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Chemistry LibreTexts

Lab 6: Atomic Hydrogen Spectroscopy

The experiment described in the main procedure uses a prism spectrograph to disperse light and the image of the emission spectrum is recorded on a photographic plate. See the attached text for a detailed description of the theory of this experiment.

Introduction

Rather than a prism with a photographic plate, you will use a monochromator, a combination of mirrors and gratings, and a photomultiplier tube as a detector. The monochromator and photomultiplier are parts of our fluorimeter, an instrument used to measure fluorescent spectra of molecules. You will be using part of this instrument, namely the monochromator and the detector. The light source of the instrument will not be used. Instead, the Hg reference spectrum will come from a Hg penlight. A hydrogen lamp (low pressure discharge) will be used as the to provide the emission spectrum. Brief description of the instrument Figures 1 and 2 show the spectrometer and the recorder-amplifier, respectively. All the control knobs in each unit are shown. The Meter (Item 5, Figure 1) shows the output of the detector and is used for zeroing the detector and monitoring the spectrum. The sample 0 and 100% adjust knobs are used to adjust the chart recorder. The filter selector (6) should be set at position (.) and the cell positioning knob (7) should be at position 2. The emission slit knob (8) controls the slit width for the emission monochromator; it determines the bandwidth of the spectrum. The knob is labeled with the effective bandwidth, not the slit width. Figure 2 shows the front of the recorder-amplifier unit. The amplifier controls the power for the photomultiplier and provides the output signal to the meter and the chart recorder. The power switch provides all the power for the spectrometer. The reference Sensitivity knob will not be used. The sample sensitivity knob (4, Figure 2) adjusts the sensitivity of the detector. It controls the gain or sensitivity of the photomultiplier. This knob is also used for zeroing the meter and the chart recorder. Figure 3 shows the schematic of the emission half of the fluorimeter. Light from the Hydrogen lamp is reflected down into the sample compartment and reflected off a mirror in the sample holder into the detector. Note: The chart recorder used in this set up is not the chart recorder provided with the instrument. It will however give you the information you need.

Experimental

Step 1. Record a reference spectrum

You must acquire a reference spectrum of the mercury vapor lamp. A small mercury penlight will be used as the reference. Look at the mercury spectrum in the text and note which lines are the most intense. To start, adjust the slit width with high scan rates of ~ 20 nm/sec to determine the optimal slit width. You do not need to record these preliminary adjustments. Starting slit widths will change with lamp alignment. Start with slit widths ~ 5-10 nm. To observe the effect of slit width try 2 mm and 15 or 20 mm, for example; so that you examine with similar increments above and below the starting width.

Procedure:

  1. Zero the chart recorder. Make sure that the entrance to the monochromator is closed. Cover the opening with the black piece of cardboard. Set the sample sensitivity knob on the amplifier to zero. Use the sample 0% adjustment on the spectrophotometer to adjust the zero on the meter. Use the zero adjustment on the chart recorder to set the zero. Set the sample sensitivity knob on the amplifier to 6 and readjust the zero on the meter.
  2. Turn on the mercury penlight that is mounted in the lamp holder. This is UV light so don’t look at it directly and always wear your goggles to protect your eyes from the UV light.
  3. Manually adjust the wavelength to 340 nm. Set the scan speed to high (100 nm/min). Watch the meter and the wavelength indicator. Note the approximate (to the nearest nm) position of each peak in nm. You will need to adjust the sample sensitivity for each peak, some are much more intense than others. Once you have identified the peaks and the appropriate gain setting for each peak, go back to each peak and scan slowly the entire spectrum. Set the chart recorder at 5 inch/min and the scan speed on the spectrometer to slow (25nm/min). The chart recorder makes marks at 20-nm interval. Use the point where the mark line rises just above the baseline as the reference point. Note: You will do all the scans at the narrowest slit width.

Step 2: Record the emission spectrum from a hydrogen lamp

  1. Turn on the H2 lamp that is mounted behind the mercury lamp. Repeat the preliminary determination of optimal slit width in step 1 with this lamp.
  2. The Balmer series is in the visible range so you will again do a survey scan from 340 – 700 nm at the high scan speed start with the sensitivity at 2 and adjust the sensitivity down if the signal goes off scale. Again there are a wide variety of intensities for the signals in the spectrum. Once you have identified the peaks, the appropriate gains, and heights, go back and slowly scan the entire spectrum using the same scan speed and chart speed as for the mercury lamp.
  3. Acquire a spectrum with both lamps on. You can use the mercury lines to calibrate the hydrogen spectrum.
  4. Now turn the mercury lamp off and acquire a spectrum of the H2 lamp by itself. Several of the Hg and H2 lines overlap, this spectrum will allow you to resolve those. Repeat the preliminary determination of optimal slit width in step 1 with the H2 lamp alone. Question: How does the bandwidth or resolution of your spectrum correlate with the slit width? Approximately and exactly?

Step 3: Record the effect of slit width on peak width

Adjust the slit width to 5 nm and then 10 nm and observe the effect of the slit width on the spectrum. Choose one small peak in the spectrum of either lamp. Scan just this peak at the 3 different slit widths. You mat have to adjust the sensitivity of the detector to keep the peak on-scale as the slits are opened wider.

Question: What is the inverse linear dispersion for the monochromator in this instrument? Your spectrum will contain both the hydrogen lines and the intense lines from the reference source. You can use these as an internal reference. How can you best determine which lines are from the reference source? Discuss which lines come from which source, and how well the combined spectrum reflects this. You should also compare your results with literature results.

Lab Report

 

Contributors

  • Josh Steele (UC Davis)