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Spectroscopy: How does light interact with matter

Page history last edited by Ben Geller 10 years, 11 months ago

Spectroscopy: How does light interact with matter?

 

In your chemistry courses and labs, you have frequently made use of various “spectroscopic” tools to identify the molecular structure of unknown compounds.  You are probably familiar with the idea that molecules can attain a higher energy state by absorbing light:

 

Because different molecules absorb light of different frequencies, one can obtain spectra that show how much light is absorbed by a particular compound as a function of the frequency (or wavelength) of the light.  Spectroscopic data like these are sometimes called “molecular fingerprints” because each compound has a unique absorption spectrum by which its identity can be discovered.  Spectroscopy is an incredibly powerful tool for investigating the physical structures of objects we cannot see with our normal vision! For example, here’s an infrared absorption spectrum for an ester:

 

 

But what determines the molecular fingerprint?  Why is it that some molecules absorb strongly at a particular frequency of light while others do not?  Let's try to answer these questions...

 

Part 1 - How does light interact with molecules?

 

 

Q1: If a molecule absorbs a light ray - where did the energy from the light go?  (Be specific - don't just say "it went to the molecule"!)







 
Q2: Last week in recitation we motivated modeling a diatomic molecule (like HCl) as a simple harmonic oscillator. What represents the molecule in that model? Thinking about everything we've explored about harmonic oscillators - how could we represent the incoming light wave in this model? (Hint: Remember the homework problem on resonance!)








 
 Q3: Connect Q1 & Q2 - what impact would the incoming light ray have on the molecule in the harmonic oscillator model? What would change about the motion? Assuming the light wave begins hitting the molecule at time t1, draw the graph of r as a function of time for the molecule as a harmonic oscillator.

 

 

 Q4: If the graph below shows the initial energy of the molecule, draw what would change about the molecule after the light wave hits it. What are the conditions on the incoming light wave in order for the oscillator to gain energy?


 

 

 

Part 2 - What about the molecule would change which light is absorbed?

 

Q5: In the first part of this problem you explored using an harmonic oscillator to model the interaction between light and a molecule. Recall the natural frequency of a harmonic oscillator is defined by: 

 

 In modeling the molecule as a harmonic oscillator, what changes to the molecule would change the values of k and m, and therefore the natural frequency? Could your reasoning explain why C=O and C-O absorb light at different wavelengths?

 

 

 

 

 

 

 

Q6: If you changed the molecule in this way, what would change about the light absorbed? 

 

 

 

 

 

 

 

 

Q7: There are multiple types of molecular oscillation.  The one we’ve been exploring up to this point has been vibrational oscillation, in which the H and Cl nuclei vibrate with respect to each other.  Another type of oscillation is electronic oscillation, which can be thought of as the oscillation of the electron density of a molecule.  In the latter case, one can think of the electron density as being attached to the nucleus by a spring.  The spring constant and the mass set the frequency, but different quantum states of the molecule have different "spring constants," and the electrons and the nuclei have different masses.  Based on this, do you have a hypothesis as to whether the frequency is larger for vibrational or electronic oscillation?  Why? 

 

 

 

 

 

 

 

 

 

 

Q8: Now let's return to the absorption spectra that started this whole discussion. If the molecule absorbed light energy at particular frequencies, how do you think the IR spectrometer produces a spectrum like the one shown?  How is it that the spectrometer allows us to "see" that light has been absorbed?

 

 

 

 

 

 

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