Chapter one showed direction doesn't move the gap for one pair; this scan generalizes the fact to a second pair. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A new pair sets a new baseline gap
Ground to upper is a pair chapter one never used. Its emission gap is 9 joules.
Eupper−Eground=9J,direction=emission
Flipping direction never changes the gap, on any pair
Chapter one showed this once, for the middle-to-upper pair. Flipping ground-to-upper from emission to absorption leaves the gap exactly where it was; only the direction label moves.
The middle-to-upper pair is chapter one's own case
This third row is the exact pair and direction chapter one already checked: gap 5 joules, direction absorption. The new ground-to-upper rows above generalize the same fact to a pair chapter one never tried.