How do collisions lead to phase change of the excited atom? In collisional broadening the excited atoms interact with the surrounding atoms and de-excitation of the excited atom takes place. How does this lead to reduced lifetime? Why is this de-excitation non-radiative? How does the phase change?
Thanks in advance.
 A: If you have an isolated atom then the electronic eigenstates are the usual atomic orbitals $1s$, $2s$, $2p$, etc$^1$. However suppose a second atom collides with our atom. As the second atom approaches its electrons perturb the first atom, and the eigenstates of the perturbed atom are no longer the $1s$ etc atomic orbitals. To a first order approximation the perturbation causes the atomic orbitals to mix with each other so the electrons will be in a superposition of the $1s$, etc states.
As the colliding atom moves away again our atom settles back into the usual atomic orbital eigenstates. However because the orbitals got mixed up during the collision the final electronic state is not necessarily the same as the state before the collision. Energy can be exchanged between the kinetic energy of the colliding atoms and the electron energy.
So for example two atoms in the ground state can collide and some of the kinetic energy can go into exciting an electron in one (or both) of the atoms. This is then an inelastic collision because the kinetic energy afterwards is less than the kinetic energy before - the missing energy is absorbed in exciting an electron.
Alternatively if one of the atoms is already in an excited state then the electronic energy can be transferred into kinetic energy so the kinetic energy after the collsiion is greater than the kinetic energy before the collision. And it's this process that is known as collisional de-excitation. The process is non-radoative because the energy of the excited state is transferred to kinetic energy instead of the energy of an emitted photon.
How much effect collisions have depends on the mean time between collisions. If the mean collision time is a lot longer than the excited state lifetime then the excited atoms will normally emit a photon before they collide. If the mean collision time is much shorter than the excitated state lifetime then the excited atoms will collide and de-excite before they have a chance to emit a photon, and light emission will be suppressed.

$^1$ this isn't strictly true but we can ignore this detail for the purposes of this question
A: The simplest way to put this is, when an atom hits another, it will cause a little bit of damage to each of the atoms. But with Newtons third law of motion, for every action there is an equal and opposite reaction, the atoms will bounce and transfer their energy to another atom through a collision and soon lots of atoms are damaged and moving (that's what make the vibrations). This would shorten the life of the molecule, which is then shortening the life of whatever all of the molecules together makes up. That could just harm the object a little bit but if the vibration was big enough, then the blow would have set more sets of atoms on a collision course weakening more areas shortening the life a lot more. Also I am looking for some green laser pointers in stores and not online so if you know a store with some or just green laser diode modules, then please let me know at calebeil@hpsvikings.org. Warning, if anybody spams me I will report them, and block them. Thank you!  
