We present detailed experimental and theoretical results on population
transfer with frequency-swept picosecond laser pulses. Here, we demon
strate that intense frequency-swept pulses, when applied in the adiaba
tic limit, lead to both more efficient and more selective excitation t
han do unmodulated laser pulses. The experimental work is performed on
quasi-two-level systems (pentacene/p-terphenyl crystal and Na vapor),
quasi-three-level systems (Na vapor), and on more complex multilevel
systems (I-2 vapor). We discuss the different characteristics of adiab
atic population transfer in both few-level, and multilevel cases, and,
in particular, present computer calculations to explore the effects o
f molecular rotations in multilevel adiabatic population transfer.