Burton J. Krohn

Identification of the SF6 transitions pumped by a CO2 laser

Abstract The quantum numbers ( J values and octahedral symmetry types) of the SF 6 transitions from the ground state to v 3 = 1 that fall within ± 1.5 G Hz of the CO 2 P(14), P(18), and P(20) laser lines have been assigned. The SF 6 absorptions nearest these three laser frequencies are R(28) A 0 2 , P(33) A 1 2 , and an F 2 component of P(59) or P(60), respectively.

Vibrational levels and anharmonicity in SF6—I. Vibrational band analysis

Abstract The vibrational spectrum of SF 6 has been recorded with a Fourier-transform i.r. spectrometer at a resolution of 0.05 cm −1 and pressure—path length products of up to 2 × 10 5 Torr-cm. Twenty-nine bands were observed. Rotational structure was resolved for 11 of these and polynomials were fitted to the observed frequencies to yield the scalar spectroscopic constants, including the band origins m and derived values of B′  B 0 and the Coriolis constants ζ. For 12 other unresolved bands accurate estimates of the origins could be made from the frequency of a sharp Q -branch edge. Three more bands (ν 3 , 2ν 1 + ν 3 , and 3ν 3 ) were not resolvable at our resolution but have been previously analyzed from Doppler-limited or sub-Doppler spectra. In addition, about 10 assignable hot bands were observed whose frequency shifts relative to the principal transitions could be accurately measured; two of these were sufficiently resolved for full scalar analyses. These frequencies were combined with results of several high-resolution Raman studies by other authors to yield the most complete data set on SF 6 vibrational levels yet obtained. Isotopic frequency shifts have also been measured. The effective Coriolis constants for combination and overtone bands of octahedral molecules are discussed.

Anharmonic splittings and vibrational energy levels of octahedral molecules: Application to the nv3 manifolds of 32SF6☆

Abstract The expression relating the Hecht vibrational splitting parameters G33 and T33 to the normal coordinate force constants has been derived for molecules of octahedral symmetry. Using a model calculation for the anharmonic energy levels that is believed to be reasonably correct, it has been possible to derive values in the anharmonic energy levels of v3 of 32SF6 up to v3 = 4. It is possible to find allowed transitions forming a multiple-photon pathway resonant with the CO2 P(20) laser line (with perhaps 50 kW power broadening) for these levels, thus reinforcing the Cantrell and Galbraith explanations for the resonant absorption processes in terms of anharmonic splittings, together with rotational splittings.

The predicted infrared spectrum and the structure of the isolated UF5 molecule

The infrared absorption spectrum by the fundamental modes of UF5 is calculated using force constants transferred from UF6 to predict the wavenumbers and using the F atom polar tensor from CH3F to predict the intensities. Calculations are made for several assumed geometrical configurations of UF5. Comparison of the predicted spectra with the recently observed spectrum, in the UF‐stretching region, of a photolysis product of UF6 isolated in an Ar matrix suggests strongly that the structure is square pyramidal (C4v) with the U atom above the F atom equatorial plane. Although the model for the intensities is simple, the calculated spectrum is expected to predict the correct order of magnitude of the intensities of the fundamentals, first overtones, and binary combination bands of UF5, and the approximate wavenumbers for the expected absorption. Some comparisons are given between the calculations for the several structures of UF5 and observed spectra of other XF5 molecules, including PF5, BrF5, and IF5.

Measurement and analysis of the infrared‐active stretching fundamental (ν3) of UF6

High‐resolution spectra of the infrared‐active stretching fundamental ν3 of 238UF6 have been obtained between 620.6 and 633.5 cm−1 using tunable semiconductor diode lasers. Interference from hot bands was suppressed by cooling the UF6 in a supersonic expansion, and useful monomer concentrations were produced with effective temperatures of <100 K. Portions of the band from P(77) to R(66) are illustrated. All transitions from the vibrational ground state have been assigned, and the Q branch has been fully analyzed. A total of 43 line frequencies and 110 frequency differences extending in J to P(77), Q(91), and R(67) has been used to fit seven spectroscopic constants. The ground‐ and excited‐state values of the rotational constant B could be individually determined, and the U–F bond length in the ground vibrational state is r0=1.9962±0.0007 A. The Q branch of 235UF6 has also been analyzed and the 235UF6–238UF6 ν3 isotope shift measured to be 0.603 79±0.000 17 cm−1. The isotope shift and the Coriolis constant...

Analysis of the ν4 (615 cm−1) region of the Fourier transform and diode laser spectra of SF6☆

A detailed analysis of the nu/sub 4/ fundamental band in SF/sub 6/ is presented. Both from the intermediate resolution (0.04 cm/sup -1/) FT-IR and high resolution (approx. 1 x 10/sup -4/ cm/sup -1/) diode laser measurements the frequencies of the rotational transitions as well as the fine splitting parameters of the individual rotational manifolds have been determined. Well-resolved rotational sublevels of several extremely high angular momentum states (J > 100) have been recorded using a kilometer optical path diode laser spectrometer, and the observed pattern of Coriolis substructures is shown to be in good agreement with the theory.

Infrared spectrum and potential constants of silicon tetrafluoride

Doppler‐limited tunable diode laser spectra of ν4 of 28SiF4 have been analyzed and the spectroscopic constants determined. In contrast to most earlier low‐resolution studies, the Coriolis constant ζ4, when combined with ζ3 as obtained from previous laser spectroscopy, yields a zeta sum that is within 5% of the expected harmonic value of 1/2. The band origins of 12 overtones and combinations have been obtained from Fourier‐transform spectra (0.04 cm−1 resolution), resulting in estimates of the anharmonicity constants and harmonic frequencies. From the Coriolis constants and the isotope shifts in ν3 we have redetermined the general quadratic force field of SiF4.

The ν2 + ν4 band of 12CF4☆

Abstract From a high-resolution diode laser spectrum of cooled 12CF4, line assignments in ν2 + ν4 at 1066.4 cm−1 have been made for tetrahedral subspecies to J = 20, and in many cases to higher J. Spectroscopic constants have been obtained from a least-squares fit of the Hamiltonian, and the relative intensities of the assigned lines have been calculated. The ground- and excited-state rotational constants, Coriolis constant, and splitting of the F1 and F2 vibrational substates have the values a.The CF bond length in the ground vibrational state is thus r 0 = 1.31752 ± 0.00007 A . The analysis of a combination band such as this provides a method of obtaining ground-state spectroscopic constants of spherical-top molecules directly from the infrared spectrum, without the necessity of measuring weak “forbidden” transitions. The assignments allow accurate predictions of the frequencies emitted by the CO2-pumped CF4 laser.

High resolution spectroscopy of the OsO4 stretching fundamental at 961 cm−1

The ν3 bands of 187Os16O4, 189Os16O4, and 192Os16O4 have been recorded using both a Michelson interferometer (resolution 0.06 cm−1) and a tunable semiconductor diode laser (resolution limited by the Doppler width, ∼0.0007 cm−1). The rotational fine structure differs from that of most other spherical‐top molecules, for only rotational levels of A symmetry exist. A total of 112 individual vibration–rotation lines in the P and R branches of the three isotopic species were calibrated against stimulated emission lines from a high‐voltage CO2 gain cell, and were used to determine three scalar and two tensor spectroscopic constants for each species; an additional scalar constant was obtained from an analysis of the Q branch of 192OsO4. The strength of P (11) A2 0 was measured for 192OsO4 and yields a vibrational transition moment for ν3 of 0.17±0.02 D. Transitions of all isotopic species that are expected to fall near CO2 laser lines in the region 949–972 cm−1 are tabulated as an aid in the interpreation of satu...

High-resolution spectroscopy of the 16-μm bending fundamental of CF4

Abstract The 16-μm bending fundamentals ( ν 4 ) of 12 CF 4 , 13 CF 4 , and 14 CF 4 have been observed at Doppler-limited resolution using a tunable PbSnSe semiconductor diode laser. The tetrahedral splittings of the rotational manifolds have been observed in all three branches, and in particular the dense and partially overlapping transitions in the Q branches have been resolved and assigned. A least-squares fit of the Hamiltonian, including off-diagonal terms, yielded five scalar and three tensor spectroscopic constants for each of the three isotopes. From these constants the upper-state rotational constant B 4 and the Coriolis constant ζ 4 have been calculated, together with some of the other molecular constants. An absorption feature at about 0.18 cm −1 to the red of the main Q branch of each isotopic species has been identified as the Q branch of ( ν 2 + ν 4 ) − ν 2 , which is the transition that lases when CF 4 is pumped by a CO 2 laser at 9.4 μm (i.e., in ν 2 + ν 4 ).