Lyman-α Wavelength Research Articles

Vacuum ultraviolet photodissociation of sulfur dioxide and its implications for oxygen production in the early Earth's atmosphere.

The emergence of molecular oxygen (O2) in the Earth's primitive atmosphere is an issue of major interest. Although the biological processes leading to its accumulation in the Earth's atmosphere are well understood, its abiotic source is still not fully established. Here, we report a new direct dissociation channel yielding S(1D) + O2(a1Δg/X3Σg-) products from vacuum ultraviolet (VUV) photodissociation of SO2 in the wavelength range between 120 and 160 nm. Experimental results show O2 production to be an important channel from SO2 VUV photodissociation, with a branching ratio of 30 ± 5% at the H Lyman-α wavelength (121.6 nm). The relatively large amounts of SO2 emitted from volcanic eruptions in the Earth's late Archaean eon imply that VUV photodissociation of SO2 could have provided a crucial additional source term in the O2 budget in the Earth's primitive atmosphere. The results could also have implications for abiotic oxygen formation on other planets with atmospheres rich in volcanically outgassed SO2.

Lyman-α source for laser cooling antihydrogen.

We present a Lyman-α laser developed for cooling trapped antihydrogen. The system is based on a pulsed Ti:sapphire laser operating at 729nm that is frequency doubled using an LBO crystal and then frequency tripled in a Kr/Ar gas cell. After frequency conversion, this system produces up to 5.7μW of average power at the Lyman-α wavelength. This laser is part of the ATRAP experiment at the antiproton decelerator in CERN.

Observed responses of mesospheric water vapor to solar cycle and dynamical forcings.

This study focuses on responses of mesospheric water vapor (H2O) to the solar cycle flux at Lyman-α wavelength and to wave forcings according to the multivariate ENSO index (MEI). The zonal-averaged responses are for latitudes from 60°S to 60°N and pressure-altitudes from 0.01 to 1.0 hPa, as obtained by multiple linear regression (MLR) analyses of time series of H2O from the Halogen Occultation Experiment (HALOE) for July 1992 to November 2005. The solar responses change from strong negative H2O values in the upper mesosphere to very weak, positive values in the tropical lower mesosphere. Those response profiles at the low latitudes agree reasonably with published results for H2O from the Microwave Limb Sounder (MLS). The distribution of seasonal H2O amplitudes corresponds well with that for temperature and is in accord with the seasonal net circulation. In general, the responses of H2O to MEI are anti-correlated with those of temperature. H2O responses to MEI are negative in the upper mesosphere and largest in the northern hemisphere; responses in the lower mesosphere are more symmetric with latitude. The H2O trends from MLR for the lower mesosphere agree with those reported from time series of microwave observations at two ground-based network stations.

Measurement of the absorption cross sections of SiCl4, SiCl3, SiCl2 and Cl at H Lyman-[formula omitted] wavelength

Atomic resonance absorption spectroscopy coupled with a shock tube is a powerful technique for studying high temperature dynamics of reactive systems. Presently, high temperature pyrolysis of SiCl4–Ar mixtures has been studied behind reflected shock waves. Using time-resolved absorption profiles at 121.6nm and a detailed reaction model, the absorption cross sections of SiCl4, SiCl3, SiCl2 and Cl have been measured. Results agree well with available data for SiCl4 and constitute, to our knowledge, the first measurements for SiCl3, SiCl2 and Cl at the Lyman-α wavelength. These data are relevant to silica particle production from SiCl4-oxidant mixtures combustion synthesis.

Polarization-dependent vacuum-ultraviolet reflectometry using elliptically polarized synchrotron radiation

In the laboratory of the Physikalisch-Technische Bundesanstalt (PTB) at the Berlin electron-storage ring BESSY II, a procedure has been developed to investigate the dependence of vacuum-ultraviolet reflection on polarization. It is based on characterizing the elliptically polarized synchrotron radiation at PTB's normal-incidence monochromator beamline for reflectometry by means of polarization-sensitive photodetectors. For this purpose, the polarization dependency in the detector responsivity was determined at a small, low, solid angle of acceptance for the synchrotron radiation, i.e., within the orbital plane of the storage ring where the degree of linear polarization is known to be almost 100%. Our method allows the polarization dependence of reflection samples to be measured with relative standard uncertainties ranging from 2.4% to 11% in the spectral range between 60 and 160 nm. The method has been applied to the optimization of polarizing mirrors at the Lyman-alpha wavelength of 121.6 nm.

The vacuum-UV absorption spectrum of SF 5CF 3; implications for its lifetime in the earth’s atmosphere

Using vacuum-UV radiation from a synchrotron source, the absorption spectrum of SF 5CF 3 has been measured in the range 50–150 nm at a resolution of 0.12 nm. The cross-section at the Lyman-α wavelength of 121.6 nm is 1.5±0.3×10 −17 cm 2 molecule −1 . The loss of SF 5CF 3 on a molecular basis from the earth’s atmosphere is dominated, not by photon-induced dissociation, but by electron attachment in the mesosphere above 60 km, yielding SF 5 −. The lifetime of SF 5CF 3 in the earth’s atmosphere, however, is determined primarily by the meterological conditions that transport it from the earth’s surface to the mesosphere, and not by processes that occur in that region. By comparison with data for SF 6, a lifetime of ca. 1000 years for SF 5CF 3 is estimated.

Absolute primary H atom quantum yield measurements in the 193.3 and 121.6 nm photodissociation of acetylene

Absolute quantum yields for primary H atom formation ( Φ H) were measured under collision-free conditions for the room-temperature gas-phase dissociation of acetylene (C 2H 2) after photoexcitation at 193.3 nm and at the H-atom Lyman-α wavelength (121.6 nm) by a pulsed laser photolysis (LP)–laser-induced fluorescence (LIF) `pump-and-probe' technique. Using HCl and CH 4 photolysis at 193.3 and 121.6 nm, respectively, as a reference, values of Φ H (193.3 nm)=0.94±0.12 and Φ H (121.6 nm)=1.04±0.16 were obtained which demonstrate that for both photolysis wavelengths the H+C 2H product channel dominates the primary C 2H 2 photochemistry.

Ultra-sensitive Detection of Hydrogen Isotopes by Lyman-α RIS

An ultra-sensitive way of detecting hydrogen isotopes (muonium, H, D, and T) is described, by in which a resonant ionization scheme through the {1S-2P-unbound} transition is utilized. A time-of-flight measurement using the pulsed lasers coupled with a mass separation enables us to distinguish any hydrogen isotope with very low backgrounds. As a real demonstration of sensitive detection, H, D, and T atoms generated simultaneously by the nuclear reactions of a 500-MeV proton beam were shown to be easily distinguished and extracted without changing any experimental condition other than the corresponding Lyman-α wavelength. In the case of a laser-induced fluorescence technique using Lyman-α or Balmer-α photons, the lowest detection limit is reported to be as low as on the order of 107-109 cm-3, or 107 cm-3, respectively. In the case of two-photon induced Balmer-α emission, the detection limit is reported to be on the same order as that by three-photon ionization, 3×109 cm-3. The detection limit of the present method is estimated to be about 7.7×103 cm-3. The method of applying Lyman-α resonant ionization spectroscopy can be recognized as an ultra-sensitive and efficient mass-selection technique.

Ultra-sensitive Detection of Hydrogen Isotopes by Lyman-.ALPHA. RIS.

An ultra-sensitive way of detecting hydrogen isotopes (muonium, H, D, and T) is described, by in which a resonant ionization scheme through the {1S-2P-unbound} transition is utilized. A time-of-flight measurement using the pulsed lasers coupled with a mass separation enables us to distinguish any hydrogen isotope with very low backgrounds. As a real demonstration of sensitive detection, H, D, and T atoms generated simultaneously by the nuclear reactions of a 500-MeV proton beam were shown to be easily distinguished and extracted without changing any experimental condition other than the corresponding Lyman-α wavelength. In the case of a laser-induced fluorescence technique using Lyman-α or Balmer-α photons, the lowest detection limit is reported to be as low as on the order of 107-109 cm-3, or 107 cm-3, respectively. In the case of two-photon induced Balmer-α emission, the detection limit is reported to be on the same order as that by three-photon ionization, 3×109 cm-3. The detection limit of the present method is estimated to be about 7.7×103 cm-3. The method of applying Lyman-α resonant ionization spectroscopy can be recognized as an ultra-sensitive and efficient mass-selection technique.

Absorption cross-sections for some atmospherically important molecules at the H atom Lyman-α wavelength (121.567 nm)

Absolute values of gas-phase absorption cross-sections for some atmospherically important molecules (HCl, H 2O, H 2S, NH 3, H 2O 2, HNCO, CH 4, CH 3Cl, CH 2 Cl 2, CHCl 3 , CHF 2 Cl, CHBr 3 , CH 3CF 2Cl and CH 3CFCl 2) have been measured at the H atom Lyman-α wavelength (121.567 nm) employing narrow band ( Δλ=0.0006 nm) laser radiation generated by resonant third-order sum-difference frequency conversion. The present values can be used to access the uncertainty associated with Lyman-α absorption cross-section values derived from available literature absorption spectra which usually have been measured with quite different and sometimes with quite low spectral resolutions.

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.

Angular resolved studies of the Lyman-α photodissociation of HCN and DCN: New dynamical insights

The photochemistries of HCN and DCN at the H(D) Lyman-α wavelength have been reinvestigated using the technique of H(D) Rydberg atom time-of-flight spectroscopy, with angular resolution of the H/D atom signal about the polarization vector of the photolysis radiation. In the case of HCN photodissociation, the previous assignment of substantial branching to H+CN(A 2Π)v=0 products is confirmed. Analysis of the profile taken under parallel polarization of the Lyman-α radiation relative to the time of flight axis reveals additional structure attributable both to a progression in CN(A) products with high rovibrational excitation (v=4–9, with N∼26–41, for all v), and to various rotationally excited levels associated with CN(B 2Σ+)v=0,1. From these various assignments an improved value for the dissociation energy, D0(H–CN)=43 710±70 cm−1, is obtained. The determined β parameter, which is a measure of the angular part of the photofragment velocity distribution about the polarization vector of the photolysis radiation, shows an increasingly parallel distribution of H atom velocities with increasing CN internal energy. DCN photolysis at the D Lyman-α wavelength yields both CN(A)v=0 and a range of rovibrationally excited CN(A) products but, in contrast to HCN, no significant branching to CN(B) products is observed. The corresponding β parameter, which is found to be relatively invariant with CN internal energy, indicates a near limiting perpendicular distribution of D atom velocities about the photolysis radiation. These results are interpreted with reference to the available ab initio calculated potential energy surfaces of A′ and A″ symmetry, and the relative propensities for excitation to, and the likely dynamics on, these various excited states is discussed.

H atom formation dynamics in the dissociation of CH3–CF2Cl (HCFC-142b) after UV and VUV laser photoexcitation

Using the laser photolysis/laser-induced fluorescence (LIF) “pump-and-probe” technique, the dynamics of H atom formation in the photodissociation of CH3–CF2Cl (HCFC-142b) after excitation at 193 nm and the Lyman-α wavelength were studied under collision-free conditions in the gas-phase at room temperature. The H atoms produced were detected by (2p2P←1s2S)-LIF using tunable narrow-band Lyman-α laser radiation (λLα≈121.6 nm) generated by resonant third-order sum-difference frequency conversion of pulsed dye laser radiation. In the VUV photodissociation experiments the Lyman-α laser radiation was used both to photodissociate the parent molecules and to detect the produced nascent H atoms via laser induced fluorescence. The following quantum yields ΦH for H atom formation were determined by a photolytic calibration method: ΦH(193 nm)=(0.06±0.02) and ΦH(Lα)=(0.53±0.12). From the measured H atom Doppler profiles the average H atom kinetic energy was determined to be ET(193 nm)=(51±10) kJ/mol and ET(Lα)=(72±4) kJ/mol, respectively.

Dissociation dynamics of HNCO and DNCO after laser photoexcitation in the vacuum ultraviolet

Using the laser photolysis–laser-induced fluorescence “pump–probe” technique, the gas-phase dissociation dynamics of HNCO(X̃ 1A)+hν→H+NCO and DNCO(X̃ 1A)+hν→D+NCO after photoexcitation at the Lyman-α wavelength were studied under collision-free conditions at room temperature. In the vacuum ultraviolet photodissociation experiments narrow band tunable Lyman-α laser radiation (λ≈121.4–121.6 nm) was used both to photodissociate the parent molecules and to detect the produced nascent H and D atom products via (2p 2P←1s 2S) laser induced fluorescence. The following quantum yields ΦH–D for H–D atom formation were determined by a photolytic calibration method: ΦH=(0.62±0.15) and ΦD=(0.51±0.17). For HNCO and DNCO the measured H–D atom Doppler line shapes can be well described by a single Gaussian function, which corresponds to a statistical Maxwell–Boltzmann-like distribution of the translational energy. From the measured H and D atom Doppler profiles the average H and D atom kinetic energy was determined to be ET(H)=(137±10) kJ/mol and ET(D)=(115±4) kJ/mol, respectively. The average kinetic energies were found to be in reasonable agreement with results from simple statistical calculations in which it is assumed that H–D atoms are produced in combination with NCO in the ground electronic state (X̃ 2Π). A dissociation mechanism is suggested in which H–D atom formation proceeds via a statistical unimolecular decay of a hot H–DNCO intermediate formed by a radiationless transition of the optically excited bound H–DNCO state to a lower-lying dissociative state.

Quantum yield for H atom formation in the methane dissociation after photoexcitation at the Lyman-α (121.6 nm) wavelength

The gas-phase photodissociation dynamics of methane after excitation at the Lyman-α wavelength (121.6 nm) was investigated under collision-free conditions at room temperature. Narrow-band tunable Lyman-α laser radiation was generated by the resonant third-order sum-difference frequency conversion of pulsed-dye laser radiation and used both to photodissociate the parent molecules and to detect the photolytically produced hydrogen atoms via (2p 2P → 1s 2S) laser-induced fluorescence. H atom Doppler profiels were recorded and the absolute quantum yield for H atom formation, Φ H = (0.47 ± 0.11), was determined by means of photolytic calibration method where the Lyman-α photolysis of H 2O was used as a reference source of well-defined H atom concentrations. This value will be used in combination with previous results from H 2 and CH yield measurements to estimate the relative importance of the different product pathways in the CH 4 photodissociation.

Photodissociation Dynamics of CHF2Cl after Photoexcitation at the Lyman-α Wavelength (121.6 nm)

The collision-free gas-phase photodissociation dynamics of CHF2Cl (HCFC-22) were studied at room temperature using the laser photolysis/laser-induced fluorescence “pump/probe” technique. Lyman-α laser radiation (λ = 121.6 nm) was used both to photodissociate the parent molecule and to detect the nascent H atom products via (2p2P ← 1s2S) laser-induced fluorescence. An absolute H atom quantum yield of ΦH = 0.77 ± 0.13 was determined by a calibration method in which H2O 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 bimodal velocity distribution suggesting the presence of two different H atom formation pathways in the Lyman-α photodissociation of CHF2Cl.

Photodissociation dynamics of the chloromethanes at the Lyman-α wavelength (121.6 nm)

The gas-phase dissociation dynamics of CH3Cl, CH2Cl2, and CHCl3 after photoexcitation at the Lyman-α wavelength (121.6 nm) were studied under collision-free conditions at room temperature. Narrow-band tunable Lyman-α laser radiation (λLα≈121.6 nm) was generated by resonant third-order sum-difference frequency conversion of pulsed-dye-laser radiation and used both to photodissociate the parent molecules and to detect the nascent H atom products via (2p2P←1s2S) laser induced fluorescence. Absolute H atom quantum yields ΦH(CH3Cl)=(0.53±0.05), ΦH(CH2Cl2)=(0.28±0.03), and ΦH(CHCl3)=(0.23±0.03) were determined employing a photolytic calibration method where the Lyman-α photolysis of H2O was used as a reference source of well-defined H atom concentrations. H atom Doppler profiles were measured for all chlorinated methanes. In the case of CH3Cl the line shapes of the profiles indicate a pronounced bimodal translational energy distribution suggesting the presence of two H atom formation mechanisms leading to a markedly different H atom translational energy release. The observed “slow” component of the H atom translational energy distribution corresponds to an average kinetic energy of (55±5) kJ/mol, while the “fast” component leads to an average kinetic energy of (320±17) kJ/mol. The relative branching ratio between the “fast” and the “slow” H atom channel was determined to be (0.71±0.15). For CH2Cl2 and CHCl3 no bimodal translational energy distributions were observed. Here the translational energy distributions could each be well described by a single Maxwell–Boltzmann distribution, corresponding to an average translational energy of (81±9) kJ/mol and (75±4) kJ/mol, respectively.

Laser system for the resonant ionization of hydrogen-like atoms produced by nuclear reactions

We report on the generation of intense pulsed VUV laser radiation of the Lyman-α wavelength of hydrogen-like atoms and muonium by a four wave mixing process in a mixture of krypton and argon gas. The pulse energy was of the order of ∼ 1 μJ/pulse at a 20 Hz repetition rate. The 200 GHz band-width corresponds to the Doppler width of muonium at 2300 K. With an additional ionizing laser, the one-photon resonant atomic transitions, 1S ⇒ 2P ⇒ unbound, can be realized for thermal hydrogen-like atoms produced by nuclear reactions as well as for muonium. Anticipating the need for long term operation, a completely solid-state laser system was chosen for the pumping, oscillators and amplifiers. The system construction and performance are described together with the results concerning resonant ionization of thermal hydrogen atoms produced by high energy nuclear reactions.

VUV spectroscopy of magnetically trapped atomic hydrogen

We discuss the experimental and theoretical aspects of absorption spectroscopy of cold atomic hydrogen gas in a magnetostatic trap using a pulsed narrow-band source (bandwidth ≈ 100 MHz) at the Lyman-α wavelength (121.6 nm). A careful analysis of the measured absorption spectra enables us to determine non-destructively the temperature and the density of the trapped gas. The development of this diagnostic technique is important for future attempts to reach Bose-Einstein condensation in trapped atomic hydrogen.

Broadly tunable vacuum-ultraviolet radiation source employing resonant enhanced sum–difference frequency mixing in krypton

Vacuum-ultraviolet (VUV) generation by resonant third-order sum–difference frequency conversion (wVUV = 2wR − wT) in krypton has been investigated. The frequency wR (λR = 212.55 nm) was resonant with the Kr two-photon transition 4p–5p[1/2, 0]. By tuning wT in the range λT = 220–860 nm it was possible to generate wVUV in the wavelength range λVUV = 200–120 nm. The conversion efficiency was enhanced in the range 121–123 nm by phase matching with a normally dispersive buffer gas (Ar); under the best phase-matched conditions employed, VUV output powers at the Lyman-α wavelength of the order of 1 kW were inferred from the estimated VUV pulse energy. This source was used to measure high-lying members of the Mg principal series near 162 nm. High resolution and excellent sensitivity of detection, when a thermionic diode detector was used, have been demonstrated.