Theoretical calculations of atom excitation dynamics by powerful (about 10 MW/cm 2 ) pico- and femtosecond laser pulses demonstrate that efficient and selective population of the desired atomic level is achieved. The conditions and the value for the maximum population degree of a desired level have been found. The mechanism of the ultrafast coherent excitation under study is able to shift 90% of lithium atoms to the 3d-level and 50% of rubidium atoms to the 7s-level. This mechanism is a convenient tool for preparing notable quantities of atoms in the required state.

The possibilities of ultrafast selective population of specific levels in the IR-active vibrational mode of a polyatomic molecule excited under collisionless conditions by high-power picosecond IR laser pulses with different shapes, amplitudes and durations are theoretically investigated. There exists a minimum pulse duration at which the population can be localized completely at a given vibrational level. For high-lying levels this minimum duration is shorter than for low-lying ones. For each level there exists such power broadening at which the conditions for population localization are practically independent of the laser pulse shape. The pumping rate of the IR-active vibrational mode may be much greater than the rate of the vibrational energy intramolecular redistribution.