Pumping and probing vibrational modulated coupled electronic coherence in HCN using short UV fs laser pulses: a 2D...

The coupled electronic-nuclear coherent dynamics induced by a short strong VUV fs pulse in the low excited electronic states of HCN is probed by transient absorption spectroscopy with a second weaker fs UV pulse. The nuclear time-dependent Schrodinger equation is solved on a 2D nuclear grid with several electronic states with a Hamiltonian including the dipole coupling to the pump and the probe electric fields. The two internal nuclear coordinates describe the motion of the light H atom. There is a band of several excited electronic states at about 8 eV above the ground state (GS) that is transiently accessed by the pump pulse. We tailored the pump so as to selectively populate the lowest 1A'' electronic state thereby the pulse creates an electronic coherence with the GS. Our simulations show that this electronic coherence is modulated by the nuclear motion and persists all the way to dissociation on the 1A'' state. Transient absorption spectra computed as a function of the delay time between the pump and the probe pulses provide a detailed probe of the electronic amplitude and its phase, as well as of the modulation of the electronic coherence by the nuclear motion, both bound and dissociative.