A Low-Cost Time-Resolved Spectrometer for the Study of Ruby Emission

Oct 23, 2017 - a separate, quiet preamplifier supply. The response time of the instrument is easily short enough to accurately resolve the few- millis...
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Technology Report Cite This: J. Chem. Educ. XXXX, XXX, XXX-XXX

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A Low-Cost Time-Resolved Spectrometer for the Study of Ruby Emission George C. McBane, Christian Cannella, and Stephanie Schaertel* Department of Chemistry, Grand Valley State University, Allendale, Michigan 49401, United States S Supporting Information *

ABSTRACT: A low-cost time-resolved emission spectrometer optimized for ruby emission is presented. The use of a Class II diode laser module as the excitation source reduces costs and hazards. The design presented here can facilitate the inclusion of timeresolved emission spectroscopy with laser excitation sources in the undergraduate laboratory curriculum. The ruby decay data show evidence of radiation trapping, an interesting optical phenomenon discussed in the research literature.

KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Fluorescence Spectroscopy, Laboratory Computing/Interfacing, Laboratory Equipment/Apparatus, Lasers, Solids, Spectroscopy

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BACKGROUND The study of ruby time-resolved emission provides an opportunity to introduce physical chemistry laboratory students to an array of topics, including lasers, electronic states, Boltzmann statistics, selection rules, emission, crystal field theory, term symbols, solid-state photophysics, time-resolved measurements, and kinetics. The physical chemistry laboratory text by Nibler et al.1 contains a ruby emission experiment that emphasizes temperature dependence and Boltzmann statistics. A paper in this Journal2 presents an experiment that emphasizes features of ruby photophysics that made ruby attractive as the lasing medium for the first laser.3 Other authors have described ruby emission experiments that demonstrate methods of detecting time-varying signals.4,5 The experiments in refs 1, 2, and 4 may present cost and/or safety barriers for laboratory programs. Reference 1 suggests the use of a high-power (Class IV) laser as the excitation source. Many physical chemistry laboratories would be able to afford only one version of such a setup. Also, the use of Class IV lasers requires very careful attention to eye safety. The setups described by Chandler et al.4 could be replicated for several laboratory groups but contain high-cost components (chopper, lock-in amplifier, photomultiplier tube). Esposti and Bizzochi2 use a commercial fluorimeter; it is unlikely that many teaching laboratories would have access to multiple fluorimeters, although the measurements are quick, which would allow groups to rotate through the use of the instrument. Light source miniaturization (notably, light-emitting diode sources and diode-pumped lasers such as those employed in laser pointers) and improvements in detector technology have © XXXX American Chemical Society and Division of Chemical Education, Inc.

allowed the construction of low-cost spectroscopic devices for teaching laboratories. Wilson and Wilson6 point out advantages of lowering the cost and reducing the “black box” property of spectroscopy in teaching settings. Reference 6 includes a helpful set of references that discuss “hand-built” (their term) colorimeters and spectrometers. References 7−14 provide additional references that describe low-cost absorption and emission spectrometer designs, including spectrometers capable of time-resolved measurements. The ruby emission experiments in refs 4 and 5 represent a welcome movement toward lowercost emission spectrometers for time-resolved emission. In this technology report, we describe a low-cost, relatively eye-safe emission lifetime spectrometer optimized for ruby emission. Our design is similar to that described in a lab handout by Heiman5 but uses a safer Class II laser module and replaces a digital oscilloscope and function generator with an inexpensive data acquisition module. The low-power and inexpensive Class II laser is made to mimic a pulsed source via an on−off circuit that controls the experiment. The design presented here is unique in its low-cost, low-hazard characteristics. Our aim is to increase accessibility to an experiment in a canonical physical chemistry laboratory textbook,1 allowing more students to obtain hands-on experience with timeresolved emission measurements for solid-state samples. To this end, the Supporting Information contains detailed information about optical bench components and circuit design Received: June 30, 2017 Revised: October 23, 2017

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DOI: 10.1021/acs.jchemed.7b00438 J. Chem. Educ. XXXX, XXX, XXX−XXX

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as well as computer codes for instrument control, data collection, and data fitting.

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HAZARDS The laser diode module excitation sources used at GVSU for this experiment were certified by the manufacturer as Class II lasers; we recommend that adopters act likewise, using certified Class II laser modules or verifying continuous-wave power below 1 milliwatt with a calibrated power meter. (Laser pointers are regulated in the United States by the Food and Drug Administration;15 however, pointers up to class 3a/3R are permitted, and pointers whose power exceeds even that limit are available for purchase.) Class II lasers are relatively safe because the human blink reflex will normally prevent eye damage, though Class II sources can cause eye damage under conditions of focused light or prolonged viewing.16,17 Good laser safety practices remain important. The optical bench should be surrounded by an enclosure to prevent stray beams from entering the room. Beams should be kept roughly horizontal. All or most components of the apparatus should have flat black surfaces to prevent reflections. The surfaces of shiny, faceted samples should be roughened. The ANSI laser standard16,17 does not require laser safety goggles for Class I and II sources, though instructors may choose to have students wear low-opticaldensity laser alignment goggles in order to instill good habits. Our apparatus presents no chemical or electrical hazards.

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Figure 1. Photograph of an optical bench, top view. Labeled parts: (a) Heat sink containing the diode laser module; black and red leads attach to a power supply through a control circuit that turns the laser on and off, allowing for data collection during the off part of the cycle. (b) Steering mirror. (c) Ruby sample attached to mount with adhesive putty. (d) Collection lens. (e) Photodiode with theater gel filter (for suppression of 532 nm light) mounted in the opening of the photodiode housing (the gel filter is not visible in the photograph). (f) Home-built preamplifier with a twisted-pair (Ethernet-style) cable leading to the instrumentation amplifier and analog-to-digital module. (g) Optical enclosure (lid removed). Original photo credit: GVSU, University Communications/Bernadine Carey-Tucker.

SAMPLES

A natural ruby sample was donated by the GVSU Geology Department. A synthetic ruby sample was purchased from an online gemstone vendor. The synthetic ruby arrived polished and faceted; it was roughened with a rotary tool to reduce multiple reflections that might exit the sample and cause stray beams. Ruby characteristics, including emission lifetimes, can vary from sample to sample depending on the Cr3+ ion concentration.18−24 Well-characterized samples with low Cr3+ concentrations are available from laser rod vendors.

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immunity and switch-selectable gain that accommodated varying brightness of emission from individual ruby samples. Instrument Response Time

Figure 2 shows the postillumination response of one of our instruments for both a synthetic ruby sample and a sample of rhodamine B in ethanol, whose fluorescence lifetime is a few nanoseconds. The rhodamine trace effectively shows the instrument temporal response function. The periodic fluctuations in the baseline of the rhodamine sample, collected at higher gain, are caused by digital noise on the USB-derived +5 V power supply to the preamplifier and can be eliminated with a separate, quiet preamplifier supply. The response time of the instrument is easily short enough to accurately resolve the fewmillisecond ruby decay.

SPECTROMETER DESIGN

The emission apparatus is described briefly here. Detailed information is given in the Supporting Information. Optical Bench

Figure 1 is a photograph of an optical bench arrangement. A detailed description is given in the Supporting Information. Electronics for Control of the Laser Duty Cycle and Data Collection

Software

The laser module power supply was switched on and off at a rate that allows ruby decay lifetimes (1−4 ms) to be observed. We used a square wave provided by the digital output of a LabJack U3-HV module to drive a transistor switch. The photocurrent from the detector was converted to a voltage by a home-built transimpedance preamplifier connected directly to the detector. The preamplifier used an 820 kΩ feedback resistor, and its bandwidth was deliberately limited to about 9 kHz in preparation for sampling at 20 kHz or lower. The preamplifier output was connected with twisted-pair cables to the differential input of a LabJack LJTickInAmp instrumentation amplifier mounted on a LabJack U3-HV data acquisition module. The instrumentation amplifier provided both noise

Home-written Python code was used for experiment control and data collection; the code is provided in the Supporting Information. Visualization

We used an oscilloscope so that students could observe the signal in real time during setup and data collection, though it is not strictly necessary.

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USE WITH STUDENTS In the most recent implementation of this experiment at GVSU, pairs of students collected ruby emission data with preconstructed ruby emission instruments. During this B

DOI: 10.1021/acs.jchemed.7b00438 J. Chem. Educ. XXXX, XXX, XXX−XXX

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was avoided, as students could easily see and understand the functions of the optical bench components. Student pairs worked to optimize the alignment of the laser and collection lens, collect ruby decay curves, and fit them. Data collection and fitting took roughly 2 hours. Students were asked to fit the data to single-exponential decays and interpret the resulting residual plots. They were able to see significant differences in effective lifetimes between natural and synthetic samples, note slight changes in observed decay times because of different optical alignments, and observe clear evidence that the singleexponential model was inadequate. The laboratory handout is included in the Supporting Information. We do not present teaching materials for the use of the ruby emission apparatus as part of a careful study of ruby solid-state photophysics because a thorough treatment of such an investigation exists in a wellknown textbook.1 Use of the spectrometer design presented in this technical report will allow adopters to perform the textbook experiment in ref 1 with significantly reduced costs and hazards.

Figure 2. Averaged decay traces showing the emission intensities from ruby and rhodamine samples. The 532 nm excitation laser turns off at t = 0. The ruby curve (red) represents 14 individual decays, and the rhodamine curve (black) represents 99 individual decays; the rhodamine data points are connected by a line to make the sharp transition more visible. The indicated gains are those of the instrumentation amplifier; the ruby trace has also been expanded vertically for easy comparison.

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DATA

Figure 3 shows a set of decay data obtained from a synthetic ruby sample along with fitted curves and residuals from both single- and double-exponential models. The residual curves make it clear that the single-exponential fit is inadequate, although the fitted curve in the lower left panel appears satisfactory on casual inspection. The residuals from the double-exponential fit, however, are satisfactory. This doubleexponential behavior is well-understood,18−24 although we have not found it mentioned in the previous pedagogical literature. It

investigation, the students were able to become familiar with electronic data collection, use software to fit data to a nonlinear model, and use the fit information to evaluate the fitting model. Additionally, the black-box nature of commercial spectrometers

Figure 3. The bottom left panel shows emission data (black) from a roughened synthetic ruby sample, fitted to a single-exponential decay (red). Corresponding residuals are shown in the top left panel. The right panels show a biexponential fit [I(t) = A1e−t/τ1 + A2e−t/τ2 + c] to the same data. Uncertainties represent standard uncertainties in the fitted parameters. The same ruby decay data appear in Figure 2. C

DOI: 10.1021/acs.jchemed.7b00438 J. Chem. Educ. XXXX, XXX, XXX−XXX

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appears because of radiation trapping: some photons emitted from within the crystal do not immediately escape to the detector but can be absorbed by other ions in the crystal and then reradiated. The effect depends upon the chromium ion concentration, the sample size, and the number and efficiency of internal reflections.18−24 Toci23 demonstrated that the shorter of the two time constants is close to the intrinsic lifetime of the excited ions. We also see evidence of radiation trapping with our natural ruby sample.

SUMMARY A low-cost, low-hazard emission spectrometer can be constructed to collect time-resolved ruby emission traces. The inexpensive spectrometer presented here can allow students to gain hands-on experience with a variety of topics that might otherwise be under-represented in the physical chemistry undergraduate laboratory curriculum. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.7b00438. Description and an additional photographs of the optical bench, parts list with cost estimates, circuit diagrams, photographs, and descriptions (ZIP, PDF) Zip file containing (i) the Python code for laser control and data collection (test7.py), (ii) a Genplot macro code to generate averaged and collated decay curves (ce050a.mac), and (iii) a Genplot macro code to fit the averaged decay curve (ce050b.mac) (Python itself may be obtained free at www.python.org, and Genplot is a free plotting and data analysis program available at www.genplot.com) (ZIP) Laboratory handout (PDF)

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REFERENCES

(1) Nibler, J. W.; Garland, C. W.; Stine, K.; Kim, J. Experiments in Physical Chemistry, 9th ed.; McGraw-Hill: New York, 2014; pp 484− 492. (2) Esposti, C. D.; Bizzocchi, L. Absorption and Emission Spectroscopy of a Lasing Material: Ruby. J. Chem. Educ. 2007, 84 (8), 1316−1318. (3) Maiman, T. H. Stimulated Optical Radiation in Ruby. Nature 1960, 187, 493−494. (4) Chandler, D. E.; Majumdar, Z. K.; Heiss, G. J.; Clegg, R. M. Ruby Crystal for Demonstrating Time- and Frequency-Domain Methods of Fluorescence Lifetime Measurements. J. Fluoresc. 2006, 16, 793−807. (5) Spectroscopy of Ruby Fluorescence. http://www.northeastern. edu/heiman/3600/RUBY.pdf (accessed October 2017). (6) Wilson, M.; Wilson, E. Authentic Performance in the Instrumental Analysis Laboratory: Building a Visible Spectrophotometer Prototype. J. Chem. Educ. 2017, 94 (1), 44−51. (7) Wahab, M. F. Fluorescence Spectroscopy in a Shoebox. J. Chem. Educ. 2007, 84 (8), 1308−1312. (8) Payne, S. J.; Zhang, G.; Demas, J. N.; Fraser, C. L.; DeGraff, B. A. Laser Phosphoroscope and Applications to Room-Temperature Phosphorescence. Appl. Spectrosc. 2011, 65 (11), 1321−1324. (9) Wigton, B. T.; Chohan, B. S.; McDonald, C.; Johnson, M.; Schunk, D.; Kreuter, R.; Sykes, D. A Portable, Low-Cost, LED Fluorimeter for Middle School, High School, and Undergraduate Chemistry Labs. J. Chem. Educ. 2011, 88 (8), 1182−1187. (10) Vietmeyer, F.; Kuno, M. Construction and Demonstration of a Low-Cost Educational Transient Absorption Spectrometer. Chem. Educ. 2013, 18, 196−202. (11) Tommey, T.; Wagner, D.; Shaw, J., Jr.; Bellamy, M. K. A Breadboard Filter Photometer/Fluorometer that Illustrates the Effects of Spectral Bandpass on Calibration Curves While Measuring FreeChlorine in Tap Water and Quinine in Tonic Water. Chem. Educ. 2014, 19, 175−181. (12) Larsen, M. C.; Perkins, R. J. Flash Photolysis Experiment of oMethyl Red as a Function of pH: A Low-Cost Experiment for the Undergraduate Physical Chemistry Lab. J. Chem. Educ. 2016, 93 (12), 2096−2100. (13) Kvittingen, E. V.; Kvittingen, L.; Melø, B. T.; Sjursnes, B. J.; Verley, R. Demonstrating Basic Properties of Spectroscopy Using a Self-Constructed Combined Fluorimeter and UV-Photometer. J. Chem. Educ. 2017, 94 (10), 1486−1491. (14) Desktop Spectrometry Starter Kit 3.0 Assembly Instructions. https://publiclab.org/notes/abdul/10-13-2016/desktop-spectrometrystarter-kit-3-0-instructions (accessed October 2017). (15) U.S. Food and Drug Administration. Does FDA regulate these new powerful laser “pointers” and are they hazardous? https://www. fda.gov/aboutfda/transparency/basics/ucm302664.htm (accessed October 2017). (16) ANSI Z136.1-2014: American National Standard for Safe Use of Lasers; Laser Institute of America: Orlando, FL, 2014. (17) ANSI Z136.5-2009: American National Standard for Safe Use of Lasers in Educational Institutions; Laser Institute of America: Orlando FL, 2009. (18) Imbusch, G. F. Energy Transfer in Ruby. Phys. Rev. 1967, 153 (2), 326−337. (19) Auzel, F.; Bonfigli, F.; Gagliari, S.; Baldacchini, G. The Interplay of Self-Trapping and Self Quenching for Resonant Transitions in Solids; Role of a Cavity. J. Lumin. 2001, 94−95, 293−297. (20) Auzel, F.; Baldacchini, G. Photon Trapping in Ruby and Lanthanide-doped Materials: Recollections and Revival. J. Lumin. 2007, 125, 25−30. (21) Guy, S. Modelization of Lifetime Measurement in the Presence of Radiation-Trapping in Solid-State Materials. Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 73, 144101. (22) Kühn, H.; Fredrich-Thornton, S. T.; Krankel, K.; Peters, R.; Petermann, K. Model for the Calculation of Radiation Trapping and Description of the Pinhole Method. Opt. Lett. 2007, 32 (13), 1908− 1910.

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Technology Report

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Stephanie Schaertel: 0000-0003-1531-4072 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Grand Valley State University for a sabbatical period for S.S. during which work was done on this project. Nick Timkovich wrote the original code upon which the current version of the interfacing program was based. Jeff Woollett designed and built the modular enclosure for the optical bench. Thomas Pentecost reviewed the manuscript and made suggestions. We thank physical chemistry laboratory students at GVSU for trying versions of the apparatus. Kevin Cole of the GVSU Geology Department provided the natural ruby sample. Alex Wong figured out how to roughen a synthetic ruby sample. We thank Mary Karpen for help with figures, including the artwork in Figures a2 and c4 in the Supporting Information. Photos in Figures 1, a2, and c4 are modified from original photographs by Bernadine CareyTucker. We thank Blair Miller for helping S.S. take the photograph for the graphical abstract. D

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(23) Toci, G. Lifetime Measurements with the Pinhole Method in Presence of Radiation Trapping: I − Theoretical Model. Appl. Phys. B: Lasers Opt. 2012, 106, 63−71. (24) Chen, C.-H.; Wu, Y.-H.; Fan, C.-P.; Wei, T.-H. Saturation of Radiation Trapping and Lifetime Measurements in Three-Level Laser Crystals. Opt. Express 2012, 20 (23), 25613−25623.

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DOI: 10.1021/acs.jchemed.7b00438 J. Chem. Educ. XXXX, XXX, XXX−XXX