[catgreat]

Calculate the velocity of a hydrogen and deuterium atom following the photolysis of HBr or DBr at 193 nm. The bond energy D(H-Br) is 366 KJ mol-1; assume that all the bromine atoms are produced in their lowest electronic state and that D(D-Br)=D(H-Br). Which of the noble gases would be most effective in relaxing the translationally excited H atoms?

[Enthalpy]

Unless there's some subtlety, I'd compute the photon energy at 193nm, compare it with the bond energy, and attribute the excess to kinetic energy - of which, as a refinement, bromine gets a small share to be computed from the conservation of momentum.

Assuming zero speed before the photolysis, even rotational, is reasonable. At room temperature or 26meV, it makes little as compared to a UV photon.

"Translationally excited" must mean "moving". Nice not to be a student any more.
To choose your noble gas, look if it shall be light or heavy so hydrogen transfers more of its momentum to it at shocks.

My doubts:
- Bromine not excited. A heavy atom, with UV light.
- Dissociation into neutral atoms.

[catgreat]

It is a physical chemistry question but an experiment. Thus I  have to work it out...Thanks a lot for your help. I don't think Uv will work. Actually, 193 nm may be laser beam or something.

[Enthalpy]

193nm is a known wavelength for a special laser, one of the few that emits in the UV.
Used for instance to produce semiconductor chips, where a short wavelength reduces diffraction at small patterns.