Why do molecules not fall apart under observation?

  • #1

I was just revising for my Quantum Mechanics A exam and was considering the 1D double well model of an H2+ molecule. As I understand it Psi2 is non zero between the wells and therefore there is some negative charge present to which the H+ nuclei are attracted resulting in a covalent bond.

But what happens when the electron is observed and found to be around either of the nuclei? i.e the wave fuction "collapses". Why does the molecule not split apart?


Answers and Replies

  • #2
First, we need to understand the kinematics of the H2+ ion. Its binding energy is about 2.78 eV (please check this). This is comparable to the energy of a 450 nanometer wavelength photon. So a blue photon could (not will) dissociate an isolated H2+ ion. Macroscopic laws like conservation of energy would prevent dissociation of an isolated ion when the photons are less than 2.78 eV (or longer than 450 nm). The electron cannot be "observed" without disturbing it (per the Uncertainty Principle). The wave function of the 3 particles is nothing more than a probability function; not the kind like rolling snake eyes with two dice. It just implies a fuzzy area where the electron's wave function is (but not where the electron is), but nothing about what its binding energy to either or both protons is at any given time. What can be said is that UV photons could dissociate it by photoelectron emission, but the probability would be based on detailed calculations of the density of final states, and that green light >= 500 nanometers long cannot dissociate it, because of conservation laws.

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