The ground state harmonic frequencies of gas phase H/DSi35Cl and H/DSi79Br have been determined by exciting single vibronic bands of the à 1A″–X̃ 1A′ electronic transition and recording the dispersed fluorescence. The jet-cooled radicals were produced in a pulsed discharge jet using H/DSiX3 (X=Cl or Br) precursors. The emission data were fitted to an anharmonic model and a normal coordinate analysis of the harmonic frequencies allowed the determination of five of the six force constants of each molecule. Using previously obtained v″=0 rotational constants and the improved force fields, average (rz) and estimated equilibrium (rez) structures were calculated for both monohalosilylenes. The validity of the force constants was evaluated by comparing calculated and observed zero-point inertial defects and by simulating the Franck–Condon profiles of the observed emission spectra in the harmonic approximation.
Emission spectroscopy, harmonic vibrational frequencies, and improved ground state structures of jet-cooled monochloro-and monobromosilylene (HSiCl and HSiBr)The jet-cooled laser induced fluorescence excitation spectrum of the à 1 AЉϪX 1 AЈ band system of DSiF has been observed using the pulsed discharge jet technique. Vibrational analysis of the spectrum yielded upper state harmonic vibrational frequencies of 1 ϭ1322, 2 ϭ444, and 3 ϭ867 cm Ϫ1 . Vibronic bands involving all of the upper state fundamentals of HSiF and DSiF have now been rotationally analyzed, allowing a determination of the excited state equilibrium structure as r e Ј(SiH) ϭ 1.526 Ϯ 0.014 Å, r e Ј(SiF) ϭ 1.597 Ϯ 0.003 Å, and e Ј(HSiF) ϭ 115.0 Ϯ 0.6°. The harmonic frequencies and centrifugal distortion constants were used to obtain harmonic force fields and average (r z ) structures for the ground and excited states. The ground state average structure was used to estimate the equilibrium structure of r e Љ(SiH) ϭ 1.528 Ϯ 0.005 Å, r e Љ(SiF) ϭ 1.603 Ϯ 0.003 Å, and e Љ(HSiF) ϭ 96.9 Ϯ 0.5°.
The optical spectrum of jet-cooled Ge2 has been observed for the first time. Laser-induced fluorescence (LIF) and wavelength resolved emission spectra were recorded using the pulsed discharge technique with a tetramethylgermane precursor. Analysis of the spectra yielded the vibrational constants ωe″=287.9(47), ωexe″=0.81(55), ωeye″=0.0037(18), ωe′=189.0(15), ωexe′=6.41(30), and Te′=20 610.8(16) cm−1. High-resolution rotationally resolved spectra of several bands of Ge274 show two strong P and R branches and two very weak Q branches. We have assigned the band system as a Hund’s case (c) Ω′=1−Ω″=1 transition from the ground Σg−3 state to a Σu−3 excited state. The bond lengths derived from the rotational constants are r0″=2.3680(1) Å and re′=2.5244(18) Å, an ∼0.16 Å increase on electronic excitation. Arguments are presented for assigning the transition to a σg2πu2→σg2πuπg electron promotion, although the observed increase in the bond length is much less than predicted by previous ab initio calculations. The absence of the 0u+–0g+ component in the spectra has been attributed to an excited state predissociation.
The ground state vibrational energy levels of jet-cooled SiCH and SiCD have been studied by a combination of laser-induced fluorescence and wavelength-resolved fluorescence techniques. The radicals were produced by a pulsed electric discharge at the exit of a supersonic expansion using tetramethylsilane or methyltrichlorosilane as the precursor. Emission spectra have been obtained by pumping both perpendicular and parallel (vibronically induced) bands, providing complementary information on the Si–C stretching and Si–C–H bending modes. Ground state energy levels up to 4000 cm−1 have been assigned and fitted using a vibrational Hamiltonian that incorporates Renner–Teller, spin–orbit, vibrational anharmonicity, and Fermi resonance interactions. The validity of the derived parameters has been tested using the isotope relations.
The electronic spectrum of germylidene (H2C=Ge), the simplest unsaturated germylene, has been observed for the first time. Jet-cooled H2CGe and D2CGe were produced by an electric discharge through tetramethylgermane diluted in argon at the exit of a supersonic expansion. High-resolution spectra of H2C74Ge and D2C74Ge, obtained from (CH3)474Ge prepared from isotopically enriched 74Ge metal, have been rotationally analyzed to yield the following r0 structures: r0″(CGe)=1.7908(2) Å, r0″(CH)=1.1022(5) Å, θ0″(HCH)=115.05(5)°, r0′(CGe)=1.914(4) Å, r0′(CH)=1.082(9) Å, and θ0′(HCH)=139.3(11)°. The 367–354 nm B̃1B2–X̃ 1A1 band system consists of prominent perpendicular bands involving the CGe stretching (ν3) and CH2 scissors (ν2) vibrations and a weaker series of vibronically induced parallel bands involving the CH2 rocking mode (ν6). Vibronic bands involving Δv=2 changes in ν6(b2) and ν4(b1) have also been assigned. The fluorescence decays of single rotational levels of the 000 band of H2C74Ge exhibit molecular quantum beats for about 70% of the levels surveyed. Density of states arguments reveal that most of the beats originate from interactions with high rovibronic levels of the ground state. In one case, hyperfine splittings in the Fourier transform of the beat pattern indicate an accidental coincidence with an excited triplet state level. The less frequent occurrence of quantum beats in germylidene compared to silylidene, where they are almost universal, can be attributed to the smaller density of ground state levels at the zero-point energy of the S2 state in the former.
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