  Ultrafast optical excitation with intense mid-infrared and terahertz pulses
has emerged as a new tool to control materials dynamically. As most experiments
are performed with femtosecond pulse excitation, typical lifetimes of most
light-induced phenomena in solids are of only few picoseconds. Yet, many
scientific applications require longer drive pulses and lifetimes. Here, we
describe a mid-infrared pump - terahertz-probe setup based on a CO$\mathrm{_2}$
laser seeded with 10.6 $\mathrm{\mu}$m wavelength pulses from an optical
parametric amplifier, itself pumped by a Ti:Al$\mathrm{_2}$O$\mathrm{_3}$
laser. The output of the seeded CO$\mathrm{_2}$ laser produces high power
pulses of nanosecond duration, which are synchronized to the femtosecond laser.
Hence, these pulses can be tuned in pulse duration by slicing their front and
back edges with semiconductor-plasma mirrors irradiated by replicas of the
femtosecond seed laser pulses. Variable pulse lengths from 5 ps to 1.3 ns are
achieved, and used in mid-infrared pump, terahertz-probe experiments with probe
pulses generated and electro-optically sampled by the femtosecond laser.
