Rotational spectroscopy of the argon dimer by time-resolved Coulomb explosion imaging of rotational wave packets.

We report time-domain rotational spectroscopy of the argon dimer, Ar 2 , by implementing time-resolved Coulomb explosion imaging of rotational wave packets. The rotational wave packets are created in Ar 2 with a linearly polarized, nonresonant, ultrashort laser pulse, and their spatiotemporal evolution is fully characterized by measuring angular distribution of the fragmented Ar + promptly ejected from Ar 2 2+ generated by the more intense probe pulse. The pump-probe measurements have been carried out up to a delay time of 16 ns. The alignment parameters, derived from the observed images, exhibit periodic oscillation lasting for more than 15 ns. The pure rotational spectrum of Ar 2 is obtained by Fourier transformation of the time traces of the alignment parameters. The frequency resolution in the spectrum is about 90 MHz, the highest ever achieved for Ar 2 . The rotational constant and the centrifugal distortion constant are determined with much improved precision than the previous experimental results: B 0 = 1.72713 ± 0.00009 GHz and D 0 = 0.0310 ± 0.0005 MHz. The present B 0 value does not match within the quoted experimental uncertainty with that from the VUV spectroscopy, so far accepted as an experimental reference to assess theories. The present improved constants would stand as new references to calibrate state-of-the-art theoretical investigations and an indispensable experimental source for the construction of an accurate empirical intermolecular potential.