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- Introduction
Luminescence spectroscopy is the spectroscopy of excited systems which
are in the process of decaying. Excited systems on the way down to the
ground state have often the chance to take alternate routes. Each step
on the way down may include two mutually exclusive events: radiative
and non-radiative decays. The probabilities of the two events add up
to the total probability of decay.
Every luminescent system is particular and differs from all others.
Even if the energy level schemes of two luminescent systems are
similar, their kinetic behavior may be very different.
The luminescence centers in solids are connected to the host and can
in principle provide information regarding the collective excitations
of the solid, such as phonons.
Luminescence spectroscopy relies on four basic measurements:
1. Absorption spectra
2. Luminescence spectra
3. Excitation spectra
4. Response to pulsed excitation
Technical improvements or breakthroughs, while providing better and
faster acquisition of data, have not, and most probably will not,
produce any "conceptually new" addition to these four basic
measurements.
- How We Measure
a. Absorption Spectra
We use an absorption spectrophotometer, which provides spectra in
optical density (OD) or in percentage of transmission. The
information is connected to small changes in the intensity of a large
signal.
b. Luminescence spectra
The exciting sources are lamps (which provide wide band excitation) or
lasers (which provide selective excitation). For CW sources, a
combined chopper and lock-in amplifier is used. For pulsed sources, a
boxcar integrator is used. The detection is done with a
photomultiplier or an infrared detector. The advantage of these
measurements is that they are absolute in the sense that they emerge
from the zero line.
c. Excitation spectra
We monitor a certain line while continuously varying the wavelength of
the exciting light. A small monochromator is set between this source
(generally a wide-source ribbon lamp) and the sample. A filter is set
between the sample and the detector to select the particular line
under observation. The combined chopper and lock-in amplifier is used
since the source is generally continuous. In principle, with a pulsed
source, a boxcar integrator could be used. The detection is done by a
photomultiplier or an IR detector.
d. Response to Pulsed Excitation
For a broad band excitation a flash tube is used. For a selective
excitation we generally use a pulsed laser. Alternatively a combined
flash-lamp and interference filter can be used. We monitor the decay
of a particular line by putting an interference filter between the
sample and the detector.
If the excitation goes through n steps before reaching the initial
level of the transition under observation, the decay contains (n+1)
exponentials. Some of these exponentials may not be observed because
they are too fast.
The detection is done by a photomultiplier or an IR detector. The
measurement may be done directly from an oscilloscope or by means of a
boxcar integrator.
- Resources
The laboratory setup for the measurements for
pulsed
excitation and for
continuous source
are available. The absorption spectrophotometer is not shown in the
drawings because it is a stand-alone instrument and does not interact
with any of the components shown in the figures. In the following
paragraphs each component will be briefly described.
3.1. Major Equipment
a. Absorption Spectrophotometer
The UV/VIS /NIR spectrophotometer in our laboratory is a Perkin-Elmer
Lambda 9. It is a fully automatic ratio-recording double-beam, double-monochromator
spectrometer.
For each monochromator there are two gratings with 1440 lines/mm and
360 lines/mm for UV/VIS and NIR ranges respectively. The sources are
deuterium lamp for UV range and tungsten-halogen for the VIS/NIR
ranges. The detectors are a photomultiplier for the UV/VIS range and a
PbS detector for NIR range. The useable wavelength range is from 185
to 3200 nm with an accuracy of 0.2 and 0.8 nm for UV/VIS and NIR
ranges respectively, and a repeatability of 0.05 and 0.2 nm for the
same ranges. The system can be operated in three modes: time drive,
scan, and wavelength. Parameters for the three modes are programmed
into the computer for the instrument. The output is in a standard
RS-232 bi-directional format.
b. Monochromator
Our laboratory is equipped with a McPherson model 2051 one-meter
scanning monochromator. The instrument is fitted with a 600 groove/mm
grating, blazed at 1.25 mm. It has a
resolution and wavelength resetability of 0.1 A with repeatability of
+/- 0.05 A. The dispersion is 8.33 A/mm with a 1200 groove/mm grating.
The scan controller used is model 789A-3, allowing to set the scanning
speed,manually with a thumbwheel from 0.1 to 999.9 angstroms per
minute. Samples are scanned to a maximum wavelength of 15,740 A. The
entrance and exit slits are adjustable from 5 mm
to 2 mm.
c. Nitrogen Laser
The nitrogen laser is a convenient and economical way to pump the dye
laser. It produces a short, nanosecond ultraviolet (337.1 nm) pulse.
Our laser is a Molectron UV 12, capable of up to 50 pulses per second
(pps). A capacitor of 20 nF at 20kV is discharged across the nitrogen
gas column by a thyratron, type JAN 8613. The average output power is
90 mW, with a pulsed energy of 2.5 mJ at 20 pps. The output pulse is
nominally 10 ns (FWHM).
d. Dye Laser
The dye laser is the Molectron DL 14. A basic dye laser consists of an
oscillator cavity with grating, front mirror, beam expander, cuvette
dye cell and nitrogen beam focusing lens. To increase system
efficiency, beam quality and amplitude stability, the DL 14 model adds
to it a dye amplifier assembly, and beamsplitters. These additions
are important for narrowband and frequency doubling applications. The
tuning range of the laser is 360-950 nm, with an absolute accuracy of
0.2 nm and a reproducibility of 0.01 nm. The energy conversion
efficiency is 15% with an amplitude stability of 5%. The output pulse
duration is 6 to 8 ns in the fundamental mode and 3 to 4 ns in the
frequency doubled mode.
e. Argon Laser
The CW laser in the spectroscopy laboratory is the Argon-ion laser,
the Omnichrome model 532. This CW air-cooled laser is capable of
providing 300 mW of output power. The instrument is a positive column
gas discharge laser using singly ionized argon as the optical gain
medium. Its plasma tube is a rugged metal-ceramic device. The
resonator is a cast aluminum alloy to stable performance and pointing
stability. Laser output is produced in nine, selectable wavelengths,
from 454 to 514 nm. We use all wavelengths in our experiments.
f. Ti-Sapphire Laser
The laboratory is equipped with a Schwartz Electro-Optics Titan-P
pulsed, tunable Ti-sapphire laser. It can be tuned from 680 nm to 940
nm, and produces 10 ns pulses with energies up to ~100 mJ. The system
is typically operated at 10 Hz. It is equipped with a second harmonic
generator crystal, resulting in laser pulses between 350 nm and 430
nm. The resonator utilizes two Ti-sapphire crystals, a multiple prism
tuning system, and a graded reflectivity mirror as the output coupler
in an unstable resonator configuration. The Ti-sapphire crystals are
pumped with a frequency-doubled pulse from a Q-switched Nd:YAG
oscillator/amplifier system, thus also making accessible laser pulses
at both 1064 nm and 532 nm.
3.2. Detectors
a. Infrared Detector
The IR detector used in continuous optical signal is of the Indium-Antimonide
(InSb) type. This is a sensitive and fast (7 ns) photodiode with
useful spectral range from 1 m to 5.4 m. The one we are using is the
Kolmar Technologies model KISDP-1-J1/DC. It is a 1x1 mm photodiode
integrated with a pre-amplifier whose bandwidth is from DC to 15 MHz.
The responsivity of the detector is better than 2x 10E4 V/W and a
spectral response (D*)
of 1E11. The Dewar can be funnel-filled with liquid nitrogen that can
hold the temperature for 12 hours.
b. Photomultiplier Tube (PMT)
The photomultiplier tubes used are the Hamamatsu types R1387 and 7102.
The useful spectral response is from 300 to 850 nm ( S-20) curve for
the R1387, and 400 to 1200 nm ( S-1) for the 7102. The PMT is powered
by a bench-top variable power supply. The voltage is adjusted, as
required, to prevent saturation of the output. The output current of
the PMT is proportional to the voltage applied to the bleeder ladder
network. Cooling for the PMT is provided by a thermoelectric cooler
that cools the tube to about 50 deg. C below ambient.
3.3. Sample Environment
a. Cryogenic Cooler
The cryogenic refrigerator operates on the Gifford-McMahon (GM)
principle using a closed helium gas cycle. The advantage of the GM is
that the compressor unit can be separated from the cold head which is
part of the sample chamber, thus allowing the flexibility of mounting
the cold head in any position. The compressor and the cold head are
connected with pressure flexible tubing. The system is filled with
helium to a pressure of 16 bar, capable of cooling the sample to 20 K.
b. Vacuum System
The vacuum system lowers the pressure of the sample chamber to about
2x10-5 Torr in two stages. In the first stage, a mechanical
"roughing" pump achieves about 30 microns of vacuum. The pressure is
further lowered by a second stage consisting of a compact air-cooled
diffusion pump, capable of 1x10-6 Torr for small volumes.
c. Sample Chamber
The sample chamber was manufactured by Janis. The sample can be easily
mounted on a pedestal and adjusted for orientation with the
monochromator input FOV and the light source beam. It has five optical
windows for versatility of beam positioning and steering. The sample
can be cooled to about 20 K in a vacuum of 10-5 Torr.
Because of the vibration and noise generated by the cryogenic pump,
the sample chamber is mounted on a concrete column standing on the
floor, weighing over 200 pounds. The column is adjustable for the
correct sample position in the line of sight with the monochromator
input slit.
3.4. Signal Conditioning
a. Pre-Amplifier
The output current of the PMT anode is read, as a voltage, with a load
resistor with respect to ground potential. The effective bandwidth
(BW) of the output pulse is inversely proportional to the product of
the resistor and all parasitic capacitance in the PMT output circuit,
including cabling. Thus to increase the bandwidth, an amplifier is put
close to the PMT output and the load resistor is made as small as
possible. The pre-amplifier is home-made using an ultra-low
distortion, high speed integrated circuit, the Analog Devices AD 8008.
The chip has a bandwidth specification of 230 MHz for a 2x voltage
gain. With the pre-amplifier in place the PMT output is 50 to 1000
W, depending on the output signal strength.
The pre-amp gain can be two or twenty. The pre-amp is powered by AA
cells.
b. Chopper
The chopper is placed at the entrance of the monochromator continuous
wave (CW) optical signal. In essence, the mechanical chopper, operated
at nominally 250 Hz, becomes carrier frequency of the optical signal,
thus removing all the DC biases introduced by the instrumentation and
amplifiers.
c. Lock-in Amplifier
The purpose of this instrument is to amplify the low level AC signal
of the PMT output, when the signal is chopped, and demodulate it to
recover the base-band optical signal. In this set-up we use the EG&G
Model 5206. It has a 1 mV to 5 V rms input
sensitivity, with a carrier frequency of 0.2 Hz to 200 kHz. Signal to
reference phase shift control can be in 0.025 degree steps from 0 to
360 degree C.
3.5. Data Acquisition
a. Box-Car Sampling System
While there are new computer methods to acquire continuous pulsed
signals, the spectroscopy laboratory is equipped with a dependable and
proven signal sampling system known as a box-car averager. It is
basically a sample-and-hold circuit which integrates the signal during
a sampling window and then stores it. The sampling window and its
position in the signal can be selected as desired. For repetitive
signals, the sampling window is shifted forward at a new portion of
the signal during the next cycle. A set of samples is thus accumulated
during the time of interest in the signal. The sampling window width
and the delay are set on the panel of the box-car averager. In this
manner background noise is not integrated because the signal is
captured only during the time of interest, and the sample-and-hold
integrator removes fluctuations (noise) during this time. This method
of signal capturing is particularly suitable when the repetitive
signal is a very small fraction of the duty cycle. The signal is
captured with a set of samples only in the interval of interest, and
then averaged over many cycles.
The instruments used are the Signal Recovery Model 4121B, with the
dual A/D converter module 4161A. This system is capable of 1.5 ns
sampling gate width, an input bandwidth of 450 MHz and a signal
repetition rate of 80 kHz, maximum. The spectroscopy laboratory has
recently acquired LabView data acquisition software from National
Instruments. The instruments above can be interfaced with LabVIEW
compatible software drivers so that the data logging will be more
automated. The gate width and delay can also be controlled by LabVIEW
software with Delay Generator Model 9650A.
b. Data Logging Computer
The output of the box-car averaging system is sent to a data logging
computer that is programmed to generate plots of intensity vs.
wavelength. These plots are formatted to be published in reports. When
we introduce LabVIEW, the data logging control and programming will be
done using a graphical interface to the program.
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