5.5: Experimental Part II - Emission Spectrum of Iodine Vapor

In this part of the experiment you will use a HeNe laser at 543.5 nm to excite iodine to the B state, and a monochromator and photomultiplier to measure the spectrum of the emission back to the ground X state. It is worth noting that this technique of "laser-induced fluorescence" is an important spectroscopic tool, not only for physical chemists, but also for analytical chemists. The sensitivity is extremely high, and this has led to a wide variety of applications. These include the measurement of trace amounts of osmium and iridium in rock samples as part of an effort to test a theory for the sudden destruction of the dinosaurs by a catastrophic collision of the earth with an asteroid!

Laser Safety

Since the laser used in this experiment can be harmful, and the apparatus is relatively expensive, a number of safety precautions must be taken.

Safety and Other Precautions

1. Don't look into the laser! This is a class IIIa/3R laser. If the beam is directed into your eye it will damage your eye before you can blink. However you will still want to avoid looking directly into the beam, or having the beam be directed at you. Any glass that the laser hits will reflect about 5% of the beam intensity. This specular reflection may accidentally be directed toward your eyes. Therefore, do not move items into or out of the laser path with the laser on. Wear laser goggles whenever the laser is on.
2. Don't vent the cell. There is no danger to you if you vent it, but you won't be able to do the experiment until the cell is evacuated again.
3. Don't kill the PMT. The Photomultiplier Tube (PMT) is an extremely sensitive light detector. If you can see the light, that's enough light to overload the PMT if there is voltage on it. Read and understand the experimental procedure before applying any voltage to the PMT. Do not turn on the voltage to the PMT if it is exposed to significant amounts of light. Also, turn up the voltage slowly as you look for signal. As long as the observed signal stays below 0.2 V the PMT will not be damaged. Again, no physical damage to you will result from overloading the PMT, only financial.

Note

The Helium-Neon laser frequency of 543.5 nm corresponds to one of the larger absorption peaks from the absorption spectrum obtained in Part I of the experiment. Notice that absorptions close to 543 nm in the absorption spectrum (Part I) are among the most intense absorptions.

Preparation of Gas Sample Cell

• A cell containing solid iodine (\(\ce{I2}\)) will be put under high vacuum to vaporize the \(\ce{I2}\) for spectroscopic analysis. 

Vacuum-Line Start-Up

Some of the following may be completed by the TA before the lab period.

1. Check that all external valves on the working and main manifold are closed to the outside atmosphere.
2. Attach an empty N₂ trap to the main manifold. Hold this trap in place until a vacuum has formed.
3. Turn on the floor vacuum pump to start generating a vacuum.
4. Turn on all valves in the manifold that are connected to gages (marked with purple tape).
5. Turn on the diffusion pump.
   • Ensure that the floor pump is on BEFORE the diffusion pump. Otherwise, the diffusion pump will generate heat and the oil within the pump will oxidize, causing long-term damage to the diffusion pump.
6. Turn on Digivac electronic monitor. Use this to actively monitor the pressure at the end of the main manifold.
7. Check the analog pressure gauge on the diffusion pump to ensure the pressure is stable.
   • Throughout the experiment, the pressure should remain near 10 mtorr if the system is closed and stable.
8. Fill a 4L dewar with liquid N₂ from the shared Chemistry department supply.
9. Place a second dewar underneath the N₂ trap on the Main Manifold and fill with liquid N₂ such that the N₂ trap is submerged in liquid N₂.
10. Place the iodine trap on the main manifold and place a third (small) dewar under the trap such that the trap is submerged.
11. Check the level of N₂ as the experiment progresses. Refill if necessary to ensure the trap remains cold.
12. Check to ensure the pressure at the end of the main manifold is near 10 mtorr before proceeding.

1.  Use the high vacuum line, with help from your TA, to evacuate the iodine vapor cell that will be used in the emission experiment.

   

   Figure \(\PageIndex{1}\): A sample cell containing solid iodine is placed in a liquid nitrogen dewar and evacuated using the main manifold of a high vacuum line. (CC-BY-SA; Kathryn Haas, Duke Chem)

2. Once the cell is well evacuated, place it on the sample holder on the laser table. The cell should be in the path of the laser and there should be nothing obstructing the light path. 

   

   Figure \(\PageIndex{2}\): A sample cell is positioned on the sample cell holder in the path of the laser. The valve on the sample chamber is positioned toward the laser and away from the detector, as shown. (CC-BY-SA; Kathryn Haas, Duke Chem)

    

Vacuum Line Shutdown

1. Turn off the diffusion pump. Wait 5 mins for the pump to cool.
   • The pump should be cool before any oxygen is introduced to the system to prevent oil in the pump from oxidizing.
2. Close all valves with purple tape.
3. Turn off Digivac monitor.
4. Turn off the floor pump.
5. Pick an external valve and open it to release the vacuum and open both the working manifold and the main manifold to the air.
   • Do this quickly after step 4. The system should not be left under vacuum without an active vacuum pump.
6. Remove N₂ trap and transfer it to the fume hood to safely evaporate excess HCl/DCl gas.
   1. Do this quickly after step 5. Leaving the N₂ trap cold after the vacuum has been released may cause liquid O₂ to condense, which is highly volatile.

Instrument Configuration

The figure below shows a schematic diagram of the instrument set up.

Figure \(\PageIndex{1}\): Instrumentation used for Laser Fluorescence Experiments. (CC-NC-BY-DUKE CHEM)

Laser: A Research Electro-Optics, Inc. HeNe laser (model 30968), which emits visible light (543.5 nm), is used to pump primarily transitions from \(\upsilon^{\prime\prime}=0\).

Monochromator: A Triax 320 monochromator is used to disperse the emission signal from the sample. The entrance and exit slits of the monochromator control the signal intensity and resolution. It is best to use very narrow slits when looking for signal of unknown intensity so you don't overload the photomultiplier tube (PMT) detector with too much light. The PMT is mounted on the exit slit, which should be set to the same width as the entrance slit. There is a slide shutter on the exit port. The manual control unit (CD-2a) can be used for tuning the monochromator when you are setting up the experiment. The monochromator is computer controlled during data collection.

PMT Power Supply: The PMT power supply allows you to introduce a voltage to the PMT. Typically you will use a voltage off 1100 volts for this specific PMT. Depending on the available time, your TA may show you the effect of PMT voltage on the observed signal.

Lock-In Amplifier: In association with Stanford Research Systems (SRS) chopper an SRS510 Lock-in Amplifier is used to improve the signal to noise ratio of the emission spectrum. The interested student should read the Bentham Instruments introduction to the principles of a lock-in amplifier in the Course Documents section of your Sakai site, or at http://www.bentham.co.uk/pdf/F225.pdf.

Spectra Acq2: This is an analog to digital (A/D) converter which takes the analog signal (current from the PMT) and converts it to a digital signal that the SynerJY computer software can read and process.

Operating the Laser Spectrometer

1. Turn on the Chopper using the power switch on the back of the power supply. It should already be set to approximately 500 Hz. If necessary, adjust settings using the frequency adjust controls to bring it to approximately 500 Hz.

   

   Figure \(\PageIndex{3}\): The chopper and its power supply are labeled. Turn on the chopper using the switch on the back side of the power supply. (CC-BY-SA; Kathryn Haas, Duke Chem)

2. Turn on the power for Amplifier and Photomultiplier Tube (PMT).
   1. Turn on the Pre-Amp power supply using the toggle switch on the box.
   2. Turn on the Lock-in Amplifier using the black switch on the front right of the box.
   3. Turn on the PMT Power Supply, but do NOT apply voltage yet.

      

      Figure \(\PageIndex{4}\): a) The Pre-Amp power supply is the black box in the center of the picture; it is turned off using the silver toggle switch.  b) the Lock-in Amplifier is the box on the left of the photo; it is powered on using the black toggle switch at the bottom right of the control panel. c) The PMT power supply is the box on the right; it is powered on using the press button at the bottom right. Do NOT touch the black switch to apply voltage to the PMT until the lights in the room are off. (CC-BY-SA; Kathryn Haas, Duke Chem)

3. Turn off the lights in the room.
4. With the lights off, turn on the laser (key on the laser power supply). Notice that while the laser beam itself is green (543.5 nm), the emitted light, seen as a beam along the length of the sample cell, is in the yellow orange, that is, at lower energy (higher wavelength) than the exiting laser. Of course what you see in the cell is the sum of all wavelengths of emitted light. To resolve that into a spectrum, you will send that light into the monochromator and scan an appropriate wavelength range.
5. Click the SynerJY icon () on the computer screen to load the SynerJY software.
6.  From the Collect menu choose Experiment Setup.

   

   Figure \(\PageIndex{5}\): Choosing Experiment Setup from the Main SynerJY Window. (CC-NC-BY-DUKE CHEM)

   • Now, just wait until the system initializes. You will briefly see two windows come and go as the software connects with the monochromator. The first to appear is a Hardware Configuration window, and Triax320PMT should be highlighted in it.

     

     Figure \(\PageIndex{6}\): Hardware Configuration. (CC-NC-BY-DUKE CHEM)

   • This window will disappear and be replaced by an Initialization Window. The software will initialize the monochromator and PMT and then that window will also disappear.

     

     Figure \(\PageIndex{7}\): The Initialization Window. (CC-NC-BY-DUKE CHEM)

   • The software will then load the Experiment Setup screen.

     

     Figure \(\PageIndex{8}\): The Setup Screen. (CC-NC-BY-DUKE CHEM)

7. Set up the optimization experiment: 
   1. Type a name for your Experiment File (use YYYYMMDD_YourInitials_opt).
   2. Press SAVE.
   3. Choose a Start Wavelength of 540 nm and an End Wavelength of 550 nm.
   4. Set the Increment to 0.2 nm.
   5. Make sure Type is set to Wavelength, and Triax320 is selected under Scanning.
   6. Click the "Monos" button () at the left of the screen and set the Front_Entrance and Front_Exit Slits each to 0.026 mm.
   7. Click on the Detectors button () button at the left of the window and make sure there is a check in the PMT Active box and set the Integration Time to 3000 ms (i.e. 3 seconds). Leave the Gain at x1.
8. Turn on the PMT
   1. Using the buttons on the PMT power supply, apply 900 volts to the PMT. Do this by typing "900" and then pressing the "Enter" button. Then switch the power to "on".
   2. On the software, click on the Real Time Control button (RTC,).

      

      Figure \(\PageIndex{9}\): Real time controller. (CC-NC-BY-DUKE CHEM)

   3. Make sure the Slits and Integration Time are set properly.
   4. Change the value in the Position box to 543.5 nm.
   5. Click on the "Run" button ().
   6. Wait a few seconds, then look at the digital readout on the lock-in. You should see less than a tenth of a millivolt signal at this point.
   7. Repeat the steps above to change the PMT voltage to 1000 V and watch the signal at the lock-in. Make sure it does not go off scale; decrease the sensitivity if needed.
   8. Bring the PMT voltage up to 1100 V, adjusting the lock-in sensitivity to obtain a good signal (several tenths of a mV) without going off scale.
   9. Click the "Cancel" button () to return to the main Experiment screen.
9. Run the optimization Experiment: Click on the "Run" button () to start the short optimization experiment.
   You will see the spectrum appear in the Display window as the scan progresses. You should see a peak at the pump wavelength. When the scan is complete you may be asked about saving files. To be safe, save all files when asked. Your scan will then appear in the main window of the SynerJY software (Figure \(\PageIndex{10}\)).
10. Double Click on the spectrum so that you can edit it. Then select Pick Peaks from the Analysis menu.

    

    Figure \(\PageIndex{10}\): The Results of the Test Scan. (CC-NC-BY-DUKE CHEM)

    1. To begin the peak analysis leave the default settings in the boxes and select Find Peaks at the bottom of the Peak Analyzer window. Then press Finish.

       

       Figure \(\PageIndex{11}\): Copy and Paste Caption here. (Copyright; author via source)

    2. If you do not like the results, alter the Height and Width values until you get a good result. In the example shown in Figure \(\PageIndex{10}\) the major peak is found at approx. 543.5 nm, demonstrating that our system is working properly.
11. Do any final optimizing of the signal (check with your TA) and start the run.
12. Create the Iodine Emission Experiment:
    1. Make sure the Increment is set to 0.2 nm.
    2. Make sure the Integration Time is still set at 3000 ms.
    3. Change the Start wavelength to 510 nm and End wavelength to 800 nm.

       

       Figure \(\PageIndex{12}\): Setting Up The Sample Run. (CC-NC-BY-DUKE CHEM)

    4. Click the Real Time Control button (RTC,) to set the monochromator to the start wavelength.
    5. Click the Run button ONCE only () and wait a few seconds (this is equivalent to setting the monochromator to the start wavelength and allowing the detector signal to relax).
    6. Then click the Cancel Button ONCE only () to return to Setup screen.
    7. Click the "Run" button () from the setup screen to start the full scan.

       

       Figure \(\PageIndex{13}\): Data collection. (CC-NC-BY-DUKE CHEM)

    8. Watch the data collection for the first 5-10 minutes to be sure everything is running correctly. The total scan will take a at least 1.5 hours. If you leave the room, close the door behind you to be sure that stray light does not get into the experiment.
13. At the end of the scan you may want to rescale the horizontal axis to see the spectrum more clearly.

    

    Figure \(\PageIndex{14}\): The FInal Spectrum. (CC-NC-BY-DUKE CHEM)

14. Repeat the Pick Peaks procedure you did earlier on the short scan. Likely you will need to adjust the parameters in the Pick Peaks dialog box to get most or all of the peaks. Below is a suggested set of options:
    1. Goal: Find Peaks
    2. Baseline Mode: Constant, Median
    3. Baseline Treatment: Auto subtract baseline
    4. Find Peaks: Enable Auto Find, Peak Filtering By Height, Threshhold at ~ 1-2%
15. When you get a good listing of the peaks look at the bottom of the data window where you will see a list of the files associated with these data (Figure \(\PageIndex{15}\)). Click on the Peak_Centers1 worksheet and you will get a table of the peaks found in the spectrum.

    

    Figure \(\PageIndex{15}\): The Peaks Table. (CC-NC-BY-DUKE CHEM)

    You can copy the appropriate data from there to an Excel spreedsheet. You should the save the spreadsheet and any screenshots that you need.
16. Shutdown:
    Before you shut down, be sure you have collected all necessary data in your ELN and in excel. Then Send a copy of the ELN to you and your partners.
    1. Turn off the laser using the key.
    2. Turn off the power to the PMT and power off the power supply.
    3. Turn off the power to the Pre-amp Power Supply
    4. Turn off the power to the Lock-in Amplifier.
    5. Turn off the Chopper.
    6. Close the SynerJY software.

Instructions for Data Analysis

You may consult your instructors and classmates during data analysis, however all work should be your own.

1. Assign at least 19 peaks in the emission spectrum based on the known pump transition. Remember, this is an emission spectrum. Longer wavelengths (lower energy) correspond to transitions from higher ground state vibrations.
   1. Convert the wavelengths of these peaks from nm to wavenumbers.
   2. Create a Birge-Sponer plot (see Equation 5.3.5) by plotting \(\Delta G_{\upsilon}^{\prime\prime}\) vs \(\left (\upsilon^{\prime\prime}+1 \right )\).
2. From the intercept of the Birge-Sponer plot, calculate \(\omega_{e}^{\prime\prime}\) (using Equation 5.3.5).
3. From the slope of the Birge-Sponer plot, calculate \(\chi_{e}^{\prime\prime}\) (using Equation 5.3.5).
4. From the Morse Potential (using Equation 5.3.7), calculate \(D_{e}^{\prime\prime}\).
5. Estimate the uncertainty in the peak positions using the standard deviations from the least squares analysis and calculate the uncertainties in \(\omega_{e}^{\prime\prime}\), \(\chi_{e}^{\prime\prime}\) and \(D_{e}^{\prime\prime}\).
6. For comparison, calculate the value of \(D_{e}^{\prime\prime}\) using Equation 5.3.9 as follows: From the wavelengths of at least three absorption lines, calculate the corresponding transition energies \(\nu_{\left ( \upsilon^{\prime\prime},\upsilon^{\prime} \right )}\) (where \(\upsilon^{\prime\prime}=0\)) in units of cm^–1. For these same transitions, calculate (\(G_{\left ( \upsilon^{\prime} \right )}-G_{\left ( 0^{\prime\prime} \right )}\)) using Equation 5.3.4 and your values of \(\omega_{e}^{\prime\prime}\), \(\omega_{e}^{\prime}\), \(\chi_{e}^{\prime\prime}\) and \(\chi_{e}^{\prime}\). Now use Equation 5.3.8 to calculate the average value of \(\nu_{el}\) for the three transitions. Finally, use Equation 5.3.9 and your value of \(D_{e}^{\prime}\) from Part I to calculate \(D_{e}^{\prime\prime}\). The value of E(I*) is known to be 7603 cm^–1 from atomic spectroscopy.¹ Which value of \(D_{e}^{\prime\prime}\) has the least uncertainty (neglect the error in E(I*))?
7. Calculate the Morse parameter (a") from Equation 5.3.7.
8. To plot the Morse potential wells, you will use the Matlab routine morse. Simply type morse and follow the screen prompts. Separate ground and excited state Morse potential curves will be plotted using your experimental data, and sent to the printer. For now, both \(R_{e}^{\prime\prime}\) and \(R_{e}^{\prime}\) are set at zero. Vibrational quantum levels will be shown for both ground and excited states.
9. The maximum intensity in the absorption spectrum occurs as the excited state vibrational wave functions overlap vertically with the ground state wave functions. A Matlab script, overlap, is used to overlay the two plots and shift the ground and excited state potential wells such that the center of \(\upsilon^{\prime\prime}=0\) overlaps with the left edge of the \(\upsilon^{\prime}=25\). At the Matlab prompt type overlap . The script will animate the overlap of the ground and excited states and the vertical transition. It will also estimate the displacement (\(R_{e}^{\prime} - R_{e}^{\prime\prime}\)) (Ångstroms) between the minima of these two potential wells along the R-axis. Save a copy of this graph (or take a screenshot) for your ELN, and record the displacement.

   Note

   The ground and excited state potential wells are overlaid so that the displacement \(R_{e}^{\prime} - R_{e}^{\prime\prime}\) may be measured. However, this gives a misleading impression of the relative positions (energies) of the two potential wells. Clearly the excited state potential well is at much higher energy than the ground state potential well (see Figure 5.3.3 and note the discontinuity in the V(R) axis). A realistic picture is shown in reference [10]. In the Introduction to Matlab Excercises, you plotted the ground and excited state potential wells for iodine using literature values for D_e, a, and R_e. You will now repeat this assignment using your results.

10. The functional form of the Morse potential may be represented by the equation \[ V\left ( R \right )=T_{e}+D_{e}\left [ 1-e^{-a\left ( R-R_{e} \right )} \right ]^2 \] where T_e is the electronic term energy (the energy at the bottom of the potential well). \(T_{e}^{\prime\prime}\) and \(T_{e}^{\prime}\) represent the electronic term energies of the ground and excited states respectively. If we set \(T_{e}^{\prime\prime}=0\), then \(T_{e}^{\prime}=\nu_{el}\). Given that \(R_{e}^{\prime\prime}=2.66\) Å (obtained from IR spectroscopy), calculate \(R_{e}^{\prime}\) using your value of \(R_{e}^{\prime} - R_{e}^{\prime\prime}\).
11. You will now use your values of \(T_{e}^{\prime\prime}\) (zero), \(T_{e}^{\prime}\) (\(=\nu_{el}\)), \(D_{e}^{\prime\prime}\), \(D_{e}^{\prime}\), a", a', \(R_{e}^{\prime\prime}\) and \(R_{e}^{\prime}\) to replot the Morse potential wells for the ground and excited states of iodine on the same graph. Return to your Matlab command window, and as instructed, press any key to continue. Enter your calculated \(T_{e}^{\prime}\). The two Morse potentials will be plotted on one graph and the vibrational quantum levels will be shown for both ground and excited states. Save a copy of this graph (or take a screenshot) for your ELN.
12. Compare all of your results with literature values (See, for example, reference. 1, p. 430-432).

Discussion Questions

Include discussion of the following in your ELN.

1. Why are the vibrational spacings different in the ground and excited states?
2. Why is the shape of the Morse potential different in the ground and excited states?
3. How is it possible to observe emission peaks at higher energies than the pump energy?