Abstract

The control of chemical reactions with ultra fast lasers is a major goal for theoreticians and experimentalists. Several different approaches to reach this goal have been developed, however, mainly by pump/probe experiments envolving excited electronic energy surfaces. Non-photoactive chemical reactions take place on the electronic ground state energy surfaces of the involved molecules. Here we report on a femtosecond quantum control experiment on the potassium dimer molecule, produced by supersonic jet expansion in a molecular beam apparatus. We have controlled the population of vibrationally hot ground state wavepackets around a prechosen mean vibrational target state following a theoretical concept proposed by Tannor and Rice^1,2 The ground state wavepackets were generated by a sequence of two femtosecond laser pulses (see Fig. 1) and probed by a third femtosecond laser pulse which transfers the molecule to its ionic ground state by resonance enhanced multiphoton ionization (REMPI) (not shown in Fig. 1). The ions are detected by a reflectron time of flight mass spectrometer. The first pump pulse generates an intermediate wavepacket in the electronic excited | B〉 state, which then starts to evolve in time. The second dump pulse stimulates at a delay time τ₁, a portion of the intermediate wavepacket back to the electronic ground state, and generates there a vibrationally excited wavepacket The mean vibrational target state (v_x in our experiment) can be chosen over a wide range, only limited by the relative position of the two involved electronic states and the shape of the potential.

© 1999 Optical Society of America