  A method is proposed to generate low emittance electron bunches from two
color laser pulses in a laser-plasma accelerator. A two-region gas structure is
used, containing a short region of a high-Z gas (e.g., krypton) for ionization
injection, followed by a longer region of a low-Z gas for post-acceleration. A
long-laser-wavelength (e.g., 5 micron) pump pulse excites plasma wake without
triggering the inner-shell electron ionization of the high-Z gas due to low
electric fields. A short-laser-wavelength (e.g., 0.4 micron) injection pulse,
located at a trapping phase of the wake, ionizes the inner-shell electrons of
the high-Z gas, resulting in ionization-induced trapping. Compared with a
single-pulse ionization injection, this scheme offers an order of magnitude
smaller residual transverse momentum of the electron bunch, which is a result
of the smaller vector potential amplitude of the injection pulse.
