Principles of Fluorescence Spectroscopy

PRINCIPLES OF FLUORESCENCE SPECTROSCOPY 113
Figure 4.19. Coaxial nanosecond flashlamp. Revised and reprinted
with permission from [52]. Copyright © 1981, American Institute of
Physics.
occurred whenever the voltage across the electrodes
reached the breakdown value. Almost all presently used
lamps are gated. The electrodes are charged to high voltage.
At the desired time the thyratron is gated on to rapidly discharge
the top electrode to ground potential. A spark discharge
occurs across the electrodes, which results in a
flash of light. These pulses are rather weak, and one frequently
has to block the room light to see the flash with the
naked eye.
Compared to ion or Ti:sapphire laser sources, flashlamps
are simple and inexpensive. Hence, there have been
considerable efforts to obtain the shortest pulse widths and
highest repetition rates. Despite these efforts, the flashes are
much wider than that available with a laser source. One of
the shortest time profiles is shown in Figure 4.20 (left),
where the full width at half maximum is 730 ps FWHM.
More typical is the 1.2-ns FWHM for a flashlamp in which
the gas is an argon–hydrogen mixture. Also typical of flashlamps
is the long tail that persists after the initial pulse. The
spectral output of the pulse lamps depends on the gas, and
the pulse width typically depends on both the type of gas
and the pressure. Hydrogen or deuterium (Figure 4.21) provides
a wide range of wavelengths in the UV, but at low
intensity. Nitrogen provides higher intensity at its peak
wavelengths, but little output between these wavelengths. In
recognition of the growing interest in red and near-infrared
(NIR) fluorescence, flashlamps have been developed with
red and NIR outputs. 53–54 Given the availability of pulsed
LDs and LEDs laser diodes in the red and NIR, there is less
motivation to develop red and NIR flashlamps.
The most significant drawback of using a flashlamp is
the low repetition rate. The fastest flashlamps have repeti-
Figure 4.20. Time-profiles of coaxial flashlamps. Revised and reprinted from [9] and [53]. Copyright © 1991, American Institute of Physics.