The predissociation of chem{N_2} excited levels is enabled by the presence of optically-inaccessible triplet states. We have recorded vacuum ultraviolet (VUV) spectra at the SOLEIL synchrotron which reveal these states through their perturbation of allowed transitions or their direct appearance due to intensity borrowing. 

Some of these measurements were recorded at 900,K in order to access high-rotational levels, other measurements investigated weak forbidden transitions at high column density. Following careful analysis, significant new information has been obtained elucidating the states responsible for the astrochemically and atmospherically signficant chem{N_2} predissociation mechanism, and allowing for improvements in its quantitiative modelling

Heays, Alan

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FORBIDDEN TRANSITIONS IN THE VUV SPECTRUM OF N2

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Forbidden transitions in the VUV spectrum of N2Alan N. Heays,1 Mingli Niu,2 Nelson de Oliveira,3Edcel J. Salumbides,2 Brenton R. Lewis,4 Wim Ubachs,2Ewine van Dishoeck11Leiden Observatory, The Netherlands2VU University Amsterdam, The Netherlands3Synchrotron SOLEIL, France4Australian National University, AustraliaMotivation and objectiveN2 photodissociation is interesting and astrochemically importantThis is predissociative – controlled by S = 1 triplet states which arenot easily observable from the S = 0 ground stateObjective: Quantify these states in absorption at high columndensity or through their perturbative effectsN2 electron energy-loss spectrumGeiger and Schröder (1969)Photoabsorption threshold: 100 nmPhotoionisation threshold: 15.58 eV / 80 nmAll states predissociate  30–100%N2 b 1u(v 0) X 1+g (v 00) photoabsorption991 992 993 994 995v′′ = 0, v′ = 014N214N15N15N2985 986 987 988 989v′′ = 0, v′ = 1978 979 980 981 982v′′ = 0, v′ = 2971.5 972.5 973.5 974.5 975.5v′′ = 0, v′ = 3Transmission(shifted)Wavelength (A˚)Mostly sharp rotational structureIn some cases predissociation broadenedN2 astronomical self-shielding92.0 92.1 92.2 92.3 92.4Wavelength (nm)0.00.20.40.60.81.0Transmission14 N214 N15 N H2 DustSharply peaked 14N2 lines quickly saturate14N15N is unaffected by a saturated 14N2 columnComparable or more important effect than shielding by H2 and dustN2 experimental linewidths of 1u states14N214N15N15N2Heays et al. (2011)Predissociation linewidth,  , varies with:Electronic state: b, c3, o3Vibrational levelIsotopologueElectron-excited emission spectrum800 900 1000 1100 1200 1300Wavelength ( )0.00.51.01.52.02.53.0Intensity (arb. units)0 1 2 3 4 5 6 7 8 9 10 11 120 1 2 3 4 5 7 8 9 10 11 12b 1 Πu (v′=1)→X(v′′)c′41 Σ+u (v′=0)→X(v′′)v′′Heays et al. (2014)Emission seen from:b 1u(v = 1; 4; 5;6; 7)b0 1+u (v = 1; 4; 7  19)c3 1u(v = 0; 1; 2)c4 1u(v = 0)c041+u (v = 0; 1; 2; 3; 4; 6)c051+u (v = 0)o3 1u(v = 0; 1; 2; 3;4)N2 potential-energy curves0.8 1.2 1.6 2.0 2.4Internuclear distance (A˚)90000100000110000120000Potentialenergy(cm−1)b1Πub′1Σ+uc41Πu / c′51Σ+uc31Πu / c′41Σ+uo31ΠuG33ΠuF33ΠuC3ΠuC′3Πu2 3Σ+uX1Σ+g + 95 000 cm−14S+2D4S+2P2D+2D1u and 1+u states absorband emit photons3u and 3+u states have anopen dissociation channelSpin-orbit coupling leads topredissociation of 1u and1+u statesKnown 3u potential-energy curves1.0 1.5 2.0 2.5Internuclear Distance (Å)9.010.011.012.0PotentialEnergy(104cm−1 )181687210012345321001CC’F IIIGN(4S)+N(2D)N(4S)+N(2P)N(2D)+N(2D)Lewis et al. (2008a)Levels known from optical spectroscopy, electron-energy loss, orinduced perturbationsThe SOLEIL/DESIRS experimentBSVUV-MVUV-FPhotodiodeUndulatorbeamHeNeFMRWInterferometric spectrometerMaximum path difference: 10 cmMaximum resolution: 0.07 cm 1/ 10 4 nmBeam bandwidth: 5 nmSample temperature: 90 – 1000K112,865112,860112,855112,850112,845112,840Wavenumber (cm−1)113,000112,800112,600112,400112,200Wavenumber (cm−1)118,000116,000114,000112,000110,000108,000106,000Wavenumber (cm−1)Krypton absorption spectrumde Oliveira et al. (2011)New photoabsorption spectra97.7 97.5 97.3 97.1 96.9 96.7 96.5 96.3Wavelength (nm)102400 102600 102800 103000 103200 103400 103600 103800Transition wavenumber ( cm−1)0.000.050.100.150.200.250.300.35Intensity(arb.)14N15Nb 1Πu(3)← X 1Σ+g (0)b 1Πu(4)← X 1Σ+g (0)N = 1.2× 1015 cm−2 T = 300 KN = 3.3× 1017 cm−2 T = 100± 20 KWeak lines become visibleHighly-excited rotational lines are suppressed14N15N D 3+u (v = 0)103540 103560 103580 103600 103620Transition wavenumber (cm−1)00.050.10.150.2Intensity(arb.units)1234P (J ′′)0 1 2 3 4 5 6 7 8R(J′′)b1Πu(4)← X(saturated)D3Σ+uΩ=1(v′ = 0)← X1Σ+g (v′′ = 0)H2H2Known in 14N2 and15N2 (Lewis et al. 2008b)14N15N b 1u(v = 4) and perturber103320 103340 103360 103380 103400 103420 103440Transition energy (cm−1 )00.050.10.150.20.25Intensity (arb. units) 14151617181920211112131415161718910111213141516∗∗R(J′′ )Q(J′′ )P(J′′ )b1Πu (v′=4)←X(v′′=0)Heays et al. (2011)N = 1:2 1015 cm 2 and T = 300KTwo extra lines14N15N b 1u(v = 4) and perturber103320 103340 103360 103380 103400 103420 103440Transition energy (cm−1 )00.050.10.150.20.25Intensity (arb. units) 14151617181920211112131415161718910111213141516∗∗R(J′′ )Q(J′′ )P(J′′ )b1Πu (v′=4)←X(v′′=0)0 5 10 15 20 25 30J0.60.40.20.00.20.40.6Reduced term values (cm−1)b(4)e-parityf-parityExtra lines are Q(17) and R(15)transitionsG 3u(v = 0) predicted by Lewis et al.(2008a)Error bars: Exp. term values.Lines: CSE model.14N15N b 1u(v=4) and G 3u(v = 0)103250 103300 103350 103400Transition wavenumber (cm−1)00.10.20.30.4Transmittance(arb.units)234567891114151618P (J ′′)123456 810Q(J ′′)173 4 5 6 7 8 9 10 13R(J ′′)Ω = 023456P (J ′′)12346Q(J ′′)0 1 2345678R(J ′′)Ω = 13456789P (J ′′)345678910Q(J ′′)138910R(J ′′)Ω = 2G33Πu(v′ = 0)← X 1Σ+g (v′′ = 0)N = 3:3 1017 cm 2 and T = 300 20KPreviously observed extra lineNow lines from 9 rotational branches: J =  1; 0; 1 and “”= 0; 1;214N15N b 1u(v=4) and G 3u(v = 0)0 5 10 15 20 25 30J103300103400103500Reduced term values (cm−1) Ω =0Ω =1Ω =2b 1Πu (v=4)G 3Πu (v=0)Parameters (cm 1)T0 103316.70(2)B 1.8057(4)D 0.000020(2)A -8.12(3)AD 0.047(4) –D – -0.133(8)o –p 0.020(4)q -0.0360(3)Information from the observed G(0) levels and the perturbed b(4)levelsOther bands in 14N15NF 3u(v = 0)104680 104700 104720 104740 10476000.050.10.150.20.250.3234567P (J ′′)1–6Q(J ′′)Ω = 02345P (J ′′)123456789Q(J ′′)0 1 2 3 4 5678R(J ′′)Ω = 143456746Ω = 2P (J ′′)Q(J ′′)R(J ′′)F33Πu(v′ = 0)← X 1Σ+g (v′′ = 0)C 3u(v = 7)100880 100900 100920 100940 100960 100980 101000Transition wavenumber (cm-1)00.020.040.060.080.10.120.14Transmittance(arb.units)23456789P (J ′′)1234567891011Q(J ′′)0123456789R(J ′′)Ω = 0345678P (J ′′)245678Q(J ′′)1 2-78R(J ′′)Ω = 2P (2) Q(1) R(0)Ω = 1C 3Πu(v′ = 7)← X 1Σ+g (v′′ = 0)Other preliminaryidentifications in14N15N:D 3+u (v = 0)D 3+u (v = 1)G 3u(v = 1)C 3u(v = 8)C 3u(v = 14) F 3u(v = 1)C 3u(v = 15)C 3u(v = 16)Other identifications in14N2 with T = 1000K(Niu et al. 2015):C 3u(v = 16) G 3u(v = 2)High temperature photoabsorption101200 101400 101600 101800 102000 102200Transition wavenumber (cm−1)0.00.10.20.30.4Transmittance(arb.units)2610141822263034P (J ′′)15913172125293337Q(J ′′)0812162024283236R(J ′′)b−X(2, 0)1591317212529P (J ′′)0 182529R(J ′′)c′4 −X(0, 1)26101418P (J ′′)159131721Q(J ′′)0812162024R(J ′′)b−X(1, 0)(Niu et al. 2015)900K ground-state excitationObserved lines as high as J = 40, v = 1High temperature photoabsorption – Linewidths0 5 10 15 20 25 30 35 400.00.10.20.30.40.5b(1)e-parityf -parityprevious results0 5 10 15 20 25 30 35 400.00.20.40.60.81.0b(2)0 5 10 15 20 25 30 35 40J0.00.20.40.60.81.01.21.41.6b(10)Naturallinewidths(cm−1FWHM) Rotational effects due to particularspin-orbit interacting levelsb 1u(2)  C 3u(8)b 1u(10)  G 3u(2)  C 3u(16)Fitted perturber parametersintermediate to what is predicted forG(2) and C(16) :B ' 1:5 cm 1A ' 30 cm 1Predissoc. width ' 20! 80 cm 114N15N C 3u(v = 8)101600 101800 102000 102200Transition wavenumber (cm–1)00.020.040.060.080.10.120.140.16Transmittance(arb.units)CSE model +0.02ExperimentH2Main saturated band:b 1u(v = 2) X (0)Broad feature:C 3u(v = 8) X (0)Confirms an existingCSE model (Lewis et al.2008a; Heays et al.2011)Summary1.0 1.5 2.0 2.5Internuclear Distance (Å)9.010.011.012.0PotentialEnergy(104cm−1)181687210012345321001CC’F IIIGN(4S)+N(2D)N(4S)+N(2P)N(2D)+N(2D)14152Newly observed levelsin 14N15N and 14N2Also levels of D 3+u– not shownThis information will be used to refine the potential-energy curves ofS = 1 states, and spin-orbit interaction with S = 0 statesLeading to an improved CSE model of N2 photodissociation forastrophysical / atmospheric purposes

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FORBIDDEN TRANSITIONS IN THE VUV SPECTRUM OF N2

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