can be of intermediate energy between the highest occupied molecular
orbital of the molecule and its lowest unoccupied molecular orbital of the
molecule. Hence, the new electronic states can serve as resonant intermediaries in the process of Raman scattering. Therefore, excitations of the
molecule that use the charge-transfer mechanism occur at much lower
energies than are normally required for the molecule to be excited. Thus,
Raman scattering can happen more readily and the signal is enhanced.
SERS is typically carried out on a “rough” metallic surface, with roughnesses on the order of 10–100 nm. Such surfaces are commonly manufactured by sputtering or evaporation of the metal onto a substrate or by
roughening of a metallic electrode surface during oxidation–reduction
cycles. Colloidal particles of the metal are also used to produce SERS.
These colloids may either be suspended in solution or adsorbed onto a
substrate. On these types of “rough” metallic surfaces, the SERS effect
extends up to tens of nanometers from the surface, allowing for the
effective probing of ultra-thin nanofilms.
8.6 NONLINEAR SPECTROSCOPIC METHODS
8.6.1 An introduction to nonlinear optics
Thus far, the optical techniques that we have discussed rely on linear
optical processes, describable in terms of a single electric field of a specific
frequency propagating through materials that can be further described by
a single refractive index or as excitation, emission, or scattering processes
involving single photons. However, not all optical processes are linear—
linear optics provides a good description of most materials over typical
intensities of light, analogous to how Newtonian mechanics provides a
good description of massive objects moving at speeds well below the
speed of light. At sufficiently high electric field strengths and/or in certain,
highly ordered materials, nonlinear effects, which involve the interaction
of multiple photons, can become important. Unlike linear optical processes, nonlinear optical processes can change the frequency of the light
moving through a material or change the instantaneous refractive index of
a material without any change in composition. Exploiting nonlinear
optical processes in the laboratory typically requires powerful pulsed
lasers that can generate brief (femtosecond to nanosecond) bursts of
extremely strong electric fields. Although the theory of nonlinear optics
is beyond the scope of this book, a cursory treatment is provided to
CHAPTER 8: Surface Characterization and Imaging Methods
294
orbital of the molecule and its lowest unoccupied molecular orbital of the
molecule. Hence, the new electronic states can serve as resonant intermediaries in the process of Raman scattering. Therefore, excitations of the
molecule that use the charge-transfer mechanism occur at much lower
energies than are normally required for the molecule to be excited. Thus,
Raman scattering can happen more readily and the signal is enhanced.
SERS is typically carried out on a “rough” metallic surface, with roughnesses on the order of 10–100 nm. Such surfaces are commonly manufactured by sputtering or evaporation of the metal onto a substrate or by
roughening of a metallic electrode surface during oxidation–reduction
cycles. Colloidal particles of the metal are also used to produce SERS.
These colloids may either be suspended in solution or adsorbed onto a
substrate. On these types of “rough” metallic surfaces, the SERS effect
extends up to tens of nanometers from the surface, allowing for the
effective probing of ultra-thin nanofilms.
8.6 NONLINEAR SPECTROSCOPIC METHODS
8.6.1 An introduction to nonlinear optics
Thus far, the optical techniques that we have discussed rely on linear
optical processes, describable in terms of a single electric field of a specific
frequency propagating through materials that can be further described by
a single refractive index or as excitation, emission, or scattering processes
involving single photons. However, not all optical processes are linear—
linear optics provides a good description of most materials over typical
intensities of light, analogous to how Newtonian mechanics provides a
good description of massive objects moving at speeds well below the
speed of light. At sufficiently high electric field strengths and/or in certain,
highly ordered materials, nonlinear effects, which involve the interaction
of multiple photons, can become important. Unlike linear optical processes, nonlinear optical processes can change the frequency of the light
moving through a material or change the instantaneous refractive index of
a material without any change in composition. Exploiting nonlinear
optical processes in the laboratory typically requires powerful pulsed
lasers that can generate brief (femtosecond to nanosecond) bursts of
extremely strong electric fields. Although the theory of nonlinear optics
is beyond the scope of this book, a cursory treatment is provided to
CHAPTER 8: Surface Characterization and Imaging Methods
294