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2090 Fall 2015 1

Optical Spectroscopy

Objective:

Observe emission spectra from a variety of sources using a spectrometer

Design and implement a procedure capable of determining the composition of a solutionthat contains two or more ionic salts

Construct a partial energy-level diagram for hydrogen

Introduction:

Optical spectroscopy involves the measurement and analysis of electromagnetic radiation

(a.k.a.light). Many of the properties of light are conveniently described by means of a classical

wave model. Within this model, light waves are characterized by such variables as frequency and

wavelength. The frequency (!), which describes the number of wave crests passing a given point

per second for the light wave, is inversely proportional to the wavelength ("), the distance

between successive wave crests. The light we can see, visible light, corresponds to a very small

portion of the electromagnetic spectrum, from about 400 nm (violet) to 800 nm (red). The lightwave frequency and wavelength are related to one another by the equation

c= "! (1)

where cis the speed of the light wave. The speed of light in a vacuum is 2.998 !108m/s.

For phenomena where the classical wave model of light proves insufficient, a particlemodel is invoked. In the particle model, light is composed of a stream of discrete particles called

photons.The energy (E) of each photon is directly proportional to its frequency and inverselyproportional to its wavelength,

!"

hchE == (2)

where the proportionality constant his Plancks constant (6.6261!1034

J#s). Therefore,

electromagnetic radiation can be described by either the wave or particle model; the modelapplied is the one that accurately describes the phenomenon being investigated.

In spectroscopy, light is used as a means of probing matter. One means of probing matterwith light uses the phenomenon of absorption. When an atom, molecule, or ion absorbs a photon,

its energy increases. The energy change of the atom must be equivalent to the energy of thephoton. Thus the absorbed wavelengths of light reveal the differences between energy levels:

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2090 Fall 2015 2

E3= 8.0 aJ !E3,1= E3 E1= 31.5 aJ

!E3,2 "1= hc/!E3,1= 6.31 nm

E2= 17.0 aJ

!E2,1= E2 E1= 22.5 aJ

!E3,1 "2= hc/!E2,1= 8.83 nm

!E2,1!E3,2= E3 E2= 9.0 aJ

E1= 39.5 aJ "3= hc/!E3,2= 22 nm

"1 "2 "3

Figure 1: For an atom with only three energy states as shown above, there areonly three wavelengths of light that can be absorbed, each illustrated by an arrow.

The values of the three absorbed wavelengths of light, as calculated from theenergy-level differences, are shown as well.

The converse of absorption is emission; when an atom emits a photon of light, its energy

decreases. The energy of the emitted photon must be equivalent to the energy change of theatom. The emitted wavelengths of light correspond to the differences between energy levels:

E3= 1.5 aJ !E1,3= E1 E3= 43.5 aJ

!E2,3 "1= hc/!E1,3= 4.56 nmE2= 16.0 aJ

!E1,2= E1 E2= 29.0 aJ

!E1,3 "2= hc/!E1,2= 6.85 nm

!E1,2

!E2,3= E2 E3= 14.5 aJ

E1= 45.0 aJ"

3= hc/!E2,3= 13.7 nm

"1 "2 "3

Figure 2: Analogous with the description in Figure 1, only three wavelengths of

light can be emitted, each illustrated by an arrow. Negative energies arise fromthe thermodynamic convention that energy released by an atom is represented by

negative !E values; the values of the emitted wavelengths calculated fromenergy-level differences are shown to the right of the energy-level diagram.

In many cases, the pattern of wavelengths absorbed or emitted by a pure substance is

characteristic of that substance. Thus the pattern of emitted or absorbed wavelengths can be usedas a means of identifying a substance, a sort of fingerprint in light.

The patterns of wavelengths absorbed or emitted by atoms, molecules, or ions are knownasspectra.Spectra may be classified as emission spectra or absorption spectra. In an emission

experiment, the source, which could be an ordinary incandescent or fluorescent light bulb, a saltin a flame, or an electrically excited gas in a tube, emits the light (see Figure 3).

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2090 Fall 2015 3

Figure 3: Typical emission experimental set-up.

The emitted light is passed through a wavelength selector (a prism or diffraction grating) toselect one wavelength. The intensity of light at this wavelength is measured at the detector.

Adjusting the wavelength selector changes the wavelength of light whose intensity is measured atthe detector. The collection of these measurements over a range of wavelengths makes up the

spectrum. If only a few characteristic wavelengths are emitted, the result is a bright-line emissionspectrum. If emission occurs at all wavelengths within a given range, the result is a continuous

emission spectrum. Continuous emission spectra result when the number of available energylevels is very large and the spacing between them approaches the infinitesimal.

In an absorption experiment, the light from a source passes through an absorbingmedium, such as a gas sample or a solution, the effect of which is to remove certain wavelengths.

The wavelengths of unabsorbed light then are passed through a wavelength selector and onto adetector (see Figure 4).

Figure 4: Typical absorption experimental set-up.

The result is a dark-line spectrum. If a band of wavelengths is absorbed, the result will be

a dark area in that part of the spectrum.In this experiment you will be using both a simple hand-held spectroscope and a

sophisticated research-grade spectrometer to acquire emission spectra from a variety of sources.Both of these instruments contain a diffraction grating (wavelength selector) and a detector. The

hand-held spectroscope presents this data in a manner that is more visually appealing, as colorson a wavelength scale. The research-grade spectrometer is more quantitative; it yields

quantitative measures of intensity as a function of wavelength. The different views of emissionspectra obtained by the two instruments complement one another.

The emission spectra of a variety of materials will be observed: a fluorescent light bulb, anincandescent light bulb, helium gas, and a variety of salt solutions. These materials will provide

examples of both continuous and line spectra.

Your purpose in this experiment is twofold. One goal is to determine the composition of asolution that contains two or more ionic salts. The second goal is to construct a partial energy-

level diagram for hydrogen (see Figures 1 and 2 for examples of this). In Part A of this

experiment, you will make a number of observations of emission spectra. In Part B, you will useyour initial observations to design procedures for determining the composition of a solution that

contains two or more ionic salts and for constructing a partial energy-level diagram for

hydrogen.

Source DetectorWavelength

selector

Source DetectorWavelength

selector

Absorbing

medium

Many s One

Many s Fewer sOne

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2090 Fall 2015 4

Experiment:When recording spectral data, include the relative intensity of each line. If your

spectrometer lacks an intensity scale, estimate the intensities by eye using a scale of 1 to 10,

where 10 means very bright and 1 means you can barely see the line. For continuous spectra,record wavelength ranges corresponding to each color that you can distinguish.

Part A: Observations of emission spectra.

Procedure:

Part A: Observation of Emission Spectra

1.

Fluorescent Light Spectrum:Observe the emission spectrum of the fluorescent light with the spectroscope and the

spectrometer. Some of the lines you are likely to see occur at 405, 436, 546, 577, 579, 615, and691 nm. Dont be concerned if you cant see all the lines; some are very faint.

2.

Incandescent Light bulb Spectrum:

Observe the emission spectrum of the incandescent light bulb with the spectroscope and

the spectrometer. You should observe a continuous spectrum because an incandescent light bulbgives off white light.

3.

Helium Spectrum:

WARNING: Because of the ultraviolet radiation emitted, you should look at the radiation sourcefor only short periods of time. Do not touch any portion of the power supply, wire leads, or

discharge tube unless the power supply is unplugged from the electrical outlet because of the

very large voltages produced by the apparatus.

There is a device in your lab consisting of a glass tube (discharge tube) filled with heliumthat is attached to a voltage source used to excite the helium gas, causing it to glow. Turn thedischarge tube on, and record the color of the glowing helium gas. Use the spectroscope and the

spectrometer to observe the helium emission spectrum. Helium lines reportedly occur at 447,502, 588, 668, and 707 nm, but some are much easier to see than others.

4. Spectra of Salt Solutions: NaCl, LiCl, KCl, CaCl2, and SrCl2

Half-fill five of the smallest test tubes in your equipment drawer with one each of the

following salt solutions: sodium chloride, lithium chloride, potassium chloride, calcium chloride,and strontium chloride. Obtain 10 cotton swabs.

Light a Bunsen burner. Adjust the air flow using the knurled knob at the bottom of theburner so as to get two distinct cones of flame. Arrange the spectrometer so that the flame is

visible in the slit.

Caution: Dont get the spectrometer too close to the flame; the heat could damage it.

Safety Precautions:

Safety goggles must be worn at all times while you are in the laboratory.

In addition to visible light, the discharge tubes emit ultraviolet radiation, which is

damaging to the eyes. While safety goggles will absorb most of this radiation, it is

recommended that you look at the radiation source for only short periods of time. The power supply to the discharge tubes develops a voltage of several thousand volts.

Do not touch any portion of the power supply, wire leads, or discharge tubes unless

the power supply is unplugged from the electrical outlet.

Always unplug the power supply from the electrical outlet prior to adjusting theposition of the discharge tubes or any other part of the apparatus.

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2090 Fall 2015 5

Use the spectrometer to observe the emission spectrum of each salt solution. Toaccomplish this, soak one end of a cotton swab in a salt solution. One student should hold the tip

of the swab in the hottest portion of the burner flame (just above the inner cone) while anotherlooks through the spectroscope at the flame. It may take a few seconds for the swab to dry out

before the intensely colored flame appears. Try not to ignite the swab. If you cannot find the slit

image when the color is intense, soak the swab again, and repeat the trial. Repeat this procedurefor each of the other salts. To avoid contamination, be sure to use a new swab when you changesalt solutions.

The emission from potassium chloride is so faint that you may have difficulty seeing anylines at all. If you cannot record the wavelength of any potassium lines, just record the color.

Part B: Analysis of Spectra

1.

Identification of Unknown Salts in Solution:

You are to design and carry out a procedure that will allow for the identification of the

unknown salts in a solution. Your unknown solution will contain two or more of the followingsalts: NaCl, LiCl, KCl, CaCl2, and SrCl2. Your instructor will provide you with your unknown.

Record the unknown number in your laboratory notebook.Prior to examining its unknown sample, you are required to test your procedure on a

mixture of known composition. Use the salt solutions available in labNaCl, LiCl, KCl, CaCl2,and SrCl2to create a mixture of known composition. Record the results of testing your

procedure. Use these data to verify (or improve) the efficacy of your procedure.

2. Partial Energy-Level Diagram for Hydrogen:

WARNING: Because of the ultraviolet radiation emitted, you should look at the radiation sourcefor only short periods of time. Do not touch any portion of the power supply, wire leads, or

discharge tube unless the power supply is unplugged from the electrical outlet because of thevery large voltages produced by the apparatus.

There is also a discharge tube filled with hydrogen in the lab. You are to design andimplement a procedure for collecting data from this discharge tube that can be used to generate a

partial energy-level diagram for the electronic states of hydrogen.For the partial energy-level diagram of hydrogen, assume that all the observed transitions

terminate at the n = 2 state; for example, if you observe two transitions, they are from state A $

n = 2 and state B $n = 2. Also, set the value of the energy of the n = 2 state to 0.545 aJ.

Available Equipment and Reagents:

To perform this experiment, you will have access to all the equipment in your lab drawer

and: cotton swabs

a spectrometer (equipped with a fiber optic cable) and a spectroscope

hydrogen and helium discharge tubes, incandescent and fluorescent lights

aqueous solutions ofNaCl, LiCl, KCl, CaCl2, and SrCl2

Waste Disposal:

All chemical waste is to be flushed down the sink with plenty of water.

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2090 Fall 2015 6

Name: _________________________________ Date: _________________________________

Lab Instructor: __________________________ Lab Section: ___________________________

EXPERIMENT 7 Optical Spectroscopy

Pre-laboratory Questions: (answers to be written in your laboratory notebook)

1. Explain the difference between continuous and line spectra.

2.

Explain the difference between absorption and emission spectra.

3. For an atom with the energy levels below, what wavelength light (in nm) will be emitted

in a transition between E5and E1 (indicated by the down arrow below)? What wavelength

of light must be absorbed to cause a transition between E2and E5 (indicated by the uparrow below)?

E5= %0.0133 aJ

E4= %0.0282 aJ

E3= %0.0656 aJ

E2= %0.332 aJ

E1= %1.827 aJ

4.

Can the atom of Question 3 absorb or emit light with a wavelength of 723 nm? Can itabsorb or emit light with a wavelength of 653 nm? If so, state which energy levels the

transition occurs between. Show all work.

5. Consider the emission spectra of the two hypothetical elements X and Z. (NOTE: On the

spectra below, the upper scale corresponds to energies in units of eV.)

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2090 Fall 2015 7

Cut out these three spectra below and paste them in your laboratory notebook to make

answering this question easier. Since the duplicate copy will be collected, make sure thatthe spectra appear on the duplicate copy.

Emission spectrum of X:

Emission spectrum of Z:

Draw a picture of the emission spectra expected from a sample containing a mixture of Xand Z on the spectrum blank below.

Emission spectrum of a mixture of X and Z:

1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

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2090 Fall 2015 8

Name: _________________________________ Date: _________________________________


EXPERIMENT 7 Optical Spectroscopy

Results/Observations:

Part A: Observations of Emission Spectra.

1. Fluorescent Light Spectrum:

Observations (i.e. color of light, wavelengths from spectrometer):

2.

Incandescent Light bulb Spectrum:


3. Helium Spectrum:


1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

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Name: ________________________________ Date: _________________________________


4. Spectra of Salt Solutions:

a.

Sodium Chloride Spectrum:Observations (i.e. color of light, wavelengths from spectrometer):

b. Lithium Chloride Spectrum:


c. Potassium Chloride Spectrum:


d. Calcium Chloride Spectrum:


e.

Strontium Chloride Spectrum:


1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

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2090 Fall 2015 10

Name: ________________________________ Date: _________________________________


Part B: Analysis of Spectra.

1.

Test of Procedure for Determining the Composition of an Unknown Salt Solution:

Test mixture contains:_______________


Unknown Salt Solution Spectrum and Identification Number:


The unknown salt solution contains:_________________________________

2.

Hydrogen Spectrum:


1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

1.7 1.8 1.9 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 eV

700 600 500 400 nm

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Name: ________________________________ Date: _________________________________


Use the observed wavelengths from the hydrogen emission spectrum to calculate the

differences between hydrogen energy levels. (Show one sample calculation. Tabulate the restof the values.)

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Name: ________________________________ Date: _________________________________


Draw a partial energy-level diagram for hydrogen.Assume that all observed transitions

terminate at the n = 2 state; for example, if you observe two transitions, they are from state An = 2 and state B n = 2. Also, set the value of the energy of n = 2 state to 0.545 aJ.

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