Photon collision with H2 molecule

In summary, photons of a given wavelength can collide with molecules and cause them to move. The translational energy of the molecule is the mass of the molecule, which means 2 protons and 2 electrons. The kinetic energy of the molecule is 1/2mv^2, which is the same as the translational energy.
  • #1
physgirl
99
0

Homework Statement


A photon of a given wavelength "lambda" collides with a hydrogen molecule that's initially at rest (photon is absorbed by H2). Assuming absorption event conserves momentum, find velocity and translational energy of H2 after the event.


Homework Equations


momentum of photon = h/wavelength
momentum of object=mv
kinetic energy=1/2mv^2


The Attempt at a Solution


For the velocity, since momentum is conserved, and H2 is initially at rest:
h/wavelength=mv
where left side of equation is for photon and right side of equation is for H2.
So I plug in values to find "v". But I'm not sure of what to put in for "m". Is it just mass of two hydrogen atoms (which is really the mass of 2 protons) or is it the reduced mass of two hydrogen atoms or protons?

(In general, I'm confused as to when to use sum of masses as opposed to reduced mass)

And then for the kinetic energy, I guess I would just use 1/2mv^2... where "v" is velocity I find in the first part and "m" is... again, I'm not sure what "m" is.
 
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  • #2
A photon of a given wavelength "lambda" collides with a hydrogen molecule . . . . translational energy of H2
m would be the mass of the hydrogen molecule, which means 2 protons and 2 electrons.
 
  • #3
Astronuc said:
m would be the mass of the hydrogen molecule, which means 2 protons and 2 electrons.

Oh... so when would you ever use the reduced mass?

Also, can't I just ignore the 2 electrons' mass since they're about 4 orders of magnitude smaller than the proton?


Also, just to verify... translational energy of a molecule is the same thing as the kinetic energy of a molecule??
 
Last edited:
  • #4
See this for a discussion on reduced mass.
http://en.wikipedia.org/wiki/Reduced_mass

and this on the two body problem - http://en.wikipedia.org/wiki/Two-body_problem

I would expect that if the photon interacted with one H atom, and the result was a change in rotation of the molecule, then one would use the reduce mass in the appropriate equation.

One could ignore the mass of the electrons and maintain reasonable approximation.
 
  • #5
Thanks for the help! I think that makes more sense.

Last question, is translational energy the same thing as a kinetic energy? It would seem so since they are both the energy due to movement, but then I also read somewhere that kinetic energy is the sum of translational, rotational, and vibrational energy? I'm not sure... :(
 

FAQ: Photon collision with H2 molecule

What is a photon collision with an H2 molecule?

A photon collision with an H2 molecule occurs when a photon (a particle of light) interacts with a molecule of hydrogen gas composed of two hydrogen atoms. This interaction can result in various outcomes, such as absorption, scattering, or chemical reactions.

Why is studying photon collisions with H2 molecules important?

Understanding photon collisions with H2 molecules is crucial for various fields, such as astrophysics, atmospheric science, and chemical kinetics. These collisions play a significant role in the absorption and scattering of light, which affects the temperature and composition of the Earth's atmosphere and other planetary atmospheres.

What factors influence the outcome of a photon collision with an H2 molecule?

The outcome of a photon collision with an H2 molecule depends on several factors, such as the energy and frequency of the photon, the orientation of the molecule, and the electronic energy levels of the molecule. These factors can determine whether the photon is absorbed, scattered, or causes a chemical reaction with the molecule.

How do scientists study photon collisions with H2 molecules?

Scientists study photon collisions with H2 molecules through various experimental and theoretical methods. Experiments involve using lasers to create controlled photon beams and measuring the resulting interactions with H2 molecules. Theoretical methods involve using mathematical models and simulations to predict the outcomes of these collisions.

Can photon collisions with H2 molecules create new molecules?

Yes, photon collisions with H2 molecules can result in the creation of new molecules through chemical reactions. For example, when a photon with enough energy collides with an H2 molecule, it can break the chemical bonds between the two hydrogen atoms, resulting in the formation of two separate hydrogen atoms. These atoms can then combine with other molecules to create new compounds.

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