The Observation of Molecular Emission (OH) from an Induction Coupled Plasma
(a demonstartion project)

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:


Figure 1:  The Vista ICP AES

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


Figure 2:  The Vista Console, complete with Spectroscopist

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


Figure 3:  The Neslab(TM) Heat Exchanger

Figure 4:  The Sample Inlet of the Vista



Figure 5:  The Plasma Torch

Shutting Down

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
 

Get familiar with the spectrometer.  Explore molecular spectroscopy. Read up on OH.   Then, in your paper, make sure to include the following points:

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

4)   Describe the following spectroscopic terms
-------           That's all you have to do!         --------