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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.
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.
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 µm. 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 µm 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.
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 µm to 5.4 µm. 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 2x10E4 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
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.
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 1µV 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.