SCIENTIFIC HIGHLIGHTS 2000-2001

If you have Internet Explorer, you can click here to see a slide show of the SPHERS scientific highlights from the network's mid-term review meeting (Netscape does not work!).

In the following, the network partners are referred to by the abbreviations given in Table 1.

On the experimental side of the project, the most important scientific highlights are

• The synthesis and first detection of the molecule BF₂OH in the gas phase by infrared spectroscopy. The doublet structure of the ₄ and 2₉ bands shows the existence of a large amplitude bending motion of the OH group. Recently, the high resolution infrared spectra of monoisotopic samples ¹⁰BF₂OH and ¹¹BF₂OH have been recorded at BUGHW. Some bands have been analyzed by PPM and BUGHW. Furthermore the microwave and millimeter-wave study of F₂BOH has been started by BUGHW in collaboration with Dr. J. Demaison at the University of Lille, France, using the experimental equipment available in Lille (BUGHW, PPM).

• Studies of the ground and v₄ = 1 excited states of H₂SiF₂ have been completed by BUGHW in collaboration Dr. J. Demaison, Lille, who contributed millimeterwave measurements and ab initio data allowing a better assessment of the experimental results.

• Spectra of ¹³CH₃35Cl and ¹³CH₃37Cl have been recorded, the analysis is in progress (BUGHW).

• Overtone and combination bands of TeH₂ in the 4500 - 7000 cm^-1 region have been recorded at BUGHW, and their analysis is ongoing jointly with PPM.

• The construction of a cavity ring-down spectroscopic experiment using a CW titanium-sapphire ring laser (UNIHEL).

• The analysis of jet-cooled ₁₂, ₇+ ₈, and ₆+ ₁₀ absorption bands of ¹²C₂H₄ was made (PPM in collaboration with ULB).

• The first high-resolution analysis of the six fundamental bands of COFCl at high resolution (PPM in collaboration with BUGHW).

• Extensive analyses of the ₇ and ₉ interacting bands of formic acid CHOOH and the detection of the second harmonic transition near 10200 cm^-1 by ICLAS. The rotational analysis of this transition has been completed at ULB and included in a paper describing the vibrational energy pattern of formic acid (PPM in collaboration with UNIJFG and ULB).

• The development of an experimental set-up called FT-ICLAS for Fourier transform-intracavity laser absorption spectroscopy. It uses a new Fourier transform spectrometer installed at ULB at Easter 2001, to record the near-infrared broadband absorption achieved with the cavity of a sapphire-titanium solid state laser. Intracavity laser absorption allows very long pathlengths to be reached, while FTS recording provides significant advantages compared to conventional spectrographs. At present, the experiment uses a commercial laser cavity, which had to be significantly modified. Electronic equipment was developed to synchronise the FTS data handling and laser pulses. The FT-ICLAS set-up has been used to record spectra of acetylene with resolution and sensitivity previously unachievable. Improvements are being brought up regularly on the now existing system. Simultaneously, the building of a new, more adequate laser cavity has been started. It has required extensive planning including, in particular, the writing of a computer program to deal with a "virtual" cavity in order to allow for selection of optics (ULB in collaboration with UNIJFG).

• A series of metrological experiments have been carried out at ULB with the aim of providing reference spectral line positions in the near-infrared range. These lines will be compiled in a spectral atlas made available to the scientific community. Spectra of acetylene and nitrous oxide were recorded and handled, so far. Results from this work are decribed in a review article with authors from ULB and UNIJFG, entitled "Vibrational spectroscopic database on acetylene, (¹²C₂H₂, ¹²C₂D₂, and ¹³C₂H₂)". This paper has been submitted to J. Phys. Chem. Ref. Data. It includes in particular an exhaustive listing of the observed vibrational energy states and complete sets of predicted vibrational energies and predicted principal rotational constants Bv up to 15000, 10000 and 12000 cm^-1 for ¹²C₂H₂, ¹²C₂D₂, and ¹³C₂H₂, respectively. The measurements are of direct relevance to other groups in the collaboration, UNIJFG and CNRSDR18.PhLAM, to calibrate their own laser data. The development of highly sensitive near-infrared spectrometers by UNIJFG and ULB motivated a collaboration with CNRSDR18.PhLAM on acetylene. At CNRSDR18.PhLAM, the photo-acoustic technique has been validated for the fine analysis of spectral line shapes of this molecule. If validated for the determination of absolute line intensities, it will become an ideal complement to FT spectrometers.

• Absorption spectroscopy of acetylene (C₂H₂ and dideuteroacetylene C₂D₂) in the 10000-12500 cm^-1 spectral region. The spectra were recorded by Intracavity Absorption Spectroscopy (ICLAS) using a Ti : Sapphire laser as amplification medium. The rovibrational analyses of the data were performed in the frame of effective Hamiltonians. In particular, an anharmonic resonance involving the stretching and bending modes of C₂D₂ is confirmed. An alternative approach based on a local mode Hamiltonian was also successfully applied to the stretching modes of C₂D₂ (UNIJFG in collaboration with ULB and UNIHEL).

• The vibrational energy pattern in propyne (methyl acetylene). On the basis of a Fourier Transform spectrum recorded at ULB and helped by ab initio calculations, an effective Hamiltonian was developed and a polyad structure of the vibrational states was put in evidence (UNIJFG in collaboration with ULB).

• The V_SiH=5-7 overtone transitions of deuterated silanes (SiHi₃D, SiH₂D₂ and SiHD₃) were recorded by ICLAS at UNIJFG. The samples were provided by BUGHW and a predoc supported by the network has performed the analysis. The evolution of the rotational structure of the V_SiH=3-6 overtone transitions of monodeuterated silane, SiHi₃D, could be discussed from complementary spectra recorded by Fourier Transform Spectroscopy at BUGHW (V_SiH= 3 and 4) and ICLAS at UNIJFG (V_SiH= 5 and 6). The analysis of these data has been completed and a paper is in preparation. Finally, the rotational analysis of the newly observed V_SiH=5 transition of SiH₃D has shown that the upper states of the transition are mostly unperturbed with characteristic features of local mode (UNIJFG in collaboration with BUGHW). For SiHi₃D, the Si-H stretching energy structure has been deeply analyzed, combining high accuracy experiments, ab initio methods, and the relations deduced from local mode concept. The unprecedented successful inclusion of electronic structure calculations in the fit of local mode overtone levels opens a wide field for forthcoming studies on related and other molecules. So far, these studies have been extended to the spectra of the asymmetric top SiH₂D₂ and the symmetric top PH₃ (CNRSDR18.PhLAM in collaboration with BUGHW, UNIJFG, UNIHEL and MPKOHLE).

• An important experimental development was achieved by successfully using VECSELs (Vertical External Cavity Systems Emitting Lasers) for ICLAS. This new system opens the near infrared region (below 1 m) to ICLAS. High quality spectra of acetylene, N₂O, and ⁷⁰GeHD₃ (V_GeH= 5) have already been recorded between 9500 and 10000 cm^-1 and are under analysis.

• The dipole moment of vibrationally excited formaldehyde (H₂CO) has been measured with a technique that has previously been proved to work for HOCl (EPFL).

• Stark-induced quantum beats have been used as a different approach to dipole moment measurements at EPFL. This new technique has been used to measure the dipole moment in the 2₅ overtone of formaldehyde, and in highly vibrationally excited water. The water measurements include the local mode vibrational states |40^-0>, |50^-0>, |80⁺0> and |40^-2>. The measurements also yield the transition dipole moment between these states and their counterparts of opposite symmetry, i.e. the transition dipole moment between |40^-0> and |40⁺0>. Comparison with calculations carried out at UCL will give valuable insight into the dipole moment surface away from the equilibrium position. It is difficult to compute this part of the dipole moment surface with high accuracy, but it is important for calculating high overtone intensities.

• A combination of high accuracy experiments and ab initio methods is currently applied to analyse the spectrum of FCO₂ (a molecule of atmospheric interest) in various spectral regions (sub-MMW, IR), and then to determine its structure. (CNRSDR18.PhLAM in collaboration with BUGHW and MPKOHLE).

• A laser-induced dispersed vibration-rotation fluorescence experiment has been optimized at UNIHEL. In acetylene spectra, an improvement of a factor of 7 has been attained in the signal- to-noise ratio. This experimental technique can be used to study ortho-para spin conversion in water, a phenomenon treated theoretically at UCL.

• The laser-induced dispersed vibration-rotation fluorescence experiment at UNIHEL has been used to study the first CH stretching overtone FTIR spectrum of diacetylene. This is a very dense spectrum. Relative to previous work, more precise wavenumber values were obtained, and more lines could be assigned. The analysis was carried out in a collaboration between UNIHEL and BUGHW.

The theoretical research has produced the following results:

• Considerable progress has been made in the development of a Hamiltonian and computer program describing the rotation and vibration of XY₃ molecules using a MORBID-type approach. In parallel, the calculation of the potential energy surface for ammonia NH₃ is in progress. A two-dimensional surface (symmetric stretch and symmetric bend, 440 data points) has been obtained at the CCSD(T) level, with extrapolation to the basis set limit and inclusion of core correlation and relativistic corrections. A full six-dimensional surface (14400 data points) has been generated at the CCSD(T)/aug-cc-pVTZ level. (BUGHW, MPKOHLE).

• Intensities of overtone and combination bands of SiH₄ and SiHCl₃ have been investigated by ab initio methods, in particular by means of many-dimensional CCSD(T) dipole moment surfaces. This work has now been extended to GeH₄. The results rationalize the experimental findings (including intensity anomalies) and highlight the reliability of high-level ab initio predictions (BUGHW,MPKOHLE).

• New assignments of H₂O and HDO spectra have been made, both for hot molecules in the mid infrared and for cold molecules in the near infrared/optical. New ab initio studies show the importance of various electronic relativistic effects, including qed corrections, on the potential for water. A new database containing all known experimental energy levels for water (over 12,000) has been constructed. Further, all quasi-degenerate ortho-para states in H2O have been analysed; this analysis produced a list of candidate states, which could be suitable for experimental observation, possibly using the experimental techniques at UNIHEL (UCL).

• A new ab initio potential energy surface for H₂S including electronic relativistic and adiabatic corrections has been developed. Nuclear motion calculations using this surface show much greater accuracy than previous studies (UCL).

• A new, global ab initio potential energy and dipole moment surfaces for the isomerizing HCN/HNC system have been calculated and used for rotation-vibration calculations extending above the barrier between the isomers. Detailed spectroscopic studies and comparison with experiment have been made (UCL).

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