Examining Spectra

Gaseous atoms excited by an electric field will emit photons with wavelengths corresponding to identifiable electron transitions.

We will observe the "spectral lines" so generated with a grating spectrometer, light source, Hydrogen, Helium and Mercury (and perhaps other) spectrum tubes with their associated power supply.

Name:

Lab Partners:

Procedure

  1. Verify that the spectrometer is properly adjusted by viewing the spectrum of the Mercury vapor tube. The green line should be at 546 nm; the two yellow lines should be at 577 and 579. If adjustment seems necessary, see the lab instructor. DO NOT leave the tube power supply on any longer than necessary to take data.

  2. Record the line wavelengths (l) of as many Hydrogen and Helium lines as you can:

    lH, 1 = nm lH, 4 = nm
    lH, 2 = nm lH, 5 = nm
    lH, 3 = nm lH, 6 = nm

    lHe, 1 = nm lHe, 4 = nm
    lHe, 2 = nm lHe, 5 = nm
    lHe, 3 = nm lHe, 6 = nm

  3. Repeat the last step for as many other spectrum tubes as are available:

    lNe, 1 = nm lNe, 4 = nm
    lNe, 2 = nm lNe, 5 = nm
    lNe, 3 = nm lNe, 6 = nm

    lAr, 1 = nm lAr, 4 = nm
    lAr, 2 = nm lAr, 5 = nm
    lAr, 3 = nm lAr, 6 = nm

    lKr, 1 = nm lKr, 4 = nm
    lKr, 2 = nm lKr, 5 = nm
    lKr, 3 = nm lKr, 6 = nm

    lHg, 1 = nm lHg, 4 = nm
    lHg, 2 = nm lHg, 5 = nm
    lHg, 3 = nm lHg, 6 = nm

Analysis

  1. For the Hydrogen and Helium data only, calculate the electron transition energy DE = h c / l for each line (recall that 1 eV = e J):

    DEH, 1 = eV DEH, 4 = eV
    DEH, 2 = eV DEH, 5 = eV
    DEH, 3 = eV DEH, 6 = eV

    DEHe, 1 = eV DEHe, 4 = eV
    DEHe, 2 = eV DEHe, 5 = eV
    DEHe, 3 = eV DEHe, 6 = eV

  2. Compute the following transition energies for Hydrogen (Z = 1, E0 = 13.6 eV). Do not round until the end of your computations, since you may have very small numbers in denominators during the calculation:

    D Eni, nf = - E0 Z2 ( 1 / nf2 - 1 / ni2 )
    DE2, 1 = eV DE6, 4 = eV
    DE3, 2 = eV DE6, 3 = eV
    DE3, 1 = eV DE6, 2 = eV
    DE4, 3 = eV DE6, 1 = eV
    DE4, 2 = eV DE7, 6 = eV
    DE4, 1 = eV DE7, 5 = eV
    DE5, 4 = eV DE7, 4 = eV
    DE5, 3 = eV DE7, 3 = eV
    DE5, 2 = eV DE7, 2 = eV
    DE5, 1 = eV DE7, 1 = eV
    DE6, 5 = eV

  3. Identify the transitions which led to the lines you saw in Hydrogen using the results of the last step:
    H, 1 ni = nf =
    H, 2 ni = nf =
    H, 3 ni = nf =
    H, 4 ni = nf =
    H, 5 ni = nf =
    H, 6 ni = nf =

  4. Multiply each of the energies you calculated for Hydrogen by 4:
    DE2, 1 = eV DE6, 4 = eV
    DE3, 2 = eV DE6, 3 = eV
    DE3, 1 = eV DE6, 2 = eV
    DE4, 3 = eV DE6, 1 = eV
    DE4, 2 = eV DE7, 6 = eV
    DE4, 1 = eV DE7, 5 = eV
    DE5, 4 = eV DE7, 4 = eV
    DE5, 3 = eV DE7, 3 = eV
    DE5, 2 = eV DE7, 2 = eV
    DE5, 1 = eV DE7, 1 = eV
    DE6, 5 = eV
    and identify the transitions for the Helium lines you observed:
    He, 1 ni = nf =
    He, 2 ni = nf =
    He, 3 ni = nf =
    He, 4 ni = nf =
    He, 5 ni = nf =
    He, 6 ni = nf =
    Why should this work?

©2004, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.

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