Experimental and coupled‐channels investigation of the radiative properties of the N2c′41Σu+ − X1Σg+ band system

Liu, Xianming; Shemansky, D. E.; Malone, C. P.; Johnson, P. V.; Ajello, J. M.; Kanik, I.; Heays, A. N.; Lewis, B. R.; Gibson, S. T.; Stark, G.

doi:10.1029/2007ja012787

Cited by 16 publications

(23 citation statements)

References 99 publications

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“…The predissociation efficiency η is then given by η = 1−τ tot /τ rad , where τ tot is the inverse of the sum of the radiative and predissociation rates. For almost all transitions, η 1: significant corrections for partial dissociation are needed only for the b − X(1, 0) and c − X(0, 0) bands near 986 and 959 Å, respectively (Lewis et al 2005b;Liu et al 2008;Sprengers et al 2004;Wu et al 2012). For example, the top panel of Fig.…”

Section: Photoabsorption and Photodissociation Spectrummentioning

confidence: 99%

Photodissociation of interstellar N₂

Heays

Visser

et al. 2013

A&A

138

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Context. Molecular nitrogen is one of the key species in the chemistry of interstellar clouds and protoplanetary disks, but its photodissociation under interstellar conditions has never been properly studied. The partitioning of nitrogen between N and N 2 controls the formation of more complex prebiotic nitrogen-containing species. Aims. The aim of this work is to gain a better understanding of the interstellar N 2 photodissociation processes based on recent detailed theoretical and experimental work and to provide accurate rates for use in chemical models. Methods. We used an approach similar to that adopted for CO in which we simulated the full high-resolution line-by-line absorption + dissociation spectrum of N 2 over the relevant 912-1000 Å wavelength range, by using a quantum-mechanical model which solves the coupled-channels Schrödinger equation. The simulated N 2 spectra were compared with the absorption spectra of H 2 , H, CO, and dust to compute photodissociation rates in various radiation fields and shielding functions. The effects of the new rates in interstellar cloud models were illustrated for diffuse and translucent clouds, a dense photon dominated region and a protoplanetary disk. Results. The unattenuated photodissociation rate in the Draine (1978, ApJS, 36, 595) radiation field assuming an N 2 excitation temperature of 50 K is 1.65 × 10 −10 s −1 , with an uncertainty of only 10%. Most of the photodissociation occurs through bands in the 957-980 Å range. The N 2 rate depends slightly on the temperature through the variation of predissociation probabilities with rotational quantum number for some bands. Shielding functions are provided for a range of H 2 and H column densities, with H 2 being much more effective than H in reducing the N 2 rate inside a cloud. Shielding by CO is not effective. The new rates are 28% lower than the previously recommended values. Nevertheless, diffuse cloud models still fail to reproduce the possible detection of interstellar N 2 except for unusually high densities and/or low incident UV radiation fields. The transition of N → N 2 occurs at nearly the same depth into a cloud as that of C + → C → CO. The orders-of-magnitude lower N 2 photodissociation rates in clouds exposed to black-body radiation fields of only 4000 K can qualitatively explain the lack of active nitrogen chemistry observed in the inner disks around cool stars. Conclusions. Accurate photodissociation rates for N 2 as a function of depth into a cloud are now available that can be applied to a wide variety of astrophysical environments.

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Section: Photoabsorption and Photodissociation Spectrummentioning

confidence: 99%

Photodissociation of interstellar N₂

Heays

Visser

et al. 2013

A&A

138

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“…The Viking Lander mission (17,18) measured the Martian atmosphere composition, revealing a very high 15 N isotopic enrichment (1.6 times terrestrial) that was suggested as deriving from dissociative recombination and photolysis of N 2 and preferential gravitational escape of the lighter isotope, 14 N (17,(19)(20)(21)(22)(23)(24)(25)(26). Recent work describing the nitrogen isotopic composition of the atmosphere of Titan has also been modeled to interpret its anomalous molecular nitrogen isotopic composition, which is a factor of two higher than the associated HCN (27).…”

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confidence: 99%

On the strong and selective isotope effect in the UV excitation of N ₂ with implications toward the nebula and Martian atmosphere

Muskatel

Remacle

Thiemens

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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Isotopic effects associated with molecular absorption are discussed with reference to natural phenomena including early solar system processes, Titan and terrestrial atmospheric chemistry, and Martian atmospheric evolution. Quantification of the physicochemical aspects of the excitation and dissociation processes may lead to enhanced understanding of these environments. Here we examine a physical basis for an additional isotope effect during photolysis of molecular nitrogen due to the coupling of valence and Rydberg excited states. The origin of this isotope effect is shown to be the coupling of diabatic electronic states of different bonding nature that occurs after the excitation of these states. This coupling is characteristic of energy regimes where two or more excited states are nearly crossing or osculating. A signature of the resultant isotope effect is a window of rapid variation in the otherwise smooth distribution of oscillator strengths vs. frequency. The reference for the discussion is the numerical solution of the time dependent Schrödinger equation for both the electronic and nuclear modes with the light field included as part of the Hamiltonian. Pumping is to all extreme UV dipole-allowed, valence and Rydberg, excited states of N 2 . The computed absorption spectra are convoluted with the solar spectrum to demonstrate the importance of including this isotope effect in planetary, interstellar molecular cloud, and nebular photochemical models. It is suggested that accidental resonance with strong discrete lines in the solar spectrum such as the CIII line at 97.703 nm can also have a marked effect.photodissociation | isotopic fractionation | UV photodissociation P hotochemical processes are particularly pervasive in nature. Measurements of isotopic compositions provide insights into a range of such processes, both terrestrial and extraterrestrial. To adequately interpret such measurements, characterization of relevant isotopically selective physicochemical processes associated with gas phase processes is desirable. In this paper we quantify an isotopic selectivity due to electronic reorganization following ligfht absorption. The results are compared to the solar spectrum for applications in nature.There are a number of photodissociation processes that have been suggested where a more basic understanding of isotope effects could aid in the development of models. A striking example of observed, but presently not fully understood oxygen isotopic distribution occurs in the high temperature calcium aluminum rich inclusions of the Allende meteorite (1) and most oxygen bearing meteorites (see, for example, review in ref.2). Although originally thought to be nucleosynthetic in origin based upon laboratory experiments, it was later suggested that the observed isotopic composition might arise from photochemical self-shielding (3-6). It has also been proposed that given the special properties of oxygen, symmetry related properties might produce the same fraction in the actual mineral formation process (7-11). A...

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“…using isotope-specific photodissociation cross sections calculated theoretically by the diabatic coupled-channel Schrödinger equation model (Liu et al 2008). To test the extent to which isotopic self-shielding due to N 2 photoionization could significantly affect the relative photoionization rates of 14 N 2 versus 15 N 14 N, and hence might play a role in determining the isotopic composition of N 2 and nitrogencontaining photochemical products in Titan's atmosphere, the measured cross sections for photoionization of 14 N 2 and 15 N 14 N were included in a one-dimensional (1D) photochemical model of Titan's atmosphere (Yung et al 1984;Liang et al 2007aLiang et al , 2007b.…”

Section: Photochemical Modelmentioning

confidence: 99%

MEASUREMENTS OF ISOTOPE EFFECTS IN THE PHOTOIONIZATION OF N ₂ AND IMPLICATIONS FOR TITAN'S ATMOSPHERE

Croteau

Randazzo

Kostko

et al. 2011

ApJ

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Isotope effects in the non-dissociative photoionization of molecular nitrogen (N 2 + hν → N 2 + + e − ) may play a role in determining the relative abundances of isotopic species containing nitrogen in interstellar clouds and planetary atmospheres but have not been previously measured. Measurements of the photoionization efficiency spectra of 14 N 2 , 15 N 14 N, and 15 N 2 from 15.5 to 18.9 eV (65.6-80.0 nm) using the Advanced Light Source at Lawrence Berkeley National Laboratory show large differences in peak energies and intensities, with the ratio of the energy-dependent photoionization cross sections, σ ( 14 N 2 )/σ ( 15 N 14 N), ranging from 0.4 to 3.5. Convolving the cross sections with the solar flux and integrating over the energies measured, the ratios of photoionization rate coefficients are J( 15 N 14 N)/J( 14 N 2 ) = 1.00 ± 0.02 and J( 15 N 2 )/J( 14 N 2 ) = 1.00 ± 0.02, suggesting that isotopic fractionation between N 2 and N 2 + should be small under such conditions. In contrast, in a one-

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Experimental and coupled‐channels investigation of the radiative properties of the N₂c′₄¹Σ_u⁺ − X¹Σ_g⁺ band system

Abstract: [1] The emission properties of the N 2 c 0 4 1 S u + À X 1 S g + band system have been investigated in a joint experimental and coupled-channels theoretical study. Relative intensities of the c 0

Cited by 16 publications

References 99 publications

Photodissociation of interstellar N₂

Photodissociation of interstellar N₂

On the strong and selective isotope effect in the UV excitation of N ₂ with implications toward the nebula and Martian atmosphere

MEASUREMENTS OF ISOTOPE EFFECTS IN THE PHOTOIONIZATION OF N ₂ AND IMPLICATIONS FOR TITAN'S ATMOSPHERE

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Experimental and coupled‐channels investigation of the radiative properties of the N2c′41Σu+ − X1Σg+ band system

Abstract: [1] The emission properties of the N 2 c 0 4 1 S u + À X 1 S g + band system have been investigated in a joint experimental and coupled-channels theoretical study. Relative intensities of the c 0

Cited by 16 publications

References 99 publications

Photodissociation of interstellar N2

Photodissociation of interstellar N2

On the strong and selective isotope effect in the UV excitation of N 2 with implications toward the nebula and Martian atmosphere

MEASUREMENTS OF ISOTOPE EFFECTS IN THE PHOTOIONIZATION OF N 2 AND IMPLICATIONS FOR TITAN'S ATMOSPHERE

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Experimental and coupled‐channels investigation of the radiative properties of the N₂c′₄¹Σ_u⁺ − X¹Σ_g⁺ band system

Photodissociation of interstellar N₂

Photodissociation of interstellar N₂

On the strong and selective isotope effect in the UV excitation of N ₂ with implications toward the nebula and Martian atmosphere

MEASUREMENTS OF ISOTOPE EFFECTS IN THE PHOTOIONIZATION OF N ₂ AND IMPLICATIONS FOR TITAN'S ATMOSPHERE