Photodissociation Of Methane Research Articles

Departure of the thermal escape rate from the jeans escape rate for atomic hydrogen at Earth, Mars, and Pluto

The recent observations of the Pluto’s upper atmosphere by the UV spectrometer Alice on the New Horizons spacecraft mission have shown that it is not in slow hydrodynamic escape as predicted by some fluid models but not by kinetic models. This instrument also detects the Lyman-alpha emission of atomic hydrogen. On Pluto, the hydrogen atoms are produced by the photodissociation of methane and reside in an extended corona around Pluto. Similar to the case at Earth and Mars, the Jeans escape should be the dominant escape process for hydrogen on Pluto due to the low value of the escape parameter at the exobase. However, because of this escape, the velocity distribution at the exobase is truncated at high velocities and the Jeans’s escape rate needs to be reduced by a factor B. The goal of this study is to calculate the value of B for the hydrogen on Pluto and check if a plane parallel model, valid to estimate B on Earth and Mars is also valid to calculate B on Pluto.We compute B with a plane parallel model for the planets’ exospheres, and with a more realistic spherical model to check the validity of the plane parallel model. We find very good agreement between the two models for the current exobase temperatures at Earth, Mars and Pluto. The departure of the thermal hydrogen escape rate from the predicted Jeans escape rate is larger for Mars and Earth than Pluto, even though the escape parameter is lower on Pluto than Mars and Earth. This difference is due to the presence of a minimum in this correction factor for an escape parameter near 3. This minimum is due to the large fraction of particles with a velocity larger than the escape velocity at low escape parameter, leading to an upward-directed velocity distribution close to the Maxwellian distribution at the exobase. The factor B can be decomposed as the product of two terms: one associated with the departure of the distribution velocity from a Maxwellian distribution at the exobase, and the second, associated with the few collisions above the exobase, reducing the escape rate. The first term has a minimum as a function of exobase temperature, while the second term is a monotonically decreasing function of exobase temperature to an asymptotic value.

Peculiarities of molecule photodissociation under influence of ultrashort electromagnetic pulses: Nonlinear dependence of probability on pulse duration

The paper considers novel behavior of molecule photodissociation probability under the influence of electromagnetic pulses in the femtosecond range. The study considers as examples the photodissociation of methane, ethylene and water molecules. The probability of photodissociation under radiation is calculated as a function of pulse duration for different values of the carrier frequency. In the limit of long pulses, the process probability, as expected, increases linearly with pulse duration. However, the dependence of photodissociation probability on pulse duration for the pulses in the femto- and sub-femtosecond ranges exhibits strong nonlinearity. The nonmonotonic effects occur at carrier pulse frequencies corresponding to the minimum values of the photodissociation cross-section. It is remarkable that the observed nonlinearity is described in the frame of the first order of perturbation theory for relatively small field amplitude and unrelated to the strong field effects. The proposed method of describing photodissociation is quite general and suitable for studying such processes under the influence of electromagnetic pulses of any duration.

Fragmentation dynamics of methane induced by femtosecond laser pulses

In this study it has been carried out theoretical simulations of ab-initio molecular dynamics of the C–H photo-dissociation of methane induced by femtosecond laser pulses. Our discussion about the reaction mechanism leading to the formation of the H and \(\hbox {CH}_{3}\) fragments is based on the rectification of the Lorentz force. Such an electric force rectification occurs via a periodical charge transfer between H and \(\hbox {CH}_{3}\) induced by the strong infrared laser pulse.

Trajectory surface-hopping study of methane photodissociation dynamics

We use the fewest switches nonadiabatic trajectory surface hopping approach to study the photodissociation of methane on its lowest singlet excited state potential surface (1 (1)T(2)) at 122 nm, with emphasis on product state branching and energy partitioning. The trajectories and couplings are based on CASSCF(8,9) calculations with an aug-cc-pvdz basis set. We demonstrate that nonadiabatic dynamics is important to describe the dissociation processes. We find that CH(3)(X (2)A(2) ("))+H and CH(2)(a (1)A(1))+H(2) are the major dissociation channels, as have been observed experimentally. CH(3)+H is mostly formed by direct dissociation that is accompanied by hopping to the ground state. CH(2)+H(2) can either be formed by hopping to the ground state to give CH(2)(a (1)A(1))+H(2) or by adiabatic dissociation to CH(2)(b (1)B(1))+H(2). In the latter case, the CH(2)(b (1)B(1)) can then undergo internal conversion to the ground singlet state by Renner-Teller induced hopping. Less important dissociation mechanisms lead to CH(2)+H+H and to CH+H(2)+H. Intersystem crossing effects, which are not included, do not seem essential to describe the experimentally observed branching behavior. About 5% of trajectories involve a roaming atom mechanism which can eventually lead to formation of products in any of the dissociation channels. Branching fractions to give H and H(2) are in good agreement with experiment, and the H atom translational energy distribution shows bimodal character which also matches observations.

Photodissociation of Methane at Lyman Alpha (121.6 nm)

Laser induced fluorescence studies of hydrogen atom using four wave mixing technique are reported for the photodissociation of CH4 and its isotopomers at Lya (121.6 nm). The source of dissociating and probe radiation is one and the same (delay time??20 nsec). The average translational energy of ejected hydrogen atoms (50 Kcal/mol) reveals that CH4 + hn??CH3 + H(2S) and CH4 + hn??CH2(a1A1) + H2(1Sg) are the main dissociation processes. The absolute quantum yield for CH4 and CD4 are the same, FH(CH4) = FD(CD4) = 0.31 0.05. If one divides the experimental H/D ratios from the isotopomers CH3D, CH2D2, CHD3 by the isotopic H/D ratios, a value 2 is obtained in all three cases. Overall, the heavier D atoms are more likely than the H atoms to remain attached to the carbon atom.

Explosive photodissociation of methane induced by ultrafast intense laser

A new type of molecular fragmentation induced by femtosecond intense laser at the intensity of 2 x 10(14) W/cm2 is reported. For the parent molecule of methane, ethylene, n-butane, and 1-butene, fluorescence from H (n = 3-->2), CH (A 2Delta, B 2Sigma-, and C 2Sigma+-->X 2Pi), or C2 (d 3Pi g-->a 3Pi u) is observed in the spectrum. It shows that the fragmentation is a universal property of neutral molecule in the intense laser field. Unlike breaking only one or two chemical bonds in conventional UV photodissociation, the fragmentation caused by the intense laser undergoes vigorous changes, breaking most of the bonds in the molecule, like an explosion. The fragments are neutral species and cannot be produced through Coulomb explosion of multiply charged ion. The laser power dependence of CH (A-->X) emission of methane on a log-log scale has a slope of 10 +/- 1. The fragmentation is thus explained as multiple channel dissociation of the superexcited state of parent molecule, which is created by multiphoton excitation.

Sensitivity studies of methane photolysis and its impact on hydrocarbon chemistry in the atmosphere of Titan

The photodissociation of methane at Lyman α (1216 Å) has been the focus of much scrutiny over the past few years. Methane photolysis leads to the formation of H2 molecules as well as H, CH, 1CH2, 3CH2, and CH3 radicals, which promote the propagation of hydrocarbon chemistry. However, recent studies [Mordaunt et al., 1993; Romani, 1996; Smith and Raulin, 1999] have not fully resolved the issue of methane photolytic product yields at this wavelength. We use a one‐dimensional photochemical model with updated chemistry to investigate the significance of these quantum yield schemes on the hydrocarbon chemistry of Titan's atmosphere, where Lyman α radiation accounts for 75% of methane photolysis longward of 1000 Å. Sensitivity studies show that while simple hydrocarbons like C2H2 (acetylene) and C2H4 (ethylene), which serve as important intermediates to the formation of more complex hydrocarbons, show virtually no variation in abundance, minor C3 molecules do show substantial sensitivity to choice of quantum yield scheme. We find that the C3H4 isomers (methylacetylene, allene) and C3H6 (propylene) display major variation in atmospheric mixing ratios under the implementation of these schemes, with a maximum variation of approximately a factor of 5 in C3H4 abundance and approximately a factor of 4 for C3H6. In these cases our nominal scheme, recommended by Romani [1996], offers an intermediate result in comparison with the other schemes. We also find that choice of pathway for non‐Lyman α methane absorption does affect hydrocarbon chemistry in the atmosphere of Titan, but this effect is minimal. A 65% variation in C2H6 (ethane) abundance, a value within observational uncertainty, is the largest divergence found for a wide range of possible non‐Lyman α photofragment quantum yields. These results will have significance in future modeling and interpretation of observations of the atmosphere of Titan.

Theoretical studies of surface reactions on metals: I. Ethyl to ethylene conversion on platinum; II. Photodissociation of methane on platinum

An embedding method designed to provide a molecular level understanding of adsorbate energetics and surface reaction mechanisms is presented. Electronic structures, including relativistic effects, are described by an ab initio formalism that permits an accurate determination of energies and adsorbate structure. Applications to catalytic and photochemical reactions on transition metal surfaces are reported. (I) In the catalytic conversion of ethyl adsorbed on platinum surfaces to ethylene, the minimum energy pathway for the transfer of hydrogen to the metal surface is calculated. A detailed description of the transition state is presented. (II) In the photodissociation of methane physisorbed on platinum, results are reported for a methane molecule interacting with an embedded cluster model of Pt(111). Configuration interaction theory is used to sort out states resulting from electron attachment to methane from lower energy states that correspond to metal excitations. Energies of the ground and excited states of the Pt(111)CH4 complex are calculated as a function of geometrical distortions and the distance of methane from the surface. Key steps in the photodissociation process are described.

Theoretical studies of surface reactions on metals.: I. Ethyl to ethylene conversion on Ni(1 0 0). II. Photodissociation of methane on platinum

The goal of this research is the development and application of theoretical techniques that will provide a molecular level understanding of surface processes, especially reaction mechanisms and energetics. Electronic structures are described by an ab initio embedding formalism that permits an accurate determination of energies and adsorbate structure. An overview of the theoretical method is presented and applications to catalytic and photochemical reactions on nickel and platinum surfaces are discussed. Preliminary studies of ethylene production from ethyl adsorbed on nickel are reported. A second study concerns the photodissociation of methane physisorbed on platinum. Results are reported for a methane molecule interacting with a Pt ring model of the surface. Configuration interaction theory is used to sort out states resulting from electron attachment to methane from lower energy states that correspond to metal excitations. A mechanism is proposed for the photodissociation process.

Photodissociation of methane on platinum

An overview of an embedding method for treating molecule-surface interactions is presented and an application to the photodissociation of methane physisorbed on platinum is discussed. Electronic structures are described by an ab initio formalism that permits an accurate determination of reaction energetics and adsorbate structure. Results are reported for a methane molecule interacting with a Pt ring model of the surface. Configuration interaction theory is used to sort out states resulting from electron attachment to methane from lower energy states that correspond to metal excitations. A mechanism is proposed for the photodissociation process.

Photochemical growing of complex organics in planetary atmospheres

UV induced syntheses of organic compounds from the main atmospheric constituents can be a very important source of organics in a given planetary environment provided the atmosphere is in a reduced state. The evolution of a CO 2 rich medium only produces very low yields of formaldehyde and related oxygenated compounds. Considering a CO rich atmosphere, the photochemical yield of O-organics formation is much higher, when the synthesis of N-organics remains difficult. The most favourable atmosphere as far as photochemical organic synthesis is concerned is a CH 4 rich milieu.. The photochemical evolution of such a CH 4 atmosphere under UV irradiation leads to a chain of various organics, the complexity of which increases together with the number of pathways involved in their formation. Their complexity also closely correlates with their UV photoabsorption spectrum: the more complex they are, the more shifted is their UV spectrum toward the visible range. Direct photodissociation of methane requires UV photon of wavelengths shorter than about 145 nm. It mainly produces ethane which absorbs UV at wavelengths shorter than about 160 nm, and acetylene, that presents an absorption spectrum extending up to 200 nm. This shift still continuously increases with further increase in number of C atoms. Unsaturated hydrocarbons with 4 and more C atoms have UV absorption characteristics including noticeable band structures in the 250–300 nm range. This trend has very important implication in the photochemical behaviour of a CH 4-rich planetary atmosphere, as it induces many catalytic processes. The occurrence of such processes is closely related to vertical atmospheric and energy deposition profiles. Titan provides a very good example of such a UV-directed organic atmospheric chemistry.

Primary product channels in the photodissociation of methane at 121.6 nm

The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of CH4(CD4) at 121.6 nm. Contrary to the previous consensus view, we find simple C–H bond fission to be the dominant primary process following excitation at this wavelength. The resulting CH3 fragments are formed with very high levels of internal excitation: Some (∼25%) possess so much internal energy that they must undergo subsequent unimolecular decay. The present experiments do not provide a unique determination of the products of this secondary decay process, but statistical arguments presented herein suggest that they will be predominantly CH and H2 fragments. Similar considerations point to a significant role for the direct three body process yielding the same products H+H2+CH. This overall pattern of energy disposal can be rationalized by assuming that most of the initially prepared CH4(Ã 1T2) molecules undergo rapid internal conversion (promoted by the Jahn–Teller distortion of this excited state) to high vibrational levels of the ground state prior to fragmentation. The realization that CH4 photodissociation at 121.6 nm yields CH3 (and CH) fragments, rather than methylene radicals, will necessitate some revision of current models of the hydrocarbon photochemistry prevailing in the atmospheres of the outer planets and some of their moons, notably Titan.

Hydrocarbon nucleation and aerosol formation in Neptune's atmosphere

Photodissociation of methane at high altitude levels in Neptune's atmosphere leads to the production of complex hydrocarbon species such as acetylene (C 2H 2), ethane (C 2H 6), methylacetylene (CH 3C 2H), propane (C 3H 8), diacetylene (C 4H 2), and butane (C 4H 8). These gases diffuse to the lower stratosphere where temperatures are low enough to initiate condensation. Particle formation may not occur readily, however, as the vapor species become supersaturated. We present a theoretical analysis of particle formation mechanisms at conditions relevant to Neptune's troposphere and stratosphere and show that hydrocarbon nucleation is very inefficient under Neptunian conditions: saturation ratios much greater than unity are required for aerosol formation by either homogeneous, heterogeneous, or ion-induced nucleation. Homogeneous nucleation will not be important for any of the hydrocarbon species considered; however, both heterogeneous and ion-induced nucleation should be possible on Neptune for most of the above species. The relative effectiveness of heterogeneous and ion-induced nucleation depends on the physical and thermodynamic properties of the particular species, the abundance of the condensable species, the temperature at which the vapor becomes supersaturated, and the number and type of condensation nuclei or ions available. Typical saturation ratios required for observable particle formation rates on Neptune range from ∼3 for heterogeneous nucleation of methane in the upper troposphere to greater than 1000 for heterogeneous nucleation of methylacetylene, diacetylene, and butane in the lower stratosphere. Thus, methane clouds may form slightly above, and stratospheric hazes far below, their saturation levels. When used in conjunction with the results of detailed models of atmospheric photochemistry, our nucleation models place realistic constraints on the altitude levels at which we expect hydrocarbon hazes or clouds to form on Neptune.

Pluto's extended atmosphere: An escape model and initial observations

We have calculated the rates of production and hydrodynamic outflow of atomic hydrogen resulting from the photodissociation of methane in the upper atmosphere of Pluto. Under the present near-perihelion conditions this yields an extended cloud of H around Pluto which is likely to be the most easily observable signature of Pluto's extended atmosphere, and thereby provide information on the extent, escape rate, and composition of Pluto's upper atmosphere. We have also performed initial observations with the IUE attempting to detect the H Ly α emission from the extended H cloud, which we use to derive upper limits to the cloud properties as a function of the cloud extent.

On the possibility of H2-near-infrared detection in comets with large gas production

Comets with large gas production offer a unique chance to observe a H2-flux of about 105 photon cm−2 s−1 sr−1 (1 Rayleigh) at wavelengths 8497.4 A, 8560.2 A and 8747.9 A-i.e., where photon counting methods are still applicable. In the following it will be shown that population of the vibrational levels, giving rise to these quadrupole overtone transitions, is dominated by photodissociation of methane, and that the emission even of quadrupole lines is not attenuated by collisional quenching. Wavelength scanning by ±1 A is shown to be enough to discriminate between cometary and atmospheric emissions by phase-sensitive subtraction techniques. Solid angle of Ω<10−7 sr has to be used, whence follows that a large ground-based telescope combined with a tilting Fabry-Perotfilter is best suited for detection of the near-infrared H2-emissions at reasonable counting rates and sufficient rejection of the atmospheric background.

Absolute Specific Photodissociation Cross Sections of CH4 in the Extreme Ultraviolet

The photodissociation of methane has been studied at 16 different incident wavelengths in the range 555–1242 Å. Absolute partial cross sections have been determined for those processes yielding excited fragments which fluoresce in the range 3500–8000 Å. The fluorescence was dispersed and found to result from the A 2Δ → X 2Π and B 2Σ → X 2Π systems of CH, the b̃ 1B1 → ã 1A1 system of CH2, and the Hα, Hβ, and Hγ transitions of H. Absolute cross sections have been assigned to the processes CH4 + hν → CH(A 2Δ) + H2(X 1Σg+) + H (1 2S1/2), CH4+hν → CH(B 2Σ) + H2(X 1Σg1) + H(1 2S1/2), and CH4 + hν → CH2 (b̃ 1B1) + H2 (X 1Σg+). Emission cross sections have been assigned to the Balmer lines Hα, Hβ, and Hγ. The process CH4 + hν → CH3 (X 2A2″) + H* is believed to be responsible for the Balmer lines. The largest cross section measured, σ = 8 × 10−20 cm2, occurred at 923 Å for the process leading to CH (A 2Δ). Cross sections of this magnitude are three orders of magnitude less than the total peak absorption cross section for methane.