: Lasers and Light Sources
Basic Spectroscopy and Analysis
In a typical setup for viewing the emission spectra of gases, a basic lab spectroscope is seen with a mercury tube as a source. Light from the source passes through an adjustable slit and is dispersed into it's components for analysis.
We begin with an introduction to use of the spectroscope. The professor will begin with a brief description of the unit immediately at the start of the lab followed by a lab in which students will view the emissions of various light sources. Do NOT be late or you will miss the introductory demonstration which will NOT be repeated (aside from which you will lose marks for late arrival).
Begin by installing a mercury spectrum tube into the power supply (ensure the supply is off before installing the tube). Orient the tube so that light from the discharge tube passes through the entrance slit of the spectroscope. Close the slit slightly and observe the zeroth order (where light from the entrance slit passes right through the spectroscope. Read the angle on the vernier - this is "true zero". Now view the spectrum which is visible when the eyepiece telescope is rotated. Adjust both telescopes to sharpen the observed lines as much as possible.
Now decrease the slit width until the spectral lines appears sharp and detailed. Note the output of this type of source and the angle observed on EACH SIDE of "zero". You will notice that "zero" on the vernier scale is not zero at all ... there is an offset error which must be compensated for all readings. If, for example, the zeroth order was observed at 358.5 degrees (now true zero), and an emission line was then found at 5.5 degrees, the actual angle of the emission line would be 7.0 degrees.
Compare the observed position of all known mercury lines with the predicted locations calculated as part of the prelab assignment. The average difference in angle will provide an error figure (i.e. +/- degrees) from which one may determine a wavelength. If, for example, the expected line (from the prelab calculations) was at 5.6 degrees and the actual line was found at 5.8 degrees the error tolerance is 0.2 degrees and this may be added/subtracted to a 'found' angle to determine the error (so that the observed angle is actually 5.8 +/- 0.2 degrees). From that figure, a wavelength range may be calculated which shows the range within which the found 'line' falls.
Analyze the data (later, after the lab session), to deduce, as well, if there is an offset error (for example, if all readings are "off" by the same amount). If so, include this error correction as part of your calibration (i.e. "Subtract xxx.x degrees from all readings"). This type of compensation can only be applied if the error is found to be the same for all readings.
DO this type of analysis for each line found and summarize in chart form in your lab report with columns as follows:
Finally, determine the average error tolerance (in degrees) for all measurements. If that error tolerance is similar for all readings, note this and apply it to all future readings.
Now, use an Incandescent Lamp source. Set-up the lamp socket on a breadboard (using clamps in the Newport kits) and screw in the long, tubular lamp. Orient the lamp so that the filament is vertical as shown in the photo and point the entrance slit of the spectroscope at the lamp. Open the slit fully and orient the spectroscope so that the continuum spectrum is visible.
Note the output of this type of source and note the relative intensity of the red, yellow, and blue ends of the spectrum as observed by your eye. Identify the first, second, and third orders as expected from the grating and note the approximate angles of these (record the angle of the red and violet ends of each order) ... the angles should agree with the formula outlined in section 2.2. Note, too, the dispersion of the spectrum at higher orders (m=2, m=3) and any possible overlap of these orders. This is the only time observation of multiple orders is required.
Deconstructing the Fluorescent Lamp
Remove the incandescent lamp and install a Fluorescent Lamp in the same socket. Adjust the slit again and note exactly what you see. The light appears as a series of discontinuous lines and bands. For each line seen, record the angle (and hence compute the wavelength of that line). For each band seen, record the angles of the edges of the band so that its apparent width (in nm) may be computed (i.e. "a cyan band was observed spanning from xxx.xx nm to yyy.yy nm"). As discussed in lectured, a fluorescent light uses a basic gas discharge to emit radiation that is absorbed by a coating on the inside of the tube which re-emits photons as white light. The basic gas is mercury vapour with a buffer gas. Note emitted bands carefully and compare them with the 'pure' mercury discharge in the first part of this lab. Comparing with the mercury spectrum determines which lines come from mercury and which do not. (By noting all bands/lines and observing those not attributed to mercury, determine where the other bands come from - perhaps the buffer gas or the phosphor coating on the inside of the tube - and include this in the lab write-up).
In the write-up, create a chart of observed lines and bands and attribute each to a source (Hg, buffer gas, phosphor) ... lines from Hg or the buffer gas are expected to be sharp (like those from the pure mercury discharge seen while calibrating the spectroscope in Part A) while emissions from a phosphor take the form of broad bands (a 'phosphor' is a complex molecule). Where a line is assigned to a gas (mercury or buffer), cite the gas and the known wavelength to justify the assignment - a bit of research will reveal common buffer gases used in fluorescent tubes.
Now that Part A and B are complete, begin the lab write-up. DO NOT wait until the end of next week since this will mean a very intense amount of work next week. Spread the workload out accordingly. The entire background section as well as most of the procedure section and a good portion of the observations may be completed NOW ... the lab write-up will consume more time than one evening to complete.
Observe the spectra of two gas discharge tubes (hydrogen and neon) by placing them in the power supply (switched OFF, of course, when the tubes are installed or removed). Do not leave the tubes ON over 30 seconds at a time (the manufacturer specifies a duty cycle of 30 seconds on, 30 seconds off): This rule must be observed for some tubes such as hydrogen (which run incredibly hot) more than for others (like neon which can run forever). Use common sense and allow the tube time to cool in between uses (especially with hydrogen)! As you observe the emitted lines, close the slit further to define them better.
Install mercury first and quickly verify the calibration from the first period. You may have to repeat the calibration procedure.
Use Hydrogen and Neon gas tubes and note the spectrum of each. In the case of neon, there may be too many lines to record, record the major (brightest) lines (at least four) and note especially the two green lines in the spectrum (so six lines are required, at a minimum). For each of these gases, compare the spectrum you have observed to the 'known' spectrum from a physics reference (such as the NIST eBook) as you did in the prelab submitted last week. Make a chart for each gas showing your recorded wavelength, an approximated tolerance for each line based on an estimate of how accurately you can read the instrument (the "+/-" degree figures discussed in part A), the accepted wavelength for this line, and the % error in your observation (the difference between your center reading and the accepted reading ratioed over the accepted value). In some cases you might well 'miss' a weak emission line - don't sweat it!
Note all lines observed in the hydrogen and neon spectra and compare to the 'known' spectrum for each. In class we have discussed the basic physics (quantum) behind these transitions. Relate these lines seen to their transitions (i.e. determine the levels involved in each transition).
For this part of the experiment you will use the OceanOptics spectrometer with OOIBASE32 software in V12. Like the manual spectrometer, it must be calibrated first using a known gas (mercury, again).
Install a mecury tube into the power supply in V12. Adjust the gain of the spectrometer via the "integration time" parameter until sharp peaks are evident for the VISIBLE (400nm to 700nm range) lines of mercury - ignore IR lines. Save both the graphical spectrum (Copy -> Graphical then paste into a graphics utility like 'paint' and save to your USB key) as well as the graphical data (File -> Save and save the data file to your USB key). The data is saved as wavelength/intensity pairs which may be imported to a spreadsheet and graphed or analyzed). Use this data to determine an offset for wavelength in order to calibrate the spectrometer.
Now, install the tube with the 'mystery' gas - each bench will receive a different one - into the power supply in V12. Use the Ocean Optics spectrometer to identify key emission lines. Again, adjust the integration time to observe sharp peaks in the visible region and save both graphical and wavelength data to your memory key. Compare to known spectra (e.g. using the NIST eBook) and identify the gas. The possible gases in the tube (manufactured by Electro-Technic Products are:
Argon Gas Bromine Vapor Carbonic Acid Carbon Dioxide Chlorine Gas Helium Gas Iodine Vapor Krypton Nitrogen Gas Oxygen Gas Water Vapor XenonGases which are _not_ included are Hydrogen, Mercury Vapor, and Neon Gas since these are used as "known" gases in the previous part of this experiment. Each tube has a serial number - be sure to record it as there are many different gases in the room! (and be sure to report it in the lab write-up).
Observed lines can be 'off' by the wavelength error determined using the mercury tube in part A. Determine the error (on several known mercury lines) and apply this as a correction factor to allow the accurate determination of actual wavelength of emission lines - this is crucial to accurate analysis.
Your analysis must attempt to 'match' no less than five lines which you must outline in a chart (showing the wavelength of the observed line and the known wavelength of the proposed unknown gas line). NEVER use "colour" or other qualitative analysis to determine the gas - use wavelengths only (hence why you saved the data, not just the graphics).
The entire lab must be WORD PROCESSED, never handwritten
The FIRST PAGE must be a title page containing nothing more than the title of the lab, the course, and the student's name and ID number
Each section must start on a NEW PAGE so that BACKGROUND starts on the top of a new page and PROCEDURE starts at the top of a different page, etc.
The lab must be submitted in a report cover (preferably either a three-hole punched cover or one with a clamp on the left side), NEVER as a stapled mass of loose papers
Failure to follow this simple outline, used for all full lab reports in this course, will result in deduction of marks
A complete tutorial on formal lab reports, including an example, can be found here
LAB CONTENTS:All FULL labs in this course will follow this basic format ...
Put this section on a separate, first page immediately following the title page. It should require about one-half page to explain the following:
Again, put this section on a separate page immediately following the abstract page. It will require several pages, and significant marks are attributed to the calculation of the mercury spectrum (below)
This will be discussed in class. DO NOT simply cut-and-paste the lab outline from here as it is insufficient.
The procedure must be complete enough, and with enough details, to allow reproduction of the same experiment. Diagrams may be required here to illustrate how equipment is setup (this is a simple lab, but more complex setups are in your future). All phases of the experiment should be outlined including the procedure for calibration, observations of various types with the spectroscope, and use of the OceanOptics spectrograph (including calibration of that unit).
For example, in the case of the OceanOptics spectrometer outline how the mercury spectral data is used to calibrate the unit so that data from the unknown tube is "corrected" to represent the correct wavelengths.
The procedure must include details of the types of observations taken and how the data was analyzed (e.g. comparison to known wavlengths, etc).
The largest portion of the total marks are associated with this section. It goes FAR beyond simply "numerical diarrhea" (i.e. blasting numbers on the page without explanation) .... many marks are associated with explaining what was done and for actual analysis of the numbers (e.g. determining error % in the spectrograph readings, analysis of the unknown by intelligent comparison with known wavlengths, etc)
NEVER dump numbers on a page without an explanation of what they mean: Try something like "Table 3: Observed fluorescent lamp emissions" and a simple few lines which explain how this was done (even if you already mentioned it in the procedure, it bears repeating in a very condensed way here). An example: "This chart contains the observed emissions from a fluorescent lamp as recorded using a manual spectroscope. The angles of emission lines were recorded and converted to wavelength using the formula outlined in the background. Since there is an error associated with the readings, a possible range for the observed wavelength is provided".
Copyright (C) Niagara College, Canada, 2003-2012 A part of the
course in the photonics programs at Niagara College
A part of the course in the photonics programs at Niagara College