Three-dimensional potential energy and dipole moment surfaces of the Cl−–H2 system are calculated ab initio by means of a coupled cluster method with single and double excitations and noniterative correction to triple excitations with augmented correlation consistent quadruple-zeta basis set supplemented with bond functions, and represented in analytical forms. Variational calculations of the energy levels up to the total angular momentum J=25 provide accurate estimations of the measured rotational spectroscopic constants of the ground van der Waals levels n=0 of the Cl−⋯H2/D2 complexes although they underestimate the red shifts of the mid-infrared spectra with v=0→v=1 vibrational excitation of the monomer. They also attest to the accuracy of effective radial interaction potentials extracted previously from experimental data using the rotational RKR procedure. Vibrational predissociation of the Cl−⋯H2/D2(v=1) complexes is shown to follow near-resonant vibrational-to-rotational energy transfer mechanism so that more than 97% of the product monomers are formed in the highest accessible rotational level. This mechanism explains the strong variation of the predissociation rate with isotopic content and nuclear spin form of the complex. Strong deviation of the observed relative abundances of ortho and para forms of the complexes from those of the monomers is qualitatively explained by the secondary ligand exchange reactions in the ionic beam, within the simple thermal equilibrium model. Positions and intensities of the hot v=0, n=1→v=1, n=1 and combination v=0, n=0→v=1, n=1 bands are predicted, and implications to the photoelectron spectroscopy of the complex are briefly discussed.
The infrared spectrum of the Al(+)-H(2) complex is recorded in the H-H stretch region (4075-4110 cm(-1)) by monitoring Al(+) photofragments. The H-H stretch band is centered at 4095.2 cm(-1), a shift of -66.0 cm(-1) from the Q(1)(0) transition of the free H(2) molecule. Altogether, 47 rovibrational transitions belonging to the parallel K(a)=0-0 and 1-1 subbands were identified and fitted using a Watson A-reduced Hamiltonian, yielding effective spectroscopic constants. The results suggest that Al(+)-H(2) has a T-shaped equilibrium configuration with the Al(+) ion attached to a slightly perturbed H(2) molecule, but that large-amplitude intermolecular vibrational motions significantly influence the rotational constants derived from an asymmetric rotor analysis. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 3.03 A, decreasing by 0.03 A when the H(2) subunit is vibrationally excited. A three-dimensional potential energy surface for Al(+)-H(2) is calculated ab initio using the coupled cluster CCSD(T) method and employed for variational calculations of the rovibrational energy levels and wave functions. Effective dissociation energies for Al(+)-H(2)(para) and Al(+)-H(2)(ortho) are predicted, respectively, to be 469.4 and 506.4 cm(-1), in good agreement with previous measurements. The calculations reproduce the experimental H-H stretch frequency to within 3.75 cm(-1), and the calculated B and C rotational constants to within approximately 2%. Agreement between experiment and theory supports both the accuracy of the ab initio potential energy surface and the interpretation of the measured spectrum.
We develop a simple analytical theory for the study of coherent control of radiationless transitions, and in particular, internal conversion leading to dissociation, in molecules possessing overlapping resonances. The method is applied to a model diatomic system. In contrast to previous studies, we consider here the control of a molecule that is allowed to decay during and after the preparation process. We use this theory to derive the shape of the laser pulse that creates the specific excited wave packet that best enhances or suppresses the radiationless transitions process. The results show the importance of resonance overlap in the molecule in order to achieve efficient coherent control over radiationless transitions via laser excitation. Specifically, resonance overlap is proven to be crucial in order to alter interference contributions to the controlled observable, and hence to achieve efficient coherent control by varying the phase of the laser field.
High-quality, ab initio potential energy functions are obtained for the interaction of bromine atoms and anions with atoms of the six rare gases (Rg) from He to Rn. The potentials of the nonrelativistic (2)Sigma(+) and (2)Pi electronic states arising from the ground-state Br((2)P)-Rg interactions are computed over a wide range of internuclear separations using a spin-restricted version of the coupled cluster method with single and double excitations and noniterative correction to triple excitations [RCCSD(T)] with an extrapolation to the complete basis set limit, from basis sets of d-aug-cc-pVQZ and d-aug-cc-pV5Z quality. These are compared with potentials derived previously from experimental measurements and ab initio calculations. The same approach is used also to refine the potentials of the Br(-)-Rg anions obtained previously [Buchachenko et al., J. Chem. Phys. 125, 064305 (2006)]. Spin-orbit coupling in the neutral species is included both ab initio and via an atomic approximation; deviations between two approaches that are large enough to affect the results significantly are observed only in the Br-Xe and Br-Rn systems. The resulting relativistic potentials are used to compute anion zero electron kinetic energy photoelectron spectra, differential scattering cross sections, and the transport coefficients of trace amounts of both anionic and neutral bromine in the rare gases. Comparison with available experimental data for all systems considered proves a very high precision of the present potentials.
Molecular excitation with incoherent light is examined using realistic turn-on time scales, and results are compared to those obtained via commonly used sudden turn-on, or pulses. Two significant results are obtained. First, in contrast to prior studies involving sudden turn-on, realistic turn-on is shown to lead to stationary coherences for natural turn-on time scales. Second, the time to reach the final stationary mixed state, known to result from incoherent excitation, is shown to depend directly on the inverse of the molecular energy level spacings, in both sudden and realistic turn-on cases. The S0 → S2/S1 internal conversion process in pyrazine is used as an example throughout. Implications for studies of natural light harvesting systems are noted.
A theoretical description of coherent control of excited state dynamics using pulse trains in the perturbative regime, as carried out in recent experiments, is presented. Analytical expressions relating the excited state populations to the pulse train control parameters are derived. Numerical examples are provided for models of pyrazine and β-carotene, and the significant role of overlapping resonances is exposed.
Natural hydrogen gas is a mixture of molecules in two distinct forms-ortho-and para-hydrogen-corresponding to the different total spin of the nuclei. [1,2] Physical and chemical properties of ortho-and para-hydrogen are different and the development of efficient techniques for separation or interconversion of the hydrogen spin-isomers is important for hydrogen gas production and storage technologies with particular relevance on the eve of the hydrogen fuel era; [2, 3] it is also essential for understanding the chemistry of the Universe [4] and critical for fundamental studies of low-temperature physics.[1] The lowerenergy para-form can be obtained by cryogenic distillation through ortho-para conversion, [5] whereas a separation technique is used to enrich the gas to obtain the more energetic ortho-form. Conventionally, the separation of the hydrogen spin-isomers (as well as the isotopes) is achieved by differential adsorption on surfaces [6,7] or by sieving in nanoporous materials. [8,9] The fractionation of the spin-isomers is known to occur due to the difference in their adsorption heats arising mostly from hindrance of the rotational motion of adsorbed or confined molecules [6,7, 10] as a result of the anisotropic interactions between the molecule and the adsorbent surface. These interactions are very complicated and are rarely known wellenough to allow for quantitative simulations of adsorption equilibria. Herein we suggest that similar separation of the spin-isomers may occur in the gas phase through the formation of van der Waals (vdW) complexes so the analysis of vdW complexes involving H 2 may elucidate the mechanisms of ortho--para separations on surfaces.The total nuclear spin of para-hydrogen, pH 2 , is zero, while the total nuclear spin of ortho-hydrogen, oH 2 , is one. Ortho-hydrogen may therefore exist in three nuclear spin states with different spin projections, whereas only one state is allowed for pH 2 . The corresponding statistical weights of the nuclear spin levels are g o = 3 and g p = 1 for oH 2 and pH 2 , respectively. The rotational angular momentum j of H 2 can be either even or odd depending on the parity of the total nuclear spin. The zero-point energy of oH 2 (j = 1) is therefore larger than that of pH 2 (j = 0) by 118.7 cm À1 (171 K). The natural ortho-para spin conversion is a very slow process and, despite the energetic difference, the relative abundance of the ortho-and para-isomers in natural gas remains close to the ratio of the statistical weights g o : g p = 3:1.[1]The gas of H 2 near an adsorbent surface is described by the equilibrium relations shown in Equations (1) and (2):and the enrichment of the gas phase by the ortho-isomer is described in Equation (3) in which the separation coefficient [7] is equal to the ratio of the equilibrium constants from Equations (1) and (2)where O and P denote the absolute partition functions Q(T) of ortho-and para-isomers in the gas phase and Õ and P are the same quantities for the adsorbed species. In a mixture of H 2 with a gas of inert ...
We develop a rigorous quantum mechanical theory for collisions of polyatomic molecular radicals with S-state atoms in the presence of an external magnetic field. The theory is based on a fully uncoupled space-fixed basis set representation of the multichannel scattering wave function. Explicit expressions are presented for the matrix elements of the scattering Hamiltonian for spin-1/2 and spin-1 polyatomic molecular radicals interacting with structureless targets. The theory is applied to calculate the cross sections and thermal rate constants for spin relaxation in low-temperature collisions of the prototypical organic molecule methylene [CH(2)(X(3)B(1))] with He atoms. To this end, two accurate three-dimensional potential energy surfaces (PESs) of the He-CH(2)(X(3)B(1)) complex are developed using the state-of-the-art coupled-cluster method including single and double excitations along with a perturbative correction for triple excitations and large basis sets. Both PESs exhibit shallow minima and are weakly anisotropic. Our calculations show that spin relaxation in collisions of CH(2), CHD, and CD(2) molecules with He atoms occurs at a much slower rate than elastic scattering over a large range of temperatures (1 μK-1 K) and magnetic fields (0.01-1 T), suggesting excellent prospects for cryogenic helium buffer-gas cooling of ground-state ortho-CH(2)(X(3)B(1)) molecules in a magnetic trap. Furthermore, we find that ortho-CH(2) undergoes collision-induced spin relaxation much more slowly than para-CH(2), which indicates that magnetic trapping can be used to separate nuclear spin isomers of open-shell polyatomic molecules.
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