The ground state energy levels and molecular structure of jet-cooled HGeCl and DGeCl from single vibronic level emission spectroscopy

Abstract

Single vibronic level dispersed fluorescence spectra of jet-cooled HGeCl and DGeCl have been recorded by laser excitation of selected bands of the A 1A"-X 1A' electronic transition. Twenty-six ground state vibrational levels of HGeCl and 42 of DGeCl were measured, assigned, and fitted to standard anharmonicity expressions, which allowed all the harmonic frequencies to be determined for both isotopomers. A normal coordinate least squares analysis obtained by fitting the harmonic frequencies yielded reliable values for five of the six force constants. The ground state effective rotational constants and force field data were combined to calculate average (rz) and approximate equilibrium (re z) structures, with re z(GeH)=1.586(1) A, re z(GeCl)=2.171(2) A, and the bond angle fixed at our CCSD(T)/aug-cc-pVTZ ab initio value of 93.9 degrees . Comparisons show that the derived bond lengths are consistent with those of the appropriate diatomic molecules in their ground electronic states and the bond angle is similar to that of germylene (GeH2). A Franck-Condon simulation of the vibrational intensities in the 0(0) (0) band emission spectrum of HGeCl using ab initio force field data shows good agreement with experiment, lending credence to the vibrational analysis of the observed spectra.

Ab initio potential energy surfaces for both the ground (X̃ A1′) and excited (Ã A1″) electronic states of HGeCl and the absorption and emission spectra of HGeCl/DGeCl

“…The vibrational energy levels are assigned by three vibrational quantum numbers ͑n 1 , n 2 , n 3 ͒, representing the H-Ge stretching, bending, and Ge-Cl stretching vibrations, respectively. The fundamental frequency of the H-Ge stretching mode ͑1831.15 cm −1 ͒ is in excellent agreement with that of Tackett et al 15 ͑1830 cm −1 ͒ and slightly lower than the earlier value of Isabel and Guillory 10 ͑1862 cm −1 ͒. The plots of 12 wave functions of HGeCl are given in Fig.…”

“…In the same figure, the calculated emission spectrum from the vibrational state à ͑0, 0, 1͒ of HGeCl is compared to the experimental data of Tackett et al 15 As expected, the spectrum is dominated by bending progressions, with 0, 1, and 2 quanta in the Ge-Cl mode, respectively. Among them, the ͑0,n 2 ,1͒ progression is the most intense, consistent with the vertical nature of the excitation.…”

“…The assignment of the vibrational levels was achieved by inspecting the nodal structures of the corresponding eigenfunctions in the Radau coordinates. 15 The good theoryexperiment agreement is held all the way to the highest experimentally assigned level ͑1, 6, 0͒ near 6000 cm −1 . 2, in which the nodal structures are clearly visible.…”

“…13,15 The only nonzero transition dipole is perpendicular to the molecular plane, and it varies smoothly with the three coordinates in the Franck-Condon region ͑not shown here͒. 13,15 The only nonzero transition dipole is perpendicular to the molecular plane, and it varies smoothly with the three coordinates in the Franck-Condon region ͑not shown here͒.…”

“…11 In 1990, laser-induced fluorescence was employed by Ito et al to investigate the à 1 AЉ-X 1 AЈ transition of HGeCl. More recently, 26 ground state vibrational levels of HGeCl and 42 of DGeCl were observed by Tackett et al 15 from single vibronic level dispersed fluorescence spectra of jet-cooled HGeCl and DGeCl by laser excitation of selected bands. The first spectra of jet-cooled HGeCl using a pulsed discharge technique was obtained about 10 years ago by Harper and Clouthier,13 and the vibrational and rotational structures of HGeCl were successfully resolved.…”