E. Zwart

The submillimeter rotation tunneling spectrum of the water dimer

Rotational tunneling transitions have been measured for the (Ka = 0, lower) → (Ka = 1, upper) and the (Ka = 1, lower) → (Ka = 2, upper) bands of (H2O)2. Although some of these transitions have been reported in an earlier publication, a more detailed discussion of the experiment and of the results is presented here. Transitions have been measured by direct absorption spectroscopy in a continuous slit nozzle expansion using either harmonics from klystrons or sidebands. These data along with previous measurements have been analyzed using an IAM-like treatment. A better determination of the A rotational constant and of the value of the largest tunneling splitting has been achieved.

A far infrared laser sideband spectrometer in the frequency region 550–2700 GHz

This paper describes a tunable far‐infrared (FIR) spectrometer. Tunable radiation is obtained by frequency mixing, fixed frequency FIR laser radiation and tunable microwave radiation in Schottky barrier diodes. An optically pumped laser and an HCN discharge laser are used as FIR sources and klystrons in the frequency range of 22–114 GHz as microwave sources. This yields an 85% coverage of the frequency region between 550 and 2700 GHz. Up to sixth order sidebands have been generated and used for spectroscopy. The ultimate sensitivity corresponds to a minimum detectable fractional absorption of 10−5 at 1‐s RC time. The applicability of the spectrometer in high‐resolution spectroscopy of transient species has been demonstrated by the observation of spectra of OD and N2H+. New laser emissions of optically pumped CH2F2 have been found and accurate frequencies have been determined for some of them.

Microwave and submillimeter spectroscopy of Ar-NH3 states correlating with Ar + NH3 (j=1,|k|=1)

Microwave and submillimeter transitions for Ar–NH3 have been observed and assigned for the ∑ and Π states correlating asymptotically with Ar+NH3 (j=1,‖k‖=1). The ∑ states are found to lie below the Π states and are separated by approximately the inversion splitting of free NH3. For the Π states the NH3 inversion tunneling is nearly quenched, being only weakly allowed through Coriolis interactions with the nearby ∑ states. The observed microwave and submillimeter spectra also allow the determination of 14N quadrupole coupling constants and relative submillimeter absorption intensities. All the above results are interpreted using a model internal–rotation inversion Hamiltonian, leading to detailed information about the anisotropy of the intermolecular potential.

The submillimeter rotation-tunneling spectrum of Ar-D2O and Ar-NH3

Abstract Two bands for Ar-D 2 O and one band for Ar-NH 3 have been observed for the first time in the frequency region 300–500 GHz. For both complexes the bands are explained in terms of a nearly free internal rotor model, following earlier investigations of these types of complexes. We found evidence that this model is correct for Ar-D 2 O. Two of the internal rotor states studied for the Ar-D 2 Ocomplex show a Coriolis interaction and the interaction constant is determined. The hyperfine constants for the studied excited state of the Ar-NH 3 complex turn out to be large.

The (Ka=0→1) submillimeter rotation-tunneling spectrum of the water dimer

Abstract The ( K a =0→1) submillimeter rotation-tunneling transitions of the water dimer have been observed. The (H 2 O) 2 clusters were produced in a slit nozzle arrangement and the transitions were detected by direct absorption of the submillimeter radiation. A number of lines in the allowed subbranches ( K a =0, E ± → K a =1, E ∓ ), ( K a =0, A 1 − → K a =1, A 1 + ) and ( K a =0, A 1 + → K a =1, A 1 −1 ) have been studied. Combination differences using microwave transitions within the ( K a =0) and ( K a =1) states allowed an unambiguous assignment of the lines. The present results allow a more accurate determination of the A -rotational constant and the tunneling splittings of ( K a =0) and ( K a =1). Further they show that the K -type doubling constant has a different sign for the lower and upper tunneling states in ( K a =1).

Determination of the electric dipole moment of HN+2

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.

The submillimeter rotation tunneling spectrum of (D2O)2

Abstract By direct absorption of submillimeter radiation in a planar supersonic jet, we have measured K a =1→ K a =2 transitions of the fully deuterated water dimer (D 2 O) 2 in the region between 350 and 430 GHz. Several previously unknown constants have been derived. The A -rotational constant is determined as 120327.492(32) MHz. The sum or the difference of the largest tunneling splittings for K a =1 and K a =2 is approximately 21 GHz. The tunneling matrix element h 2 v has been found to be a factor of 100 smaller than in (H 2 O) 2

An infrared—far-infrared double resonance study on (NH3)2 in a jet

Abstract An infrared—far-infrared double resonance experiment on (NH 3 ) 2 in a jet was performed. From the results, the nature of the tunneling splitting detected in former IR and FIR studies was identified. It was found that the (NH 3 ) 2 complex exhibits the following feasible motions: internal rotation of each monomer, interchange tunneling, and inversion tunneling. The rigid structure was classified in C s , where a plane of symmetry is assumed. The FIR transitions reported so far and three new bands reported in this work could be assigned within the group G 144 . It is found that the inversion splitting for some of the bands is almost quenched. Upon excitation of the ν 2 umbrella motion the inversion splitting amounts to 3.7 cm −1 .

Determination of the electric dipole moment of KrH

Abstract By measuring the isotope shifts of the rotational g R -factors of different isotopic species the electric dipole moment of the molecular ion KrH + was determined. For that purpose we studied the Zeeman effect of the J = 1 ← 0 transition of KrH + at 494.5 GHz and the J = 2 ← 1 transition of KrD + at 503 GHz. In a magnetic field of 4.950 T the rotational transition splits into two Δ M = ±1 components, resulting in splittings of approximately 41.8 MHz (KrH + ) and 21.1 MHz (KrD + ). It turned out that there is a major influence of the zero-point vibrations on the experimentally determined electric dipole moment, which results in an uncertainty of the interpretation of the experimental data. The ions were generated in a modified anomalous discharge placed in a superconducting magnet. The transitions were observed by direct absorption of the harmonics of a 70-GHz klystron, using the tunable FIR sideband spectrometer at Nijmegen university.