234 research outputs found
Bound state calculations of the van der waals nh3−ne complex and first microwave detection of the missing (para)-nh3−ne nuclear spin isomer
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Previous issue date: 24The microwave spectrum of the NHNe van der Waals complex has been observed in a supersonic molecular jet expansion via broadband (2-8 GHz) chirped-pulse Fourier-transform microwave spectroscopy. Together with the well-known lines related to the ()-NHNe spectrum\footnote{J. van Wijngaarden and W. Jäger, \textbf{115}, 6504 (2001).}, new transitions were detected and attributed to the missing ()-NHNe nuclear spin isomer. The assignments were guided by the rovibrational bound state ( = 0 ... 10) calculations based on the recent NHNe intermolecular potential surface\footnote{J. Loreau and A. van der Avoird, \textbf{143}, 184303 (2015).}. The hyperfine structure arising from quadrupole N nucleus of NHNe was resolved, and the quadrupole coupling constant associated with the ()-NH subunit was precisely determined. This constant provided the dynamical information about the angular orientation of ammonia indicating that the average angle between the axis of NH and intermolecular axis is about 68. Similar results for the deuterated isotopologue, NDNe, were also obtained thus confirming and extending the analysis for the parent NHNe complex
Jet-cooled infrared laser spectroscopy in the umbrella ν2 vibration region of NH3: Improving the potential energy surface model of the NH3−Ar van der Waals complex
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Previous issue date: 6Taking advantage of our sensitive laser spectrometer coupled to a pulsed slit jetfootnote{P. Asselin, Y. Berger, T. R. Huet, R. Motiyenko, L. Margulès, R. J. Hendricks, M. R. Tarbutt, S. Tokunaga, B. Darquié, PCCP 19, 4576 (2017),} we recorded near the nub{2} vibration a series of rovibrational transitions of the chem{NH_3-Ar} van der Waals (vdW) complex. These transitions involve in the ground vibrational state several internal rotor states corresponding to the orthochem{NH_3} and parachem{NH_3} spin modifications of the complex. They are labeled by (j,k), (j,k), (j,k) and (j,k) where (K=0) and (K=1) indicate the projection K of the total rotational angular momentum J on the vdW axis, the superscripts s and a designate a symmetric or antisymmetric chem{NH_3} inversion wave function, and j, k quantum numbers indicate the correlation between the internal-rotor state of the complex and the j, k rotational state of the free chem{NH_3} monomer. Five bands have been identified, only one of which was partly observed beforefootnote{G. T. Fraser, A.S. Pine and W. A. Kreiner, J. Chem. Phys. 94, 7061 (1991).}. They include transitions starting from the (j=0 or j=1) state without any internal angular momentum, consequently they can be assigned from the band contour of a linear-molecule-like K=0, J=1 transition. The energies and splittings of the rovibrational levels of the 0 spectrum derived from the analysis of the , (j=1) (j=0), k=0 bands and mostly of the , and (j=1)(j=1), k=1 bands bring relevant information about the nub{2} dependence of the chem{NH_3-Ar} interaction, the rovibrational dynamics of the chem{NH_3-Ar} complex and provide a sensitive test of a recently developed 4D potential energy surface that includes explicitly its dependence on the umbrella motionfootnote{J. Loreau, J. Liévin, Y. Scribano and A. van der Avoird, J. Chem. Phys. 141, 224303 (2014).}
Intermolecular dynamics of binary nh3-rare gas complexes in the ν2 umbrella mode region of nh3 : rovibrational jet-cooled laser spectroscopy and ab initio calculations
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Previous issue date: 23The large number of spectroscopic and theoretical studies of \chem{NH_3}-Rg complexes reported for three decades clearly proves that they are good prototypes to investigate intermolecular dynamics of van der Waals interactions. The inversion coordinate for which the inversion-tunneling splitting in \chem{NH_3} is not quenched in the case of \chem{NH_3}-Rg represents a valuable probe of asymmetry in the intermolecular potential energy surface (IPES).\footnote{G. T. Fraser, D. D. Nelson Jr, A. Charo and W. Klemperer, J. Chem. Phys. 82, 2535 (1985).} A recent high resolution infrared laser jet- cooled study realized in the \nub{2} umbrella mode region of \chem{NH_3}-Ar enabled to detect five ortho and para bands and to provide accurate band centres and upper state rotational constants for the ortho ones.\footnote{P. Asselin, Y. Belkhodja, A. Jabri, A. Potapov, J. Loreau and A. van der Avoird, Mol . Phys. 116, 3642 (2018)} The puzzling para bands observed in the region of the lower and upper components of the inversion splitting doublets have been assigned by comparison with calculations of vibration-rotation-tunneling (VRT) levels based on a 4D PES that includes the umbrella inversion motion.\footnote{J. Loreau and A. van der Avoird, J. Chem. Phys. 143,184303 (2015)}
The present study aims to investigate more in detail the intense ortho bands of the \chem{NH_3}-Rg family (Rg= Ne, Ar, Kr, Xe) taking advantage of recent improvements in the pulsed slit expansion to reduce line widths and to increase gain in absorption. These advances enabled to derive accurate excited rotational and quartic parameters and l-type doubling constant q for the four binary \chem{NH_3}-Rg complexes from rovibrational analyses of the ortho (j=1,k=0) (j=1,k=0) transition. Using a pseudodiatomic model, structural parameters and force constants of the \chem{NH_3}-Rg family could be estimated from the experimental set of spectroscopic constants obtained and then compared with those calculated using ab initio VRT levels, transition frequencies and line strengths from 4D PES of \chem{NH_3}-Rg. The predictive character of the IPES is discussed on the grounds of band origins, rovibrational states and l-type doubling contribution
Bound State Calculations Of The Van Der Waals Nh<sub>3</sub>−ne Complex And First Microwave Detection Of The Missing (para)-nh<sub>3</sub>−ne Nuclear Spin Isomer
No full textThe microwave spectrum of the NHNe van der Waals complex has been observed in a supersonic molecular jet expansion via broadband (2-8 GHz) chirped-pulse Fourier-transform microwave spectroscopy. Together with the well-known lines related to the ()-NHNe spectrum\footnote{J. van Wijngaarden and W. Jäger, \textbf{115}, 6504 (2001).}, new transitions were detected and attributed to the missing ()-NHNe nuclear spin isomer. The assignments were guided by the rovibrational bound state ( = 0 ... 10) calculations based on the recent NHNe intermolecular potential surface\footnote{J. Loreau and A. van der Avoird, \textbf{143}, 184303 (2015).}. The hyperfine structure arising from quadrupole N nucleus of NHNe was resolved, and the quadrupole coupling constant associated with the ()-NH subunit was precisely determined. This constant provided the dynamical information about the angular orientation of ammonia indicating that the average angle between the axis of NH and intermolecular axis is about 68.Made available in DSpace on 2021-09-24T21:09:39Z (GMT). No. of bitstreams: 2
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Previous issue date: 2021-06-2
Ab initio potential-energy surface and rovibrational states of the HCN-HCl complex
No full textA four-dimensional intermolecular potential-energy surface has been calculated for the HCN-HCl complex, with the use of the coupled cluster method with single and double excitations and noniterative inclusion of triples. Data for more than 13 000 geometries were represented by an angular expansion in terms of coupled spherical harmonics; the dependence of the expansion coefficients on the intermolecular distance R was described by the reproducing kernel Hilbert space method. The global minimum with D-e=1565 cm(-1) and R-e=7.47a(0) has a linear HCN-HCl hydrogen-bonded structure with HCl as the donor. A secondary hydrogen-bonded equilibrium structure with D-e=564 cm(-1) and R-e=8.21a(0) has a T-shaped geometry with HCN as the donor and the acceptor HCl molecule nearly perpendicular to the intermolecular axis. This potential surface was used in a variational approach to compute a series of bound states of the isotopomers HCN-(HCl)-Cl-35, DCN-(HCl)-Cl-35, and HCN-(HCl)-Cl-37 for total angular momentum J=0,1,2 and spectroscopic parities e, f. The results could be analyzed in terms of the approximate quantum numbers of a linear polyatomic molecule with two coupled bend modes, plus a quantum number for the intermolecular stretch vibration. They are in good agreement with the recent high resolution spectrum of Larsen [Phys. Chem. Chem. Phys. 7, 1953 (2005)] in the region of 330 cm(-1) corresponding to the HCl libration. The (partly anomalous) effects of isotopic substitutions on the properties of the complex were explained with the aid of the calculations
Simulating rotationally inelastic collisions using a direct simulation Monte Carlo method
No full textCross sections for He NH3 rotationally inelastic collisions in text file format. Results from a direct simulation Monte Carlo simulation of a supersonic expansion in HDF5 format. Python scripts to generate paper figures from simulation results. A new approach to simulating rotational cooling using a Direct Simulation Monte Carlo (DSMC) method is described and applied to the rotational cooling of ammonia seeded into a helium supersonic jet. The method makes use of {\it ab initio} rotational state changing cross sections calculated as a function of collision energy. Each particle in the DSMC simulations is labelled with a vector of rotational populations that evolves with time. Transfer of energy into translation is calculated from the mean energy transfer for this population at the specified collision energy. The simulations are compared with a continuum model for the on-axis density, temperature and velocity; rotational temperature as a function of distance from the nozzle is in accord with expectations from experimental measurements. The method could be applied to other types of gas mixture dynamics under non-uniform conditions, such as buffer gas cooling of NH3 by He
THE AMMONIA DIMER REVISITED
No full textAuthor Institution: Missouri University of Science and Technology; Rolla, MO 65409-0010; Radboud University, 6525 AJ Nijmegen, The NetherlandsThe conclusion from microwave spectra by Nelson, Fraser, and Klemperer nderline{\textbf{83}} 6201 (1985)} that the ammonia dimer has a nearly cyclic structure led to much debate about the issue of whether (NH) is hydrogen bonded. This structure was surprising because most \textit{ab initio} calculations led to a classical, nearly linear, hydrogen-bonded structure. An obvious explanation of the discrepancy between the outcome of these calculations and the microwave data which led Nelson \textit{et al.} to their "surprising structure'' might be the effect of vibrational averaging: the electronic structure calculations focus on finding the minimum of the intermolecular potential, the experiment gives a vibrationally averaged structure. Isotope substitution studies seemed to indicate, however, that the complex is nearly rigid. Additional data became available from high-resolution molecular beam far-infrared spectroscopy in the Saykally group nderline{\textbf{97}} 4727 (1992)}. These spectra, displaying large tunneling splittings, indicate that the complex is very floppy. The seemingly contradictory experimental data were explained when it became possible nderline{\textbf{101}} 8430 (1994); E.~H.~T.~Olthof, A.~van der Avoird, P.~E.~S.~Wormer, J.~G.~Loeser, and R.~J.~Saykally \textit{J.~Chem.~Phys.} nderline{\textbf{101}} 8443 (1994)} to calculate the vibration-rotation-tunneling (VRT) states of the complex on a six-dimensional intermolecular potential surface. The potential used was a simple model potential, with parameters fitted to the far-infrared data. Now, for the first time, a six-dimensional potential was computed by high level \textit{ab initio} methods and this potential will be used in calculations of the VRT states of (NH) and (ND). So, we will finally be able to answer the question whether the conclusions from the model calculations are indeed a valid explanation of the experimental data
Para-ortho Hydrogen Conversion; Solving A 90-year Old Mystery
No full textIt is well known among spectroscopists that hydrogen has two modifications: para-H and ortho-H. Pure para-H can be produced by leading ``normal'' H, a 3:1 ortho:para mixture, over a catalyst at low temperature. It is perhaps less well known that para-ortho H conversion is also catalyzed by collisions with paramagnetic molecules, such as O.
Almost ninety years ago Farkas and Sachsse measured the rate coefficient of para-ortho H conversion in gas mixtures with O.[1] In the same year, 1933, it was proposed by Wigner [2] that it is the magnetic dipole-dipole coupling between the electron spin of O and the nuclear spins of the two protons in H that is responsible for the conversion. In asymmetric collisions this coupling makes the two H-nuclei inequivalent and mixes the nuclear spin functions of para- and ortho-H, as well as their rotational states with even and odd values. Another mechanism, suggested to be much more effective, was proposed later: the exchange interaction with the open-shell O induces spin density into the electronic wavefunction of H. In most collisions the spin density is different at the two H-nuclei, which makes them inequivalent by different hyperfine interactions through the Fermi contact term.
An important application of para-H is in NMR spectroscopy and its imaging variant, MRI. By adding para-H to the sample the sensitivity of NMR can be increased by four orders of magnitude by a phenomenon called para-hydrogen induced polarization (PHIP). Para-ortho H conversion by O in the gas phase was remeasured in 2014 in view of this application. A detailed and quantitative understanding of the conversion process was still lacking, however.
We theoretically investigated the para-ortho H conversion by collisions with O in a first principles approach.[3] Both mechanisms were taken into account and the corresponding coupling terms were quantitatively evaluated as functions of the geometry of the O-H collision complex by means of \textit{ab initio} electronic structure calculations. Then they were included in nearly exact quantum mechanical coupled-channels scattering calculations for the collisions between O and H, which yielded the para-ortho H conversion cross sections and the rate coefficients for temperatures up to 400\,K. The conversion rate and its temperature dependence are in good agreement with the values measured in H-O gas mixtures. The calculations provide detailed insight into the conversion process.
[1] L. Farkas and H. Sachsse, Z. Phys. Chem. B {\bf 23}, 1 (1933). [2] E. Wigner, Z. Phys. Chem. B {\bf 23}, 28 (1933). [3] X. Zhang, T. Karman, G.~C. Groenenboom, and A. van der Avoird, Nat. Sci. (2021); https://doi.org/10.1002/ntls.10002.Made available in DSpace on 2021-09-24T21:08:52Z (GMT). No. of bitstreams: 2
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Previous issue date: 2021-06-21Made available in DSpace on 2022-01-21T16:10:34Z (GMT). No. of bitstreams: 4
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Previous issue date: 2021-06-2
Mapping partial wave dynamics in scattering resonances by rotational de-excitation collisions
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Authors: Tim de Jongh, Quan Shuai, Grite L. Abma, Stach Kuijpers, Matthieu Besemer, Ad van der Avoird, Gerrit C. Groenenboom, Sebastiaan Y.T. van de Meerakke
Fine-structure spectrum of O2-rare gas Van der Waals molecules
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