In addition to this
‘‘electromagnetic eld enhancement,’’ the electronic interaction
between the Raman molecule and the metal can
result in an increase of the Raman cross-section itself, called ‘‘chemical or electronic enhancement.’’ Several
mechanisms are discussed to account for this chemical
SERS enhancement, such as a resonance Raman effect, which becomes operative due to possibly shifted and
broadened electronic levels in the adsorbed molecule
compared to the ‘‘free’’ one or due to a new (charge
transfer) electronic transition in the metal–molecule system