Wave-packet dynamics in the Li₂ E(¹Σ _g ⁺) shelf state: Simultaneous observation of vibrational and rotational recurrences with single rovibronic control of an intermediate state

A three-step excitation sequence is used to study the wave-packet dynamics in the E(1 Σ_g ⁺) "shelf" state of lithium dimer. In the first excitation step, a continuous wave (cw) dye laser prepares a single rovibrational level (v=14, J=22) in the intermediate ⁷Li ₂ A (¹ Σ_u ⁺) state. Ultrafast excitation of this single level with a 200 fs laser pulse centered at 803 nm creates a rovibrational wave packet (v=13-16; J=21 and 23) in the shelf region of the E(¹Σ_g ⁺) state. The motion of this three-dimensional wave packet is probed via ionization by a second ultrafast laser pulse of the same color. The initial cw excitation step allows precise control of the states that compose the wave packet. Fourier analysis of the pump-probe transients shows 15 frequency components that correspond to energy differences between the levels that constitute the wave packet. Because of the large rotational energy splitting, the rotational beats occur in the same frequency range as the vibrational beats. Experiments performed with parallel and perpendicular pump-probe polarizations provide a "magic angle" transient in which only the pure vibrational beats are observed, thus aiding in the spectroscopic assignment. The observed beat frequencies agree well with conventional high resolution frequency-domain spectroscopy. Applications of the intermediate-state control of the initial wave packet are discussed.