A. V. Potapov

A comprehensive experimental and theoretical study of H2−CO spectra

A detailed description of a new ab initio interaction potential energy surfaces for the H2-CO complex computed on a six-dimensional grid (i.e., including the dependence on the H-H and C-O separations) is presented. The interaction energies were first calculated using the coupled-cluster method with single, double, and noniterative triple excitations and large basis sets, followed by an extrapolation procedure. Next, a contribution from iterative triple and noniterative quadruple excitations was added from calculations in smaller basis sets. The resulting interaction energies were then averaged over the ground-state and both ground- and first-excited-states vibrational wave functions of H2 and CO, respectively. The two resulting four-dimensional potential energy surfaces were fitted by analytic expressions. Theoretical infrared spectra calculated from these surfaces have already been shown [P. Jankowski, A. R. W. McKellar, and K. Szalewicz, Science 336, 1147 (2012)] to agree extremely well, to within a few hundredth of wavenumber, with the experimental spectra of the para and orthoH2-CO complex. In the latter case, this agreement enabled an assignment of the experimental spectrum, ten years after it had been measured. In the present paper, we provide details concerning the development of the surfaces and the process of spectral line assignment. Furthermore, we assign some transitions for paraH2-CO that have not been assigned earlier. A completely new element of the present work are experimental investigations of the orthoH2-CO complex using microwave spectroscopy. Vast parts of the measured spectrum have been interpreted by comparisons with the infrared experiments, including new low-temperature ones, and theoretical spectrum. Better understanding of the spectra of both para and orthoH2-CO complexes provides a solid foundation for a new search of the bound H2-CO complex in space.

Rotational Spectroscopy of the CO-para-H2 Molecular Complex

The rotational spectrum of the CO-para-H2 van der Waals complex, produced using a molecular jet expansion, was observed with two different techniques: OROTRON intracavity millimeter-wave spectroscopy and pulsed Fourier transform microwave spectroscopy. Thirteen transitions in the frequency range from 80 to 130 GHz and two transitions in the 14 GHz region were measured and assigned, allowing for a precise determination of the corresponding energy level positions of CO-para-H2. The data obtained enable further radio astronomical searches for this molecular complex and provide a sensitive test of the currently best available intermolecular potential energy surface for the CO-H2 system.

Millimeter-wave spectroscopy of weakly bound molecular complexes: Isotopologues of He-CO

Van der Waals complexes consisting of helium atoms and carbon monoxide molecules, He-12C16O, He-13C16O, He-12C18O, and He-13C18O are studied in the frequency range 110–140 GHz on an intracavity orotron-based spectrometer. Ten new lines, which correspond to rotational transitions and transitions to the bending vibration of the complex, are detected and identified as lines belonging to the R and P branches. The positions of the energy levels of isotopologue of the He-CO complex are refined by joint analysis of both the transition frequencies measured in this study and previously published microwave and infrared data. Isotopic dependences of the positions of the He-CO energy levels are obtained. These results can serve as the starting point for studying small helium clusters with embedded CO molecules.

Microwave spectroscopy of the weakly bound CO-ortho-D2 molecular complex

A van der Waals complex that consists of a deuterium molecule in the ortho state and a carbon monoxide molecule, CO-ortho-D2, was studied in the frequency range 85–130 GHz with the help of an intracavity orotron-based spectrometer. Nine new lines, which correspond to rotational transitions and belong to the R and Q branches, were measured and identified. The positions of the rotational energy levels of CO-ortho-D2 were refined by comparative analysis of the transition frequencies measured in this work and previously reported microwave and infrared data.

Rotational study of the CH4-CO complex: Millimeter-wave measurements and ab initio calculations.

The rotational spectrum of the van der Waals complex CH4-CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 110-145 GHz. Newly observed and assigned transitions belong to the K = 2-1 subband correlating with the rotationless jCH4 = 0 ground state and the K = 2-1 and K = 0-1 subbands correlating with the jCH4 = 2 excited state of free methane. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the CH4-CO complex. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of CH4-CO have been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations [CCSD(T)-F12a] and an augmented correlation-consistent triple zeta (aVTZ) basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the CH4 face closest to the CO subunit and binding energy De = 177.82 cm(-1). The bound rovibrational levels of the CH4-CO complex were calculated for total angular momentum J = 0-6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D0 are 91.32, 94.46, and 104.21 cm(-1) for A (jCH4 = 0), F (jCH4 = 1), and E (jCH4 = 2) nuclear spin modifications of CH4-CO, respectively.

Electronic energy transfer between dye molecules in structured solutions of H2O and D2O

The structure of H2O+D2O solutions was studied by correlation spectroscopy of scattered light. The correlation function and size of scatterers were both found to depend nonmonotonically on the D2O concentration in the H2O+D2O mixture. Processes of transfer of electronic excitation energy between dye molecules of different types in H2O+D2O solutions were studied. The efficiency of these processes was found to depend extremally on the concentration of the components of the solution. A fractal distribution of the interacting dye molecules is ascertained from the experimental data. The dependence of the fractal dimensionality of the dye solutions on the D2O concentration in the D2O+H2O mixture is determined.

The problem of the structure (state of helium) in small HeN-CO clusters

A second-order perturbation theory, developed for calculating the energy levels of the He-CO binary complex, is applied to small HeN-CO clusters with N = 2−4, the helium atoms being considered as a single bound object. The interaction potential between the CO molecule and HeN is represented as a linear expansion in Legendre polynomials, in which the free rotation limit is chosen as the zero approximation and the angular dependence of the interaction is considered as a small perturbation. By fitting calculated rotational transitions to experimental values it was possible to determine the optimal parameters of the potential and to achieve good agreement (to within less than 1%) between calculated and experimental energy levels. As a result, the shape of the angular anisotropy of the interaction potential is obtained for various clusters. It turns out that the minimum of the potential energy is smoothly shifted from an angle between the axes of the CO molecule and the cluster of θ = 100° in He-CO to θ = 180° (the oxygen end) in He3-CO and He4-CO clusters. Under the assumption that the distribution of helium atoms with respect to the cluster axis is cylindrically symmetric, the structure of the cluster can be represented as a pyramid with the CO molecule at the vertex.

Millimeter-wave spectroscopy of the weakly bound molecular complex NH3-N2

The rotational spectrum of the van der Waals NH3-N2 complex is studied in the frequency range of 112–130 GHz. The transitions are measured in a cold molecular beam with an intracavity spectrometer based on an orotron. A total of six new transitions of different spin modifications of the complex are recorded. Molecular parameters of the K = 0 ground state are determined for the orthoNH3-orthoN2 modification.

Cluster formation in dispersion systems: Oil and micellar systems

Solutions of hydrated reversed micelles and mixtures of distillate cracking residue and light catalytic gas oil are studied by the methods of correlation spectroscopy of scattered light and polarized fluorescence, respectively. The temperature dependences of the particle size of the disperse phase are obtained. It is found that the particle size increases with temperature in the range from room temperature to the critical temperature, which depends on the composition of the system under investigation, at temperatures above the critical temperature, the particle size decreases. These dependences can be explained by the formation of clusters from particles of the disperse phase at temperatures below the critical temperature and subsequent destruction of these clusters. Therefore, the structures of model (micellar) and natural (oil) disperse systems are compared for the first time, and the correlation between their behavior upon heating is revealed.

Association of rhodamine 6G in light and heavy water solutions

The absorption characteristics of rhodamine 6G molecules in light and heavy water solutions are studied. The association efficiencies of dye molecules and the structure and the binding energy of complexes formed are determined in relation to the solvent composition. The structure of H2O +D2O solutions is studied using correlation and low-frequency Raman spectroscopy. The distribution of dissolved complex molecules in the water matrix is shown to have inhomogeneities, which are responsible for a high association efficiency. The characteristic size of these inhomogeneities is estimated. The fractal dimension of the structural formations of light and heavy water molecules is determined as a function of their concentration.