Notes: The vacuum-UV and visible spectroscopy of SiF4 using fluorescence excitation and dispersed emission techniques is reported. The fluorescence excitation spectrum has been recorded following excitation with synchrotron radiation from the BESSY 1, Berlin source in the energy range 10–30 eV with an average resolution of ∼0.05 eV. By comparison with vacuum-UV absorption and electron energy loss spectra, all the peaks in the Rydberg spectra that photodissociate to a fluorescing state of a fragment have been assigned. Dispersed emission spectra have been recorded at the energies of all the peaks in the excitation spectra. Four different decay channels are observed: (a) SiF3 fluorescence in the range 380–650 nm for photon energies around 13.0 eV, (b) SiF2 a˜ 3B1–X˜ 1A1 phosphorescence in the range 360–440 nm for photon energies in the range 15.2–18.0 eV, (c) SiF2 A˜ 1B1–X˜ 1A1 fluorescence in the range 210–270 nm for photon energies in the range 17.0–20.0 eV, and (d) emission from the D˜ 2A1 state of SiF4+ predominantly in the range 280–350 nm for photon energies greater than 21.5 eV. These assignments are confirmed by action spectra in which the excitation energy of the vacuum-UV radiation is scanned with detection at a specific (dispersed) wavelength. Using the single-bunch mode of the synchrotron, lifetimes of all the emitting states have been measured. The lifetimes of the unassigned emitting state in SiF3, the A˜ 1B1 state of SiF2, and the D˜ 2A1 state of SiF4+ are 3.9±0.7, 11.2±1.5, and 9.16±0.02 ns, respectively. This is the first measurement of the lifetimes of these excited states in SiF3 and SiF2. The decay from the a˜ 3B1 state of SiF2 has a fast component of 2.6±0.4 ns. We conclude that the lifetime of the a˜ 3B1 state of SiF2 is either as low as 2.6 ns or too high (τ〉∼200 ns) to measure with the timing profile of the single-bunch mode of BESSY 1. If the latter interpretation is correct, as seems likely for a spin-forbidden phosphorescence to the 1A1 ground state, the 2.6 ns component could be the lifetime of intersystem crossing from higher vibrational levels of the a˜ 3B1 state of SiF2 into its ground state. © 1997 American Institute of Physics.

Notes: The infrared spectrum of 3-hydroxy-2-methyl-4-pyrone reveals an O-H stretching frequency roughly 200 cm−1 lower than that of a typical alcohol group. The frequency lowering results from intramolecular hydrogen bonding between the alcohol and ketone groups. In this work, the stretching and bending vibrations of the O-H group in 3-hydroxy-2-methyl-4-pyrone are studied with a theoretical methodology more rigorous than the conventional harmonic approximation. A two-dimensional potential energy surface in internal coordinates corresponding to different hydrogen positions in the plane of the molecule is calculated with the use of the second order Møller-Plesset perturbation theory. To include all possible variations in kinetic energy in a large amplitude vibrational mode, g-matrix elements with variable values are employed. The analytical expression for the Hamiltonian matrix elements of the two-dimensional vibrational problem in a basis of shifted Gaussian functions is derived. Expectation values for the O-H stretch nuclear states are variationally determined with the use of shifted Gaussian functions as the basis set. The results of the calculations are compared with the recent matrix-isolation infrared (IR) spectroscopic results. The calculated transition frequency corresponding to the in-plane O-H stretching is found to be in good agreement with the experimental value. © 1998 American Institute of Physics.

Notes: Threshold photoelectron–photoion coincidence (TPEPICO), photoion–fluorescence coincidence (PIFCO), and threshold photoelectron–fluorescence coincidence (TPEFCO) spectroscopies have been used to measure, state selectively, the decay pathways of all the valence states of four gas-phase tetrahedral ion CF+4, SiF+4, SiCl+4, and GeCl+4 in the range 11–26 eV. Vacuum UV radiation from a synchrotron source dispersed by a 5 m normal-incidence McPherson monochromator ionizes the parent molecule, and electrons and ions are detected by threshold electron analysis and time-of-flight mass spectrometry, respectively. Undispersed fluorescence from the interaction region can also be detected, allowing the three different types of coincidence experiment to be performed. The optimum resolution of the monochromator is matched to that of the threshold analyzer, and this work improves on preliminary results using a 1 m Seya monochromator [Chem. Phys. 174, 441 and 453 (1993)] where the resolution of the spectra was limited by that of the optical source. TPEPICO spectra are recorded continuously as a function of photon energy, allowing both threshold photoelectron spectra and yields of all the fragment ions to be obtained. Kinetic energy releases can also be measured at fixed photon energies with good time resolution. PIFCO and TPEFCO spectra are recorded at fixed photon energies. The former experiment can yield the fate of the lower electronic state of the parent ion to which fluorescence occurs.The latter experiment yields the lifetime of the fluorescing state; with sufficient resolution of the photoionizing radiation, the lifetime is specific to one vibrational level of the emitting electronic state. For CF+4 and SiF+4 work has concentrated on the third and fourth excited states, C˜ 2T2 and D˜ 2A1, of which only the C˜ state of SiF+4 does not decay radiatively. Vibrationally state-selected fluorescence quantum yields and lifetimes have been measured for four levels of the C˜ state of CF+4, and absolute values of radiative and nonradiative decay rates have been evaluated for these levels. Jahn–Teller distortion of the C˜ state of SiF+4 from Td to C3v geometry assists internal conversion of the C˜ 2T2 state into high vibrational levels of the B˜ 2E state, and is an efficient route for nonradiative decay. A non-Franck–Condon distribution of intensities is observed in the threshold photoelectron spectrum of the D˜ 2A1 state of CF+4, due to autoionization from a high-lying Rydberg state of neutral CF4. For the two chloride molecules, SiCl4 and GeCl4, fragmentation of the ground (X˜) and the first four excited states (A˜–D˜) of the parent ion have been studied at slightly lower resolution. For SiCl+4 an important result is confirmation of the stability of its electronic ground state with respect to dissociation to SiCl+3+Cl. By contrast, a substantial part of the Franck–Condon zone of the ground state of GeCl+4 is energetically unstable with respect to GeCl+3+Cl. Radiative decay from the C˜ 2T2 state of both ions is an important process. The decay dynamics of all the valence states of this family of tetrahedral ions are reviewed. Dynamical, rather than statistical, processes generally dominate, and reasons for these surprising phenomena are discussed. © 1994 American Institute of Physics.

Notes: Many vibration–rotation components of the ν2 fundamental of the SiH+3 cation have been measured using diode laser velocity modulation spectroscopy. By comparing the intensities of these components with their nuclear statistical weights, the ion is shown to be planar (D3h). The position of the ν2 band center is 838.0674(7) cm−1 and the ground state rotational constant is B0=5.2153(1) cm−1, r0=0.1462 nm. These results are compared with ab initio calculations.