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.

Introduction

PreLab (To be done PRIOR to the lab)

• Review notes on the use of a spectroscope on the SOP area of the technology site (a link can be found on the course home page).
• For further information on the use of a spectrometer read this reference (Password-protected PDF).
• Review notes on the use of the OceanOptics spectrometer and the OOIBASE32 software on the SOP area of the technology site.
• Read section 2.2 of Csele on the operation of the spectroscope.
• The following prelab assignment (worth 10% of the total lab mark), is due at the beginning of the first lab period. Late marks are not assigned if the prelab it is not received at the beginning of the lab: you lose 10% of the total lab marks immediately with no recourse if it is not received upon entering the lab (extensions will NOT be given to "print it out" in the lab ... be prepared with the hardcopy already printed).
  1. Produce a table listing the major emission lines for the spectrum for hydrogen, mercury, and neon gas emissions. These tables must contain an accurate listing of major emission lines (in nm) - use the NIST eBook for this (see the course notes section of the course home page). Do NOT cut-and-paste the NIST table, but rather list only persistent neutral-atom visible lines (in the 400-700nm range).
  2. Compute the expected angle of deflection (in degrees), for the first-order, for all emission lines of mercury. The grating has 7500 lines/inch (convert to lines/mm first then compute d in m). Show all work for one visible line only (including formulae used), then list a simple table of wavelength and expected angle in degrees for all three visible lines.
  Bring TWO copies of the prelab to the lab: one is to be submitted as you enter the lab for marks, one is required for use in the lab.
  
  
• Finally, be sure to bring a USB memory key to save the files from the OceanOptics spectrometer in Part D.

Part A: Calibrating the Spectroscope and Determining Error

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:

Part B: Spectrum of Common Light Sources

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.

Part C: Line Spectra Of Gases

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).

Part D: Identification of an Unknown Gas

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
Xenon 

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).

Lab Report

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:

Abstract


Put this section on a separate, first page immediately following the title page. It should require about one-half page to explain the following:


• Summarize the INTENT of the laboratory (why we are doing this lab and what we are determining here - be sure to mention the intent of all parts of the experiment)
• Summarize HOW the lab is to be done (e.g. "using a manual spectroscope ...." and "an unknown gas was determined by capturing the emission spectrum using an Ocean ....")
• Summarize the RESULTS of the experiment (including the estimated error of the manual spectroscope, observations of the fluorescent lamp, and analysis of the unknown gas)
Background


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)


• A description of how a spectroscope works (include a diagram of a spectroscope that resembles ours - i.e. using a transmission grating) and relevant grating formulae, etc
• A description of the expected output of blackbody sources (such as an incandescent lamp) and Boltzmann distributions of energies vs. temperature
• The expected output of gas discharge sources with the specific example of hydrogen and the origins of the lines (relate to the Bohr atomic model for hydrogen, and transitions between energy levels)
• From the prelab: list major emission lines for Hydrogen, Mercury, and Neon gases as determined using the NIST eBook.
• IMPORTANT: Calculate the exact emission wavelengths of mercury as follows:
  1. Obtain the energy levels of the neutral Hg atom from the NIST eBook (mercurytable5) and convert to eV using this link. Where two energy levels are very close (less than 5 cm^-1 difference between them), treat them as the same level. For example, the two mercury energy levels at 71333 cm^-1 and 71336 cm^-1 are essentially the same energy level at 8.84eV ... three significant figures are required.
     Know how to convert from cm^-1 to eV, but you may "cheat" by examining the questions at the end of chapter 2 in Csele.
  2. Determine the photon energies (in eV) which correspond to visible transitions in the range 400nm to 700nm.
  3. Compute the wavelength of only transitions in the visible range (i.e. with an energy difference between the upper and lower levels in the range computed in the last step).
  
  The ability to calculate wavelength given upper and lower energy levels for a particular transition is a vital part of this course and will be employed extensively in this course (i.e. learn to do it now as you will be using it further).

  Procedure
  

  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).
  

  Observations
  

  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".
  

• Outline the calibration procedure used and numbers as follows:
  1. Indicate where "true zero" is (measured degrees, and explain this as well)
  2. Use a chart to show how closely the observed mercury lines correspond to the predicted positions. Use the headings "KNOWN MERCURY LINE (nm)", "PREDICTED ANGLE" (This is taken from the prelab), "OBSERVED ANGLE", "ERROR (degrees)", "OBSERVED WAVELENGTH (nm)", "ERROR %". Remember to correct the observed angle for "true zero" as discussed in the lab (i.e. it is NOT simply the angle you read directly from the vernier scale). The error estimate is based on wavelength and can be found by (Known λ - Observed λ) / Known λ *100%.
  3. Report the average error tolerance, in degrees (it will be used in further observations).
• The observed output of an incandescent lamp including relative intensity distribution (did the spectrum resemble that expected from a blackbody source?)
• Observations of higher grating orders when using the incandescent lamp (in general, angles where the red/violet ends of the bands for each order were found, and an explanation of what was seen: did any orders overlap?).
• The output of the fluorescent lamp specifically major emission bands (a chart as discussed in this lab). Identify those belonging to mercury and those not from mercury. For lines not belonging to mercury, predict the source (phosphor, or buffer gas, and cite references as required). To do this, use a chart with the headings "OBSERVED COLOUR", "OBSERVED ANGLE", "OBSERVED WAVELENGTH", "KNOWN LINE SOURCE", "KNOWN LINE nm". If a line corresponds to mercury, list the KNOWN LINE SOURCE as "MERCURY" and fill-in the corresponding known wavelength from the NIST tables. If a line is from a different source such as PHOSPHOR or BUFFER GAS, list that as the source (however the KNOWN LINE nm might not be filled-in then).
• The output of the hydrogen and neon gas discharge tubes used (a chart, as discussed in the lab procedure above, essentially identical to that used with mercury). Determine the percentage error in each observation of 'known' lines (Hydrogen, Neon) and report whether the known wavelength of the observed transition falls within the range of the observed wavelength (with the observed angle +/- the error tolerance determined during the calibration step and so the wavelength range found accordingly).
• Observations of the emissions from the unknown tube in the form of a chart (be sure to record the serial number of the tube). Outline the calibration procedure for the OceanOptics spectrometer as well (which was done before observation of the unknown ... no purpose is served by calibration if it is not applied before observations are analyzed).

Conclusion


• For continuous sources like the incandescent lamp describe the relative distribution of light energy and relate to the Boltzmann distribution (Planck's prediction) for energies.
• For the fluorescent lamp, identify each band and determine which come from mercury (The main gas in the tube). Summarize and reiterate key data (a chart, as outlined in the observations) and propose where the other observed lines come from (another gas in the tube - identify this gas, which is called a 'buffer gas', by researching how fluorescent tubes are made and relating spectral observations to verify this).
• Determine the unknown gas in the discharge tube and include a paragraph describing your rationale for identifying it (e.g. # of lines, wavelengths, error justification - are observed lines within tolerances determined?). Use a chart as per below and conclude which gas is contained within the tube based on how closely the known emissions from a suspect gas matches the observed lines.

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