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from the solvent. In the present case, the time resolution can be as short as 40 fs,
although this time resolution is degraded by the need to use optical filters to block
out scattered excitation radiation.
The up-conversion method is the best established and highest time resolution
method for time-resolved fluorescence. It can, however, be time consuming, espe-
cially when fluorescence is detected at many wavelengths to generate a time-
dependent fluorescence spectrum. Alternative broadband detection methods are
available, including broadband up-conversion [ 39 ], Kerr gate detection [ 40 - 43 ]
and streak camera measurements [ 44 ].
2.2 Transient Absorption
There are many different forms of the transient absorption experiment, however, in
its essence it is a two pulse experiment [ 36 , 45 ]. The first pulse excites the sample
(the pump), while the second time delayed pulse (the probe) monitors the sample
absorption as a function of time. In the simplest case, the pump excites a molecule
from its ground state to an excited state. If the probe wavelength is at the same
wavelength as the pump (and therefore the ground state absorption of the sample),
its transmittance is increased after the pump and will decrease as a function of delay
time as (or if) the ground state is repopulated. If the probe wavelength is set at a
wavelength at which the newly formed excited state absorbs, then the transmittance
will be decreased instantaneously by the pump pulse and will increase as the excited
state decays. If the wavelength of the probe is set to a wavelength where the excited
state emits fluorescence, then the probe pulse can stimulate emission from the
excited state. This acts as a gain mechanism for the transmitted probe, so the
apparent probe transmittance increases as the excited state is created and decreases
as it decays. Of course in many important cases, including the CPs considered here,
the excited state goes on to generate new species (e.g., by proton transfer or
isomerisation), and the appearance and decay kinetics of these states can also be
monitored. In general, these processes overlap one another in both time and wave-
length requiring sophisticated analysis methods.
The laser apparatus used for transient absorption differs in some respects from
that used in fluorescence, principally because to achieve a few per cent change in
transmission, a moderately intense pump pulse is required (typically a 1 m J pulse
is focused into a 10 4 cm 2 cross-sectional area in the sample, compared to the
nanojoule pulse energy used for background-free fluorescence up-conversion mea-
surements). Thus, the preferred source is an amplified laser with a reduced repeti-
tion rate of a few kilohertz, to allow for sample recovery between pulses. In most
cases, the change in absorbance as a result of the pump laser absorption should be
kept small (a few per cent at most); hence, it is necessary to use difference methods
normalised for the pump intensity and extensive signal averaging. Ideally this is
done on a shot by shot basis with alternate pulses being used to record pumped and
un-pumped sample transmission by the probe. With this methodology and a stable
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