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Electronic and vibrational excitation of N2 by O+ and N+
Volume 173, number 2,3
CHEMICAL PHYSICS LETTERS
5 October 1990
Electronic and vibrational excitation of N2 by O+ and N+ A.R. Lee I, C.S. Enos and A.G. Brenton MarsSpectrumetryResearch Unit, UniversityCollege of Swansea, Swansea SA2 8PP, UK Received 12 July 1990; in final form 28 July 1990
High-resolution spectroscopy has been performed on O+ and N+ ions in collision with N, molecules. The spectra for these ions display vastly different characters. O+ ions appear to excite only the first positive band (X ‘Cl -tB ‘IIs) of Nb with frequent accompaniment of simultaneous atomic transitions. The spectra for N+, on the other hand, reveal excitation of four different molecular bands of Nt, but simultaneous atomic and molecular transitions are completely absent.
1. Introduction Interactions of N+ and Of ions with Nz and O2 molecules play an important role in upper atmospheric phenomena. High-resolution translational energy spectroscopy is a valuable tool in helping to identify the relevant processes, and complements photo-emission and cross-section measurements. The translational energy spectra (TES) for N+ and O+ in collision with O2 have the spiked appearance of a large number of resonant or near-resonant transitions which can be readily and unambiguously identified [ I 1. The spectra presented in this communication with N, as a target gas display a very different character, with vibrational structures which are largely well resolved. Interpretation is less straight forward as spectral structures resulting from transitions between different electronic states sometimes overlap. This is further complicated by simultaneous molecular and atomic transitions which imbue the spectra with their unusual appearance. Proton excitation of Nz produces relatively simple spectra dominated by the Lyman-Birge-Hopfield X ‘C: + a ‘II* transition [ 2 1. Singlet-triplet transitions of Nz are spin-forbidden for proton impact because of the unavailability of electrons for spin exchange [ 3-51. Collision with projectile ions with non-singlet states add an extra dimension to these encounters. Singlet’ Permanent address: Physics Department, La Trobe University, Victoria 3083, Australia.
230
triplet transitions associated with the Vegard-Kaplan (X ‘El -A ‘Z: ) and the first positive (X ‘1: -+B 311,) bands [ 61 become prominent, and the spectra are further enhanced by simultaneous molecular and atomic transitions which make the spectra less amenable to easy analysis at initial attempts. Experience with the less complicated spectra involving CO as a target [ 5 I, and awareness of the likely adherence to the Wigner spin rule [ 3 1, enables one to identify the features of the energy loss spectra for N+ and 0’ on N1 with added confidence.
2. Experimental The experiments were performed on a novel double-focusing mass and translational energy spectrometer [ 71 based on a symmetrical arrangement of two identical analysers. The instrument has been described in a recent publication [ 8 1. Briefly, N+ and Of ions were generated from Nz and O2 gas, respectively, in a low-pressure electron-impact ion source from which they were extracted and accelerated to 3 keV. Following mass analysis by a magnetic sector, the ions were passed through an electrostatic monochromator and a collision gas cell containing low pressure ( = 10v3 Torr) target Nz gas to ensure near single collision conditions. As a test for single collision conditions TES spectra of O+ and N’ on Nz were recorded at pressures of 101, 25% and 50% of the actual pressure chosen. The ratio be-
0009-2614/90/$03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland )
Volume 173, number 2,3
5 October 1990
CHEMICAL PHYSICS LETTERS
tween the relative peak heights in the product spectrum remained constant indicating single collision conditions predominate. Energy analysis of the scattered ions was carried out by scanning in tandem two electrostatic energy analysers, the second of which was used to eliminate low level interferences which may be caused by ion and/or neutral reflections from walls and slits.
3. Results and discussion The translational energy spectra for O+ on Nz are shown in figs. la and lb. The TES spectrum for the full energy range is displayed in fig. la to give an overall view of the spectrum, while a contracted energy range with enhanced energy resolution of the vibrational features is given in fig. lb. A negative value of the energy defect AE refers to energy loss in which translational energy is converted into internal energy, and a positive value of A,? refers to superelastic transitions in which internal energy is converted into translational energy. The peaks A and B observed at A/Z= 2 1.7 eV result from atomic transitions 0 + ( 2D * ‘P) between the metastable states of the O+ ion, with spectroscopic energy of 1.69 eV [ 9 1. The forbidden doublet-quartet atomic transitions are not observed. The dominant structure I, which peaks at an energy loss of 7.9 eV, can be identified with the first positive transition (X ‘xi + B ‘I&), for which the spectroscopic energy for the O0 transition is 7.39 eV [ 61. The average spacing between four vibrational peaks v’= 1 to 5 is measured at 0.20 eV, which is in reasonable agreement with the listed value of w,=O.215 eV [6]. The highest peak thus appears to correspond to the vibrational level v’= 3 of the upper electronic state. The shoulder at the high energy end of this structure is probably caused by underlying transitions X ‘C: -+a ‘II, (Lyman-Birge-Hoplield) and X ‘Zg’+ w ‘AUwhich are observed in proton collision with NZ [ 21. Structure 11,which peaks at an energy of 3.4 eV to the endothermic side of the peak (v’ = 3) of structure I, is attributed to the simultaneous atomic and molecular transitions 0+ (4S)+O’ (‘D) , N2(X ‘C:)+N2(B 31-1,).
,
‘D- ‘5
'P-‘s
-5
0
I
I
-20
-15
-10
+5
*JO
Energy Change BE (eV1
-i4
-iz
-io
-‘a
4
Energy Change AE WI Fig. 1.Translational energy spectra of O+ in collision with N2.
The spectroscopic energy for the atomic transition is 3.33 eV [ 81. The two structures (1 and II) retain similar basic shapes, and the average spacing between the five vibrational levels v’= I to 5 of structure II is measured at 0.20? 0.02 eV which in accord with the spectroscopic value for this transition, and incompatible with transition to the C 311,state, for which the vibrational energy separation is z 0.25 eV. 231
Volume 173, number 2,3
CHEMICAL PHYSICS LETTERS
This type of simultaneous transition is prevalent in the TES spectrum involving CO as molecular target [I ] and can be unambiguously identified in those situations. Structure III appears to be of similar general shape as I and II. The measured vibrational spacings are also comparable, being 0.20 eV between the five vibrational levels labelled U’= 1 to 5. The highest peak (v’= 3) is shifted by 1.7 eV to the exothermic side of the highest peak (v’~3) of structure I, which prompted us to identify structure III with the simultaneous transitions O+(‘P)+O+(‘D), Nz(X ‘C:bNdB
5 October 1990
2,3 ... . Transition to the C 3III,state from the ground X ‘Z: state of N2, which has also been observed in the TES spectrum of C+ on Nz, is not apparent in the present spectra. The spectra for N+ on NZare shown in figs. 2a and 2b. The peaks observed at AE= + 2.2 eV result from the N+(‘S * ‘D) transitions between the metastable singlet states. The forbidden singlet-triplet transitions ‘D ++ 3P (1.90 eV) and ‘S * 3P (4.05 eV) are not observed. The spectrum for N+ as projectile is distinctly different from those for Of and C+ as projectiles in that there are no simultaneous atomic and molecular transitions observed in this
3IIr),
with spectroscopic energy of 1.69 eV for the atomic transition. Structure IV, which peaks at ~4.4 eV, or 3.5 eV to the exothermic side of structure I, is not well resolved. There is no listed electronic state at that energy, which is below that for the A ‘Z: state (6.22 eV) or upper electronic state of the Vegard-Kaplan band, and we have attributed structure IV to the following simultaneous transitions:
with spectroscopic energy of 3.33 eV for the atomic transition. This is in agreement with the observed energy shift of 3.5 eV, particularly in view of the effect of the profile of the main beam which would have shifted the peak of structure IV away from I. Structure IV is most likely to be associated with the simultaneous transitions
N,(X ‘C:)+Nz(B
-is
-iO
-j
ij
Energy Change
is
AE (eV1
‘l-l,,
for which the spectroscopic energy for the atomic transition is 5.02 eV. It is interesting to note that four observed simultaneous transitions appear to involve the first positive band. The ground 4Sand excited *D and *P states of O+ are all variously involved. There is no sign of the Vegard-Kaplan (X ‘Zz +A ‘Zz ) excitation which has been clearly observed in the TES spectrum of C+ on N, [ lo], with monotonically rising intensities for the well-resolved vibrational levels v’= 0, 1, 232
40
-12
-11
-10
-9
-8
-7
Energy Change AE (eV) Fig. 2. Translational energy spectra of N+ in collision with Nt.
Volume 173, number 2,3
CHEMICAL PHYSICS LETTERS
5 October 1990
Table I Observed reaction channels and single and simultaneous transitions, with corresponding observedenergylossesAEand spectroscopic values. For channels involving molecular transitions, energiesare given for designated vibrational levels u’for the upper state Reaction channel
Observed energy A.5 (W
Spectroscopic value (eV)
fig. 1
A: B: v: IV: III: I: II:
0+(2P)+N2(X’Z:)+O+(ZD)+N2(X’Z:) 0+(2D)+NI(X’ZI+)+O+(ZP)tN2(X’Ts+) 0+(2P)tN2(X’Z~,v=O)~O+(4S)tN~(B311s,u’=3) 0+(2D)tN2(X’Z,+,u=0)+0+(4S)tN~(B3~,,u’=3) 0+(2P)+Nr(X’Z~,v=O)+O+(2D)tNI(B311,,u’=O) 0+(4S)+N~(X’Z~,~=O)-t0+(4S)tN~(B~~g,u’=O) 0+(4S)+N2(X%:,~=0)+0+(2D)tNz(B311p,v’=O)
-1.7 1.7 z2.7 a’ 24.4 8’ 5.6 7.3 10.7
-1.69 1.69 3.0 4.6 5.70 1.39 10.72
fig. 2
A: B: II: I: III: Iv:
N+(‘S)tNz(X’Z:)-+N+(‘D)+N,(X’~:) N+(‘D)tN,(X’Z;)+N+(‘S)tN,(X’Z:) N+(‘P)+Nz(X ‘Z,+)+N+(‘P)+N~(A ‘Z:, u’=O) N+(3P)tN,(X’C~)+N+(3P)tN2(B311,,u’=O) N+(3P)tNz(X ‘Z:)+N+(3P)+N,(a ‘II,, u’=O) N+(3P)tN,(X’~~)~N+(‘P)tN2(C31-IU,u’=O)
-2.2 2.2 6.2 7.3 8.6 11.0
-2.15 2.15 6.22 7.39 8.59 Il.05
‘1 The discrepancy is due largely to the shift of the observed peak position on account of the main beam profile.
particular case. There is only one broad composite structure which can be subdivided into four basic regions. They all correspond to molecular transitions in which the N+ ion remains in its triplet ground 3P state. The vibrational peaks are best resolved in region I, the position of the highest peak (u’= 3) of which is measured at 7.9 eV. This region is identified with the first positive band (X’C: +B ‘IJ) for which the spectroscopic energy for the O-O transition is 7.39 eV. The measured average vibrational spacing between the six levels U’=O to 5 is 0.205 eV, compared with the listed value of o, = 0.2 15 eV [ 61. Region II, which represents the rising slope of the main structure, may be attributed to the VegardKaplan (X ‘Z.$+A ‘C; ) transition. The measured average spacing between the assigned vibrational peaks u’= 1 to 5 is 0.17 eV, compared with the spectroscopic value of o&= 0.18 1 eV. The positions of the vibrational peaks are diff%ult to locate from fig. la, but can be located fairly accurately from their displacement from peak v’= 3 of region I (fig. 1b). This gives an energy of 6.2 eV for the v’=O peak, which compares with the corresponding spectroscopic value of 6.22 eV for the Vegard-Kaplan band. This feature is very similar to that observed for the TES spectrum of Cf on N2 [9].
Region III is identified mainly with the LymanBirge-Hopfleld (X “Cl -ta III,) transition. The average vibrational spacing between the assigned peaks Y’= 3 to 6 is measured at 0.20 eV, compared with the spectroscopic value of w,= 0.2 10 eV. The position of the assigned U’= 3 peak is measured at 9.2 eV, which is compatible with the spectroscopic energy of 8.59 eV for the O-Otransition. Region IV, which joins in to region III, contains peaks which are more widely spaced, and this region is attributed to the singlet-triplet transition X ‘El +C 311U, which has also been observed but far more distinctly in the TES spectrum of C+ on N2 [ 10 1. The average spacing between the assigned vibrational levels v’=O to 3 is measured at 0.25 eV, which agrees with the spectroscopic value of o, = 0.254 eV. The position of the Y’= 0 peak is measured by its displacement from the Y’= 3 peak of region I at 11.0 eV, which agrees with the spectroscopic value of 11.05 eV for the O-Otransition. In conclusion, we note that collision of N2 with O+ produces mainly first positive transitions (X ‘&! -+B 311s), either on their own, or accompanied by simultaneous atomic transition between the 4S, ‘D and 2P states of the O+ ion. Other molecular transitions are not apparent. By contrast, col233
Volume 173, number 2,3
CHEMICAL PHYSlCS LEITERS
N, with N+ results in four distinct molecular transitions, the Vegard-Kaplan (X ‘Z: + A 3C: ), the first positive (X ‘Ez +B %“), the Lyman-Birge-Hopfield (X ‘2: +a ‘I&) and excitation to the C 311U state, all of which are clearly identified. No simultaneous atomic and molecular transitions have been observed in these spectra. The identified reaction channels are summarized in table 1. lisions of
References [ I] A.R. Lee, C.S. Enos and A.G. Brenton, to be published. [2] J.H. Moore Jr., J. Geophys. Res. 77 (1972) 5567.
234
5 October 1990
[3] E. Wignet, Nachr. Akad. Wiss. Goettingen, Math.-Physik, KI, IIa (1927) 375. (41 J.H. Moore Jr., Phys. Rev. A 8 (1973) 2359. [5] A.R. Lee, C.S. Enos and A.G. Brenton, Intern. J. Mass Spectrom. Ion Processes, to be published. [6] K.P. Huber and G. Her&erg, Constants of diatomic molecules (Van Nostrand Reinhold, New York, 1979). [7] J.H. Bcynon, A.G. Brenton and L.C.E. Taylor, Intern. J. Mass Spectrom. Ion Processes 64 (1985) 237. [8] P. Jonathan, M. Hamdan, A.G. Brenton and G.D. Will&, Chem.Phys. 119 (1988) 159. [ 91 C.E. Moore, Atomic energy levels, NBS Circular 467 (US GPO, Washington, 1949). [ 1OlA.R. Lee, C.S. Enos and A.G. Brenton, Rapid Commun. Mass Spectrom., to be published.
CHEMICAL PHYSICS LETTERS
5 October 1990
Electronic and vibrational excitation of N2 by O+ and N+ A.R. Lee I, C.S. Enos and A.G. Brenton MarsSpectrumetryResearch Unit, UniversityCollege of Swansea, Swansea SA2 8PP, UK Received 12 July 1990; in final form 28 July 1990
High-resolution spectroscopy has been performed on O+ and N+ ions in collision with N, molecules. The spectra for these ions display vastly different characters. O+ ions appear to excite only the first positive band (X ‘Cl -tB ‘IIs) of Nb with frequent accompaniment of simultaneous atomic transitions. The spectra for N+, on the other hand, reveal excitation of four different molecular bands of Nt, but simultaneous atomic and molecular transitions are completely absent.
1. Introduction Interactions of N+ and Of ions with Nz and O2 molecules play an important role in upper atmospheric phenomena. High-resolution translational energy spectroscopy is a valuable tool in helping to identify the relevant processes, and complements photo-emission and cross-section measurements. The translational energy spectra (TES) for N+ and O+ in collision with O2 have the spiked appearance of a large number of resonant or near-resonant transitions which can be readily and unambiguously identified [ I 1. The spectra presented in this communication with N, as a target gas display a very different character, with vibrational structures which are largely well resolved. Interpretation is less straight forward as spectral structures resulting from transitions between different electronic states sometimes overlap. This is further complicated by simultaneous molecular and atomic transitions which imbue the spectra with their unusual appearance. Proton excitation of Nz produces relatively simple spectra dominated by the Lyman-Birge-Hopfield X ‘C: + a ‘II* transition [ 2 1. Singlet-triplet transitions of Nz are spin-forbidden for proton impact because of the unavailability of electrons for spin exchange [ 3-51. Collision with projectile ions with non-singlet states add an extra dimension to these encounters. Singlet’ Permanent address: Physics Department, La Trobe University, Victoria 3083, Australia.
230
triplet transitions associated with the Vegard-Kaplan (X ‘El -A ‘Z: ) and the first positive (X ‘1: -+B 311,) bands [ 61 become prominent, and the spectra are further enhanced by simultaneous molecular and atomic transitions which make the spectra less amenable to easy analysis at initial attempts. Experience with the less complicated spectra involving CO as a target [ 5 I, and awareness of the likely adherence to the Wigner spin rule [ 3 1, enables one to identify the features of the energy loss spectra for N+ and 0’ on N1 with added confidence.
2. Experimental The experiments were performed on a novel double-focusing mass and translational energy spectrometer [ 71 based on a symmetrical arrangement of two identical analysers. The instrument has been described in a recent publication [ 8 1. Briefly, N+ and Of ions were generated from Nz and O2 gas, respectively, in a low-pressure electron-impact ion source from which they were extracted and accelerated to 3 keV. Following mass analysis by a magnetic sector, the ions were passed through an electrostatic monochromator and a collision gas cell containing low pressure ( = 10v3 Torr) target Nz gas to ensure near single collision conditions. As a test for single collision conditions TES spectra of O+ and N’ on Nz were recorded at pressures of 101, 25% and 50% of the actual pressure chosen. The ratio be-
0009-2614/90/$03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland )
Volume 173, number 2,3
5 October 1990
CHEMICAL PHYSICS LETTERS
tween the relative peak heights in the product spectrum remained constant indicating single collision conditions predominate. Energy analysis of the scattered ions was carried out by scanning in tandem two electrostatic energy analysers, the second of which was used to eliminate low level interferences which may be caused by ion and/or neutral reflections from walls and slits.
3. Results and discussion The translational energy spectra for O+ on Nz are shown in figs. la and lb. The TES spectrum for the full energy range is displayed in fig. la to give an overall view of the spectrum, while a contracted energy range with enhanced energy resolution of the vibrational features is given in fig. lb. A negative value of the energy defect AE refers to energy loss in which translational energy is converted into internal energy, and a positive value of A,? refers to superelastic transitions in which internal energy is converted into translational energy. The peaks A and B observed at A/Z= 2 1.7 eV result from atomic transitions 0 + ( 2D * ‘P) between the metastable states of the O+ ion, with spectroscopic energy of 1.69 eV [ 9 1. The forbidden doublet-quartet atomic transitions are not observed. The dominant structure I, which peaks at an energy loss of 7.9 eV, can be identified with the first positive transition (X ‘xi + B ‘I&), for which the spectroscopic energy for the O0 transition is 7.39 eV [ 61. The average spacing between four vibrational peaks v’= 1 to 5 is measured at 0.20 eV, which is in reasonable agreement with the listed value of w,=O.215 eV [6]. The highest peak thus appears to correspond to the vibrational level v’= 3 of the upper electronic state. The shoulder at the high energy end of this structure is probably caused by underlying transitions X ‘C: -+a ‘II, (Lyman-Birge-Hoplield) and X ‘Zg’+ w ‘AUwhich are observed in proton collision with NZ [ 21. Structure 11,which peaks at an energy of 3.4 eV to the endothermic side of the peak (v’ = 3) of structure I, is attributed to the simultaneous atomic and molecular transitions 0+ (4S)+O’ (‘D) , N2(X ‘C:)+N2(B 31-1,).
,
‘D- ‘5
'P-‘s
-5
0
I
I
-20
-15
-10
+5
*JO
Energy Change BE (eV1
-i4
-iz
-io
-‘a
4
Energy Change AE WI Fig. 1.Translational energy spectra of O+ in collision with N2.
The spectroscopic energy for the atomic transition is 3.33 eV [ 81. The two structures (1 and II) retain similar basic shapes, and the average spacing between the five vibrational levels v’= I to 5 of structure II is measured at 0.20? 0.02 eV which in accord with the spectroscopic value for this transition, and incompatible with transition to the C 311,state, for which the vibrational energy separation is z 0.25 eV. 231
Volume 173, number 2,3
CHEMICAL PHYSICS LETTERS
This type of simultaneous transition is prevalent in the TES spectrum involving CO as molecular target [I ] and can be unambiguously identified in those situations. Structure III appears to be of similar general shape as I and II. The measured vibrational spacings are also comparable, being 0.20 eV between the five vibrational levels labelled U’= 1 to 5. The highest peak (v’= 3) is shifted by 1.7 eV to the exothermic side of the highest peak (v’~3) of structure I, which prompted us to identify structure III with the simultaneous transitions O+(‘P)+O+(‘D), Nz(X ‘C:bNdB
5 October 1990
2,3 ... . Transition to the C 3III,state from the ground X ‘Z: state of N2, which has also been observed in the TES spectrum of C+ on Nz, is not apparent in the present spectra. The spectra for N+ on NZare shown in figs. 2a and 2b. The peaks observed at AE= + 2.2 eV result from the N+(‘S * ‘D) transitions between the metastable singlet states. The forbidden singlet-triplet transitions ‘D ++ 3P (1.90 eV) and ‘S * 3P (4.05 eV) are not observed. The spectrum for N+ as projectile is distinctly different from those for Of and C+ as projectiles in that there are no simultaneous atomic and molecular transitions observed in this
3IIr),
with spectroscopic energy of 1.69 eV for the atomic transition. Structure IV, which peaks at ~4.4 eV, or 3.5 eV to the exothermic side of structure I, is not well resolved. There is no listed electronic state at that energy, which is below that for the A ‘Z: state (6.22 eV) or upper electronic state of the Vegard-Kaplan band, and we have attributed structure IV to the following simultaneous transitions:
with spectroscopic energy of 3.33 eV for the atomic transition. This is in agreement with the observed energy shift of 3.5 eV, particularly in view of the effect of the profile of the main beam which would have shifted the peak of structure IV away from I. Structure IV is most likely to be associated with the simultaneous transitions
N,(X ‘C:)+Nz(B
-is
-iO
-j
ij
Energy Change
is
AE (eV1
‘l-l,,
for which the spectroscopic energy for the atomic transition is 5.02 eV. It is interesting to note that four observed simultaneous transitions appear to involve the first positive band. The ground 4Sand excited *D and *P states of O+ are all variously involved. There is no sign of the Vegard-Kaplan (X ‘Zz +A ‘Zz ) excitation which has been clearly observed in the TES spectrum of C+ on N, [ lo], with monotonically rising intensities for the well-resolved vibrational levels v’= 0, 1, 232
40
-12
-11
-10
-9
-8
-7
Energy Change AE (eV) Fig. 2. Translational energy spectra of N+ in collision with Nt.
Volume 173, number 2,3
CHEMICAL PHYSICS LETTERS
5 October 1990
Table I Observed reaction channels and single and simultaneous transitions, with corresponding observedenergylossesAEand spectroscopic values. For channels involving molecular transitions, energiesare given for designated vibrational levels u’for the upper state Reaction channel
Observed energy A.5 (W
Spectroscopic value (eV)
fig. 1
A: B: v: IV: III: I: II:
0+(2P)+N2(X’Z:)+O+(ZD)+N2(X’Z:) 0+(2D)+NI(X’ZI+)+O+(ZP)tN2(X’Ts+) 0+(2P)tN2(X’Z~,v=O)~O+(4S)tN~(B311s,u’=3) 0+(2D)tN2(X’Z,+,u=0)+0+(4S)tN~(B3~,,u’=3) 0+(2P)+Nr(X’Z~,v=O)+O+(2D)tNI(B311,,u’=O) 0+(4S)+N~(X’Z~,~=O)-t0+(4S)tN~(B~~g,u’=O) 0+(4S)+N2(X%:,~=0)+0+(2D)tNz(B311p,v’=O)
-1.7 1.7 z2.7 a’ 24.4 8’ 5.6 7.3 10.7
-1.69 1.69 3.0 4.6 5.70 1.39 10.72
fig. 2
A: B: II: I: III: Iv:
N+(‘S)tNz(X’Z:)-+N+(‘D)+N,(X’~:) N+(‘D)tN,(X’Z;)+N+(‘S)tN,(X’Z:) N+(‘P)+Nz(X ‘Z,+)+N+(‘P)+N~(A ‘Z:, u’=O) N+(3P)tN,(X’C~)+N+(3P)tN2(B311,,u’=O) N+(3P)tNz(X ‘Z:)+N+(3P)+N,(a ‘II,, u’=O) N+(3P)tN,(X’~~)~N+(‘P)tN2(C31-IU,u’=O)
-2.2 2.2 6.2 7.3 8.6 11.0
-2.15 2.15 6.22 7.39 8.59 Il.05
‘1 The discrepancy is due largely to the shift of the observed peak position on account of the main beam profile.
particular case. There is only one broad composite structure which can be subdivided into four basic regions. They all correspond to molecular transitions in which the N+ ion remains in its triplet ground 3P state. The vibrational peaks are best resolved in region I, the position of the highest peak (u’= 3) of which is measured at 7.9 eV. This region is identified with the first positive band (X’C: +B ‘IJ) for which the spectroscopic energy for the O-O transition is 7.39 eV. The measured average vibrational spacing between the six levels U’=O to 5 is 0.205 eV, compared with the listed value of o, = 0.2 15 eV [ 61. Region II, which represents the rising slope of the main structure, may be attributed to the VegardKaplan (X ‘Z.$+A ‘C; ) transition. The measured average spacing between the assigned vibrational peaks u’= 1 to 5 is 0.17 eV, compared with the spectroscopic value of o&= 0.18 1 eV. The positions of the vibrational peaks are diff%ult to locate from fig. la, but can be located fairly accurately from their displacement from peak v’= 3 of region I (fig. 1b). This gives an energy of 6.2 eV for the v’=O peak, which compares with the corresponding spectroscopic value of 6.22 eV for the Vegard-Kaplan band. This feature is very similar to that observed for the TES spectrum of Cf on N2 [9].
Region III is identified mainly with the LymanBirge-Hopfleld (X “Cl -ta III,) transition. The average vibrational spacing between the assigned peaks Y’= 3 to 6 is measured at 0.20 eV, compared with the spectroscopic value of w,= 0.2 10 eV. The position of the assigned U’= 3 peak is measured at 9.2 eV, which is compatible with the spectroscopic energy of 8.59 eV for the O-Otransition. Region IV, which joins in to region III, contains peaks which are more widely spaced, and this region is attributed to the singlet-triplet transition X ‘El +C 311U, which has also been observed but far more distinctly in the TES spectrum of C+ on N2 [ 10 1. The average spacing between the assigned vibrational levels v’=O to 3 is measured at 0.25 eV, which agrees with the spectroscopic value of o, = 0.254 eV. The position of the Y’= 0 peak is measured by its displacement from the Y’= 3 peak of region I at 11.0 eV, which agrees with the spectroscopic value of 11.05 eV for the O-Otransition. In conclusion, we note that collision of N2 with O+ produces mainly first positive transitions (X ‘&! -+B 311s), either on their own, or accompanied by simultaneous atomic transition between the 4S, ‘D and 2P states of the O+ ion. Other molecular transitions are not apparent. By contrast, col233
Volume 173, number 2,3
CHEMICAL PHYSlCS LEITERS
N, with N+ results in four distinct molecular transitions, the Vegard-Kaplan (X ‘Z: + A 3C: ), the first positive (X ‘Ez +B %“), the Lyman-Birge-Hopfield (X ‘2: +a ‘I&) and excitation to the C 311U state, all of which are clearly identified. No simultaneous atomic and molecular transitions have been observed in these spectra. The identified reaction channels are summarized in table 1. lisions of
References [ I] A.R. Lee, C.S. Enos and A.G. Brenton, to be published. [2] J.H. Moore Jr., J. Geophys. Res. 77 (1972) 5567.
234
5 October 1990
[3] E. Wignet, Nachr. Akad. Wiss. Goettingen, Math.-Physik, KI, IIa (1927) 375. (41 J.H. Moore Jr., Phys. Rev. A 8 (1973) 2359. [5] A.R. Lee, C.S. Enos and A.G. Brenton, Intern. J. Mass Spectrom. Ion Processes, to be published. [6] K.P. Huber and G. Her&erg, Constants of diatomic molecules (Van Nostrand Reinhold, New York, 1979). [7] J.H. Bcynon, A.G. Brenton and L.C.E. Taylor, Intern. J. Mass Spectrom. Ion Processes 64 (1985) 237. [8] P. Jonathan, M. Hamdan, A.G. Brenton and G.D. Will&, Chem.Phys. 119 (1988) 159. [ 91 C.E. Moore, Atomic energy levels, NBS Circular 467 (US GPO, Washington, 1949). [ 1OlA.R. Lee, C.S. Enos and A.G. Brenton, Rapid Commun. Mass Spectrom., to be published.