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 lab marks immediately with no recourse if it is not received upon entering the lab.
  • Produce a table listing the major emission lines for the spectrum for hydrogen, mercury, sodium and neon gas emissions. These tables must contain an accurate listing of major emission lines (in nm), (the NIST eBook is an excellent source of data - see the course notes section of the course home page). Do NOT cut-and-paste the NIST table, but rather list only visible lines in the top 10% of maximum intensity.
  • Compute the expected angle of deflection, 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 line only (including formulae used), then list a simple table of wavelength and expected angle in degrees.
• As well, be sure to bring a USB memory key to save the files from the OceanOptics spectrometer in Part D.

Part A: Calibrating the Spectroscope

We begin with an introduction to use of the spectroscope. The professor will begin with a brief description of the unit followed by a lab in which students will view the emissions of various light sources.

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" is not zero at all ... there is an offset error which must be compensated. Once the error is determined, and "true" zero is found, the spectroscope will be calibrated (the procedure is in the SOP you read for the prelab).

Compare the expected position of known mercury lines with the predicted location 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 was at 5.6 degrees and the actual line was found at 5.8 degrees the 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.

DO this type of analysis for each line found: report the found angle, range of certainty based on the error figure for the angle, and the range of possible wavelengths for the 'found' line.

Identify the first, second, and third orders as expected from the grating and note the angles of these ... the angles should agree with the formula outlined in section 2.2 of Csele (but be careful when calculating line spacing d - see the example on page 24).

Part B: 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 ... 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.

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 bands. For each band seen, record the centre angle (wavelength) and edges of the band so that its apparent width (in nm) may be computed. 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/bands and attribute each to a source (Hg, buffer gas, phosphor) ... lines from Hg or the buffer gas are expected to be sharp (what would be expected from a phosphor emission ??). Where a line is assigned to a gas, cite the gas and the known wavelength to justify the assignment.

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 several gas discharge tubes 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 for longer). 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 verify the calibration from the last 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). 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 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 V14A. Like the manual spectrometer, it must be calibrated first using a known gas (mercury, again).

Install a mecury tube into the power supply in V14A. 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 V14A. 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. Each tube is either a pure, common gas, or a common vapour (nothing too weird like methane gas, I promise, the most complex molecules are triatomic), NONE are metal vapours except mercury, this will help narrow the search. 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 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.

Lab Report

Background

• A description of how a spectroscope works and relevant grating formulae, etc
• A description of the expected output of blackbody sources 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)
• List major emission lines for Hydrogen, Mercury, and Neon gases and cite the source. Wavelengths must be accurate to 1nm or better and a quality reference such as the NIST eBook must be used.

Procedure


This will be discussed in class. DO NOT simply cut-and-paste the lab outline from here
This procedure must include details about the calibration procedures used for each spectrometer. 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.


Observations

• Outline the calibration procedure used and numbers (where "true zero" is, how closely the observed mercury lines correspond to the predicted positions, and finally an estimate of how accurately the wavelength of a spectral line can be determined: the "+/-" figure). Include observations of higher grating orders as well.
• The observed output of an incandescent lamp including relative intensity distribution
• The output of the fluorescent lamp specifically major emission bands (center wavelength and spectral width). Identify those belonging to mercury and those not from mercury. For lines not belonging to mercury, predict the source (cite references as required).
• The output of all gas discharge tubes used (a chart, as discussed in the lab procedure above). Determine the percentage error in each observation of 'known' lines (Hydrogen, Neon). As well determine a tolerance for reading the instrument. If you missed a line, simply specify the accepted wavelength and explain why.
• Observations of the emissions from the unknown tube (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 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?)

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