Molecular Beam Electric Resonance Research Articles

Impact of sub-Doppler measurements on centrifugal-distortion terms: rotational spectrum of methyl fluoride revisited.

Methyl fluoride is a prototypical symmetric-top molecule. Despite the fact that its rotational spectrum has been largely investigated, centrifugal-distortion constants were determined only up to the sextic terms. The present investigation demonstrates the importance of sub-Doppler measurements in the terahertz region not only for deriving higher-order centrifugal-distortion terms but also for revising the parameters available in the literature. The Lamb-dip technique has been exploited for obtaining sub-Doppler resolution in the 102 GHz to 1.2 THz frequency range, thus allowing for the improvement of all spectroscopic parameters. Furthermore, the hyperfine structure due to fluorine and hydrogens has been resolved, thus enabling the determination of the corresponding spin-rotation constants with an accuracy rivaling that obtained by molecular-beam electric resonance measurements and of the dipolar spin-spin coupling constants for the first time. The prediction and analysis of hyperfine structures were guided and supported by high-level quantum-chemical calculations of the parameters involved.

Hyperfine spectrum of RbBr

The molecular beam electric resonance technique has been used to conduct a high precision examination of the hyperfine spectrum of 85Rb79Br and 85Rb81Br. Coupling constants for the nuclear electric quadrupole interactions, the spin–rotation interactions, the tensor and scalar spin–spin interactions, and their dependence on vibrational and rotational state have been determined. A slightly improved value of the electric dipole moment is also obtained, and the inclusion of an electric hexadecapole interaction for the rubidium nucleus is found to somewhat improve the fit.

Sensitivity enhancement and low-field spin relaxation in singlet NMR

The singlet states of nuclear spin-1/2 pairs often display extended lifetimes that can be an order of magnitude longer than conventional relaxation times. We show that, in favourable circumstances, acquisition of the NMR signal during an extended multiple spin-echo train, followed by suitable data processing, enhances the signal-to-noise ratio of singlet NMR by up to an order of magnitude. The achievable enhancement depends on the transverse relaxation time constant, the magnetic field inhomogeneity, and the acceptable degradation in digital spectral resolution. We use the combination of singlet NMR and multiple spin-echo data acquisition to study the low-field nuclear relaxation processes of (15)N-labelled nitrous oxide ((15)N(2)O) in solution. A general relaxation theory for coupled 2-spin-1/2 systems in low magnetic field is developed. Experimental trajectories of the nuclear spin observables are compared with theoretical expressions, including dipole-dipole and spin-rotation relaxation mechanisms. The estimated values of the spin-rotation tensors are compared with previous estimations from NMR and molecular beam electric resonance.

Hyperfine spectrum of RbI

The molecular beam electric resonance technique has been used to conduct a high precision examination of the hyperfine spectrum of RbI. Coupling constants for the nuclear electric quadrupole interactions, the spin-rotation interactions, the tensor and scalar spin-spin interactions, and their dependence on vibrational and rotational state have been determined.

Hyperfine spectrum of NaF

The molecular beam electric resonance technique has been used to conduct a high precision examination of the hyperfine spectrum of 23Na 19F. Coupling constants for the nuclear electric quadrupole interactions, the spin–rotation interactions, the tensor and scalar spin–spin interactions, and their dependence on vibrational and rotational state have been determined.

Nitric Oxide Beam Intensity Oscillations Induced by the Combined Action of a Static and a Radio Frequency Electric Field

This paper details an experimental and theoretical investigation in which a simplified version of the molecular beam electric resonance technique is employed that requires the use of a C-field only. In the experiment the forward intensity of a NO beam is measured as a function of the frequency of the oscillating electric field over the 900-1460 kHz range. Specifically, the interaction of the NO beam with a radio frequency (rf) field of 1.12 kV/m amplitude and -610 kV/m (2) of gradient at the horizontal plane during 72 micros produces a series of oscillations in the transmitted beam intensity. The theoretical analysis shows how the interaction between a beam of NO molecules and both a static and oscillating rf field produces interferences in the forward beam intensity and how the observed interferences are due to superposition of molecular internal states. Furthermore, the interference model reproduces satisfactorily the observed beam intensity oscillations. The present technique could be useful for the development of new schemes to achieve coherent control of molecular processes using radiowaves.

The Stark effect in the A1Σ +– X1Σ +(0, 0) band of barium monoxide, BaO

The optical Stark spectrum of the A 1Σ– X 1Σ +(0, 0) band of BaO was recorded and analyzed to determine the permanent electric dipole moments, μ, for the A 1Σ( ν = 0) and X 1Σ +( ν = 0) states to be 3.125(16) D and 7.869(17) D, respectively. The molecular beam electric resonance data for the X 1Σ +( ν = 0) states [L. Wharton, M. Kaufman, W. Klemperer, J. Chem. Phys. 37 (1962) 621] was re-analyzed to produce μ of 7.9530(4) D. The systematic difference between the MBER and optical Stark values is discussed.

Hyperfine spectra of KBr and KI

The molecular beam electric resonance technique has been used to conduct a high precision examination of the hyperfine spectra of 39K 79Br, 39K 81Br and 39K 127I. Coupling constants for the nuclear electric quadrupole interactions, the spin–rotation interactions, the tensor and scalar spin–spin interactions, as well as the electric dipole moment of KI, and their dependence on vibrational and rotational state have been determined. A few transitions observed for 41K 127I show a small shift in the iodine nuclear electric quadrupole interaction, and the fit improves somewhat with the inclusion of an iodine nuclear electric hexadecapole interaction term.

Isotopic studies of the binary complex He–OCS

Rotational transitions of four isotopologues of the He–OCS van der Waals complex were investigated using a pulsed molecular beam Fourier transform microwave spectrometer, following the original molecular beam electric resonance study of the normal isotopologue of this complex [K. Higgins, W. Klemperer, J. Chem. Phys. 110 (1999) 1383]. This includes 10, 9, and 8 rotational transitions for He–OC 34S, He–O 13CS, and He–OC 33S, respectively, as well as four additional transitions for the normal isotopologue. In the case of the He–OC 33S isotopologue, 29 33S nuclear quadrupole hyperfine components were observed and analyzed. The new isotopic data are of importance for the further refinement of the He–OCS potential energy surface.

Interaction of a supersonic NO beam with static and resonant RF fields: Simple theoretical model to account for molecular interferences

In previous experimental works from this laboratory two unexpected phenomena were reported: (i) a depletion of ca. 40% in the total intensity of a pulsed He seeded NO beam when these molecules passed a homogeneous and a resonant oscillating RF electric field and (ii) a beam splitting of ca. 0.5° when the transverse beam profile is measured, under the same experimental conditions. In this work a model based on molecular beam interferences is introduced which satisfactorily accounts for these two observations. It is shown how the experimental set-up a simple device used as C-field in early molecular beam electric resonance experiments, can be employed as molecular interferometer to investigate matter–wave interferences in beams of polar molecules.

Nuclear electric quadrupole moments of Rb from the hyperfine spectrum of RbF

The molecular beam electric resonance technique has been used to examine the hyperfine spectrum of RbF. The Rb nuclear electric quadrupole interaction, the spin-rotation interactions, and tensor and scalar spin-spin interactions have been measured for both Rb isotopes, including their dependence on vibrational and rotational states. Transition frequencies have been determined to a precision of better than 1 Hz in many cases. The magnetic interactions in the two isotopomers are consistent with what is expected from the known masses and magnetic dipole moments. In the case of the Rb nuclear electric quadrupole interaction, adjustments have been made for a small isotopomer shift, and for the ratio of the effective nuclear electric quadrupole moments, Q(87Rb)Q(85Rb) = 0.483 830 1+/-0.000 001 8. The effective quadrupole interaction includes a pseudoquadrupole interaction that may be significant at this level of precision, but cannot be distinguished experimentally.

Hyperfine spectrum of RbCl

The molecular beam electric resonance technique has been used to conduct a high precision examination of the hyperfine spectrum of the four isotopomers of RbCl. Coupling constants for the nuclear electric quadrupole interactions, the spin-rotation interactions, the tensor and scalar spin-spin interactions, and a rubidium nuclear octupole interaction, and their dependence on vibrational and rotational states have been determined. The dominant interaction, the rubidium nuclear electric quadrupole interaction, shows a small shift with substitution of the chlorine isotope.

Multiconfiguration SCF electric field gradients in (NO) 2 and its singly charged ions

Electric field gradients (EFGs) at the N and O nuclei in the complexes (NO) 2 + , (NO) 2 and (NO) 2 - have been calculated by using the complete-active-space self-consistent field (CASSCF) method with correlation-consistent basis sets. The energetics obtained at given level of accuracy were found to be consistent with previous experimental and theoretical findings. Changes in the EFGs at the nuclei due to the formation of the complex from diatomic species were used to interpret the character of bonding in the triad of dimers. Direct comparison with (molecular beam electric resonance) spectroscopic data was made for NO and its dimer: 14N nuclear quadrupole coupling constants and the electric dipole moment of (NO) 2 appeared to be in reasonable accord.

Molecular beam electric resonance without A and B fields: NO versus (NO) 2 depletion spectra

Using our spectroscopic technique based on molecular beam electric resonance without A and B fields [Chem. Phys. Lett. 341 (2001) 495] depletion spectra of both NO and (NO) 2 were measured. The experimental results show an energy shift of the order of 10 −8 cm −1 between the maxima of their (central) band spectra. This resolution enables to estimate changes in the NO dipole moment of the order of 0.6%, as this molecule forms the dimer. It is suggested that this technique could be used to investigate molecular rearrangements in gas-phase dynamical processes as well as to resolve variations in molecular bond distances in solvation or cluster processes.

Molecular beam electric resonance without A- and B-fields

We report on a novel phenomenon observed during the passage of a pulsed NO beam seeded in He through a combined homogeneous, static electric and RF field, denoted as C-field in a standard molecular beam electric resonance (MBER) experiment. Although we refrain from the state selective A- and B-fields, which are considered crucial for a MBER experiment, the transmitted intensity exhibits as a function of the RF frequency conspicuous dips at resonance frequencies that depend strictly linearly on the static field strength E 0. Their spectral width is by a factor of 4 smaller than the time of flight broadening. Both, the resonance frequencies and their linear field dependence can be precisely predicted applying a simple expression for the Stark effect to a \(\Delta M = \pm 1\) transition of a single rotational state (J = 3/2) of the electronic ground state \(^2\Pi_{1/2}\). However, this formula is valid only in the high field limit (E 0 > 1000 kV/m) while the employed field (E 0 = 1.47 kV/m) was in the extreme low field domain where the large \(\Lambda\) type doubling and hyperfine coupling lead to a purely quadratic Stark effect. We assume that the phenomenon is due to a yet unknown collective rather than to an isolated particle process.

The hyperfine interactions in CsF

The molecular beam electric resonance technique has been used to examine the hyperfine spectrum of CsF to determine the nuclear quadrupole interaction of the cesium nucleus. A total of 95 transitions in vibrational states v=0−5 and rotational states J=1−8 have been included in a fit to determine the cesium nuclear quadrupole and spin–rotation interactions, the fluorine spin–rotation interaction, and the tensor and scalar parts of the spin–spin interaction. Vibration and rotation dependencies of these constants have been determined, allowing correction for zero point vibration effects. This experimental Cs nuclear quadrupole coupling constant when combined with the electric field gradient calculated using a relativistic coupled cluster method yields a nuclear quadrupole moment of the Cs nucleus equal to eQ=−3.43098 mbarn. The vibrational dependence of the coupling constant is smaller than the theoretical estimate. The coupling constants we have determined are the following: eQCsqCs=1245.598(10)−14.322(25)(v+1/2)+0.080(14)(v+1/2)2+0.0040(22)(v+1/2)3−0.00209(59)J(J+1)+0.00048(40)(v+1/2)J(J+1), cCs=0.66177(14)−0.01509(28)(v+1/2)+0.000550(94)(v+1/2)2, cF=15.08163(84)−0.1744(14)(v+1/2)+0.00234(41)(v+1/2)2−0.000093(13)J(J+1), c3=0.92713(53)−0.00917(93)(v+1/2)+0.00097(29)(v+1/2)2, c4=0.62745(30)−0.00903(22)(v+1/2). All values are in kHz units, with one standard deviation uncertainty estimates in the last two digits shown in ( ).

Evidence for a nuclear hexadecapole interaction in the hyperfine spectrum of LiI

The molecular beam electric resonance technique has been used to examine the hyperfine spectrum of LiI7 to determine the nuclear hexadecapole interaction of the iodine nucleus. The nuclear magnetic octupole interaction was also considered but found to be marginally significant. A total of 172 transitions in vibrational states 0-3 and rotational states 1-6 have been included in a fit to determine the iodine nuclear quadrupole, spin-rotation, and hexadecapole interactions, the lithium quadrupole and spin-rotation interactions, and the tensor and scalar parts of the spin-spin interaction. Vibration and rotation dependencies of these constants have been determined. The results include: eHh=−0.0151(30), eQIqI=−194351.212(17)−8279.521(46)(v+1/2)+100.616(34)(v+1/2)2−0.3949(73)(v+1/2)3−6.41977(50)J(J+1)+0.10593(33)(v+1/2)J(J+1),eQLiqLi=172.613(52)−3.26(14)(v+1/2)+0.00145(87)J(J+1),cI=6.80260(32)+0.00303(49)(v+1/2)−0.000118(13)J(J+1), cLi=0.75872(72)−0.0088(11)(v+1/2), c3=0.62834(68)−0.0050(11)(v+1/2), c4=0.06223(36)+0.00041(26)(v+1/2), and eΩIωI′=0.000112(73), all in kHz with one standard deviation uncertainties for the last 2 digits in ( ).

Nuclear electric quadrupole moment of6Li

The molecular beam electric resonance technique has been used to examine the hyperfine spectrum of ${}^{6}{\mathrm{Li}}^{19}\mathrm{F}$ for the purpose of obtaining an improved value of the ratio of the electric quadrupole moments of the two lithium nuclei. A total of 29 transitions in vibrational states 0-2 and rotational states 1-4 have been included in a fit to determine the Li nuclear quadrupole interaction along with the magnetic spin-rotation and spin-spin interactions. The magnetic interactions are consistent with values calculated from the previously reported ${}^{7}{\mathrm{Li}}^{19}\mathrm{F}$ values, but those have been refitted to take advantage of the new information from ${}^{6}{\mathrm{Li}}^{19}\mathrm{F}$. The electric quadrupole moment ratio of the two lithium isotopes determined from measurements on the two forms of LiF is ${Q(}^{6}\mathrm{Li}{)/Q(}^{7}\mathrm{Li})=0.020161\ifmmode\pm\else\textpm\fi{}0.000013$ (one standard deviation estimate).

Precise Measurement of the Pure Rotational Submillimeter-Wave Spectrum of HCl and DCl in Theirv= 0, 1 States

High-resolution sub-Doppler Lamb-dip measurements were performed on the low-Jpure rotational transitions of the hydrogen chloride isotopomers H35Cl, H37Cl, D35Cl, and D37Cl in the submillimeter-wave region up to 646 GHz. For theJ= 1–0 transitions of the two HCl isotopomers, the hyperfine splitting due to the hydrogen nuclear spin–rotation interaction is resolved. Furthermore Doppler-limited lines of the DClJ= 3 ← 2 transition around 965 GHz as well as hyperfine-resolved rotational transitions in the first excited vibrational state were recorded up to 1.22 THz. The new frequencies were analyzed in a global fit together with FIR data yielding a set of mass-invariant rotational parameters. Isotopically invariant hyperfine parameters were obtained also from the global fit. Inclusion of the precise results from molecular beam electric resonance measurements allowed the determination of higher orders of the vibrational and rotational expansion coefficients of the chlorine and hydrogen hyperfine interactions. The precise transition frequencies reported here should be useful as secondary calibration standards in the submillimeter-wave and terahertz region.

The Fourier transform infrared spectrum of the boron trifluoride-carbon dioxide complex

The Fourier transform infrared spectra from 4000 to 250 cm −1 of mixtures of boron trifluoride and carbon dioxide, trapped in nitrogen and argon matrices at cryogenic temperatures, have been recorded. Many bands have been assigned to a proposed 1:1 molecular complex, and the spectrum is consistent with the complex having an asymmetric top structure. This proposed structure is in agreement with the conclusions of an earlier gas phase molecular beam electric resonance study and with our own ab initio theoretical calculations.