Investigation On Spectroscopic Parameters And Molecular Constants For Some Diatomic Mlecules

Molecular spectroscopy is an important tool to investigate the molecular structure and properties. By the understanding of the molecular spectroscopy, we can obtain the molecular vibraitonal energy levels, rotational energy levels, et al. In addition, using the molecular spectroscopy, various molecules which exists in the fixed star, planet, comets and star-forming regions can be detected. Then, the physical environment in the interstellar space can be investigated.Molecular potential energy curve is one of the important branches in atomic and molecular physics. It gives one absolute description of molecular and electronic energy with Born-Oppenheimer approximation, namely molecular energy, geometry, force constants and spectrum properties.In the present work, the spectroscopic parameters and molecular constants for 11 diatomic molecules (P2, PN, AsN, PO, PO+, CN, HI, PH, C12, As2 and CS) are calculated. The effects on the spectroscopic parameters and molecular constants by the isotope effect, the Davidson modification and core-valence correlation and relativistic corrections are considered. And the extrapolation is also investgated.First, the potential energy curves (PECs) of the P2 (X'Sg+, a3Su+ and A1∏g), PN (X'S+, A1∏and D1△) and AsN (X1S+) are calculated using the multi-reference configuration interaction method (MRCI) with Dunning's correlation consistent basis sets in MOLPRO 2008.1 program package. The basis set used for N and P atoms is aug-cc-pV6Z and for As atom is aug-cc-pV5Z. With the aid of module VIBROT in Molcas program package, the PECs are fitted and the spectroscopic parameters (De, Re, we, wexe, ae and Be) for each electronic state are determined. The complete vibrational states are also obtained by fitting the PECs. For each vibrational state, the vibrational level G(v), inertial rotation constant Bv and centrifugal distortion constant Dv are calculated when J=0. The results are compared with the other theoretical and experimental values available, and the comparison shows that excellent agreement exists between the present results and the available experiments.Then, the effects on the spectroscopic parameters and molecular constants by the isotope effect are evaluated in the present work. The MRCI method and the correlation consistent basis sets are used to calculate the PECs for the PO (X2∏, b4S- and C2S-), PO+(X1S+), CN(X2S+ and A2∏), HI(X1S+), PH (X3S-,a1△and A3∏) and Cl2(X1Sg+). The basis set used for Cl, P, O and H atoms is aug-cc-pV6Z, for C and N atoms is cc-pV5Z and for I atom is aug-cc-pV5Z-pp. By fitting the PECs obtained here, the spectroscopic parameters and the complete G(v), Bv and Dv for the isotope species are calculated when J=0. Most of them are reported for the first time.Third, the effects on the spectroscopic parameters and molecular constants by the Davidson, core-valence correlation and relativistic corrections are also considered in the present work. The same MRCI method and the correlation consistent basis sets are adopted to investigate the PECs for the As2 (X1Sg+, e3△u, a3Su- and d3∏g), CS (X1S+, a3∏, a'3S+, d3△, e3S-) and P2 (X1Sg+, w3△u and W1△u). The basis set used for P, C and H atoms is aug-cc-pV6Z and for As atom is aug-cc-pv5z. To obtain more accurate results, the Davidson modification and core-valence correlation and relativistic corrections are included. For the As2 molecule, the relativistic correction is made at the level of cc-pV5Z basis set. For the CS radical, the core-valence correlation correction is carried out with the cc-pCV5Z basis set. And the relativistic correction is done at the level of cc-pV5Z basis set. The way to consider the relativistic correction is to use the second-order Douglas-Kroll Hamiltonian approximation. Fitting the PECs including the Davidson modification and core-valence correlation and relativistic corrections, the spectroscopic parameters and molecular constants are determined.Finally, using the two-point energy extrapolation formula, the totle energies calculated are extrapolated to the CBS limit. In detail, for the As2 molecule, the extrapolations are based on the aug-cc-pVQZ and aug-cc-pV5Z basis sets, and for the CS and P2 molecules, the extrapolations are based on the aug-cc-pV5Z and aug-cc-pV6Z basis sets. Then using the PECs obtained by the extrapolation, the corresponding spectroscopic parameters and molecular constants are calculated.On the whole, by comparing the present results with the experiments, the Davidson, core-valence correlation, relativistic corrections and the extrapolation obviously improve the spectroscopic parameters quality.