Coherent Excitation of the CH Stretching Vibrations in C2H4+: The Role of the Derivative Coupling Studied by the Quantum Ehrenfest Method

We report nonadiabatic dynamics computations on C₂H₄⁺ initiated on a coherent superposition of the five lowest cationic states, employing the Quantum Ehrenfest method. In addition to the totally symmetric carbon–carbon double bond stretch and carbon-hydrogen stretches, we see that the three non-totally symmetric modes become stimulated; torsion and three different CH stretching patterns. Thus, a coherent superposition of states, of the type involved in an attochemistry experiment, leads to the stimulation of specific non-totally symmetric motions. The computations were also performed on the specific combination of the A and C states. In each case normal mode 9 (cis-asymmetric H₂CCH₂ stretch), out of the set of non-totally-symmetric normal modes, dominates. Thus, we can steer the nuclear motion along specific non-totally symmetric normal modes using a defined coherent superposition.