Pure Rotational Spectroscopy Of Rare Gas Dimers Based On Rotational Wave Packet Imaging

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: \emph{B}$_{0}$ = 1.72713(9) GHz and \emph{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

Proton transfer or hemibonding? The structure and stability of radical cation clusters

The basin hopping search algorithm in conjunction with second-order Moller-Plesset perturbation theory is used to determine the lowest energy structures of the radical cation clusters (NH_3)_n^+, (H_2O)_n^+, (HF)_n^+, (PH_3)_n^+, (H_2S)_n^+ and (HCl)_n^+, where n=2-4. The energies of the most stable structures are subsequently evaluated using coupled cluster theory in conjunction with the aug-cc-pVTZ basis set. These cationic clusters can adopt two distinct structural types, with some clusters showing an unusual type of bonding, often referred to as hemibonding, while other clusters undergo proton transfer to give an ion and radical. It is found that proton transfer based structures are preferred by the (NH_3)_n+, (H_2O)_n^+, and (HF)_n^+ clusters while hemibonded structures are favoured by (PH_3)_n^+, (H_2S)_n^+ and (HCl)_n^+. These trends can be attributed to the relative strengths of the molecules and molecular cations as Brønsted bases and acids, respectively, and the strength of the interaction between the ion and radical in the ion-radical clusters

Assessment of density functional approximations for the hemibonded structure of water dimer radical cation

Due to the severe self-interaction errors associated with some density functional approximations, conventional density functionals often fail to dissociate the hemibonded structure of water dimer radical cation (H2O)2+ into the correct fragments: H2O and H2O+. Consequently, the binding energy of the hemibonded structure (H2O)2+ is not well-defined. For a comprehensive comparison of different functionals for this system, we propose three criteria: (i) The binding energies, (ii) the relative energies between the conformers of the water dimer radical cation, and (iii) the dissociation curves predicted by different functionals. The long-range corrected (LC) double-hybrid functional, omegaB97X-2(LP) [J.-D. Chai and M. Head-Gordon, J. Chem. Phys., 2009, 131, 174105.], is shown to perform reasonably well based on these three criteria. Reasons that LC hybrid functionals generally work better than conventional density functionals for hemibonded systems are also explained in this work.Comment: 10 pages, 5 figures, 4 table

Surface Affinity of the Hydronium Ion: The Effective Fragment Potential and Umbrella Sampling

The surface affinity of the hydronium ion in water is investigated with umbrella sampling and classical molecular dynamics simulations, in which the system is described with the effective fragment potential (EFP). The solvated hydronium ion is also explored using second order perturbation theory for the hydronium ion and the empirical TIP5P potential for the waters. Umbrella sampling is used to analyze the surface affinity of the hydronium ion, varying the number of solvent water molecules from 32 to 256. Umbrella sampling with the EFP method predicts the hydronium ion to most probably lie about halfway between the center and edge of the water cluster, independent of the cluster size. Umbrella sampling using MP2 for the hydronium ion and TIP5P for the solvating waters predicts that the solvated proton most probably lies about 0.5–2.0 Å from the edge of the water cluster independent of the cluster size

Rotational And Vibrational Wave Packet Imaging Spectroscopy: Broad Bandwidth, High-resolution Spectra And Dynamics Of Weakly Bound Molecular Dimers, <span Class="roman">ar</span>_{<span Class="roman">2</span>}, (<span Class="roman">n</span><sub><s

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 \textgreater 1 THz bandwidth are achieved simultaneously. We have measured high-resolution spectra of \chem{Ar_2}, (\chem{N_2})$_{2}$, and (\chem{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 (\textless 150 GHz) show well-resolved structures. In the case of (\chem{N_2})$_{2}$ and (\chem{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 (\chem{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