Molecular Beam Electric Resonance Method Research Articles

Avoided-crossing molecular-beam study of the torsion-rotation energy levels of CH 3CF 3

The avoided-crossing molecular-beam electric resonance method has been used to determine the leading parameters in the torsion-rotation Hamiltonian of CH 3CF 3, which was selected as a prototype for the case of a symmetric rotor with small (≤ 500 kHz) internal rotor splittings. Stark anticrossings have been studied for ( K= ±2↔∓ 1) with J= 2−6; hyperfine anticrossings have been studied for ( K= ±2↔0),(±2↔±1), and (±)↔0) with J≤2. A detailed investigation of the hyperfine case has been carried out involving selection rules, relative intensity calculations, Zeeman studies, and combination differences. Several pure rotational transitions for the lowest two torsional states have been measured between 93 and 114 GHz with a mm-wave spectrometer. The ( J= 1 ← 0) transition in the ground torsional state has been measured with the molecular beam spectrometer. Stark and Zeeman studies have been carried out with conventional molecular beam techniques. It has been determined that the effective rotational constant A eff=5502.904(3) MHz, the effective height of the threefold barrier to internal rotation V eff=1093 (11) cm −1 and the moment of inertia of the methyl top I α=3.21(4) Å 2. It has been found that the electric dipole moment μ=2.34720(13) D and the distortion dipole moment constant μ D=3.220(11) μD. The magnitudes and signs of the molecular g-factors have been obtained: g∥=−0.0226(13) nm and g ⊥=−0.117(1) nm. In addition, values have been determined for the B-rotational constant and several distortion constants.

The rotational spectrum, barrier to internal rotation, and structure of NH3–N2O

The rotational spectrum of NH3–N2O has been observed by the molecular beam electric resonance method. The spectrum is characteristic of a T-shaped complex in which the nitrogen of the NH3 subunit is directed toward the N2O subunit. The NH3 unit exhibits nearly free internal rotation about its C3 axis. The A internal rotor state transitions were fit to Watson’s asymmetrical top Hamiltonian and the following spectroscopic constants were determined: A(MHz)=12 722.5(5), δK (MHz)=0.22(2), B(MHz)=4 083.5(2), δJ(MHz) =0.007(1), C(MHz)=3 070.8(2), ΔK(MHz) =−0.3(2), ΔJ(MHz) =0.016(9), ΔJK(MHz) =0.32(3), μa(D) =1.514(9), μb(D) =0.09(9). The E internal rotor state transitions were fit to the Hamiltonian of Kilb, Lin, and Wilson. For the E internal rotor state the rotational constants A [12 743(64) MHz], B[4077.1(12) MHz], and C[3069.9(9) MHz] were determined, as well as the height of the barrier to internal rotation, V3 [12.5(25) cm−1]. The distance between the nitrogen of the NH3 to the center of mass of N2O is 3.088 Å. The angle between the C3 axis of NH3 and the line joining the centers of mass of the NH3 and N2O subunits is 13°. The C3 axis of NH3 is pointed towards the nitrogen end of the N2O subunit.

The Zeeman spectrum of the OH 2Π [formula omitted] state

The J = 1 2 and J = 3 2 states of the X 2Π 1 2 level of OH have been studied in magnetic fields up to 0.8 T. The molecular beam electric resonance method was used. The Λ-doubling Zeeman transitions have been observed for the J = 1 2 singular level for the first time. The results were combined with data from the literature, which enabled a rather uncorrelated fit of all the g-factors with improved accuracies. The experimental value of the electronic spin g-factor is compared with the theoretical prediction.

The Stark and Zeeman effects in methyl silane

The Stark and Zeeman effects in methyl silane in the ground vibronic state have been studied in detail using the molecular-beam electric-resonance method. For a symmetric rotor without internal rotation, the rotational dependence of the effective dipole moment for matrix elements diagonal in J has been shown by Watson, Takami, and Oka to have the form μ Q = μ 0 + μ J J( J + 1) + μ K K 2. It is shown here that, to this order, a complete characterization of the Stark effect requires only one more parameter, namely, the effective anisotropy ( α | - α ⊥) eff in the polarizability. From Stark measurements alone, the true anisotropy cannot be separated from the additional dipole distortion constant shown by Aliev and Mikhaylov to enter dipole matrix elements off-diagonal in J. By studying nine different transitions ( J, K, m J ) → (J, K, m J ± 1) in CH 3 28SiH 3, values were obtained for the four Stark parameters: μ 0 = 0.7345600(33) D, μ J = 8.83(35) μD, μ K = −32.82(37) μD, and ( α | - α ⊥) eff = 1.99(16) × 10 −24 cm 3. These errors reflect only the internal consistency in the data; the absolute error in μ 0 is 32 μD. The modification of the Stark effect by internal rotation is discussed; it is shown that the only significant effect here is to modify the interpretation of μ 0. The change in μ 0 upon isotopic substitution of 30Si for 28Si was determined: μ 0( 30Si) - μ 0( 28Si) = 67.0(2.0) μD . A study of molecular magnetic effects in CH 3 28SiH 3 has yielded the two molecular g factors, g ⊥ = −0.036391(21) nm and g | = −0.10667(13) nm, as well as the anisotropy in the susceptibility (χ | - χ ⊥) = −44.9(2.3) × 10 −30 J T 2 . The molecular quadrupole moment has been calculated.

Sign of the 41K electric quadrupole moment determined by ENDOR of F centres in KCl

ENDOR measurements of F centres in KCl with a highly sensitive spectrometer allowed the measurement of the quadrupole line positions of the nearest 41K and 39K neighbours to ± 0.5 kHz. The sign of the quadrupole interaction constant was determined for 41K and 39K. It turned out to be positive for both nuclei. Hence the sign of the 41K quadrupole moment is positive in agreement with the only other determination so far by Bonczyk and Hughes with the molecular beam electric resonance method.

The molecular beam electric resonance spectrum of OPF3

The molecular-beam electric resonance spectrum of phosphoryl fluoride (16OPF3) has been investigated. A hyperfine study of (K = 0) multiplets has been combined with earlier magnetic resonance data and chemical shift arguments to obtain four spin–rotation constants and two tensor spin–spin constants. The results (in kHz) are: [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], dFF = −2.3(9), and dFF = 4.1(9). A study of molecular magnetic effects has yielded the two molecular g-factors [Formula: see text] and [Formula: see text] as well as the anisotropy in the susceptibility [Formula: see text]. The molecular quadrupole moment has been calculated. From a study of the Stark effect, the electric dipole moment has been determined for the ground vibrational state and for the (ν5 = 1) and (ν6 = 1) fundamentals. The results are: μ = 1.86847(10) D, (μ5 − μ)/μ = −3.49(4) × 10−3, and (μ6 − μ)/μ = −0.65(4) × 10−3. For each of these two excited vibrational states, the l-doubling constants qt, and DqJ(t) (t = 5,6) have been obtained. This work demonstrates that the molecular beam electric resonance method can be applied to symmetric tops with relatively large room temperature rotational partition functions by reducing the rotational temperature to a few degrees kelvin with the seeded beam technique.

Hyperfine structure, electric and magnetic properties of 14N 216O in the ground and first excited bending vibrational state

The molecular beam electric resonance method has been used to obtain the components of the electric quadrupole coupling tensor ( eQq zz ), of the electric dipole moment along the molecular axis (μ el), the spin—rotation constant ( c ⊥) and the molecular g-factor perpendicular to the molecular axis ( g ⊥), and the anisotropy in the magnetic susceptibility ( x | − x ⊥) for the ground vibrational state of nitrous oxide ( 14N 14 N 16O). The same quantities, except x | − x ⊥, have also been determined for the (01 10) excited vibrational state. For this state the l-doubling constants q ν 2 and μ ν 2 , perpendicular anisotropy ( eQq xx − eQq yy ) in nuclear quadrupole tensor, the anisotropy ( c | — c ⊥) in the spin-rotation constant, and the molecular g-factor ( g |) parallel to the molecular axis have been obtained.

Avoided Crossings in Molecular-Beam Electric-Resonance Spectroscopy: The Observation of Forbidden (ΔK=±1, ±2, ±3) Transitions in Phosphoryl Fluoride (OPF3)

Normally forbidden transitions obeying the selection rules $\ensuremath{\Delta}K=\ifmmode\pm\else\textpm\fi{}1, \ifmmode\pm\else\textpm\fi{}2, \mathrm{and} \ifmmode\pm\else\textpm\fi{}3$ have been observed in OP${\mathrm{F}}_{3}$ in the ground vibronic state by a new avoided-crossing technique based on the molecular-beam electric-resonance method. It is shown how this technique can be used in suitable symmetric rotors to study the $K$-dependent terms in the rotational Hamiltonian, the effects of centrifugal distortion on the total electric dipole moment, and the nuclear hyperfine effects off-diagnoal in $K$.

Electric and magnetic properties of carbon monoxide by molecular-beam electric-resonance spectroscopy

The molecular-beam electric-resonance method was used to investigate the Stark-Zeeman spectra of the J = 1, v = 0 state of CO. Along with the most abundant species 12C 16O we studied the 13C 16O and 12C 18O molecules. The electric dipole moment, the magnetic g J factor, the magneitc susceptibility anisotropy, and the spin-rotation constant of carbon-13 were obtained. The experimental results were used to determine the polarity of the electronic charge distribution (C −O + was found), the vibrational dpendence of the electric dipole moment, and the molecular quadrupole moment.

The radiofrequency spectra of LiF by the molecular beam electric resonance method

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Pure-state molecular beams: Production of rotationally, vibrationally, and translationally selected CsF beams

Beams of CsF in selected vibrational (v=0,1,2,3,4) and rotational (J,M=1,0) states, with narrow velocity distributions, have been produced via an improved microwave molecular beam electric resonance (MBER) method utilizing ’’displacement focusing.’’ The apparatus consists of an effusive graphite oven source, a slotted-disk velocity selector, and two electrostatic quadrupole fields. The first field focuses the (J,M) = (2,0) state through an off-axis collimating orifice into a microwave cavity. Here microwave radiation stimulates the J=2→1 transition for a specified v state. Then a second (’’refocusing’’) quadrupole delivers the specified (v,J,M) = (v,1,0) state to an image point (a collimated surface ionization mass filter detector). The final focused and state-selected beam, say (0,1,0), has a purity ≳95% with respect to J,M. However, the best over-all (v,J,M) state purity achieved was 83% (0,1,0), with 17% of ’’background’’ [of which 4% is actually (0,1,0), 4% is (1,1,0), 4% (v,1,0) with v?2, and the remaining 5% unselected (scattered) CsF]. Using the present effusive oven source, the best intensity achieved for a state-selected (0,1,0) beam (of velocity 3.65×104 cm sec−1) was 1×105 molecule sec−1, corresponding to a flux density of 1.3×109 molecule cm−2 sec−1.

A Molecular Beam Electric Resonance Study of the Hyperfine Λ Doubling Spectrum of OH, OD, SH, and SD

The molecular beam electric resonance method was employed to obtain a complete set of hyperfine Λ doubling transitions of the free radicals OH, OD, SH, and SD. The observed spectra could be explained very well by the degenerate perturbation theory adapted to the 2Π state. The experimental results include fine and hyperfine coupling constants, the electric dipole moments could be explained very well by the degenerate perturbation theory adapted to the 2Π state. The constants agree well with ab initio calculations.

The hyperfine Λ-doubling spectrum of 14N16O and 15N16O

The molecular beam electric resonance method was used for the investigation of the hyperfine Λ-doubling transitions ΔJ = 0, ΔF = 0, ±1 for a number of J values of both the 2II1/2 and the 2II3/2 states of the molecules 14N16O and 15N16O. The observed spectrum is explained using the degenerate perturbation theory introduced by Freed (1). This theory is adapted for a 2II molecule and includes contributions up to third order in fine and hyperfine structure. The agreement between observed and calculated values is satisfactory.

Electrical and magnetic properties of OCS in the (01 10) vibrational state measured by molecular-beam electric-resonance spectroscopy

The Δ J = 0, l-doublet transitions in the first excited state of the bending vibration of 16O 12C 32S have been investigated using the molecular-beam electric-resonance method. The l-doubling constant up to second order in J( J+1) has been derived. The electric and magnetic properties obtained are the electric dipole moment and the magnetic constants g ⊥- g |- g ⊥, g xx - g yy , gc ⊥-χ |, and χ xx -χ yy

Electric Resonance Spectrum of NaLi

Δ M J =± 1 transitions from the J =1, MJ =0 and the J =2, MJ =0 molecular rotational states of 23Na7Li have been observed by the molecular beam electric resonance method at 1–15 MHz. With an experimental linewidth of 7.5 kHz, the nuclear hyperfine pattern consisted of six unresolved doublets. In the beam, originating from a supersonic source, only one vibrational state was observed, which was shown to be the ground vibrational state by measurement of a 23Na6Li spectrum. The ground vibronic Stark coefficient μ02/B0 and the molecular electric dipole moment μ0, the equilibrium dipole moment μe, the molecular electric polarizabilities αs and αa, and the two nuclear electric quadrupole coupling constants eqQ were determined in 23Na7Li (using the reported rotational constant B0=0.394 cm−1) to be: μ02/B0=2740± 5 Å3#α3=40± 5 Å3μ0=0.463± 0.002 Dαa=24± 2 Å3μe=0.46 ± 0.01 D(eqQ)Na=−749± 4 kHz|(eqQ)Li|=28± 4 kHz.

Radiofrequency and Microwave Spectrum of the Hydrogen Fluoride Dimer; a Nonrigid Molecule

The radiofrequency and microwave spectra of the K=0 states of (HF)2, (DF)2, and HFDF have been studied by the molecular beam electric resonance method. A unique hydrogen tunnelling motion involving the breaking and reforming of the hydrogen bond causes a splitting of rotational energy levels for (HF)2 and (DF)2, but not for HFDF. The electric dipole selection rules and nuclear spin statistics for the tunnelling molecules have been derived from a consideration of an extended permutation-inversion group. Rotational constants, tunneling doublings, electric dipole moments, and deuterium quadrupole coupling constants have been determined from the observed spectra of the K=0 states. (HF)2(DF)2HFDF(B+C)/2 (MHz)6504.8±2.06252.194±0.0026500.1±0.1ν (MHz)19 776±121579.877 ±0.004μa (D)2.987±0.0032.9919±0.00063.029±0.003(eqQ)Da (KHz)···110±8270±30These results are interpreted with a semirigid, nonlinear model of the dimer geometry. The F—F distance is 2.79 ± 0.05 Å, and the end hydrogen fluoride unit is bent from 60 to 70° from the F—F axis. Large amplitude motion of the hydrogen and deuterium atoms occurs. Higher hydrogen fluoride polymers have been studied by mass spectroscopy and electric deflection methods. All were observed to be nonpolar which is interpreted to imply cyclic structures.

Vibrational energy distribution of CsF produced by reactive beam scattering of Cs by SF 6 and SF 4

The molecular beam electric resonance method has been used to measure the vibrational energy distribution of CsF produced in the chemical reactions Cs + SF 6 and SF 4. The reaction Cs + SF 6 yields a perfect Boltzmann distribution for the products CsF. For the reaction Cs + SF 4 a more complicated distribution has been found which, according to the molecular structure of SF 4, can be explained as a superposition of two Boltzmann distributions of different vibrational temperatures.

Molekularstrahl-Experimente an zweiatomigen Molekülen in angeregten Schwingungszuständen: Elektrisches Dipolmoment und Quadrupolkopplungskonstante von CsF bisv=8

Radio frequency spectra of CsF in the rotational stateJ=1 have been measured for the vibrational statesv=0, 1,..., 8 using the molecular beam electric resonance method. The analysis of the spectra yields the electric dipole moment μv and the quadrupole coupling constanteqvQ connected with the quadrupole moment of the Cs nucleus. The results are: $$\begin{gathered} \mu _\upsilon = 7.8478 + 0.07026(\upsilon + 1/2) + 0.000195(\upsilon + 1/2)^2 debye \hfill \\ eq_\upsilon Q/h = 1245.2 - 16.2(\upsilon + 1/2) + 0.31(\upsilon + 1/2)^2 kHz. \hfill \\ \end{gathered} $$

Molecular-beam measurement of the electric dipole moment and the quadrupole coupling constant of CsF in vibrational states up to ι′ = 8

The molecular-beam electric-resonance method has been used to measure the radio frequency spectra of CsF in the rotational state J = 1 and vibrational states ι′ = 0. 1. … 8. The analysis of the spectra yields the electric dipole moment and the quadrupole coupling constant as functions of ι′.

Radio-Frequency Spectra of NaCl by the Molecular-Beam Electric Resonance Method

The molecular-beam electric resonance method served to measure the radio-frequency spectra of NaCl and to determine the hyperfine constants and the dipole moments for the υ = 0, 1, 2, and 3 vibrational levels of Na35Cl, and for the υ = 0 and 1 vibrational levels of Na37Cl.