Tomas Foldes

The FANTASIO+ set-up to investigate jet-cooled molecules: focus on overtone bands of the acetylene dimer

The experimental set-up FANTASIO, for ‘Fourier trANsform, Tunable diode and quadrupole mAss spectrometers interfaced to a Supersonic expansIOn’ (M. Herman, K. Didriche, D. Hurtmans, B. Kizil, P. Macko, A. Rizopoulos and P. Van Poucke, Mol. Phys. 105, 815 (2007)) built in Brussels has been updated. The turbomolecular pumping system of the supersonic expansion has been doubled and new mirrors, with reflectivity 99.999% instead of 99.99%, have been set in the CW-cavity ring down spectrometer probing jet-cooled molecules. The changes all together result in a signal to noise increased by up to a factor 10, around 1.5 µm. These improvements are demonstrated with various acetylene data in the 2CH excitation range, including the assignment of a new sub-band of acetylene–Ar, with K′–K′′ = 2–3. The focus is set on the acetylene dimer. Overtone sub-bands, with b- and a-type structures, are identified for the first time in the literature. They are assigned to vibrational excitation in the hat and body units of the T shaped dimer, respectively. The relevance of the overtone data on acetylene dimers for space remote sensing is highlighted.

Experimental 2CH excitation in acetylene-containing van der Waals complexes

Spectroscopic results are presented concerning the 2CH excitation around 1.5 µm in van der Waals complexes of acetylene (C2H2) with Ar, Kr, N2, CO2, N2O and C2H2. Many are reviewed from the literature, with some updates. Previously unpublished results are also presented, concerning the mechanism of formation of C2H2–Ar in the supersonic jet, the assignment of new spectral structures in C2H2–N2O, and the first observation of 2CH excitation in C2H2–Ne, C2H2–H2O, C2H2–D2O and (C2H2) n . Lifetimes of these 2CH vibrationally excited dimers are discussed.

Fluorine negative ion density measurement in a dual frequency capacitive plasma etch reactor by cavity ring-down spectroscopy

The authors wish to thank Lam Research Corporation for donation of equipment and financial support.

High resolution spectroscopy of the Ar-D2O and Ar-HDO molecular complexes in the near-infrared range

Three rovibrational bands of Ar-D2O and two rovibrational bands of Ar-HDO were observed in the 1.5 μm range by continuous wave cavity ringdown spectroscopy. Their analyses led to the determination of rotational constants for the upper states and vibrational shifts indicating that the potential energy surface is only slightly affected by the vibrational excitation. Some Coriolis couplings were identified. The observed lines were fitted to retrieve a 3.5 ns lifetime of the upper state, showing that even with a triple or double excitation quanta in the water subunit, the Ar-D2O and Ar-HDO complexes are long-lived species.

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.

Torsional excitation in the 2CH vibrational overtone of the C2H2?CO2 and C2H2?N2O van der Waals complexes

Infrared spectra of the weakly-bound C2H2-CO2 and C2H2-N2O complexes in the region of the 2CH acetylene overtone band (∼1.52 µm) were recorded using CW-cavity ring down spectroscopy in a continuous supersonic expansion. A new, c-type combination band is observed in each case. The rotational analysis of low J, K lines is performed and rotational constants are obtained. The band origins are 40.491(2) and 40.778(2) cm−1 higher in energy than the 2CH excitation bands for C2H2-CO2 and C2H2-N2O, respectively. The combination band is assigned in each case as involving intermolecular torsional excitation combined to 2CH. The values of the torsional vibrational frequency and of the xCH/torsion anharmonicity constant are briefly discussed.

Low-temperature high-resolution absorption spectrum of 14NH3 in the ν1+ν3 band region (1.51 μm)

Jet-cooled spectra of 14NH3 and 15NH3 in natural abundance were recorded using cavity ring-down (CRDS, 6584–6670 cm−1) and cavity enhanced absorption (CEAS, 6530–6700 cm−1) spectroscopy. Line broadening effects in the CRDS spectrum allowed lines with J″-values between 0 and 3 to be identified. Intensity ratios in 14NH3 between the jet-cooled CRDS and literature room-temperature data from Sung et al. (J. Quant. Spectrosc. Radiat. Transfer, 113 (2012), 1066) further assisted the line assignments. Ground state combination differences were extensively used to support the assignments, providing reliable values for J, K and inversion symmetry of the ground state vibrational levels. CEAS data helped in this respect for the lowest J lines, some of which are saturated in the CRDS spectrum. Further information on a/s doublets arose from the observed spectral structures. Thirty-two transitions of 14NH3 were assigned in this way and a limited but significant number (19) of changes in the assignments results, compared to Sung et al. or to Cacciani et al. (J. Quant. Spectrosc. Radiat. Transfer, 113 (2012), 1084). Sixteen known and 25 new low-J transitions were identified for 15NH3 in the CRDS spectrum but the much scarcer literature information did not allow for any more refined assignment. The present line position measurements improve on literature values published for 15NH3 and on some line positions for 14NH3.

Communication: A rotationally resolved (2OH) overtone band in the water dimer (H2O)2

Cw-CRDS spectra of water-rare gas supersonic expansions were recorded between 7229 and 7262 cm(-1). The effective absorption pathlength was about 1 km in jet-cooled gas and the resolution about 1 × 10(-4) cm(-1). Many well-resolved structures are observed that could be assigned from experimental evidence to H2O-Ar/Kr bands. Eight broader unresolved features are more specifically reported and assigned to small H2O multimers, in good agreement and refining previous observations by Nizkorodov et al. [J. Chem. Phys. 122, 194316 (2005)]. Among these, the band at 7256.5 cm(-1) is shown to be a Q branch of the water dimer with accompanying R and very weak P lines. The band is assigned to a K(a) = 0 ← 1 transition and rotationally analyzed, leading to a restricted set of upper state rotational constants. The upper state lifetime (60 ± 3 ps) is extracted from the linewidths.

High-resolution overtone spectra of molecular complexes

A high-resolution spectrum of the acetylene–water complex has been recorded in the overtone range. Two bands of C2H2–D2O were analysed, corresponding to the overtone excitations of either the acetylene or the water units. The vibrational shifts and the upper states rotational constants were retrieved, demonstrating that the geometry of the complex is only slightly modified by the excitation. A larger linewidth was observed for the 2CH band than for the 2OD + DOD band, probably due to the direct coupling of the 2CH excitation with the dissociation coordinate.

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.