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Perfluorobutane is an inert, high-density colorless gas and has a high Global Warming Potential value of 4800.
At about the same temperature of -1.7 ⁰C, it has a density of 11.2 kg/m3, opposed to 2 kg/m3 for CO2 (both at 1 atm.)
Edit: Perflenapent (C5F12) has a molar mass of 288 g/mol, opposed to perfluorobutane with 238 g/mol, and according to this saturation curve could still be a gas at 25 kPa and 0⁰ C.
So this gas could cause a (considerable) higher pressure.
So if we ever will have serious intentions to terraform Mars, here are the gases to start with!
But first we could begin in a modest way, for instance with Ganges Cavus, a collapse feature lying in the eastern part of the Valles Marineris system of canyons.

Ganges Cavus

Ganges Cavus, from File:Ganges Chasma

From the edges to its floor it is about 5 km deep (calculated with Mars Trek), and calculated with the Mars Atmosphere Model the atmospheric pressure at the edge would be 0.692 kPa.
With that pressure (of the CO2 atmosphere) as a starting point at its edges, what would be the pressure at the floor of Ganges Cavus, if it was filled with perfluorobutane?

Liquid water of 15⁰ C would need a pressure of 1.8 kPa, water of 20⁰ C would need 2.4 kPa. (Steamtablesonline)
10⁰ C would need 1.3 kPa.

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What you are looking for is scale height--the distance required to reduce atmospheric pressure to 1/e of what it was.

Mars has a scale height of 11.1km with an atmosphere of 95% CO2. I'll assume it's all CO2. Your perfluorobutane has a density 5.6x as high. Scale height is linear on molecular weight, thus in the crater the scale height is 1.98 km. Thus we get 2.52 scale heights within the crater, e2.52 = 12.46 the density at the bottom of the crater as at the top.

Combine this with your pressure at the crater rim and we have a pressure of 8,622 Pa at the bottom of the crater. Not enough to avoid spacesuits.

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  • $\begingroup$ Thank you, I've made an edit for another fluorocarbon ($C_5F_1$$_2$) that could more than double the pressure, so you could adjust your answer to that. $\endgroup$
    – Cornelis
    Aug 31 '20 at 9:46
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    $\begingroup$ @Cornelisinspace You should not keep moving the goalposts by changing your question like that after an answer is posted. $\endgroup$ Aug 31 '20 at 10:15
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    $\begingroup$ @Cornelisinspace I don't recognize the formula to look up the density, but I explained the math, there should be enough there to figure it out for any other gas. Note that doubling the density will far more than double the pressure. $\endgroup$ Sep 1 '20 at 2:33
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    $\begingroup$ It should be noted that 8.6 kPa is slightly above the Armstrong limit (6.3 kPa), so you could maybe survive uncomfortably on supplemental oxygen without a pressure suit for a few minutes. $\endgroup$ Jan 2 at 15:34
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    $\begingroup$ @WaterMolecule of possible relevance; What is the deepest place on Mars? Do humans need pressurized suits there? and very slightly Mars' atmosphere and the triple point of water $\endgroup$
    – uhoh
    Jan 3 at 22:55

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