Opacity data for HCN and HNC from a new ab initio line list

A new extensive ab initio rotation-vibration HCN/HNC line list is presented. The line list contains rotation-vibration energy levels, line frequencies, and line strengths for transitions between states with energy less than 18,000 cm-1 and with J≤60. This line list greatly improves the quality and range of HCN/HNC data available. It is presently the most extensive and most accurate ab initio HCN/HNC line list in existence. It is hoped that this data set will be used in models of C star atmospheres and elsewhere

Spectroscopically determined potential energy surface of H216O up to 25 000 cm–1

A potential energy surface for the major isotopomer of water is constructed by fitting to observed vibration–rotation energy levels of the system using the exact kinetic energy operator nuclear motion program DVR3D. The starting point for the fit is the ab initio Born–Oppenheimer surface of Partridge and Schwenke [J. Chem. Phys. 106, 4618 (1997)] and corrections to it: both one- and two-electron relativistic effects, a correction to the height of the barrier to linearity, allowance for the Lamb shift and the inclusion of both adiabatic and nonadiabatic non-Born–Oppenheimer corrections. Fits are made by scaling the starting potential by a morphing function, the parameters of which are optimized. Two fitted potentials are presented which only differ significantly in their treatment of rotational nonadiabatic effects. Energy levels up to 25 468 cm–1 with J = 0, 2, and 5 are fitted with only 20 parameters. The resulting potentials predict experimentally known levels with J≤10 with a standard deviation of 0.1 cm–1, and are only slightly worse for J = 20, for which rotational nonadiabatic effects are significant. The fits showed that around 100 known energy levels are probably the result of misassignments. Analysis of misassigned levels above 20 000 cm–1 leads to the reassignment of 23 transitions

Ab initio rotation–vibration spectra of HCN and HNC

We have calculated an ab initio HCN/HNC linelist for all transitions up to J=25 and 18 000 cm−1 above the zero point energy. This linelist contains more than 200 million lines each with frequencies and transition dipoles. The linelist has been calculated using our semi-global HCN/HNC VQZANO+PES and dipole moment surface, which were reported in van Mourik et al. (J. Chem. Phys. 115 (2001) 3706). With this linelist we synthesise absorption spectra of HCN and HNC at 298 K and we present the band centre and band transition dipoles for the bands which are major features in these spectra. Several of the HCN bands and many of the HNC bands have not been previously studied. Our line intensities reproduce via fully ab initio methods the unusual intensity structure of the HCN CN stretch fundamental (0001) for the first time and also the forbidden (0220) HCN bending overtone. We also compare the J=1→0 pure rotational transition dipole in the HCN/HNC ground and vibrationally excited states with experimental and existing ab initio results

Beyond the Born-Oppenheimer approximation: high-resolution overtone spectroscopy of H2D+ and D2H+

Transitions to overtone 2v2 and 2v3, and combination v2 + v3 vibrations in jet-cooled H2D+ and D2H+ molecular ions have been measured for the first time by high-resolution IR spectroscopy. The source of these ions is a pulsed slit jet supersonic discharge, which allows for efficient generation, rotational cooling, and high frequency (100 KHz) concentration modulation for detection via sensitive lock-in detection methods. Isotopic substitution and high-resolution overtone spectroscopy in this fundamental molecular ion permit a systematic, first principles investigation of Born–Oppenheimer "breakdown" effects due to large amplitude vibrational motion as well as provide rigorous tests of approximate theoretical methods beyond the Born–Oppenheimer level. The observed overtone transitions are in remarkably good agreement (<0.1 cm–1) with non-Born–Oppenheimer ab initio theoretical predictions, with small but systematic deviations for 2v2, 2v + 3v, and 2v3 excited states indicating directions for further improvement in such treatments. Spectroscopic assignment and analysis of the isotopomeric transitions reveals strong Coriolis mixing between near resonant 2v3 and 2v + 3v vibrations in D2H+. Population-independent line intensity ratios for transitions from common lower states indicate excellent overall agreement with theoretical predictions for D2H+, but with statistically significant discrepancies noted for H2D+. Finally, H2D+ versus D2H+ isotopomer populations are analyzed as a function of D2/H2 mixing ratio and can be well described by steady state kinetics in the slit discharge expansion

Ab initio rotation–vibration energy levels of triatomics to spectroscopic accuracy

The factors that need to be taken into account to achieve spectroscopic accuracy for triatomic molecules are considered focusing on H3+ and water as examples. The magnitude of the adiabatic and non-adiabatic corrections to the Born–Oppenheimer approximation is illustrated for both molecules, and methods of including them ab initio are discussed. Electronic relativistic effects are not important for H3+, but are for water for which the magnitude of the various effects is discussed. For H3+ inclusion of rotational non-adiabatic effects means that levels can be generated to an accuracy approaching 0.01 cm−1; for water the error is still dominated by the error in the correlation energy in the electronic structure calculation. Prospects for improving this aspect of the calculation are discussed

Two-electron relativistic corrections to the potential energy surface, and vibration-rotation levels of water

Two-electron relativistic corrections to the ground-state electronic energy of water are determined as a function of geometry at over 300 points. The corrections include the two-electron Darwin term (D2) of the Coulomb–Pauli Hamiltonian, obtained at the cc-pVQZ CCSD(T) level of theory, as well as the Gaunt and Breit corrections, calculated perturbationally using four-component fully variational Dirac–Hartree–Fock (DHF) wavefunctions and two different basis sets. Based on the calculated energy points, fitted relativistic correction surfaces are constructed. These surfaces are used with a high-accuracy ab initio nonrelativistic Born–Oppenheimer (BO) potential energy hypersurface to calculate vibrational band origins and rotational term values for H216O. The calculations suggest that these two-electron relativistic corrections, which go beyond the usual kinetic relativistic effects and which have so far been neglected in rovibrational calculations on light many-electron molecular systems, have a substantial influence on the rotation–vibration levels of water. The three effects considered have markedly different characteristics for the stretching and bending levels, which often leads to fortuitous cancellation of errors. The effect of the Breit interaction on the rovibrational levels is intermediate between the effect of the kinetic relativistic correction and that of the one-electron Lamb-shift effect

Analysis of hot D2O emission using spectroscopically determined potentials

Fourier transform emission spectra of D2O vapor were recorded at a temperature of 1500 °C in the wavenumber range 380–1880 cm–1. 15 346 lines were measured, of which the majority were identified as belonging to D2O. The spectrum was analyzed using variational nuclear motion calculations based on spectroscopically determined potential-energy surfaces. Initial assignments were made using a potential surface obtained by fitting a high accuracy ab initio potential. The new assignments were used to refine the potential surface, resulting in additional assignments. A total of 6400 D2O transitions were assigned and 2144 new D2O energy levels were obtained. Transitions involving the 42 and 52 bending states, with band origins of 4589.30 (±0.02) and 5679.6 (±0.1) cm–1, respectively, were assigned for the first time

Water line parameters for weak lines in the range 9,000–12,700 cm-1

A total of 7923 transitions previously derived from long pathlength, Fourier transform spectra of pure water vapor (Schermaul et al., J. Mol. Spectrosc. 211 (2002) 169) have been refitted and reanalyzed using a newly calculated variational linelist. Of these, 6600 lines are weaker than 1 × 10−24 cm/molecule, for which reliable intensities are obtained. These weak lines include 1082 lines, largely due to H216O, which have not been previously observed. A total of 7156 lines were assigned resulting in 329 new energy levels for H216O spread over 32 vibrational levels. Estimates are also given for the band origins of the (022), (140), and (051) vibrational states