Mode specific internal and direct rotational predissociation in HeHF, HeDF, and HeHCl: van der Waals complexes in the weak binding limit

Quantum study of the rovibrational relaxation of HF by collision with 4He on a new potential energy surface

The HF molecule is considered the main reservoir of fluorine in the interstellar medium (ISM). Also, the interactions of this molecule with the most common atoms and molecules in the ISM have attracted great interest from the astrochemical community. Collisions between HF and helium have recently caused controversy following a study using a two-dimensional SAPT potential energy surface (PES) that exhibited large discrepancies with previous scattering calculations based on more recent ab initio potentials. To address this issue, our current work aims to develop the most precise three-dimensional PES for the HF+He system. We employ the size-consistent CCSD(T) method in conjunction with the aug-cc-pV6Z basis set. The main features of the new PES as well as the bound states of the He-HF complex are compared to the existing data. The new PES is then utilised to conduct close coupling calculations that demonstrate He-HF as a good instance of vibration-rotation near resonant energy transfer. The novel rate coefficients will be accessible via the BASECOL database, and the use of the new PES is advised when describing HF in helium droplets.

High-resolution infrared spectroscopy of jet cooled cyclobutyl in the α-CH stretch region: large-amplitude puckering dynamics in a 4-membered ring radical

Gas-phase cyclobutyl radical (c-C4H7) is generated at a rotational temperature of Trot = 26(1) K in a slit-jet discharge mixture of 70% Ne/30% He and 0.5-0.6% cyclobromobutane (c-C4H7Br). A fully rovibrationally resolved absorption spectrum of the α-CH stretch fundamental band between 3062.9 cm-1 to 3075.7 cm-1 is obtained and analyzed, yielding first precision structural and dynamical information for this novel radical species. The α-CH stretch band origin is determined to be 3068.7887(4) cm-1, which implies only a modest (≈0.8 cm-1) blue shift from rotationally unresolved infrared spectroscopic studies of cyclobutyl radicals in liquid He droplets [A. R. Brown, P. R. Franke and G. E. Douberly, J. Phys. Chem. A, 2017, 121, 7576-7587]. Of particular dynamical interest, a one-dimensional potential energy surface with respect to the ring puckering coordinate is computed at CCSD(T)/ANO2 level of theory and reveals a double minimum Cs puckered geometry, separated by an exceedingly shallow planar C2v transition state barrier (Ebarr ≈ 1 cm-1). Numerical solutions on this double minimum potential yield a zero-point energy for the ground state (Ezero-point ≈ 27 cm-1) greatly in excess of the interconversion barrier. This is indicative of highly delocalized, large amplitude motion of the four-membered ring structure, for which proper vibrationally averaging of the moment of inertia tensor reproduces the experimentally determined inertial defect remarkably well. Finally, intensity alternation in the experimental spectrum due to nuclear spin statistics upon exchange of three indistinguishable H atom pairs (IH = ½) matches Ka + Kc = even : odd = 36 : 28 predictions, implying that the unpaired electron in the radical center lies in an out-of-plane pπ orbital. Thus, high-resolution infrared spectroscopy provides first experimental confirmation of a shallow double minimum ring puckering potential with a highly delocalized ground state wave function peaked at a planar C2v transition state geometry consistent with a cyclobutyl π radical.

Rotational-vibrational resonance states

Resonance states are characterized by an energy that is above the lowest dissociation threshold of the potential energy hypersurface of the system and thus resonances have finite lifetimes. All molecules possess a large number of long- and short-lived resonance (quasibound) states. A considerable number of rotational-vibrational resonance states are accessible not only via quantum-chemical computations but also by spectroscopic and scattering experiments. In a number of chemical applications, most prominently in spectroscopy and reaction dynamics, consideration of rotational-vibrational resonance states is becoming more and more common. There are different first-principles techniques to compute and rationalize rotational-vibrational resonance states: one can perform scattering calculations or one can arrive at rovibrational resonances using variational or variational-like techniques based on methods developed for determining bound eigenstates. The latter approaches can be based either on the Hermitian (L², square integrable) or non-Hermitian (non-L²) formalisms of quantum mechanics. This Perspective reviews the basic concepts related to and the relevance of shape and Feshbach-type rotational-vibrational resonance states, discusses theoretical methods and computational tools allowing their efficient determination, and shows numerical examples from the authors' previous studies on the identification and characterization of rotational-vibrational resonances of polyatomic molecular systems.

Attosecond timing of electron emission from a molecular shape resonance

Shape resonances in physics and chemistry arise from the spatial confinement of a particle by a potential barrier. In molecular photoionization, these barriers prevent the electron from escaping instantaneously, so that nuclei may move and modify the potential, thereby affecting the ionization process. By using an attosecond two-color interferometric approach in combination with high spectral resolution, we have captured the changes induced by the nuclear motion on the centrifugal barrier that sustains the well-known shape resonance in valence-ionized N2. We show that despite the nuclear motion altering the bond length by only 2%, which leads to tiny changes in the potential barrier, the corresponding change in the ionization time can be as large as 200 attoseconds. This result poses limits to the concept of instantaneous electronic transitions in molecules, which is at the basis of the Franck-Condon principle of molecular spectroscopy.

A unified theory of weak-field coherent control of the behavior of a resonance state

A unified weak-field control scheme to modify the two properties that determine the whole behavior of a resonance state, namely the lifetime and the asymptotic fragment distribution produced upon resonance decay, is proposed. Control is exerted through quantum interference induced between overlapping resonances of the system, by exciting two different energies at which the resonances overlap. The scheme applies a laser field consisting of a first pulse that excites the energy of the resonance to be controlled, and two additional pulses that excite another different energy to induce interference, with a delay time with respect to the first pulse. Each of the two additional pulses is used to control one of the two resonance properties, by adjusting its corresponding delay time: with a relatively short delay time the second pulse controls the resonance lifetime, while with a very long delay time the third pulse modifies the asymptotic fragment distribution produced. The efficiency of the control of each resonance property is found to be strongly dependent on the choice of the second interfering energy, which allows for a more flexible control optimization by choosing a different energy for each property. The theory underlying the interference mechanism of the control scheme is developed and presented, and is applied to analyze and explain the results obtained. The present scheme thus appears to be a useful tool for controlling resonance-mediated molecular processes.

Weak-Field Coherent Control of Molecular Photofragment State Distributions

It is known that the long-time energy-resolved photofragment state distribution produced upon photodissociation of a molecule cannot be modified in the weak-field limit for a fixed pump pulse spectral profile. This work, however, demonstrates both computationally and mathematically that the above limitation can be circumvented in practice when the molecule presents overlapping resonances. It is shown that when two or more energies where the resonances overlap are excited by different laser pulses delayed in time, interference is induced between the product fragment states associated with the different energies populated. The occurrence of interference is found to be independent of the delay time between the pulses exciting the different energies. Thus, as demonstrated, this finding makes it possible to modify the fragment distribution at a given energy, as far in time and as many times as desired.

Nuclear spin/parity dependent spectroscopy and predissociation dynamics in vOH = 2 ← 0 overtone excited Ne-H2O clusters: Theory and experiment

Vibrationally state selective overtone spectroscopy and state- and nuclear spin-dependent predissociation dynamics of weakly bound ortho- and para-Ne-H₂O complexes (D_0(ortho) = 34.66 cm^-1 and D_0(para) = 31.67 cm^-1) are reported, based on near-infrared excitation of van der Waals cluster bands correlating with v_OH = 2 ← 0 overtone transitions (|02^-〉 and |02⁺〉) out of the ortho (1₀₁) and para (0₀₀) internal rotor states of the H₂O moiety. Quantum theoretical calculations for nuclear motion on a high level potential energy surface [CCSD(T)/VnZf12 (n = 3, 4)], corrected for basis set superposition error and extrapolated to the complete basis set (CBS) limit, are employed to successfully predict and assign Π-Σ, Σ-Σ, and Σ-Π infrared bands in the spectra, where Σ or Π represent approximate projections of the body-fixed H₂O angular momentum along the Ne-H₂O internuclear axis. IR-UV pump-probe experimental capabilities permit real-time measurements of the vibrational predissociation dynamics, which indicate facile intramolecular vibrational energy transfer from the H₂O v_OH = 2 overtone vibrations into the VdWs (van der Waals) dissociation coordinate on the τ_prediss = 15-25 ns time scale. Whereas all predicted strong transitions in the ortho-Ne-H₂O complexes are readily detected and assigned, vibrationally mediated photolysis spectra for the corresponding para-Ne-H₂O bands are surprisingly absent despite ab initio predictions of Q-branch intensities with S/N > 20-40. Such behavior signals the presence of highly selective nuclear spin ortho-para predissociation dynamics in the upper state, for which we offer a simple mechanism based on Ne-atom mediated intramolecular vibrational relaxation in the H₂O subunit (i.e., |02^±〉 → {|01^±〉; v₂ = 2}), which is confirmed by the ab initio energy level predictions and the nascent OH rotational (N), spin orbit (Π_1/2,_3/2), and lambda doublet product distributions.

Unravelling the mechanisms of interference between overlapping resonances

The enhancement of the resonance lifetime that occurs upon interference of two overlapping resonances excited coherently by two pulses with delayed time has been investigated as a function of the pulse temporal width and the delay time between the pulses. A general law predicting quantitatively the optimal delay time that maximizes the lifetime enhancement of the two resonances has been established in terms of the pulse width and of the lifetimes of both resonances when they are excited isolatedly. The specific form of the law and all the results found can be closely related to the characteristic features of the mechanism of interference between the overlapping resonances, providing a detailed understanding on how the mechanism operates. The proposed law is envisioned as a useful tool to design experimental strategies to control the resonance lifetime.

The structure of a resonance state

The existence of a structure in a resonance state is systematically investigated. A resonance structure is defined as the energy dependence across the resonance width of the fragment state distributions produced upon resonance decay. Different types of resonances, both isolated and overlapping ones, have been explored for this purpose. It is found that isolated resonances do not present an appreciable energy dependence on the product state distributions. On the contrary, overlapping resonances exhibit a clear structure regarding the fragment distributions, which becomes increasingly more pronounced as the intensity of the overlap between the resonances increases. Such an energy dependence of the product distributions arises from the quantum interference between the amplitudes of the overlapping resonances, as demonstrated formally here by the equations derived from the condition of resonance overlap. The application of the present effect to the control of the fragment state distributions produced in a wide variety of molecular processes governed by resonance states is envisioned.

Sub-Doppler spectroscopy of the trans-HOCO radical in the OH stretching mode

Rovibrational spectroscopy of the fundamental OH stretching mode of the trans-HOCO radical has been studied via sub-Doppler high-resolution infrared laser absorption in a discharge slit-jet expansion. The trans-HOCO radical is formed by discharge dissociation of H2O to form OH, which then combines with CO and cools in the Ne expansion to a rotational temperature of 13.0(6) K. Rigorous assignment of both a-type and b-type spectral transitions is made possible by two-line combination differences from microwave studies, with full rovibrational analysis of the spectrum based on a Watson asymmetric top Hamiltonian. Additionally, fine structure splittings of each line due to electron spin are completely resolved, thus permitting all three ε(aa), ε(bb), ε(cc) spin-rotation constants to be experimentally determined in the vibrationally excited state. Furthermore, as both a- and b-type transitions for trans-HOCO are observed for the first time, the ratio of transition dipole moment projections along the a and b principal axes is determined to be μ(a)/μ(b) = 1.78(5), which is in close agreement with density functional quantum theoretical predictions (B3LYP/6-311++g(3df,3pd), μ(a)/μ(b) = 1.85). Finally, we note the energetic possibility in the excited OH stretch state for predissociation dynamics (i.e., trans-HOCO → H + CO2), with the present sub-Doppler line widths providing a rigorous upper limit of >2.7 ns for the predissociation lifetime.

Defining the hydrogen bond: An account (IUPAC Technical Report)

The term “hydrogen bond” has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material scientists, there has been a continual debate about what this term means. This debate has intensified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following X–H···Y hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some characteristics that are observed in typical hydrogen-bonding environments.

Probing the dependence of long-range, four-atom interactions on intermolecular orientation: 2. Molecular deuterium and iodine monochloride

Laser-induced fluorescence and action spectroscopy experiments have identified multiple conformers of the D2...ICl van der Waals complex for both ortho-D2 (o-D2) and para-D2 (p-D2). As with the analogous H2...ICl van der Waals complexes [Darr, J. P.; Crowther, A. C.; Loomis, R. A.; Ray, S. E.; McCoy, A. B. J. Phys. Chem. A 2007, 111, 13387], the C2v conformer with the deuterium molecule localized at the iodine atom end of the dihalogen is significantly more stable than the asymmetric conformer that has the deuterium positioned orthogonally to the ICl bond axis, D0'' = 223.9(2.4) versus 97.3(8)-103.9(3) cm(-1) for p-D2...I(35)Cl(X, v''=0). For both conformers, complexes containing p-D2 are found to be more strongly bound than those with o-D2. The electronically excited D2...ICl(A, v') and D2...ICl(B, v') complexes are found to have equilibrium geometries that are nearly the same as those of the ground-state asymmetric structures. Calculated D2...ICl(B, v'=3) energies and probability amplitudes obtained using a simple scaled He + ICl(B, v'=3) potential provide clues to the nature of the different excited-state levels accessed.

Intermolecular potential energy surface and spectra of He–HCl with generalization to other rare gas–hydrogen halide complexes

A two-dimensional (rigid monomer) intermolecular potential energy surface (PES) of the He-HCl complex has been obtained from ab initio calculations utilizing the symmetry-adapted perturbation theory (SAPT) and an spdfg basis set including midbond functions. The bond length in HCl was chosen to be equal to the expectation value in the ground vibrational state of isolated HCl. The rigid-monomer potential should be a very good approximation to the complete (three-dimensional) potential for H-Cl distances corresponding to the lowest vibrational levels of the monomer since the He-HCl interaction energy was found to be only weakly dependent on the HCl bond length in this region, at least as compared to systems such as Ar-HF. The calculated points were fitted using an analytic function with ab initio computed asymptotic coefficients. As expected, the complex is loosely bound, with the dispersion energy providing the majority of the attraction. Our SAPT PES agrees with the semiempirical PES of Willey et al. [J. Chem. Phys. 96, 898 (1992)], in finding that, atypically for rare gas-hydrogen halide complexes including the lighter halide atoms, the global minimum is on the Cl side (with intermonomer separation 3.35 A and depth of 32.8 cm(-1)), rather than on the H side, where there is only a local minimum (3.85 A, 30.8 cm(-1)). The ordering of the minima was confirmed by single-point calculations in larger basis sets and complete basis set extrapolations, and also using higher levels of theory. We show that the opposite findings in the recent calculations of Zhang and Shi [J. Mol. Struct: THEOCHEM 589, 89 (2002)] are due to the lack of midbond functions in their basis set. Despite the closeness in depth of the two linear minima, the existence of a relatively high barrier between them invalidates the assumption of isotropy, a feature of some literature potentials. The trends concerning the locations of minima within the family of rare gas-hydrogen halide complexes are rationalized in terms of the physical components of the intermolecular forces and related to monomer properties. The accuracy of the SAPT PES was tested by performing calculations of rovibrational levels. The transition frequencies obtained were found to be in excellent agreement (to within 0.02 cm(-1)) with the measurements of Lovejoy and Nesbitt [J. Chem. Phys. 93, 5387 (1990)]. The SAPT PES predicts a dissociation energy for the complex of 7.74 cm(-1) which is probably more accurate than the experimental value of 10.1+/-1.2 cm(-1). Our analysis of the ground-state rovibrational wave function shows that the He-HCl configuration is favored over the He-ClH configuration despite the ordering of minima. This is due to the greater volume of the well in the former case. We have also determined positions and widths of three low-lying resonance states through scattering calculations. These predictions are expected to be more accurate than values derived from experiment.

Theoretical study of the He-HF+ complex. I. The two asymptotically degenerate ground state potential energy surfaces

Two three-dimensional potential energy surfaces (PESs) are reported for the cationic complex He-HF+; they are degenerate for linear geometries of the complex and correlate with the doubly degenerate X2Pi ground state of the HF+monomer. The PESs are computed from the interaction energies of the neutral dimer and the ionization potentials of the He-HF complex and the HF molecule. Ionization potentials are obtained from the outer valence Green's function (OVGF) method, while the energies of the neutral species are computed by means of the single and double coupled-cluster method with perturbative triples [CCSD(T)]. For comparison, interaction energies of the ionic complex were computed also by the use of the partially spin-restricted variant of the CCSD(T) method. After asymptotic scaling of the OVGF results, good agreement is found between the two methods. A single global minimum is found in the PES, for the linear He-HF+ geometry. The well depth and equilibrium separation are 2.240 A and 1631.3 cm(-1), respectively, at an HF+ bond length r=1.0012 A, in rather good agreement with results of Schmelz and Rosmus [Chem. Phys. Lett. 220, 117 (1994)]. The well depth depends much more strongly on the internuclear H-F separation than in the neutral He-HF complex and the global minimum in a full three-dimensional PES occurs at r=1.0273 A.

Bound state spectroscopy of NH–He

The NH-He van der Waals complex was characterized via laser excitation of bands associated with the NH A (3)Pi-X (3)Sigma(-) transition. It was demonstrated that the ground state supports a bound level with a rotational constant of B"=0.334(2) cm(-1). These results are in agreement with the predictions of recent high-level theoretical calculations. Spin-orbit predissociation of the excited complex was observed, and the spectra yield insights regarding the NH(A)+He potential energy surfaces.

HF dimer in small helium clusters: interchange-tunneling dynamics in a quantum environment

We present diffusion quantum Monte Carlo calculations of the interchange tunneling splitting of (4)He(n)(HF)(2) clusters, n = 1-10. The tunneling splitting decreases rapidly for n = 1-4 clusters, and much more slowly for n>4. The decrease calculated for (4)He(n)(HF)(2) represents 74% of the reduction in the tunneling splitting measured recently for HF dimer in nanodroplets of more than 2000 He atoms. The first four He atoms quench the interchange tunneling very efficiently by virtue of occupying the equatorial ring which encircles the C(2h) transition state of the tunneling pathway.