That's a contradiction - a feeling of being energized IS alertness, and usually mental focus as well.
That being said, caffeine is xanthine, which acts on the neurotransmitter adenosine, which in turn acts as an inhibitor of dopamine, norepinephrine and epinephrine. Since xanthine/caffeine is an antagonist at adenosine, adenosine inhibits much less of earlier mentioned drugs. Dopamine generally involves the brains pleasure/reward areas, norepinephrine which involves wakefulness and appetite, and epinephrine, which is also known as adrenaline.