  We present quantum mechanical explanations for unresolved phenomena observed
in field-free molecular alignment by a femtosecond laser pulse. Quantum phase
analysis of molecular rotational states reveals the physical origin of the
following phenomena: strong alignment peaks appear periodically, and the
temporal shape of each alignment peak changes in an orderly fashion depending
on molecular species; the strongest alignment is not achieved at the first
peak; the transition between aligned and anti-aligned states is very fast
compared to the time scale of rotational dynamics. These features are
understood in a unified way analogous to that describing a
carrier-envelope-phase-stabilized mode-locked laser.
