Femtosecond wave-packet propagation in spin-orbit-coupled electronic states of ^39,39K₂ and ^39,41K₂

Applying femtosecond pump-probe spectroscopy, we investigated via three-photon ionization (3PI) and high mass selection the vibrational dynamics of the potassium dimer’s electronic A¹Σ_u⁺ state separately for two of its isotopoes, ^39,39K₂ and ^39,41K₂. The fast oscillation with T_A=500 fs, observed for both isotopes, reflects the wave-packet propagation prepared on the potential-energy surface of the A state. The long-time dynamics, however, of the isotopes is totally different. While for ^39,39K₂ a beat structure with T_BS=10 ps is superimposed, for ^39,41K₂ a rather fast decay and revivals at 38 ps, 60 ps, and 82 ps could be resolved. A detailed Fourier analysis of the 200-ps scans with a resolution of 0.1 cm^-1 enables the identification of the excited vibrational levels of the A state in detail, including their energetic shifts due to spin-orbit coupling with the crossing b³Π_u state. Theoretical simulations of the pump-probe spectra on the basis of fully quantum-dynamical calculations reproduce well the experimental data. The reason for the slight differences can be identified as deviations between the real potential-energy surfaces and the ab initio data, used for the simulations and demonstrates the very high sensitivity of the femtosecond spectroscopy to investigate vibrational states and their perturbation. Furthermore, the theoretical investigations reveal the details of the ultrafast intersystem crossing process in real time. © 1996 The American Physical Society.