Vacuum ultraviolet photochemistry of CH4 and isotopomers. II. Product channel fields and absorption spectra

Unraveling the Photodissociation Branching and Pathways of Methane at 118 nm by Imaging the CH3, CH2, and CH Fragments

Under irradiation of a vacuum ultraviolet (VUV) photon, methane dissociates and yields multiple fragments. This photochemical behavior is not only of fundamental importance but also with wide-ranging implications in several branches of science. Despite that and numerous previous investigations, the product channel branching is still under debate, and the underlying dissociation mechanisms remain elusive. In this study, the photofragment imaging technique was exploited for the first time to map out the momentum and anisotropy parameter distributions of the CH3, CH2, and CH fragments at the 118 nm photolysis wavelength (10.48 eV photon energy). In conjunction with previously reported results of the H atom fragment at 121.6 nm (10.2 eV), a complete set of product channel branching in both two-body and three-body fragmentations is accurately determined. We concluded that extensive nonadiabatic transitions partake in the processes with two-body fragmentations accounting for more than 90% of overall photodissociation, for which the channel branching values for CH2 + H2 and CH3 + H are about 0.66 and 0.25, respectively. Careful kinematic analysis enables us to untangle the intertwined triple fragmentations into the CH2(X̃ 3B1 and ã 1A1) + H + H and CH(X2Π) + H + H2 channels and to evidence their underlying sequential (or stepwise) mechanisms. With the aid of electronic correlation and prior theoretical calculations of the potential energy surfaces, the most probable or dominant dissociation pathways are elucidated. Comparisons with fragmentary reports in the literature on various photochemical aspects are also documented, and discrepancies are clarified. This comprehensive study benchmarks the VUV photochemistry of methane and advances our understanding of this important process.

Pump-probe strategy for instantaneous 2D detection of CH3 in flames using a single laser

Visualization of the reaction zone of flames using CH radicals as markers is restricted by the low concentration of CH in fuel-lean conditions. To address this, methyl radicals (CH₃) are employed as a substitution of CH in premixed methane/air flames. A pump-probe method was adopted with the pump laser photolyzing CH₃ and the probe laser detecting the photolyzed CH (X²Π) fragments. Laser excitation scans were performed to ensure that the fluorescence detected was from CH only. Visualization of the reaction zone of flames was accomplished by a CH₃ photofragmentation laser-induced fluorescence technique in fuel-lean conditions (the equivalence ratio of 0.4), where CH planar laser-induced fluorescence did not work in both laminar and turbulent jet flames. The proposed pump-probe method of CH₃ can be used to visualize the reaction zone of hydrocarbon combustion under both fuel-lean and fuel-rich conditions with a superior signal-to-noise ratio.

Theoretical Investigations on the Possibility of Prebiotic HCN Formation via O-Addition Reactions

Until now, reactions between methane photolysis products (CH₃^•, CH₂) and active N atom or reactive NO radical are proposed as routes of HCN formation in the prebiotic Earth. Scientists think that the reducing atmosphere of primitive Earth was made of H₂, He, N₂, NO, CH₄, H₂O, CO₂, etc., and there was no molecular oxygen. However, it has been evident from experiments that the vacuum ultraviolet (VUV) photolysis of CO₂ can produce atomic oxygen. Therefore, it can be presumed that atomic oxygen was likely present in early Earth's atmosphere. Was there any impact of atomic oxygen in production of early atmospheric HCN for the emergence of life? To hunt for the answer, we have employed computational methods to study the mechanism and kinetics of CH₃NO + O(¹D) and CH₂NO^• + O(³P) addition reactions. Current study suggests that the addition of O(¹D) into nitrosomethane (CH₃NO) and the addition of O(³P) into nitrosomethylene radical (CH₂NO^•) can efficiently produce HCN through an effectively barrierless pathway. At STP, Bartis-Widom phenomenological loss rate coefficients of O(¹D) and O(³P) are obtained as 2.47 × 10^-12 and 4.67 × 10^-11 cm³ molecule^-1 s^-1, respectively. We propose that addition reactions of atomic oxygen with CH₃NO and CH₂NO^• might act as a potential source for early atmospheric HCN.

Ultraviolet photochemistry of ethane: implications for the atmospheric chemistry of the gas giants

Chemical processing in the stratospheres of the gas giants is driven by incident vacuum ultraviolet (VUV) light. Ethane is an important constituent in the atmospheres of the gas giants in our solar system. The present work describes translational spectroscopy studies of the VUV photochemistry of ethane using tuneable radiation in the wavelength range 112 ≤ λ ≤ 126 nm from a free electron laser and event-triggered, fast-framing, multi-mass imaging detection methods. Contributions from at least five primary photofragmentation pathways yielding CH2, CH3 and/or H atom products are demonstrated and interpreted in terms of unimolecular decay following rapid non-adiabatic coupling to the ground state potential energy surface. These data serve to highlight parallels with methane photochemistry and limitations in contemporary models of the photoinduced stratospheric chemistry of the gas giants. The work identifies additional photochemical reactions that require incorporation into next generation extraterrestrial atmospheric chemistry models which should help rationalise hitherto unexplained aspects of the atmospheric ethane/acetylene ratios revealed by the Cassini-Huygens fly-by of Jupiter.

CH3NO as a potential intermediate for early atmospheric HCN: a quantum chemical insight

Hydrogen cyanide (HCN) has played a central role in the production of several biological molecules under prebiotic conditions on primitive Earth. Previously, K. J. Zahnle (J. Geophys. Res.: Atmos., 1986, 91, 2819) and Tian et al. (Earth Planet. Sci. Lett., 2011, 308, 417) emphasized that HCN production in the early Earth's CH₄-rich atmosphere could have been possible through the reaction between active nitrogen atoms (N) and methane photolysis products. Here, we have proposed alternative pathways for the formation of early atmospheric HCN via the decomposition of CH₃NO as an intermediate. In the early Earth's O₂-free atmosphere, CH₃˙ could preferentially attach to NO, which was generated via early atmospheric volcanism or lightning and photochemical processes. We have quantum chemically explored both unimolecular and bimolecular decomposition pathways of CH₃NO via the assistance of another CH₃NO molecule and via H₂O, NH₃, HCl, HCOOH, HNO₃ and H₂SO₄ catalysis. Both energetic and kinetic analyses reveal that H₂SO₄ is more efficient in this regard than other atmospheric species. Overall, it has been suggested that the proposed bimolecular decomposition pathways might have been alternative pathways for the formation of HCN under certain conditions on prebiotic Earth, while the unimolecular decomposition of CH₃NO could lead to the formation of HCN in the high temperature volcanic environment on early Earth.

Photoinduced C–H bond fission in prototypical organic molecules and radicals

We survey and assess current knowledge regarding the primary photochemistry of hydrocarbon molecules and radicals.

Possibility of methane conversion into heavier hydrocarbons using nanosecond lasers

Effect of nanosecond lasers on the methane dissociation is experimentally studied by using three different laser wavelengths at 248 nm, 355 nm and 532 nm. C2H2 generation is measured as a major reaction product in experiments and the energy consumptions in production of this component are measured as 5.8 MJ/mol, 3.1 MJ/mol and 69.0 MJ/mol, for 355 nm, 532 nm and 248 nm wavelengths, respectively. The mechanism of conversion and production of new stable hydrocarbons is also theoretically investigated. It is found that in theoretical calculations, the ion-molecule reactions should be included and this leads to a unique approach in proper explanation of the experimental measurements.

13C and 15N fractionation of CH4/N2 mixtures during photochemical aerosol formation: Relevance to Titan

The ratios of the stable isotopes that comprise each chemical species in Titan's atmosphere provide critical information towards understanding the processes taking place within its modern and ancient atmosphere. Several stable isotope pairs, including 12C/13C and 14N/15N, have been measured in situ or probed spectroscopically by Cassini-borne instruments, space telescopes, or through ground-based observations. Current attempts to model the observed isotope ratios incorporate fractionation resulting from atmospheric diffusion, hydrodynamic escape, and primary photochemical processes. However, the effect of a potentially critical pathway for isotopic fractionation - organic aerosol formation and subsequent deposition onto the surface of Titan - has not been considered due to insufficient data regarding fractionation during aerosol formation. To better understand the nature of this process, we have conducted a laboratory study to measure the isotopic fractionation associated with the formation of Titan aerosol analogs, commonly referred to as 'tholins', via far-UV irradiation of several methane (CH4) and dinitrogen (N2) mixtures. Analysis of the δ13C and δ15N isotopic signatures of the photochemical aerosol products using an isotope ratio mass spectrometer (IRMS) show that fractionation direction and magnitude are dependent on the initial bulk composition of the gas mixture. In general, the aerosols showed enrichment in 13C and 14N, and the observed fractionation trends can provide insight into the chemical mechanisms controlling photochemical aerosol formation.

Neutral-fragmentation paths of methane induced by intense ultrashort IR laser pulses: ab initio molecular orbital approach

Instantaneous (laser-field-dependent) potential energy curves leading to neutral fragmentations of methane were calculated at several laser intensities from 1.4 × 10(13) to 1.2 × 10(14) W/cm(2) (from 1.0 × 10(10) to 3.0 × 10(10) V/m) using ab initio molecular orbital (MO) methods to validate the observation of neutral fragmentations induced by intense femtosecond IR pulses (Kong et al. J. Chem. Phys. 2006, 125, 133320). Two fragmentation paths, CH(2) + 2H and CH(2) + H(2), in (1)T(2) superexcited states that are located in the energy range of 12-16 eV were considered as the reaction paths because these states are responsible for Jahn-Teller distortion opening up reaction paths during ultrashort pulses. As field intensity increased, the low-lying excited (1)A(1) states originated from the Jahn-Teller (1)T(2) states were substantially stabilized along the neutral-fragment path CH(4) → CH(2) + 2H and were located below the ionization threshold. On the other hand, the low-lying excited (1)B(1) states, which also originate from the Jahn-Teller (1)T(2) states, were embedded on the ionized state along the dissociation path to CH(2) + H(2). This indicates that ionic fragments, rather than neutral ones, are produced along the CH(2) + H(2) path. The computational results support neutral fragmentations through superexcited states proposed by Kong et al.

Reaction dynamics on the formation of 1- and 3-cyanopropylene in the crossed beams reaction of ground-state cyano radicals (CN) with propylene (C3H6) and its deuterated isotopologues

Crossed molecular beams experiments were utilized to explore the chemical reaction dynamics of ground-state cyano radicals, CN(X(2)Sigma(+)), with propylene (CH3CHCH2) together with two d3-isotopologues (CD3CHCH2, CH3CDCD2) as potential pathways to form organic nitriles under single collision conditions in the atmosphere of Saturn's moon Titan and in the interstellar medium. On the basis of the center-of-mass translational and angular distributions, the reaction dynamics were deduced to be indirect and commenced via an addition of the electrophilic cyano radical with its radical center to the alpha-carbon atom of the propylene molecule yielding a doublet radical intermediate: CH3CHCH2CN. Crossed beam experiments with propylene-1,1,2-d3 (CH3CDCD2) and propylene-3,3,3-d3 (CD3CHCH2) indicated that the reaction intermediates CH3CDCD2CN (from propylene-1,1,2-d3) and CD3CHCH2CN (from propylene-3,3,3-d3) eject both atomic hydrogen through tight exit transition states located about 40-50 kJ mol(-1) above the separated products: 3-butenenitrile [H2CCDCD2CN] (25%), and cis/trans-2-butenenitrile [CD3CHCHCN] (75%), respectively, plus atomic hydrogen. Applications of our results to the chemical processing of cold molecular clouds like TMC-1 and OMC-1 are also presented.

Hydrogen Migration and Vinylidene Pathway for Formation of Methane in the 193 nm Photodissociation of Propene: CH3CHCH2 and CD3CDCD2

Photodissociation channels and the final product yields from the 193 nm photolysis of propene-h6 (CH(2)=CHCH(3)) and propene-d6 (CD(2)=CDCD(3)) have been investigated, employing gas chromatography, mass spectroscopy, and flame ionization (GC/MS/FID) detection methods. The yields of methane as well as butadiene relative to ethane show considerable variations when propene-h6 or propene-d6 are photolyzed. This suggests significant variances in the relative importance of primary photolytic processes and/or secondary radical reactions, occurring subsequent to the photolysis. Theoretical calculations suggest the potential occurrence of an intramolecular dissociation through a mechanism involving vinylidene formation, accompanied by an ethylenic H-migration through the pi-orbitals. This process affects the final yields of methane-h4 versus methane-d4 with respect to other products. The product yields from previous studies of the 193 nm photolysis of methyl vinyl ketone-h6 and -d6 (CH(2)=CHCOCH(3), CD(2)=CDCOCD(3)), alternative precursors for generating methyl and vinyl radicals, are compared with the current results for propene.

Production yields of H(D) atoms in the reactions of N2(AΣu+3) with C2H2, C2H4, and their deuterated variants

The production yields of H(D) atoms in the reactions of N(2)(A (3)Sigma(u) (+)) with C(2)H(2), C(2)H(4), and their deuterated variants were determined. N(2)(A (3)Sigma(u) (+)) was produced by excitation transfer between Xe(6s[32](1)) and ground-state N(2) followed by collisional relaxation. Xe(6s[32](1)) was produced by two-photon laser excitation of Xe(6p[12](0)) followed by concomitant amplified spontaneous emission. H(D) atoms were detected by using vacuum-ultraviolet laser-induced fluorescence (LIF). The H(D)-atom yields were evaluated from the LIF intensities and the overall rate constants for the quenching, which were determined from the temporal profiles of the NO tracer emission. The absolute yields were evaluated by assuming that the yield for NH(3)(ND(3)) is 0.9. Although no HD isotope effects were observed in the overall rate constants, there were isotope effects in the H(D)-atom yields. The H-atom yields for C(2)H(2) and C(2)H(4) were 0.52 and 0.30, respectively, while the D-atom yields for C(2)D(2) and C(2)D(4) were 0.33 and 0.13, respectively. The presence of isotope effects in yields suggests that H(2)(D(2)) molecular elimination processes are competing and that molecular elimination is more dominant in deuterated species than in hydrides.

First ultraviolet absorption band of methane: Anab initiostudy

Quantum mechanical calculations of the cross sections for photodissociation of CH4 and CD4 in the 1t2-->3s band are presented. The potential energy surfaces for the three states correlating with the 1 1T2 state at tetrahedral geometries are calculated. The elements of the (3x3) matrix representing the electronic Hamiltonian in the diabatic basis are expanded in powers of nuclear coordinates, up to the second order. The expansion coefficients are based on accurate multireference configuration interaction calculations. The electronically nonadiabatic dynamics is treated with the multiconfiguration time-dependent Hartree approach. All nine internal degrees of methane are included in the quantum dynamics simulations. The calculated cross section agrees well with experiment. Semiclassical calculations using the reflection principle suggest that the peaks in the spectrum correspond to the three adiabatic electronic states correlating with the 1 1T2 state at Td geometries. However, the non-Born-Oppenheimer terms in the Hamiltonian have a strong effect on the positions of the peaks in the absorption spectrum. The results of semiclassical calculations, which neglect these terms, are therefore quite different from the accurate quantum results and experiment.

Rate Constants and H Atom Branching Ratios of the Gas-Phase Reactions of Methylidyne CH(X2Π) Radical with a Series of Alkanes

The reactions of the CH radical with several alkanes were studied, at room temperature, in a low-pressure fast-flow reactor. CH(X2Pi, v = 0) radicals were obtained from the reaction of CHBr(3) with potassium atoms. The overall rate constants at 300 K are (0.76 +/- 0.20) x 10(-10) [Fleurat-Lessard, P.; Rayez, J. C.; Bergeat, A.; Loison, J. C. Chem. Phys. 2002, 279, 87],1 (1.60 +/- 0.60) x 10(-10)[Galland, N.; Caralp, F.; Hannachi, Y.; Bergeat, A.; Loison, J.-C. J. Phys. Chem. A 2003, 107, 5419],2 (2.20 +/- 0.80) x 10(-10), (2.80 +/- 0.80) x 10(-10), (3.20 +/- 0.80) x 10(-10), (3.30 +/- 0.60) x 10(-10), and (3.60 +/- 0.80) x 10(-10) cm3 molecule(-1) s(-1), (errors refer to +/-2sigma) for methane, ethane, propane, n-butane, n-pentane, neo-pentane, and n-hexane respectively. The experimental overall rate constants correspond to those obtained using a simple classical capture theory. Absolute atomic hydrogen production was determined by V.U.V. resonance fluorescence, with H production from the CH + CH4 reaction being used as a reference. Observed H branching ratios were for CH4, 1.00[Fleurat-Lessard, P.; Rayez, J. C.; Bergeat, A.; Loison, J. C. Chem. Phys. 2002, 279, 87];1 C(2)H(6), 0.22 +/- 0.08 [Galland, N.; Caralp, F.; Hannachi, Y.; Bergeat, A.; Loison, J.-C. J. Phys. Chem. A 2003, 107, 5419];2 C(3)H(8), 0.19 +/- 0.07; C(4)H(10) (n-butane), 0.14 +/- 0.06; C(5)H(12) (n-pentane), 0.52 +/- 0.08; C(5)H(12) (neo-pentane), 0.51 +/- 0.08; C(5)H(12) (iso-pentane), 0.12 +/- 0.06; C(6)H(14) (n-hexane), 0.06 +/- 0.04.

Photodissociation of methane: Exploring potential energy surfaces

The potential energy surface for the first excited singlet state (S(1)) of methane is explored using multireference singles and doubles configuration interaction calculations, employing a valence triple zeta basis set. A larger valence quadruple zeta basis is used to calculate the vertical excitation energy and dissociation energies. All stationary points found on the S(1) surface are saddle points and have imaginary frequencies for symmetry-breaking vibrations. By studying several two-dimensional cuts through the potential energy surfaces, it is argued that CH(4) in the S(1) state will distort to planar structures. Several conical intersection seams between the ground state surface S(0) and the S(1) surface have been identified at planar geometries. The conical intersections provide electronically nonadiabatic pathways towards products CH(3)((approximately)X (2)A"(2))+H, CH(2)((approximately)a (1)A(1))+H(2), or CH(2)((approximately)X (3)B(1))+H+H. The present results thereby make it plausible that the CH(3)((approximately)X (2)A"(2))+H and CH(2)((approximately)a (1)A(1))+H(2) channels are major dissociation channels, as has been observed experimentally.

H(D)-atom yields in the quenching of Xe(6s[3∕2]1) by methane, ethane, ethene, ethyne, and their deuterated isotopologues

The yields for the production of H(D) atoms in the reactions of Xe(6s[3/2]1) with simple hydrocarbons and their deuterated variants were determined. Xe(6s[32](1)) was produced by two-photon laser excitation of Xe(6p[1/2]0) followed by concomitant amplified spontaneous emission. H(D) atoms are detected using a vacuum-ultraviolet laser-induced fluorescence (LIF) technique. The H(D)-atom yields were evaluated from the LIF intensities and the overall rate constants for the quenching, which were determined from the temporal profile measurements of the resonance fluorescence from Xe(6s[3/2](1)). HD isotope effects were observed not only in the overall rate constants but also in the H(D)-atom yields. The yields for CH4, C2H4, and C2H2 were determined to be 0.89, 1.43, 1.03, respectively, while those for CD4, C2D4, and C2D2 were found to be smaller; 0.63, 0.86, and 0.79, respectively. The HD yield ratio for CH2D2 was 1.76. The presence of the isotope effects both in the rate constants and the yields suggests that electronic-to-electronic energy transfer processes and abstractive processes are competing.

Vacuum-ultraviolet spectroscopy of poly(methylphenylsilylene)

The first vacuum-ultraviolet spectrum of a polysilylene (chain-type polysilane) with aromatic substituents is presented. Assignments of the absorption bands of the model compound poly(methylphenylsilylene) are based on previous experimental data and theoretical electronic band structure calculations for poly(alkylsilylenes) and on ultraviolet spectra of phenyl-containing monomers and polymers. Although aryl orbitals mix with the sigma-conjugated orbitals located along the catenated silicon backbone, some transitions are largely localized on the phenyl groups. These assignments elucidate the nature of the bonding in polysilylenes and should be useful in understanding photodegradation mechanisms and in the design of related new optical materials.

State-resolved dynamics of photofragmentation

We discuss experiments on the dynamics of photodissociation that employ methods to select the energy, sometimes quantum states, of the reactant and to determine the quantum states and energy, sometimes also the orientation and alignment, of products. A summary of new advances of experimental methods is followed by applications to photodissociation of various types. Representative examples of simple bond fission, molecular elimination, and three-body dissociation with determined electronic states-sometimes the orientation of their angular momentum-of product atoms or distributions of electronic and internal states of product molecules illustrate the detailed information and insight that one can derive from such experiments. Photodissociation of van der Waals complexes, ions, species adsorbed on surfaces, and species in solution is excluded from this review.