Ultrafast Non-adiabatic Dynamics and Electronic Population Control in CH₃I⁺

The methyl iodide cation CH₃I⁺ is a compelling model system to investigate complex coupled nuclear electron dynamics on ultrafast time scales. The molecule is open shell, highly spin-orbit coupled, and exhibits sizable derivative coupling between experimentally accessible states. Here we present a construction of a spin-diabatic vibronic coupling Hamiltonian for the X²E_3/2, X²E_1/2 and A²A₁ states of the molecule. Using MCTDH quantum simulations we report a detailed analyses of the electronic and nuclear coherences in these states upon ionization, and develop a Stimulated Raman Adiabatic Passage (STIRAP) scheme to control the population between the X²E_3/2 and X²E_1/2 states, using the Iodine 4s core ionized state as an intermediate. We report greater than 95% population control in MCTDH simulations using 40 fs laser pulses. We comment on the expected limitations of the scheme, as well as its potential applicability to a wide range of molecules, thus opening up a new realm into molecular STIRAP control.