Oxygen isotope fractionation in the vacuum ultraviolet photodissociation of carbon monoxide: Wavelength, pressure, and temperature dependency (original) (raw)
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Chemical Physics
The carbon and oxygen isotope fractionations occurring during photodissociation of carbon monoxide (CO) by vacuum ultraviolet photons (91 to 107 nm) at 80K were measured and the isotopic fractionations due to direct and indirect predissociation processes individually quantified. The isotopic fractionations depend on the photodissociation wavelength. Slope values (δ' 17 O/ δ' 18 O) in oxygen three-isotope space range from 0.75 to 1.1. The isotopic composition of the products depends on the dissociation dynamics at the upper electronic state (perturbation and coupling associated with that state), which in turn modulates the isotope effect (in this case an enrichment of minor isotopes) inside the gas column due to the saturation of major isotopologue (isotope self-shielding). An explanation in terms of isotope self-shielding would require a quantum yield of one for photodissociation of all isotopologues which is not consistent with the data.
Isotope dependent predissociation in the C1Σ+, v=0 and v=1 states of CO
The European Physical Journal D, 2001
Rotationally resolved spectral lines in the C−X(1, 0) band of carbon monoxide are investigated under high resolution using a coherent vacuum ultraviolet laser source, continuously tunable near 107 nm. Transition frequencies are determined by calibrating against a reference standard of iodine lines, recorded with saturation spectroscopy in the visible range, yielding an absolute accuracy of 0.003 cm −1 in the vacuum ultraviolet. Improved molecular constants for the excited state are derived and no effects of perturbation are found at the present level of accuracy. Line broadening measurements result in information on the excited state lifetime of the C 1 Σ + , v = 1 state for five natural isotopomers of carbon monoxide: τ ( 12 C 17 O) = 280 ps, τ ( 12 C 18 O) = 210 ps, τ ( 13 C 16 O) = 295 ps, τ ( 13 C 17 O) = 160 ps, and τ ( 13 C 18 O) = 150 ps. Within the accuracy of the present measurements no effects of J-dependent lifetimes were observed, for neither of the isotopomers. In addition direct time domain measurements of the lifetime of the C 1 Σ + , v = 0 and v = 1 states of the main isotopomer are performed in a pump-probe experiment using a picosecond VUV-laser, yielding τ ( 12 C 16 O) = 1780 ps for v = 0 and τ ( 12 C 16 O) = 625 ps for v = 1. For C 1 Σ + , v = 0 in 12 C 16 O and 13 C 16 O the same lifetime is found; this lifetime matches experimental values of the oscillator strength and hence supports previous results showing pure radiative decay in this state; the error margins however do not exclude some low level of predissociation. The measurements indicate that the C 1 Σ + , v = 0 state of the 13 C 18 O isotopomer is predissociated with an estimated yield of 17% (i.e. above the level of predissociation for 12 C 16 O.) From the combined data predissociation yields upon excitation of the C 1 Σ + , v = 1 state are derived, lying in the range 0.84-0.91 for the five less abundant isotopomers; for the main 12 C 16 O isotopomer a strongly deviating predissociation yield of 0.65 is deduced.
Further Evidence for Chemical Fractionation from Ultraviolet Observations of Carbon Monoxide
The Astrophysical Journal, 2003
Ultraviolet absorption from interstellar 12 CO and 13 CO was detected toward ρ Oph A and χ Oph. The measurements were obtained at medium resolution with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Column density ratios, N ( 12 CO)/N ( 13 CO), of 125 ± 23 and 117 ± 35 were derived for the sight lines toward ρ Oph A and χ Oph, respectively. A value of 1100 ± 600 for the ratio N ( 12 C 16 O)/N ( 12 C 18 O) toward ρ Oph A was also obtained. Absorption from vibrationally excited H 2 (v ′′ = 3) was clearly seen toward this star as well.
Investigations of the photochemical isotope equilibrium between O2, CO2 and O3
Atmospheric Chemistry and Physics, 2007
Contrary to tropospheric CO 2 whose oxygen isotopic composition follows a standard mass dependent relationship, i.e. δ 17 O∼0.5 δ 18 O, stratospheric CO 2 is preferentially enriched in 17 O, leading to a strikingly different relation: δ 17 O∼1.7 δ 18 O. It has been shown repeatedly that the isotope anomaly is inherited from O 3 via photolytically produced O(1 D) that undergoes isotope exchange with CO 2 and the anomaly may well serve as a tracer of stratospheric chemistry if details of the exchange mechanism are understood. We have studied the photochemical isotope equilibrium in UV-irradiated O 2-CO 2 and O 3-CO 2 mixtures to quantify the transfer of the anomaly from O 3 to CO 2 at room temperature. By following the time evolution of the oxygen isotopic compositions of CO 2 and O 2 under varying initial isotopic compositions of both, O 2 /O 3 and CO 2 , the isotope equilibria between the two reservoirs were determined. A very strong dependence of the isotope equilibrium on the O 2 /CO 2-ratio was established. Equilibrium enrichments of 17 O and 18 O in CO 2 relative to O 2 diminish with increasing CO 2 content, and this reduction in the equilibrium enrichments does not follow a standard mass dependent relation. When molecular oxygen exceeds the amount of CO 2 by a factor of about 20, 17 O and 18 O in equilibrated CO 2 are enriched by (142±4)‰ and (146±4)‰, respectively, at room temperature and at a pressure of 225 hPa, independent of the initial isotopic compositions of CO 2 and O 2 or O 3. From these findings we derive a simple and general relation between the starting isotopic compositions and amounts of O 2 and CO 2 and the observed slope in a three oxygen isotope diagram. Predictions from this relation are compared with published laboratory and atmospheric data.
Predissociation rates in carbon monoxide: dependence on rotational state, parity and isotope
Chemical Physics, 1994
A high-resolution spectroscopic study of carbon monoxide has been performed using a narrow-band and tunable extreme ultraviolet laser source in the wavelength range 91-97 nm. For three isotopes 12C'60, r3Cr60 and "C'*O the (4px)L IfI, u=O, (4po)K'Z+, v=O and (3sa)W'lT, u=O states were investigated. The 'II, u=2, (3so)W'D, v=2 and (3dx)L"D, v=l states were studied for r2Cr60 and r3Cr60. The 'II, v=O state at 109564.58 cm-r, the (6~0) IX+, v=O state at 109173.68 cm-', the (Spa) IX+, v=O stateat 107174.44cm-', andthe (4do) 'Z+, v=O stateat 105676.30cm-r were excitedonlyforthe mainisotope r2Cr60. Previously unknown states are found at 107365.87 cm-' ('Z') for r2CL60 and at 103203.07 cm-' ('II) for '3C'80. Indirect evidence is found for a perturber state at 107708 f 2 cm-r for r2CL60, while spectroscopic information on the E '4 v= 5 state of r3C's0 is deduced from a deperturbation analysis. A rotational-state and isotope-dependent predissociation rate is found for almost all states. A parity dependence is observed for the 4px)L IfI, v= 0 and ( 3~0) W *lI, v=O states, where the predissociation rate of the l-L component varies proportional to J(J+ 1). This effect is attributed to a coupling with the repulsive part ofthe D' rE+ state. Several examples of accidental predissociations have been found, particularly in the (4do) 'C+, v=O state. 0301-0104/94/S 07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDZ 030 1-O 104(93)E0400-P K.S.E. Eikema et al. /Chemical Physics l&(1994) 217-245 219 KD*P 4 valve timing Dye laser Nd:YAG medium CO beam
Investigations of the photochemical isotope equilibrium between O2, CO2 and O3
Atmospheric Chemistry and Physics Discussions, 2006
Contrary to tropospheric CO 2 whose oxygen isotopic composition follows a standard mass dependent relationship, i.e. δ 17 O∼0.5 δ 18 O, stratospheric CO 2 is preferentially enriched in 17 O, leading to a strikingly different relation: δ 17 O∼1.7 δ 18 O. It has been shown repeatedly that the isotope anomaly is inherited from O 3 via photolytically produced O( 1 D) that undergoes isotope exchange with CO 2 and the anomaly may well serve as a tracer of stratospheric chemistry if details of the exchange mechanism are understood. We have studied the photochemical isotope equilibrium in UV-irradiated O 2 -CO 2 and O 3 -CO 2 mixtures to quantify the transfer of the anomaly from O 3 to CO 2 at room temperature. By following the time evolution of the oxygen isotopic compositions of CO 2 and O 2 under varying initial isotopic compositions of both, O 2 /O 3 and CO 2 , the isotope equilibria between the two reservoirs were determined. A very strong dependence of the isotope equilibrium on the O 2 /CO 2 -ratio was established. Equilibrium enrichments of 17 O and 18 O in CO 2 relative to O 2 diminish with increasing CO 2 content, and this reduction in the equilibrium enrichments does not follow a standard mass dependent relation. When molecular oxygen exceeds the amount of CO 2 by a factor of about 20, 17 O and 18 O in equilibrated CO 2 are enriched by (142±4)‰ and (146±4)‰, respectively, at room temperature and at a pressure of 225 hPa, independent of the initial isotopic compositions of CO 2 and O 2 or O 3 . From these findings we derive a simple and general relation between the starting isotopic compositions and amounts of O 2 and CO 2 and the observed slope in a three oxygen isotope diagram. Predictions from this relation are compared with published laboratory and atmospheric data.
Geophysical Research Letters, 2000
Photodissociation of CO2 by ultraviolet light (λ = 185 nm) generates CO and O2, which are unusually enriched (more than 100‰) in 17O. The dissociation takes place through a spin forbidden process during transition from a singlet to a triplet state, the latter lying on a repulsive potential energy surface. The 17O isotopic enrichment is a primary process associated with this transition and could be due to near resonant spin-orbit coupling of the low energy vibrational levels of the 16O12C17O molecule in the singlet state with those of the triplet state near the zone of transition. In contrast, photodissociation at shorter wavelengths (λ < 160 nm) involves no spin violation and produces CO and O2 which are fractionated in a conventional mass dependent fashion. The proposed explanation is further supported using 13C enriched CO2; in this case the products are enriched in both heavy isotopes but about 100‰ more in 18O. The 17O enrichment in CO and O2 generated by CO2 photolysis in a range of UV wavelengths may be a useful tracer in delineating processes in the atmospheres of Earth and Mars.
Isotope effects in photodissociation: Chemical reaction dynamics and implications for atmospheres
Advances in Quantum Chemistry, Vol 55: Applications of Theoretical Methods To Atmospheric Science, 2008
Obtaining the absorption and/or photodissociation cross section is a threefold challenge: computing the electronic potential energy surfaces, interpolating the potentials, and finding the cross section either by timedependent or time-independent methods. We review electronic structure methods used for computing accurate potential energy surfaces for the electronic ground and accessible excited state as well as coupling between them (electronic transition dipole moments and diabatic coupling). Methods used for interpolation are discussed. The time-independent methods are based on the reflection principle and implicitly involve the short time approximation. In the time-dependent methods the time-dependent Schrödinger equation is solved exactly and the method considers the effect of dynamics away from the Franck-Condon region. We illustrate the presented methods using small molecules (HCl, N 2 O, OCS and HCHO) and their isotopic analogues.
2006
Abstract. Contrary to tropospheric CO2 whose oxygen iso-topic composition follows a standard mass dependent rela-tionship, i.e. δ17O∼0.5 δ18O, stratospheric CO2 is preferen-tially enriched in 17O, leading to a strikingly different rela-tion: δ17O∼1.7 δ18O. It has been shown repeatedly that the isotope anomaly is inherited from O3 via photolytically pro-duced O(1 D) that undergoes isotope exchange with CO2 and the anomaly may well serve as a tracer of stratospheric chem-istry if details of the exchange mechanism are understood. We have studied the photochemical isotope equilibrium in UV-irradiated O2-CO2 and O3-CO2 mixtures to quantify the transfer of the anomaly from O3 to CO2 at room tempera-ture. By following the time evolution of the oxygen isotopic compositions of CO2 and O2 under varying initial isotopic compositions of both, O2/O3 and CO2, the isotope equilibria