Principles of Fluorescence Spectroscopy

126 TIME-DOMAIN LIFETIME MEASUREMENTS
Figure 4.35. Schematic of a streak camera with wavelength resolution.
From [152].
plished at high speed using deflection plates within the
detector (Figure 4.35). In the example shown, the light is
dispersed by wavelength in a line across the front of the
photocathode. Hence, streak cameras can provide simultaneous
measurements of both wavelength and time-resolved
decays. Such data are valuable when studying time-dependent
spectral relaxation or samples which contain fluorophores
emitting at different wavelengths.
The time resolution obtainable with a steak camera is
illustrated in Figure 4.36. The instrument response function
is superior to that found with the fastest MCP PMTs. Earlier-generation
streak camera were delicate, and difficult to
use and synchronize with laser pulses. This situation has
changed, and streak cameras have become more widely
used, especially for simultaneous measurement of subnanosecond
decays over a range of emission wavelength.
One example is the decay of PyDMA (Figure 4.37), which
decays rapidly due to exciplex formation and solvent relaxation
around the charge-transfer (CT) complex. 161 Pyrene in
Figure 4.36. Comparison of the instrument response functions of a
streak camera and two MCP PMTs (R3809U and R2809U). The 600
nm dye laser pulse width was 2 ps. Revised from [158].
Figure 4.37. Time and wavelength-dependent intensity decays of
dimethyl-(4-pyren-1-yl-phenyl)amine (PyDMA) measured with a
streak camera. Revised from [161].
the excited state forms a charge-transfer complex, an exciplex,
with the linked dimethylphenyl group. The chargetransfer
complex is polar and shows a time-dependent spectral
shift to longer wavelengths. This shift occurs rapidly
and is complete in less than 500 ps. The time-dependent
shift to longer wavelength is due to reorientation of the solvent
around the CT state. The high temporal resolution of
the streak camera allows recording of the complete emission
spectra over intervals as short as 10 ps. The data can
also be displayed in a format where the axes are time and
wavelength and the color reveals the intensity (Figure 4.38).
An important development in streak camera technology
is the introduction of the photon-counting streak camera
(PCSC). 162 These devices provide single-photon detection
with high time resolution (14 ps) and simultaneous wavelength
resolution. The photon counts can be collected at
high rates because pulse pileup and dead time are not problems
with these instruments. A PCSC functions the same as