Photolysis Of CH Research Articles

New Estimates of Nitrogen Fixation on Early Earth.

Fixed nitrogen species generated by the early Earth's atmosphere are thought to be critical to the emergence of life and the sustenance of early metabolisms. A previous study estimated nitrogen fixation in the Hadean Earth's N2/CO2-dominated atmosphere; however, that previous study only considered a limited chemical network that produces NOx species (i.e., no HCN formation) via the thermochemical dissociation of N2 and CO2 in lightning flashes, followed by photochemistry. Here, we present an updated model of nitrogen fixation on Hadean Earth. We use the Chemical Equilibrium with Applications (CEA) thermochemical model to estimate lightning-induced NO and HCN formation and an updated version of KINETICS, the 1-D Caltech/JPL photochemical model, to assess the photochemical production of fixed nitrogen species that rain out into the Earth's early ocean. Our updated photochemical model contains hydrocarbon and nitrile chemistry, and we use a Geant4 simulation platform to consider nitrogen fixation stimulated by solar energetic particle deposition throughout the atmosphere. We study the impact of a novel reaction pathway for generating HCN via HCN2, inspired by the experimental results which suggest that reactions with CH radicals (from CH4 photolysis) may facilitate the incorporation of N into the molecular structure of aerosols. When the HCN2 reactions are added, we find that the HCN rainout rate rises by a factor of five in our 1-bar case and is about the same in our 2- and 12-bar cases. Finally, we estimate the equilibrium concentration of fixed nitrogen species under a kinetic steady state in the Hadean ocean, considering loss by hydrothermal vent circulation, photoreduction, and hydrolysis. These results inform our understanding of environments that may have been relevant to the formation of life on Earth, as well as processes that could lead to the emergence of life elsewhere in the universe.

Meridional Variations of C2H2 in Jupiter's Stratosphere From Juno UVS Observations

AbstractThe Ultraviolet Spectrograph (UVS) instrument on the Juno mission records far‐ultraviolet reflected sunlight from Jupiter. These spectra are sensitive to the abundances of chemical species in the upper atmosphere and to the distribution of the stratospheric haze layer. We combine observations from the first 30 perijoves of the mission in order to study the meridional distribution of acetylene (C2H2) in Jupiter's stratosphere. We find that the abundance of C2H2 decreases toward the poles by a factor of 2–4, in agreement with previous analyses of mid‐infrared spectra. This result is expected from insolation rates: near the equator, the UV solar flux is higher, allowing more C2H2 to be generated from the UV photolysis of CH4. The decrease in abundance toward the poles suggests that horizontal mixing rates are not rapid enough to homogenize the latitudinal distribution.

Vacuum ultraviolet photolysis of CH4:O2 ice mixtures and the formation of CO and CO2

Vacuum ultraviolet (VUV) photolysis of CH4:O2 ice mixtures at 10 K using 121.6, 157, and 170-nm wavelengths was performed to examine photochemical reactions for the oxidation of hydrocarbon species. Ice samples with three component ratios were prepared: CH4-dominated (CH4:O2 = 100:1), mixed (CH4:O2 = 1:1), and O2-dominated (CH4:O2 = 1:100) ice samples. Photochemical behavior with dependence on the photolysis wavelength and component ratio was observed. In addition, the column densities of CO and CO2 formed by the VUV photolysis of the three ice samples and the abundance ratios of CO and CO2 as a function of the photon dosage and photolysis wavelength were examined. The results may imply a correlation between the unknown depletion of interstellar oxygen and the formation of CO and CO2.

VUV Photolysis of CH4–H2O mixture in methane-rich ices: Formation of large complex organic molecules in astronomical environments

The present work aims to highlight the influence of water molecules in the photo-decomposition of methane ice and reveal the photoproducts formed in solid phase upon VUV irradiation of CH4–H2O mixture in methane-rich ices. The analysis of our IR spectra shows that even with very low concentrations of water in methane ices, several oxygenated hydrocarbons are formed as photoproducts derived from the photodecomposition of water and methane at cryogenic temperatures. We show that both alka[e]nes and oxygen bearing organics are efficiently formed at temperatures as low as 3 K. However, while the IR signatures of the alka[e]nes such as C2H6, C2H4 and C2H2 dominate the IR spectra of the irradiated CH4–H2O ices at temperatures lower than 50 K, the heating of the sample to 110 K reveals the formation of large carbon chain complex organic molecules such as ethanol, pronanol, propanal and glycolaldehyde.

Chemical Composition of Gas-Phase Positive Ions during Laboratory Simulations of Titan’s Haze Formation

The organic haze in the atmosphere of Saturn’s moon Titan affects the moon’s atmospheric and surface properties. Ions are known to play an important role in haze formation chemistry; however, the role of ions in laboratory simulations of haze formation is poorly characterized. Here, we use a high-resolution time-of-flight mass spectrometer with an atmospheric pressure interface to chemically characterize the ambient gas-phase cations formed as haze is produced by the far ultraviolet photolysis of CH4 in N2. These experiments show that the chemical composition of the cations is complex. High molecular weight ions up to m/z 400 are observed, with organic nitrogen ions, including ions with multiple nitrogen atoms, accounting for the majority of the identified ion peaks. Many of the ions identified in this work have been observed in Titan’s upper atmosphere by the Cassini Ion and Neutral Mass Spectrometer. While the chemistry in this study is likely more representative of Titan’s lower atmosphere than of its ...

3D modeling of organic haze in Pluto’s atmosphere

The New Horizons spacecraft, which flew by Pluto on July 14, 2015, revealed the presence of haze in Pluto’s atmosphere that were formed by CH4/N2 photochemistry at high altitudes in Pluto’s atmosphere, as on Titan and Triton. In order to help the analysis of the observations and further investigate the formation of organic haze and its evolution at global scales, we have implemented a simple parameterization of the formation of organic haze in our Pluto General Circulation Model. The production of haze in our model is based on the different steps of aerosol formation as understood on Titan and Triton: photolysis of CH4 in the upper atmosphere by Lyman-α UV radiation, production of various gaseous species, and conversion into solid particles through accumulation and aggregation processes. The simulations use properties of aerosols similar to those observed in the detached haze layer on Titan. We compared two reference simulations ran with a particle radius of 50 nm: with, and without South Pole N2 condensation. We discuss the impact of the particle radius and the lifetime of the precursors on the haze distribution. We simulate CH4 photolysis and the haze formation up to 600 km above the surface. Results show that CH4 photolysis in Pluto’s atmosphere in 2015 occurred mostly in the sunlit summer hemisphere with a peak at an altitude of 250 km, though the interplanetary source of Lyman-α flux can induce some photolysis even in the Winter hemisphere. We obtained an extensive haze up to altitudes comparable with the observations, and with non-negligible densities up to 500 km altitude. In both reference simulations, the haze density is not strongly impacted by the meridional circulation. With No South Pole N2 condensation, the maximum nadir opacity and haze extent is obtained at the North Pole. With South Pole N2 condensation, the descending parcel of air above the South Pole leads to a latitudinally more homogeneous haze density with a slight density peak at the South Pole. The visible opacities obtained from the computed mass of haze, which is about 2–4×10−7 g cm−2 in the summer hemisphere, are similar for most of the simulation cases and in the range of 0.001-0.01, which is consistent with recent observations of Pluto and their interpretation.

FORMATION OF N 3 , CH 3 , HCN, AND HNC FROM THE FAR-UV PHOTOLYSIS OF CH 4 IN NITROGEN ICE

The irradiation of pure solid N2 at 3 K with far-ultraviolet light from a synchrotron produced infrared absorption lines at 1657.7, 1655.6, and 1652.4 cm−1 and an ultraviolet absorption line at 272.0 nm, which are characteristic of the product N3. The threshold wavelength at which N3 was generated was 145.6 ± 2.9 nm, corresponding to an energy of 8.52 ± 0.17 eV. The photolysis of isotopically labeled 15N2 at 3 K consistently led to the formation of 15N3 with the same threshold wavelength of 145.6 ± 2.9 nm for its formation. The photolysis of CH4 in nitrogen ice in low concentrations also led to the formation of N3, together with CH3, HCN, and HNC, with the same threshold wavelength of 145.6 ± 2.9 nm. These results indicate that N3 radicals may play an important role in the photochemistry of nitrogen ices in astronomical environments.

Photolysis of CH₃CHO at 248 nm: evidence of triple fragmentation from primary quantum yield of CH₃ and HCO radicals and H atoms.

Radical quantum yields have been measured following the 248 nm photolysis of acetaldehyde, CH3CHO. HCO radical and H atom yields have been quantified by time resolved continuous wave Cavity Ring Down Spectroscopy in the near infrared following their conversion to HO2 radicals by reaction with O2. The CH3 radical yield has been determined using the same technique following their conversion into CH3O2. Absolute yields have been deduced for HCO radicals and H atoms through fitting of time resolved HO2 profiles, obtained under various O2 concentrations, to a complex model, while the CH3 yield has been determined relative to the CH3 yield from 248 nm photolysis of CH3I. Time resolved HO2 profiles under very low O2 concentrations suggest that another unknown HO2 forming reaction path exists in this reaction system besides the conversion of HCO radicals and H atoms by reaction with O2. HO2 profiles can be well reproduced under a large range of experimental conditions with the following quantum yields: CH3CHO + hν(248nm) → CH3CHO*, CH3CHO* → CH3 + HCO ϕ(1a) = 0.125 ± 0.03, CH3CHO* → CH3 + H + CO ϕ(1e) = 0.205 ± 0.04, CH3CHO*[Formula: see text]CH3CO + HO2 ϕ(1f) = 0.07 ± 0.01. The CH3O2 quantum yield has been determined in separate experiments as ϕ(CH₃) = 0.33 ± 0.03 and is in excellent agreement with the CH3 yields derived from the HO2 measurements considering that the triple fragmentation (R1e) is an important reaction path in the 248 nm photolysis of CH3CHO. From arithmetic considerations taking into account the HO2 and CH3 measurements we deduce a remaining quantum yield for the molecular pathway: CH3CHO* → CH4 + CO ϕ(1b) = 0.6. All experiments can be consistently explained with absence of the formerly considered pathway: CH3CHO* → CH3CO + H ϕ(1c) = 0.

Ly[formula omitted]@Pluto

The Alice instrument on New Horizons will perform several observations of Pluto’s far-ultraviolet (FUV) airglow emissions during its July 2015 flyby. While Pluto’s atmosphere is dominated by N2, simulations suggest that the brightest airglow signal at Pluto will actually be due to Lyman alpha (Lyα) emissions of atomic hydrogen. This is because H atoms, produced at lower altitudes due to the photolysis of CH4 and other hydrocarbons, rise up above the homopause to become an important constituent of the atmosphere at high altitudes, and are able to scatter the very bright Lyα lines from the Sun and the interplanetary medium (IPM). The IPM Lyα signal at Earth is very much less than direct solar Lyα, but IPM Lyα falls off much more slowly than r-2, so that at Pluto’s distance from the Sun the two sources are of comparable strength. Detailed simulations of its Lyα emissions indicate that Pluto will appear dark against the IPM background, but that enough contrast exists for the useful extraction of H densities from the Alice observations. As viewed on approach (or from the inner solar system), the Lyα brightness of the disk of Pluto is expected to be ∼30R, against an IPM background of ∼90R.

Two roaming pathways in the photolysis of CH3CHO between 328 and 308 nm

We attribute the two product-state distributions previously seen in CH3CHO photodissociation to CH3-roaming and H-roaming, unifying all previous experimental results.

Impact of a new wavelength-dependent representation of methane photolysis branching ratios on the modeling of Titan’s atmospheric photochemistry

A new wavelength-dependent model for CH4 photolysis branching ratios is proposed, based on the values measured recently by Gans et al. (Gans, B. et al. [2011]. Phys. Chem. Chem. Phys. 13, 8140–8152). We quantify the impact of this representation on the predictions of a photochemical model of Titan’s atmosphere, on their precision, and compare to earlier representations. Although the observed effects on the mole fraction of the species are small (never larger than 50%), it is possible to draw some recommendations for further studies: (i) the Ly-α branching ratios of Wang et al. (Wang, J.H. et al. [2000]. J. Chem. Phys. 113, 4146–4152) used in recent models overestimate the CH2:CH3 ratio, a factor to which a lot of species are sensitive; (ii) the description of out-of-Ly-α branching ratios by the “100% CH3” scenario has to be avoided, as it can bias significantly the mole fractions of some important species (C3H8); and (iii) complementary experimental data in the 130–140nm range would be useful to constrain the models in the Ly-α deprived 500–700km altitude range.

Observations of upper tropospheric acetylene on Saturn: No apparent correlation with 2000 km-sized thunderstorms

Thunderstorm activity has been observed on Saturn via radio emissions from lightning discharges and optical detections of the lightning flashes on the planet's nightside. Thunderstorms provide extreme environments in which specific atmospheric chemistry can be induced—namely through energy release via lightning discharges, and fast vertical transport resulting in rapid advection of tropospheric species. It is thus theorised that certain atmospheric trace species – such as C2H2, HCN, and CO – can be generated in the troposphere by large bursts of energy in the form of lightning, and transported upward towards the upper troposphere by the extreme dynamics of thunderstorms, where they should be observable by satellite instruments. In this work, high-spectral-resolution Cassini/CIRS observations from October 2005 through April 2009 are used to study whether there is an observable increase in upper tropospheric acetylene in regions of known normal thunderstorm activity. Using both individual measurements in which there is known thunderstorm activity, as well as large coadditions of data to study latitudinal-dependencies over the full disc, no systematic enhancement in upper tropospheric (100mbar) C2H2 was detected around regions in which there are known occurrences of normally sized (2000km) thunderstorms, or in normally sized thunderstorm-prone regions such as 40°S. It is likely that the magnitude of the enhancement theorised is too generous or that enhancements are not advected into the upper troposphere as vertical mixing rates in models would suggest, since Cassini/CIRS can only detect C2H2 above the 200mbar level—although the massive northern hemisphere thunderstorm of 2010/2011 seems able to decrease stratospheric concentrations of C2H2. From this, it can be asserted that lightning from normal thunderstorm activity cannot be the key source for upper tropospheric C2H2 on Saturn, since the upper-tropospheric concentrations retrieved agree with the concentrations stemming from the photolysis of CH4 (2–3ppbv) from solar radiation penetrating through the Saturnian atmosphere, with an upper limit for lightning-induced C2H2 volume mixing ratio of 10−9.

The Formation of Sulfate and Elemental Sulfur Aerosols under Varying Laboratory Conditions: Implications for Early Earth

The presence of sulfur mass-independent fractionation (S-MIF) in sediments more than 2.45 × 10(9) years old is thought to be evidence for an early anoxic atmosphere. Photolysis of sulfur dioxide (SO(2)) by UV light with λ < 220 nm has been shown in models and some initial laboratory studies to create a S-MIF; however, sulfur must leave the atmosphere in at least two chemically different forms to preserve any S-MIF signature. Two commonly cited examples of chemically different sulfur species that could have exited the atmosphere are elemental sulfur (S(8)) and sulfuric acid (H(2)SO(4)) aerosols. Here, we use real-time aerosol mass spectrometry to directly detect the sulfur-containing aerosols formed when SO(2) either photolyzes at wavelengths from 115 to 400 nm, to simulate the UV solar spectrum, or interacts with high-energy electrons, to simulate lightning. We found that sulfur-containing aerosols form under all laboratory conditions. Further, the addition of a reducing gas, in our experiments hydrogen (H(2)) or methane (CH(4)), increased the formation of S(8). With UV photolysis, formation of S(8) aerosols is highly dependent on the initial SO(2) pressure; and S(8) is only formed at a 2% SO(2) mixing ratio and greater in the absence of a reductant, and at a 0.2% SO(2) mixing ratio and greater in the presence of 1000 ppmv CH(4). We also found that organosulfur compounds are formed from the photolysis of CH(4) and moderate amounts of SO(2). The implications for sulfur aerosols on early Earth are discussed. Key Words: S-MIF-Archean atmosphere-Early Earth-Sulfur aerosols.

On the abundance of non-cometary HCN on Jupiter

Using one-dimensional thermochemical/photochemical kinetics and transport models, we examine the chemistry of nitrogen-bearing species in the Jovian troposphere in an attempt to explain the low observational upper limit for HCN. We track the dominant mechanisms for interconversion of N2-NH3 and HCN-NH3 in the deep, high-temperature troposphere and predict the rate-limiting step for the quenching of HCN at cooler tropospheric altitudes. Consistent with some other investigations that were based solely on time-scale arguments, our models suggest that transport-induced quenching of thermochemically derived HCN leads to very small predicted mole fractions of hydrogen cyanide in Jupiter's upper troposphere. By the same token, photochemical production of HCN is ineffective in Jupiter's troposphere: CH4-NH3 coupling is inhibited by the physical separation of the CH4 photolysis region in the upper stratosphere from the NH3 photolysis and condensation region in the troposphere, and C2H2-NH3 coupling is inhibited by the low tropospheric abundance of C2H2. The upper limits from infrared and submillimetre observations can be used to place constraints on the production of HCN and other species from lightning and thundershock sources.

Reduction in Haze Formation Rate on Prebiotic Earth in the Presence of Hydrogen

Recent attempts to resolve the faint young Sun paradox have focused on an early Earth atmosphere with elevated levels of the greenhouse gases methane (CH(4)) and carbon dioxide (CO(2)) that could have provided adequate warming to Earth's surface. On Titan, the photolysis of CH(4) has been shown to create a thick haze layer that cools its surface. Unlike Titan, however, early Earth's atmosphere likely contained high amounts of CO(2) and hydrogen (H(2)). In this work, we examine haze formation in an early Earth atmosphere composed of CO(2), H(2), N(2), and CH(4), with a CO(2)/CH(4) ratio of 10 and a H(2)/CO(2) ratio of up to 15. To initiate aerosol formation, a broad-spectrum ultraviolet (UV) energy source with emission at Lyman-alpha was used to simulate the solar spectrum. Aerosol composition and total aerosol mass produced as a function of reagent gas were measured with an aerosol mass spectrometer (AMS). Results show an order of magnitude decrease in haze production with the addition of H(2), with no significant change in the chemical composition of the haze. We calculate that the presence of H(2) on early Earth could thus have favored warmer surface temperatures and yet allowed photochemical haze formation to deliver complex organic species to early Earth's surface.

Optical and chemical properties of tholins

AbstractFor over three decades tholins have been synthesized from mixtures of the cosmically abundant gases CH4, C2H6, NH3, H2O, HCHO, N2, and H2, previously in the Laboratory for Planetary Studies at Cornell University and in recent years at NASA Ames Research Center. The tholin synthesized by UV light or spark discharge on sequential and non-sequential pyrolysis GC-MS revealed hundreds of compounds, and on hydrolysis produced a large number of amino acids including racemic protein amino acids. Optical constants have been measured of many of the tholins, tholins produced from a condensed mixture of water and ethane at 77 K, poly HCN, and Titan tholin produced on electrical discharge through a mixture of 90% N2 and 10% CH4. Its optical constants were measured from soft x-rays to microwave for the first time.Here we report the absorption properties of Titan tholin that is produced in the temperature range 135 to 178 K where tholins are produced by magnetospheric charged particles, then pass through lower temperature at 70 K and finally to the ground at 95 K. While descending to the ground, it gets coated and processed on the way by other sources of energy such as long UV and cosmic rays. It is therefore expected that the stable products of CH4 photolysis react with Titan tholin to recycle the CH4 supply in Titan's atmosphere. Furthermore, the reactions of gaseous C2H6 with the reactive materials on the surface of the tholin could incorporate atmospheric C2H6 into the tholin and therefore might reduce the deposition rate of C2H6 onto the ground of Titan.

Lyman-α photodissociation of CH3CFCl2 (HCFC-141b): Quantum yield and translational energy of hydrogen atoms

The collision-free, room temperature gas-phase photodissociation dynamics of CH3CFCl2 (HCFC-141b) was studied using Lyman-α laser radiation (121.6 nm) by the laser photolysis/laser-induced fluorescence ‘pump/probe’ technique. Lyman-α radiation was used both to photodissociate the parent molecule and to detect the nascent H atom products via (2p 2P → 1s 2S) laser-induced fluorescence. Absolute H atom quantum yield, ϕH = (0.39 ± 0.09) was determined by calibration method in which CH4 photolysis at 121.6 nm was used as a reference source of well-defined H atom concentrations. The line shapes of the measured H atom Doppler profiles indicate a Gaussian velocity distribution suggesting the presence of indirect H atom formation pathways in the Lyman-α photodissociation of CH3CFCl2. The average kinetic energy of H atoms calculated from Doppler profiles was found to be E T(lab) = (50 ± 3) kJ/mol. The nearly statistical translational energy together with the observed Maxwell-Boltzmann velocity distribution indicates that for CH3CFCl2 the H atom forming dissociation process comes closer to the statistical limit.

Photolysis of CH3C(O)CH3 (248 nm, 266 nm), CH3C(O)C2H5 (248 nm) and CH3C(O)Br (248 nm): pressure dependent quantum yields of CH3 formation

The formation of CH(3) in the 248 or 266 nm photolysis of acetone (CH(3)C(O)CH(3)), 2-butanone (methylethylketone, MEK, CH(3)C(O)C(2)H(5)) and acetyl bromide (CH(3)C(O)Br) was examined using the pulsed photolytic generation of the radical and its detection by transient absorption spectroscopy at 216.4 nm. Experiments were carried out at room temperature (298 +/- 3 K) and at pressures between approximately 5 and 1500 Torr N(2). Quantum yields for CH(3) formation were derived relative to CH(3)I photolysis at the same wavelength in back-to-back experiments. For acetone at 248 nm, the yield of CH(3) was greater than unity at low pressures (1.42 +/- 0.15 extrapolated to zero pressure) confirming that a substantial fraction of the CH(3)CO co-product can dissociate to CH(3) + CO under these conditions. At pressures close to atmospheric the quantum yield approached unity, indicative of almost complete collisional relaxation of the CH(3)CO radical. Measurements of increasing CH(3)CO yield with pressure confirmed this. Contrasting results were obtained at 266 nm, where the yields of CH(3) (and CH(3)CO) were close to unity (0.93 +/- 0.1) and independent of pressure, strongly suggesting that nascent CH(3)CO is insufficiently activated to decompose on the time scales of these experiments at 298 K. In the 248 nm photolysis of CH(3)C(O)Br, CH(3) was observed with a pressure independent quantum yield of 0.92 +/- 0.1 and CH(3)CO remained below the detection limit, suggesting that CH(3)CO generated from CH(3)COBr photolysis at 248 nm is too highly activated to be quenched by collision. Similar to CH(3)C(O)CH(3), the photolysis of CH(3)C(O)C(2)H(5) at 248 nm revealed pressure dependent yields of CH(3), decreasing from 0.45 at zero pressure to 0.19 at pressures greater than 1000 Torr with a concomitant increase in the CH(3)CO yield. As part of this study, the absorption cross section of CH(3) at 216.4 nm (instrumental resolution of 0.5 nm) was measured to be (4.27 +/- 0.2) x 10(-17) cm(2) molecule(-1) and that of C(2)H(5) at 222 nm was (2.5 +/- 0.6) x 10(-18) cm(2) molecule(-1). An absorption spectrum of gas-phase CH(3)C(O)Br (210-305 nm) is also reported for the first time.

Photodissociation thresholds of OH produced from CH 3OH in solid neon and argon

Photodissociation thresholds of OH from CH 3OH in solid Ne and Ar were determined via photolysis of CH 3OH/Ne and CH 3OH/Ar (1/200) samples in situ at 4 K. The samples were irradiated with VUV synchrotron radiation after dispersion by a Seya-Namioka monochromator. The OH photo-product was detected by means of laser-induced fluorescence technique. The increase of fluorescent intensity of OH was monitored and recorded after the matrix sample was irradiated at different wavelengths for 3–5 min. Photodissociation threshold energies of 7.13±0.02 eV (174.0±0.5 nm) and 7.08±0.04 eV (175.0±1.0 nm) were measured for OH production of CH 3OH in solid Ne and Ar, respectively.

Vacuum ultraviolet photochemistry of methane, silane and germane

The photochemistry of jet-cooled CH4, SiH4 and GeH4 molecules following excitation at the Lyman-α wavelength (121.6 nm) has been investigated by high resolution photofragment translational spectroscopy methods. Complementary ab initio calculations of selected portions of the potential energy surfaces for the various components of the 1T2 and 3T2 excited states arising from the 3sa1←1t2 electron promotion are presented in the case of CH4. The form of the H atom recoil velocity distribution arising in the 121.6 nm photolysis of CH4 is rationalised in terms of initial excitation to both the 21A′ and 11A″ excited states (Jahn–Teller components of the degenerate 1T2 state), followed by a range of decay mechanisms. CH4(21A′) molecules can decay adiabatically, ia sequential extension of first one, then a second, C–H bond with eventual formation of two H atoms and CH2(ã1A1) products, or after internal conversion (IC) to the ground state. The H + CH3() products resulting from the IC process display a recoil velocity distribution characterised by an anisotropy parameter β∽ + 2, implying that the fragmentation involves irreversible extension of the C–H bond along which the transition dipole points at the instant of photon absorption. Fragmentation of CH4(11A″) molecules to H + CH3() products proceeds ia intersystem crossing to the lowest 3A′ potential energy surface. The recoil anisotropy of these products (β∽ − 0.45) implies that this radiationless process also occurs on a timescale that is rapid compared to the parent rotational period. Both single H–C bond fission channels may yield CH3() products with such high levels of internal excitation that they are unstable with respect to further unimolecular decay; any H atoms that result from this secondary decay must contribute to the observed yield of slow H atoms with β∽0. All H atoms resulting from Lyman-α photolysis of both SiH4 and GeH4 have (low) kinetic energies and little or no recoil anisotropy, compatible with their being formed ia three body fragmentation to, primarily, H + H + SiH2/GeH2(1A1) products. Faster H atoms are evident in the total kinetic energy release (TKER) spectra obtained following 157.6 nm photoexcitation of SiH4, but the power dependence of this fast H atom signal implies that these arise as a result of a two photon process involving initial formation of SiH2 + H2 products and subsequent photolysis of the nascent silylene fragments.