Collisional Energy Transfer Research Articles

Investigation of the generation of Xe 3680 nm ASE by Kr- Xe energy transfer

In this work, a ∼215 nm laser was used to pump mixture gas of Kr and Xe, and a 3680 nm ASE, which is corresponding to the transition of 5d[1/2]1-6p[1/2]1 of Xe, was found. This process was identified as the following: Kr was excited from ground state to 5p[3/2]2 state by absorbing two photon of ∼215 nm, and transferred to 5s[3/2]2 state with ASE emission of 760 nm; Xe(5d[1/2]1) was then populated by near-resonance energy transfer collision with Kr(5s[3/2]2), consequently ASE of 3680 nm was generated. By analyzing the intensity of ASE at 3680 nm, the experiment shows the optimal total pressure and optimal ratio of Kr to Xe. The collision reaction rate constant was evaluated to be 9.6±1.4×10−10cm3s−1 by simulating delay time under different Xe pressures. Under the pressure of 50 torr Kr and 50 torr Xe, gain coefficient of 3680 nm was measured to be 0.035 ± 0.009 cm−1, which make this system a potential candidate for high power middle infrared laser.

Collisional energy transfer: general discussion.

The first page of this article is displayed as the abstract.

Collisional energy transfer in the CO-CO system.

An accurate determination of the physical conditions in astrophysical environments relies on the modeling of molecular spectra. In such environments, densities can be so low (n ≪ 1010 cm−3) that local thermodynamical equilibrium conditions cannot be maintained. Hence, radiative and collisional properties of molecules are needed to correctly model molecular spectra. For comets at large heliocentric distances, the production of carbon monoxide (CO) gas is found to be larger than the production of water, so that molecular excitation will be induced by collisions with CO molecules. This paper presents new scattering calculations for the collisional energy transfer in CO–CO collisions. Using the quantum coupled states approach, cross sections and rate coefficients are provided between the first 37 rotational states of the CO–CO system. Cross sections were calculated for energies up to 800 cm−1, and excitation rate coefficients were derived for temperatures up to 100 K. In comparison with data available in the literature, significant differences were found, especially for the dominant transitions. Due to the high cost of the calculations, we also investigated the possibility of using an alternative statistical approach to extend our calculations both in terms of rotational states and temperatures considered. The use of these new collisional data should help in accurately deriving the physical conditions in CO-dominated comets.

Spiers Memorial Lecture: Theory of unimolecular reactions.

One hundred years ago, at an earlier Faraday Discussion meeting, Lindemann presented a mechanism that provides the foundation for contemplating the pressure dependence of unimolecular reactions. Since that time, our ability to model and predict the kinetics of such reactions has grown in leaps and bounds through the synergy of ever more sophisticated experimental studies with increasingly high level theoretical analyses. This review begins with a brief historical overview of the progress from the Lindemann mechanism to the master equation, which now provides the cornerstone for most theoretical analyses. The current status of ab initio transition state theory based implementations of the master equation is then reviewed, beginning with a discussion of the energy resolved chemical conversion rates, followed by a review of the pressure dependence of the thermal kinetics. The latter discussion focusses on the collisional energy and angular momentum transfer rates as well as the complexities of non-thermal effects and of multiple-well multiple-channel reactions. The synergy with recent state-of-the-art experiments is used to motivate these discussions. Attempts to automate the calculations are also briefly reviewed. Throughout the discussion, we provide our perspective on current understanding and continuing challenges and opportunities. One key challenge relates to the coupling of sequences of reactions, which frequently leads to deviations from the classic presumption of thermal reactants.

Differential Condensation of Methane Isotopologues Leading to Isotopic Enrichment under Non-equilibrium Gas-Surface Collision Conditions.

We examine the initial differential sticking probability of CH4 and CD4 on CH4 and CD4 ices under nonequilibrium flow conditions using a combination of experimental methods and numerical simulations. The experimental methods include time-resolved in situ reflection–absorption infrared spectroscopy (RAIRS) for monitoring on-surface gaseous condensation and complementary King and Wells mass spectrometry techniques for monitoring sticking probabilities that provide confirmatory results via a second independent measurement method. Seeded supersonic beams are employed so that the entrained CH4 and CD4 have the same incident velocity but different kinetic energies and momenta. We found that as the incident velocity of CH4 and CD4 increases, the sticking probabilities for both molecules on a CH4 condensed film decrease systematically, but that preferential sticking and condensation occur for CD4. These observations differ when condensed CD4 is used as the target interface, indicating that the film’s phonon and rovibrational densities of states, and collisional energy transfer cross sections, have a role in differential energy accommodation between isotopically substituted incident species. Lastly, we employed a mixed incident supersonic beam composed of both CH4 and CD4 in a 3:1 ratio and measured the condensate composition as well as the sticking probability. When doing so, we see the same effect in the condensed mixed film, supporting an isotopic enrichment of the heavier isotope. We propose that enhanced multi-phonon interactions and inelastic cross sections between the incident CD4 projectile and the CH4 film allow for more efficacious gas–surface energy transfer. VENUS code MD simulations show the same sticking probability differences between isotopologues as observed in the gas–surface scattering experiments. Ongoing analyses of these trajectories will provide additional insights into energy and momentum transfer between the incident species and the interface. These results offer a new route for isotope enrichment via preferential condensation of heavier isotopes and isotopologues during gas–surface collisions under specifically selected substrate, gas-mixture, and incident velocity conditions. They also yield valuable insights into gaseous condensation under non-equilibrium conditions such as occur in aircraft flight in low-temperature environments. Moreover, these results can help to explain the increased abundance of deuterium in solar system planets and can be incorporated into astrophysical models of interstellar icy dust grain surface processes.

Fluorocarbon-driven photosensitizer assembly decodes energy conversion pathway for suppressing breast tumor

Despite enlarging the clinical applicability of organic photosensitizers in photodynamic therapy (PDT), current nanoencapsulation technology prefers to improve photothermal conversion rather than photochemical conversion from photon energy to reactive oxygen species (ROS)-represented chemical energy. To address it, a fluorocarbon-driven IR780 assembly was constructed to unlock the photochemical conversion pathway for suppressing breast tumor. Systematic experiments and molecular dynamic simulations successfully validated the feasibility and structure characteristics of fluorocarbon-driven IR780 assembly, where fluorocarbon-contained molecules assisted IR780 to assemble into nanoparticles via the energy-driven process. The orderly IR780 configuration in this assembly suppressed photothermal-represented nonradiative relaxation and considerably elevated collisional energy transfer for ROS production. As well, fluorocarbon could bound and release oxygen for mitigating hypoxia and improving ROS production, which united with hyaluronic acid stabilizer-mediated active targeting and fluorocarbon-mediated endosomal escape to cooperatively support the high-efficient PDT. Moreover, these inspiring characteristics triggered robust immunogenicity associated with immunogenic cell death, which eventually repressed the growth and metastasis of breast cancers especially when combining with anti-PD-L1. This unprecedented source design based on PDT principle paves a general method to develop desirable phototheranostic nanoagents.

Ab Initio and RRKM/Master Equation Analysis of the Photolysis and Thermal Unimolecular Decomposition of Bromoacetaldehyde

Bromoacetaldehyde (BrCH2CHO) is a major stable brominated organic intermediate of the bromine-ethylene addition reaction during the arctic bromine explosion events. Similar to acetaldehyde, which has been recently identified as a source of organic acids in the troposphere, it may be subjected to photo-tautomerization initially forming brominated vinyl compounds. In this study, we investigate the unimolecular reactions of BrCH2CHO under both photolytic and thermal conditions using high-level quantum chemical calculations and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation analysis. The unimolecular decomposition of BrCH2CHO takes place through 14 dissociation and isomerization channels along a potential energy surface involving eight wells. Under the assumption of singlet ground-state potential energy surface-dominated photodynamics, the primary photodissociation yields of BrCH2CHO are investigated under both collision-free and collision energy transfer conditions. At atmospheric pressure and under tropospheric actinic flux conditions at ground level, depending on the assumed collisional energy transfer parameter, 150 cm-1 < ⟨ΔEdown⟩ < 450 cm-1, 78-33% of BrCH2CHO undergoes direct photodissociation instead of collisional deactivation at an excitation wavelength of 320 nm. This is significantly higher than the 14% reported for acetaldehyde, hence indicating a strong effect of bromine substitution on the product photolysis yield that is related to additional favorable Br and HBr forming dissociation channels. In contrast to the overall photodissociation quantum yield, the relative branching fractions of the photodissociation products are less dependent on the collisional energy transfer parameter. For a representative value of ⟨ΔEdown⟩ = 300 cm-1 and an excitation wavelength of 320 nm, with 27% for C-C bond fission, 11% for C-Br bond fission, 7% for HBr elimination, and only below 2% each for a consecutive O-Br fission reaction and the photo-tautomerization channel yielding brominated vinyl alcohol, the photodissociation is markedly different from the acetaldehyde case. Finally, as brominated halogenated compounds are of interest for flame inhibition purposes, thermal multichannel unimolecular rate constants were calculated for temperatures in the range from 500 to 2000 K. At a temperature of 2000 K and ambient pressure, the two main reaction channels are the C-Br and C-C bond fissions, contributing 35 and 43% to the total reaction flux, respectively.

Quasi-classical trajectory study of inelastic collision energy transfer between H2CO and H2 on a full-dimensional potential energy surface

The inelastic collision energy transfer between H2CO and H2 was investigated using QCT method based on a new accurate full-dimensional PES, which was constructed using PIP-NN method based on ∼195000 ab initio points calculated at CCSD(T)/cc-pVTZ level. The dynamic results for different collision energy, vibration and/or rotation energy of H2CO(H2) show that the histograms of energy transfer obey biexponential-like decays, indicating that there exist energy transfers between H2CO and H2. Moreover, the energy transfers of H2CO(H2) increased with the increase of collision energy, vibration and/or rotational energy of H2CO(H2). These results are beneficial to understand the intermolecular inelastic collision.

A Time-Independent Quantum Approach to Ro-vibrationally Inelastic Scattering between Atoms and Triatomic Molecules

A full-dimensional time-independent quantum mechanical theory for ro-vibrationally inelastic scattering of triatomic molecules with atoms is formulated. The Jacobi-Radau coordinate system used in the calculation allows not only a near perfect description of the vibrational problem but also the adaptation of the exchange symmetry for A2B type triatoms. The S-matrix elements are obtained by solving the close-coupling equations with contracted basis using the log-derivative method. This method is applied to the inelastic scattering of the water molecule by a chlorine atom, which sheds light on the energy gap law in energy transfer in atom-triatom collisions.

Spectroscopic evaluation of vibrational temperature and electron density in reduced pressure radio frequency nitrogen plasma

The optical emission spectroscopy technique is used to determine the vibrational temperature of the second positive band system, N_{2} (C,upsilon^{^{prime}} - B,upsilon^{^{primeprime}}) in the wavelength range 367.1–380.5 nm by using the line-ratio and Boltzmann plot methods. The electron temperature is evaluated from the intensity ratio of the selected molecular bands corresponding to N_{2}^{ + } (B,upsilon - X, upsilon^{^{prime}} , 391.44 nm), and, N_{2} (C,upsilon^{^{prime}} - B,upsilon^{^{primeprime}}, 375.4 nm) transitions, respectively. The selected bands have a different threshold of excitation energies and thus serve as a sensitive indicator of the electron energy distribution function (EEDF). The electron density has been determined from the intensity ratio of the molecular transitions corresponding to N_{2}^{ + } (B,upsilon - X, upsilon^{^{prime}} , 391.44 nm), and, N_{2} (C,upsilon^{^{prime}} - B,upsilon^{^{primeprime}}, 380.5 nm) for different levels of pressure and radio frequency power. The results show that the vibrational temperature decreases with increasing nitrogen fill pressure and radio frequency power. However, the electron temperature increases with radio frequency power and reduces with fill pressure. The electron density increases both with nitrogen fill pressure and radio frequency power that attributes to the effective collisional transfer of energy producing electron impact ionization. Plasma parameters show a significant dependence on discharge conditions and can be fine-tuned for specific surface treatments.Article HighlightsSpectrum analysis of RF-driven nitrogen plasma for varying discharge conditionsEvaluation of vibrational temperature using line-ratio and Boltzmann plot methodsComparison of vibrational temperatures for line-ratio and Boltzmann plot methodsEvaluation of electron temperature and density using the intensity-ratio of bandsCorrelation of temperature and density with varying fill pressure and RF power

Two-Dimensional Master Equation Modeling of Some Multichannel Unimolecular Reactions.

Multichannel thermal decomposition reactions of n-propyl radicals, 1-pentyl radicals, and toluene are investigated by solving a two-dimensional master equation formulated as a function of total energy (E) and angular momentum (J). The primary aim of this study is to elucidate the role of angular momentum in the kinetics of multichannel unimolecular reactions. The collisional transition processes of the reactants colliding with argon are characterized based on the classical trajectory calculations and implemented in the master equation. The rate constants calculated by using the two-dimensional master equation are compared with those of one-dimensional master equations. The consequence of the explicit treatment of angular momentum depends on the J dependence of the microscopic rate constants and is particularly emphasized in the thermal decomposition of toluene, for which the C-H and C-C bond fission channels are considered. The centrifugal effect is insignificant in the energetically favored C-H bond fission but is substantial in the energetically higher C-C bond fission, which causes rotational channel switching of the microscopic rate constants. The proper treatment of the J-dependent channel coupling effect, weak collisional transfer of J, and initial-J-dependent collisional energy transfer are found to be essential for predicting the branching fractions at low pressures.

A phase diagram for energy flow-limited reactivity.

Intramolecular energy flow (also known as intramolecular vibrational redistribution or IVR) is often assumed in Rice-Ramsperger-Kassel-Marcus, transition state, collisional energy transfer, and other rate calculations not to be an impediment to reaction. In contrast, experimental spectroscopy, computational results, and models based on Anderson localization have shown that ergodicity is achieved rather slowly during molecular energy flow. The statistical assumption in rate theories might easily fail due to quantum localization. Here, we develop a simple model for the interplay of IVR and energy transfer and simulate the model with near-exact quantum dynamics for a 10-degree of freedom system composed of two five-mode molecular fragments. The calculations are facilitated by applying the van Vleck transformation to local random matrix models of the vibrational Hamiltonian. We find that there is a rather sharp "phase transition" as a function of molecular anharmonicity "a" between a region of facile energy transfer and a region limited by IVR and incomplete accessibility of the state space (classically, the phase space). The very narrow transition range of the order parameter a happens to lie right in the middle of the range expected for molecular torsion, bending, and stretching vibrations, thus demonstrating that reactive energy transfer dynamics several kBT above the thermal energy occurs not far from the localization boundary, with implications for controllability of reactions.

The Kinetics of O2 Singlet Electronic States in the Upper and Middle Atmosphere During Energetic Electron Precipitation

AbstractThe electronic kinetics of O2 singlet states a1Δg and b1 in the upper and middle atmosphere of the Earth during energetic electron precipitation is considered. Intramolecular and intermolecular electron energy transfers in inelastic collisions of electronically excited molecular oxygen and molecular nitrogen with O2 and N2 molecules are taken into account in the calculations. The calculations indicate an influence of atmospheric density increase on the calculated volume intensity of 762 nm emission. For the first time, it is shown the dependence of the calculated column intensity ratios of 1.27 μm and 762 nm bands on the energy of auroral and relativistic electrons (40 keV to 10 MeV) and on time of the precipitation. Applying the calculated integral intensities of the bands, we have calculated column intensities of 762 nm and 1.27 μm bands for 12 most intensive events of energetic electron precipitations observed in the polar atmosphere during 2003–2014.

Supercollisions of fast H-atom with ethylene on an accurate full-dimensional potential energy surface.

The collisions transferring large portions of energy are often called supercollisions. In the H + C2H2 reactive system, the rovibrationally cold C2H2 molecule can be activated with substantial internal excitations by its collision with a translationally hot H atom. It is interesting to investigate the mechanisms of collisional energy transfer in other important reactions of H with hydrocarbons. Here, an accurate, global, full-dimensional potential energy surface (PES) of H + C2H4 was constructed by the fundamental invariant neural network fitting based on roughly 100 000 UCCSD(T)-F12a/aug-cc-pVTZ data points. Extensive quasi-classical trajectory calculations were carried out on the full-dimensional PES to investigate the energy transfer process in collisions of the translationally hot H atoms with C2H4 in a wide range of collision energies. The computed function of the energy-transfer probability is not a simple exponential decay function but exhibits large magnitudes in the region of a large amount of energy transfer, indicating the signature of supercollisions. The supercollisions among non-complex-forming nonreactive (prompt) trajectories are frustrated complex-forming processes in which the incoming H atom penetrates into C2H4 with a small C-H distance but promptly and directly leaves C2H4. The complex-forming supercollisions, in which either the attacking H atom leaves (complex-forming nonreactive collisions) or one of the original H atoms of C2H4 leaves (complex-forming reactive trajectories), dominate large energy transfer from the translational energy to internal excitation of molecule. The current work sheds valuable light on the energy transfer of this important reaction in the combustion and may motivate related experimental investigations.

Inelastic scattering in isotopologues of O2-Ar: the effects of mass, symmetry, and density of states.

The two species considered here, O2 (oxygen molecule) and Ar (argon-atom), are both abundant components of Earth's atmosphere and hence familiar collision partners in this medium. O2 is quite reactive and extensively involved in atmospheric chemistry, including Chapman's cycle of the formation and destruction of ozone; while Ar, like N2, typically plays the nevertheless crucial role of inert collider. Inert species can provide stabilization to metastable encounter-complexes through the energy transfer associated with inelastic collisions. The interplay of collision frequency and energy transfer efficiency, with state lifetimes and species concentrations, contributes to the rich and varied chemistry and dynamics found in diverse environments ranging from planetary atmospheres to the interstellar and circumstellar media. The nature and density of bound and resonance states, coupled electronic states, symmetry, and nuclear spin-statistics can all play a role. Here, we systematically investigate some of those factors by looking at the O2-Ar system, comparing rigorous quantum-scattering calculations for the 16O16O-40Ar, 18O16O-40Ar, and 18O18O-40Ar isotope combinations. A new accurate potential energy surface was constructed for this purpose holding the O2 bond distance at its vibrationally averaged distance.

On the CN production through a spark-plug discharge in air-CO2 mixture

This work presents a simulation study of the cyanide production through an electric discharge in a vehicular spark plug in dry air containing typical outdoor amounts of CO 2 . For such a task, a previously reported 2D theoretical and numerical framework is used. Simulations are carried out at atmospheric pressure. The starting gas mixture is formed by molecular nitrogen, oxygen, and CO 2 (3996:999:5) ratio. The mathematical formalism includes heat and species diffusion and convection considering sub-model for energy transfer in electronic, atomic, and molecular collisions. The plasmo-chemical kinetics model includes 69 species and 710 collision processes . Principal pathways for the cyanide formation are discussed. The spatial and temporal evolution of species of interest are also reported.

Collisional stabilization of ion-molecule association complexes in He, H2, or N2 buffer gases

The role of collisional energy transfer in ion-molecule association reactions is analyzed. Rate constants for the formation of adducts between HCl and NO3−(HNO3)x (H2SO4)y or HSO4−(HNO3)x (H2SO4)y (with x = 0–2 and y = 0–2) in the buffer gases H2, He, and N2 and at temperatures between 150 and 300 K are considered. Quantum-chemical calculations of molecular parameters and statistical unimolecular rate theory are combined to model low-pressure rate constants whereas ion-molecule capture theory provides high-pressure rate constants. The comparison with experimental results indicates that energy transfer is dominated by overall collision numbers while weak-collision effects are only of minor importance. On the other hand, often neglected falloff effects between termolecular and bimolecular reaction behavior have to be taken into account.

Modeling third-body effects in the thermal decomposition of H2O2

The thermal decomposition of hydrogen peroxide (H2O2) in seven bath gases (M = He, Ar, H2, N2, CO, CH4, and H2O) has been studied by classical trajectory calculations of the collisional energy transfer processes and master equation analyses of the pressure-dependent rate constants. The energy transfer processes are modeled with the range parameter of the exponential down model and collision frequency for energy transfer. Both of the two quantities are calculated from the collisional trajectories propagated on the potential energy surfaces directly evaluated by the optimized spin-component-scaled MP2 method. The master equation calculations using these parameters were found to give reasonable descriptions of the rate constants at low pressures. The calculated relative third-body efficiencies agree well with the available experimental data for M = He, Ar, and N2 but the efficiency calculated for M = H2O appears to be overestimated at low temperature. The calculated rate constants are represented by the limiting high-pressure rate constant of k∞ = 6.7 × 1014 exp(−24800 K/T) s−1, limiting low-pressure rate constants for M = Ar and N2 of k0(Ar) = 3.65 × 108 (T/K)−4.691 exp(−26470 K/T) cm3 molecule−1 s−1 and k0(N2) = 8.21 × 109 (T/K)−5.034 exp(−26600 K/T) cm3 molecule−1 s−1, the center broadening factor of Fcent = 0.7 exp(− T/3400 K), and the tabulated relative third-body efficiencies. The pressure-dependent rate constants calculated for multicomponent bath gases are reasonably reproduced by the traditional linear mixture rule, whereas the mixture rule based on the reduced pressure is found to provide a more precise description.

An Oxygen-Enriched Photodynamic Nanospray for Postsurgical Tumor Regression

Postoperative local recurrence and metastasis are non-negligible challenges in clinical cancer treatment. Photodynamic therapy (PDT) has presented a great potential in preventing cancer recurrence owing to its noninvasiveness and high specificity for local irradiation of tumor sites. However, the application of conventional PDT is often limited by insufficient oxygen supply, making it difficult to achieve high PDT efficacy. Herein, we combined liposomes with photosensitizer indocyanine green (ICG) and perfluorooctyl bromide (PFOB) to develop a new oxygen-enriched photodynamic nanospray (Lip-PFOB-ICG) for cancer postoperative treatment. The Lip-PFOB-ICG not only has good biocompatibility but also enhanced the PDT effect under near-infrared light. More importantly, PFOB can continuously absorb oxygen, thus improving the collision energy transfer between the ICG photosensitizer and oxygen, and significantly inhibit local tumor recurrence in the subcutaneous tumor recurrence model. This oxygen-enriched photodynamic nanospray strategy may open up new avenues for effective postoperative cancer therapy in the clinic.

Mode‐to‐Mode Collision Energy Transfer from Vibrationally Excited C6F6 to NO/N2 Mixed Bath with the Development of New Potential Energy Functions

AbstractChemical dynamics simulations are performed to study collisional intermolecular energy transfer (IET) from highly vibrationally excited C6F6 to NO/N2 mixed baths equilibrated at 300 K. Two baths with a respective total of 200 and 1000 molecules are considered. The simulations are performed with very accurate intramolecular and intermolecular potential energy parameters either taken from literature or developed. A new simulation methodology is implemented to prepare a three‐component bath system. There is a rise in temperature during the IET dynamics in the smaller bath. The rotational ΔT is observed as 85 K in this bath and is much higher than 20 K obtained at 600 ps from the large bath simulation. However, both the simulations show less average energy transfer as compared to the one obtained from pure N2 bath simulation done previously [J. Chem. Phys. 2014, 140, 194103]. The deviation is less for the large bath simulation. The rotational degree of freedom for NO is found less effective towards IET compared to N2, whereas, the center‐of‐mass translational and vibrational modes behave similar to those of N2.