Dual mode improves NSOM images Reflection near-field scanning optical microscopy (NSOM) suffers from S/N and obstructed view problems, but researchers at the University of California-Santa Barbara think they can circumvent them. Steven K. Burrato and Kenneth D. Weston describe a "dual mode" NSOM technique that reduces some of the difficulties associated with reflection NSOM. In dual-mode NSOM, a 1 x 2 bidirectional coupler is used with the 50% lead connected to an Ar+ laser and a photomultiplier tube and the 100% lead coupled to the NSOM tip. Because the reflected light is also collected by the illumination fiber, the light passes through the tip twice and is strongly attenuated. For each dualmode image, the researchers obtained a plot of the NSOM signal as a function of the tip-sample gap (the "retract curve").
The dual-mode images were easier to interpret than those obtained in conventional illumination mode. The researchers suggest that the improved contrast and resolution result from a decrease in the effective aperture. Interference effects between the incident and
Image of a multilayer hafnium aryl-(4-diethylphosphonate)10-decylphosphoric acid ether film with the z-contrast removed.
flected light strongly influence the detected signal in the dual-mode geometry and can result in z-contrast (that is not true optical contrast) The retract curve can be used to subtract this z-contrast from the NSOM image
The dual-mode NSOM technique is used to image a patterned aluminum film on glass, a grating standard, and a ninelayer self-assembled multilayer film on a silicon substrate. (J. Phys. Chem. B 1997, 101, 5684-91)
Breaking the SEIRA rules
Multiresonant for the sodium salt, whereas the reverse is true for the copper salt. The SEIRA spectra vibrational of the />-nitrobenzoic acid on copper and In the physical world, rules aren't made to spectroscopies be broken. If situations arise in which a rule silver surfaces were similar, so the interacdoesn't work, maybe it's time for a new—or tions of the />-nitrobenzoate with the surface Most existing multiple-resonance spectroscopic methods rely on electronic transiare similar, regardless of the surface comat least modified—one. Surface-enhanced tions. Only a limited number of composition. The Idaho researchers show that IR absorption (SEIRA) seems to be such a pounds have the sharp electronic transithe location and intensity of the antisymsituation. The currently proposed surface tions necessary for good selectivity. John metric carboxylate stretching band depend selection rule for SEIRA says that vibraC. Wright and his colleagues at the Union the local environment and that caution tions for which the change in dipole moversity of Wisconsin-Madison describe a should be used when attributing such difment is perpendicular to the metal surface new family of vibrational multiresonant ferences to selection rules. will be observable and that vibrations with nonlinear spectroscopies. Vibrational changes parallel to the surface will be forThey suggest that molecules adsorbed transitions are inherently more selective bidden. Peter R Griffiths and Gregory T. to rough sites on the metal islands could be than electronic transitions, because the Merklin of the University of Idahofindthat expected to have bands in the IR spectra environmentally induced broadening this selection rule may not apply in all that supposedly violate the electromagnetic tends to be less and the transitions tend circumstances. field selection rule. They also say that the to be highly characteristic of individual SEIRA spectra of 4-nitrobenzoic acid on standard model for the adsorption of pcomponents. copper and silverfilmswere compared with nitrobenzoic acid on silver islands is overThe researchers describe double IR simplified and that a better model should diffuse reflection IR spectra of sodium and resonance, vibrationally enhanced Raman, include structural elements that would copper /Miitrobenzoate. In the diffusereflection spectra, the antisymmetric bands cause the breakdown of the selection rules. and vibrationally enhanced two-photon resonance, all of which are four-wave mixing are more intense than the symmetric bands (J. Phys. Chem. B 1997,101,5810-13) processes. These vibrational processes involve nuclear rather than electronic polarizations and occur because the polarization induced in a molecule by an applied electromagneticfieldis nonlinear at highfieldintensities. Each method involves two vibrational resonances (which can be used to enhance selectivity) and a third resonance that involves electronic states that may or may not be These on.
proaches may provide component and mode selectivity well OP
The model for electromagnetic enhancement in SEIRA. Analytical
reduction of inhomogeneous broadening (Af)f)l Sfypctwsc 1997 51 949-58) Chemistry News & Features, October 1, 1997 5 8 5 A
