CH Stretch Research Articles

Vibrational Coupling Pathways in Methanol As Revealed by Coherence-Converted Population Transfer Fourier Transform Microwave Infrared Double-Resonance Spectroscopy

Coherence-converted population transfer infrared-microwave double-resonance spectroscopy is used to record the infrared spectra of jet-cooled CH(3)OH and CH(3)OD. Population transfer induced by a pulsed IR laser is detected by Fourier transform microwave spectroscopy background-free using a two-MW pulse sequence. The observed spectrum of CH(3)OH in the nu(3) symmetric CH stretch region contains 12 interacting vibrational bands, whereas in CH(3)OD, only one vibrational band is observed in the same interval (2750-2900 cm(-1)). The bright state, responsible for the transitions observed in this region, is not just nu(3) but also contains an admixture of the binary CH bending combinations, particularly 2nu(5). The lack of interacting bands in CH(3)OD confirms that in CH(3)OH the binary combinations of the OH bend (nu(6)) and a CH bend (nu(4), nu(5), nu(10)) act as doorway states linking the bright state to higher order combination vibrations involving torsional excitation. A time-dependent interpretation of the frequency-resolved spectra reveals a fast (approximately 200 fs) initial decay of the bright state followed by a slower (1-2 ps) redistribution among the lower frequency modes.

Molecular Restructuring of Water and Lipids upon the Interaction of DNA with Lipid Monolayers

Understanding the molecular mechanism of DNA/lipid interaction is critical in optimizing the use of lipid cofactors in gene therapy. Here, we address this question by employing label-free vibrational sum frequency (VSF) spectroscopy to study the interaction of DNA with lipid monolayers of the cationic lipids DPTAP(1,2-dipalmitoyl-3-trimethylammonium-propane) and diC14-amidine as well as the zwitterionic lipid DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) in the presence and absence of calcium. Our approach has the advantage both of allowing us to explicitly probe intermolecular interactions and of providing insight into the structure of water and lipids around DNA at the lipid interface. We find, by examination of the OD stretch of interfacial D(2)O, that water structure differs markedly between systems containing DNA adsorbed to cationic and those that contain DNA adsorbed to zwitterionic lipid monolayers (in the presence or absence of Ca(2+)). The spectral response of interfacial water in the cationic system is consistent with a highly structured, undercoordinated, structural 'type' of water. Further, by investigation of CH stretch modes of the diC14-amidine lipid tails, we demonstrate that the adsorption of DNA to this lipid leads to increased ordering of lipid tails.

Phospholipid Monolayers Probed by Vibrational Sum Frequency Spectroscopy: Instability of Unsaturated Phospholipids

The surface specific technique vibrational sum frequency spectroscopy has been applied to in situ studies of the degradation of Langmuir monolayers of 1,2-diacyl-phosphocholines with various degrees of unsaturation in the aliphatic chains. To monitor the degradation of the phospholipids, the time-dependent change of the monolayer area at constant surface pressure and the sum frequency intensity of the vinyl CH stretch at the carbon-carbon double bonds were measured. The data show a rapid degradation of monolayers of phospholipids carrying unsaturated aliphatic chains compared to the stable lipids carrying fully saturated chains when exposed to the ambient laboratory air. In addition, the degradation of the phospholipids can be inhibited by purging the ambient air with nitrogen. This instability may be attributed to spontaneous degradation by oxidation mediated by various reactive species in the air. To further elucidate the process of lipid oxidation in biological membranes artificial Langmuir monolayers probed by a surface specific spectroscopic technique as in this study can serve as a model system for studying the degradation/oxidation of cell membrane constituents.

(HCN)m−Mn (M = K, Ca, Sr): Vibrational Excitation Induced Solvation and Desolvation of Dopants in and on Helium Nanodroplets

Infrared (IR) laser spectroscopy is used to probe the rotational and vibrational dynamics of the (HCN)(m)-M(n) (M = K, Ca, Sr) complexes, either solvated within or bound to the surface of helium nanodroplets. The IR spectra of the (HCN)(m)-K (m = 1-3), HCN-Sr, and HCN-Ca complexes have the signature of a surface species, similar to the previously reported spectra of HCN-M (M = Na, K, Rb, Cs) [Douberly, G. E.; Miller, R. E. J. Phys. Chem. A 2007, 111, 7292.]. A second band in the HCN-Ca spectrum is assigned to a solvated complex. The relative intensities of the two HCN-Ca bands are droplet size dependent, with the solvated species being favored in larger droplets. IR-IR double resonance spectroscopy is used to probe the interconversion of the two distinct HCN-Ca populations. While only a surface-bound HCN-Sr species is initially produced, CH stretch vibrational excitation results in a population transfer to a solvated state. Complexes containing multiple HCN molecules and one Sr atom are surface-bound, while the nu(1) (HCN)(2)Ca spectrum has both the solvated and surface-bound signatures. All HCN-(Ca,Sr)(n) (n > or = 2) complexes are solvated following cluster formation in the droplet. Density-functional calculations of helium nanodroplets interacting with the HCN-M show surface binding for M = Na with a binding energy of 95 cm(-1). The calculations predict a fully solvated complex for M = Ca. For M = Sr, a 2.2 cm(-1) barrier is predicted between nearly isoenergetic surface binding and solvated states.

Thermal Confinement and Temperature-Dependent Absorption in Resonant Infrared Ablation of Frozen Liquid Targets

The mechanism of matrix-assisted resonant infrared laser ablation of frozen aqueous and methanol solutions of polyethylene glycol was investigated by time-resolved plume shadowgraphy and ablation yield measurements. A picosecond free-electron laser was tuned to two wavelengths resonant with the target matrixes, one (2940 nm) that is resonant with the −OH stretch in both liquid water and methanol and the other (3450 nm) that is resonant with the −CH stretch in methanol. The plume images showed gross similarities, differing only in the time required for the shockwave to appear and in the velocity of the shock front. Primary material ejection typically commences 15−25 μs after the ablation laser pulse arrives and lasts for hundreds of μs. In all three cases studied here, the ablation plume appears to consist entirely of vapor with no droplets or solid particles. The ablation yield is found to be linear in fluence for frozen methanol but quadratic in fluence for frozen water. This dependence can be understood...

Blue shifted hydrogen bond in 3-methylindole·CHX3 complexes (X = Cl, F)

Although the first experimental report on the blue shifted hydrogen bond in gas phase appeared about a decade ago, not many examples have been reported in the literature thereafter. Computational studies on systems exhibiting such blue shifted hydrogen bond however have been abundant since then. Many of these theoretical predictions remain to be verified experimentally. In this work we present an example of blue shifted hydrogen bond observed in the weakly bound complexes of 3-methylindole and CHX(3) (X = F, Cl). The complexes were prepared using the supersonic jet expansion method and studied using the laser spectroscopic methods. The key findings were that these complexes exhibit C-Hpi type hydrogen bonding interaction and the CH is bound to the phenyl part of the aromatic plane. The CH stretch was found to be blue shifted by 2 and 16 cm(-1) in the case of CHCl(3) and CHF(3), respectively. Ab initio calculations along with atoms-in-molecule analysis and natural bond orbital analysis support the experimental findings. The computed results at the DFT/MP2 level also indicated that the IR intensity of the H-bond donor CH-stretch increases by two to three orders of magnitude for the CHCl(3) complex whereas for the fluoroform complex the same decreases by an order of magnitude, which are consistent with the trend reported in the case of C-HO type of blue shifting hydrogen bonds.

Micro-Spectroscopy of Biomolecules and Cells at Variable Pressure in a Micro-Capillary

Combining Raman microscopy with a micro-capillary compartment enables spectroscopic studies of small amounts of biological material at variable pressure. We present experiments over the pressure range from atmospheric pressure to 4 kBar in a micro-capillary that use less than 100 nanoliters of sample. We investigate pressure dependent structural changes in DMPC and POPC lipid systems through measurements of the Raman spectrum in the CH2 stretch band and fingerprint regions. The micro-capillary also allows to enclose living cells and to optically interrogate them through a microscope. This is illustrated through Raman spectroscopy and direct optical imaging of individual red blood cells at variable pressure.

Quasiclassical trajectory calculations of correlated product distributions for the F+CHD3(v1=,1) reactions using an ab initio potential energy surface

We report quasiclassical trajectory (QCT) calculations of the correlated product distributions and branching ratios of the reactions F+CHD(3)(v(1)=0,1)-->HF(v)+CD(3)(v) and DF(v)+CHD(2)(v) using a recently published ab initio-based full-dimensional global potential energy surface [G. Czako et al., J. Chem. Phys. 130, 084301 (2009)]. Harmonic normal mode analysis is done for the methyl products to determine the classical actions of each normal mode and then standard histogram binning and Gaussian binning (GB) methods are employed to obtain quantum state-resolved probabilities of the products. QCT calculations have been performed for both the vibrationally ground state and the CH stretching excited F+CHD(3)(v(1)=0,1) reactions at eight different collision energies in the 0.5-7.0 kcal/mol range. HF and DF vibrationally state-resolved rotational and angular distributions, CD(3) and CHD(2) mode-specific vibrational distributions, and correlated vibrationally state-specific distributions for the product pairs have been obtained and the correlated results were compared to the experiment. We find that the use of GB can be advantageous especially in the threshold regions. The CH stretching excitation in the reactant does not change the CD(3) vibrational distributions significantly, whereas the HF molecules become vibrationally and rotationally hotter. On the other hand in the case of the DF+CHD(2) channel the initially excited CH stretch appears mainly "intact" in the CHD(2) product and the DF distributions are virtually the same as formed from the ground state CHD(3) reaction. The computed results qualitatively agree with recent crossed molecular beam experiment [W. Zhang et al., Science 325, 303 (2009)] that (a) CHD(2)(v(1)=1) is the most populated product state of the F+CHD(3)(v(1)=1) reaction and this reaction produces much less CHD(2)(v=0) compared to the reaction F+CHD(3)(v=0); (b) the cross section ratio of CHD(2)(v(1)=1):CHD(2)(v=0) formed from the reactions F+CHD(3)(v(1)=1):F+CHD(3)(v=0) is less than 1 and shows little collision energy dependency; (c) the reactant CH stretch excitation increases the DF:HF ratio at low collision energies; (d) the correlated vibrational and angular distributions for DF(v)+CHD(2)(v(1)=0,1) from the ground state and stretch-excited reactions, respectively, are almost identical.

Wavelength and Time-Resolved Imaging of Material Ejection in Infrared Matrix-Assisted Laser Desorption

The dynamics of glycerol ablation at atmospheric pressure was studied using fast photography. A mid-infrared optical parametric oscillator was used to irradiate a droplet of glycerol at normal incidence. The wavelength of the infrared source was tunable and was varied between 2.7 and 3.5 microm for the studies. After an adjustable delay, an excimer pumped dye laser was used to illuminate the expanding plume, and the 90 degrees scattered light was imaged with a high-speed CMOS camera. The time delay between the IR and UV lasers was varied up to 1 ms with a particular emphasis in the early stages of plume evolution below 1 micros. The threshold fluence for plume formation varied between 1000 and 6000 J/m(2), and the minimum fluence corresponded to the OH stretch absorption of glycerol near 3.0 microm, which also corresponded to the greatest scattered light signal and duration of material emission. The velocity of the expanding plume was measured and ranged from >300 m/s near the OH stretch absorption to <100 m/s near the 3.4 microm CH stretch. Plume modeling calculations suggest that material removal is driven by phase explosion in the stress confinement regime that is at a maximum near the wavelength of the OH stretch absorption.

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.

Molecular Orientation and Intermolecular Interaction in Alanine on Cu(001)

Molecular orientation of L-alanine and structure model of racemic alanine on Cu(001) have been studied by RAIRS, LEED, and STM. The appearance of characteristic frequencies of the NH(2) wagging, the symmetric stretch of carboxylate group (COO(-)), the symmetric CH(3) stretch, and the C*-H (C* denotes a chiral carbon) bending modes in RAIRS is consistent with an adsorption configuration in which alanine molecule is bonded to three Cu atoms through the two oxygen atoms of carboxylate group and the nitrogen atom of amino group in its anionic form with the methyl group standing up over near bridge sites. In the adlayer, hydrogen bonds between the hydrogen atoms of the amino group and the oxygen atom of the carboxylate group contribute to assemble D(L)-alanine molecules along [130] ([310]) as well as [110] directions with a c(2 x 4) periodicity. In the adsorption of racemic mixtures, DL-alanine, D- and L-alanine molecules segregate to form their own c(2 x 4) domains, thus creating the boundary lines along the [110] direction. A submolecular resolution STM image of these domains exhibits distinction between some functional groups and the surrounding intermolecular hydrogen bonds along the [130] or [310] direction. In the submolecular resolution STM, it is most reasonably interpreted that intermolecular network between adsorption alanines is connected by N-H(1)...O(2) and N-H(2)...O(2) hydrogen bonds to form each homochiral domain and the bond at the domain boundary is enhanced by scanning tip which is most probably modified.

A quadruple wavelength IR sensor system for label-free tumour screening

In this contribution, we present a novel quadruple wavelength infrared sensor system for measuring the CH2-symmetric/CH2-antisymmetric stretch ratio of biological cells. This ratio can be used as a marker to distinguish between healthy and carcinoma cells. Compared to common techniques our system does not require specific labelling or staining, or the use of expensive liquid nitrogen cooled IR spectrometers. Two of the four wavelengths, 3.33 and 3.57 µm, are used as reference points and the other two represent CH2-symmetric and CH2-antisymmetric stretch absorption (respectively at 3.51 and 3.42 µm). IR absorbance spectra recordings of healthy (MDCK) and malignant (Caki-1) epithelial kidney cells with a conventional IR spectroscope showed significant differences in the absorbance ratio 3.51 µm/3.42 µm (CH2-symmetric/CH2-antisymmetric stretch). The sensor has been validated by measuring the CH2 stretch ratio of yeast samples both by IR spectroscopy and the sensor system. The methods yielded similar results. The application potential of our system is demonstrated by CH2 stretch measurements on healthy and carcinoma epithelial kidney cells. Measurements on both cell types yielded significant differences in CH2 stretch ratio. The sensor has the potential to be further developed into a fast, low-cost and label-free screening system for suspicious biopsy samples.

Compact coherent anti-Stokes Raman scattering microscope based on a picosecond two-color Er:fiber laser system

We present a compact coherent anti-Stokes Raman scattering microscope based on a widely tunable picosecond Er:fiber laser. Intense and bandwidth-limited 1 ps pump pulses at a center wavelength of 775 nm are generated via frequency mixing within the broadband fundamental at 1.55 microm. Narrowband Stokes pulses are obtained by frequency shifting of solitons in a highly nonlinear bulk fiber and subsequent second-harmonic generation. The tuning range from 850 nm to 1100 nm gives access to vibrational resonances between 1150 cm(-1) and 3800 cm(-1). A first imaging application in the spectral region of CH stretch vibrations is demonstrated.

Jet-Cooled Electronic and Vibrational Spectroscopy of Crown Ethers: Benzo-15-Crown-5 Ether and 4′-Amino-Benzo-15-Crown-5 Ether

Laser-induced fluorescence (LIF), ultraviolet hole-burning (UVHB), and resonant ion-dip infrared (RIDIR) spectroscopies were carried out on isolated benzo-15-crown-5 ether (B15C) and 4'-amino-benzo-15-crown-5 ether (ABC) cooled in a supersonic expansion. Three conformational isomers of B15C and four of ABC were observed and spectroscopically characterized. Full optimizations and harmonic frequency calculations were undertaken for the full set of almost 1700 conformational minima identified in a molecular mechanics force field search. When compared with TDDFT predictions, the S(0)-S(1) origin positions serve as a useful diagnostic of the conformation of the crown ether near the phenyl ring responsible for the UV absorption and to the position of the NH(2) substituent. In-plane orientations for the beta carbons produce red-shifted S(0)-S(1) origins, while out-of-plane "buckling" produces substantial blue shifts of 600 cm(-1) or more. Comparison between the alkyl CH stretch spectra of B15C and ABC divide the spectra into common subgroups shared by the two molecules. The high-frequency CH stretch transitions (above 2930 cm(-1)) reflect the number of CH...O interactions, which in turn track in a general way the degree of buckling of the crown. On this basis, assignments of each of the observed conformational isomers to a class of structure can be made. All the observed structures have some degree of buckling to them, indicating that in the absence of a strong-binding partner, the crown folds in on itself to gain additional stabilization from weak dispersive and CH...O interactions.

IR−UV Double-Resonance of Methyl Radicals and a Determination of the Detection Sensitivity of REMPI Bands

A novel method is exploited in this report to directly determine the relative detection sensitivity of the (2+1) resonance-enhanced multiphoton ionization (REMPI) bands of CH(3) and CHD(2) radicals. The basic idea is based on the simple fact that in an infrared (IR) absorption process the number of molecules being pumped from the lower state must be the same as the number of molecules in the excited upper state. Hence, the measured intensities of the respective REMPI bands should directly reflect their relative detection sensitivities. In order to ensure the processes involved and better quantify the measurements, extensive IR-UV double resonance spectroscopy was also performed. Using the REMPI-IR scheme, the IR spectrum of the v(1) fundamental (CH stretch) of CHD(2) was obtained and assigned for the first time. Using the IR-REMPI approach, high-resolution (2+1) REMPI spectra via the Rydberg 3p states of both radicals were demonstrated in a rotationally specific manner for both the origin and vibronic-excited bands, from which the predissociation rates of the Rydberg 3p states were deduced.

The adiabatic approximation as a diagnostic tool for torsion–vibration dynamics

The adiabatic separation of large-amplitude torsional motion from small-amplitude vibrations is applied as an aid in interpreting the results of fully coupled quantum calculations on a model methanol Hamiltonian. Comparison is made with prior work on nitromethane [D. Cavagnat, L. Lespade, J. Chem. Phys. 106 (1997) 7946]. Even though the torsional potentials are very different, both molecules show a transition from adiabatic to diabatic behavior when the CH stretch is excited to ν CH = 4 or higher. In the adiabatic approximation, the effective torsional potentials for the various CH stretch vibrational states do not cross, but the CH vibrational amplitude moves from one bond to the next as a function of torsional angle. In the diabatic approximation, the effective torsional potentials do cross, but the distribution of the CH vibrational amplitude remains approximately constant in the vicinity of the crossing. The transition to diabatic behavior is promoted by the normal mode to local mode transition, and the relevant adiabatic and diabatic effective torsional potentials are determined by the torsion–vibration coupling. The torsion–vibration couplings in the four overtone manifolds considered (methanol OH, CH, nitromethane CH, and hydrogen peroxide OH) are large, reaching 265–500 cm −1 by ν XH = 6, and are of generally similar magnitude. The largest torsion–vibration couplings involve the first Fourier term in the torsional angle (cos γ for the CH stretch in methanol and the OH stretch in HOOH), whereas higher Fourier terms (cos2 γ in nitromethane and cos3 γ for the OH stretch of methanol) result in somewhat weaker coupling. Nonadiabatic matrix elements in methanol couple the torsional and vibrational energies and they exhibit a slow fall-off with coupling order.

The action modes of Lippia sidoides (Cham) essential oil as penetration enhancers on snake skin

The aim of this work was to study the effect of Lippia sidoides essential oil (LSEO) on stratum corneum lipids and the permeation of salicylic acid. DSC and FTIR spectroscopy were applied. LSEO 1% (v/v) significantly enhanced salicylic acid flux through snake skin. According to the DSC curves changes in the transition temperature of the lipids were observed indicating that LSEO can interact with stratum corneum. The IR spectrum of skin treated with LSEO showed a decrease in the peak intensity for CH2 stretchings (2920–2850 cm–1) however the peak positions did not alter suggesting the extraction of the lipids.

Improper Hydrogen-Bonding CH·Y Interactions in Binary Methanol Systems As Studied by FTIR and Raman Spectroscopy

Fourier Transform infrared spectroscopy and Raman spectroscopy have been used to investigate hydrogen bonding of methanol in different solvents with an aim to explore potential experimental evidence for improper hydrogen bonding involving the methyl group of methanol as suggested by various computational studies. Pure methanol and solutions of methanol in water, acetonitrile, carbon tetrachloride, deuterium oxide, and deuterated acetonitrile have been studied over a range of concentrations. Wavenumber shifts of the CH stretching vibrations were examined to determine if the CH from methanol participates in hydrogen bonding. New concepts of the vibrational wavenumber and integrated intensity at infinite dilution are proposed and given the respective symbols nu(CH(o)) and C(j,CH)*(o). Using the results obtained for methanol in carbon tetrachloride as a reference, shifts in nu(CH(o)) of methanol to higher wavenumbers (blue shifts) were observed in each of the other solvents studied, with the shifts being greatest for the methanol-water interactions. The shifts in vibrational wavenumber suggest possible improper hydrogen bonding, although at this stage a definitive conclusion is not possible. The C(j,CH)*(o) results show that there is no distinguishable change in the methanol CH stretch integrated intensity in carbon tetrachloride and acetonitrile, while there is a significant decrease in the methanol CH stretch integrated intensity in the water solutions.

Fabrication of SERS Active Gold Nanoarrays for the Study of Adsorbed Phospholipids

In this study Surface-enhanced Raman scattering (SERS) was employed to study phospholipid molecules adsorbed on nanometre-sized Au structures. Highly ordered gold nano-rod arrays were produced using anodized aluminum oxide templates and electrochemical deposition. The nanorods were imaged with scanning electron microscopy and had an aspect ratio of 10. Raman spectra of 4-aminothiophenol on the prepared substrates showed an enhancement of 10ˆ2 when compared with a smooth gold surface. Lipids were deposited on the nanoarrays and peaks at 2850 and 2927 cm-1 in the Raman spectra were identified as CH2 and CH3 stretches respectively. It has been demonstrated that nano-rod textured electrodes are excellent substrate for SERS of biomolecules at metal surfaces.

Surface-enhanced IR–visible sum frequency generation vibrational spectroscopy

Surface-enhanced IR-visible sum frequency generation (SFG) was studied using Ag and Au films over nanospheres (AgFON and AuFON), which provided large area of reproducible nanoscale structures with well-defined morphology. SFG vibrational spectra for a self-assembled monolayer of octadecanethiol were investigated on spheres with diameters ranging from 300 nm to 620 nm. With an input wavelength of 532 nm, a sphere diameter near 360 nm produced the maximum enhancement, which was consistent with the localized surface-plasmon resonant wavelength identified by minimum reflectivity. The measured second-order susceptibility for the asymmetric CH3 stretch mode of octadecanethiol was enhanced by up to 27 times on AgFON, which corresponded to an SFG enhancement factor of approximately 730. The SFG enhancement factor for AuFON was a quarter of that from AgFON. The SFG enhancement factor for the symmetric CH3 mode was about 10 times smaller than that for the asymmetric CH3 mode. This difference can be explained by the highly directional electrical-field parallel to the surface, created at the junction between nanospheres. Polarization-dependent studies also indicated that excitations with the electrical field parallel to the surface was the main contribution to the observed surface enhancements, and the electrical field perpendicular to the surface offered little enhancement.