In this experiment, a commercial (and expensive) Atomic Emission Spectometer, a Varian Vista CCD Simultaneous ICP-AES, will be used to observe the optical emission from a highly-excited gas sample.  The Spectrometer itself is usually used to detect trace amounts of elements, which after being atomized and heated, show characteristic sharp emission lines.  We saw such lines in the spectrum of our Hydrogen lamp in a previous lab.

The spectrometer itself is an unfriendly looking box with few visible controls:

and, of course, is completely under the control of a computer:

The instrument itself requires a setup taking approximately 1/2 an hour, which (hopefully) the TA will have performed for you.  Your job begins with

• Check to make sure that the Argon tank is open and has at least 500 psi in the tank and delivers 80psi to the instrument.(Ar is the major constituent of the plasma)
• Check that the Heat Exchanger is in operation:

Figure 3:  The Neslab^(TM) Heat Exchanger

• Check that the exhaust hose is moving air and that the room lights are on (This increases airflow in the hood system)
• Make sure that the sample tubing is configures properly on the peristaltic pump and that the Inlet tube (front) has slightly less pressure that the outlet tube.  The sample inlet system looks like this:

Figure 4:  The Sample Inlet of the Vista

• Sample prep:  Our sample will be quite exotic,  pure water!  Why?  Because we should be able to see, H atoms (which we know all about), O atoms, a lot of Ar* and Ar⁺ lines, and if we are lucky, OH molecular emission! We will detect identify this emmission by its spectral characteristics, and, to do this, we will use two different spectrometers (hense two computer systems). One spectrometer will be the internal CCD/Eschelle spectrometer of the Vista itself, and the other is a PChem lab addition: an Ocean Optics micro-spectrometer coupled to the VISTA instrument via a fiber optic.

The Dell computer hooked up to the Ocean Optics Spectrophotometer has an icon on the desktop labeled "Ocean Optics Spec." Double click on this icon to open the Ocean Optics software. Once the program has opened, click on the button labeled "Data Acquisition". (It looks like a spectra with three blue peaks). This will tell the spectrophotometer to begin acquiring data
The Ocean Optics Spectrophotometer has two measurement channels, Master and Slave. These are located on the side of the small Ocean Optics Spectrometer box. The data is obtained through a fiber optic cable. You will be using the "Master" channel, which has wavelength range of approx. 205nm to 520nm. The "Slave" channel has a wavelength range of 365-900, which is too high to see a lot of the OH emission spectra. Once the Ocean Optics software has started acquiring data, you can watch the real time emission on the Ocean Optics software.
On the desktop of the HP computer associated with the Vista spectrometer, you must double-click on the icon labeled "vista.exe." This will start the program that operates the plasma spectrometer. A window will open with three buttons to choose from: Worksheet, Instrument, and Exit. At the bottom of the screen, inside of the grey border, will be the ongoing actions taken by the plasma spectrometer. As you will note, the ICP torch has a Argon purge delay that must be completed before the torch is lit. This will take approximately 20 minutes.
When the Argon purge delay is finished, click on the "Worksheet" button. It will prompt you to choose if you want a new or an existing worksheet. Select the open option and open the file named "OH emission.vws." This file has a wavelength for Al displayed in the "summary" window. The Vista series spectrometer does not allow you to perform a broadband scan. You are only allowed to look at certain "windows" for various elemental emission wavelengths. There is no way to tell this window to look at a molecular line. In this case, we shall be looking at emission lines for Al that have OH lines in close proximity as interferences.
• When you have 'prepared' your sample, turn the Plasma ON.  This is accomplished with the Instrument Settings window by clicking on the Plasma Icon.  After a few seconds ( There are serveral warm-up delays before the plasma is actually ignited ) you should see this...

• To take a spectra with the Vista, click on the "Edit Method" button. Select the "Conditions" tab. You will now see a listing of the operational parameters of the instrument. To measure a spectra, click on the "Read Spectrum" icon. (It looks like a set of 3 blue peaks under a black arrow) The Vista will now obtain a spectra. Once it has finished, look in the Al window off to your right. The molecular emission peak will most likely be very broad and centered around 309.23 nm. To zoom in, click and drag a box around the region you would like to see magnified. This can be repeated several times to get the desired level of magnification. To unzoom, right-click with your mouse and select the "Unzoom" feature. It is also possible to place the cursor over a desired region of the spectrum and read off the nm and intensity values for that spectral region in the grey bar below the spectra boxes.
• You may now wish to modify some of the instrumental parameters to obtain a better signal. You may modify Power, Plasma Flow, Auxillary flow, Nebulizer Flow, and viewing height. Please don not lower the power below 0.7 kW. Note that each time you change or modify a parameter, you must click on the "Read Spectrum" icon for the parameters to be changed.
• You can note the changes each parameters has on the broadband spectra measured by the Ocean Optics Spectrometer. Adjust the parameters of the plasma to maximize your OH signal. To zoom in on a specific region on the Ocean Optics Spectra, click on the "Set Scale" button. (It resembles two crossed red arrows) This will open up a window that will allow you to change the min and max values for the x-axis(wavelength). This will zoom in on the wavelength region specified by the new max and min wavelength values. To unzoom, repeat the above procedure, but type in the old min and max values for the x-axis.
• You are now ready to save your broadband scan from the Ocean Optics of your optimized OH emission signal. Click on "File", and then select "Save." You are given the choice to save as either a sample or an experiment. If you save as a sample, the file is able to be opened in Excel for data manipulation. By choosing the "sample" option, a new window will open prompting you to enter the name of the save file. Type in the desired name in the blank area labeled "File Name". Click on the Okay button to save your file. It is important to note that you are only saving what you currently see on the screen and not the current complete scan. If you have zoomed in on a region, the software will save the spectra of that zoomed-in region. To save the complete spectra, make sure you Unzoom before saving.

To close the software, click on "File" and select "exit". Once you are finished using the Ocean Optics Spectrophotometer, you are ready to turn off the ICP torch and shut down the instrument. Close the "Method Editor" window. A prompt will ask you if you want to update the window, click "OK". Now click on the icon that resembles an ICP torch with a red X through it. (Or press F4) This will turn the plasma off. Now close the main window. If a prompt asks you if you want to save changes, click "YES". It is very important to turn off the gas before you leave. Leave the heat exchanger running for approximately 5-10 minutes to cool down the system.

The Spectra...

The plasma is hot!  So hot that most molecules are completely dissociated into atoms.  This contributes to the analytic utility of the the instrument and method.  But because there is so much water in the plasma, a few OH molecules survive and emit in the UV region of the electromagnetic spectrum.  As opposed to atoms, molecules emit in bands:

Figure 6:  A simulated spectrum of OH radicals in a 4000K Plasma

When you look at greater detail, the spectrum appears more 'atomilike' in that it has a bunch of sharp lines (maybe too many)

Figure 7:  A portion of the OH spectrum shown at 0.02 nm resolution.  The OH spectral data corresponding to this figure is in ASCII (wavelength, Intensity) columns.

I give you this data because it is not very easy to get a 'big picture' of the emission observed in our VISTA spectrometer and the Ocean Optics device does not have the greatest resolution.  The Vista instruments' best view of the spectrum comes in what can be called an eschelle spectrum, or eschellogram, that your TA will help you find in the instrument menu.  What you see in this is a large number of sharp lines, almost all of which are atomic.  A few molecular lines creep in, and are considered 'interferences'.  OH is a common interference, because of the solvent, water.  But OH lines are weaker than atomic lines because the intensity of the electronic emission is spread out over vibrational and rotational states (see Figure 6,7), unlike that in atoms.

Lots of information is available about the transition you have observed in OH
 

1)    Use the VISTA to find at least one (hopefully a few) OH emission lines in the water/Ar plasma, with the proviso that it is in the range of wavelengths covered in Figure 7.  Identify the exact wavelength of the emission, and redraw Figure 7 with stars (or other symbols) marking the lines you have seen.
 

2)   Assign the transitions in part 1)  in terms of ALL all the quantum numbers in the upper and lower states.  You might need help from a spectroscopist in this assignment of the rotational quantum numbers of the upper and lower states as the details of the rotational energy of the upper and lower states in the transition are quite different from one another.   Here is a tabulation of the transition wavelengths (in air)  of the A-X (0,0) band of the OH radical if both the upper and lower states are treated as Hund's case (a) (J, the total angular momentum is used as a quantum number), and here is a tabulation if they are both treated as  Hund's case (b) (N, the intertial rotational angular momentum is used as a quantum number).  Warning:  case (b) is not for the faint of heart.

3)   Describe the function (in the VISTA) of

•             The Eschelle grating
•             The CCD
•             The RF power generator
•             The Nebulizer
•             The Peristaltic Pump

•             Hund's Cases
•             Radical
•             ²S -> ²P  transition
•             (0,0) Band