We report time-domain rotational spectroscopy of the argon dimer, Ar2, by implementing time-resolved Coulomb explosion imaging of rotational wave packets. The rotational wave packets are created in Ar2 with a...

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We have developed a wave packet imaging-based, broad bandwidth, high-resolution spectroscopic method for weakly bound molecular dimers. In the present method, rotational and intermolecular vibrational wave packet motion is induced in the molecular dimer, via impulsive stimulated Raman scattering upon femtosecond, broad bandwidth pulse irradiation. The subsequent rotational/vibrational motion is observed as a molecular movie, utilizing time-resolved Coulomb explosion imaging. Rotational and vibrational Raman spectra are obtained as Fourier transform of the observed time-dependent image parameters. In our present setup, 80 MHz frequency resolution and >1 THz bandwidth are achieved simultaneously. We have measured high-resolution spectra of Ar 2 , (N 2 ) 2 , and (CH 4 ) 2 , while all of them are difficult targets for microwave spectroscopy due to their no or small permanent dipole. All measured spectra in the region of rotational transitions (<150 GHz) show well-resolved structures. In the case of (N 2 ) 2 and (CH 4 ) 2 , in which monomer units can rotate almost freely in the dimer, rotational constants vary with internal rotational states. This suggests internal motions govern the effective structures of the dimers. In addition to the rotational structure, the spectrum of (N 2 ) 2 shows a 250 GHz oscillation, which can be attributed to the fundamental band of an intermolecular vibration. These results indicate that the present approach is a powerful approach to study large-amplitude intermolecular dynamics. Details of the experimental setup and spectral analyses will be presented.

We report time-domain rotational spectroscopy of argon dimer and krypton dimer by implementing time-resolved Coulomb explosion imaging of rotational wave packets. The rotational wave packets are created in the dimers with a ultrashort laser pulse, and their spatiotemporal evolution is fully characterized by measuring angular distribution of the fragment ions. 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. 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 presision than the previous experimental results: B 0 = 1.72713(9) GHz and D 0 = 0.0310(5) 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. Spectrum of the krypton dimer will be also reported.