Harold W. Galbraith

Towards an explanation of collisionless multiple-photon laser dissociation of SF6

Abstract The rapidity and high degree of molecular vibrational excitation by the absorption of ir laser light in SF 6 and other molecules may be due in large part to the anharmonic splitting of excited vibrational states. Anharmonic splitting of an overtone or combination vibrational level ( i ) is possible only in molecules with degenerate vibrational states, ( ii ) can be comparable in magnitude to the net anharmonic shift of the level, ( iii ) is generally much larger than the rotational shifts which have previously been proposed as an explanation for the dissociation of SF 6 . We find that consecutive nearly resonant transitions are possible in SF 6 up to υ 3 = 5 to 10.

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.

The ν3 Q branch of SF6 at high resolution: Assignment of the levels pumped by P(16) of the CO2 laser☆

Abstract The Q branch of the ν 3 stretching fundamental of 32 SF 6 (ca. 947.6–948.3 cm −1 ) has been recorded at 109–142 K with a resolution of −4 cm −1 , and the individual transitions have been assigned. Several series of lines arising from different subbranches can be followed up to high J values ( J ≈ 80 in one case). Some of these subbranches form prominent band-heads that dominate the gross structure of the Q branch, especially at higher temperatures. A moderately strong transition at 947.742 cm −1 , displaced −8 MHz from the P (16) CO 2 laser line, has been assigned as Q(38) F 1 0 + E 0 + F 2 0 ; 39 other transitions that fall within ±300 MHz of the laser line have also been identified. The Q branch of 33 SF 6 , present in natural abundance (0.76%), has been recored and analyzed; the isotope shift is 8.97 cm −1 .

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...

Effects of anharmonic splitting upon collisionless multiple-photon laser excitation of SF6☆

Abstract Qualitative agreement of the dependence of molecular excitation upon laser frequency, between non-perturbative quantum-mechanical calculations and experimental measurements of the collisionless multiple-photon excitation of a few vibrational quanta in SF 6 by a CO 2 laser, results when anharmonic splitting of the vibrational energy levels is taken into account.

Collisionless multiple photon excitation of SF6: A comparison of anharmonic oscillators with and without octahedral splitting in the presence of rotational effects

We reinvestigate the importance of octahedral splitting in multiple photon absorption by SF6 gas in view of the recent measurement and analysis of the 3ν3 spectrum by Kildal. A simple anharmonic oscillator model which includes rotational effects to first order is an excellent approximation to the more complicated octahedral splitting model. Indeed it is the rotational splittings which are essential to the physics of multiple photon absorption modeling.

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 ).

Line frequency expressions for triply degenerate fundamentals of spherical top molecules appropriate for large angular momentum

The results show that the cluster representation accounts for higher approximations to line positions in rotational manifolds for spherical top octahedral molecules like SF/sub 6/. The mixing of branches has been explained in terms of the mixing of clusters. Such a viewpoint leads to simple perturbative formulas involving 3-J symbols for the dominant approximation excited state ..nu../sub 3/ = 1 energies and the off-diagonal correction to these energies that results from the weak coupling of pure rotational angular momentum R of the different branches. These formulas predict the rotational level structure for 30 less than or equal to J less than or equal to 90 with an accuracy that exceeds the sensitivity of most diode laser measurements. The rotational spectra of SF/sub 6/-like molecules can be predicted for high J without having to calculate symmetry adapted vector coupling coefficients or doagonalize matrices. Because of time reversal symmetry, the methods also apply to heavy tetrahedral molecules like CF/sub 4/, OsO/sub 4/, or SiF/sub 4/.