In the (spontaneous) Raman process, incident light $\hbar \omega_1$ scatters and transfers some energy $\hbar \omega$ to a vibrational excitation of molecule or solid.

Typically this is a very rare process, and only happens to one of every $10^9$ photons or so. So even if a sample is irradiated with a ultrafast pump laser of high intensity, Raman processes tend to not be significant enough to move the system out of equilibrium. This is in comparison to something like direct absorption processes which easily bring the system out of equilibrium and often dominate the response.

But the discussion above is for spontaneous Raman processes and makes me wonder, what about stimulated Raman processes? In the case of stimulated Raman, two photons come in with $\omega_1-\omega_2=\omega$, causing vibrational excitations to be much more efficiently created.

So my questions are

  1. In practical cases, what is the efficiency of Stimulated Raman processes? In other words, for a given number of pump photon $n_1$ and Stokes photon $n_2$, what are some ballpark numbers for the number of vibrational excitations created?

  2. Can stimulated Raman processes be so strong as to move a system out of equilibrium in a pump-probe setup?

  • $\begingroup$ Vibrational relaxation is, except in a rather dilute gas, extremely fast. I`ll say no chance. $\endgroup$ – Karl Nov 16 '19 at 7:52
  • $\begingroup$ @Karl, what about in a pump-probe setup like in Q2? Usually they look at femtoseconds there, which is also pretty fast. $\endgroup$ – user157879 Nov 16 '19 at 7:55
  • $\begingroup$ even the faintest absorption drives a system out of equillibrium. Do you mean you want to get a significant saturation effect? $\endgroup$ – Karl Nov 16 '19 at 7:58
  • $\begingroup$ @Karl yes that's more along what I was thing, some significant saturation on the ultrafast time scale $\endgroup$ – user157879 Nov 16 '19 at 13:19

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