Isomerization HCN → HNC in the electronic ground state using chirp-optimized mid-IR laser pulses: 4D quantum dynamics.

The laser-induced isomerization of HCN is studied starting from a four-nuclear-degrees-of-freedom (4D) Hamiltonian, which includes the rotation (χ) of H around a fixed CN axis. This Hamiltonian reproduces the known exact quantized rovibrational states, including Coriolis coupling. We study the laser-induced dynamics using linearly chirped mid-IR laser pulses with a temporal duration of ≃1 picosecond. The initial state is the vibrational ground state of HCN with J = K = 0, where K is the quantum number associated with vibrational angular momentum and the rotation of H around the CN axis. Over-barrier excitation via fundamental or first overtone bending modes is studied. An optimized pump-dump pulse, which is fully tailored for a fixed χ angle, is also employed and leads to considerable isomerization. Multiphoton excitation via the fundamental bending mode creates a rotational wave packet in the χ angle, whereas multiphoton excitation via the overtone bending mode preserves the uniform distribution in χ. The influence of the χ angle on the isomerization yield is discussed in comparison with a planar 3D model of the isomerization.