The effects of molecular vibration on the yield of high-order harmonic generation.

It is well-accepted that the high-order harmonic spectrum is the results of interference between many attosecond pulses. Each of the attosecond pulse is produced by a three-step process taking place within one laser cycle. For light molecules such as H$_{2}$, the first step is the ionization of one electron. When the freed electron returns to the H$_{2}^{+}$, the internuclear distance is changed. This may cause the electron to miss the ion during its revisit, thus reducing its probability to recombine with the parent ion. As a result, the high harmonic generation yield is lower for H$_{2}$ than D$_{2}$, since D$_{2}$ has a longer vibration period ($\sim $21 fs) than that of H$_{2}$ ($\sim $15 fs). Here we report, to the best of our knowledge, the first experimental observation of the effects of vibration on the yield of HHG in molecules. We compared the high-order harmonic spectra of H$_{2}$, HD and D$_{2}$. The shortest pulses were $\sim $8 fs, which is almost the same as one half of the vibration period of H$_{2}$. Using such short pulses assures that the internuclear distances of all three types of molecules are in the increasing phase of a cycle when the harmonics are generated. From the HHG spectra it is evident that the yield of D$_{2}$ is a factor of two higher than that of H$_{2}$, while that of HD is in between. This is consistent with the theoretical predictions.