David A. Hostutler

The electronic spectrum, molecular structure, and oscillatory fluorescence decay of jet-cooled germylidene (H2C=74Ge), the simplest unsaturated germylene

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) A, r0″(CH)=1.1022(5) A, θ0″(HCH)=115.05(5)°, r0′(CGe)=1.914(4) A, r0′(CH)=1.082(9) A, and θ0′(HCH)=139.3(11)°. The 367–354 nm B1B2–X 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 q...

PULSED DISCHARGE JET SPECTROSCOPY OF DSIF AND THE EQUILIBRIUM MOLECULAR STRUCTURE OF MONOFLUOROSILYLENE

The jet-cooled laser induced fluorescence excitation spectrum of the A 1A′′−X 1A 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 ω1=1322, ω2=444, and ω3=867 cm−1. 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 re′(SiH)=1.526±0.014 A, re′(SiF)=1.597±0.003 A, and θe′(HSiF)=115.0±0.6°. The harmonic frequencies and centrifugal distortion constants were used to obtain harmonic force fields and average (rz) structures for the ground and excited states. The ground state average structure was used to estimate the equilibrium structure of re″(SiH)=1.528±0.005 A, re″(SiF)=1.603±0.003 A, and θe″(HSiF)=96.9±0.5°.

The ground state of germylidene (H2C=Ge)

Single vibronic level emission spectra have been obtained by pumping individual vibronic bands in the B1B2–X1A1 electronic transitions of jet-cooled H2CGe and D2CGe. Analysis of the spectra gave for the first time the frequencies of the following ground state fundamentals: for H2CGe, ν2=1238 cm−1, ν3=782 cm−1, ν4=673 cm−1, ν6=351 cm−1, and for D2CGe, ν1=2089 cm−1, ν2=1000 cm−1, ν3=694 cm−1, ν4=520 cm−1, and ν6=282 cm−1. The vibrational data have been combined with our previously determined molecular structures to obtain approximate harmonic force fields for the ground and excited states. Franck–Condon simulations of the emission spectra in the harmonic approximation show reasonable agreement with experiment, except for unexpected activity in the out-of-plane bending mode. This is attributed to a very anharmonic out-of-plane bending potential in the S2 excited state. Germylidene also has a very low ground state CH2 rocking frequency (ν6) which is along the normal coordinate leading to isomerization to th...

Orbital angular momentum (Renner–Teller) effects in the 2Πi ground state of silicon methylidyne (SiCH)

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.

Exploring the Bermuda triangle of homonuclear diatomic spectroscopy: The electronic spectrum and structure of Ge2

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 74Ge2 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 3Σg− state to a 3Σu− excited state. The bond lengths derived from the rotational constants are r0″=2.3680(1) A and re′=2.5244(18) A, an ∼0.16 A 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 ...

The permanent electric dipole moments of iron monoxide, FeO

The Q(4) and R(4) branch features of the (0,0)D (5)Delta(4)-X (5)Delta(4) band system and the Q(3) and R(3) branch features of the (0,0)D (5)Delta(3)-X (5)Delta(3) band system of iron monoxide FeO have been studied by optical Stark spectroscopy. The Stark splittings in the high resolution laser induced fluorescence spectra were analyzed to produce values for the magnitude of the permanent electric dipole moments /mu/ of 4.50+/-0.03, 4.29+/-0.05, 2.53+/-0.04, and 2.58+/-0.06 D for the X (5)Delta(4) (nu=0), X (5)Delta(3) (nu=0), D (5)Delta(4) (nu=0), and D (5)Delta(3) (nu=0) states, respectively. The results are compared to several ab initio predictions and to FeC. The qualitative trends are explained in terms of a molecular orbital correlation picture.

Single vibronic level emission spectroscopy of jet-cooled HSiF and DSiF

Using the technique of single vibronic level emission spectroscopy, the ground state vibrational manifolds of jet-cooled HSiF and DSiF have been studied. The radicals were produced in a pulsed electric discharge jet using trifluorosilane (HSiF3 or DSiF3) as the precursor. The gas phase ground state harmonic vibrational frequencies of both isotopomers have been determined for the first time. A normal coordinate analysis using the vibrational frequencies and literature values for the centrifugal distortion constants allowed the determination of all six ground state force constants. Our previous ground state rotational constants have been combined with the calculated harmonic contributions to the α constants to obtain an average (rz) structure and an estimate of the equilibrium (rez) structure. The reliability of the force constants has been evaluated by Franck–Condon simulations of the emission spectra and comparisons of the calculated and experimentally determined inertial defects.

Emission spectroscopy, harmonic vibrational frequencies, and improved ground state structures of jet-cooled monochloro- and monobromosilylene (HSiCl and HSiBr)

The ground state harmonic frequencies of gas phase H/DSi35Cl and H/DSi79Br have been determined by exciting single vibronic bands of the A 1A″–X 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.

The laser-induced fluorescence spectrum, Renner–Teller effect, and molecular quantum beats in the à 2Πi-X̃ 2Πi transition of the jet-cooled HCCSe free radical

The selenoketyl (HCCSe) radical has been positively identified for the first time as a product of an electric discharge through selenophene vapor. Laser-induced fluorescence, wavelength resolved emission, and fluorescence decay studies of jet-cooled HCCSe and DCCSe have given a detailed picture of the ground and excited state. The 418-400 nm band system of the HCCSe radical is assigned as A (2)Pi(i)-X (2)Pi(i) and the available evidence suggests that the radical is linear in the ground state and quasilinear in the excited state. The fluorescence decays of some upper state rotational levels show field-free molecular quantum beats, ascribed to an internal conversion interaction with high vibrational levels of the ground state. A comparison of the molecular structures and bonding in the HCCX (X=O,S,Se) free radicals shows that nonlinear ground state HCCO is best described as the ketenyl radical (H[Single Bond]C[Double Bond]C[Double Bond]O) with the unpaired electron on the terminal carbon atom, whereas HCCS and HCCSe have linear ground state acetylenic (H[Single Bond]C[Triple Bond]C[Single Bond]X) structures with the unpaired electron on the heteroatom. On electronic excitation, B (2)Pi HCCO reverts to the linear acetylenic structure, and A (2)Pi HCCS and HCCSe become quasilinear with the allenic structure.