  Pair production in inhomogeneous electric fields with symmetrical frequency
chirp is studied numerically using the Dirac-Heisenberg-Wigner formalism. We
investigate high- and low-frequency modes and consider two carrier envelope
phases. Momentum spectrum is sensitive to chirp causing different interference
effect for different spatial scales as well as the carrier phase of the
external field. The reduced particle number is in general enhanced with
increasing chirp. The effect of spatial scale of the field on the reduced
particle number is also examined. It is found that it is enhanced at small
spatial scale but is almost unchangeable at large spatial scales for the
considered field parameters. On the other hand, at small spatial scale, the
reduced particle number is enhanced by one or two orders when chirp is applied
with the exception of cosine low-frequency field which is only a few times
larger. Moreover it is found that the reduced particle number is further
increased by symmetrical chirp at about two times by comparing to the usual
asymmetrical chirp in high frequency field.
