  The proposal of generating high quality electron bunches via ionization
injection triggered by an counter propagating laser pulse inside a beam driven
plasma wake is examined via two-dimensional particle-in-cell simulations. It is
shown that electron bunches obtained using this technique can have extremely
small slice energy spread, because each slice is mainly composed of electrons
ionized at the same time. Another remarkable advantage is that the injection
distance is changeable. A bunch with normalized emittance of 3.3 nm, slice
energy spread of 15 keV and brightness of $7.2\times 10^{18}$ A m$^{-2}$
rad$^{-2}$ is obtained with an optimal injection length which is achieved by
adjusting the launch time of the drive beam or by changing the laser focal
position. This makes the scheme a promising approach to generate high quality
electron bunches for the fifth generation light source.
