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Characterization
of Actinic Power Dependence in Systems which Undergo Irreversible Multiphoton-Induced
Photochemistry.
Masthay, Mark1
and Helvenston, Merritt2
Murray State University, Murray, KY 42071-33461
New Mexico Highlands University, Las Vegas, NM 877012
Abstract-
We present a model we have recently developed to characterize actinic
power dependence in systems which undergo irreversible multiphoton-induced
photochemistry upon irradiation with actinic wavelengths at which the
one-photon (Beer's Law) absorbance is large. We demonstrate that
(1) the majority of multiphoton-induced photochemistry occurs early
in the optical path of optically-dense (high absorbance) samples since
(a) the probability of multiphoton absorption is proportional to In
(I = actinic intensity; n = number of photons absorbed
by a single molecule) and (b) I decreases exponentially along
the optical path due to the strong Beer's Law absorbance in such
systems. In accord with (1), we also demonstrate that (2) the total
amount of multiphoton-induced photochemistry which occurs (as characterized
by the normalized drop in absorbance  A/A)
is enhanced in low as compared to high optical density samples. Accordingly,
since the absorbance drop is more rapid at higher actinic powers (leading
to a lower average absorbance over an irradiation interval t
when higher powers are used), n will be seriously overestimated
using the commonly-employed experimental protocols of exposure for identical
irradiation intervals t
or to identical actinic dose It.
We thus conclude that (3) actinic power dependence measurements obtained
using identical A/A
at different powers are more reliable than those obtained using identical
t
or It.
We illustrate that this is indeed the case with a set of representative
calculations for the 532 nm-induced biphotonic purple-to-blue photoconversion
of bacteriorhodopsin which we have recently characterized in our laboratory.
We anticipate that our model will be applicable to a wide range of systems
in which irreversible photochemistry is induced by multiphoton absorption
at actinic wavelengths at which the effective one-photon cross section
is large.
Keywords: bacteriorhodopsin,
multiphoton, photochemistry
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