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We present detailed experimental and theoretical results on population transfer with frequency-swept picosecond laser pulses. Here, we demonstrate that intense frequency-swept pulses, when applied in the adiabatic limit, lead to both more efficient and more selective excitation than 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 (I2 vapor). We discuss the different characteristics of adiabatic population transfer in both few-level, and multilevel cases, and, in particular, present computer calculations to explore the effects of molecular rotations in multilevel adiabatic population transfer.

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Frequency modulated picosecond laser pulses are applied to the X→B electronic transition in I2 vapor. In the adiabatic limit, these pulses generate more excitation in I2 vapor than do ‘‘transform-limited’’ laser pulses. Multilevel effects make it important to match the pulse bandwidth to the material absorption spectrum, as demonstrated experimentally.