106
K. Kneipp et al.
and often also the Raman scattered photons, match electronic transitions in the molecule. This gives rise to a stronger Raman signal for those vibrational modes which are
related to the electronic transition. But, fluorescence may be excited at the same time,
together with the Raman scattering, and the strong fluorescence signal can interfere
with the Raman Stokes light.
A critical parameter that determines the applicability of a spectroscopic effect
for structural probing is its cross section. Raman cross sections range from 10 −31 to
10 −29 cm 2 per molecule. In the case of RRS, cross sections become typically 2–6
orders of magnitude larger. Infrared absorption appears at cross sections from 10 −22
up to 10 −20 cm 2 , i.e. about 10 orders of magnitude larger than non-resonant Raman
cross sections. Overall, even the best cross sections achievable for vibrational spectroscopy are still orders of magnitude below those used in electronic absorption and in
fluorescence which can be typically obtained at cross sections of 10 −17 –10 −16 cm 2 .
Despite the high molecular structural information content in vibrational spectra, the
low cross sections, particularly those of Raman scattering, represent a considerable
disadvantage for all applications of vibrational spectroscopy. This applies particularly for two-photon excitation. Cross sections on the order of 10 −65 cm 4 s/photon
for HRS—compared to 10 −50 cm 4 s/photon typical of two-photon fluorescence—
make the utilization of HRS as a practical spectroscopic tool nearly impossible. This
situation has changed dramatically during the last 15 years. Exciting opportunities
for gaining and improving vibrational signals arise, when spectroscopy takes place
in strongly enhanced local optical fields of plasmonic nanostructures [4, 17, 18].
Field enhancement in such metal structures can be understood in terms of resonant
excitation of high-Q-factor surface plasmon polaritons or/and by field concentration due to the lightening rod effect [19]. Some effects and observations exploiting
enhanced optical fields, can be explained by looking at metal nanostructures as optical nanoantenna which can direct and further enhance local optical fields. [20, 21].
We discuss “surface enhancement” for different vibrational spectroscopic methods
in more detail in the following sections.
2.2.1 Surface Enhanced Raman Scattering
Surface-enhanced Raman scattering (SERS) is probably the most prominent
observation to demonstrate the capabilities of plasmon supported spectroscopy.
In analogy to normal, non-surface–enhanced Raman scattering, the number of
Stokes photons per second n SERS
S
in surface-enhanced Raman scattering can be
calculated as
n
SERS
S
= N 0 σ
SERS n L
(2.1)
where σ SERS
S
describes an effective cross section of the surface enhanced Raman
process, S denotes the Stokes scattering. n L is the Photon flux density of the excitation laser. N 0 is the number of molecules in the vibrational ground state, which
are involved in the SERS Stokes process. The cross section for Raman scattering is
K. Kneipp et al.
and often also the Raman scattered photons, match electronic transitions in the molecule. This gives rise to a stronger Raman signal for those vibrational modes which are
related to the electronic transition. But, fluorescence may be excited at the same time,
together with the Raman scattering, and the strong fluorescence signal can interfere
with the Raman Stokes light.
A critical parameter that determines the applicability of a spectroscopic effect
for structural probing is its cross section. Raman cross sections range from 10 −31 to
10 −29 cm 2 per molecule. In the case of RRS, cross sections become typically 2–6
orders of magnitude larger. Infrared absorption appears at cross sections from 10 −22
up to 10 −20 cm 2 , i.e. about 10 orders of magnitude larger than non-resonant Raman
cross sections. Overall, even the best cross sections achievable for vibrational spectroscopy are still orders of magnitude below those used in electronic absorption and in
fluorescence which can be typically obtained at cross sections of 10 −17 –10 −16 cm 2 .
Despite the high molecular structural information content in vibrational spectra, the
low cross sections, particularly those of Raman scattering, represent a considerable
disadvantage for all applications of vibrational spectroscopy. This applies particularly for two-photon excitation. Cross sections on the order of 10 −65 cm 4 s/photon
for HRS—compared to 10 −50 cm 4 s/photon typical of two-photon fluorescence—
make the utilization of HRS as a practical spectroscopic tool nearly impossible. This
situation has changed dramatically during the last 15 years. Exciting opportunities
for gaining and improving vibrational signals arise, when spectroscopy takes place
in strongly enhanced local optical fields of plasmonic nanostructures [4, 17, 18].
Field enhancement in such metal structures can be understood in terms of resonant
excitation of high-Q-factor surface plasmon polaritons or/and by field concentration due to the lightening rod effect [19]. Some effects and observations exploiting
enhanced optical fields, can be explained by looking at metal nanostructures as optical nanoantenna which can direct and further enhance local optical fields. [20, 21].
We discuss “surface enhancement” for different vibrational spectroscopic methods
in more detail in the following sections.
2.2.1 Surface Enhanced Raman Scattering
Surface-enhanced Raman scattering (SERS) is probably the most prominent
observation to demonstrate the capabilities of plasmon supported spectroscopy.
In analogy to normal, non-surface–enhanced Raman scattering, the number of
Stokes photons per second n SERS
S
in surface-enhanced Raman scattering can be
calculated as
n
SERS
S
= N 0 σ
SERS n L
(2.1)
where σ SERS
S
describes an effective cross section of the surface enhanced Raman
process, S denotes the Stokes scattering. n L is the Photon flux density of the excitation laser. N 0 is the number of molecules in the vibrational ground state, which
are involved in the SERS Stokes process. The cross section for Raman scattering is