Nuclear Dynamics in Polyatomic Molecules and High-Order Harmonic Generation

High-order harmonic generation in molecular gases is accompanied by short-time evolution of the nuclear vibrational wave function. Using normal coordinate representation, I derive a simple analytical theory of short-time autocorrelation functions and apply it to a test set of 15 small molecules. The results explain large isotope effects observed in CH4. At the harmonic cutoff in 800 nm driving field, nuclear dynamics reduces the emission intensity from NO and NO2 molecules by more than 50%. Autocorrelation functions are sensitive to the initial vibrational state, with the nodal structure of the initial vibrational wave packet reflected in the frequency spectrum of the harmonics.