Understanding the high-resolution spectral signature of the N2-H2O van der Waals complex in the 2OH stretch region.

We present the observation of the N2-H2O van der Waals complex in the 2OH stretch overtone region. The high-resolution jet cooled spectra were measured using a sensitive continuous wave cavity ringdown spectrometer. Several bands were observed and vibrationally assigned in terms of ν1, ν2, and ν3, the vibrational quantum numbers of the isolated H2O molecule, as (ν1'ν2'ν3')←(ν1″ν2″ν3″)=(200)←(000) and (101) ← (000). A combination band involving the excitation of the in-plane bending motion of N2 and the (101) vibration of water is also reported. The spectra were analyzed using a set of four asymmetric top rotors, each associated with a nuclear spin isomer. Several local perturbations of the (101) vibrational state were observed. These perturbations were assigned to the presence of the nearby (200) vibrational state and to the combination of (200) with intermolecular modes.

Spectroscopic study of the tunneling dynamics in N2-water observed in the O-D stretch region.

The O-D stretch rovibrational spectra of N2-D2O and N2-DOH were measured and analyzed. A combination band involving the in-plane N2 bending vibration was also observed. These bands were recorded using a pulsed-slit supersonic jet expansion and a mid-infrared tunable optical parametric oscillator. The spectra were analyzed by considering the feasible tunneling motions, and transitions were fitted to independent asymmetric rotors for each tunneling state. The rotational constants of the four tunneling components of N2-D2O were retrieved for the excited vibrational states. A two order of magnitude increase in the tunneling splittings is observed for the asymmetric O-D stretch (ν3 in D2O) excitation compared to the symmetric stretch (ν1 in D2O) and to the ground vibrational state. This last finding indicates that the ν3 vibrational state is likely perturbed by a combination state that includes ν1. Finally, the observation of a local perturbation in the ν3 vibrational band, affecting the positions of few rovibrational levels, provides an experimental lower limit of the dissociation energy of the complex, D0 > 120 cm-1.

Design and characteristics of a cavity-enhanced Fourier-transform spectrometer based on a supercontinuum source.

We report the in-house fabrication of a high-resolution Fourier-transform spectrometer (FTS) for the spectroscopy of molecules in the gas phase at resolutions down to 0.002 cm-1 working in the spectral range from 5880 cm-1 (1.7 µm) to 15 380 cm-1 (650 nm). The FTS employs a supercontinuum as a broadband light source and a He:Ne laser with a homemade frequency-stabilization scheme as the spatial reference for the sampling of the interferogram on a constant optical path difference (OPD) grid. The sampling of the two lasers is performed at constant time intervals, and the resampling process is performed at the software level. The resampling of the interferogram on a constant OPD grid relies on cubic approximations of the He:Ne interference pattern to determine its zero-crossings. The use of an invariant in the sampling process allows us to perform on-the-fly data treatment. Both the hardware aspect and the data processing are described with, in each case, an original approach. We also report the successful coupling of the FTS with a high finesse optical cavity with effective mirror reflectivities of 99.76%, allowing us to reach sensitivities down to 6.5 × 10-8 cm-1 with a root-mean-square accuracy of 0.0017 cm-1 on the position of the Doppler-broadened transitions with a mean transition width of 0.046 cm-1 for spectra recorded at a spectral resolution of 0.015 cm-1. The sensitivity of the instrument per spectral element, once normalized, represents the best sensitivity reported in the literature for Fourier-transform incoherent broadband cavity-enhanced absorption spectroscopy with a supercontinuum light source.

Structure and dynamics of the radical cation of ethane arising from the Jahn-Teller and pseudo-Jahn-Teller effects.

The pulsed-field-ionization zero-kinetic-energy photoelectron spectrum of C2H6 has been recorded in the region of the adiabatic ionization threshold. The partially rotationally resolved spectrum indicates the existence of several vibronic states of C2H6+ with less than 600 cm-1 of internal excitation. The analysis of the rotational structures assisted by ab initio calculations enabled the determination of the adiabatic ionization energy of C2H6 and the investigation of the structure and dynamics of C2H6+ at low energies. The ground state of C2H6+ is found to be a 2Ag state of diborane-like structure with strongly mixed (a1g)-1 and (eg)-1 configurations. The vibrational structure reveals the importance of large-amplitude nuclear motions involving the diborane distortion modes, the C-C stretching motion, and the internal rotation at elongated C-C distances. The spectrum is analyzed in the light of the information obtained in earlier studies of C2H6+ by ab initio quantum chemistry, EPR spectroscopy and photoelectron spectroscopy.

New Infrared Bands of the C-Bonded Isomer of OC-N2O and Determination of Two Intermolecular Frequencies.

Three combination bands involving intermolecular modes of the most stable isomer of the OC-N2O van der Waals complex have been observed, two in the carbon monoxide CO stretch region (∼2150 cm-1) and one in the ν3 asymmetric stretch region of N2O (∼2223 cm-1). Vibrational assignment is achieved by comparison with data recently published ( Barclay , A. ; et al. Chem. Phys. Lett. 2016 , 62 , 651 ), concerning OC-CO2. Two of these bands involve the same intermolecular mode, one with the CO stretch and the other with the ν3 asymmetric stretch of N2O. The values of intermolecular frequency deduced from these two bands agree very well, showing no significant dependence on intramolecular vibrational excitation.

Detection of a higher energy isomer of the CO-N2O van der Waals complex and determination of two of its intermolecular frequencies.

Infrared spectra in the carbon monoxide CO stretch region (∼2150 cm-1) and in the ν3 asymmetric stretch region of N2O (∼2223 cm-1) are assigned to the previously unobserved O-bonded form of the CO-N2O dimer ("isomer 2"). This van der Waals complex has a planar skewed T-shaped structure like that of the previously observed C-bonded form ("isomer 1"), but with the CO rotated by 180°. The effective intermolecular distance between the centers of mass is 3.51 Å for isomer 2 as compared to 3.88 Å for isomer 1. In addition to the fundamental band, two combination bands are observed for isomer 2, yielding values for two intermolecular vibrational modes: 14.502(5) cm-1 for the coupled disrotatory motion or the uncoupled CO rock and 21.219(5) cm-1 for the out-of-plane rock. We show that the published ab initio study on this system is inadequate in predicting the intermolecular frequencies for isomer 2 of CO-N2O.

High resolution overtone spectroscopy of the acetylene van der Waals dimer, ((12)C2H2)2.

CW-cavity ring down spectroscopy was used to record in a free jet expansion the spectrum of the absorption band in ((12)C(2)H(2))(2) with origin at 6547.6 cm(-1). It is a perpendicular band and corresponds to 2CH excitation in the hat unit of the T-shaped dimer. Calibration (better than ±1 × 10(-3) cm(-1) accuracy) and ring-down time (130 µs) were improved compared to a previous contribution (Didriche et al. Mol. Phys., 2010, 108, 2158-2164). A line-by-line analysis was achieved. Three series of lines were identified involving levels with A(1)(+), E(+) and B(1)(+) ground state tunneling symmetries, confirming the spectral and symmetry analyses reported in the literature for the 1CH excitation band (Fraser et al. J. Chem. Phys., 1988, 89, 6028-6045). 164 vibration-rotation-tunneling lines were assigned in the K(a)' - K(a)'' = 2 - 3, 0 - 1, 2 - 1 and 4 - 3 sub-bands and effective rigid rotor vibration-rotation constants were obtained by simultaneously fitting 1CH and 2CH lines with the same symmetry series. Perturbations affecting the K(a) stacks, in particular, are reported. The tunneling frequency in 2CH is estimated to be ν(tunn)(2CH) = 270 MHz for the K(a) = 0 stack. The rotational temperature is determined to be 23 K from relative line intensities and the lifetime of the dimer in the 2CH hat state is estimated to be 1 ns from individual line widths.

Demonstration of cavity enhanced FTIR spectroscopy using a femtosecond laser absorption source.

A proof of principle experiment was performed by recording the cavity enhanced absorption spectrum of the weak b-X transition of molecular oxygen in the atmosphere using a Ti:Sa femtosecond laser as an absorption source and a high resolution continuous scan Fourier transform interferometer. The cavity was mode matched and either continuously scanned or stabilized at the so-called magic point. An optimal rms noise equivalent absorption of 3x10(-7) cm(-1) Hz(-1/2) was reached in the latter case, corresponding to alpha(min)=3x10(-7) cm(-1).

Investigation of the C2H2-CO2 van der Waals complex in the overtone range using cw cavity ring-down spectroscopy.

A slit nozzle supersonic expansion containing acetylene [492 SCCM (SCCM denotes cubic centimeter per minute at STP)] and carbon dioxide (740 SCCM) seeded into Ar (837 SCCM) is investigated using cw-cavity ring-down spectroscopy, in the 1.5 microm range. The C(2)H(2)-CO(2) van der Waals complex is observed around the nu(1) + nu(3) acetylenic band. The rotational temperature is estimated to be close to 60 K from the comparison between observed and simulated spectra. The analysis of the main, perturbed B-type band centered near 6,549.280 cm(-1), is performed. It is attributed to a dimer with the known planar, C(2v) geometry. The present overtone data, involving ground state levels with higher J/K states (J < or = 35 and K(a) < or = 20) than previously reported, are combined to 3 microm data [D. G. Prichard, R. N. Nandi, J. S. Muenter, and B. J. Howard, J. Chem. Phys. 89, 1245 (1988); Z. S. Huang and R. E. Miller, Chem. Phys. 132, 185 (1989)] to determine improved ground state parameters. The major perturbations affecting the upper state are accounted for through C-type Coriolis resonances involving one dark state, whose symmetry must therefore be A(1). Upper state constants are obtained for the bright and dark states. The dependence upon vibrational excitation is demonstrated to arise from excitation in the acetylene unit, only, for the former, but cannot be unravelled for the latter.

(12)C2H2-Ar van der Waals complex.

New theoretical and experimental results on the acetylene-Ar van der Waals complex are presented and the literature is reviewed. New ab initio calculations at the MP2 level were performed using large basis sets with diffuse functions and taking into account the basis set superposition error. It was found that the structure of acetylene is not significantly altered by the complexation and that its vibrational frequencies are only slightly lowered. Finally, it was observed that the calculated properties of the complex (structure, vibrational spectrum, bond dissociation energy) are not sensitive to the structure imposed on acetylene. Experimentally, acetylene-Ar was produced in a supersonic expansion under experimental conditions corresponding to 9 K rotational temperature. Thanks to the performances of CW-CRDS detection, the K(a) = 0 <-- 1, 1 <-- 0, and 2 <-- 1 sub-bands of the nu(1) + nu(3) band could be recorded and resolved and most of their lines assigned. Upper-state rotational constants were fitted, however not including the upper K(a) = 2 state, which shows K-doubling the opposite of the expected. The Lorentzian width of most line profiles sets the mean lifetime to some 7.5 ns. Local perturbations affecting line positions and/or line widths are demonstrated. Additional series of lines tentatively attributed to acetylene-Ar are discussed.