Phase and amplitude control in the formation and detection of rotational wave packets in the E1Σg+ state of Li2

Femtosecond laser pulse amplitude/phase masking techniques are employed to control the formation and detection of rotational wave packets in the electronic E (1)Sigma(g)(+) state of lithium dimer. The wave packets are prepared by coherent excitation of rovibronic E (1)Sigma(g)(+) a ( VE,JE) states of Li-2 from a single intermediate state, A (1)Sigma(u)(+) (nu(A) = 11, J(A) = 28), and probed by time-resolved photoionization. In the detection step, the wave packet is projected onto the X (2)Sigma(g)(+) state of Li-2(+). New resonance structure in the X (2)Sigma(u)(+) ionic state continuum is obtained by measuring the wave packet signal modulation amplitude as a function of the frequencies removed from the spectrally dispersed probe pulse by insertion of a wire mask in a single-grating pulse shaper. A split glass phase mask inserted into the pulse shaper is used to produce step function changes in the spectral phase of the pulse. The phase relation among the wave packet states is varied by changing the relative phases of spectral components in the pump pulse and is monitored by measuring the changes in the phase of the rotational wave packet recurrences using an unmodified probe pulse. By altering the relative phases among the wave packet components, the spatial distribution of the initial wave packet probability density is varied, resulting in phase-dependent "alignment" of the probability density in angular space. Phase changes in the signal recurrences are also observed when a phase modified pulse is used in the wave packet detection step after wave packet preparation with an unmodified pulse. The formation and detection of the wave packets is discussed in terms of quantum interference between different excitation routes. The relative phase factors encoded in a single optical pulse (pump dr probe) are transferred into the interference term of the measured signal through the molecule-photon interaction. (C) 1998 American Institute of Physics. [S0021-9606(98)00222-0].