Intersystem crossing (ISC), formally forbidden within nonrelativistic 9 quantum theory, is the mechanism by which a molecule changes its spin state. It plays an 10 important role in the excited state decay dynamics of many molecular systems and not 11 just those containing heavy elements. In the simplest case, ISC is driven by direct spin− 12 orbit coupling between two states of different multiplicities. This coupling is usually 13 assumed to remain unchanged by vibrational motion. It is also often presumed that spin-14 allowed radiationless transitions, i.e. internal conversion, and the nonadiabatic coupling 15 that drives them, can be considered separately from ISC and spin−orbit coupling owing 16 to the vastly different time scales upon which these processes are assumed to occur. 17 However, these assumptions are too restrictive. Indeed, the strong mixing brought about 18 by the simultaneous presence of nonadiabatic and spin−orbit coupling means that often 19 the spin, electronic, and vibrational dynamics cannot be described independently. Instead of considering a simple ladder of states, 20 as depicted in a Jablonski diagram, one must consider the more complicated spin-vibronic levels. Despite the basic ideas being 21 outlined in the 1960s, it is only with the advent of high-level theory and femtosecond spectroscopy that the importance of the 22 45 3.1.2. Time-Dependent Methods I 46 3.1.3. Including Temperature J 47 3.2. Quantum Dynamics Methods J 48 3.2.1. Multi-Configurational Time-Dependent 49 Hartree Approach J 50 3.2.2. Including Temperature K 51 3.2.3. Spin-Vibronic Model Hamiltonian L 52 3.3. On-the-f ly Dynamics Methods M 53 3.3.1. Trajectory Surface Hopping M 54 3.3.2. Methods Based upon Gaussian Wave-55

Femtosecond high-resolution pump-probe experiments have been used together with theoretical ab initio quantum calculations and wave packet dynamics simulations to decode an optimal femtosecond pulse that is generated from adaptive learning algorithms. This pulse is designed to maximize the yield of the organometallic ion CpMn(CO)3 while hindering the competing fragmentation. The sequential excitation and ionization of the target ion are accomplished by an optimized field consisting of two dominant subpulses with optimal frequencies and time delays.