An ultrafast electronic excitation of N2 in the vacuum ultraviolet creates a nonstationary coherent linear superposition of interacting valence and Rydberg states resulting in a net oscillating dipole moment. There is therefore a linear response to an electrical field that can be queried by varying the time delay between the pump and a second optical probe pulse. Both the pump and probe pulses are included in our computation as part of the Hamiltonian, and the time-dependent wave function for both electronic and nuclear dynamics is computed using a grid representation for the internuclear coordinate. Even on an ultrafast time scale there are several processes that can be discerned beyond the expected coherence oscillations. In particular, the coupling between the excited valence and Rydberg states of the same symmetry is very evident and can be directly probed by varying the delay between pulse and probe. For quite a number of vibrations the nuclear motion does not dephase the electronic disequilibrium. However, the nuclear motion does modulate the dipolar response by taking the wave packet in and out of the Franck-Condon region and by its strong influence on the coupling of the Rydberg and valence states. A distinct isotope effect arises from the dependence of the interstate coupling on the nuclear mass.

Crump Institute for Molecular Imaging and Department of Molecular and Medical Pharmacology David Geffen School of Medicine and Department of Chemistry and Biochemistry University of California Los Angeles California 90095 United States

Département de Chimie B6c Université de Liège B4000 Liège Belgium

Department of Radiation and Chemical Physics Institute of Physics Academy of Sciences of the Czech Republic 18221 Praha 8 Czech Republic

The Fritz Haber Center for Molecular Dynamics and Institute of Chemistry The Hebrew University of Jerusalem Jerusalem 91904 Israel

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