The first high resolution spectra of a rare gas–H2O cluster have been observed using a tunable far infrared laser to probe the vibration–rotation–tunneling levels of Ar–H2O formed in a continuous planar supersonic jet. The high sensitivity of this spectrometer facilitated extensive measurements of two perpendicular subbands which are assigned to transitions from the ground state to the upper component of a hydrogen exchange tunneling doublet (c-type) at 21 cm^−1, and to vb1 =1+ (b-type) at 25 cm^−1, the lower tunneling component of a bending vibration which is perpendicular to the tunneling coordinate. The tunneling splitting is shown to be in the range 2.5–7 cm^−1 and the lower tunneling component of the excited bending vibration lies between 39 and 43 cm^−1 above the ground state of the complex. The experimentally determined center-of-mass separation (Rc.m. =3.75 A) and harmonic stretching force constant (ks =0.0134 mdyn/A) are compared to those of related first and second row hydrides. The large amplitude motions occurring within this complex make it difficult to establish its structure.

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Tunable far infrared laser spectroscopy of van der Waals bonds: Vibration–rotation–tunneling spectra of Ar–H2O

Defined most conveniently in terms of modern technology, the far infrared (FIR) region of the spectrum extends from the frequencies where “standard” submillimeter techniques begin to fail ( - 10 cm - ’ = 0.3 THz) to those where lead-salt infrared diode laser technology becomes operative ( -350 cm- ’ = 10.5 THz). In this frequency range, pure rotational spectra of light molecules and torsional (hindered rotation) spectra of nonrigid molecules have been studied for many years. In terms of wavelength, this spectral region (30-1000 ,um) is centered near the peak of the room temperature (3-300 K) blackbody emission spectrum, and has accordingly been of great importance in the development of modern physics. In terms of energy, photons in this region (0.03-1.0 kcal/mole) are less energetic than essentially all known hydrogen bond (and, of course, chemical bond) strengths, but are about the same magnitude as typical van der Waals bond strengths. The extremely high resolution ( > 1 x 106) tradition

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Tunable far infrared laser spectrometers

The technique of direct laser absorption spectroscopy in fast ion beams has been employed for the determination of absolute integrated band intensities (S0v) for the ν3 fundamental bands of H3O+ and NH+4. In addition, the absolute band intensities for the ν1 fundamental bands of HN+2 and HCO+ have been remeasured. The values obtained in units of cm−2 atm−1 at STP are 1880(290) and 580(90) for the ν1 fundamentals of HN+2 and HCO+, respectively; and 4000(800) and 1220(190) for the ν3 fundamentals of H3O+ and NH+4, respectively. Comparisons with ab initio results are presented.

Absolute infrared vibrational band intensities of molecular ions determined by direct laser absorption spectroscopy in fast ion beams

The J=0–1 rotational transition of XeH+ and the J=1–2 transition of XeD+, which both occur near 390 GHz, have been studied by microwave absorption spectroscopy For each of these all nine naturally occurring stable isotopes of xenon were detected The magnetic hyperfine structure for the 129Xe forms and the electric quadrupole–magnetic hyperfine structure of both 131Xe forms were fully resolved and analyzed to determine the CI and eQq0 constants The scaled spin–rotation parameter CI/(gIB) is found to be practically identical in XeH+ and HI, as it is also in the isoelectronic pair KrD+ –DBr/HBr The available magnetic field was too small to resolve the rotational Zeeman effect, but the rotational g factor of XeH+ was estimated from Zeeman broadening The mass‐independent Dunham parameters U01, ΔH01, and ΔXe01 were determined from the very accurate microwave frequencies combined with higher order Dunham coefficients from published Fourier‐transform infrared (FTIR) spectroscopy of XeH+  Although it was not

Submillimeter wave spectroscopy of XeH+ and XeD+

The electric dipole moment of the linear molecular ion HN+ 2 was determined by measuring the isotope shifts of the rotational g factors of different isotopic species. We studied the Zeeman effect of the R(6) rotational transition at 650 GHz. In a magnetic field of 5.4 T the rotational transition split into two components, separated by 2.2–2.5 MHz. The g R factors were determined for H14N14N+, H14N15N+, and H15N14N+. The dipole moment for H14N14N+ was determined as (3.4±0.2) D, which is in excellent agreement with the theoretical value. The transitions were observed by direct absorption spectroscopy with a tunable FIR sideband spectrometer. The ions were generated in a modified anomalous discharge placed in a superconducting magnet.

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Determination of the electric dipole moment of HN+2

The details of the first experimental determination of the dipole moment of a molecular ion from the rotational Zeeman effect are presented, along with an assessment of the ultimate accuracy of the technique.

K. B. Laughlin

Papers

Determination of the Dipole Moments of Molecular Ions from the Rotational Zeeman Effect by Tunable Far-Infrared Laser Spectroscopy