Temperature-dependent vibrational dephasing in molecular crystals: a picosecond cars study of naphthalene

The temperature dependence of picosecond CARS of vibrational excitons in naphthalene is presented. Below ≈40 K relaxation is dominated by spontaneous emission of phonons. Above 40 K high-frequency phonons induce dephasing. The results are consistent with a phonon-promoted energy transfer process.     

General theory of vibrational mode mixing and frequency shifts in resonance raman scattering

A general formalism is given for treating vibrational mode mixing, frequency shifts, and atomic equilibrium position shifts under electronic excitation in resonance Raman scattering. The theory is exact for first-order scattering at T = 0 K for all linear and quadratic electron-phonon coupling strengths. Numerical results illustrating mode mixing are presented.     

A new theory of determination of vibrational raman intensities in crystal lattices

Quantum-chemical methods have been used for the calculation of the intensities of Raman-active internal and external modes in crystalline oxamide. In a first step the frequencies and eigenvectors of the modes in oxamide are calculated by use of the concept of a flexible vibrating molecule within the intermolecular atom-atom forces. The quantum-mechanical aspects are considered and finally, a semiempirical method is extended to cover the relative Raman intensities of the external and internal modes.     

Discrete resonance raman scattering and vibrational relaxation in Ca2 in solid Kr

Raman scattering is found to be strongly enhanced when the exciting laser is in resonance with discrete vibronic transitions of Ca₂ in solid Kr. Relaxation of the high vibrational levels is competitive with the rate of local phonon relaxation, and excitation in the phonon sideband produces no relaxed zero phonon line (ZPL) re-emission from levels above υ = 6.     

Theory of raman scattering from molecules. Summation over vibrational states

Upper and lower bounds of the vibrational sums involved in the polanzability tensor are derived. The lower bound leads to a simplified formula applicable to resonance Raman scattering in both weak and strong coupling cases. A new expansion of the vibrational sum is introduced and useful expressions for excitation profiles are obtained.     

Picosecond time scale optical coherence experiments in mixed molecular crystals: pentacene in naphthalene

Picosecond time scale high-power pulse optical coherence measurements including photon echo and the stimulated photon echo are obtained with a mode-locked dye laser synchronously pumped by a double Q-switched and mode-locked Nd: YAG laser. Effects on coherence arising from excitation with gaussian laser pulses rather than square pulses are examined. Preliminary echo decay measurements are reported.     

Evidence for excited electronic state interference in resonance raman scattering

The complex resonance Raman spectra of molecular bromine have been analyzed quantitatively and a clear demonstration of interference in the Raman intensity from the B(³11₀⁺_u) and ¹17_1u excited states has been found.     

Effects of frequency change on vibronically induced resonance raman scattering

Frequency change effects on vibronically induced resonance Raman scattering (VIRRS) have been theoretically studied. It is shown that the excitation energy profile consists of a succession of doublets. The Raman spectrum of pyrazine resonant to the S₁ state has been calculated using the derived analytical expressions for the cross section.     

Triplet electron spin dephasing in the disordered chemically mixed molecular crystal dichlorobenzene/dibromobenzene

Optically detected spin-locking experiments on the excited triplet state of p-dichlorobenzene (DCB) in p-dibromo-benzene (DBB) single crystals reveal at various DCB concentrations temperature-activated dephasing processes for the DCB monomers and one of the aggregates. The activation energies of 27 and 38 cm⁻¹ correspond to the energy separation of nearest-neighbour host molecules within induced energy funnels. Within the large concentration range 0.0001 to 0.06 mol/mol no influence of concentration on the spin dephasing could be observed.     

Optical dephasing in a glass-like system: a photon echo study of pentacene in benzoic acid

Optical absorption and picosecond photon echo experiments are used to study the dephasing of pentacene in benzoic acid. It is shown that, while the absorption spectrum of pentacene is effected by proton transfer in the benzoic acid dimer, the dephasing is caused by elastic and inelastic phonon scattering processes.     

On the experimental estimation of surface enhanced raman scattering (SERS) cross sections by vibrational pumping

We present an in-depth analysis of the experimental estimation of cross-sections in surface enhanced raman scattering (SERS) by vibrational pumping. The paper highlights the advantages and disadvantages of the technique, pinpoints the main aspects and limitations, and provides the underlying physical concepts to interpret the experimental results. Examples for several commonly used SERS probes are given, and a discussion on future possible developments is also presented. Obtaining good estimates of SERS cross-sections is, in general, an extremely hard problem and has been a longstanding ambition of the SERS community for reasons that go from the purely applied (quantification of signals) to the more fundamental (comparisons of theoretical electromagnetic enhancement factors with experiment). Any method that can produce a standard protocol for the estimation of cross-sections is, accordingly, of great interest and an effort to understand its principles and limitations is required.     

The phase transition and reorientational fluctuations in pyridinium iodide crystal investigated by raman scattering

The order-disorder character of the transition in pyridinium iodide at 247 K is apparent from the relaxation of the zone-centre selection rules in the Raman spectra. Divergence of the librational modes bandwidth suggests that fluctuations in the out-of-plane orientation of the pyridinium ions exist in both the low- and high-temperature phases     

Herzberg-teller effect and intensity distribution in the absorption and raman scattering resonance spectra of polyatomic molecules

This paper studies the influence of the Herzberg-Teller effect on the intensity distribution in one- and two-photon absorption and Raman scattering resonance spectra of substituted benzenes. A quantum mechanical analysis of the intensity distribution in the spectra of phenylacetylene and pyrazine is accomplished using the general approach suggested in [1, 2]. Due to the inclusion of vibronic coupling in the calculation, one can explain the presence of lines (as well as their combinations and overtones) arising from one-quantum excitation of nontotally symmetric vibrations. The calculated data are in good agreement with the experimental data. Significantly, the two-photon absorption spectrum of pyrazine in the region of the¹A_1g→¹B_3u long-wave singlet-singlet transition is explained only by the Herzberg-Teller effect. Saratov State Pedagogical Institute. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, pp. 304–309, March–April, 1995. Translated by L. Smolina     

Moderate-resolution two-photon spectrum of the naphthalene crystal and naphthalene in durene

The two-photon absorption spectra at ～ 5 cm^?1 resolution are presented for the naphthalene crystal and naphthalene in durene at 2°K in polarized light. A rich vibronic spectrum of the ¹B_2u(3200 Å) naphthalene state is observed and some of the strongest vibronic origins are from in-plane vibrations.     

Derivation from coupled channel equations of the resonance raman scattering amplitude for a diatomic molecule

The coupled equations of molecular collision theory can be formulated in such a way as to yield the amplitude for resonance Raman scattering of light by a diatomic molecule. Two variants of the method can be adopted, characterized by different choices for the potential matrices. All molecular potentials, either analytical or numerical, are treated on the same footing. Tests of accuracy are presented for the case of harmonic potentials intersected by linear potentials, for which an exact analytical solution can be given.     

Normal mode analysis of Pyrococcus furiosus rubredoxin via nuclear resonance vibrational spectroscopy (NRVS) and resonance raman spectroscopy

We have used (57)Fe nuclear resonance vibrational spectroscopy (NRVS) to study the Fe(S(cys))(4) site in reduced and oxidized rubredoxin (Rd) from Pyrococcus furiosus (Pf). The oxidized form has also been investigated by resonance Raman spectroscopy. In the oxidized Rd NRVS, strong asymmetric Fe-S stretching modes are observed between 355 and 375 cm(-1); upon reduction these modes shift to 300-320 cm(-1). This is the first observation of Fe-S stretching modes in a reduced Rd. The peak in S-Fe-S bend mode intensity is at approximately 150 cm(-1) for the oxidized protein and only slightly lower in the reduced case. A third band occurs near 70 cm(-1) for both samples; this is assigned primarily as a collective motion of entire cysteine residues with respect to the central Fe. The (57)Fe partial vibrational density of states (PVDOS) were interpreted by normal mode analysis with optimization of Urey-Bradley force fields. The three main bands were qualitatively reproduced using a D(2)(d) Fe(SC)(4) model. A C(1) Fe(SCC)(4) model based on crystallographic coordinates was then used to simulate the splitting of the asymmetric stretching band into at least 3 components. Finally, a model employing complete cysteines and 2 additional neighboring atoms was used to reproduce the detailed structure of the PVDOS in the Fe-S stretch region. These results confirm the delocalization of the dynamic properties of the redox-active Fe site. Depending on the molecular model employed, the force constant K(Fe-S) for Fe-S stretching modes ranged from 1.24 to 1.32 mdyn/A. K(Fe-S) is clearly diminished in reduced Rd; values from approximately 0.89 to 1.00 mdyn/A were derived from different models. In contrast, in the final models the force constants for S-Fe-S bending motion, H(S-Fe-S), were 0.18 mdyn/A for oxidized Rd and 0.15 mdyn/A for reduced Rd. The NRVS technique demonstrates great promise for the observation and quantitative interpretation of the dynamical properties of Fe-S proteins.     

A study of alpha-helix hydration in polypeptides, proteins, and viruses using vibrational raman optical activity

A vibrational Raman optical activity (ROA) study, supplemented by protein X-ray crystal structure data, of alpha-helices in polypeptides, proteins, and viruses has suggested that ROA bands in the extended amide III spectral region may be used to distinguish between two types of right-handed alpha-helix. One type, associated with a positive ROA band at approximately 1300 cm(-1), dominates in hydrophobic environments and appears to be unhydrated; the other, associated with a positive ROA band at approximately 1340 cm(-1), dominates in hydrophilic environments and appears to be hydrated. Evidence is presented to support the hypothesis that unhydrated alpha-helix corresponds to the canonical conformation alpha(c) and hydrated alpha-helix to a more open conformation alpha(o) stabilized by hydrogen bonding of a water molecule or a hydrophilic side chain to the peptide carbonyl. Alpha-helical poly(L-lysine) and poly(L-ornithine) in aqueous solution and poly(L-alanine) in dichloracetic acid display both bands, but alpha-helical poly(l-glutamic acid) in aqueous solution and poly(gamma-benzyl L-glutamate) in CHCl(3) display only the approximately 1340 cm(-1) band and so may exist purely as alpha(o) due to enhanced stabilization of this conformation by particular side chain characteristics. The ROA spectrum of poly(beta-benzyl L-aspartate) in CHCl(3) reveals that it exists in a single left-handed alpha-helical state more analogous to alpha(o) than to alpha(c).     

A study of the resonance nonlinear scattering of silver atomic clusters

The Ag cluster in solution has been prepared by a microwave high-pressure procedure. There is a maximum resonance Rayleigh scattering (RRS) peak at 425 nm (7.06 x 10(14) Hz) and a smaller RRS peak at 850 nm (1/2 x 7.06 x 10(14) Hz) by synchronous scattering at lambda(ex) = lambda(em). It is a nonlinear optical medium. When scattering the emission wavelength (lambda(em)) as the excited wavelength (lambda(ex)) at 425 nm (7.06 x 10(14) Hz), a maximum RRS peak was observed at 425 nm (7.06 x 10(14) Hz). A 1/2 fraction scattering peak at 850 nm (1/2 x 7.06 x 10(14) Hz) was also investigated. When lambda(ex) is at 850 nm (3.53 x 10(14) Hz), a frequency-doubling scattering peak at 425 nm is observed, and a RRS peak is displayed at 850 nm (3.53 x 10(14) Hz). When the lambda(ex) is two times longer than the maximum RRS wavelength (the strongest wavelength of pump band lambda(pb)) of an atomic cluster, a nonlinear scattering frequency-doubling peak (lambda(pb)) exhibits at half of the lambda(ex), which is stronger than the RRS peak. This phenomenon is defined as pump effect.     

Pulse radiolytic study of the triplet excited state of naphthalene in mixed solvents

The triplet excited state of naphthalene (³N^*) has been produced by irradiating solutions of naphthalene in mixtures of isopropanol-formamide, acetonitrile-formamide and N,N-dimethylformamide — formamide mixtures with 2 microsecond electron pulses. The yield and decay kinetics of the triplet have been studied over a wide range of compositions of the mixtures. An attempt has been made to explain the variation in the yields and decay rate constants on the basis of the micro properties of the solvent mixtures.