Concepts from Chemical Reaction Kinetics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 245

If several components are absorbing simultaneously in the sample, then

I 1 10 − i ε v C i Z . I abs = I 0 − I = − (5.188)

The Stark-Einstein law explicitly identifies that only molecules that are electron-
ically excited take part in photochemical reactions, that is, those that are chemically
“active” must be distinguished from those that are “excited” by photons. Excited
species can lose energy by non-chemical pathways and via thermal reactions.The effi-
ciency of a photochemical reaction was defined by Einstein as quantum efficiency , φ ,

number of molecules formed
number of quanta absorbed .

φ = (5.189)

This concept can be extended to all physical and chemical processes following the
absorption of light. It can therefore be identified as a means of keeping tabs on the
partitioning of absorbed quanta into the various modes.
Consider a molecule B that received a quantum of light energy to form the excited
species B ∗

B ∗ .

B + hv −→ (5.190)

The absorption spectrum for the molecule is the plot of absorbance ( A) versus wave-
length ( ) . Since the absorbance depends on the nature of the

functional groups in the molecule that are photoexcited, the absorption spectrum
of each compound can be considered to be its fingerprint. These functional groups
are called chromophores . The wavelength at which maximum absorption is possi-
ble (designated

λ ) or frequency ( ν

are listed in standard handbooks ( CRC
HandbookofChemistryandPhysics , 1994). Organic compounds with fused aromatic
rings or unsaturated heteroatom functionalities have generally high absorbances.
A molecule in its excited state can undergo four main types of primary photochem-
ical processes.

λ max ) and the corresponding ε

Fluorescence: B ∗ −→ ν , B + h

Dissociation: B ∗ −→ P 1 + P 2 ,

Quenching: B ∗ + M −→ B + M,

Energy transfer: B ∗ + C ∗ ,

C −→ B +

Reaction: B ∗ + C −→ S 1 + S 2 .

Eachoftheprocessesgivenabovehasa φ i associatedwithit.Thevalueofthequantum

efficiency is a function of the wavelength and denoted by . In the atmosphere

(troposphere) the range of wavelengths of interest is between 280 and 730 nm, since
most of the UV wavelengths < 280 nm are blocked by the stratospheric ozone layer.
Beyond 730 nm no reactions of interest take place. In the aquatic environment (lakes,
rivers, and oceans) the upper range is rarely above 400 nm. The general form of the

φ λ