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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 Sensors
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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