Collisional Energy Transfer Research Articles

Kinetics of optically pumped Kr metastables.

Optically-pumped atomic gas lasers that utilize metastable excited states of rare gas atoms (Rg(*)) have been demonstrated using both pulsed and CW pump sources. These devices resemble diode-pumped alkali vapor lasers, but have the advantage of using a chemically inert lasing medium. Collisional energy transfer is needed to sustain a population inversion, and He is used as the transfer agent. Consequently, values for the Kr(*)+He state-to-state energy transfer rate constants are needed for the analysis and prediction of laser performance characteristics. In the present study, we report He energy transfer rate constants for Kr(*) in the 5p[5/2](2), 5p[5/2](3), 5p[1/2](1), and 5s[3/2](1) states.

Bichromatic laser-induced quadrupole-dipole collisional energy transfer in Ca-Sr.

We consider the response of a laser-induced quadrupole-dipole collisional system driven by a strong dressing laser field with the aim of calculating the collisional cross section of a weak inducing laser probe. The addition of a second driving field to the traditional arrangement will cause magnitude changes of the spectra and modify the profile. The calculation results show that the bichromatic laser-induced collisional energy-transfer process can be an efficient way to probe Stark splitting of both the final state and intermediate state. The magnitude and position of the splitting spectral lines are strongly dependent on the intensity of the dressing laser field. The peak cross section almost reduces by a factor of 2 with the presence of the dressing laser. Also, in the antistatic wing, bright and dark lines are periodic, appearing with the increasing of the dressing laser intensity.

Characteristics of kinetic energy transfer in collisions between a fragile nano-particle and a rigid particle on a surface

The characteristics of kinetic energy transfer during a collision between a rigid target particle on a surface and a fragile bullet particle (BP) moving at a high velocity were analyzed by Molecular Dynamics (MD) simulation. Ar, N2, and CO2 BPs were considered; their size, temperature and velocity were varied over a wide range. The fraction (FK) of kinetic energy transferred from the BP to the target particle was almost independent of BP size or velocity, but was sensitively dependent on BP type and temperature. The rigidity or the average potential energy of the BP was a dominant factor determining the post-collision behavior of the target particle and FK. FK was almost linearly proportional to the average potential energy per molecule of the BP.

Dissociation of 1,1,1-trifluoroethane is an intrinsic RRKM process: classical trajectories and successful master equation modeling.

Rate constants for thermal decomposition of 1,1,1-trifluoroethane (CH3CF3) in the high-temperature falloff region were previously reported to have an unusual pressure dependence that could not be explained by Rice-Ramsperger-Kassel-Marcus (RRKM) theory in combination with unimolecular master equation analysis. This study investigates the dynamics of the CH3CF3 dissociation and the energy transfer of CH3CF3 in collisions with Ar and Kr by classical trajectory calculations on a global potential energy surface constructed from a large number of quantum chemical calculations. The simulations showed that the ensemble-averaged CH3CF3 populations decay with single exponential profiles that have rate constants close to those predicted by RRKM theory, indicating that the microcanonical ensemble is maintained during decomposition. The trajectory calculation also indicated that a significant portion of the HF product is formed in its vibrationally excited state. Such observation motivated this study to correct some of the reported rate constants for the CH3CF3 decomposition. With the correction applied, the experimental rate constants were well reproduced by the RRKM/master equation calculation using the collisional energy transfer parameters that were also obtained from trajectory calculations. Overall, the title reaction is demonstrated to be another successful example of RRKM/master equation modeling.

Optically pumped microplasma rare gas laser.

The optically pumped rare-gas metastable laser is a chemically inert analogue to three-state optically pumped alkali laser systems. The concept requires efficient generation of electronically excited metastable atoms in a continuous-wave (CW) electric discharge in flowing gas mixtures near atmospheric pressure. We have observed CW optical gain and laser oscillation at 912.3 nm using a linear micro-discharge array to generate metastable Ar(4s, 1s(5)) atoms at atmospheric pressure. We observed the optical excitation of the 1s(5) → 2p(9) transition at 811.5 nm and the corresponding fluorescence, optical gain and laser oscillation on the 2p(10) ↔ 1s(5) transition at 912.3 nm, following 2p(9)→2p(10) collisional energy transfer. A steady-state kinetics model indicates efficient collisional coupling within the Ar(4s) manifold.

Maximizing kinetic energy transfer in one-dimensional many-body collisions

The main problem discussed in this paper involves a simple one-dimensional two-body collision, in which the problem can be extended into a chain of one-dimensional many-body collisions. The result is quite interesting, as it provides us with a thorough mathematical understanding that will help in designing a chain system for maximum energy transfer for a range of collision types. In this paper, we will show that there is a way to improve the kinetic energy transfer between two masses, and the idea can be applied recursively. However, this method only works for a certain range of collision types, which is indicated by a range of coefficients of restitution. Although the concept of momentum, elastic and inelastic collision, as well as Newton’s laws, are taught in junior college physics, especially in Singapore schools, students in this level are not expected to be able to do this problem quantitatively, as it requires rigorous mathematics, including calculus. Nevertheless, this paper provides nice analytical steps that address some common misconceptions in students’ way of thinking about one-dimensional collisions.

Studies on laser-assisted Penning ionization by the optogalvanic effect in Ne/Eu hollow cathode discharge

Laser-assisted Penning ionization (LAPI) is detected in a Ne/Eu hollow cathode (HC) discharge lamp using the pulsed optogalvanic (OG) method. In the Ne/Eu discharge, doubly ionized europium excited energy levels Eu[4f(7)(P(7/2,5/2)6)] lie within the thermal limit (∼kT) from the laser-excited neon's energy level [2p(5)(P3/202)3p or 2p(8) (in Paschen notation)] lying at 149,848 cm(-1). Therefore, Penning ionization (PI) of europium atoms likely to occur into its highly excited ionic states is investigated. To probe the PI of europium, the temporal profiles of its counterpart neon OG signal are studied as a function of discharge current for the transitions (1s(4)→2p(8)) and (1s(2)→2p(2)), corresponding to 650.65 and 659.89 nm wavelengths, respectively. It is observed that PI of europium alters the overall discharge characteristics significantly and, hence, modifies the temporal profile of the OG signals accordingly. The quasi-resonant ionizing energy transfer collisions between laser-excited Ne 2p(8) atoms and electronically excited europium P(9/2)10 atoms are used to explain the LAPI mechanism. Such LAPI studies carried out in HC discharge could be useful for the discharge of a metal-vapor laser with appropriate Penning mixtures.

"Photo" Chemistry Without Light?

In the early seventies, Giuseppe Cilento (Sao Paulo University), Emil White (Johns Hopkins University) and Angelo Lamola (AT&T Bell Laboratories) postulated that typical photochemical reactions could occur in parts of living organisms if coupled to enzymatic sources of electronically excited products. Their paradoxical hypothesis of photochemistry without light was chemically anchored on the synthesis and weak chemiluminescence of several 1,2-dioxetanes, unstable cyclic peroxides whose thermal cleavage produces long-lived and reactive triplet carbonyls. Collisional reactions or energy transfer of triplet species to cellular targets could eventually result in photo products that potentially trigger normal or pathological responses. These ideas flourished in the labs of various researchers who attempted to explain the presence and biological roles of dark secondary metabolites, including plant hormones, pyrimidine dimers, alkaloid lumi-isomers, protein adducts, and mitochondrial permeators, thereby broadening the field of photobiology.

"Photo"chemistry Without Light?

In the early seventies, Giuseppe Cilento (Sao Paulo University), Emil White (Johns Hopkins University) and Angelo Lamola (AT&T Bell Laboratories) postulated that typical photochemical reactions could occur in parts of living organisms if coupled to enzymatic sources of electronically excited products. Their paradoxical hypothesis of photochemistry without light was chemically anchored on the synthesis and weak chemiluminescence of several 1,2-dioxetanes, unstable cyclic peroxides whose thermal cleavage produces long-lived and reactive triplet carbonyls. Collisional reactions or energy transfer of triplet species to cellular targets could eventually result in photo products that potentially trigger normal or pathological responses. These ideas flourished in the labs of various researchers who attempted to explain the presence and biological roles of dark secondary metabolites, including plant hormones, pyrimidine dimers, alkaloid lumi-isomers, protein adducts, and mitochondrial permeators, thereby broadening the field of photobiology.

Na2高位振动态与CO2分子间碰撞能量转移的研究

Na₂高位振动态与CO₂分子间碰撞能量转移的研究

Mechanistic details of energy transfer and soft landing in ala2-H(+) collisions with a F-SAM surface.

Previous chemical dynamics simulations (Phys. Chem. Chem. Phys., 2014, 16, 23769-23778) were analyzed to delineate atomistic details for collision of N-protonated dialanine (ala2-H(+)) with a C8 perfluorinated self-assembled monolayer (F-SAM) surface. Initial collision energies Ei of 5-70 eV and incident angles θi of 0° and 45°, with the surface normal, were considered. Four trajectory types were identified: (1) direct scattering; (2) temporary sticking/physisorption on top of the surface; (3) temporary penetration of the surface with additional physisorption on the surface; and (4) trapping on/in the surface, by physisorption or surface penetration, when the trajectory is terminated. Direct scattering increases from 12 to 100% as Ei is increased from 5 to 70 eV. For the direct scattering at 70 eV, at least one ala2-H(+) heavy atom penetrated the surface for all of the trajectories. For ∼33% of the trajectories all eleven of the ala2-H(+) heavy atoms penetrated the F-SAM at the time of deepest penetration. The importance of trapping decreased with increase in Ei, decreasing from 84 to 0% with Ei increase from 5 to 70 eV at θi = 0°. Somewhat surprisingly, the collisional energy transfers to the F-SAM surface and ala2-H(+) are overall insensitive to the trajectory type. The energy transfer to ala2-H(+) is primarily to vibration, with the transfer to rotation ∼10% or less. Adsorption and then trapping of ala2-H(+) is primarily a multi-step process, and the following five trapping mechanisms were identified: (i) physisorption-penetration-physisorption (phys-pen-phys); (ii) penetration-physisorption-penetration (pen-phys-pen); (iii) penetration-physisorption (pen-phys); (iv) physisorption-penetration (phys-pen); and (v) only physisorption (phys). For Ei = 5 eV, the pen-phys-pen, pen-phys, phys-pen, and phys trapping mechanisms have similar probabilities. For 13.5 eV, the phys-pen mechanism, important at 5 eV, is unimportant. The radius of gyration of ala2-H(+) was calculated once it is trapped on/in the F-SAM surface and trapping decreases the ion's compactness, in part by breaking hydrogen bonds. The ala2-H(+) + F-SAM simulations are compared with the penetration and trapping dynamics found in previous simulations of projectile + organic surface collisions.

Experimental determination of the rate of V-V collisional relaxation in (14)N2 in its ground (X(1)Σ(g)(+)) electronic state between 77 and 300 K.

We propose new values for the V-V collisional energy transfer rate constant of (14)N2 (X(1)Σg(+)) at different temperatures in collisions between molecules in the v = 1 and v = 0 vibrational states. The values were obtained experimentally by means of a time-resolved double resonance pump-probe stimulated Raman setup in which the stimulated Raman technique was used for both the pump and the probe stage. The main feature of the experiment is the fact that population pumping is done with rotational (and thus spin) selectivity, that is, only molecules belonging to the ortho spin variety of (14)N2 are promoted to v = 1. The probe stage is then used to monitor the decay of this ortho rotational population placed in v = 1 and the emergence of a para population in that same vibrational level. Since the only possible mechanism for the arrival of para population to v = 1 is collisional V-V energy transfer, the evolution of the ortho/para ratio in v = 1 is used to quantify the rate constant of the process. The measurements were conducted at 77, 136, 226 and 300 K. The 300 K value had been measured and calculated before by other authors, but a spread larger than an order of magnitude existed between their results. Our proposed value is more precise than previous ones and lies near the mid-point of that interval. The low-temperature values of the rate constant are reported here for the first time. The availability of data at several temperatures has allowed us to unequivocally determine the existence of a strong negative temperature dependence of the rate constant between 77 and 300 K that exhibits a linear behavior in a Landau-Teller plot in this temperature interval.

High resolution IR diode laser study of collisional energy transfer between highly vibrationally excited monofluorobenzene and CO2: the effect of donor fluorination on strong collision energy transfer.

Collisional energy transfer between vibrational ground state CO2 and highly vibrationally excited monofluorobenzene (MFB) was studied using narrow bandwidth (0.0003 cm(-1)) IR diode laser absorption spectroscopy. Highly vibrationally excited MFB with E' = ∼41,000 cm(-1) was prepared by 248 nm UV excitation followed by rapid radiationless internal conversion to the electronic ground state (S1→S0*). The amount of vibrational energy transferred from hot MFB into rotations and translations of CO2 via collisions was measured by probing the scattered CO2 using the IR diode laser. The absolute state specific energy transfer rate constants and scattering probabilities for single collisions between hot MFB and CO2 were measured and used to determine the energy transfer probability distribution function, P(E,E'), in the large ΔE region. P(E,E') was then fit to a bi-exponential function and extrapolated to the low ΔE region. P(E,E') and the biexponential fit data were used to determine the partitioning between weak and strong collisions as well as investigate molecular properties responsible for large collisional energy transfer events. Fermi's Golden rule was used to model the shape of P(E,E') and identify which donor vibrational motions are primarily responsible for energy transfer. In general, the results suggest that low-frequency MFB vibrational modes are primarily responsible for strong collisions, and govern the shape and magnitude of P(E,E'). Where deviations from this general trend occur, vibrational modes with large negative anharmonicity constants are more efficient energy gateways than modes with similar frequency, while vibrational modes with large positive anharmonicity constants are less efficient at energy transfer than modes of similar frequency.

Unexpected temporal evolution of atomic spectral lines of aluminum in a laser induced breakdown spectroscopy experiment

The temporal evolution of the laser induced breakdown (LIBS) signal of a pure aluminum sample was studied under nitrogen and air atmospheres. In addition to the usual decrease of signal due to plasma cooling, unexpected temporal evolutions were observed for a spectral lines of aluminum, which revealed the existence of collisional energy transfer effects. Furthermore, molecular bands of AlN and AlO were observed in the LIBS spectra, indicating recombination of aluminum with the ambient gas. Within the experimental conditions reported in this study, both collisional energy transfer and recombination processes occurred around 1.5μs after the laser shot. This highlights the possible influence of collisional and chemical effects inside the plasma that can play a role on LIBS signals.

Near-infrared emission spectra of TeS, TeSe and Te2

Emission spectra of the radicals TeS, TeSe and Te2 in the near-infrared spectral region have been measured with a high-resolution Fourier-transform spectrometer. The molecules were generated in a fast-flow system by reacting microwave-discharged mixtures of Tex, Sx, and/or Sex vapour and Ar carrier gas and excited by energy transfer and energy pooling processes in collisions with metastable oxygen O2(a1Δg). The b1Σ+(b0+)→X3Σ−(X10+,X21) electric dipole transitions of TeS and TeSe and the b1Σ+g(b0+g)→X3Σ−g(X21g) magnetic dipole transition of Te2 were measured at medium and high spectral resolution. A very weak emission at 3356cm−1 observed in the spectrum of TeSe was identified to be the 0–0 band of the hitherto unknown a1Δ(a2)→X3Σ−(X21) transition of the molecule. Analyses of the spectra have yielded a number of new or improved spectroscopic parameters of the molecules.

Collisional energy transfer in highly excited molecules.

The excitation/de-excitation step in the Lindemann mechanism is investigated in detail using model development and classical trajectory studies based on a realistic potential energy surface. The model, based on a soft-sphere/line-of-centers approach and using elements of Landau-Teller theory and phase space theory, correctly predicts most aspects of the joint probability distribution P(ΔE,ΔJ) for the collisional excitation and de-excitation process in the argon-allyl system. The classical trajectories both confirm the validity of the model and provide insight into the energy transfer. The potential employed was based on a previously available ab initio intramolecular potential for the allyl fit to 97418 allyl electronic energies and an intermolecular potential fit to 286 Ar-allyl energies. Intramolecular energies were calculated at the CCSD(T)/AVTZ level of theory, while intermolecular energies were calculated at the MP2/AVTZ level of theory. Trajectories were calculated for each of four starting allyl isomers and for an initial rotational level of Ji = 0 as well as for Ji taken from a microcanonical distribution. Despite a dissimilarity in Ar-allyl potentials for fixed Ar-allyl geometries, energy transfer properties starting from four different isomers were found to be remarkably alike. A contributing factor appears to be that the orientation-averaged potentials are almost identical. The model we have developed suggests that most hydrocarbons should have similar energy transfer properties, scaled by differences in the potential offset of the atom-hydrogen interaction. Available data corroborate this suggestion.

Trajectory study of energy transfer and unimolecular dissociation of highly excited allyl with argon.

The influence of rotational excitation on energy transfer in single collisions of allyl with argon and on allyl dissociation is investigated. About 90,000 classical scattering simulations are performed in order to determine collision-induced changes in internal energy and in allyl rotational angular momentum. Dissociation is studied by means of about 50,000 additional trajectories evolved for the isolated allyl under three different conditions: allyl with no angular momentum (J = 0); allyl with the same microcanonically sampled initial conditions used for the collisions (J*); allyl evolving from the corresponding exit conditions after the collision. The potential energy surface is the sum of an intramolecular potential and an interaction one, and it has already been used in a previous work on allyl-argon scattering (Conte, R.; Houston, P. L.; Bowman, J. M. J. Phys. Chem. A 2013, 117, 14028-14041). Energy transfer data show that increased initial rotation favors, on average, increased relaxation of the excited molecule. The availability of a high-level intramolecular potential energy surface permits us to study the dependence of energy transfer on the type of starting allyl isomer. A turning point analysis is presented, and highly efficient collisions are detected. Collision-induced variations in the allyl rotational angular momentum may be quite large and are found to be distributed according to three regimes. The roles of rotational angular momentum, collision, and type of isomer on allyl unimolecular dissociation are considered by looking at dissociations times, kinetic energies of the fragments, and branching ratios. Generally, rotational angular momentum has a strong influence on the dissociation dynamics, while the single collision and the type of starting isomer are less influential.

Direct analysis of complex mixtures by mass spectrometry

Mixture analysis can provide information on individual components if the sample is first subjected to chromatographic separation. Two critical capabilities, soft ionization and the ability to mass-select and then dissociate ions of a particular m/z, coalesced in 1975, allowing direct analysis of complex mixtures by mass spectrometry. Chemical ionization was used as the soft ionization method and mass-analysis used the ion kinetic energy spectrometer (MIKES). Soft molecular ionization produces a set of ions that are structural surrogates of the neutral molecules; they can be mass-selected and allowed to spontaneously dissociate (i.e. as metastable ions) or fragmented upon energy transfer e.g. in the course of collision-induced dissociation (CID). The second stage of mass analysis provides information about the atomic connectivity in the precursor ion and by implication in the original molecule. This review focuses on the development of complex mixture analysis by mass spectrometry and allied topics. Discussion of the activation techniques associated with (collision-induced dissociation, surface-induced dissociation and metastable ion dissociation) emphasizes the importance of energy transfer phenomena and the internal energies distributions of ions to explain the observed mass spectra. The translational to internal energy transfer in collisions is readily accessed in the MIKES where the second stage mass analyzer is a kinetic energy/charge analyzer. Collisions in the keV range can also be used to change ion the charge state via the processes of charge exchange, electron stripping, or charge inversion. New ionization sources for analysis of non-volatile compounds that were introduced during the time period (1975–1990) of this review included secondary ion mass spectrometry, plasma desorption and field ionization and they are briefly discussed. Several types of scans were developed to rapidly access information of the individual components, including chemically specific scans (e.g. neutral loss scans of mass 30 for nitro compounds).

H2O maser pumping: The effect of quasi-resonance energy transfer in collisions between H2 and H2O molecules

The effect of quasi-resonance energy transfer in collisions between H2 and H2O molecules in H2O maser sources is investigated. New data on the state-to-state rate coefficients for collisional transitions for H2O and H2 molecules are used in the calculations. The results of ortho-H2O level population inversion calculations for the 22.2-, 380-, 439-, and 621-GHz transitions are presented. The ortho-H2O level population inversion is shown to depend significantly on the population distribution of the para-H2 J = 0 and 2 rotational levels. The possibility of quasi-resonance energy transfer in collisions between H2 molecules at highly excited rotational-vibrational levels and H2O molecules is considered. The quasi-resonance energy transfer effect can play a significant role in pumping H2O masers in the central regions of active galactic nuclei and in star-forming regions.

Pressure-dependent branching in the reaction of 1CH2 with C2H4 and other reactions on the C3H6 potential energy surface

A number of reactions on the C3H6 potential energy surface (PES) are of central importance to various combustion environments. High level theoretical methods were used to predict the temperature and pressure dependent kinetics for these reactions. The rovibrational properties of the key stationary points in the systems were determined with the CCSD(T)/cc-pVTZ method. High accuracy energies for these stationary points were obtained via the consideration of basis set, higher-order correlation, core–valence, anharmonic vibrational, relativistic, and diagonal Born–Oppenheimer corrections. Variable reaction coordinate transition state theory (VRC-TST) was employed to treat the barrierless channels on the PES, while, for channels possessing a distinct barrier, rate coefficients were instead obtained with conventional transition state theory employing rigid-rotor harmonic oscillator assumptions. For the VRC-TST calculations, the interaction energies required for the evaluation of the reactive flux were evaluated with the CASPT2/cc-pVTZ approach for the orientational sampling, coupled with one-dimensional corrections based on higher accuracy evaluations along the minimum energy path. A priori predictions for the temperature and pressure-dependent rate coefficients were obtained through master-equation calculations incorporating these TST predictions for the microcanonical rate coefficients coupled with a simple model for the collisional energy transfer rates. These theoretical predictions compare favorably with the limited experimental data. The analysis yields modified Arrhenius rate expressions for the 1CH2+C2H4, C2H3+CH3, CH2CHCH2+H, CH3CCH2+H, and CH3CHCH+H recombination reactions as well as for the dissociations and isomerizations of propene and cyclopropane.