The absorption process

The energy of a molecule can be considered to be part rotational, part vibrational and part electronic. A molecule can only have one of a discrete series of energy values. Energy increments corresponding to changes in a molecule’s electronic energy are large, those corresponding to changes in vibrational energy are intermediate in size and those corres-ponding to changes in rotational energy are small. This is indicated diagrammatically inFig. 3.1. When molecules collide with each other in the liquid or gaseous state, or are in contact with each other in the solid state, there can be transfer of rotational or vibrational energy between molecules and this is accompanied by transitions from one rotational or vibrational energy level to another within each molecule.

Molecules can obtain energy from radiation as well as from other molecules. When a photon passes within the vicinity of a molecule, there is a finite probability that it will be captured by that molecule, i.e. be absorbed. If the photon is captured, then the energy of the molecule must increase by an amount corresponding to the energy of the photon. If the photon is of long wavelength (>20mm), in the far infrared/microwave region of the spectrum, then its energy is low and its absorption can only cause a transition in the energy of the molecule from one rotational energy level to another. If the photon is in the

50

infrared region(<20mm), then its absorption will cause a transition from one vibrational level to another.

Photons in the visible/photosynthetic part of the spectrum have suffi-cient energy to bring about, when they are absorbed, transitions from one electronic energy level (usually the ground state) to another. The first event in the absorption of light by the aquatic medium is, therefore, the capture of a photon by some molecule in the medium and the simultan-eous transition of an electron in that molecule from the ground state to an excited state.

Within a complex molecule such as chlorophyll or any of the other photosynthetic pigments, there is usually more than one possible elec-tronic energy transition that can occur. For example, inFig. 3.1excitation can occur up to electronic energy level I or to level II: to be more precise, excitation will occur to one of the many vibrational/rotational levels belonging to the given electronic energy level. Any given electronic tran-sition is preferentially excited by light that has an amount of energy per

Fig. 3.1 Absorption of a photon raises an electron from the ground state to one of two possible excited singlet states, depending on the wavelength (and therefore energy) of the photon. Wavelengths l1 and l2 are each within a separate absorption band in the absorption spectrum of the molecule (l2>l1).

3.1 The absorption process 51

photon corresponding to the energy required for the transition. Chloro-phyll, as we shall see later, has two main absorption bands, in the red and in the blue regions of the spectrum. Absorption of a blue photon leads to transition to a substantially higher electronic energy level than absorption of a red photon. Because of the multiplicity of vibrational/rotational states, these two electronic energy states nevertheless overlap: i.e. the highest vibrational/rotational levels of the lower electronic state can have energies as high as the lowest level of the upper electronic state (Fig. 3.1).

As a consequence, immediately after absorption of a blue photon there is a very rapid series of transitions downwards through the various rota-tional/vibrational levels (associated with transfer of small increments of rotational/vibrational energy to adjoining molecules) until the lower elec-tronic energy state (referred to as the lowest excited singlet state) is reached.

It is the energy in this lowest excited singlet state that is used in photosynthesis and it is because an excited chlorophyll molecule usually ends up in this state anyway, that allabsorbedvisible quanta are equiva-lent, i.e. a red quantum, or even a green quantum if it succeeds in being absorbed, brings about as much photosynthesis as a blue quantum. This is also the reason why, in the context of primary production, it is more meaningful to express irradiance in quanta m²s¹than in W m².

In the case of light-absorbing molecules in the aquatic medium that are not part of the photosynthetic system, and that cannot therefore transfer their energy to a reaction centre for use in photosynthesis, there is likely to be a transition from the lowest excited singlet state to the correspond-ing triplet state by interaction with a paramagnetic molecule such as oxygen. In a singlet state, the two members of every pair of electrons in the molecule have opposite spin (þ½ and –½) so that the resultant spin is zero: in a triplet state, the members of a pair of electrons in the molecule have the same spin so that the resultant spin is one. The excited triplet state is at a lower energy level and is also much longer lived than the excited singlet state, the average lifetime of the excited molecules being 10⁵to 10⁴s in the former case, compared to 10⁹to 10⁸s in the latter.

Eventually, by interaction with a paramagnetic molecule such as oxygen, the excited molecule is returned to a singlet state by spin reversal and, by vibrational/rotational interactions with the surrounding molecules, it undergoes a downwards transition to one of the upper vibrational levels of the ground state. The electronic energy is thus dissipated as heat energy.

Excitation energy, whether in the photosynthetic system or in a mol-ecule outside it, can be lost by re-emission of radiation. An electronically excited molecule, after undergoing the radiationless transition to the

lowest level of the lowest excited singlet state, can then undergo a transi-tion to one of the vibratransi-tional/rotatransi-tional levels of the ground state by re-emitting a photon of light. This phenomenon is referred to as fluorescence. In living, photosynthesizing algal cells, only a very small proportion, ~1%, of the absorbed light is lost in this way. Most of the absorbed energy, whether captured initially by chlorophyll, carotenoid or biliprotein, is transferred by inductive resonance to the reaction centres where it is used to bring about biochemical changes (seeChapter 8).

If photosynthesis is inhibited, say with dichlorophenylmethylurea (DCMU), then fluorescence increases to about 3% of the absorbed light. The non-photosynthetic, light-absorbing material of the aquatic medium can also re-emit some of its absorbed energy as fluorescence, but the fluorescence yield (quanta emitted/quanta absorbed) is again very low: most of the excited molecules are converted to the triplet state (see above), and from there to the ground state before they can emit a photon.

Thus, most of the light energy absorbed by the aquatic ecosystem, after existing for a very brief period as electronic excitation energy, ends up either as heat (vibrational/rotational energy distributed among all the molecules of the system) or as chemical energy in the form of photosyn-thetically produced biomass. Only a tiny proportion is turned back into light again by fluorescence, and even this is for the most part re-absorbed before it can escape from the system.