Stimulated Raman Spectra Research Articles

Ultrafast Hydrogen-Bonding Dynamics in the Electronic Excited State of Photoactive Yellow Protein Revealed by Femtosecond Stimulated Raman Spectroscopy

The ultrafast structural dynamics in the electronic excited state of photoactive yellow protein (PYP) is studied by femtosecond stimulated Raman spectroscopy. Stimulated Raman spectra in the electronic excited state, S(1), can be obtained by using a Raman pump pulse in resonance with the S(1)-S(0) transition. This is confirmed by comparing the experimental results with numerical calculations based on the density matrix treatment. We also investigate the hydrogen-bonding network surrounding the wild-type (WT)-PYP chromophore in the ground and excited states by comparing its stimulated Raman spectra with those of the E46Q-PYP mutant. We focus on the relative intensity of the Raman band at 1555 cm(-1), which includes both vinyl bond C═C stretching and ring vibrations and is sensitive to the hydrogen-bonding network around the phenolic oxygen of the chromophore. The relative intensity for the WT-PYP decreases after actinic excitation within the 150 fs time resolution and reaches a similar intensity to that for E46Q-PYP. These observations indicate that the WT-PYP hydrogen-bonding network is immediately rearranged in the electronic excited state to form a structure similar to that of E46Q-PYP.

High‐resolution stimulated Raman spectroscopy and analysis of the ν1/ν5 (C–H) stretching dyad of C2H4

The high‐resolution stimulated Raman spectra of the ν1/ν5 C–H stretching bands of C2H4 have been recorded and analyzed by means of the tensorial formalism developed in Dijon for X2Y4 asymmetric‐top molecules. A total of 689 lines (428 for ν5 and 261 for ν1) were assigned and fitted as a dyad including Coriolis coupling constants. We obtained a global root mean square deviation of 4.39 × 10− 3 cm− 1 (4.61 × 10− 3 cm− 1 for ν1, 4.25 × 10− 3 cm− 1 for ν5). The nearby 2ν2 band, extrapolated from ν2, was included in the analysis. However, no interaction parameter involving it could be fitted. The analysis is quite satisfactory, although some parts of ν5 are not very well reproduced, probably indicating some yet unidentified resonances. This region is indeed quite dense, with many interacting dark states that cannot be included at present. Copyright © 2012 John Wiley & Sons, Ltd.

Laser-initiated iodine radical chemistry in single microdroplets

Iodine radical reactions in single free-falling microdroplets of iodododecane, initiated using UV laser photolysis, are probed using Raman spectroscopy. Stimulated Raman spectra, with 532nm laser excitation, are recorded at varying time delays from the UV pulse. I atom recombination reactions lead to I2 that changes the optical properties of the microdroplet ultimately quenching the Raman signal. This quenching is observed over ∼10ns, which is about the time resolution of the two-laser experiment. Although the kinetics are too rapid to be measured in current laser configuration, it demonstrates that radical kinetics can be followed in single microdroplets.

Scanning cavity-enhanced droplet spectroscopy: Tuning of the excitation laser for obtaining a continuous Raman spectrum

Scanning cavity-enhanced droplet spectroscopy (S-CEDS) has been developed to obtain Raman spectra of liquid media with high sensitivity. Cavity-enhanced Raman spectra of a water droplet are measured by irradiation of a laser with nine different wavelengths. Each of the spectra consists of a series of intense and narrow peaks resulting from the cavity-enhancement conditions, and the envelope of their peak apexes displays a stimulated Raman spectrum of liquid water. A series of continuous Raman spectra of liquid water is obtained by scanning the laser wavelength, which dissolves the discreteness of each spectrum. S-CEDS provides a novel method to obtain stimulated Raman spectra of liquid media keeping their continuous spectral shapes.

Polarized IR and Raman spectra of Ca2MgSi2O7, Ca2ZnSi2O7 and Sr2MgSi2O7 single crystals: Temperature-dependent studies of commensurate to incommensurate and incommensurate to normal phase transitions

IR and Raman spectra of Ca2MgSi2O7, Ca2ZnSi2O7 and Sr2MgSi2O7 oriented single crystals and powders have been measured. The temperature dependence of phonons has been established in the 4–650K range. Discussion of the results has been performed on the basis of factor group approach for the tetragonal P4̄21m (D2d3) space group with Z=2. The assignment of the bands observed to the internal and external modes has been made on their polarization behaviour, phonon energy calculations and literature data. The evidences of the LO–TO splitting and commensurate (C) to incommensurate (IC) as well as incommensurate (IC) to normal (N) phase transitions have been found. As a representative example, the results obtained for the spontaneous Raman scattering have also been used in the discussion of the stimulated Raman spectra of Ca2ZnSi2O7.

Collisional line widths of autoperturbed N 2: Measurements and quantum calculations

In this work we present new experimental and theoretical values for the line broadening coefficients of the Q-branch Raman lines of autoperturbed N 2. For the experimental determination of the coefficients, high resolution stimulated Raman spectra of the Q-branch of N 2 at different pressures were obtained at 77, 194 and 298 K. Simultaneously, quantum dynamical calculations, performed on two potential energy surfaces, were carried out for the system between 77 and 298 K, rendering a set of theoretical line broadening coefficients that could be directly compared to those obtained from the present measurements and the previous ones. Within the limit of considering the colliding molecules distinguishable we discuss the ortho and para contributions to the pressure broadening cross-sections. Because such calculations are time consuming we indicate routes to circumvent this difficulty. We observe a reasonable agreement between theoretical and experimental values of the collisional line broadening coefficients at all the studied temperatures.

The ν5 band of 12CH 313CH 3: High resolution stimulated Raman spectrum and global three band analysis

Rotationally resolved spectrum of 12CH 3 13CH 3 in the region of ν 5 vibrational fundamental (C C stretch) was observed using stimulated Raman spectroscopy. This spectrum was analyzed with data from the ν 12 fundamental and transitions from the ν 6, 2 ν 6– ν 6, and 3 ν 6 torsional bands using a 3-state fit. One torsional component of the ν 5 fundamental is perturbed, interacting with its partner in the ν 6 = 4 of the torsional stack of the ground vibrational state. As for normal ethane, the coupling was successfully modeled using a Fermi-type interaction. The results mirror that of 12CH 3 12CH 3 in that the inclusion of the Fermi-type interaction reduces the required number of terms in the Fourier expansion of the torsional potential for the ground vibrational state from three (in the 2-state fit) to one, only the term in the barrier height is required.

Polarized IR and Raman spectra, temperature dependence of phonons and lattice dynamic calculations for M′2M″Ge2O7 pyrogermanates (M′ = Sr, Ba; M″ = Mg, Zn)

AbstractInfrared and Raman spectra of the polycrystalline samples and single crystals of Sr2ZnGe2O7, Ba2ZnGe2O7, Ba2MgGe2O7 and Sr2MgGe2O7 were measured. The temperature dependence of phonons was studied in the range 4–295 K. The discussion of the results is based on the factor group approach for the tetragonal $P\overline{4}2_{1} m (D_{2d}^{3})$ space group with Z = 2. The discussion of the internal vibrations of the Ge2O7 unit and external modes is made on the basis of the literature data and phonon calculations. The results obtained for the spontaneous Raman scattering were used in the discussion of the stimulated Raman spectra of the studied materials. Copyright © 2010 John Wiley & Sons, Ltd.

Femtosecond Stimulated Raman Spectroscopy of Flavin after Optical Excitation

In blue-light photoreceptors using flavin (BLUF), the signaling state is formed already within several 100 ps after illumination, with only small changes of the absorption spectrum. The accompanying structural evolution can, in principle, be monitored by femtosecond stimulated Raman spectroscopy (FSRS). The method is used here to characterize the excited-state properties of riboflavin and flavin adenine dinucleotide in polar solvents. Raman modes are observed in the range 90-1800 cm(-1) for the electronic ground state S(0) and upon excitation to the S(1) state, and modes >1000 cm(-1) of both states are assigned with the help of quantum-chemical calculations. Line shapes are shown to depend sensitively on resonance conditions. They are affected by wavepacket motion in any of the participating electronic states, resulting in complex amplitude modulation of the stimulated Raman spectra. Wavepackets in S(1) can be marked, and thus isolated, by stimulated-emission pumping with the picosecond Raman pulses. Excited-state absorption spectra are obtained from a quantitative comparison of broadband transient fluorescence and absorption. In this way, the resonance conditions for FSRS are determined. Early differences of the emission spectrum depend on excess vibrational energy, and solvation is seen as dynamic Stokes shift of the emission band. The nπ* state is evidenced only through changes of emission oscillator strength during solvation. S(1) quenching by adenine is seen with all methods in terms of dynamics, not by spectral intermediates.

Two-dimensional femtosecond stimulated Raman spectroscopy: Observation of cascading Raman signals in acetonitrile

A new methodology for two-dimensional Raman spectroscopy-termed two-dimensional femtosecond stimulated Raman spectroscopy (2D-FSRS)-is presented and experimental results for acetonitrile are discussed. 2D-FSRS can potentially observe molecular anharmonicity by measuring the modulation of the frequency of a probed Raman mode, at frequency omega(hi), by the coherent motion of an impulsively driven mode, at frequency omega(low). In acetonitrile, the signal is generated by driving the CCN bend (379 cm(-1)) and CC stretch (920 cm(-1)) into coherence via impulsive stimulated Raman scattering and subsequently probing the stimulated Raman spectrum of the CC stretch, the CN stretch (2250 cm(-1)) and the CH stretch (2942 cm(-1)). The resultant signal can be generated by two alternative mechanisms: a fifth-order Raman process that would directly probe anharmonic coupling between the two modes, or a third-order cascade in which a third-order coherent Raman process produces a field that goes on to participate in a third-order stimulated Raman transition. The third-order cascade is shown to dominate the 2D-FSRS spectrum as determined by comparison with the predicted magnitude of the two signals, the 2D spectrum of a mixed isotope experiment, and the concentration dependence of the signal. In acetonitrile, theoretical calculations of the vibrational anharmonicity indicate that the third-order cascade signal should be 10(4) times larger than the fifth-order Raman signal. 2D-FSRS signals are observed between acetonitrile's CCN bend, of E symmetry, and several different A(1) modes but are forbidden by symmetry in the fifth-order pathway. A 2D-FSRS spectrum of a 50:50 mixture of acetonitrile and d(3)-acetonitrile shows equivalent intensity for intramolecular coupling peaks and intermolecular coupling peaks, indicating that the observed signal cannot be probing molecular anharmonicity. Finally, the magnitudes of the 2D-FSRS peaks are observed to be proportional to the square of the number density, supporting the cascade mechanism.

Polarised IR and Raman spectra of non‐centrosymmetric Na3Li(MoO4)2· 6H2O crystal—a new Raman laser material

AbstractPolarised IR and Raman spectra of Na3Li(MoO4)2· 6H2O single crystal were measured. Discussion of the results is based on the factor group approach for the trigonal R 3c(C3v6) space group with Z = 2. The assignment of the observed bands was performed on the basis of their polarisation behaviour and literature data. The obtained results for the spontaneous Raman scattering were used in the analysis of the stimulated Raman spectra of the material studied—a new Raman laser crystal. The promoting modes of the stimulated effect were identified. Copyright © 2009 John Wiley & Sons, Ltd.

Lineshape analysis of stimulated Raman spectra of the near‐nozzle region of free jet expansions

AbstractAn anomalous lineshape of stimulated Raman spectra obtained from the region very close to the nozzle of supersonic pulsed expansions of nitrogen is presented. High‐resolution Raman spectra of the Q branch of the fundamental vibration mode of N2 have been recorded from two different nitrogen expansions at T0 = 295 K and P0 = 1.5–3.5 bar, the lasers crossing the jet axis in the range z/D = 0.25–1.25, where D is the effective nozzle diameter. The combination of Doppler shifts and strong gradients of density and temperature in the near‐nozzle region yield an inhomogeneous broadening and a double peak structure of the recorded Raman line profiles. The comparison of the experimental results with the simulation of the Raman spectrum from this region provides valuable information about the near‐nozzle flow field. The lineshape described here is different from another reported previously in the literature, which is based on a depletion of the density of free molecules on the axis due to condensation. Copyright © 2009 John Wiley & Sons, Ltd.

High‐resolution stimulated Raman spectroscopy and analysis of the 2ν10 overtone symmetric motion of C2H4

AbstractThe high‐resolution stimulated Raman spectrum of the 2ν10 band located at 1664.16 cm−1 of C2H4 has been reanalyzed, thanks to the tensorial formalism developed in Dijon for X2Y4 asymmetric‐top molecules. A total of 191 lines were assigned and fitted as a single band without including perturbations such as Fermi or Coriolis coupling constants. We obtained a global root mean square deviation of 8.5 × 10−3 cm−1. Further investigations are required to include interactions with the ν2 and ν7 + ν10 bands. Copyright © 2009 John Wiley & Sons, Ltd.

Origin of negative and dispersive features in anti-Stokes and resonance femtosecond stimulated Raman spectroscopy

Femtosecond stimulated Raman spectroscopy is extended to probe ground state anti-Stokes vibrational features. Off resonance, negative anti-Stokes features are seen that are the mirror image of the positive Stokes side spectra. On resonance, the observed dispersive lineshapes are dramatically dependent on the frequencies of the picosecond pump and femtosecond probe pulses used to generate the stimulated Raman spectra. These observations are explained by the contributions of the inverse Raman and hot luminescence four-wave mixing processes discussed by Sun et al. [J. Chem. Phys. 128, 144114 (2008)], which contribute to the overall femtosecond stimulated Raman signal.

Polarized Raman and IR spectra of non‐centrosymmetric PbB4O7 single crystal

AbstractPolarized Raman and IR spectra of a PbB4O7 single crystal were measured. The obtained spectra are discussed within the factor group approach for the orthorhombic P21nm(C2v7) space group with Z = 2 assuming that the crystal structure is built up of the (B4O7)∞2− framework and Pb2+ ions. It has been shown that vibrations of borate and Pb2+ units are observed above 240 and below 160 cm−1, respectively. The results obtained for the spontaneous Raman scattering have also been used in the discussion of the stimulated Raman spectra of the material studied—a new Raman‐laser crystal. The obtained results revealed that mainly translational motions of Pb2+ ions participate efficiently in the SRS effect. Copyright © 2008 John Wiley & Sons, Ltd.

Polarised IR and Raman spectra of non-centrosymmetric Na 3Li(SeO 4) 2·6H 2O crystal—A new Raman laser material

Polarised IR and Raman spectra of Na 3Li(SeO 4) 2·6H 2O single crystal have been recorded. Discussion of the results has been based on the factor group approach for the trigonal R3c ( C 3v 6) space group with Z = 2. The obtained results for the spontaneous Raman scattering have been used in the discussion of the stimulated Raman spectra of the material studied—a new Raman laser crystal.

Femtosecond Stimulated Raman Study of Energy Transfer in Allophycocyanin

Allophycocyanin (APC) is a light-harvesting protein found in cyanobacteria and red algae. APC along with other phycobiliproteins such as phycocyanin (PC) and phycoerythrin (PE) constitutes phycobilisome and plays an important role in collecting light and cascading the excitation energy into a reaction center where the light energy is converted into chemical potential. Thus it is important to understand how excitation energy is transferred between the phycobiliproteins and inside them. APC is a trimer of αβ subunit, each of which contains a linear tetrapyrrole chromophore, phycocyanobilin (PCB), covalently bound to a cystein residue (Fig. 1). Close proximity (~20 A) between α-84 and nearby β-84 in APC can cause the electronic states of the chromophores to interact to form split exciton states. In this case, interexciton dynamics mediates the energy transfer. On the other hand, it is also possible that energy transfer from α-84 to β-84 occurs via dipole-dipole coupling between the two chromophores (Forster model). The intermediate range of distance between the PCBs in APC makes both models quite feasible for an energy transfer mechanism and has generated much controversy. Here we report on the excited state dynamics of APC measured by femtosecond stimulated Raman spectroscopy. Femtosecond stimulated Raman spectroscopy (FSRS) reveals the structure of fast-evolving molecules by providing vibrational spectra with excellent spectral resolution on a femtosecond time scale. We prepare the electronic excited state of APC by 40 nJ, 30 fs actinic pump pulse at 1 kHz, centered at 610 nm. The femtosecond stimulated Raman spectra of the excited state APC are obtained at a series of time delays by coupling 0.5 μJ/pulse, 2 ps, 800 nm Raman pump pulse and 4 nJ/pulse, 20 fs, broadband continuum Raman probe pulse. The detailed description of the detection system and how to generate the laser pulses has been published elsewhere. APC was isolated from the filamentous cyanobacterium Anabaena variabilis and used in phosphate buffer solution. Figure 2 presents the FSRS spectra of the ground state and the excited state APC at selected time delays. The excited state spectra were obtained by subtracting the spectra of buffer solution and the ground state APC which accounts for the unexcited 85% of APC. The ground state FSRS spectrum is in excellent agreement with the resonance Raman spectra previously reported at UV wavelengths. The excited state spectra show broad and dispersive features without any well-defined peaks. The amplitude of the dispersive features increases up to ~80 fs and decreases gradually, completely gone by 5 ps (not shown). McCamant et al. found that dispersive lineshapes appear when the Raman pump and probe pulses drive resonant stimulated emission from the excited state and create vibrational coherence on the electronic ground state surface, which they termed the Raman initiated by nonlinear emission (RINE). The RINE spectra contain the information on the vibrational frequencies of the electronic ground state whose dynamics is determined by the evolution of the excited state population. From this model, McCamant et al. successfully extracted the vibrational spectra of the ground state bacteriorhodopsin, which evolve with the population changes in the excited state. Adopting a similar method described in ref. 14, we fit the excited state FSRS spectra of APC to the RINE model. From

Vibrational Spectroscopy and Dynamics in the CH-Stretch Region of Fluorene by IVR-Assisted, Ionization-Gain Stimulated Raman Spectroscopy

We report stimulated Raman spectra at 0.2 and 0.03 cm(-1) resolution in the CH-stretching region of jet-cooled fluorene. The results were obtained by a version of ionization-gain stimulated Raman spectroscopy in which resonant two-photon ionization probing of the state-population changes arising from stimulated Raman transitions is assisted by the process of intramolecular vibrational redistribution (IVR) in the Raman-excited molecule. The fluorene spectra reveal extensive vibrational coupling interactions involving both the aliphatic and aromatic CH-stretching first excited states with nearby background states. Results pertaining to the symmetric aliphatic CH-stretching fundamental are consistent with a tier model of IVR and point to vibrational energy flow out of the CH stretch on a approximately 1 ps time scale with subsequent redistribution on a approximately 5 ps time scale.

Wave packet theory of dynamic stimulated Raman spectra in femtosecond pump-probe spectroscopy

The quantum theory for stimulated Raman spectroscopy from a moving wave packet using the third-order density matrix and polarization is derived. The theory applies, in particular, to the new technique of femtosecond broadband stimulated Raman spectroscopy (FSRS). In the general case, a femtosecond actinic pump pulse first prepares a moving wave packet on an excited state surface which is then interrogated with a coupled pair of picosecond Raman pump pulse and a femtosecond Raman probe pulse and the Raman gain in the direction of the probe pulse is measured. It is shown that the third-order polarization in the time domain, whose Fourier transform governs the Raman gain, is given simply by the overlap of a first-order wave packet created by the Raman pump on the upper electronic state with a second-order wave packet on the initial electronic state that is created by the coupling of the Raman pump and probe fields acting on the molecule. Calculations are performed on model potentials to illustrate and interpret the FSRS spectra.

High‐resolution stimulated Raman spectroscopy and analysis of the ν1 stretching band of GeD4

AbstractThe high‐resolution stimulated Raman spectrum of the ν1 band of GeD4 with natural isotopic abundance germanium has been recorded. It has been analyzed as part of the ν1/ν3 stretching dyad. The ν1 and ν3 band centers have been deduced for all the isotopologues. Copyright © 2006 John Wiley & Sons, Ltd.