Collision-induced Transitions Research Articles

Collision-induced Hopf-type bifurcation reversible transitions in a dual-wavelength femtosecond fiber laser.

Collisions refer to a striking nonlinear interaction process in dissipative systems, revealing the particle-like properties of solitons. In dual-wavelength mode-locked fiber lasers, collisions are inherent and periodic. However, how collisions influence the dynamical transitions in the dual-wavelength mode-locked state has not yet been explored. In our work, dispersion management triggers the complex interactions between solitons in the cavity. We reveal the smooth or Hopf-type bifurcation reversible transitions of dual-color soliton molecules (SMs) during the collision by the real-time spectral measurement technique of time-stretch Fourier transform. The reversible transitions between stationary SMs and vibrating SMs, reveal that the cavity parameters pass through a bifurcation point in the collision process without active external intervention. The numerical results confirm the universality of collision-induced bifurcation behavior. These findings provide new insights into collision dynamics in dual-wavelength ultrafast fiber lasers. Furthermore, the study of inter-molecular collisions is of great significance for other branches of nonlinear science.

Collisional excitation of hyperfine levels of OH by hydrogen atoms

ABSTRACT Observations of transitions between the hyperfine levels of the hydroxyl radical (OH) can provide crucial information on the physical conditions in interstellar clouds. Accurate modelling of the spectra requires calculated rate coefficients for the excitation of OH by H atoms, which is often present in molecular clouds in addition to the dominant H2 molecule. In this work, rate coefficients for the (de-)excitation of hyperfine levels of OH through collisions with hydrogen atoms are presented. In previous work, nuclear-spin-free scattering calculations were carried out; these took account of the fact that four electronic states (1A′, 1A″, 3A′, and 3A″) arise from the interaction of OH(X2Π) with H(2S). Because of the deep H2O($\tilde{X}^1A^{\prime }$) well, inelastic transitions can occur through direct collisions or by formation and decay of a collision complex. The rates of collision-induced hyperfine transitions were computed by the recoupling method and the MJ randomization approximations, respectively. These data will be useful in astrophysical models of OH excitation.

Collisional excitation of isotopologues of carbon monoxide by molecular hydrogen

ABSTRACTRate coefficients for transitions between the 14 lowest rotational levels, having energies less than 500 K, of the most abundant isotopologue of carbon monoxide,12C16O, caused by collisions with para-H2 and ortho-H2 are reported for temperatures between 5 and 400 K. These data were computed through time-independent close-coupled scattering calculations employing the high-quality potential energy surface (PES) recently calculated by Faure et al. Rate coefficients for collision-induced transitions are required for radiative transfer modelling of molecules present in interstellar clouds under non-equilibrium conditions. In addition, rate coefficients were computed for other carbon monoxide isotopologues, 13C16O, 12C17O, and 12C18O. The PESs for these systems were obtained by transforming the PES for 12C16O–H2 PES. The rotational transitions of 12C17O isotopologue often display resolved hyperfine structure in astronomical spectra. Rate coefficients for collision-induced transitions between the rotational–hyperfine levels of this isotopologue have been computed by the recoupling technique. A simple radiative transfer calculation was performed to illustrate the utility of using such an approach to relate observed intensities in astronomical spectra with molecular abundances.

Simultaneous collision-induced transitions in H2O+CO2 gas mixtures

A collision induced absorption (CIA) band has been measured near 6000 cm−1 in a spectrum of humidified CO2 recorded by cavity ring down spectroscopy (CRDS) at low pressure (<1 atm). This absorption adds a contribution to the H2O+CO2 continuum mostly originating from far wings of the CO2 and H2O resonance lines broadened by collisions with H2O and with CO2, respectively. The observed CIA corresponds to a simultaneous excitation of 12CO2 and H2O colliding molecules in the ν3 antisymmetric and ν1 symmetric stretching mode, respectively. CRDS spectra recorded near 5940 cm−1 with a highly enriched 13CO2 sample provide a confirmation of the assignment since the measured CIA isotopic spectral shift (of about −68 cm−1) coincides with that between the ν3 bands of 12CO2 and 13CO2. The integrated binary coefficient of the two CIA is evaluated and found to be on the order of 2.3 × 10−3 cm−2amagat−2. Classical molecular dynamics simulations (CMDS) of the considered CIA are also presented, based on the dominant dipole induction mechanism associated with the vibrational matrix elements of the dipole of CO2 (ν3) and isotropic polarizability of H2O (ν1). The results of the calculations are found in good agreement with the observations, thus further validating the attribution of the observed CIA structure to the above mentioned double transitions.

The interaction of methylene with molecular hydrogen: potential energy surface and inelastic collisions

ABSTRACT The potential energy surface describing the interaction of methylene (CH) in its ground electronic state with molecular hydrogen has been calculated through explicitly correlated coupled cluster theory with inclusion of single, double, and (perturbatively) triple excitations [RCCSD(T)-F12a] and a correlation-consistent aug-cc-pVTZ basis. The calculated points for various geometries of the CH–H complex were fit to a functional form suitable for use in time-independent quantum scattering calculations of cross sections for collision-induced transitions between CH rotational levels. The well depth of the CH–H complex was found to equal 92.1 cm in a planar geometry and intermolecular separation . A representative set of cross sections is presented and compared with cross sections for the corresponding transitions in methylene induced by collisions with helium.

Soft landing of metal clusters on graphite: a molecular dynamics study

Structure and stability of nanometer-sized Ag887, Au887 and Ti787 clusters soft-landed on graphite (at deposition energies Edep = 0.001 − 5.0 eV per atom) are studied by means of molecular dynamics simulations. Parameters for the cluster–surface interactions are derived from complementary ab initio calculations. The shape and the contact angle of deposited clusters are systematically analyzed for different deposition energies and temperature regimes. The Ag887 cluster deposited at Edep ≲ 0.1 eV/atom undergoes collision-induced plastic deformation, thus acquiring an ellipsoidal shape with the contact angle close to 180°. In contrast, Au887 and Ti787 clusters undergo a collision-induced melting phase transition followed by their recrystallization; these processes lead to the formation of the droplet-like shapes of the clusters in a form of truncated spheroids. At larger deposition energies all clusters flatten over the surface and eventually disintegrate at Edep ≈ 0.75 − 1.0 eV/atom (for Ag887 and Au887) and ≈3 eV/atom (for Ti787). It is found also that the shape of deposited clusters is strongly influenced by the strength of cluster–substrate interaction and the corresponding interaction mechanism, namely the weak van der Waals interaction between metal and carbon atoms or the van der Waals interaction with an onset of covalent bonding. Similar phenomena should arise in the deposition of clusters made of other elements, which interact with a substrate by one of the above-described mechanisms.Graphical abstract

Quantum dynamics of O17 in collision with ortho- and para- O17O17

We report full quantum scattering cross sections for the peculiar $^{17}\mathrm{O}+^{17}\mathrm{O}^{17}\mathrm{O}$ system, at relatively low collision energies. We consider different types of collision-induced transitions, as the indistinguishability of the three nuclei allows for the mixing of reactive, inelastic, and elastic processes. Furthermore, due to the nonzero nuclear spin of $^{17}\mathrm{O}$ and the existence of nuclear spin isomers ortho- and para-${\mathrm{O}}_{2}$, we pay particular attention to transitions between these two species, that is, the ortho-para conversion process. We find that the corresponding cross section has a magnitude comparable to that of the ${\mathrm{H}}^{+}+{\mathrm{H}}_{2}$ counterpart.

The effect of CO−H2O collisions in the rotational excitation of cometary CO

ABSTRACT We present the first accurate rate coefficients for the rotational excitation of CO by H2O in the kinetic temperature range 5–100 K. The statistical adiabatic channel method (SACM) is combined with a high-level rigid-rotor CO−H2O intermolecular potential energy surface. Transitions among the first 11 rotational levels of CO and the first 8 rotational levels of both para-H2O and ortho-H2O are considered. Our rate coefficients are compared to previous data from the literature and they are also incorporated in a simple non-LTE model of cometary coma including collision-induced transitions, solar radiative pumping and radiative decay. We find that the uncertainties in the collision data have significant influence on the CO population distribution for H2O densities in the range 103–108 cm−3. We also show that the rotational distribution of H2O plays an important role in CO excitation (owing to correlated energy transfer in both CO and H2O), while the impact of the ortho-to-para ratio of H2O is found to be negligible.

Theoretical investigation of rotationally inelastic collisions of CH(X2Π) with hydrogen atoms.

We report calculations of state-to-state cross sections for collision-induced rotational transitions of CH(X2Π) with atomic hydrogen. These calculations employed the four adiabatic potential energy surfaces correlating CH(X2Π) + H(2S), computed in this work through the multi-reference configuration interaction method [MRCISD + Q(Davidson)]. Because of the presence of deep wells on three of the potential energy surfaces, the scattering calculations were carried out using the quantum statistical method of Manolopoulos and co-workers [Chem. Phys. Lett. 343, 356 (2001)]. The computed cross sections included contributions from only direct scattering since the CH2 collision complex is expected to decay predominantly to C + H2. Rotationally energy transfer rate constants were computed for this system since these are required for astrophysical modeling.

Theoretical investigation of rotationally inelastic collisions of CH(X2Π) with molecular hydrogen.

We report calculations of state-to-state cross sections for collision-induced rotational transitions of CH(X2Π) with molecular hydrogen. These calculations employed the diabatic matrix elements of the interaction potential determined by Dagdigian [J. Chem. Phys. 145, 114301 (2016)], which employed the multi-reference configuration-interaction method [MRCISD+Q(Davidson)]. Because of the presence of a deep well on the lower potential energy surface, the scattering calculations were carried out using the quantum statistical method of Manolopoulos and co-workers [Chem. Phys. Lett. 343, 356 (2001)]. The computed cross sections included contributions from direct scattering, as well as from the formation and decay of a collision complex. The magnitude of latter contribution was found to decrease significantly with increasing collision energy. Rotationally energy transfer rate constants were computed for this system since these are required for astrochemical modeling.

Non-adiabatic transitions between first tier ion-pair states of I2 induced by collisions with Ar atoms

Non-adiabatic transitions from the I2(E0g+, D0u+, β1g and D′2g) states to first tier ion-pair (IP) states induced by collisions with Ar atoms have been investigated. The stepwise laser excitation schemes have been used for selective population of close-lying vibronic states: E, vE=13, D, vD=18, β, vβ=19 and D′2g, vD′=23. Rate constants of population of the first tier ion-pair states from the optically populated rovibronic states as well as rate constants of depopulation of the states have been determined. Analysis of the mechanism of the transitions has been carried out.We find that the λlummax≈3410Å luminescence band in the case of optical population of the D and β states cannot be described assuming the collision-induced non-adiabatic transitions only. We conclude that there is an additional radiative channel for population of the states, probably, due to the formation of the ArI2(IP) van der Waals complexes.

Communication: Rotational excitation of HCl by H: Rigid rotor vs. reactive approaches.

We report fully quantum time-independent calculations of cross sections for the collisional excitation of HCl by H, an astrophysically relevant process. Our calculations are based on the Bian-Werner ClH2 potential energy surface and include the possibility of HCl destruction through reactive collisions. The strongest collision-induced rotational HCl transitions are those with Δj = 1, and the magnitude of the HCl-H inelastic cross sections is of the same order of magnitude as the HCl-H2 ones. Results of exact calculations, i.e., including the reactive channels, are compared to pure inelastic calculations based on the rigid rotor approximation. A very good agreement is found between the two approaches over the whole energy range 10-3000 cm(-1). At the highest collisional energies, where the reaction takes place, the rigid rotor approach slightly overestimates the cross sections, as expected. Hence, the rigid rotor approach is found to be reliable at interstellar temperatures.

Predictive a priori pressure-dependent kinetics.

The ability to predict the pressure dependence of chemical reaction rates would be a great boon to kinetic modeling of processes such as combustion and atmospheric chemistry. This pressure dependence is intimately related to the rate of collision-induced transitions in energy E and angular momentum J. We present a scheme for predicting this pressure dependence based on coupling trajectory-based determinations of moments of the E,J-resolved collisional transfer rates with the two-dimensional master equation. This completely a priori procedure provides a means for proceeding beyond the empiricism of prior work. The requisite microcanonical dissociation rates are obtained from ab initio transition state theory. Predictions for the CH4 = CH3 + H and C2H3 = C2H2 + H reaction systems are in excellent agreement with experiment.

Highly Correlated Electronic Structure Calculations of the He–C3 van der Waals Complex and Collision-Induced Rotational Transitions of C3

An accurate 2D ab initio potential energy surface of the He-C3 collisional system is calculated using the supermolecular coupled-cluster method with up to perturbative quadruple excitations, CCSDT(Q). This interaction potential is then incorporated in full close-coupling calculations of rotational excitation/de-excitation cross sections in He + C3 collisions for rotational levels j = 0, 2, ..., 10 and collision energies up to 1000 cm(-1). Corresponding rate coefficients are reported for temperature between 1 and 100 K. Results are found to be in excellent agreement with available theoretical data that were restricted to the temperature range of 5-15 K. Implications of the computed rate coefficients to astrophysical models of C3 and carbon clusters in interstellar and circumstellar environments are discussed.

Near-resonant rotational energy transfer in HCl–H2 inelastic collisions

We present a new four-dimensional (4D) potential energy surface for the HCl-H2 van der Waals system. Both molecules were treated as rigid rotors. Potential energy surface was obtained from electronic structure calculations using a coupled cluster with single, double, and perturbative triple excitations method. The four atoms were described using the augmented correlation-consistent quadruple zeta basis set and bond functions were placed at mid-distance between the HCl and H2 centers of mass for a better description of the van der Waals interaction. The global minimum is characterized by the well depth of 213.38 cm(-1) corresponding to the T-shape structure with H2 molecule on the H side of the HCl molecule. The dissociation energies D0 are 34.7 cm(-1) and 42.3 cm(-1) for the complex with para- and ortho-H2, respectively. These theoretical results obtained using our new PES are in good agreement with experimental values [D. T. Anderson, M. Schuder, and D. J. Nesbitt, Chem. Phys. 239, 253 (1998)]. Close coupling calculations of the inelastic integral rotational cross sections of HCl in collisions with para-H2 and ortho-H2 were performed at low and intermediate collisional energies. Significant differences exist between para- and ortho-H2 results. The strongest collision-induced rotational HCl transitions are the transitions with Δj = 1 for collisions with both para-H2 and ortho-H2. Rotational relaxation of HCl in collision with para-H2 in the rotationally excited states j = 2 is dominated by near-resonant energy transfer.

Non-adiabatic transitions from I2(${E} {\rm 0}_{g}^{+} $Eg+ and ${D}0_{u}^{+} $Du+) states induced by collisions with M = I2(${X}0_{g}^ + $Xg+) and H2O

The stepwise two-step two-color and three-step three-color laser excitation schemes are used for selective population of rovibronic levels of the first-tier ion-pair E0(g)(+) and D0(u)(+) states of molecular iodine and studies of non-adiabatic transitions to the D and E states induced by collisions with M = I(2)(X) and H(2)O. Collection and analysis of the luminescence after excitation of the v(E) = 8, 13 and v(D) = 13, 18 vibronic levels of the E and D states in the pure iodine vapor and the gas-phase mixtures with H(2)O provide rate constants for the non-adiabatic transitions to the D and E state induced by collisions with these molecules. Vibrational distributions for the [formula: see text] collision-induced non-adiabatic transitions (CINATs) are obtained. Rather strong λ(lum)(max) ≈ 3400 Å luminescence band is observed in the I(2) + H(2)O mixtures, whereas its intensity is ~100 times less in pure iodine vapor. Radiative lifetimes and quenching rate constants of the I(2)(E,v(E) = 8, 13 and D,v(D) = 13, 18) vibronic state are also determined. Rate constants of the [formula: see text], v(E) = 8-54, CINATs are measured again and compared with those obtained earlier. New data confirm resonance characters of the CINATs found in our laboratory about 10 years ago. Possible reasons of differences between rate constant values obtained in this and earlier works are discussed. It is shown, in particular, that differences in rate constants of non-resonant CINATs are due to admixture of water vapor in iodine.

Rotational Energy Transfer of SH(X2Π, v′′=0, J′′=0.5-10.5) by Collision with Ar: Λ-Doublet Resolved Transition Propensity

The behavior of Λ-doublet resolved rotational energy transfer (RET) by Ar collisions within the SH(X(2)Π, v''=0) state is characterized. The matrix elements of terms in the interaction potential responsible for interference effects are calculated to explain the propensity rules for collision-induced transitions within and between spin-orbit manifolds. In this manner, the physical mechanisms responsible for the F(1)-F(1), F(2)-F(2), and F(1)-F(2) transitions may be reasonably identified. As collision energy increases, the propensity for collisional population of the final e or f level is replaced by the e/f-conserving propensity. Such a change in propensity rule can be predicted in terms of energy sudden approximation at high J limit for the pure Hund's case scheme.

Theoretical investigation of rotationally inelastic collisions of the methyl radical with helium

Rotationally inelastic collisions of the CH(3) molecule in its ground X(2)A(2)'' electronic state have been investigated. We have determined a potential energy surface (PES) for the interaction of rigid CH(3), frozen at its equilibrium geometry, with a helium atom, using a coupled-cluster method that includes all single and double excitations, as well as perturbative contributions of connected triple excitations [RCCSD(T)]. The anisotropy of the PES is dominated by repulsion of the helium by the hydrogen atoms. The dissociation energy D(e) was computed to equal 27.0 cm(-1). At the global minimum, the helium atom lies in the CH(3) plane between two C-H bonds at an atom-molecule separation R = 6.52 bohr. Cross sections for collision-induced rotational transitions have been determined through quantum scattering calculations for both nuclear spin modifications. Rotationally inelastic collisions can cause a change in the rotational angular momentum n and its body-frame projection k. Because of the anisotropy of the PES due to the hydrogen atoms, there is a strong propensity for Δk = ±3 transitions. Thermal rate constants for state-specific total collisional removal have also been determined.

The rotational excitation of the interstellar HNC by para- and ortho-H2

Rotational excitation of the interstellar HNC due to collisions with H(2) is investigated. We present a new four dimensional (4D) potential energy surface for the HNC-H(2) collisional system. Both molecules were treated as rigid rotors. Interaction energy was obtained from the electronic structure calculations using a single and double-excitation coupled cluster method with perturbative contributions from connected triple excitations [CCSD(T)]. The five atoms were described using the aug-cc-pVTZ basis sets. Bond functions were placed at mid-distance between the HNC center of mass and the center of mass of H(2) for a better description of the van der Waals interaction. Close coupling calculations of the inelastic integral cross sections of HNC in collisions with para-H(2) and ortho-H(2) were calculated for kinetic energies up to 800 cm(-1). After Boltzmann thermal averaging, rate coefficients were obtained for temperatures ranging from 5 to 100 K. Significant differences exist between para- and ortho-H(2) results. The strongest collision-induced rotational HNC transitions are the transitions with Δj = 1 for collisions with para-H(2) and with ortho-H(2). The new rate coefficients should induce important consequences on the determination of HNC abundance in the interstellar medium. In particular, we expect that they will help to solve the interstellar problem of relative abundance of the HCN and HNC isomers.

Dynamics and mechanism of the non-adiabatic transitions from the ungerade I2(D0+u) state induced by collisions with rare gas atoms

The stepwise three-photon two-color laser excitation scheme is used for selective population of the first-tier ion-pair D0(+)(u) state of molecular iodine. Collection and analysis of the luminescence after the excitation of the v(D) = 6, 8, 13 and 18 vibronic levels of the D state in the pure iodine vapor and the gas-phase mixtures with He, Ar and Xe provide the total and, whenever possible, partial rate constants for the collision-induced non-adiabatic transitions to the other ion-pair states of the first tier. Comparison with the analogous data obtained previously for the non-adiabatic transitions from the E0(+)(g) state reveals the similarity between two cases. For He, the D ↔ E transitions are preferable, whereas for Ar and Xe transitions to the D' and β states dominate at v(D) = 6, 8 and 13, in accord with the statistical considerations. Efficient population of the δ state at v(D) = 18 in Ar and Xe is the most prominent non-statistical feature observed. The vibrational product state distributions for the D → E transitions are also obtained. In contrast to the previously studied E → D transition, they show significant positive vibronic energy transfer. The measurements for He and Ar are accompanied by the quantum scattering calculations that reproduce well the main qualitative features of the experimental results.