Exam III

April 9, 2001

Questions 1-5 are worth 15 points each (75 points total)

Questions 6-12 are each worth 5 points each (25 points each)

• Answer any five of these questions.

• The basic processes should be described, not simply drawn and labeled.
• You should certainly explain the origin of the bandwidth and red shift of fluorescence and should certainly differentiate between fluorescence and phosphorescence.
  • remarks: basic diagram should show ground singlet and one (or more) excited singlet and a triplet state
  • diagram should show vibrational levels and excitation to many such states
  • vibrational relaxation, quickly to the lowest vibrational level of S1
  • fluorescence is then from here to many vibrational levels of S0
  • this is basis of the red shift (often not well described on exam)
  • non-radiational transition to triple (intersystem crossing)
  • phosphorescence is from the triplet to S0
  • fluorescence is fast (nsec) and phosphorescence is slow (micro to secs to mins)

• very briefly describe how the instrument operates
  • remarks: really need a diagram of interferometer (source, beam splitter, two mirrors-one moving, detector)
  • need some clear reference to interference pattern in the light
  • need some clear description of where and how the sample is introduced
  • helpful to add red light interference for accurately determining mirror location

• a. A STM (Scanning Tunneling Microscope)
• b. A SEM (Scanning Electron Microscope)
• Your answer should discuss both the hardware and what the device actually does.
• (What can it measure?)
  • remarks-- in both cases it is essential to tell how this becomes an imaging microscope, scanning a sample
  • both are raster scan devices, slowly moving across sample, collecting data
  • data is then played back as an enlarged image based on the information collected
  • STM needs to refer to current at very small distances
  • STM needs to mention probe positioning (piezo electric elements)
  • SEM needs to mention focusing beam to very fine point with magnetic lenses
  • SEM needs to include mention of detected signal (Xrays, secondary electrons)

• What does it look like?
• How does it provide mass resolution?
  • remarks-- basic diagram of rods, reference to RF and DC voltages
  • ion trajectories, mostly unstable
  • only selected ion mass will reach detector
  • I did accept description of quadrupole ion trap

• A sample has IR absorption lines at 725, 2105 and 3220 cm-1.
• A Raman spectrometer uses a 540 nm laser source.
• What do you expect to see in the Raman Spectrum
  • (at least one actual wavelength or wavenumber should be calculated.)
• You should describe and compare Stokes vs. Antistokes lines.
• Tell briefly why one or more additional Raman lines might be observed and why one or two of the IR lines might have no Raman counterpart?
  • remarks-- this needs a calculation (one or more lines)
    • 540 nm light is 18518.5 cm-1 (1/wavelength)
    • Stokes line is reduced by 725 cm-1 (or by 2105, 3220 cm-1) to 17793 (16413, 15298) cm-1
    • Wavelength is 562 (609.3 or 653.7 nm)
    • Antistokes: add 725 cm-1 to get 19243.5 cm-1 or 519.7 nm (two more lines likely)
  • Stoke are red shifted, Antistokes are blue shifted
  • Stokes are more intense (why) although all Raman is relatively weak
  • IR and Raman have different selection rules
    • molecule could have more vibrations that are IR inactive (but seen in Raman)
    • some of the IR modes might be Raman inactive
    • (optional) May also see combination and/or overtone bands and IR/ Raman have different rules.

• These answers should be short and specific.
• They are, remember, 5 point questions.

• What advantages do Raman methods have over IR
  • remarks-- a number of answers, mostly center around shift of vibrational data to visible region (solvents, windows, better detectors, good lasers)
  • don't need continuum source or wavelength that matches sample-- any good, intense laser is useful
  • Raman sees transitional that IR wont (but also vice versa, really a tradeoff)

• remarks-- most answers gave the number of normal modes (3N-5 or 3N-6) but didn't tell what a normal mode is.
• normal mode is basically a vibration in which many or all atoms move simultaneously, at the same frequency, and in a consistent phase relationship.
• this mode can absorb energy (absorption peak)
• basically, vibrational energy is not focused in individual chemical bonds.

• write an expression for a Fourier Series
  • f(t) = (SUM) A_n sin (2 pi n nu_o t) + usually a similar sum over cosines
  • sorry about the non-Greek notation for SUM (n=0 to infinity or large integer) , pi, and frequency nu_o

• name three types of ionization source commonly used in mass spectrometers
  • electron impact (collision with electrons)
  • chemical ionization (ionize gas like CH4 and let it ionize the sample)
  • others: field ionization, MALDI and laser ionization, in atomic MS a plasma source

• Briefly sketch the light path and very briefly explain how that can give you spectra from liquids, solids (including powders)
  • see text, but light enters and reflects often from top and bottom of the prism
  • total internal reflection is important part of the answer
    • (perhaps better stated and no transmission-- see below)
  • at the sample / prism interface, light penetrates sample and can undergo absorption in lieu of reflection

• After all, both X-rays and UV radiation can penetrate deep within a sample.
  • remarks-- many answers discussed PES but skipped the actual question
  • Electrons are generated (kicked loose) below the surface by photoelectric effect since the radiation gets well below the surface
  • However, these relatively low energy electrons won't be able to get through more than a few layers of atoms and so they are never detected
  • the only photoelectrons we see are those that originate very near the surface

• (Why is a FT instrument much faster than a dispersive IR instrument.)
  • Basically we simultaneously collect data at all wavelengths with FT-IR
  • we need to run long enough for a respectable S/N ratio
  • dispersive instrument must wait for acceptable S/N ratio at each wavelength and that takes much longer
  • Multiplexing really means collecting or transmitting different data simultaneously (or nearly simultaneously)