The importance of ground-state vibronic mixing in molecular Raman scattering

We consider Albrecht's theory for Raman scattering of fundamentals in the far and pre-resonance regions. Destructive interferences inherent to the A and B terms augment the conventionally suppressed C term dramatically. Raman excitation profiles for the ν₁(a_1g) and ν₆(e_2g fundamentals in benzene can be well fitted with theoretical C-term profiles involving the ¹E_1u(π-π^*) state at 1800 A.     

Single dibenzoterrylene molecules in naphthalene and 2,3-dimethylnaphthalene crystals: vibronic spectra

Fluorescence excitation spectra of single dibenzoterrylene (DBT) molecules embedded in naphthalene (N) and 2,3-dimethylnaphthalene (2,3-DMN) crystals were studied at 5 K. The frequencies characterizing the vibronic structure of single DBT molecules in an N crystal agree with the theoretical prediction for the isolated DBT molecule. The 'dipolar' disorder encountered in 2,3-DMN crystals leads to a broad distribution of frequencies of the (0,0) lines of single DBT molecules. Moreover, the observed vibronic frequencies and intensities in the spectrum of DBT in 2,3-DMN crystals are slightly different to those in an N crystal. We conclude that the structure of DBT molecules in a 2,3-DMN crystal is disturbed in comparison with isolated DBT and the main change concerns its central tetracene moiety.     

Vibronic coupling in naphthalene anion: vibronic coupling density analysis for totally symmetric vibrational modes

Vibronic coupling, or electron-phonon coupling, of naphthalene is calculated. A method of vibronic coupling density analysis, which has been proposed for the vibronic coupling of the Jahn-Teller active modes in a Jahn-Teller molecule, is extended for totally symmetric vibrational modes of a molecule including a non-Jahn-Teller molecule. Contrary to non-totally-symmetric modes, orbital relaxation upon a charge transfer plays a crucial role in the vibronic coupling calculation for the totally symmetric modes. The method is applied for the ground state of the naphthalene anion to compare with that of the benzene anion. The relationship between the vibronic coupling density and a nuclear Fukui function is also discussed.     

Photostability of electronically excited polyacenes: a case study of vibronic coupling in the naphthalene radical cation

Novel issues of electronic nonadiabatic coupling in the excited state dynamics of prototypical naphthalene radical cation of polycyclic aromatic hydrocarbon of the polyacene family are theoretically investigated. A benchmark ab initio quantum dynamical study is performed and its complex vibronic spectra and nonradiative decay are examined. The findings are in very good accord with the experiment, unambiguously establishing the crucial role of intricate electron-nuclear coupling in the photoinduced dynamical processes of this system.     

Vibronic coupling in benzene cation and anion: vibronic coupling and frontier electron density in Jahn-Teller molecules

Vibronic coupling constants of Jahn-Teller molecules, benzene radical cation and anion, are computed as matrix elements of the electronic part of the vibronic coupling operator using the electronic wave functions calculated by generalized restricted Hartree-Fock and state-averaged complete active space self-consistent-field methods. The calculated vibronic coupling constants for benzene cation agree well with the experimental and theoretical values. Vibronic coupling density analysis, which illustrates the local properties of the coupling, is performed in order to explain the order of magnitude of the coupling constant from view of the electronic and vibrational structures. This analysis reveals that the couplings of the e2g2 and e2g3 modes in which the large displacements locate on C-C bonds are strong in the cation. On the other hand, they are greatly weakened in the anion because of the decrease of electron density in the region of the C-C bonds, which originates from the antibonding nature of the singly occupied molecular orbital of the anion. However, the difference of the electronic structure has a little influence on the vibronic coupling of the e2g4 mode. These results indicate that the vibronic coupling depends not only on the direction of the nuclear displacement but also on the frontier electron density.     

Single molecule level detection of allophycocyanin by surface enhanced resonance Raman scattering

Single molecule level detection of the near-infrared fluorescent protein allophycocyanin (APC) has been achieved using surface enhanced resonance Raman scattering (SERRS). The detection limit using the peak height of the 440 cm(-1) band was 1 x 10(-13) mol l(-1), compared to 2 x 10(-12) mol l(-1) for the fluorescence peak at 660 nm.     

Single particle scattering and momentum distributions in solid hydrogen

The incoherent inelastic neutron scattering function of solid para-hydrogen was recorded. It was shown that at momentum transfers above 5Å⁻¹ the data can be interpreted as recoil spectra from single particle scattering at hydrogen molecules. The evaluation of the momentum distribution and kinetic energy of the scattering particles from such data is discussed.     

Raman scattering in GeS2 glass and its crystalline polymorphs compared

Raman spectrum of glassy GeS₂ and low-resolution ones of polycrystalline α- and β-GeS₂ were studied. It was shown that the medium range structure of glassy GeS₂ is similar to the three-dimensional structure of β-GeS₂. Our conclusion of similarity of medium range order of glassy GeS₂ and β-GeS₂ was also confirmed by detection of β-GeS₂ microcrystals grown from glassy GeS₂ at annealing temperature sufficiently below glass transition temperature.     

Single vibronic level fluorescence and relaxation spectra of s-tetrazine-d0 and d2 vapors

The third strongest cold band in the 5500Å absorption spectrum of s-tetrazine vapor is assigned to 6b₀² by analysis of single-vibronic-level (SVL) fluorescence spectra obtained with a tunable cw dye laser. Fermi resonance model calculations for the doublet 6a₀¹…6b₀² simulate the perturbations of relative intensities in ν_6a^″ fluorescence progressions which arise fron interference between the two terms of the Franck-Condon factors. Addition of a quartic anharmonic term to the quadratic ν_6b^′ potential accounts for the observed spacing of the observed 6b¹ and 6b² levels and suggests a likely candidate for the heretofore missing 6b₀⁴ member of the Fermi resonance triad. Weak bands with in-plane polarization are revealed in fluorescence and contribute to a list of newly measured frequencies of the ground state. Experiments with added n-pentane show that rotational relaxation proceeds at the hard-sphere rate while vibrational relaxation is about five times slower. Vibrational relaxation exhibits non-statistical distribution in the early stages; it is rather distinctive when starting from the zero-level of the excited state, but is reminiscent of earlier work (on benzene) when starting from the 16b¹ level. Identification of the level 6b¹ is supported by these relaxation experiments.     

Resonance Raman scattering of rhodamine 6G as calculated by time-dependent density functional theory: vibronic and solvent effects

The geometries, UV-vis absorption spectra, and resonance Raman (RR) intensities have been determined for the S1 and S3 excited states of rhodamine 6G (R6G) in vacuum and ethanol by means of DFT/TDDFT methodologies with the aim of better understanding the structures and properties of the excited states. The RR spectra have been simulated from the vibronic theory of RR scattering as well as within the short-time approximation, while the solvent effects have been modeled using the polarizable continuum model. The S1 and S3 states of R6G present UV-vis absorption bands with similar vibronic structure, i.e., a shoulder at smaller wavelengths, although this shoulder is relatively more intense and more sensitive to the solvent in the case of S3. These differences are corroborated by the larger geometry relaxations upon excitation for S3 and the fact that the charge transfer of S3 is reduced in ethanol. Moreover, the differences between S1 and S3 are magnified when considering the RR spectra. On one hand, the RR spectrum of R6G in resonance with the S0 --> S1 transition presents many transitions of which the relative intensities strongly vary when the excitation wavelength gets closer to the maximum of absorption. The RR spectrum of R6G in resonance with S1 is however little influenced by the solvent. On the other hand, the RR spectrum of R6G in resonance with the S0 --> S3 transition displays only a few bands, strongly depends on the solvent, and is little affected when changing the excitation wavelength within the limits of the absorption band. As a consequence, the short-time approximation is suitable to reproduce the RR spectrum of R6G in resonance with S3 for a broad range of excitation wavelengths, whereas the vibronic theory approach is needed for describing the RR spectrum of R6G in resonance with S1 close to resonance.     

Surface enhanced Raman scattering of benzotriazole: a molecular orientational study

Surface enhanced Raman scattering of benzotriazole in aqueous silver sol at pH approximately 9 has been investigated. The spectra of the molecule at various concentrations were recorded and the maximum enhancement was observed with a 10(-5) M solution. From the comparison of the surface enhanced Raman spectra with the conventional Raman spectra and applying "surface selection rule", it is inferred that the molecular ion interacts with the silver sol via the nitrogen atoms of the benzotriazole ring and the molecule assumes a near "end-on" orientation.     

Surface-enhanced Raman scattering: a new optical probe in molecular biophysics and biomedicine

Sensitive and detailed molecular structural information plays an increasing role in molecular biophysics and molecular medicine. Therefore, vibrational spectroscopic techniques, such as Raman scattering, which provide high structural information content are of growing interest in biophysical and biomedical research. Raman spectroscopy can be revolutionized when the inelastic scattering process takes place in the very close vicinity of metal nanostructures. Under these conditions, strongly increased Raman signals can be obtained due to resonances between optical fields and the collective oscillations of the free electrons in the metal. This effect of surface-enhanced Raman scattering (SERS) allows us to push vibrational spectroscopy to new limits in detection sensitivity, lateral resolution, and molecular structural selectivity. This opens up exciting perspectives also in molecular biospectroscopy. This article highlights three directions where SERS can offer interesting new capabilities. This includes SERS as a technique for detecting and tracking a single molecule, a SERS-based nanosensor for probing the chemical composition and the pH value in a live cell, and the effect of so-called surface-enhanced Raman optical activity, which provides information on the chiral organization of molecules on surfaces.     

Lifetime measurements of two-photon excited vibronic levels in the low pressure limit: naphthalene

Single vibronic lifetimes of two-photon excited states are measured in the low pressure limit for naphthalene. The two-photon spectrum, besides yielding new states in S₁, is also free of the interference from the neighborhood of the S₂, known from one-photon experiments, and hence unperturbed lifetimes can be measured to higher excess energies. The overall pattern of lifetimes with excess energy including both one-photon and two-photon states shows that the vibronic structure is subordinate to an overall monotonic decrease, much as found earlier for β-naphthylamine. This indicates, at least for naphthalene, that radiationless processes into triplets are dominated by Franck—Condon factors and not by vibronic inductions and promoting modes.     

Single vibronic level photochemistry of NO2 in the 2491 Å system

A photochemical decomposition study of NO₂(B?²B₂) in specified vibronic states is reported. The O(¹D)-atom yields below and above the dissociation limit (40973 cm^?1) have been measured. The role played by the rotational excitation energy and the vibrational excitation modes in promoting the predissociation is discussed.     

Experimental IR and Raman spectra and quantum chemical studies of molecular structures, conformers and vibrational characteristics of L-ascorbic acid and its anion and cation

IR and spectra of the L-ascorbic acid (L-AA) also known as vitamin C have been recorded in the region 4000-50 cm(-1). In order to make vibrational assignments of the observed IR and Raman bands computations were carried out by employing the RHF and DFT methods to calculate the molecular geometries and harmonic vibrational frequencies along with other related parameters for the neutral L-AA and its singly charged anionic (L-AA(-)) and cationic (L-AA(+)) species. Significant changes have been found for different characteristics of a number of vibrational modes. The four ν(O-H) modes of the L-AA molecule are found in the order ν(O(9)-H(10))>ν(O(19)-H(20))>ν(O(7)-H(8))>ν(O(14)-H(15)) which could be due to complexity of hydrogen bonding in the lactone ring and the side chain. The CO stretching wavenumber (ν(46)) decreases by 151 cm(-1) in going from the neutral to the anionic species whereas it increases by 151 cm(-1) in going from the anionic to the cationic species. The anionic radicals have less kinetic stabilities and high chemical reactivity as compared to the neutral molecule. It is found that the cationic radical of L-AA is kinetically least stable and chemically most reactive as compared to its neutral and anionic species.     

Single molecular detection of a perylene dye dispersed in a Langmuir-Blodgett fatty acid monolayer using surface-enhanced resonance Raman scattering

The Langmuir-Blodgett (LB) monolayer technique was used to fabricate single molecule LB monolayer containing bis(phenethylimido)perylene (PhPTCD), a red dye dispersed in arachidic acid (AA) with an average doping of 1 molecule per microm2. The monolayer was transferred onto Ag island films to obtain spatially resolved surface-enhanced resonance Raman scattering (SERRS) spectra. The mixed LB monolayers were fabricated with a concentration, on average, of 1, 6, 19 and 118 PhPTCD molecules per microm2 in AA. The AA provides a two-dimensional host matrix whose background signal does not interfere with the detection of the probe molecule's SERRS signal. The properties of the single molecule detection were investigated using micro-Raman with a 514.5-nm laser line. The Ag island surfaces coated with the LB monolayer were mapped with spatial steps of 3 microm and global chemical imaging of the most intense SERRS band in the spectrum was also recorded. The SERRS and surface-enhanced fluorescence (SEF) of the neat and single molecule LB monolayer were recorded in a temperature range from liquid nitrogen to + 200 degrees C. Neat PhPTCD LB monolayer spectra served as reference for the identification of characteristic signatures of the single molecule behavior. The spatial resolution of Raman-microscopy experiments, the multiplicative effect of resonance Raman and SERRS, and the high sensitivity of the new dispersive Raman instruments, allow SERRS to be part of the family of single molecular spectroscopies.     

The production and spectroscopy of molecular ions isolated in solid neon

Studies of the molecular spectra of small polyatomic molecular ions are stlll in therr infancy. The availability of survey spectra would facilitate the search for individual vibrational and vibronic bands of gas-phase ion species using sophisticated, highly sensitive laser techniques. Deeectable concentrations not only of simpee ions such as CO⁺ ₂ and H⁺ ₂ but also of dimer cations and anions such as O⁺ ₄ and O^- ₄ have been stabilized in solid neon when the parett molecule is codeposited with a beam of excited neon atoms. The vibrational fundamentals heretofore observed for the simpee ions isolated in solid neon lie very close to the positions of the corresponding gas-phase band centers. Molecular spectroscopic data are not yet available for the gas-phase dimer ions. Therefore, it is difficutt to estimate how closely the neon-matrix vibrational frequencies here reported for a numbrr of dimer ions correspond to the gas-phase band centers. Some guidance is available from comparison of the argon-matrix spectra of small molecules hydrogen-bonded to HF with the gasphaee spectra of theee hydrogen-bonded species [55]. In theee studies, the effect of the argon matrix is to enhance the apparent hydrogen-bond strength; the HF stretching frequency is lowered by 1 to 5% from the gas-phase value, and the absorptions contributed by the flexing of the HF with respect to the other molecule are raised by 5% or more. Since neon matrices are generally less perturbing than argon matrices, the deviation from the gas-phase frequencies shoudd be somewhat less in neon-matrix studies. In the present experiments, only the high frequency stretching fundamentals have been observed, suggesting that matrix shifss shoudd amoutt to less than 3 or 4%. Therefore, matrix isolation studies such as theee promise to provide a valuable new tool for the detection and spectroscopic characterization of small molecular ions and cluster ions.     

Single vibronic level emission spectroscopic studies of the ground state energy levels and molecular structures of jet-cooled HGeBr, DGeBr, HGeI, and DGeI

Single vibronic level dispersed fluorescence spectra of jet-cooled HGeBr, DGeBr, HGeI, and DGeI have been obtained by laser excitation of selected bands of the A (1)A(")-X (1)A(') electronic transition. The measured ground state vibrational intervals were assigned and fitted to anharmonicity expressions, which allowed the harmonic frequencies to be determined for both isotopomers. In some cases, lack of a suitable range of emission data necessitated that some of the anharmonicity constants and vibrational frequencies be estimated from those of HGeClDGeCl and the corresponding silylenes (HSiX). Harmonic force fields were obtained for both molecules, although only four of the six force constants could be determined. The ground state effective rotational constants and force field data were combined to calculate average (r(z)) and approximate equilibrium (r(e) (z)) structures. For HGeBr r(e) (z)(GeH)=1.593(9) A, r(e) (z)(GeBr)=2.325(21) A, and the bond angle was fixed at our CCSD(T)/aug-cc-pVTZ ab initio value of 93.6 degrees . For HGeI we obtained r(e) (z)(GeH)=1.589(1) A, r(e) (z)(GeI)=2.525(5) A, and bond angle=93.2 degrees . Franck-Condon simulations of the emission spectra using ab initio Cartesian displacement coordinates reproduce the observed intensity distributions satisfactorily. The trends in structural parameters in the halogermylenes and halosilylenes can be readily understood based on the electronegativity of the halogen substituent.