Beam Maser Spectrometer Research Articles

Quadrupole coupling measurements on35ClNO

Hyperfine structure for J = 0 → 1 and J = 1 → 2 transitions of 35ClNO was measured using a pulsed-beam, Fourier transform spectrometer and beam-maser spectrometer. The high resolution microwave spectra were analysed to obtain eQq aa(Cl) = -49·05(4), eQq bb(Cl) = 30·0(4·0), eQq aa(N) = 0·98(6) and eQq bb(N) = -4·78(22) MHz. The quadrupole coupling values are discussed in relation to results in other small molecules and nitrosyl complexes. Rotational constants obtained from the measurements were B = 5737·70(6) and C = 5376·32(6) MHz.

Rectangular flat-roof microwave resonator for molecular beam masers

A rectangular flat-roof and plane mirror combination type of microwave open resonator, with a mirror spacing of a single half-wavelength ( lambda =12.5 mm), is investigated experimentally in the context of molecular beam masers. The flat-roof resonator possesses the property of a radiation field that is extensive in the direction of the mirror crease, but concentrated in the transverse direction by an amount dictated by the roof-top pitch, down to a value of about a wavelength. The associated reduction of diffraction loss leads to an increase of Q value of almost twice that of a flat mirror resonator of similar dimensions. Resonances are easy to find and are insensitive to rotations of the roof edge about the plane mirror. The resonator is tunable over several GHz. A lifting of the frequency degeneracy of mode polarisation is reported. the application of the flat-roof resonator is illustrated by results obtained by its incorporation into a high-resolution molecular beam maser spectrometer and oscillator. It could also find an application in pulsed molecular beam Fourier transform spectroscopy.

Beam-maser spectroscopy on cyanoacetylene-D

The J=1→0 transition frequencies were measured for NCCCD using a beam-maser spectrometer. Hyperfine structure from 14N and D was completely resolved. Analysis of the data directly gives the 14N and deuterium quadrupole coupling constant along the internuclear axis eqQ(14N)=−4318.0±0.1 kHz, eqQ(D)=203.5±1.5 kHz. From the data the spin-rotation constant on 14N was also measured: C(14N)=1.1±0.2 kHz. The spin-rotation data were used to calculate paramagnetic chemical shift tensor components σPxx= σPyy=−593±40 ppm. These experimentally determined tensor components were combined with calculated diamagnetic tensor components to give the isotropic shift σav=18±27 ppm.

Beam maser spectroscopy on CD3CN

The J=1→0 rotational transitions of CD3CN were measured using a two-cavity molecular beam maser spectrometer. This spectrometer employs the Ramsey method of separated oscillating fields to obtain very high resolutions and allows precise measurements of the hyperfine structure splittings. The nitrogen quadrupole coupling constant is eqaaQ(N)=−4229.2±0.6 kHz and the deuterium quadrupole constant is eqaaQ(D)=−55.1±0.4 kHz. Assuming cylinderical symmetry about the C–D bond axis we obtain the C–D bond-axis deuterium quadrupole coupling strength of eqzzQ(D)=169.6±4.5 kHz. The b-axis rotational constant, and nitrogen and deuterium spin-rotation interaction strengths are also reported.

Deuterium quadrupole coupling in BD3CO

The hyperfine structure of the J=1→0 rotational transitions of 10BH3CO, 11BH3CO, and 11BD3CO was observed with a molecular beam maser spectrometer. Rotational constants, quadrupole coupling, spin rotation, and spin-spin interaction constants were determined from the data analysis. The deuterium quadrupole coupling constant in 11BD3CO, was determined to be eQqaa(D)=−48.5±2.3 kHz and eQqzz(D)=116.9±5.4 kHz. The procedure for determining the matrix elements of the spin-spin interaction between the equivalent deuterium nuclei is presented.

Beam maser measurements of hyperfine structure in chloroacetylene- d

The J = 1-0 transitions in 35ClCCD were measured using a beam maser spectrometer giving a molecular resonance linewidth of 3 kHz (FWHM). Analysis of the data yielded quadrupole coupling strengths eqQ(D) = 208.5 ± 1.5, eqQ( 35Cl)= 79 739.5 ± 1 kHz and the 35Cl spinrotation interaction strength C(Cl) 1.3 ± 0.1 kHz. The rotational constant obtained is B = 5 186 973.9 ± 1.0 kHz. The accuracy of measured molecular parameters is improved by a factor of 20 or better.

Beam maser spectroscopy on CH3CCD and CD3CCH

The J = 1→0 transition frequencies in CH3CCD and CD3CCH were measured using a beam maser spectrometer. Hyperfine structure from deuterium quadrupole coupling was well resolved. The measured deuterium quadrupole coupling tensor components along the symmetry axis are eQqaa = 228±2 kHz for CH3CCD and eQqaa = −55.0±0.5 kHz for CD3CCH. The coupling strength along the bond direction for CD3CCH is eQqzz = 174±6 kHz. These results are compared with the other deuterium quadrupole coupling values. Measured rotational constants are B = 7 778 170±1 kHz for CH3CCD and B = 7 355 767±1 kHz for CD3CCH.

Beam maser measurements of HDO hyperfine structure

The 431←432, 532←533, and 321←414 transition in HDO were measured in a beam maser spectrometer. Accurate values for rotational transiton frequencies, the deuterium quadrupole coupling tensor, and hydrogen spin-rotation tensor were obtained from the present data combined with previous results on other transitions. Resonance line widths of 2 kHz were obtained for most of the data. Measured quadrupole coupling tensor elements eqQ(D)gg and spin-rotation tensor elements C(X)gg in kHz are as follows: eqQ(D)aa = 276.45±0.88, eqQ(D)bb = −110.97±1.46, eqQ(D)cc = −165.77±1.10, C(D)aa = −1.33±0.20, C(D)bb = −4.38±0.36, C(D)cc = −2.99±0.24, C(H)aa = −58.42±0.47, C(H)bb = −5.46±0.83, C(H)cc = −24.11±0.55.

Beam maser spectroscopy on J = 1 → 2, K = 1, and K = 0 transitions in CH3CN and CH3 13CN

J = 1 → 2 transitions near 36.8 GHz in CH3CN and CH3 13CN were measured using a beam maser spectrometer. K = 1 transitions were observed with a 10 kHz linewidth (FWHM) and K = 0 transitions were observable with a two-cavity Ramsey system with about 3 kHz resolution. The measured difference between a axis and c axis nitrogen spin–rotation tensor components is Ca–Cc = 4.6±1.0 kHz for CH3CN and 4.5±1.0 kHz for CH3 13CN. The c axis nitrogen spin–rotation tensor component is Cc = 2.0±0.4 kHz. Nitrogen chemical shift tensor components of σa = 247 ppm and σc = −165 ppm are obtained from the data and calculated diamagnetic contributions. The measured paramagnetic contributions to the nitrogen chemical shift tensor are σap = −98±15 ppm and σcp = −600±120 ppm. Nitrogen quadrupole coupling strength is eqQ = −4224.3±4.0 kHz and rotational transition frequencies are: 36 795 476.9±3.0 kHz (CH3CN, K = 0), 36 794 769.3±1.5 kHz (CH3CN, K = 1), 36 777 282.6±3.4 kHz (CH3 13CN, K = 0), and 36 776 573.3±1.6 kHz (CH3 13CN, K = 1). This data, combined with previous J = 0 → 1 data yields accurate rotation and distortion constants for CH3CN and CH3 13CN as follows:

Beam maser rotational relaxation measurements on OCS

Rotational relaxation cross section measurements were made for J = 0, 1 and 2 rotational states of OCS using a beam maser spectrometer. Individual rotational states were selected using electrostatic fields and cross sections for scattering and relaxation of these states by polar and non-polar scattering gasses were obtained. The angular resolution was carefully controlled so that cross sections can be compared for the different rotational states. An analysis of dipolar contributions to scattering cross sections in terms of relaxation matrix elements is presented and this is consistent with some of the observed differences in pure-states cross sections.

Observation of the dynamic Stark effect at microwave frequencies in a molecular beam maser

Multiplet splitting of the main doublet of the 14NH 3, J = K = 1 inversion line has been observed in a molecular beam maser spectrometer when a strong microwave radiation field is introduced into the maser cavity. This phenomenon is attributed to the dynamic Stark effect at microwave frequencies.

Measurements of T1 and T2 relaxation cross sections for HDO

Rotational relaxation cross sections for the 220, 221, and superposition states of HDO were measured using a beam maser spectrometer. Scattering gases used were H2O, D2O, HDO (50%), HCl, CH3F, NH3, OCS, CO2, N2, and Ar. For HDO–NH3 scattering, pure state cross sections were much larger than the superposition state cross section. For other scattering gases, pure state cross sections were slightly larger than the superposition state cross section.

Angular resolution and isotope studies for inversion relaxation in NH3

Inversion relaxation cross sections for the J=3, K=2 upper state of NH3 were measured using a beam maser spectrometer. Scattering gases were NH3, ND3, HCl, and H2O. For NH3–NH3 scattering the angular resolution was varied from 1.5° to 0.2° with little effect on cross sections. Cross sections for NH3 and ND3 as scattering gases were equal within experimental error. The result that beam maser cross sections are relatively independent of angular resolution is discussed. Calaculated angular resolution corrections are compared with values of beam attenuation cross sections and total cross sections obtained from the present data. Comparison is made between our pure state (σ1) and superposition state (σ11) cross sections and recent gas-cell data.

Spin–rotation measurements and magnetic shielding in CH2CF2

Rotational transitions 101→000, 212→111, 211→212, and 321→322 of 1,1 difluoroethylene were studied using a molecular beam maser spectrometer. Emission transitions were observed with 6 kHz linewidth (FWHM) and hyperfine structure due to fluorine and hydrogen interactions was resolved. The diagonal elements of the fluorine spin–rotation tensor are Maa=−13.4±3.6 kHz, Mbb=−10.94±0.68 kHz and Mcc=−6.06±0.68 kHz. This data is used to determine paramagnetic contributions to the fluorine chemical shift tensor. Analysis of the data yielded line centers for the rotational transitions of 15 774 348±3 kHz for 101→000, 26 465 055±5 for 212→111, and 14 419 906±3 kHz for 321→322, and 15 250 615±3 kHz for 211→212. Results of semiempirical calculations of diamagnetic contributions to the shielding tenors are presented for CH2CF2 and other molecules. Comparisons of total shielding tensors are made for related molecules.

Beam maser measurements of rotational relaxation of H2CO

Inelastic scattering cross sections for the upper and lower states of J=1 and J=2, K-type doublets of formaldehyde were measured using a beam maser spectrometer. Scattering gasses used were He, Ar, H2, N2, CO2, HCl, NH3, CH3F, and CF3H. Systematic variations in cross sections with the symmetry and dipole moments of scattering molecules are observed. Anderson’s theory of pressure broadening, with slight modification, is used to interpret the data. This model gives reasonably good agreement with most of the observed effects.

Beam maser measurements of hyperfine structure in 14N2O

High resolution measurements of hyperfine structure on the J = 1 → 0 rotational transitions in 14N2O were made with a molecular beam maser spectrometer. All seven hyperfine components arising from quadrupole interactions by two nitrogen nuclei were completely resolved. This permitted determination of both quadrupole coupling constants as well as the two spin–rotation constants. Values of the best least-squares fit parameters are (eqQ)end = −776.7±1.0 kHz, (eqQ)central = −269.4±1.8 kHz, Mend = −2.35±0.20 kHz, and Mcentral = −2.90±0.26 kHz. The spin–rotation constants are used to calculate magnetic shielding parameters for the two nitrogen nuclear sites. The 1 → 0 rotational transition frequency is 25 123 246 kHz.

Relaxation cross section measurements on NH 3 and lower state focussing in a beam maser

Scattering cross for the lower state of the J=3, K=2 inversion doublet of NH 3 were measured using a beam maser spectrometer. The observed relaxation cross sections were the same as those obtained previously for the upper inversion state. These results support previous interpretations of observed relaxation cross sections for coherent superposition states. A coaxial electrostatic focusser state selector for beam maser spectroscopy which provides efficient focussing of states with a negative Stark effect is described.

Rotational energy distribution in a nozzle beam

The fraction of beam molecules in J = 0, J = 1, and J = 2 rotational states of OCS was measured as a function of pressure behind a nozzle beam source. The fractional populations were obtained from the intensities of microwave emission and absorption signals observed with a beam maser spectrometer. Total beam intensities were measured also. Large deviations from a Boltzmann rotational energy distribution were observed. The ``effective rotational excitation temperature'' obtained from fractional population was near 15°K for J = 0, 40°K for J = 1, and 170°K for J = 2 with 150 Torr nozzle pressure.

Measurements of rotational relaxation cross sections for H2CO

Cross sections for the 111, 110, 212, and 211 rotational states and for total beam attenuation of H2CO were measured using a beam maser spectrometer. Significant inelastic scattering in the forward direction was obtained for He and CF3H. Significant inelastic contributions were not observed using H2 as a scattering gas. Observed cross sections for the 111 state are larger than those for the 110 state. The relations of the present cross sections to various models for obtaining a low excitation temperature for the 111-110 doublet of formaldehyde in interstellar space is discussed.

A reinvestigation of the hyperfine structure in fluoroform

The hyperfine structure of the J = 1 - 0 rotational transition of the CF 3H and CF 3D molecules has been reinvestigated with a high resolution beam maser spectrometer equipped with a nozzle source. Considerable gain in S/N ratio compared with previous measurements performed with a normal effuser source is obtained. The experimental result eliminate discrepancies between the results of previous investigations.