Theoretical Studies On Potential Energy Surfaces And Spectra For Small Semiconductor Molecules

In recent years, some molecules containing silicon and germanium atoms have attacted much attention in experimental and theoretical studies because they play important roles as intermediates in semiconductor growth processes. However, the lack of spectra of these molecules will prevent us from better understanding the mechanism of semiconductor grouth. Based on the accurate potential energy surfaces (PESs), we can further study the spectra of these molecules. In this work, we have constructed reliable potential energy surfaces for both the ground (X1A’) and excited (A1A") states of the semiconductor molecules HGeCI, HGeBr and HSiCl by using the state-of-the-art ab initio calculations. Based on the calculated potential energy surfaces, the vibrational bound states of three molecules were obtained by employing the efficient Lanczos algorithm. In addition, we also calculated the absorption and emission spectra. The comparison between our calculated spectra and experimental observations can help us evaluate the quality of PESs and instruct experimental scientists to get better knowledge of molecular spectra.In this work, for HGeCl and HGeBr molecules, ab initio potential energy and transition dipole moment surfaces for the ground and excited electronic states were computed at the multi-reference configuration-interaction level with Davidson correction (MRCI+Q) using the augmented correlation-consistent polarized valence quadruple zeta basis sets (AVQZ). For HSiCl molecule, the single and double excitation coupled-cluster CCSD(T) and MRCI+Q methods with basis set of AVQZ were used to study these two states, respectively. Besides, we also calculated the transition dipole moment between these two states by using MRCI+Q method.The calculated results show that the agreement between our theoretical and experimental molecular equilibriums is very good. Based on the potential energy surfaces, all the vibrational bound state energy levels of three molecules were obtained using Lanczos propagation. By inspecting the nodal structures of the corresponding eigenfunctions, the assignment of the vibrational levels can be achieved. The calculated energy levels are in good agreement with the experimental values. For instance, for the ground (X1A’) state of HGeBr, our theoretical fundamental frequencies are287.65,689.84and1835.61cm-1, which are very consistent with the experimental values of291,694and1835cm-1. Moreover, our results also indicate some energy levels which are not found in experiment. For the excited (A1A") states of kindred molecules, the appropriate selection of active space of CASSCF calculation is very important to the accuracy of PESs. For HGeCl and HGeBr, we used a larger number of the active orbitals (14) with22electrons for CASSCF calculations. And for HSiCl, a number of the active orbitals (12) with18electrons was used. We found that, after extending the active space, the PESs are well improved. For example, for HSiCl molecule, our results showed that the calculated low-lying energy levels of excited state are in better agreement with experimental values than the previous theoretical results. Especially for the higher vibrational levels, when compared with experiment, the largest error is less than50cm-1for our results. But the largest error is larger than220cm-1in the previous theoretical research. In addition, with the calculated transition dipole moment, the absorption and emission spectra of three molecules were calculated using an efficient single Lanczos propagation method and are in reasonable agreement with the available observed spectra except the emission spectrum from the vibrational level A(0,1,0) of HSiCl. Note that, our emission spectrum is in good agreement with the previous theoretical results. Therefore, we hypothesize that some other vibrational states might mix into the emission spectrum and the spectral intensities were influenced.All the comparisons with previous experimental and theoretical results demonstrate the high accuracy of our ab initio PESs. And our theoretical results here will provide important insights for the experiment in the future.