Thomas Vanfleteren

Communication: Overtone (2NH) spectroscopy of NH3–Ar

The Π (1(1)) ← Σ (0(0)) 2NH (ν1+ν3) band of the NH3-Ar van der Waals complex formed in a supersonic jet expansion, with origin at 6628 cm(-1) was recorded at high-resolution using cavity ring down spectroscopy. The analysis leads to upper state rotational constants and J-dependent predissociation lifetimes estimated from linewidth analysis, with a mean value about 0.6 ns.

High-resolution, near-infrared CW-CRDS, and ab initio investigations of N2O–HDO

We have investigated the N2O–HDO molecular complex using ab initio calculations at the CCSD(T)-F12a/aug-cc-pVTZ level of theory and using cavity ring-down spectroscopy to probe an HDO/N2O/Ar supersonic jet around 1.58 μm. A single a-type vibrational band was observed, 13 cm−1 redshifted compared to the OH+OD excited band in HDO, and 173 vibration-rotation lines were assigned (Trot ≈ 20 K). A weighted fit of existing microwave and present near infrared (NIR) data was achieved using a standard Watsons Hamiltonian (σ = 1.26), producing ground and excited states rotational constants. The comparison of the former with those calculated ab initio suggests a planar geometry in which the OD rather than the OH bond in water is almost parallel to NNO. The equilibrium geometry and dissociation energy (De = –11.7 kJ/mol) of the water–nitrous oxide complex were calculated. The calculations further demonstrate and allow characterising another minimum, 404 cm−1 (ΔE0) higher in energy. Harmonic vibrational frequencies and dissociation energies, D0, were calculated for various conformers and isotopic forms of the complex, in both minima. The absence of N2O–D2O from dedicated NIR experiments is reported and discussed.

The Water Dimer Investigated in the 2OH Spectral Range Using Cavity Ring-Down Spectroscopy.

Two setups based on CW cavity ring-down spectroscopy were used at Bruxelles and Rennes to record jet-cooled water dimer absorption between 7188 and 7285, and between 7357 and 7386 cm(-1). Some 19 absorption features are reported, significantly more than in the literature. Limited high-resolution information is available due to strong overlap between neighboring vibration-rotation-tunneling (VRT) structures and to spectral broadening induced by short upper state vibrational predissociation lifetimes, likely to range between 100 and 20 ps. Rotational band contours analyses are performed to assign the partly resolved VRT structures to the v1v2v3,vfvb = 000,11; 200,00; 000,20; and 101,00 zero-order vibrational states. Their wavenumbers are found to be 7192.34, 7225.86, 7240.57, and 7256.99 cm(-1), respectively. Both so-called acceptor-switching tunneling components are involved in the assignments whose tentative character is discussed.

Overtone spectroscopy of molecular complexes containing small polyatomic molecules

The literature on the high-resolution spectroscopic investigation of molecular complexes containing small polyatomic species excited in their vibrational overtones is reviewed. They turn out to be complexes containing acetylene, ammonia and water, mainly excited in their 2CH, 2NH and 2OH vibrations, respectively. The majority of results published on these systems was obtained using an instrumental set-up based on cw-cavity ring-down spectroscopy, built in the ‘Laboratoire de Chimie quantique et Photophysique’ at the ‘Université libre de Bruxelles’ (CQP/ULB) and named FANTASIO+, which is described. It allowed retrieving upper state vibrational predissociation lifetimes, which are highlighted together with more results. The sequence of experiments at CQP/ULB prior and along the line of those supporting the investigation of molecular complexes is briefly illustrated.

Overtone, 2NH (ν1+ν3) spectroscopy of NH3–Ar and NH3–Kr

We have recorded between 6561 and 6671 cm–1 the spectrum of jet-cooled ammonia seeded in Ne, Ar and Kr, using continuous wave cavity ring-down spectroscopy (CW-CRDS). The equivalent absorption pathlength was around 750 m. Three bands are assigned to Π(11)←Σ(00), Π(11)←Π(10) and Δ(?)←Π(10) in the 2NH, ν1+ν3←GS transition of the ortho NH3–Ar dimer. They are rotationally analysed and a simultaneous fitting procedure, together with one far-infrared ground state band from the literature is successfully achieved. A possible Mj-dependent cooling process is reported. Only the first of these bands is observed in the ortho NH3–Kr dimer, and rotationally analysed. Individual line perturbations and anomalous line broadening effects are reported. A J-dependent vibrational predissociation lifetime with a mean value around 0.6 ns is obtained for the Π(11) sub-state in NH3–Ar and NH3–Kr. Two additional bands are assigned to NH3–Ar involving close Π upper sub-states. A group of close bands from the para dimer is identified in NH3–Ar. The energy of all observed ortho and para sub-states is extracted from the analysis. Finally, more bands are reported but their carriers could not be identified.

Line positions and intensities in the 7400–8600 cm−1 region of the ammonia spectrum

ABSTRACT The positions and intensities of 1936 lines observed in the range 7400– of two absorption spectra of ammonia recorded at high resolution using Fourier transform spectroscopy are reported. The accuracy of these line positions is estimated to range from 0.001 to from the lower to the upper limits of the spectral range considered, while the accuracy of the line intensities is estimated to be around 10% or better. Also reported are less-accurately measured positions and intensities of 1985 lines retrieved from these two spectra or from only one of them. These results are compared with the data measured recently in a spectrum recorded in 1980 at the Kitt Peak National Solar Observatory [Barton et al., J. Mol. Spectrosc. 325, 7 (2016)] and provided in HITRAN 2016, as well as line positions and intensities measured in this work in the same Kitt Peak spectrum. GRAPHICAL ABSTRACT