Isotope effect in the high harmonics of water*

We present evidence that bending motion launched by strong field ionization of the inner valence $3a_1$ orbital of H$_2$O strongly affects the high harmonic spectrum. The measured high harmonics of H$_2$O and D$_2$O are modeled consistently with solutions of the time-dependent Schr\"odinger equation for several different laser intensities. This result introduces a new method to characterize weights of different ionic states prepared by strong field ionization.