D. P. DiLella

Surface‐enhanced Raman spectroscopy of benzene and benzene‐d6 adsorbed on silver

The surface‐enhanced Raman spectrum of benzene and benzene‐d6 adsorbed on a silver film vapor‐deposited onto a low‐temperature substrate is reported. Many lines which correlate with Raman‐inactive and even infrared and Raman‐silent modes of the free molecule are observed with high intensity in the spectrum. The local symmetry which would cause those particular, otherwise inactive, vibrations to appear is C3v(σd), which corresponds to a benzene molecule lying flat atop an equilateral triangle of silver atoms so that the normals to three alternate edges of benzene point to the centers of the three atoms. An alternative explanation for the activity of these normally silent modes is presented which does not depend on the adsorption site symmetry but which postulates instead that an unusally steep electric field gradient exists near the surface of the metal which causes modes spanned by the same representations which span the elements of a third rank tensor A to become active. These, together with the vibratio...

Diatomic and triatomic scandium and diatomic manganese: A resonance Raman study

Resonance and preresonance Raman spectra of Mn and Sc containing argon matrices are reported. For scandium, we assign the progressions to the diatomic and triatomic molecules. The former is found to have vibrational constants ω″e=239.9 cm−1 and ω″ex″e=0.93 cm−1. The latter is best described as an equilateral or near equilateral triangle with a symmetric stretching vibrational frequency of 248 cm−1 and a degenerate (asymmetric stretch–bend) vibrational frequency of 150 cm−1. In addition a low lying electronic state 395 cm−1 above the ground state is attributed to Sc3 based on the observation of a second progression. In Mn containing matrices one identifies three progressions with nearly equivalent vibrational spacings and assigns them to Mn2. The ground state vibrational constants of Mn2 are found to be ω″e=124.7 cm−1 and ω″ex″e=0.24 cm−1. Two more progressions resulting from transitions to states 135.7 and 196.7 cm−1 above the ground state are interpreted by assuming Mn2 to be an antiferromagnetic dimer p...

Dititanium and divanadium

The Ar+ laser‐excited spectrum of matrix‐isolated V2 consists of a resonance Raman progression with ωe=537.5 cm−1 and ωexe=4.2 cm−1. With increasing laser power several members of an anti‐Stokes progression and sequence components on both Stokes and anti‐Stokes members of this progression were observed and attributed to transitions originating from vibrationally excited stated populated as a result of laser irradiation. A second system with ωe=508 and ωexe=3.3 cm−1 also grew in with increasing laser power and 496.5 nm excitation. This was interpreted as a resonance Raman progression within an electronically excited state (A) of V2. The electronic resonance Raman spectrum corresponding to the A→X transition was also observed as well as one to another low‐lying electronic excited state. The v′=0 level of state A was found to lie 1860 cm−1 above the v″=0 level of the ground state. The multiple photon nature of the transitions discussed above was determined by performing laser power measurements. The resonanc...

Di‐iron and nickeliron

Resonance Raman progressions assigned to Fe2 and NiFe isolated in solid Ar and Kr are reported. ωe″ and ωe″xe″ for the two molecules are found to be 299.6 and 1.4 cm−1 and 320.0 and 1.32 cm−1, respectively. In addition, an anti‐Stokes progression of inordinate intensity was observed for Fe2. On the basis of a laser power study this was suggested to arise from consecutive two‐photon absorption processes which are efficient in this case as a result of very long vibrational relaxation lifetimes of the excited vibrational states of the electronic ground state of Fe2. No spectrum of Fe2 was detected in either solid N2 or solid methane, supporting previous reports that Fe2 reacts with these substances to form complexes. With high resolution, each of the bands of the resonance Raman progression of Fe2 is observed to be a multiplet. Some of the multiplet structure results from the expected isotope effect but additional structure is seen because for a particular value of Δv″ there are transition with different ini...

Enhanced Raman spectroscopy of CO adsorbed on vapor‐deposited silver

Raman spectra of CO condensed on vapor‐deposited Ag at 11 K show two signals in the CO stretching region, one originating from a monolayer next to the metal, the other from the bulk CO film above it. In addition, a low‐frequency band associated with the former was detected, as were overtones of both the low‐ and high‐frequency bands of the surface CO. Raman scattering by the former was found to be some 106 times more intense than by the latter on a per molecule basis. The state of polarization of the tow signals was investigated as a function of angle of incidence, as was the excitation profile of the signal from the CO next to the metal, using the bulk CO signal as a reference. The behavior of the latter could only be understood when one took into account coherence effects arising from the superposition of rays directly incident on or scattered by a molecule with those reflected from the metal before impinging on, or after scattering from, the molecule. These effects could give rise to apparent polarizin...

Surface‐enhanced Raman spectroscopy of CO adsorbed on colloidal silver particles

The surface‐enhanced Raman spectrum of CO adsorbed on colloidal silver particles of about 100 A diameter is reported. The spectrum resembles that observed for CO adsorbed on vapor‐deposited silver. Only three strong Raman lines appear, at 2113, 160, and 64 cm−1. These are assigned, respectively, to the CO stretching, Ag–C stretching, and AgCO bending vibrations. The excitation profile of the 2113 cm−1 line follows closely the plasmon absorption spectrum of the silver colloid, increasing as one goes from the red to the violet; that of the 160 cm−1 line shows a more gradual increase in the same direction. The two types of behavior are accounted for by means of a model which considers the enhancement of the Raman signal to be due to coupling of the adsorbate vibration with the conduction–electron surface plasma mode through a charge transfer from the adsorbate to the metal, synchronous with the vibration. The two types of excitation profile are generated by assuming that for the AgC vibration, positive displ...

Dichromium and trichromium

Resonance and preresonance Raman spectra are obtained from chromium‐containing argon matrics. Two progressions are assigned to Cr2 and Cr3, the former with optical constants ω″e=427.5 and ω″ex″e=15.75 cm−1 and the latter with ω″e=313 and ω″ex″e=2 cm−1. When not in resonance two other lines at 123 and 226 cm−1 were observed and ascribed to the bending and asymmetric stretching vibration of Cr3, while the vibration forming the progression was taken to be the symmetric stretch. From this, Cr3 is deduced to be a molecule with C2v symmetry but with an apical angle near 60°, suggesting that it is perhaps a Jahn–Teller distorted D3h molecule.

Raman spectra of solid films—I: Metal-free phthalocyanine

Abstract The preresonance and resonance Raman spectra of solid films of metal-free phthalocyanine have been studied. A vibrational description of prominent spectral features is given in terms of the macrocycle (inner ring) and isoindole moieties. The observed changes in relative intensities of Raman lines with different excitation frequencies are also discussed.

Intense quadrupole transitions in the spectra of molecules near metal surfaces

The fluorescence, Raman, and fluorescence excitation spectra of diphenyloctatetraene and diphenylhexatriene condensed on rough silver are reported. The fluorescence excitation spectra reveal that on silver the normally dipole‐forbidden 1Ag←X 1Ag absorption takes place in addition to the allowed 1Bu←X 1Ag process. This is attributed to the large electric field gradient which is present near an illuminated metal surface causing the intensity of quadrupole absorptions to become unusually great. The possible contribution of this field gradient to two‐photon processes in centrosymmetric molecules and to the intensity of bands in infrared and EELS spectra is also noted. In addition, the relative fluorescence intensities of these molecules adsorbed on rough and smooth silver are discussed in terms of the effectiveness of nonradiative decay into metal channels.

Vibrational Spectroscopy of Molecules Adsorbed on Vapor-Deposited Metals

In the present chapter we will present the SERS spectra of a number of molecules adsorbed on silver and lithium metals vapor-deposited in vacuum onto a cooled substrate. The results will be used to illustrate a number of points.