Collision-free Conditions Research Articles

Simple Culling Methods for Continuous Collision Detection of Deforming Triangles

We present a simple and efficient approach for continuous collision detection of deforming triangles based on conservative advancement. The efficiency of our approach is due to a sequence of simple collision-free conditions for deforming triangles. In our experiment, we show that our CCD algorithm achieves 2-30 times performance improvement over existing algorithms for triangle primitives.

Photofragmentation Translational Spectroscopy of Methyl Azide (CH3N3) Photolysis at 193 nm: Molecular and Radical Channel Product Branching Ratio

We describe molecular-beam photofragment translational spectroscopy (PTS) experiments using electron impact (EI) ionization product detection to investigate the 193 nm photodissociation of methyl azide (CH(3)N(3)) under collision-free conditions. These experiments are used to derive the branching ratio between channels 1 and 2 [(1) radical channel: CH(3)N(3) + hν (λ = 193 nm) → CH(3) + N(3); (2) molecular channel: CH(3)N(3) + hν (λ = 193 nm) → CH(3)N + N(2)], which have been reported in a previous VUV-photoionization based PTS study. (1) Using electron impact ionization cross sections and ion fragmentation ratios for the various detected products, we derive the branching ratio (X(CH(3)-N(3)))/(X(CH(3)N-N(2))) = (0.017 ± 0.004)/(0.983 ± 0.004). Based on analysis of the kinetic energy release in the radical channel, we find that the cyclic form of N(3) is the dominant product in the radical channel. Only a small fraction of the radical channel produces ground state linear N(3).

Stark-induced adiabatic Raman passage for preparing polarized molecules

We propose a method based on Stark-induced adiabatic Raman passage (SARP) for preparing vibrationally excited molecules with known orientation and alignment for future dynamical stereochemistry studies. This method utilizes the (J, M)-state dependent dynamic Stark shifts of rovibrational levels induced by delayed but overlapping pump and Stokes pulses of unequal intensities. Under collision-free conditions, our calculations show that we can achieve complete population transfer to an excited vibrational level (v > 0) of the H(2) molecule in its ground electronic state. Specifically, the H(2) (v = 1, J = 2, M = 0) level can be prepared with complete population transfer from the (v = 0, J = 0, M = 0) level using the S(0) branch of the Raman transition with visible pump and Stoke laser pulses, each polarized parallel to the z axis (uniaxial π-π Raman pumping). Similarly, H(2) (v = 1, J = 2, M = ±2) can be prepared using SARP with a left circularly polarized pump and a right circularly (or vice versa) polarized Stokes wave propagating along the z axis (σ(±)-σ(∓) Raman pumping). This technique requires phase coherent nanosecond pulses with unequal intensity between the pump and the Stokes pulses, one being four or more times greater than the other. A peak intensity of ~16 GW/cm(2) for the stronger pulse is required to generate the desirable sweep of the Raman resonance frequency. These conditions may be fulfilled using red and green laser pulses with the duration of a few nanoseconds and optical energies of ~12 and 60 mJ within a focused beam of diameter ~0.25 mm. Additionally, complete population transfer to the v = 4 vibrational level is predicted to be possible using SARP with a 355-nm pump and a near infrared Stokes laser with accessible pulse energies.

ChemInform Abstract: Jet Spectroscopy and Excited State Dynamics of Benzyl and Substituted Benzyl Radicals.

Benzyl and its p‐fluoro and p‐methyl derivatives are produced by the ArF laser (193 nm) photolysis of their chlorides in the supersonic free jet. The spectroscopy and excited state dynamics of these radicals are studied by the laser induced fluorescence (LIF) method under the collision free condition. The assignments of vibronic bands are carried out from the LIF excitation and dispersed spectra and the vibrational energies of the D1 state are determined. The excitation spectrum of p‐fluorobenzyl shows quite similar vibrational structure to that of p‐fluorotoluene up to about 1000 cm−1 from the 000 band, which indicates that D2 of p‐fluorobenzyl lies about 1000 cm−1 above D1 and no vibronic coupling exists lower than this energy. On the other hand, benzyl and p‐methylbenzyl show very complicated and irregular vibronic structures in excitation spectra, which are not similar to those of toluene and p‐xylene. This complication is explained by the D1–D2 vibronic coupling caused by low lying D2 states in these radicals. Time profiles of the emission intensity of p‐fluorobenzyl and p‐methylbenzyl show single exponential decay and their lifetimes do not indicate significant dependence on vibronic levels. On the other hand, benzyl shows dual exponential decay, which is interpreted by intermediate coupling case behavior.

Ultraviolet photochemistry of buta-1,3- and buta-1,2-dienes: Laser spectroscopic absolute hydrogen atom quantum yield and translational energy distribution measurements

Using pulsed H-atom Lyman-alpha laser-induced fluorescence spectroscopy along with a photolytic calibration approach, absolute H-atom product quantum yields of phi(H-b13d) = (0.32+/-0.04) and phi(H-b12d) = (0.36+/-0.04) were measured under collision-free conditions for the 193 nm gas-phase laser flash photolysis of buta-1,3- and buta-1,2-diene at room temperature, which demonstrate that nascent H-atom formation is of comparable importance for both parent molecules. Comparison of the available energy fraction, f(T-b13d) = (0.22+/-0.03) and f(T-b12d) = (0.13+/-0.01), released as H+C(4)H(5) product translational energy with results of impulsive and statistical energy partitioning modeling calculations indicates that for both, buta-1,3- and buta-1,2-diene, H-atom formation is preceded by internal conversion to the respective electronic ground state (S(0)) potential energy surfaces. In addition, values of sigma(b-1,3-d-L alpha) = (3.5+/-0.2)x10(-17) cm(2) and sigma(b-1,2-d-L alpha) = (4.4+/-0.2)x10(-17) cm(2) for the previously unknown Lyman-alpha (121.6 nm) radiation photoabsorption cross sections of buta-1,3- and buta-1,2-diene in the gas-phase were determined.

Preparation of polarized molecules using coherent infrared multicolor ladder excitation

A density matrix treatment is presented for a general process of preparing polarized molecules through their coherent interaction with two or more infrared photons of different frequencies, each tuned to cause a transition between real levels. This process, which might be called infrared stimulated Raman adiabatic passage, allows complete population transfer to selected rotational-vibrational levels and controls the direction of the rotational angular momentum vector J of the molecule with the possibility of preparing higher moments of the J spatial distribution. HCl molecules in a supersonic molecular beam are considered as a candidate system. Theory predicts that under collision-free conditions two infrared laser pulses of microsecond duration and milliwatt power are able to achieve complete population transfer and alignment of HCl (v=2, J=2, and M=0) for mutually parallel excitation and HCl (v=2, J=2, and M=+/-1) for mutually perpendicular excitation. Orientation of the HCl (v=2, J=2, and M=+/-2) can also be achieved using two circularly polarized pulses of the same helicity. For simplicity, our treatment ignores nuclear spin depolarization, which would be the case for molecules such as (12)C(16)O and (12)C(16)OO(2). Polarized molecules in higher vibrational levels can be prepared using additional infrared pulses.

A method for mapping the boundaries of collision-free reachable workspaces

This paper proposes a method for mapping the boundaries of manipulators' collision-free reachable workspaces. Knowledge of manipulators' actual available workspaces is important in applications and design, e.g., the pre-surgery planning for medical robots, the design parameter optimization for manipulators, etc. Algorithms for mapping the boundaries of reachable workspaces have been well developed in previous works, but we have not seen any method that takes into account the obstacles' effects on the manipulators' available workspaces. In this paper, the concept of collision-free reachable workspace is delivered. The obstacle and the end-effecter are modeled as intersections of convex implicit surfaces, which in this paper are defined as convex superquadrics. The minimum distance query between each two superquadrics is collapsed to a convex optimization problem. By employing an interior-point method, the collision-free conditions are obtained and integrated with the continuation method, creating an algorithm which enables the boundary mapping of collision-free reachable workspaces. To demonstrate and validate our approach, a planar three-bar manipulator is tested with the proposed algorithm; the boundaries of the manipulator's collision-free reachable workspaces are calculated and compared with the results of the manipulator's reachable workspaces.

Determining the size-dependent structure of ligand-free gold-cluster ions

Ligand-free metal clusters can be prepared over a wide size range, but only in comparatively small amounts. Determining their size-dependent properties has therefore required the development of experimental methods that allow characterization of sample sizes comprising only a few thousand mass-selected particles under well-defined collision-free conditions. In this review, we describe the application of these methods to the geometric structural determination of Au(n)(+) and Au(n)(-) with n = 3-20. Geometries were assigned by comparing experimental data, primarily from ion-mobility spectrometry and trapped ion electron diffraction, to structural models from quantum chemical calculations.

Decomposition of 1,1‐dimethyl‐1‐silacyclobutane on a tungsten filament—evidence of both ring CC and ring SiC bond cleavages

The decomposition of 1,1-dimethyl-1-silacyclobutane (DMSCB) on a heated tungsten filament has been studied using vacuum ultraviolet laser single photon ionization time-of-flight mass spectrometry. It is found that the decomposition of DMSCB on the W filament to form ethene and 1,1-dimethylsilene is a catalytic process. In addition, two other decomposition channels exist to produce methyl radicals via the Si-CH(3) bond cleavage and to form propene (or cyclopropane)/dimethylsilylene. It has been demonstrated that both the formation of ethene and that of propene are stepwise processes initiated by the cleavage of a ring C-C bond and a ring Si-C bond, respectively, to form diradical intermediates, followed by the breaking of the remaining central bonds in the diradicals. The formation of ethene via an initial cleavage of a ring C-C bond is dominant over that of propene via an initial cleavage of a ring Si-C bond. When the collision-free condition is voided, secondary reactions in the gas-phase produce various methyl-substituted 1,3-disilacyclobutane molecules. The dominant of all is found to be 1,1,3,3-tetramethyl-1,3-disilacyclobutane originated from the dimerization of 1,1-dimethylsilene.

Electronic relaxation processes of all-trans-2,4,6,8-decatetraene vapour revealed by the measurements of the S1 and S2 fluorescence

Fluorescence and excitation spectra of all-trans-2,4,6,8-decatetraene vapour have been measured at different buffer gas pressures from 0 to 1.07 × 105 Pa (800 Torr). The quantum yield of the S1 fluorescence (ΦF(S1)) increases with increasing the buffer gas pressure, while that of the S2 fluorescence (ΦF(S2)) decreases slightly. ΦF(S1) decreases significantly with increasing the excitation energy under the collision free condition, while ΦF(S2) decreases only slightly. The pressure dependence of the fluorescence yields is interpreted in terms of the relaxation model involving the reversible internal conversion between S1 and S2 and the vibrational relaxation in the S1 manifold. The reverse S1 → S2 internal conversion rate is shown to be very slow compared with the forward S2 → S1 internal conversion rate.

Unusual features in electron attachment to chlorodifluoroacetic acid (CClF 2COOH): Strong dissociative electron attachment near 0 eV and associative attachment at 0.75 eV

Electron attachment to gas phase CClF 2COOH (M) under collision free conditions exhibits very unusual features as the non-decomposed parent anion M − (associative attachment) is observed on the mass spectrometric time scale at a resonance located at 0.75 eV while strong dissociative electron attachment (DEA) reactions already occur within a resonant feature close to 0 eV. Long lived parent anions M − are usually only formed (if at all) at energies close to 0 eV and when no DEA channels are energetically accessible. The two by far dominant DEA reactions are due to the cleavage of the C–Cl bond with the excess charge localised on either of the fragments to form the closed shell anions CF 2COOH − or Cl −. Electron attachment near 0 eV also leads to more complex DEA reactions like the loss of HF or CF 2, etc., which are associated with substantial geometrical and electronic rearrangement. The thermal attachment rate constant is derived by means of an electron swarm experiment as k = 1.5 × 10 −10 cm 3 molec. −1 s −1.

Time-dependent depolarization of aligned HD molecules

An aligned sample of HD(v = 1, J = 2, M(J) = 0) molecules is prepared under collision-free conditions using the S(0) stimulated Raman pumping transition. Subsequent coupling to the spins of the deuteron I(D) and the proton I(H) causes the initial degree of alignment to oscillate and decrease as monitored over the time range from 0-13 mus via the O2 line of the [2 + 1] REMPI E,F(1)Sigma-X(1)Sigma (0,1) band. The time dependence of the rotational alignment is also calculated using both a hierarchical coupling scheme in which the rotational angular momentum J is regarded first to couple to I(D), and then the resultant F(i) to couple to I(H), to form the total angular momentum F and a non-hierarchical coupling scheme in which the HD energy level structure is not assumed to be diagonal in the |I(H)(JI(D))F(i)FM(F)> basis set. The experimental data is in good agreement with the non-hierarchical calculation but not with the hierarchical calculation, as expected for this system. Additionally, we calculate the time dependence of the H and D nuclear spin polarizations.

THEORETICAL STUDIES OF ANHARMONIC EFFECT IN THE RICE–RAMSPERGER–KASSEL–MARCUS THEORY

The purpose of this paper is to propose a theoretical approach, which can evaluate anharmonic effects on the rate constants within the transition state theory. In treating unimolecular reactions under collision-free conditions by using the RRKM theory, we make use of the inverse Laplace transformation of the partition functions to obtain both the total number of states for the activated complex and density of states for the reactant with the Morse oscillator potential. To demonstrate the applications of our theoretical approach, we choose some model systems and a real reaction as examples.

The anharmonic effect study of coupled Morse oscillators for the unimolecular reaction

The importance of anharmonic effect in dissociation of molecular systems especially clusters has been noted. In this paper, we shall study the effect of coupled anharmonic oscillator of the standard bilinear form (SBF) Morse oscillator (MO) potential on unimolecular reaction. We shall use the systematic theoretical approach, YL method, proposed by Yao and Lin (YAO L, et. al. J Phys Chem A, 2007, 111(29): 6722–6729), which can evaluate anharmonic effects on the rate constants based on the transition state theory. In treating the anharmonic effect with the Morse oscillator potential on unimolecular reactions under collision-free conditions by using the RRKM (Rice-Ramsperger-Kassel-Marcus) theory, the inverse Laplace transformation of the partition functions was used to obtain the total amount of state and density of state by using the first-order and the second-order approximations of the saddle-point method. To demonstrate the anharmonic effect of the SBF Morse model, we choose some model systems and a real reaction as examples.

The laser-induced fluorescence spectroscopy of TiF in the ultraviolet region

The laser-induced fluorescence (LIF) spectrum of jet-cooled 48TiF has been obtained in the wavelength region of 245–270 nm for the first time. Six pairs of vibronic bands were observed and assigned to two new transitions [37.8] 4Φ-X 4Φ and 4Δ-X 4Φ. Rotational analysis was carried out for the ( ν′ = 0–3 to ν″ = 0) vibrational bands of the [37.8] 4Φ 3/2-X 4Φ 3/2 and [37.8] 4Φ 5/2-X 4Φ 5/2 subbands, and also, the ( ν′, 0) and ( ν′+1, 0) vibrational bands of the 4Δ 1/2-X 4Φ 3/2 and 4Δ 3/2-X 4Φ 5/2 subbands. The effective equilibrium molecular constants for the [37.8] 4Φ 3/2 and [37.8] 4Φ 3/2 upper states were determined. In addition, lifetime measurements were carried out for all of the observed bands under collision-free conditions. On the basis of the spectroscopic constants and lifetime measurements, the electronic transitions involved in the observed high-lying electronic states are discussed.

Preparation of oriented and aligned H2 and HD by stimulated Raman pumping

Stimulated Raman pumping has been used to prepare oriented and aligned samples of H(2)(nu=1,J=1,2,3) and HD(nu=1,J=2) under collision-free conditions using the (1,0) S(0), S(1), Q(1), Q(2), and O(3) lines. The M-sublevel anisotropies were interrogated by polarized [2+1] resonance-enhanced multiphoton ionization via the (0,1) O(2), O(3), and S(1) lines of the E,F (1)Sigma(g) (+)-X (1)Sigma(g) (+) system. The optical excitation schemes employed in this study generate highly oriented and aligned molecular ensembles. We show that the H(2)(nu=1,J=2,M=0) and H(2)(nu=1,J=2,M=2) samples retain their initial polarization for greater than 100 ns and are therefore suitable candidates for targets or projectiles in future scattering experiments.

A directly-dissociative stepwise reaction mechanism for gas-phase peroxyacetic acid

The dissociation of peroxyacetic acid (PAA, CH 3C(O)OOH) was studied by UV laser photolysis in the gas phase under collision-free conditions. Three pathways are proposed for PAA dissociation that ultimately produce CH 3 + CO 2 + OH. Two pathways are stepwise with sequential bond dissociation and the other pathway is concerted with simultaneous fission of the same bonds. The laser-induced fluorescence (LIF) spectrum of OH was analyzed after 266 and 240 nm photolysis of PAA. OH is produced in the 2Π ground electronic state with no vibrational energy ( v ″ = 0 ) and rotational distributions that peak at N″ ≈ 3–4 and extend to N″ = 11 at both photolysis wavelengths. Spin-orbit population ratios of the ground electronic state were slightly above unity for both studies. The average OH Λ-doublet ratio was 1.07 ± 0.14 after 266 nm photolysis, which indicates that OH is produced by torsional forces. At 240 nm PAA photolysis, the Λ-doublet ratio increased from unity at N″ = 4 to a statistical value of ∼2 at N″ = 11. Average OH translational energies determined from Doppler profiles were 34 ± 2 and 43 ± 2 kcal mol −1 at 266 and 240 nm photolysis, respectively. The average speed of OH is used to determine the total translational energy of products for the proposed stepwise and concerted pathways. The calculated total translational energies are compared to thermochemical available energy calculations for each pathway to determine the viability of the reactions. Experimental results indicate that dissociation occurs from an excited state, so a comparison of 266 and 240 nm photolysis data is made to discover the direct versus indirect nature of the mechanism. The UV spectra of PAA, acetic acid, hydrogen peroxide, and methylhydroperoxide are reviewed in light of the results. This study concludes that PAA photolysis occurs through initial stepwise O O bond dissociation by a directly-dissociative mechanism from a highly-repulsive electronic state like the hydroperoxides and an indirect mechanism from an exit channel barrier similar to acetic acid is not accessed at these wavelengths.

Dynamic simulation of the ion sheath in the presence of fast monoenergetic electrons

This paper contains interesting simulation results concerning the nonstationary potential distribution inside the sheath formed between a plasma and a biased electrode in the presence of a fast group of electrons. The influence of these fast monoenergetic electrons on the ion dynamics of a planar sheath is investigated assuming that the ion, electron, and fast electron components act as three coupled fluids. Under collision-free conditions and in the presence of a fast electron beam, the evolution of the potential and ion density distribution as a function of length is found. These profiles show that in the plasma with a group of fast monoenergetic electrons, a nonmonotonic distribution of the potential could be formed in front of the cathode in which its maximum is larger than the biased electrode potential with the passage of time. Results indicate that the formation of this maximum potential depends strongly on the ratio of the density of non-Maxwellian (fast) electrons to the density of the Maxwellian (plasma bulk) electrons.

The laser-induced fluorescence spectroscopy of CoF in the ultra violet region

The laser-induced fluorescence (LIF) spectrum of jet-cooled CoF has been obtained in the wavelength region of 260–290 nm for the first time. Seventeen vibronic bands were observed and assigned to three types of transition from the ground state to upper states Ω′ = 3, 4, 5 by rotational analysis. A new vibrational transition with the 0–0 band at 34697.22 cm −1 has been assigned as the [34.7] 3Γ 5–X 3Φ 4 transition and effective equilibrium molecular constants for the upper state have been determined. In addition, lifetime measurements have been carried out for most of the bands under collision-free conditions. On the basis of the spectroscopic data and lifetime measurements, the electronic structures of these possible high-lying electronic states are discussed.

Dissociative photoionization of ClN 3 using high-resolution synchrotron radiation: The N–Cl bond energy in ClN 3

High-resolution synchrotron-radiation-based photoionization mass spectrometry was applied to study the dissociative photoionization of ClN 3 under collision-free molecular beam conditions at ionization energies between 10 and 17 eV. No parent ion (ClN 3 +) could be detected under our experimental conditions. This suggests that the ground and excited states of ClN 3 + are weakly bound or repulsive, a conclusion supported by electronic structure calculations also reported here. We recorded photoionization yield spectra at m/ z = 49, 42, 35 and 14 from which we extracted the appearance potentials for NCl +, N 3 +, Cl +, and N +. The appearance potential of NCl + (10.17 ± 0.02 eV) observed here is close to the previously reported ionization potential of ClN 3 obtained form photoelectron spectroscopy. Using the theoretically calculated binding energy of ClN 3 + (0.2 eV), we derive an estimate of the adiabatic ionization potential of ClN 3 = 9.97 ± 0.02 eV. The measured appearance potentials for N 3 +, Cl +, and N + provide three independent determinations of the Cl–N bond energy in ClN 3, which agree within their respective error limits. The observations of this work are consistent with a new value of the N–Cl bond energy in ClN 3, D 0(Cl–N 3) = 1.86 ± 0.05 eV, 0.3 eV lower than previously reported values, which are however experimentally derived upper limits. The bond energy reported here is consistent with high level ab initio (CCSD(T)) electronic structure calculations extrapolated to the complete basis set limit, which yield a value: D 0(Cl–N 3) = 1.87 eV.