Immediate action right after damage

Question

Damp Power (psionic):

Manifesting Time: 1 immediate action
An amber aura quickly spreads over your body, moderating the effects of the environment. When you manifest this power, you are protected from the vagaries of destructive energies directed against you. All variable numeric effects of the next power, psi-like ability, spell, or spell-like ability that affects you before the end of your next turn is decreased to its minimum possible value. For instance, if you are struck by an energy ray augmented to deal 5d6 points of damage while protected by damp power, you would take only 5 points of damage.

Can I use an Immediate Action right after attack/damage roll?

Answer 1

The SRD description of an immediate action says you can perform an immediate action at "any time", but this has some logical limitations.

The most common interpretation is that the last chance you have to declare an immediate action to interrupt another event is after the event you are responding to has been declared, but before it has been resolved. You could manifest Damp Power in response to an enemy mage starting to cast a spell (say, if you identify it by its components to be a damaging spell), but once damage has been rolled and dealt, it has already happened, and can no longer be interrupted.

Answer 2

Yes, if you want.

Immediate Action: Much like a swift action, an immediate action consumes a very small amount of time, but represents a larger expenditure of effort and energy than a free action. Unlike a swift action, an immediate action can be performed at any time—even if it’s not your turn. Using an immediate action on your turn is the same as using a swift action, and counts as your swift action for that turn. You cannot use another immediate action or a swift action until after your next turn if you have used an immediate action when it is not currently your turn (effectively, using an immediate action before your turn is equivalent to using your swift action for the coming turn). You also cannot use an immediate action if you are currently flat-footed.

The key phrase here is at any time so as long as you haven't used your swift action, you can use an immediate action whenever you want.

In the example you gave above if you waited until after the damage was rolled then the Damp Power wouldn't work on that damage but would work on any that followed.