Using Ab Initio Investigate The Structure And Spectrum Of (RbCs)_n(n≥2)

Compare to ultracold atoms, ultracold polar molecules are expected to have many more properties and applications due to its rich vibrational states. So in recent years, people pay more attention to the field of ultracold polar molecule. In this rapidly develop field, due to its larger permanent dipole moments we can efficient manipulations the internal and external degrees of freedom of ultracold polar molecules by means of external electric field; and it has the anisotropic long-range dipole dipole interaction, etc. Therefore, ultracold polar molecules can be widely applied in the high resolution spectroscopy, quantum information processing, precision measurement, ultracold chemistry, BCS, superfluidity, many body problems and so on. In the past decades ultracold polar molecule have been extensively investigated both theoretically and experimentally. On the experimential side ultracold polar molecules have been produced mainly through photoassociation or Feshbash resonance. However, on the theoretical side have been calculated the dynamics behavior of ultracold polar molecules, which can be used to explain the experimental phenomena deeply and provide controllable parameters for experiment and a good experiment platform to many-body problem research. Up to now, various quantum chemistry calculations have been reported on energetics and geometries of different alkali clusters. But the results of calculations can only explain the experiment qualitatively because the accuracy is not precise enough, insufficient to provide optimal control parameters for the experiment. Therefore, as an extension, this paper is mainly using Gaussian09 software based on ab initio principle study the interacting of many-body system (RbCs)n, using full-dimensional quantum dynamics simulate the geometry and infrared absorption spectroscopy of the molecule (RbCs)2.The main thesis of this study including the following third sections:First, using Gaussia09 software based on the ab initio with coupled cluster method and considering correlate electrons which included in the basis set to optimize the minimum structure of ultracold molecule (RbCs)2 and compare with the existing structure has been reported. Then calculate the frequency of this stable structure, get the normal modes and the corresponding vibrational frequencies.Second, using full dimensional quantum dynamics simulate the IR absorption spectrum of ultracold molecule (RbCs)2. Firstly, scaning the potential energy surface and dipole moment surface of ultracold molecule (RbCs)2 with Gaussian09 software, expand and fit the potential energy surface and dipole moment surface to a polynomials through cluster expansion method and the least squares method, respectively. Then simulate the IR absorption spectra of ultracold molecule (RbCs)2 employed full dimensional propagations of the vibrational wave functions by means of multi-configuration time-dependent Hartree method (MCTDH). The infrared absorption spectrum obtained by Fourier transform of the dipole autocorrelation function to unravel the important details of the interactions between ultracold molecules RbCs and to provide reliable experimental parameters.Third, using Gaussian09 software based on ab initio principle with appropriate basis sets optimize the minimum structure of larger cluster (RbCs)n (n= 3,4,5), then calculate the frequency and study the vibrational and IR absorption spectrum of larger cluster (RbCs)n (n= 3,4,5,......). Further analysis of interactions between ultracold RbCs molecules.