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Experimentally, this can be achieved in a fluorescence spectrometer by placing a

polariser in the excitation path in order to excite the sample with polarised light.

A second polariser is placed between the sample and the detector with its axis either

parallel or perpendicular to the axis of the excitation polariser. The emitted light is

either partially polarised or entirely unpolarised. This loss of polarisation is called

fluorescence depolarisation.

Absorption of polarised light by a chromophore is highest when the plane of polar-

isation is parallel to theabsorption dipole momentmA(Fig. 12.15). More generally, the

probability of absorption of exciting polarised light by a chromophore is proportional to

cos^2 y,withybeing the angle between the direction of polarisation and the absorption

dipole moment. Fluorescence emission, in contrast, does notdepend on the absorption

dipole moment, but on thetransition dipole momentmE.Usually,mAandmEare tilted

against each other by about 10to 40. The probability of emission of polarised light at

Direction of propagation

Polariser

Evectors

Fig. 12.14Generation of linearly polarised light.

Fig. 12.15Absorption dipole momentmA(describing the probability of photon absorption) and transition dipole

momentmE(describing the probability for photon emission) for any chromophore are usually not parallel.

Absorption of linearly polarised light varies with cos^2 yand is at its maximum parallel tomA. Emission of linearly

polarised light varies with sin^2 fand is highest at a perpendicular orientation tomE.

505 12.3 Fluorescence spectroscopy