The threshold-photoelectron spectrum of SiH2: experiment and modeling with MCTDH method

Abstract

The threshold-photoelectron spectrum of silylene (SiH₂) has been successfully recorded using synchrotron radiation in the range 8–11 eV. The experimental spectrum spans the region of the ⁺²A₁ ← ¹A¹, ⁺²A₁ ← ã³B₁, Ã⁺²B₁ ← ¹A₁, and Ã⁺²B₁ ← ã³B₁ photoionizing transitions. Several features are observed including an isolated band and a Franck–Condon progression. The spectrum is modeled retrieving through ab initio calculations the potential energy surfaces of the four electronic states involved: the ground ¹A₁ and the triplet ã³B₁ electronic states of the neutral and the still poorly characterized ground ⁺²A₁ and first excited Ã⁺²B₁ electronic states of the cation. Calculated threshold photoelectron spectra between 8 and 14 eV are obtained Fourier transforming the auto-correlation function computed by wavepacket propagation with the MCTDH method. A satisfactory agreement is achieved with the experimental threshold photoelectron spectrum allowing us to identify the Franck–Condon active mode as the ν₂ bending mode. The experimental frequency of this mode in the cation is determined to be 870 ± 10 cm⁻¹ and is in good agreement with its calculated value. Adiabatic ionization energy of the ⁺²A₁ ← ¹A₁ and ⁺²A₁ ← ã³B₁ photoionizing transitions are determined to be 9.16(1) and 8.23(1) eV, respectively. A new value of 0.94(1) eV is obtained for the energy separation between the ground ¹A₁ and the triplet ã³B₁ electronic states of neutral SiH₂.