Vibrational wave packets in the C ¹Π_u state of Cs₂: Two color pump-probe experiments

Two color, pump-probe laser experiments on the ∼100 fs time scale have been applied to examining the dynamics of vibrational wave packets in the C ¹Π_u state of Cs₂. Wave packets consisting of more than 20 C state vibrational levels are produced with an initial pulse having a central wavelength between ∼620 and 680 nm. The temporal history of the wave packet is inferred from the time and energy-integrated photoelectron signal produced when the excited state is photoionized by a time-delayed pulse centered at 605, 610, 615, 617, or 620 nm. Because of the difference in equilibrium internuclear separations for the Cs₂(C) and Cs ₂⁺(X) states (ΔR_e≈0.75 Å), wave packets are readily observed (signal-to-noise ratio > 10) without the need to resort to mass selection techniques. Frequency analysis of the wave packet data shows a dominant (fundamental) component that decreases from 29 to ∼28.3 cm^-1 as the pump wavelength is tuned from 627 to 641 nm. Other spectral modes at ∼23.5 and ∼34 cm^-1 and a weaker term at twice the fundamental frequency are also observed and quantum mechanical calculations of the wavepacket motion are in accord with the experimental results.