Transition from Hund's coupling case (a) to case (b) in CaF ( A( 2Π Ω )) , obtained from the Ca( 3 P J)+ CF 2 Cl 2→ CaF( A( 2Π Ω ))+ CFCl 2 reaction, induced by collisional energy (original) (raw)
Journal of Physical Chemistry A, 2003
Chemiluminescent reactions of hyperthermal Ca( 3 P) with CF 2 Cl 2 and CF 2 dCCl 2 were studied in a beam-gas arrangement under single collision conditions. Emissions associated with the A( 2 Π Ω ) f X( 2 Σ + ) and the B( 2 Σ + ) f X( 2 Σ + ) transitions from CaCl and CaF were observed for both reactions. The chemical and electronic branching ratios were determined for these reactions, and different results were obtained for each one. The different behavior was rationalized by a simple MO pictures. For the case of the reaction with CF 2 Cl 2 it was assumed that an electron from Ca( 3 P) is transferred to a σ*(C-Cl) orbital in CF 2 Cl 2 which, at higher translational energies can also enter into a σ*(C-F) orbital of the same molecule. In both cases the molecular anion produced is short-lived and will undergo fast decay to Clor Fto yield CaCl and CaF. For the reaction with CF 2 dCCl 2 the electron from Ca( 3 P) is transferred to a π* orbital of the reagent molecule that generates a relatively stable molecular anion with 2 Π symmetry. This anion subsequently cross over several repulsive 2 Σ surfaces associated with σ* orbitals of the C-Cl and the C-F bonds, to dissociate into Clor Fto produce CaCl and CaF. The electronic branching ratios are in good agreement with statistical distribution based on information theory approach, assuming the rigid rotor harmonic oscillator (RROH) approximation for the reaction with CF 2 Cl 2 and the formation of a collision complex for the reaction with CF 2 dCCl 2 .
Chemical Physics, 1997
Two different experimental techniques are used to measure the lifetime of excited electronic states of CF3 X+ (X = F, H, C1, Br) in the gas phase with excitation energies in the range 20-23 eV. Using the single-bunch mode of a synchrotron source, the first technique utilises pulsed photon excitation to excite the parent neutral molecule, and the decay of the fluorescence is observed in real time. This technique detects time-resolved fluorescence from all emitters, both neutrals and ions, excited at the photon energy, and is therefore a poor method to measure the lifetime of a parent ion if the ionisation quantum yield at the incident energy is less than unity. Also using tunable vacuum-UV radiation from a synchrotron source but now in the multi-bunch, quasi-continuous mode, the second technique detects coincidences between threshold photoelectrons and fluorescent photons. Because this experiment only detects coincidences in ions and not in neutrals, the decay of the fluorescence in real time is only due to the ionic component of the emission. In this way lifetimes of emitting states of molecular ions, absent of contributions from neutrals, can be measured. With sufficient resolution in both photon source and threshold electron analyser, vibrationally state-selected lifetimes can be measured. We use this coincidence technique to measure the lifetimes of the (~2T 2 state of CF4 + (u I = 6), the f)2A I state of CF3 H+ and the F. 2A 1 state of CF3C1 + to be 8.4, 12.6 and 9.4 ns, respectively. The fluorescence quantum yield of the F. 2A l state of CF 3 Br ÷ is too small for signal to be observed in the coincidence experiment. These results are compared with those obtained from pulsed photoexcitation of CF3X, and some conclusions are drawn about the nature of the emitters when CF3X is excited at the appropriate energy. In the cases of CF 4 and CF3H emission is solely in the parent ion. With CF3C1 parent ion emission is a minor component and the main emitters are excited states of the CF radical. With CF3Br emissions are only due to neutrals with, again, excited states of CF being the main contributor.
24. X-ray excited optical luminescence (XEOL) studies of CaF2 at the Ca L2,3-edge
Journal of Electron Spectroscopy and Related Phenomena, 2001
We report a study of the optical response of CaF specimens (crystal and powder) with excitation photon energy tuned 2 across the Ca Ledge. The luminescence was in turn used to monitor the absorption across the Ca Ledge (optical XAFS). 3,2 It is found that the photoluminescence yield produces an inverted spectrum. This observation is interpreted in terms of total absorption (thickness effect), the change in decay dynamics below and above the edge and the atomic origin of the luminescence. By comparing crystal and powder results, we find that the surface component of CaF exhibits a positive shift 2 (|0.15 eV) relative to the bulk at the p–d resonance at the Ca Ledge. A DFT calculation has been carried out and it is in 3,2 good accord with the experiment. The implications of these results are discussed.
Chemical Physics, 1982
Photoion-photoelectron coincidence spectra are reported for CFaI in the region of se g2E,, and the z*E,, electronic states of the molecular cation (i.e. 10.2-11.8 ev). WiLti this energy range CI$X 'Ai) + 1(X ZPsn) are the only accessible fragments. The kinetics and the translational energy release of this dissociation process are examined. The enthalpy of formation of CF; is found to be AH8 (CF$) = 402 * 5 kJ mar I, in agreement with earIier literature values. The irrpiications of the present study for the recently reported muItiphoton dissociation ofCFaIt are discussed.
The Journal of Chemical Physics, 1999
Efficient long-range collisional energy transfer between the E0g+(3P2) and D0u+(3P2) ion-pair states of I2, induced by H2O, observed using high-resolution Fourier transform emission spectroscopy J. Chem. Phys. 135, 114302 (2011) Energy transfer of highly vibrationally excited naphthalene: Collisions with CHF3, CF4, and Kr J. Chem. Phys. 135, 054311 (2011) H+ versus D+ transfer from HOD+ to N2: Mode-and bond-selective effects J. Chem. Phys. 135, 044305 (2011) Multimode calculations of rovibrational energies and dipole transition intensities for polyatomic molecules with torsional motion: Application to H2O2 J. Chem. Phys. 135, 014308 State-to-state dynamics at the gas-liquid metal interface: Rotationally and electronically inelastic scattering of NO[21/2(0.5)] from molten gallium
Arxiv preprint arXiv: …, 2011
We present a study of dissociative electron attachment and vibrational excitation processes in electron collisions with the CF 3 Cl molecule. The calculations are based on the two-dimensional nuclear dynamics including the C-Cl symmetric stretch coordinate and the CF 3 symmetric deformation (umbrella) coordinate. The complex potential energy surfaces are calculated using the ab initio R-matrix method. The results for dissociative attachment and vibrational excitation of the umbrella mode agree quite well with experiment while the cross section for excitation of the C-Cl symmetric stretch vibrations is about a factor of three low as compared to experimental data. 1 12 Dissociative electron attachment (DEA) to polyatomic molecules typically involves mul-13 tidimensional nuclear dynamics. However, because of big computational work necessary 14 to obtain multidimensional complex (i.e., including both real and imaginary parts) energy 15 surfaces, most of theoretical DEA calculations were performed in one-dimensional approx-16 imation. In these calculations it is usually assumed that the DEA process involves one 17 reaction (dissociating) coordinate, roughly corresponding to one of the normal modes of the 18 target molecule. This approximation is sometimes too crude, and sometimes completely un-19 justified. Therefore, a lot of effort was devoted recently to calculations of multidimensional 20 DEA dynamics [1-6]. These calculations address two important problems in the physics of 21 DEA processes. First, we want to know which dissociation channels are the most important 22
The Journal of Physical Chemistry A, 1998
The F + CH 3 COCl and H + ICH 2 COCl reaction systems were studied by the infrared chemiluminescence method in a flow reactor. The primary reaction of F + CH 3 COCl gives a nascent HF(V) distribution of P 1-P 3) 21:52:27. A linear surprisal analysis gives P 0) 3 and 〈f v (HF)〉) 0.60, which is typical for H abstraction reactions by F atoms. The C-H bond energy in acetyl chloride is estimated as e101.2 kcal mol-1 , from the highest HF(V, J) level populated in the primary reaction. The H + ICH 2 COCl primary reaction leads to HI + CH 2 COCl. The secondary F + CH 2 COCl and H + CH 2 COCl reactions give chemically activated FCH 2 COCl*/CH 3 COCl* molecules. The 1,2-HCl elimination channel is the dominant unimolecular pathway for both reactions under our experimental conditions. The HCl(V) distribution from CH 3 COCl* is P 1-P 4) 39:32:20:9. Surprisal analysis was used to estimate the P 0 value as 36% and 〈f v (HCl)〉) 0.12. The reaction time had to be increased from e0.2 ms to g0.5 ms to record the HCl(V) emission from F + CH 2-COCl, and the best distribution was P 1-P 4) 68:24:5:3. The estimated 〈f v (HCl)〉 was only 0.06 which is a lower limit due to HCl(V) relaxation. The CO(V) 1 f 0) emission could also be observed from this reaction with an intensity that was typically less than 10% of the HCl(V) emission. Ab initio calculations for FCH 2-COCl at MP2/6-31G* level give the threshold energy for HCl elimination as 61 kcal mol-1 , which is 12 kcal mol-1 larger than that for CH 3 COCl at the same level. The threshold energies for the other reactions of FCH 2 COCl are 81.0 for CO elimination, 82.5 for CC dissociation, and 78.4 for C-Cl dissociation. RRKM and ab initio calculations indicate that CO formation results from the FCH 2 COCl f FCH 2 + COCl dissociation step followed by COCl f CO + Cl. For CH 3 COCl*, with 105 kcal mol-1 energy, HCl elimination accounts for 98% of the total reaction and CC dissociation accounts for the rest. The C-Cl dissociation channel is not important for either molecule at these energies.
Unimolecular Reactions of CF2ClCFClCH2F and CF2ClCF2CH2Cl: Observation of ClF Interchange
The Journal of Physical Chemistry A, 2008
The unimolecular reactions of CF 2 ClCFClCH 2 F and CF 2 ClCF 2 CH 2 Cl molecules formed with 87 and 91 kcal mol-1 , respectively, of vibrational energy from the recombination of CF 2 ClCFCl with CH 2 F and CF 2 ClCF 2 with CH 2 Cl at room temperature have been studied by the chemical activation technique. The 2,3-and 1,2-ClF interchange reactions compete with 2,3-ClH and 2,3-FH elimination reactions. The total unimolecular rate constant for CF 2 ClCF 2 CH 2 Cl is 0.54 (0.15 × 10 4 s-1 with branching fractions for 1,2-ClF interchange of 0.03 and 0.97 for 2,3-FH elimination. The total rate constant for CF 2 ClCFClCH 2 F is 1.35 (0.39 × 10 4 s-1 with branching fractions of 0.20 for 2,3-ClF interchange, 0.71 for 2,3-ClH elimination and 0.09 for 2,3-FH elimination; the products from 1,2-ClF interchange could be observed, but the rate constant was too small to be measured. The D(CH 2 F-CFClCF 2 Cl) and D(CH 2 Cl-CF 2 CF 2 Cl) were evaluated by calculations for some isodesmic reactions and isomerization energies of CF 3 CFClCH 2 Cl as 84 and 88 kcal mol-1 , respectively; these values give the average energies of formed molecules at 298 K as noted above. Density functional theory was used to assign vibrational frequencies and moments of inertia for the molecules and their transition states. These results were combined with statistical unimolecular reaction theory to assign threshold energies from the experimental rate constants for ClF interchange, ClH elimination and FH elimination. These assignments are compared with results from previous chemical activation experiments with CF 3 CFClCH 2 Cl, CF 3 CF 2 CH 3, CF 3 CFClCH 3 and CF 2 ClCF 2 CH 3 .
Two types of additional absorption spectra in CaF2 crystals
Soviet Physics Journal, 1969
A study of two types of additional absorption spectra observed in colored CaF 2 crystals is reported. The spectral shape for the additively colored crystal is governed by the experimental conditions; during photochemical coloring, it depends on the thermal prehistory of the sample. In all cases, a spectrum consisting of bands at 370 and 560 nm appears in crystals having a band near 200 nm. An additional band appears at 560 nm in the Smakula spectrum at high-irradiation doses; there are three stages in the increase of this band with increasing dose. A similar dependence on the radiation dose is observed for the 560-nmband in synthesized CaF 2 crystals. It is suggested that this band is due to defects produced in the lattice during the irradiation.
The Journal of Physical Chemistry A, 2007
The reaction kinetics of chlorine atoms with a series of partially fluorinated straight-chain alcohols, CF 3 -CH 2 CH 2 OH (1), CF 3 CF 2 CH 2 OH (2), CHF 2 CF 2 CH 2 OH (3), and CF 3 CHFCF 2 CH 2 OH (4), were studied in the gas phase over the temperature range of 273-363 K by using very low-pressure reactor mass spectrometry. The absolute rate coefficients were given by the expressions (in cm 3 molecule -1 s -1 ): k 1 ) (4.42 ( 0.48) × 10 -11 exp(-255 ( 20/T); k 1 (303) ) (1.90 ( 0.17) × 10 -11 , k 2 ) (2.23 ( 0.31) × 10 -11 exp(-1065 ( 106/ T); k 2 (303) ) (6.78 ( 0.63) × 10 -13 , k 3 ) (8.51 ( 0.62) × 10 -12 exp(-681 ( 72/T); k 3 (303) ) (9.00 ( 0.82) × 10 -13 and k 4 ) (6.18 ( 0.84) × 10 -12 exp(-736 ( 42/T); k 4 (303) ) (5.36 ( 0.51) × 10 -13 . The quoted 2σ uncertainties include the systematic errors. All title reactions proceed via a hydrogen atom metathesis mechanism leading to HCl. Moreover, the oxidation of the primarily produced radicals was investigated, and the end products were the corresponding aldehydes (R F -CHO; R F ) -CH 2 CF 3 , -CF 2 CF 3 , -CF 2 CHF 2 , and -CF 2 CHFCF 3 ), providing a strong experimental indication that the primary reactions proceed mainly via the abstraction of a methylenic hydrogen adjacent to a hydroxyl group. Finally, the bond strengths and ionization potentials for the title compounds were determined by density functional theory calculations, which also suggest that the R-methylenic hydrogen is mainly under abstraction by Cl atoms. The correlation of room-temperature rate coefficients with ionization potentials for a set of 27 molecules, comprising fluorinated C2-C5 ethers and C2-C4 alcohols, is good with an average deviation of a factor of 2, and is given by the expression log(k) (in cm 3 molecule -1 s -1 ) ) (5.8 ( 1.4) -(1.56 ( 0.13) × (ionization potential (in eV)).
Reaction Dynamics of Translational and Electronic Excitation In Ca + SF6 Collisions
Laser Chemistry, 1989
Relative values of the total chemiluminescence cross.section for the crossed molecular beam Ca∗+SF6→CaF(A2Π)+SF6 reaction have been measured as a function of collision energy. Both metastable calcium atom states 3P1 were produced by low voltage de-discharge while translational excitation of the SF6 beam was achieved via the seeding technique. Whereas chemiluminescence yield from the CaF(A2Π) was observed from both of the metastable atom reactions, no chemiluminescence signal was detected from the ground atom reaction even though enough translational excitation was used to produc.e that excited product. By changing the discharge conditions different metastable concentrations were produced to measure the state-to-state cross-section for both 3P and 1D reactions, as a function of collision energy, up to 0.6 eV. Laser induced fluorescence of the atomic 4s5s 3S1←4s4p3PJ∘(J=0,1,2,3) lines have been measured to account for the metastable population in the J=1 level.
Vacuum-UV fluorescence spectroscopy of CF3X(X=F,H,Cl,Br) in the range 10–30 eV
Chemical Physics, 1997
The vacuum-UV and visible spectroscopy of CF3X (X z F,H,CI,Br) using fluorescence excitation and dispersed emission techniques is reported. Fluorescence excitation spectra have been recorded following excitation using dispersed synchrotron radiation in the energy range 10-30 eV at an average resolution of-0.06 eV, By comparison with vacuum-UV absorption and electron energy loss spectra, assignments have been made for all the peaks in the Rydberg spectra of the titled molecules that photodissociate to a fluorescing state of a fragment. Dispersed emission spectra have been recorded at the energies of all the peaks in the excitation spectra. Three distinct decay channels are observed: (a) CF 3 fluorescence for photon energies in the range 10.5 to 13.5 eV, (b) CF 2 fluorescence for photon energies in the range 14.7 to 15.7 eV, (c) for photon energies in the range 20-23 eV parent ion fluorescence in the case of CF 4 and CF3H, predominantly CF emission in the case of CFaC1 and CF3Br. These assignments are confirmed by action spectra in which the energy of the vacuum-UV radiation is tuned for detection at a specific (dispersed) wavelength. These results confirm those reported in the previous paper from fluorescence coincidence experiments that parent-ion fluorescence is the major component of the emission for CF 4 and CF3H excited in this latter energy range of 20-23 eV, whereas it represents only a minor component for CF3C1 and a negligibly-small component for CF3Br. * Corresponding author. pulsed photoexcitation and measuring the decay of the fluorescence in real time, or by observing the decay of the signal in real time in a threshold photoelectron-fluorescence photon coincidence experiment. The advantages and disadvantages of these two techniques, and their application to the emission from CF3X photoexcited at = 20-23 eV, form the basis of the previous paper [5]. Both from the value(s) of the lifetimes and from the appearance of the fluorescence excitation spectrum when different optical filters are placed in front of the photomultiplier tube [1-4], the nature of the emitters have been inferred, albeit indirectly.
Organometallics, 2007
Laser-ablated group 4 transition metal atoms react with CF 4 to form triplet state electron-deficient FC÷MF 3 methylidyne complexes, which are identified by their infrared spectra and comparison to density functional vibrational frequency calculations of stable possible products. Of particular interest in these complexes are the strong C-X bonds and carbon-metal π bonding. The two unpaired electrons on carbon are drawn to the electron-deficient transition metal center, forming a partially filled triple bond, which is approximately equal in length to a classical CdM double bond. Reactions with carbon tetrachloride form the analogous ClC÷MCl 3 complexes, whereas reactions with CF 2 Cl 2 form a mixture of FC÷MFCl 2 and ClC÷MF 2 Cl species. The FC÷MFCl 2 complexes involving more R-Cl transfer are favored in the reaction of excited metal atoms during sample deposition, but UV irradiation photoisomerizes FC÷MFCl 2 to the lower energy ClC÷MF 2 Cl complexes with more R-F transfer to the metal center.
Organometallics, 2007
Laser-ablated group 4 transition metal atoms react with CX 1 X 2 3 (X) F, Cl, Br, or I) to form triplet state X 1 C÷MX 2 3 and X 2 C÷MX 1 X 2 2 complexes, which are identified by their infrared spectra, carbon-13 isotopic shifts, and comparison to density functional vibrational frequency calculations. The higher energy product with more heavier halogen migration converts to the lower energy product with more R-F transfer on UV irradiation. Two features are of particular interest in these product complexes: The first is the presence of a very strong C-X bond, and the second is that the two unpaired electrons on carbon are shared with the transition metal center, forming an electron-deficient triple bond. Comparisons are made to the analogous reaction products with CF 4 , CCl 4 , and CF 2 Cl 2 .
Canadian Journal of Chemistry, 1994
The reactions of H atoms with CCl3, CF2Cl, and CH2CH2Cl radicals have been studied in a flow reactor at 300 and 475 K by observation of the infrared emission from the HCl and HF products. These reactions were observed as secondary reactions from the H + CCl3Br, CF2ClBr, and CH2Cl–CH2I chemical systems. The conditions in the flow reactor were controlled so that the nascent vibrational distributions of HCl and HF were recorded. The pattern of vibrational energy disposal to HCl was used to differentiate between Cl atom abstraction and recombination–elimination mechanisms. The H atom reactions with CCl3 and CF2Cl radicals occur only via a recombination–elimination mechanism and give HCl(υ) or HF(υ) in a unimolecular step. Thus, the Cl atom abstraction reactions must have ≥3.0 kcal mol−1 higher activation energy than the recombination reaction. From observation of the ratio of the HCl and HF products from CHF2Cl*, the difference in threshold energies for HF and HCl elimination was determ...