Spectroscopic Analysis of Light

Spectroscopic Analysis of Light

SPECTROSCOPIC ANALYSIS OF LIGHT

OBJECTIVES

Spectroscopic analysis of light emitted by glowing bodies of gasses such as stars and nebulae can provide an enormous wealth of information about these objects. Estimations of size, temperature, composition, distance and age of stars can be made despite the insurmountable distance that separates us. In this exercise, you will learn the basics of how spectroscopic analysis of light from glowing gasses can aid in identifying their atomic composition.

Part I: You will map the visible spectrum of atomic hydrogen, the most common element in the universe.

Part II: You will identify two unknown elements by observing their spectra and comparing their fingerprints to the color spectra of four elements.

INTRODUCTION

1. ATOMIC EMISSION SPECTRA

This laboratory project will investigate the atomic processes by which light is emitted. For example, light from an incandescent bulb is generated by the motion of electrons in the metal filament that are stimulated by the passage of an electric current through the bulb. These electrons constantly bump into the atoms in the filament causing them (the atoms) to vibrate and radiate energy.

Fluorescent lamps, such as the ones illuminating the classrooms, operate a little differently. These lamps, or gas tubes, are in a class called “discharge lamps”. The light sources used in this lab belong to the same class. A high voltage placed between the two electrodes of the gas tubes provides the energy for the excitation of the gas. We describe the process by which this happens in terms of the promotion of electrons within the gas species to orbitals with higher energy. Thus, we now say the atoms of the gas are in a higher energy state. In the process by which these excited (energized) electrons return to their ground state, energy is emitted in the form of radiation with specific wavelengths (or frequencies). In the case of atomic hydrogen, some of these emissions are in the visible range.

For the first part of this experiment, you will use a hydrogen gas discharge tube. This tube will be placed in a box where a high voltage differential will be applied between the ends of the tube to excite the gas and make it glow. The hydrogen in this tube is diatomic (H2), and a small amount of these molecules will be broken into atomic hydrogen (H) which will undergo the process of excitation and emission in the visible spectrum.

1.1 The Hydrogen Spectrum vs. the Energy Level Diagram

The energy level diagram below (Figure 1) shows the electron transitions that result in the visible emission spectrum of atomic hydrogen. The energy states of the hydrogen atom are represented by horizontal lines. The electron in a high energy state emits light, and drops down to a lower energy state. This series of transitions is called the Balmer series, which is characterized by transitions to Energy Level 2. We can assign the specific transitions between energy levels to a spectral line of a given wavelength because the difference in energy is related to the wavelength or the frequency as:

Equation I:

where h is Planck’s constant: 4.1167 x10-15 eV x s, c is the speed of light: 3 x 108 m / s, is the wavelength,  is the frequency and Eis the difference in the specified energy levels, which can be obtained by simple subtraction. For example E32 = 12.10 - 10.20 = 1.9 eV.

Figure 1. Energy Level Diagram for the Hydrogen atom.

The emission spectrum of an element depends on its allowed electron energy states. Since each element has its own energy level diagram, it will also have its unique spectral signature and can be identified with accurate measurements!

2. OBSERVING AN EMISSION SPECTRA

You now have a working knowledge of emission spectra. But how do we separate the colors? As you have observed, light from a bulb can be passed through a prism to generate a continuous spectrum, in which colors are arranged in order. By contrast, if the emission from a gas discharge lamp is passed through a prism, the resulting spectrum will not be continuous but instead appear as a series of isolated colored lines, because emissions occur only in specific frequencies. A spectrum of this sort is called an emission line spectrum and can be generated for many chemical elements by exciting the species. Each line in the spectrum corresponds to a specific wavelength and the set of spectral lines is unique to each element. For example, all hydrogen atoms always gives a set of lines with the same set of wavelengths, which is different from the lines produced by sodium or by helium.

2.1 Using a Diffraction Grating

For the first part of the experiment you will use a diffraction grating. Recall how shining a laser light through a diffraction grating creates an interference pattern on a nearby screen (Figure 2.).

Figure 2. Interference from a double slit

The condition for constructive interference is that light waves from the laser should be in phase. In Figure 1 you can see that two light waves constructively interfere when the distance that they travel (from slits to screen) are multiples of one wavelength apart. In other words, the distance from BD – AD=n

What happens when light composed of several frequencies pass through the slits of a diffraction grating? You can see how the condition for constructive interference is wavelength dependent. Hence, the separation of the lines in the interference pattern will also depend on the wavelength of the radiation. With the aid of trigonometry, we can arrive at an equation that relates the wavelength  (for constructive interference) to the angle of diffraction.

Equation II:

for a double slit grating, where d is the slit separation and n is the order, which is related to the integer number wavelengths apart. Equation II tells us that for longer wavelengths, sin  is bigger, so  will also be bigger. This means that the bright lines produced from longer wavelengths are further apart from the axis (OC in Figure 2) than the bright lines produced from shorter wavelengths. So in effect, we have a color grating!

The diffraction gratings that you will be using have more than two slits. In fact, the gratings have 300 slits per mm! This, however, does not affect Equation II. The first order bright line is the one closest to the center, and so on. Refer to Figure 3. A simple conversion will show you that the slit separation, d, for the grating is 3333.3 nm.

Figure 3. Diffraction using a continuous light source (light bulb) and a gas discharge tube. The zero order (n = 0) has no diffraction.

PROCEDURE

Part I. Preparing the spectrometer

We will first focus the spectrometer and make sure that the telescope and collimator are 180° apart, completely in line.

Figure 4.Alignment of the telescope and collimator (Step 3).

1.Look through the eyepiece lens on the telescope tube. Ensure that the crosshair lines are aiming from top to bottom and from left to right. If they are not aligned, notify your T.A.

2.Make sure that the slit is vertical. Turn the aperture knob such that the slit is half-way open.

3.Position the telescope tube and the collimator tube in line with each other.

Figure 4: Schematic of the spectrometer. Refer to this figure for assistance during the procedure.

4.By looking through the telescope you should be able to see a slit of white light. Bring the slit into focus by simultaneously adjusting the length of the telescope and collimator. You adjust the lengths by turning knobs 6 and 7.

1. If you don’t observe the slit:
2. Ensure that the slit is half-way open
3. Ensure that the telescope and the tube are correctly aligned.
4. Adjust knobs 5 and 6 and look for the image through the telescope.

5.Adjust the slit width until the white light, as seen through the telescope, is narrow enough to be seen.

6.Align the left part of the slit with the crosshair, and tighten knob 3.

7.Loosen knob 1, which adjusts the base of the spectrometer with the Vernier scale. To the best of your ability, align the 0 mark on the Vernier with the 90° mark on the spectrometer table. This will aid in the recording of angles. Tighten knob 1. You can do fine adjustments by turning knob 2. The scales should appear as in Figure 4.

8.Loosen knob 5 and align the white line on the stage that holds the grating mount with the optical path between the telescope and collimator. Tighten knob 5.

Part II. Observations of the Hydrogen Spectrum

Warning
The power supply operates at a very high voltage (10,000 volts)! To avoid a nasty shock, you must turn off the power supply when you change tubes. Turn it back on only when the tube has been attached and you are ready to make observations. Also, the tubes may get very hot. To protect your fingers and the lamps, use the protective gear to place and remove them from the power supply. After you finish with a tube return it to the rack on your lab bench.

1.Carefully place the hydrogen tube (using a paper towel to hold it) in the power supply and turn it on. Ensure that the light from the hydrogen tube enters the slit!!

2.Carefully insert the diffraction grating into the holder

3.If everything is aligned correctly, you should be able to observe a bright pink line (n = 0). The left side should be aligned with the crosshair.

4.Loosen knob 3 and turn the telescope either way and search for the first color line. Roughly center the line with the crosshair. Tighten knob 3 and turn knob 4 for fine adjustment.

5.Taking a measurement: Figure 6 provides an example for steps b and c.

a.Remember your staring point should be 90°. So you must measure from this starting value. Your 90° should be aligned with the 0 on the Vernier.

b.Read the angle that aligns with the 0 mark on the Vernier to the best ½ of a degree (e.g.: 155.0).

c.On the Vernier scale, carefully determine which line most closely aligns with any of the lines from the degree scale. Use the magnifier and the desk lamp to aid you. Add this to your measurement. The Vernier scale is in minutes of degrees. 60 minutes = 1°. You should convert this reading to a fraction of a degree (e.g.: 15 minutes = 0.25°).

Figure 6. Example for measuring an angle with the Vernier scale.

6.Record the angle in the data table for hydrogen. Remember you are measuring from the 90° mark! Follow the example in Figure 6 when you record your angles in the table.

7.Repeat steps 1-5 for each color line.

8.If you turn the telescope past the red line, you will observe a repetition of the color patterns! This is the second order (n = 2) diffraction pattern. Record the angle for the second order red line only.

Part III. Identification of Unknown Elements.

You are going to identify two unknowns by observing their emission spectra and comparing them with the actual spectra. The colored spectra are those of elements of the noble gas family. Because of the number of electrons they have, these elements rarely form compounds with other elements. These gases have very interesting and detailed spectra and show a nice contrast when one is compared with the other. In general, since the emission lines are due to electronic transitions, the more electrons an element has, the greater the number of observed emission lines. The number of electrons for each of these elements is given below.

• Helium: 2 electrons
• Neon: 10 electrons
• Krypton: 36 electrons
• Xenon: 54 electrons

Place the gas tube containing the unknown in the power supply and turn it on.

Observe the spectrum, first with a wide slit and then with a narrow slit. Make sure the slit is aligned correctly with the gas tube. Carefully shift the gas discharge power supply until the lines appear as bright as possible. For this purpose you will not have to take scale readings.

For this part of the experiment, you will replace the diffraction grating mount with the prism mount as in Figure 7.

Figure 7. Schematic for the correct setting of the diffraction grating and prism. The prism should be in place for this part of the experiment.

The reason for using the prism is that it will not produce multiple patterns as the grating. Instead it will produce single brighter fringes of the emission lines, making observations easier.

Hints for identifying your unknown

Before you observe the spectrum of your unknown, study the printed spectra available at your lab bench. Note the general features of each spectrum. You can see the trend of more emission lines for elements with more electrons. Note the color of the strongest emissions- the brightest lines-for each element,since not all lines in the reference spectra may be visible. When looking at the spectrum of your unknown, look for these strong emissions first. Keep in mind that because of the way the human eye works, some lines in the blue-green boundary may “appear” to be greener.

For example, Krypton has a very strong single yellow emission line, but Neon also emits in the yellow. However, for the case of Neon there are multiple lines in the yellow region and Neon also has many more emission lines in the red region than Krypton. In a similar fashion you can identify the key characteristics and patterns of each element to help you identify your unknown correctly.

Try to reduce the number of possibilities first. More than likely you will have to examine the spectra more carefully again to make the final choice between the two best candidates. You should not try to compare every single emission line, it is too time consuming and tedious. There is space provided for making colored sketches of your unknown, if you wish to do so.

Data Sheets- Spectroscopic Analysis of Light

Your name:______Section:______

Laboratory Instructor:______Date: ______

Data Sheet 1: Hydrogen Spectrum

Below is a blank chart that you can use to make a sketch of your hydrogen spectrum, if you find this useful.

Emission line observations for hydrogen

Angle /

Color

/ Calculated Wavelength (Eq. II) / Table wavelength / Transition Energy
x / Ex2
1st Order
(n= 1)
(n= 2) / Red

Show your work below

Question 1. Below is a table of accurate wavelengths for all visible lines in the spectrum of atomic hydrogen.

Visible Emission Lines of Hydrogen (H)

Wavelength (nm) Color Intensity

410.2 violet bright, sharp

434.1 violet bright, sharp

486.1 blue-green bright, sharp

656.3 red bright, sharp

Can youmatch these with lines you observed? For each match, enter the accurate wavelength value under "Table Wavelength" in Data Sheet 1.

Question 2. Using your calculated wavelength, calculate Ex2 for all observed lines in the hydrogen spectrum, and record your answer in the last column of the table. Using the hydrogen energy level diagram (Figure 1.), assign the initial excited state (x) and record it in the last column. Show your work.

Question 3. Light is a form of energy. When the atom gives off light, does it gain or lose energy? What inside of the atom may be gaining or losing the energy?

Data Sheet 2: Identifying Unknown Elements

The unknowns will be labeled with one of these letters: A, B, C, D.

Below are two blank charts that you can use to make sketches of your unknown spectra, if you find this useful.

Indicate which elements are your unknowns.

Unknown Letter: ______Element:______

Unknown Letter: ______Element:______

Question 4. Briefly explain your reasoning to arrive at the identity of your unknowns. Include in your explanation some specific characteristics of the spectra that you used to arrive at your conclusion.

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