Ground State Of CH Research Articles

High resolution observations of the hot core in G29.96–0.02

We present high angular resolution observations obtained with the Owens Valley and the IRAM Plateau de Bure millimeter-wave interferometers toward the hot core in G29.96-0.02. We observed the ground state CH 3 CN(6-5), CH 1 3 3CN(6-5), vibrationally excited (ν 8 = 1) CH 3 CN(6-5), and the C 1 8 O(1-0) rotational transitions, as well as the 2.7 mm continuum emission. Our continuum maps show evidence of a compact source barely resolved whose diameter we estimate to be about 0.06 pc and whose emission mechanism is dominated by thermal emission from warm dust. Both the ground state and the ν 8 = 1 methyl cyanide lines, as well as other serendipituosly detected molecular transitions, arise from a compact source at the same position as the 2.7 mm continuum emission. The C 1 8 O observations sample the structure and kinematics of the molecular surroundings of the hot core and from the C 1 8 O data we estimate a gas mass of about 1.I × 10 3 M O . in a region with a diameter of 0.32 pc, corresponding to an average number density of about 10 6 cm - 3 . Our data show evidence of both a temperature and density gradient in the hot core and its molecular surroundings. The density gradient, in particular, is consistent with the infalling scenario suggested by the presence of an East-West oriented velocity gradient, which is however of opposite sign in CH 3 CN and C 1 8 O. We tentatively interpret the C 1 8 O velocity gradient as associated with infall, whereas the CH 3 CN gradient, consistent with that measured in NH 3 by Cesaroni et al. (1998). is likely to trace a massive rotating disk.

Photodissociation of the methane–argon complex. II. Vibrational predissociation dynamics, spectral linewidths and fragment state distributions

We calculated the cross sections for vibrational predissociation of methane–Ar induced by excitation of the methane ν3 mode. We used the ab initio CH4–Ar potential depending explicitly on the ν3 and ν1 normal coordinates of the CH4 monomer that is presented in the preceding paper. It was found that dissociation into CH4 fragments excited in the ν1 mode—a V→V′ process with very low kinetic energy release—strongly dominates over direct dissociation into Ar and ground state CH4, and is responsible for the line broadening observed experimentally. The strong variation of the linewidths, observed as well as calculated, for the van der Waals levels excited in combination with the ν3 mode is related to the opening up of appropriate ν1 dissociation channels and the occurrence of rotational resonances in the ν1 continuum in the energy range of the quasibound ν3 levels. The rotational state distributions of the emerging ν1 excited methane fragment are predicted.

Vibrational predissociation dynamics of methane-Ar: an ab initio approach.

We calculated the cross sections for vibrational predissociation of methane-Ar induced by excitation of the methane nu 3 mode with the aid of an ab initio CH4-Ar potential depending explicitly on the nu 3 and nu 1 normal coordinates of the CH4 monomer. We found that dissociation into CH4 fragments excited in the nu 1 mode, a V-->V' process with very low kinetic energy release, strongly dominates over direct dissociation into Ar and ground state CH4, and is responsible for the line broadening observed experimentally. The (observed and calculated) strong variation of the line widths for the Van der Waals levels excited in combination with the nu 3 mode (giving states of A, F and E symmetry) is related to the opening up of appropriate nu 1 dissociation channels and the occurrence of rotational resonances in the nu 1 continuum in the energy range of the quasi-bound nu 3 levels.

Inner-shell photoabsorption spectra

Based on multi-scattering self-consistent field method, the photoabsorption spectra near C 1s threshold of CH4(near-threshold structure) have been studied with the broken symmetry from To. The most possible geometry of the core excited CH4** is determined as the C3v symmetry; the core excited CH4** molecules have the same bond angle (109.5o) as that of the ground state CH4 and consist of one shorter bond length (1.91 a. u.) and three longer bond lengths (2.04 a. u.). The three longer bond lengths are almost the same as that of the ground state CH4, and the averaged bond length (2.01 a. u.) of core excited CH4** is shorter than that of the ground state CH4(2.06 a. u.).

Quantum dynamics of the O(3P)+CH4→OH+CH3 reaction: An application of the rotating bond umbrella model and spectral transform subspace iteration

We have applied the rotating bond umbrella (RBU) model to perform time-independent quantum scattering calculations of the O(3P)+CH4→OH+CH3 reaction based on a realistic analytic potential energy surface. The calculations are carried out in hypercylindrical coordinates with a log-derivative method incorporating a guided spectral transform (GST) subspace iteration technique. A single sector hyperspherical projection method is used for applying the boundary conditions. The results show that ground-state CH4 gives CH3 that is rotationally cold. For CH4 initially vibrationally excited in the C–H stretch or the H–CH3 bending mode, a bimodal CH3 rotational distribution has been observed. The product OH is a little vibrationally excited, while the umbrella mode of CH3 is moderately excited. Vibrational excitation enhances the reactivity substantially. The calculated rate constants are in good agreement with experimental measurements.

Alteration of Cl spin–orbit branching ratios via photodissociation of pre-excited fundamental CH 3 stretch of CH 3CFCl 2

The fundamental symmetric CH 3 stretch ( ν 2) in the ground electronic state of CH 3CFCl 2 is efficiently prepared by stimulated Raman excitation. The excited molecules are photodissociated by ∼235 nm photons that further tag Cl( 2 P 3/2 ) and Cl( 2 P 1/2 ) [Cl *] photofragments via (2+1) resonantly enhanced multiphoton ionization. The yield of both photofragments increases as a result of pre-excitation, indicative of energy flow from the excited C–H to the C–Cl bond and hence a better Franck–Condon overlap between the components of the wavefunction along the C–Cl coordinate. The Cl */Cl branching ratio is 0.60±0.09 in ∼235 nm photolysis of vibrationally excited molecules and 0.40±0.07 in that of vibrationless ground state CH 3CFCl 2.

Experimental and computational study of CH, CH *, and OH * in an axisymmetric laminar diffusion flame

In this study, we extend the results of previous combined numerical and experimental investigations of an axisymmetric laminar diffusion flame in which difference Raman spectroscopy, laser-induced fluorescence (LIF), and a multidimensional flame model were used to generate profiles of the temperature and major and minor species. A procedure is outlined by which the number densities of ground-state CH (X(sup 2)II) excited-state CH (A(sup 2)Delta, denoted CH*), and excited-state OH (A(sup 2)Sigma, denoted OH*) are measured and modeled. CH* and OH* number densities are deconvoluted from line-of-sight flame-emission measurements. Ground-state CH is measured using linear LIF. The computations are done with GRI Mech 2.11 as well as an alternate hydrocarbon mechanism. In both cases, additional reactions for the production and consumption of CH* and OH* are added from recent kinetic studies. Collisional quenching and spontaneous emission are responsible for the de-excitation of the excited-state radicals. As with our previous investigations, GRI Mech 2.11 continues to produce very good agreement with the overall flame length observed in the experiments, while significantly under predicting the flame lift-off height. The alternate kinetic scheme is much more accurate in predicting lift-off height but overpredicts the over-all flame length. Ground-state CH profiles predicted with GRI Mech 2.11 are in excellent agreement with the corresponding measurements, regarding both spatial distribution and absolute concentration (measured at 4 ppm) of the CH radical. Calculations of the excited-state species show reasonable agreement with the measurements as far as spatial distribution and overall characteristics are concerned. For OH*, the measured peak mole fraction, 1.3 x 10(exp -8), compared well with computed peaks, while the measured peak level for CH*, 2 x 10(exp -9), was severely underpredicted by both kinetic schemes, indicating that the formation and destruction kinetics associated with excited-state species in flames require further research.

The effect of nitric oxide on premixed flames of CH 4, C 2H 6, C 2H 4, and C 2H 2

Nitric oxide was doped into premixed, stoichiometric, 10 torr flames of CH 4/O 2/N 2, C 2H 6/O 2/N 2, C 2H 4/O 2/N 2, and C 2H 2/O 2/N 2. Spatial profiles of electronic ground state CH, OH, NO, CN, NCO, NH and metastable state 3C 2 were obtained by laser-induced fluorescence. The peak temperatures of the flames were maintained at 1800 K by varying the N 2 buffer gas mole fraction. Comparisons of the profiles among the different fuels illustrate differences in the hydrocarbon flame structure that lead to different NO reactivities in these flame systems. Concentrations of the nitrogen-containing intermediates are similar in the methane, ethane, and ethylene flames, but are considerably larger in the acetylene flame, indicating a much greater NO reactivity in this flame. The species profiles are modeled using three different kinetic mechanisms; all of these have significant shortcomings.

Photodissociation of vibrationally excited CH 3Cl: modification of the dissociation dynamics

Vibrationally excited CH 3Cl is prepared by laser excitation in the fourth CH stretch overtone transition and photodissociated at fixed wavelengths near 238 nm which also probe the Cl( 2 P 3 2 ) and Cl( 2 P 1 2 ) photofragments through (2 + 1) resonance enhanced multiphoton ionization in a time-of-flight mass spectrometer. Jet cooling reveals two partially resolved bands, which are assigned as transitions to the local-mode [5,0,0] A 1,E and [3,0,0] + 4 ν 2 (A 1,E) levels. The ratio of Cl( 2 P 1 2 ) to Cl( 2 P 3 2 ) spin-orbit populations was found to be 1.08 ± 0.08 for 238 nm photolysis of vibrationally excited CH 3Cl, significantly larger than the spin-orbit population ratio (0.27 ± 0.01) in Cl fragments from the 235 nm photolysis of ground state CH 3Cl.

Photodissociation spectroscopy and dynamics of the HCCO free radical

The photodissociation spectroscopy and dynamics of the HCCO radical have been investigated using fast radical beam photofragment translational spectroscopy. An electronic band with origin at 33 424 cm−1 has been identified. This band exhibits rotational resolution near the band origin, but the well-defined rovibronic structure is homogeneously broadened at higher photon energies. Based on the rotational structure this band is assigned to the B̃ 2Π←X̃ 2A′′ transition. Photofragment translational energy and angular distributions were obtained at several excitation energies. At excitation energies close to the origin, the excited, spin-forbidden CH(a 4Σ−)+CO channel dominates, while the ground state CH(X 2Π)+CO channel is the major channel at higher photon energies. The translational energy distributions provide evidence of competition between intersystem crossing and internal conversion dissociation mechanisms, with some evidence for nonstatistical dynamics in the CH(X 2Π)+CO channel. This work yields an improved heat of formation for HCCO, ΔHf,2980=1.83±0.03 eV.

Ultraviolet photodissociation of the HCCO radical studied by fast radical beam photofragment translational spectroscopy

The ultraviolet photolysis of jet-cooled mass-selected ketenyl radicals has been investigated using the technique of fast radical beam photofragment translational spectroscopy. The C̃2Π(2A″)–X̃2A″ photofragment yield cross section spans 33 400–48 000 cm−1 and exhibits resolved resonances and broad continua. Dissociation produces both ground and excited state CH radicals in association with ground state CO fragments; there is no evidence for H atom elimination. Analysis of the photofragment kinetic energy release spectra yield a value for the C–C bond dissociation energy and heat of formation of HCCO: D0(HC–CO)=3.14±0.03 eV (72.4±0.7 kcal/mol) and ΔHf,00(HCCO)=1.82±0.03 eV (42.0±0.7 kcal/mol).

A detailed study of conformations in the ground state of CH+4

The results of Coulomb explosion imaging for cold CH+4 molecular ions are converted to the molecular conformation probability density. This is the first complete conversion of such data for a relatively complex molecule. The results are compared with the corresponding predicted potential energy surfaces which manifest a Jahn–Teller symmetry breaking. The density of conformations along a nuclear rearrangement path is deduced and the comparison with the theory is satisfactory in almost every respect, except for the density near the minimum of the theoretical potential

Collisional excitation of CH 4 and CCl 4: Wavefunction overlap effects

The collisional excitation of neutral ground state CH 4 and CCl 4 molecules is studied in 1.8 keV collisions with H 2 +, N 2 +, O 2 + and CO 2 + ions using ion high-resolution energy loss spectrometry. Excitation of CCl 4 occurs in the case of collisions with projectile ions of Σ symmetry (H 2 + and N 2 +). No energy loss structure is observed when the projectile ions have Π symmetry (O 2 + and CO 2 +). In the case of CH 4 molecules, energy loss structure is weakly observed only in the case of collisions with H 2 + ions. No energy loss is observed with any of the other projectile ions. The results are understood as manifestations of wavefunction overlap effects.

The S(1A g)–S1(1B2u) vibronic transition in benzene: An ab initio study

The four e2g false origin bands and the a1g progression of the S0(1Ag)–S1(1B2u) transition in benzene are simulated ab initio with the recently introduced configuration interaction singles (CIS) with 6-31G orbitals. The ground and excited state CC and CH bond lengths are optimized and compared with the experiment; the CC bond elongation upon excitation is found to be slightly underestimated. The vibrational force fields are calculated at the stationary points of S0 and S1. The 1Ag force field is calculated at the Hartree–Fock level while the 1B2u force field is calculated at the CIS level of theory. The two force fields are scaled to fit the experimental frequencies and the normal mode rotation upon excitation, i.e., the Duschinsky matrix, is obtained. In agreement with previous empirical fitting of the S1(1B2u) vibrational frequencies, the Duschinsky matrix is found to be nearly diagonal with the exception of the b2u modes submatrix which shows a large amount of mixing. The mixing of the b2u modes is larger before scaling but is subsequently reduced after scaling. The normal modes and the optimized geometries are used to calculate the amount of displacement, upon excitation, of the equilibrium position of the totally symmetric modes. This displacement causes the Franck–Condon progression and is slightly underestimated by the calculation. The intensity of the four e2g false origins in the absorption spectrum of S1 is calculated and the Herzberg–Teller intensities of the four bands are found to be very close to the experiment. In particular, the relative intensity of the CCC bend (ν6) and CC stretch (ν8) bands is nicely reproduced. This result is discussed in light of similar calculations at the semiempirical level of theory. We conclude that CIS can be of great value for the unravelling of vibronic spectra of conjugated systems.

Laser-induced fluorescence measurements on the C2 Sigma +-X2IIr transition of the CH radical produced by a microwave excited process plasma

Laser-induced fluorescence (LIF) measurements were made in a microwave (2.45 GHz) excited a-C:H deposition plasma using the uv beam of a frequency-double cw dye laser. Miscellaneous vibrational and rotational transitions of the electronically excited CH band system C2 Sigma +-X2IIr at lambda approximately= 314 nm were investigated. Spatially resolved rotational, vibrational and translational temperatures as well as drift velocities and densities of ground state CH molecules have been determined. The most important molecular constants for the transitions under investigation have been compiled and the evaluation procedure of UF signals is described. Deposition rates of a-C:H layers are measured by observing the temporal development of the interference fringe system on the substrate. The growth rate is compared with the CH flux to the target. A simplified model of the discharge plasma explains the measured data in a roughly quantitative manner.

Measurement of rate constants for methylidyne(a4.SIGMA.)

A method has been developed to follow the concentration of CH(a {sup 4}{Sigma}) by measuring the chemi-ions formed in its reaction with oxygen atoms. The CH (a {sup 4}{Sigma}) is made by multiphoton dissociation of bromoform. Excess methane is present to act as a scavenger of ground state CH. The rate constant for the reaction of CH(a {sup 4}{Sigma}) with O{sub 2} is (2.6 {plus_minus} 0.5) x 10{sup {minus}11} cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}. No reaction could be observed between CH(a {sup 4}{Sigma}) and CH{sub 4}, H{sub 2}, or D{sub 2}. These rate constants are at least 10{sup 2}-10{sup 3} times smaller than the corresponding values for CH(X{sup 2}{pi}). The low reactivity of CH(a {sup 4}{Sigma}) with H{sub 2} and D{sub 2} confirms a prediction by Brooks and Schaefer. 15 refs., 4 figs., 1 tab.

Direct measurement of rate constants for the reactions of CH and CD with HCN and DCN

Multiphoton laser photolysis of CHBr 3 (or CDBr 3) at 266 nm is utilized for the production of ground state CH(X 2Π) (or CD)radicals in the gas phase. The relative concentration of CH (or CD) is monitored by laser-induced fluorescence near 430 nm. Absolute rate constants are determined by addition of HCN or DCN to the slowly flowing gas mixture. Heating of the reaction cell permits temperature studies between 293 and 676 K. The following Arrhenius expressions are derived: k = (5.0±0.4) × 10 −11 ×exp[(500±30)/ T] cm 3 s −1 for CH+HCN; k = (8.1±1.7)×10 −11 exp[(420±80)/ T] cm 3 s −1 for CH+DCN; k = (9.1±1.5)×10 −11 exp[(420±70)/ T] cm 3 s −1 for CD+HCN and k = (5.4±0.7)×10 −11 exp[(560±50)/ T] cm 3 s −1 for CD+DCN. The measured rate constants for the CH + HCN reaction do not vary with total pressure over the range 35–300 Torr at 296 K. The present observations are consistent with CH insertion into the CH bond in HCN followed by decomposition of the adduct to H+HCCN as the dominant reactive pathway. The rate measurements on mixed isotope species (i.e. CH+DCN or CD+HCN) support the conclusion that exchange reactions do occur to some extent above 475 K as in CH+DCN→CD+HCN and CD+HCN→CH+DCN. The result of the totally deuterated reaction (i.e. CD+DCN) indicates that the inverse isotope effect is important.

OH and CH profiles in a 10 Torr methane / oxygen flame: experiment and flame modeling

Relative ground state CH and OH concentrations and OH rotational temperatures have been obtained in a premixed, 10 Torr, CH 4/O 2 flame, at an equivalence ratio of 0.9 using laser-induced fluorescence. Experimental results are compared to a flame modeling calculation incorporating a published chemical kinetic mechanism. Agreement between the experimental and calculated OH profile is excellent. The calculation closely approximates the location of the experimental CH concentration maximum and the width of the CH profile. The flame model indicates that even though the flame front is clearly separated from the burner, a significant amount of reaction occurs at and below the burner surface. These reactions are driven by highly reactive radical species diffusing from the flame front.

Kinetics of methylidyne (CH X 2Π) radical reactions with ammonia and methylamines

Laser photolysis/laser-induced fluorescence (LIF) was employed to measure the rate constants for CH (X 2Π) radical reactions with NH 3, CH 3NH 2, (CH 3) 2NH, and (CH 3) 3N. The ground state CH radicals were generated by the multiphoton dissociation of CHBr 3 at 266 nm. The relative CH concentration was probed by LIF at 429.8 nm. The experiments were performed on flowing gas mixtures thus allowing pressure and temperature studies to be performed. No variation of the measured rate constants with pressure was observed over the range 30 to 100 Torr total pressure (mostly argon). The following Arrhenius equations, covering the temperature range 297 to 677 K, are derived: CH + NH 3, k 2 = (8.6±0.6 )×10 −11 exp [(230±30) / T] cm 3 s −1; CH + CH 3NH 2, k 2 = (3.1±0.2)×10 −10 exp [(170±30)/ T] cm 3s −1; CH + (CH 3) 2NH, k 2 = (2.4±0.2)×10 −10 exp [(290±30)/ T] cm 3 s −1; CH + (CH 3) 3N, k 2 = (2.6±0.2)×10 −10 exp [ (230±30)/ T] cm 3 s −1. The observed temperature and pressure dependences are in accord with an insertion-elimination mechanism. On a per bond basis, CH reactivity toward NH bonds is only slightly larger than its reactivity toward CH bonds. The observed large rate constants for the methylamine reactions could be attributed to the change in ionization potential (lowering) and polarizability (increasing) as compared with alkanes.

Evidence that CH(a 4Σ) is the precursor of chemi-ionization in hydrocarbon oxidations

The reaction responsible for chemi-ion formation in hydrocarbon flames, CH + O→HCO + + e−, may require that the CH be in its metastable a 4Σ state. Recently measured rate constants for reactions of ground state CH(X 2Π) with various molecules are incompatible with the empirical rate law for chemi-ionization in the acetylene-oxygen atom system. Participation of CH (a 4Σ) in chemi-ion formation could explain several unusual features observed previously.