Do ionisation of atoms release photons? I was reading through my text book about Fluorescence. It said "inside a fluorescent bulb, the mecury atoms collide with each other and with electrons get excited and ionised. During de-excitation, they release a UV photon" 
Does this mean the ionisation give out photons? If yes, how is it possible as the de-excitation cannot happen because the electron has permanently left. 
Another question, how does this ion gain its electron back? By going to negative terminal and gaining an electron or just catch an electron that comes to collide it? When this does happen, does the electron just "sit" in the vacant space or work its way through from valence shell?
High school student here. Keep it simple and understandable
Thanks
 A: You are correct that an electron has to drop back into the original level (or some available level) to produce a photon.  It doesn't matter where that electron comes from -- remember that in your lightbulb example,  electric current is flowing thru the tube.  Any free electron will give off a photon as it gets bound to the ionized atom. 
Now, strictly speaking,  ionization is not necessary for fluorescence to occur.  An external force such as an electric current might simply provide enough energy for a low-orbital electron to "jump" to a much higher-orbital.  That's an unstable state of the atom (because there's an empty lower orbital), so at some point that same electron falls back to its original level, giving off a photon.   Most lasers work on this basic principle, by the way.
A: "Does ionization lead to fluorescence?", you ask.
However, please note that the ionization itself do not mean fluorescence - that comes after. My post is about what happens to the atom after an electron has been ejected.
Yes indeed! Lets consider another example than your light bulb one. When radiation is very high-energy, it is sometimes referred to as "ionizing radiation". Gamma rays and X-rays are ionizing. This can be, and has been, exploited to make hand-held instruments for elemental determination. The technique I am talking of is called "X-ray fluorescence".
The principle for this instrument is quite simple. X-rays are generated within the instrument itself. These X-rays are then directed out of the "gun", and aimed at whatever material we wish to analyze. These X-rays hit the speciment, and then several things happen.
Remember that X-rays have very high energy. Also remember that the valence electrons of an atom are those easiest to remove. The closer we come to the nucleus, the more energy is needed to remove electrons. The X-rays in these guns are high enough in energy to remove the inner-most electrons of atoms; the radiation hits one of those electrons, and they are simple removed from the atom. Here comes the interesting part. As the electron is gone, the atom is left with an electron vacancy. This is quite unstable, and another electron fills up the vacancy. Usually the gap-filling electron comes from the adjacent energy level, or the next one. 
When this electron falls down to fill the vacancy, energy is sent out. This is referred to as fluorescence. The energy of the fluorescence depends on the energy difference between the electron vacancy and the energy level (and orbital) of the gap-filling electron. Since the energy differences between energy levels and orbitals are characteristic for each element in the periodic table, measuring the emitted light gives us information about the elemental composition in whatever the X-ray gun was aiming at. These guns are often used by toy stores to analyse imported toys to make sure there are no harmful elements present. 
To sum up, ionizing an atom leads to fluorescence, and this has many applications. I mentioned just one in this post, but you can probably read about others online. I know this answer maybe was a little off-topic, but I just wanted to add some context to a simple "yes, it does" answer. 
A: The collisions involving the Hg atoms cause electrons to be removed from the Hg atom (ionization).  The electrons become free electrons.   Ionization does not emit photons.
Meanwhile, A Hg ion and a free electron will find each other and the electron is captured.  Generally, the electron will make several transitions on its way to the ground state, each transition releasing a photon.  The UV transitions that are responsible for fluorescence are generally from free state to an excited state, or from an excited state to a lower excited state. 
With a source of power available, a steady state condition can be achieved in which the gas supports a certain constant (in time) density of ions and electrons: a plasma.  The tube glows.  
