Raman Overtone Research Articles

Insight into deposition duration-varied crystallinity, optical parameters, morphology and electrical properties of solution-processed nanostructured CdSSe thin films

Nanostructured cadmium sulpho selenide (CdSSe) films have been grown onto glass substrates for different deposition durations via a facile solution-based method known as chemical bath deposition (CBD). XRD studies revealed that the crystallites in the films possessed hexagonal (wurtzite) structures with predominant orientation along the (002) plane. The crystallite size increased from ∼5.95 nm to ∼16.69 nm with the increase in deposition duration from 1 h to 4 h. The absorbance, transmittance and reflectance measurements were carried out. A transparency of ∼(50–80) % was obtained for the films. The direct optical energy band gap was found to decrease from 1.982 eV to 1.952 eV with the increment of deposition duration from 1 h to 4 h. SEM micrographs of the films revealed the presence of nano spherical-shaped grains with an average size ranging from ∼305 nm to ∼612 nm. HRTEM images illustrated the presence of compact nano-spherical grains and nano-crystallites distributed in random orientations. Raman spectroscopy divulged the presence of CdSe and CdS vibrational Raman modes and overtones in the films. PL spectra revealed a prominent red emission band along with weak luminescent bands in the yellow-red emission region. The presence of Cd–Se and Cd–S bonds in the films was validated using FTIR analysis. Electrical investigation revealed the linear current‒voltage relationship for the films with the rise in conductivity with deposition duration. Hence, deposition duration stands out as a vital parameter for modulating the properties of CdSSe thin films for use in photovoltaic devices and optoelectronics.

Synchrotron-based UV resonance Raman spectroscopy probes size confinement, termination effects, and anharmonicity of carbon atomic wires

Vibrational and electronic properties and anharmonicity of sp-carbon atomic wires (i.e., polyynes) have been studied using resonance Raman spectroscopy, focusing on the confinement effects connected to their structure (length and termination). We exploited the fine tunability of the synchrotron radiation to resonantly excite short H-, CH3-, and CN-capped carbon atomic wires at their vibronic transitions in the deep UV. We report for the first time the resonance Raman spectra of size-selected CH3-capped polyynic wires (HCnCH3, n = 8–12) and CN-capped wires (HC6CN and HC12CN). We observed that the degree of π-electron conjugation increases with the wire length and is related to the specific termination. The analysis of multiple quanta Raman overtone bands shows that the anharmonic correction of the vibrational potential energy becomes increasingly relevant as the wire length increases. Finally, studying the vibronic lines of the UV absorption spectra allows the determination of an effective displacement parameter between the minima of the ground and the low-lying excited state, proving that the strong electron-phonon coupling of these systems is sensitive to chain length and termination.

Nitrogen Doping of Carbon Nanotubes Synthesized in Flowing Acetylene and Ammonia

A method has been demonstrated for doping carbon nanotubes (CNTs) with nitrogen during synthesis in an acetylene + ammonia atmosphere at different temperatures in the range 550–750°C, and conditions have been found for effective nitrogen doping of CNTs depending on synthesis temperature. The concentrations of various elements in the synthesized CNTs have been determined by X-ray photoelectron spectroscopy. Raman measurements with the use of a Raman microscope have made it possible to follow variations in the relative intensities of the G and D bands as functions of nitrogen dopant concentration in the CNTs. We have detected strong Raman overtone combination satellites whose parameters depend on the structural perfection of the synthesized CNTs.

Dye-enhanced third upconversion Raman overtone in the ultraviolet region under intense excitation of a 532 nm laser

We report, to our best knowledge, the first observation of enhanced third upconversion Raman overtone of water in the ultraviolet region 240-380nm by Rhodamine 6G (R6G) under intense excitation of a 532nm laser. Its spectral peak is mostly around 288nm. This ultraviolet spectrum has also been obtained for purified water on the Raman spectrometer LRS-3 and fluorescence spectrometer RF-5301PC, which have photomultiplier tubes. The spectral profile is approximately a triangle in most cases. Moreover, an intense and broad background is also observed to appear in the spectrum. Additionally, the conventional visible luminescence spectrum of R6G is discovered to coexist with the ultraviolet one. In comparison, the visible spectrum is much more intense.

Preparation of a selected high vibrational energy level of isolated molecules.

Stark induced adiabatic Raman passage (SARP) allows us to prepare an appreciable concentration of isolated molecules in a specific, high-lying vibrational level. The process has general applicability, and, as a demonstration, we transfer nearly 100 percent of the HD (v = 0, J = 0) in a supersonically expanded molecular beam of HD molecules to HD (v = 4, J = 0). This is achieved with a sequence of partially overlapping nanosecond pump (355 nm) and Stokes (680 nm) single-mode laser pulses of unequal intensities. By comparing our experimental data with our theoretical calculations, we are able to draw two important conclusions: (1) using SARP a large population (>1010 molecules per laser pulse) is prepared in the (v = 4, J = 0) level of HD and (2) the polarizability α00,40 (≅0.6 × 10-41 C m2 V-1) for the (v = 0, J = 0) to (v = 4, J = 0) Raman overtone transition is only about five times smaller than α00,10 for the (v = 0, J = 0) to (v = 1, J = 0) fundamental Raman transition. Moreover, the SARP process selects a specific rotational level in the vibrational manifold and can prepare one or a phased linear combination of magnetic sublevels (M states) within the selected vibrational-rotational level. This capability of preparing selected, highly excited vibrational levels of molecules under collision-free conditions opens new opportunities for fundamental scattering experiments.

The 2ν3 Raman overtone of sulfur hexafluoride: Absolute spectra, pressure effects, and polarizability properties

Of the six normal vibrations of SF6, ν3 has a key role in the mechanisms of radiative forcing. This vibration, though inactive in Raman, shows up through the transition 2ν3 allowing for a complementary view on the asymmetric stretch of the molecule. Here, we look back into this topic, which has already caught some interest in the past but with some points been left out. We make a systematic incoherent-light-scattering analysis of the overtone with the use of different gas pressures and polarization orientations for the incident beam. Absolute-scale isotropic and anisotropic spectra are reported along with natural and pressure-induced widths and shifts, and other spectral features such as the peaks corresponding to the (experimentally indistinguishable) interfering channels Eg and F2g hitherto seen solely as two-photon IR-absorption features. We make the first-ever prediction of the SF6 polarizability second derivative with respect to the ν3-mode coordinate and we develop a heuristic argument to explain why the superposition of the three degenerate stretching motions that are related to the ν3 mode cannot but generate a polarized Raman band.

More light on the 2(ν5) Raman overtone of SF6: can a weak anisotropic spectrum be due to a strong transition anisotropy?

Long known as a fully polarized band with a near vanishing depolarization ratio [ηs = 0.05, W. Holzer and R. Ouillon, Chem. Phys. Lett. 24, 589 (1974)], the 2ν5 Raman overtone of SF6 has so far been considered as of having a prohibitively weak anisotropic spectrum [D. P. Shelton and L. Ulivi, J. Chem. Phys. 89, 149 (1988)]. Here, we report the first anisotropic spectrum of this overtone, at room temperature and for 13 gas densities ranging between 2 and 27 amagat. This spectrum is 10 times broader and 50 times weaker than the isotropic counterpart of the overtone [D. Kremer, F. Rachet, and M. Chrysos, J. Chem. Phys. 138, 174308 (2013)] and its profile much more sensitive to pressure effects than the profile of the isotropic spectrum. From our measurements an accurate value for the anisotropy matrix-element |⟨000020|Δα|000000⟩| was derived and this value was found to be comparable to that of the mean-polarizability 000020α¯000000. Among other conclusions our study offers compelling evidence that, in Raman spectroscopy, highly polarized bands or tiny depolarization ratios are not necessarily incompatible with large polarizability anisotropy transition matrix-elements. Our findings and the way to analyze them suggest that new strategies should be developed on the basis of the complementarity inherent in independent incoherent Raman experiments that run with two different incident-beam polarizations, and on concerted efforts to ab initio calculate accurate data for first and second polarizability derivatives. Values for these derivatives are still rarities in the literature of SF6.

Resonance Raman overtones reveal vibrational displacements and dynamics of crystalline and amorphous poly(3-hexylthiophene) chains in fullerene blends

Resonance Raman spectra of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester blend thin films display progressions of overtone and combination bands (up to two harmonics) involving the dominant symmetric C=C backbone stretching mode of P3HT that encode excited state vibrational displacements and dynamics information. Contributions from both crystalline (aggregated) and amorphous (unaggregated) P3HT domains are resolved and intensities are analyzed using the time-dependent theory of spectroscopy. Raman spectra, excitation profiles, and absorption spectra are simulated with the same parameters using a single electronic state description for each P3HT form. Time-dependent wavepacket overlaps expose vibrational coherence on sub-100 fs timescales, which is usually difficult to extract from conventional ultrafast pump-probe spectra and transients of polymer∕fullerene blends. The results demonstrate the potential of simpler CW resonance Raman approaches to uncover excited state geometry changes and early vibrational dynamics from distinct morphological forms in polymer∕fullerene blends.

Resonance Raman Overtone Intensities and Electron–Phonon Coupling Strengths in Semiconductor Nanocrystals

For linear electron-phonon coupling, the Huang-Rhys factor, S, gives the intensity ratio of the one-quantum vibronic transition (0 → 1) to the purely electronic origin transition (0 → 0) in a vibrationally resolved, zero-temperature absorption or emission spectrum. It is often assumed that the overtone to fundamental integrated intensity ratio in resonance Raman scattering of semiconductor nanocrystals is equal to or proportional to S, or that S may be determined from the overtone intensity in some other straightforward manner. In fact, this is not generally possible because of different excitation profiles for overtones and fundamentals, differential sensitivity of overtones and fundamentals to electronic dephasing, and interference effects from partially overlapping electronic transitions. Here we examine the relationship between the Huang-Rhys factor and the overtone to fundamental intensity ratio through spectroscopic simulations using parameters appropriate to II-VI semiconductor nanocrystals such as CdSe. A simple equation relating the overtone to fundamental Raman intensity ratio to the Huang-Rhys factor is obtained only in the case of a single resonant electronic state, excitation at the maximum of the inhomogeneously broadened absorption band, and a homogeneous line width small compared with the phonon frequency.

Thickness and stacking geometry effects on high frequency overtone and combination Raman modes of graphene

We investigate two high frequency Raman overtone and combination modes of graphene named 2D' and 2D + G bands, and located at ~3240 and ~ 4260 cm–1, respectively. The graphene thickness and stacking geometry effects for these two modes are systematically studied. The features of the 2D' band, which arises from intravalley double resonance, are not sensitive to the variation of thickness with single Lorentzian peak and fixed linewidth. We explain it theoretically by calculating the phonon dispersion mode in k‐space and find that the flat band region of longitudinal optical phonon near Γ point is the mechanism leading to the 2D' band nonsplit. With the thickness increasing, the band position exhibits blueshift and the linewidth increases for the 2D + G band. With changing thickness and stacking geometry of graphene, the intensities of these two high‐frequency bands show obvious different evolution compared with that of G band. Copyright © 2012 John Wiley & Sons, Ltd.

Probing the stiffness of the simplest double hydrogen bond: The symmetric hydrogen bond modes of jet-cooled formic acid dimer

Formic acid dimer is held together and kept planar by two strong hydrogen bonds, which give rise to intermolecular vibrations. Raman active fundamentals, overtones, and combination bands involving out-of-plane bending and stretching vibrations of the hydrogen bonds are recorded under jet-cooled, vacuum-isolated conditions between 100 and 750 cm(-1) and assigned with the help of isotope substitution. Individual anharmonicity effects are shown to be very small (x(i,j) = -(1+/-2) cm(-1)), where they are accessible by experiment. However, they may accumulate to substantial differences between harmonic and anharmonic fundamental excitations. Preliminary experimental evidence for the most elusive fundamental vibration of formic acid dimer, symmetric OH torsion, is presented. A rigorous experimental reference frame for existing and future high level quantum chemical and dynamical treatments of this important prototype system is provided. The effects of clustering beyond the dimer on the low frequency dynamics are found to be small, whereas argon coating gives rise to blueshifts.

Determination of Exciton-Phonon Coupling Elements in Single-Walled Carbon Nanotubes by Raman Overtone Analysis

Raman excitation profiles are obtained and compared for carbon nanotube radial breathing mode (RBM) fundamental and overtone vibrations for 5 specific chiralities. Fitting of the Raman excitation data is performed using Raman transform theory. The Huang-Rhys factors obtained from the modeling are directly related to the magnitude of the RBM exciton-phonon coupling element, which is shown to be in a weak coupling limit. The values of exciton-phonon coupling strengths and the possible role of revealed non-Condon effects are in agreement with quantum-chemical modeling.

Contribution of the asymmetric stretch, ν3B1, to the fundamental Raman spectrum of water

The OD-stretching overtone from liquid D2O, 2nu, and the fundamental OD stretch from dilute HDO, both display high-frequency depolarization ratio minima, but the fundamental OD stretch from neat D2O displays a maximum, at the equivalent position. The rhoL minima arises from the decreased depolarization ratio produced by the absence of B1 modes. The fundamentals of HDO are of A species, and the 2nu overtone of D2O only involves A1 species, e.g., 2nu3B1 has A1 species via B1 x B1 = A1. A and A1 modes display small rhoL values which produce minima in rhoL near 2665 cm(-1) for HDO, and near 5250 cm(-1) for the D2O overtone. These minima give way to a depolarization ratio maximum when the depolarized, rhoL = 34, nu3B1 fundamental, makes its appearance in D2O at 2650 cm(-1). Fundamental and overtone depolarization ratios were used to determine the nu3B1 contribution to the depolarization ratio of the fundamental OD stretch; a value of approximately 28% resulted at 2655 cm(-1). Liquid H2O displays completely analogous features; a value of approximately 20% resulted for it at 3660 cm(-1). Nonhydrogen-bonded nu3B1, and more strongly hydrogen-bonded nu3B1, modes are also indicated for D2O and H2O. A rigorous test of the current results can be accomplished by measuring the depolarization ratio of the extraordinarily weak second Raman overtone, 3nu, recently detected for D2O.

Raman Combinations and Stretching Overtones from Water, Heavy Water, and NaCl in Water at Shifts to ca. 7000 cm-1

Photographic Raman spectra were obtained at shifts to ca. 7000 cm−1 for pure water and for a saturated aqueous solution of NaCl using argon ion laser excitation. Raman spectra were also obtained photoelectrically for H2O and D2O between ca. 2500 and ca. 7000 cm−1 using 248-nm excimer laser excitation and boxcar detection. Overtone and combination assignments are presented for H2O and D2O. The first IR OH-stretching overtone from water occurs 215 cm−1 above the first Raman OH-stretching overtone because the IR overtones are dominated by asymmetric stretching. The second OH-stretching Raman overtone from water is estimated to occur near 10,020 ± 20 cm−1, with 9950 cm−1 as a lower limit.

Fifth-order two-dimensional Raman spectroscopy: A new direct probe of the liquid state

The experimental challenges of performing high-order non-linear spectroscopies have been met using diffractive optics to allow passive phase locking of all six interacting laser fields and true phase-sensitive detection. Improvements in signal to noise, use of phase contrast, as well as geometrical phase matching and polarization have made it possible to isolate systematically the pure nuclear fifth-order Raman response. Using CS2 as a model system of a simple liquid, the two-time correlation of the probed liquid modes or bath memory function is found to decay faster than the free-induction decay associated with one-dimensional spectroscopic probes of the same modes. This observation is in sharp contrast to other two-dimensional spectroscopies and is related to the unique application of a two-quantum transition or Raman overtone for the rephasing pathway. Both theory and experiment have converged on this point, as well as a pronounced ridge along the probe time axis that is related to population decay of the excited modes. Recent advances in theoretical treatments of the correlation function have shown this spectroscopy to contain a wealth of information imbedded in the specific form of the two-dimensional spectrum. The extremely sensitive nature of this experiment stems from the involvement of a Raman overtone that gives the experiment direct access to the all important anharmonic terms in the intermolecular potential. As such, this form of spectroscopy harbours great promise to provide a rigorous benchmark for developing liquid state theories. The experimental details, current state of understanding of the experiment, interpretation and pitfalls, as well as an overview of the various theoretical efforts are given. The area is at a critical cross-road in advancing the spectroscopy to other liquids and associated complex systems. Some speculations on what the future holds are given in this context. The onus is clearly on experimentalists to advance this method and new technologies will be needed to do so--in which directly probing the dynamical structure of liquid water is the ultimate challenge.

Diffractive optics-based six-wave mixing: Heterodyne detection of the full χ(5) tensor of liquid CS2

This work exploits the passive phase stabilization of diffractive optics to implement heterodyne detection of the complete χ(5) tensor of liquid CS2 as an example of a simple liquid. This approach permits the use of two different colors for the excitation, probe, and detection beam protocols and enables full optimization of the signal with respect to discrimination against lower order cascaded third-order responses. This work extends the previous study of polarization selectivity, in combination with heterodyne detection, to all independent polarization components to provide further insight into the origins of the fifth-order response and its connection to the multitime correlation of the liquid dynamics. The characteristic feature that clearly distinguishes the direct fifth-order response from lower order cascades is the pronounced ridge along the τ4 axis (probe pulse delay) with very rapid decay along the τ2 axis (excitation pulse delay). This observation is in contrast to recent related work using one-color homodyne detection. With the determination of the direct fifth-order and cascaded third-order signal amplitudes made possible by heterodyne detection, this difference can be attributed to cross terms between the direct fifth-order and cascaded third-order terms inherent to homodyne detection under phase matching conditions used to discriminate against cascades. In support of theoretical treatments, the previously predicted enhancement of rephasing pathways for certain polarization components has been observed. However, even for these tensor elements the remarkable feature is the very rapid decay in the nuclear coherence along τ2. The experiment is predicated on the ability of a 2-quantum transition involving the Raman overtone to rephase the nuclear coherence. These findings indicate that the nuclear motions, in the frequency range accessed, are strongly damped and draw into question the validity of the overtone as a viable pathway for rephasing. With the isolation of the direct fifth-order Raman response, new information regarding relaxation and dephasing pathways in liquids can be determined for the highest frequency modes. The results are in very good agreement with a recent finite field molecular dynamics simulation of liquid CS2 with respect to the polarization dependence of signal magnitudes, relative cascade signal amplitudes, and qualitative agreement with respect to the predicted temporal profiles.

Protein Tuning of Excited-State Charge-Transfer Dynamics in Azurin

Resonance Raman spectra of azurin from Alcaligenes denitrificans have been measured at wavelengths throughout the S(Cys) → Cu charge-transfer absorption band to examine the effects of structure and environment on the excited-state charge-transfer dynamics. Measurement of the depolarization ratios suggest that the Raman scattering is enhanced by a single electronic transition. Self-consistent analysis of the absorption spectrum and the resonance Raman fundamental, overtone, and combination bands yields a total inner-sphere reorganization energy of 0.26 ± 0.02 eV, similar to that for Pseudomonas aeruginosa. The reorganization energy is distributed along similar vibrational modes in the two azurins, although the contribution from each mode is different. To understand these differences, molecular comparisons of the known structures and molecular modeling of the electrostatic field within the protein were performed. The results show that the two proteins have very similar structures, although the copper site itself has a slightly more tetrahedral geometry in A. denitrificans. The resonance Raman spectral differences between these two azurins are interpreted to arise from this copper site structural difference and a greater distribution of charges and/or dipoles near the coordinating histidines in A. denitrificans.

Resonance Raman analysis of charge-transfer reorganization energies in a covalent dicyanoethylene-aza-adamantane

Resonance Raman cross sections have been measured for a covalent electron donor-acceptor substituted aza-adamantane. Excitation near the maximum of the strong charge-transfer absorption results in resonance enhancement of a number of Raman fundamentals, overtones, and combination bands. The reorganization energy in each molecular mode and the solvent contribution to the electronic linewidth are determined through quantitative modeling of the absorption, Raman, and fluorescence spectra, which are sensitive to solvent dynamics on different time scales. The results suggest that the solvent-induced broadening in methanol has components that fluctuate on a wide range of timescales.

Ultrafast quantum dynamics and resonance Raman spectroscopy of photoexcited I2(B) in large argon and xenon clusters

The early quantum dynamics following the B(3Π0u+)←X photoexcitation of I2 in large rare gas clusters is studied and the resonance Raman spectrum of these systems is calculated by a novel time-dependent quantum mechanical simulation approach. The method used is the classically based separable potential (CSP) approximation, in which classical molecular dynamics simulations are used in a first step to determine an effective time-dependent separable potential for each mode, then followed by quantum wavepacket calculations using these potentials. In the simulations for I2(Ar)n and I2(Xe)n, with n=17, 47, all the modes are treated quantum mechanically. The Raman overtone intensities are computed from the multidimensional time-dependent wavepacket for each system, and the results are compared with experimental data on I2 in Ar matrices and in liquid Xe. The main findings include: (i) Due to wavepacket dephasing effects the Raman spectra are determined well before the iodine atoms hit the rare gas ‘‘wall’’ at about 80 fs after photoexcitation. (ii) No recurrencies are found in the correlation functions for I2(Ar)n. A very weak recurrence event is found for I2(Xe)n. (iii) The simulations for I2(Ar)17 (first solvation layer) and for I2(Ar)47 (second solvation shell) show differences corresponding to moderate cluster size effects on the Raman spectra. (iv) It is estimated that coupling to the B″(1Π1u) state or to the a(1g) state have a small effect on the Raman intensities. (v) For I2(Ar)47, the results are in very good quantitative agreement with I2/Ar matrix experiments. The I2(Xe)n results are in qualitative agreement with experiments on I2 in liquid Xe. The reported calculations represent a first modeling of resonance Raman spectra by quantum dynamical simulations that include all degrees of freedom in large systems, and they demonstrate the power of the CSP method in this respect.

Femtosecond photodissociation dynamics of nitroethane and 1-nitropropane in the gas and solution phases from resonance Raman intensity analysis

We have obtained resonance Raman spectra of nitroethane and 1-nitropropane in the gas phase and in cyclohexane solution with excitation within the broad B-band absorption around 200 nm. The resonance Raman spectra are dominated by a long overtone progression in the nominal NO2 symmetric stretch vibrational mode. The initial short-time (femtosecond) photodissociation dynamics of the excitedelectronic state associated with the B-band absorption changes mainly in the two N–O bond lengths in the Franck–Condon region. Time-dependent wavepacket calculations have been carried out to simulate both the absorption spectra and the resonance Raman intensities. These calculations show that very fast predissociation competes with wavepacket motion out of the Franck–Condon region, and or there is a significant change in the transition dipole moment along the nominal NO2 symmetric stretch coordinate, that must be taken into account to correctly model the higher overtone resonance Raman intensities. Since the resonance Raman intensity patterns and calculation parameters are very similar for both gas and solution phase nitroethane and 1-nitropropane, electronic dephasing due tosolvent collisions does notappear tobe a significantfactor in determining the resonance Raman overtone intensities up to v = 9.