When noble gas atoms are driven by linearly polarized intense nearinfrared (NIR) laser pulses containing multiple optical cycles, a train of attosecond pulses can be generated, as discussed in Chapter 7. The separation between adjacent pulses is half an optical cycle of the driving laser pulses, which is 1.3 fs for Ti:Sapphire lasers. Attosecond pulse trains with such a short inter-pulse spacing have limited use in pump-probe experiments where the process to be studied may last longer than the pulse spacing. Instead, single isolated attosecond pulses are highly desirable for initiating and probing the dynamics.