8
A. G. RYDER
Figure 5. The absolute quantum yields for neat (bottom) and dilute (top) crude oils. The cutoff wavelength is
an optical measure of the oil weight; heavy oils have a large cutoff wavelength. The cutoff wavelength is
defined as being the point at which the optical density reaches a value of 3.0 for a 2mm optical pathlength.
Quantum yields decrease greatly with increasing excitation wavelength due to internal conversion. Quenching
from chromophores interactions causes neat crude oils to have much smaller quantum yields than dilute crude
oils, especially for heavy oils. For the dilute case, the quantum yields of heavy oils are lower due to the greater
fraction of large, aromatic chromophores. Reproduced with permission from Ref [30] © 1995, Society for
Applied Spectroscopy.
The balance between collisional energy transfer and quenching is the dominant
feature affecting crude oil fluorescence. Downare and Mullins
18 calculated (from
emission spectra) and plotted the ratio of energy transfer to total emission against
excitation wavelengths for three representative oils (Figure 6). This plot shows how
influential the selection of excitation wavelength is on the photophysics of crude oil
emission. This has obvious consequences for the consideration of excitation sources for
petroleum fluorescence applications. For several studies in the 1970’s and 80’s, which
employ 316 nm or 337 nm excitation sources, emission arises largely (>75%) from
energy transfer, while more recent studies employing semiconductor light sources (380
A. G. RYDER
Figure 5. The absolute quantum yields for neat (bottom) and dilute (top) crude oils. The cutoff wavelength is
an optical measure of the oil weight; heavy oils have a large cutoff wavelength. The cutoff wavelength is
defined as being the point at which the optical density reaches a value of 3.0 for a 2mm optical pathlength.
Quantum yields decrease greatly with increasing excitation wavelength due to internal conversion. Quenching
from chromophores interactions causes neat crude oils to have much smaller quantum yields than dilute crude
oils, especially for heavy oils. For the dilute case, the quantum yields of heavy oils are lower due to the greater
fraction of large, aromatic chromophores. Reproduced with permission from Ref [30] © 1995, Society for
Applied Spectroscopy.
The balance between collisional energy transfer and quenching is the dominant
feature affecting crude oil fluorescence. Downare and Mullins
18 calculated (from
emission spectra) and plotted the ratio of energy transfer to total emission against
excitation wavelengths for three representative oils (Figure 6). This plot shows how
influential the selection of excitation wavelength is on the photophysics of crude oil
emission. This has obvious consequences for the consideration of excitation sources for
petroleum fluorescence applications. For several studies in the 1970’s and 80’s, which
employ 316 nm or 337 nm excitation sources, emission arises largely (>75%) from
energy transfer, while more recent studies employing semiconductor light sources (380