous to ground-state resonance Raman spectroscopy, the relative intensities of the Raman bands reveal displacements of the upper potential energy surface due to changes in the bonding pattern upon S _n ← S₁ electronic excitation, and therefore provide a sensitive probe of the ultrafast dynamics in the higher-lying state, S _n. Raman gain profiles from the wavelength-dependent FSRS spectrum of the model compound 2,5-diphenylthiophene (DPT) reveal several modes with large displacement in the upper potential energy surface, including strong enhancement of a delocalized C-S-C stretching and ring deformation mode. The experimental results provide a benchmark for comparison with calculated spectra using time-dependent density functional theory (TD-DFT) and equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD), where the calculations are based on the time-dependent formalism for resonance Raman spectroscopy. The simulated spectra are obtained from S₁-S _n transition strengths and the energy gradients of the upper (S _n) potential energy surfaces along the S₁ normal mode coordinates. The experimental results provide a stringent test of the computational approach, and indicate important limitations based on the level of theory and basis set. This work provides a foundation for making more accurate assignments of resonance-enhanced excited-state Raman spectra, as well as extracting novel information about higher-lying excited states in the transient absorption spectrum of a molecule.

Difference Bands in Time-Resolved Femtosecond Stimulated Raman Spectra of Photoexcited Intermolecular Electron Transfer from Chloronaphthalene to Tetracyanoethylene

The time-resolved femtosecond stimulated Raman spectra (FSRS) of a charge transfer (CT) excited noncovalent complex tetracyanoethylene:1-chloronaphthalene (TCNE:ClN) in dichloromethane (DCM) is reported with 40 fs time resolution. In the frequency domain, five FSRS peaks are observed with frequencies of 534, 858, 1069, 1392, and 1926 cm^-1. The most intense peaks at 534 and 1392 cm^-1 correspond to fundamentals while the features at 858, 1069, and 1926 cm^-1 are attributed to a difference frequency, an overtone and a combination frequency of the fundamentals, respectively. The frequency of the 1392 cm^-1 fundamental corresponding to the central C═C stretch of TCNE^•- is red-shifted from the frequency of the steady state radical due to the close proximity and electron affinity of the countercation. The observation of a FSRS band at a difference frequency is analyzed. This analysis lends evidence for alternative nonlinear pathways of inverse Raman gain scattering (IRGS) or vertical-FSRS (VFSRS) which may contribute to the time-evolving FSRS spectrum on-resonance. Impulsive stimulated Raman measurements of the complex show coherent oscillations of the stimulated emission with frequencies of 153, 278, and 534 cm^-1. The 278 cm^-1 mode corresponds to Cl bending of the dichloromethane solvent. The center frequency of the 278 cm^-1 mode is modulated by a frequency of ∼30 cm^-1 which is attributed to the effect of librational motion of the dichloromethane solvent as it reorganizes around the nascent contact ion pair. The 153 ± 15 cm^-1 mode corresponds to an out-of-plane bending motion of TCNE. This motion modulates the intermolecular separation of the contact ion pair and thereby the overlap of the frontier orbitals which is crucial for rapid charge recombination in 5.9 ± 0.2 ps. High time-frequency resolution vibrational spectra provide unique molecular details regarding charge localization and recombination.

Stimulated Raman Scattering: From Bulk to Nano

Stimulated Raman scattering (SRS) describes a family of techniques first discovered and developed in the 1960s. Whereas the nascent history of the technique is parallel to that of laser light sources, recent advances have spurred a resurgence in its use and development that has spanned across scientific fields and spatial scales. SRS is a nonlinear technique that probes the same vibrational modes of molecules that are seen in spontaneous Raman scattering. While spontaneous Raman scattering is an incoherent technique, SRS is a coherent process, and this fact provides several advantages over conventional Raman techniques, among which are much stronger signals and the ability to time-resolve the vibrational motions. Technological improvements in pulse generation and detection strategies have allowed SRS to probe increasingly smaller volumes and shorter time scales. This has enabled SRS research to move from its original domain, of probing bulk media, to imaging biological tissues and single cells at the micro scale, and, ultimately, to characterizing samples with subdiffraction resolution at the nanoscale. In this Review, we give an overview of the history of the technique, outline its basic properties, and present historical and current uses at multiple length scales to underline the utility of SRS to the molecular sciences.

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.

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.

Energy flow between spectral components in 2D broadband stimulated Raman spectroscopy

We introduce a general theoretical description of non resonant impulsive femtosecond stimulated Raman spectroscopy in a multimode harmonic model. In this technique an ultrashort actinic pulse creates coherences of low frequency modes and is followed by a paired narrowband Raman pulse and a broadband probe pulse. Using closed-time-path-loop (CTPL) diagrams, the response on both the red and the blue sides of the broadband pulse with respect to the narrowband Raman pulse is calculated, the process couples high and low frequency modes, which share the same ground state. The transmitted intensity oscillates between the red and the blue side, while the total number of photons is conserved. The total energy of the probe signal is periodically modulated in time by the coherence created in the low frequency modes.

Femtosecond time-resolved impulsive stimulated Raman spectroscopy using sub-7-fs pulses: Apparatus and applications

We describe details of the setup for time-resolved impulsive stimulated Raman spectroscopy (TR-ISRS). In this method, snapshot molecular vibrational spectra of the photoreaction transients are captured via time-domain Raman probing using ultrashort pulses. Our instrument features transform-limited sub-7-fs pulses to impulsively excite and probe coherent nuclear wavepacket motions, allowing us to observe vibrational fingerprints of transient species from the terahertz to 3000-cm(-1) region with high sensitivity. Key optical components for the best spectroscopic performance are discussed. The TR-ISRS measurements for the excited states of diphenylacetylene in cyclohexane are demonstrated, highlighting the capability of our setup to track femtosecond dynamics of all the Raman-active fundamental molecular vibrations.

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.

On the Resolution Limit of Femtosecond Stimulated Raman Spectroscopy: Modelling Fifth-Order Signals with Overlapping Pulses

Femtosecond stimulated Raman scattering (FSRS) spectroscopy is a powerful pump-probe technique that can track electronic and vibrational dynamics with high spectral and temporal resolution. The investigation of extremely short-lived species, however, implies deciphering complex signals and is ultimately hampered by unwanted nonlinear effects once the time resolution limit is approached and the pulses overlap temporally. Using the loop diagrams formalism we calculate the fifth-order response of a model system and address the limiting case where the relevant dynamics timescale is comparable to the pump-pulse duration and, consequently, the pump and the probe overlap temporally. We find that in this regime, additional diagrams that do not contribute for temporally well separated pulses need to be taken into account, giving rise to new time-dependent features, even in the absence of photoinduced dynamics and for negative delays.

Optimally shaped narrowband picosecond pulses for femtosecond stimulated Raman spectroscopy

A comparison between a Fabry-Pérot etalon filter and a conventional grating filter for producing the picosecond (ps) Raman pump pulses for femtosecond stimulated Raman spectroscopy (FSRS) is presented. It is shown that for pulses of equal energy the etalon filter produces Raman signals twice as large as that of the grating filter while suppressing the electronically resonant background signal. The time asymmetric profile of the etalon-generated pulse is shown to be responsible for both of these observations. A theoretical discussion is presented which quantitatively supports this hypothesis. It is concluded that etalons are the ideal method for the generation of narrowband ps pulses for FSRS because of the optical simplicity, efficiency, improved FSRS intensity and reduced backgrounds.

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.

Primary processes in heme-based sensor proteins

A wide and still rapidly increasing range of heme-based sensor proteins has been discovered over the last two decades. At the molecular level, these proteins function as bistable switches in which the catalytic activity of an enzymatic domain is altered mostly by binding or dissociation of small gaseous ligands (O2, NO or CO) to the heme in a sensor domain. The initial "signal" at the heme level is subsequently transmitted within the protein to the catalytic site, ultimately leading to adapted expression levels of specific proteins. Making use of the photolability of the heme-ligand bond that mimics thermal dissociation, early processes in this intra-protein signaling pathway can be followed using ultrafast optical spectroscopic techniques; they also occur on timescales accessible to molecular dynamics simulations. Experimental studies performed over the last decade on proteins including the sensors FixL (O2), CooA (CO) and soluble guanylate cyclase (NO) are reviewed with an emphasis on emerging general mechanisms. After heme-ligand bond breaking, the ligand can escape from the heme pocket and eventually from the protein, or rebind directly to the heme. Remarkably, in all sensor proteins the rebinding, specifically of the sensed ligand, is highly efficient. This "ligand trap" property possibly provides means to smoothen the effects of fast environmental fluctuations on the switching frequency. For 6-coordinate proteins, where exchange between an internal heme-bound residue and external gaseous ligands occurs, the study of early processes starting from the unliganded form indicates that mobility of the internal ligand may facilitate signal transfer. This article is part of a Special Issue entitled: Oxygen Binding and Sensing Proteins.

Ultrafast-laser-induced backward stimulated Raman scattering for tracing atmospheric gases

By combining tunable broadband pulse generation with the technique of nonlinear spectral compression we demonstrate a prototype scheme for highly selective detection of air molecules by backward stimulated Raman scattering. The experimental results allow to extrapolate the laser parameters required for standoff sensing based on the recently demonstrated backward atmospheric lasing.

Ultrafast conformational dynamics of pyranine during excited state proton transfer in aqueous solution revealed by femtosecond stimulated Raman spectroscopy

Proton transfer reaction plays an essential role in a myriad of chemical and biological processes, and to reveal the choreography of the proton motion intra- and intermolecularly, a spectroscopic technique capable of capturing molecular structural snapshots on the intrinsic time scale of proton transfer motions is needed. The photoacid pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, HPTS) serves as a paradigm case to dissect excited state proton transfer (ESPT) events in aqueous solution, triggered precisely by photoexcitation. We have used femtosecond stimulated Raman spectroscopy (FSRS) to yield novel insights into the ultrafast conformational dynamics of photoexcited HPTS in complex with water and acetate molecules. Marker bands attributed to the deprotonated form of HPTS (1139 cm(-1), ∼220 fs rise) appear earlier and faster than the monomer acetic acid peak (864 cm(-1), ∼530 fs rise), indicating that water molecules actively participate in the ESPT chain. Several key low-frequency modes at 106, 150, 195, and 321 cm(-1) have been identified to facilitate ESPT at different stages from 300 fs, 1 ps, to 6 ps and beyond, having distinctive dynamics contributing through hydrogen bonds with 0, 1, and more intervening water molecules. The time-resolved FSRS spectroscopy renders a direct approach to observe the reactive coupling between the vibrational degrees of freedom of photoexcited HPTS in action, therefore revealing the anharmonicity matrix both within HPTS and between HPTS and the neighboring acceptor molecules. The observed excited state conformational dynamics are along the ESPT multidimensional reaction coordinate and are responsible for the photoacidity of HPTS in aqueous solution.

Efficient spectral shift and compression of femtosecond pulses by parametric amplification of chirped light

We present a method for an efficient spectral shift and compression of pulses from a femtosecond laser system. The method enables generation of broadly tunable (615-985 nm) narrow bandwidth (≈10 cm(-1)) pulses from the femtosecond pulses at 1030 nm. It employs a direct parametric amplification--without spectral filtering--of highly chirped white light by a narrow bandwidth (<5 cm(-1)) 515 nm pump pulse. The system, when pumped with just 200 μJ of the fundamental femtosecond pulse energy, generates pulses with energies of 3-9 μJ and an excellent beam quality in the entire tuning range.