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Vacuum ultraviolet emission from highly excited states of molecular nitrogen
JOURNAL OF MOLECULAR SPECTROSCOPY lM,390-394
(1989)
Vacuum Ultraviolet Emission from Highly Excited States of Molecular Nitrogen JEAN-YVESRONCIN Equipe de Spectroscopic (C.N.R.S.. U.A. 171), Vniversitb de Saint Etienne et Lyon I, E.M.S.E., 42023 Saint Etienne Cedex, France FRANCOISE LAUNAY Obsewatoire de Paris, Section de Meudon [C. N.R.S., U.A. 812), 92195 Meudon Principaf Cedex, France AND KOUICHI YOSHINO Harvard-Smithsonian Centerfor Astrophysics, Cambridge, Massachusetts 02138 Five new emission bands have been analyzed at high resolution in the short wavelength range of the vacuum ultraviolet emission spectrum of molecular nitrogen. Four of them are assigned to the band system cb ‘2: (o’ = 0) + X’Z: (u” = O-3). The fifth band, at 82.6562 nm, corresponds to a transition from a highly excited state which was not known before but which is presently detected also in absorption at high resolution in 14N2and “N2. The new state is of ‘Z : symmetry and is identified as the first term of a Rydberg series converging toward the A ‘II, state of Nl. Both excited electronic states lie above the limit of dissociation of N2 into N(20) +N(20). They arc the only states in this energy range which are not totally predissociated. 0 1989 Academic Press, Inc.
The first extended investigation of the vacuum ultraviolet (WV) high resolution emission spectrum of N2 was done by Tilford and Wilkinson in 1964 (I). Recently, using a low pressure discharge which reduces self-absorption at short wavelengths, Roncin et al. extended the investigation down to 82 nm and reported tens of new bands (2,3), all of them except the one at 82.6562 nm being identified. The spectrum was obtained on the 10-m spectrograph of the Meudon Observatory. The puzzling emission band at 82.6562 nm exhibits clear-cut R and P branches with straightforward J numbering, as shown in Fig. 1. The wavenumbers of the lines are listed in Table I. The deduced value of the rotational constant BO of the excited state is found to be 1.786 cm-‘, very close to the BO value 1.735 cm-’ for the A%, state of N: (4). As all bands that correspond to transitions ending at II” = 0 are selfabsorbed it was reasonable to assume that the observed band corresponds to a transition ending at # = 1, i.e., (u’, 1). In the medium resolution absorption spectra reported by Carter (5) and Giirtler et al. (6) no feature can be ascribed to the expected (v’, 0) band. The absorption spectrum of Nz had been partly analyzed at high resolution in the spectral range around 8 1 nm but many features were lefi unassigned ( 7,8). Looking 0022-2852189 $3.00 Cowi@
0 1989 by Academic Press, Inc.
All rights of reproduction in any form reserved.
390
391
VUV SPECTRA OF Nz
Ar 1
82.53460 I
82.63649 82.6
I
82.7
I
nm
I
I
FIG, 1. Emission spectrum of 14N2around 82.6 nm.
carefully at an absorption spectrum of 14N2,obtained with the 6.65-m spectrograph at Cambridge, Massachusetts, we have sorted out R and P branches, also listed in Table I, of an extremely weak band, on the long wavelength side of the 7f + Xband, at 81.0942 nm. The spacing between the absorption band and the emission band is exactly equal to the first vibrational spacing of the ground state. A densitometer trace of the absorption band is shown in Fig. 2. In the absorption spectrum of “Nz a single peak at 8 1.0804 nm can be ascribed to the R head of the isotope band, the P branch
I
I
81.07
1
1
I
1
81.12
I
c
nm FIG. 2. Absorption spectrum of “N2 around 8 1.O nm.
392
RONCIN, LAUNAY, AND YOSHINO TABLE I Wavenumbers (cm-‘) and Line Assignments for the Bands ‘Z: t-t X’Z: (v’ = 0, v”) of i4N2 absorption J
(0.0)
R
123 123 123 123 123 123 123
emission
P
316.49 319.55 322.48 325.02 326.67 328.70 328.70
123 123 123 123 123 123
(0,)
R
309.46 304.71 299.66 294.67 289.20 282.89
120 120 120 120 120
) P
986.71 990.16 992.69 995.12 997.17
120 120 120 120 120 120 120 120
979.09 974.81 970.11 965.06 959.69 953.71 947.33 940.58
J
0 1 2 3 4 5 b 7 8
TABLE II Wavenumbers (cm-‘) and Line Assignments for the Bands cb ‘2: + X’S; Observed in the Emission Spectrum of N2
0 1
P
R
119 944.32 119 119 119 119
2 3 4 5
936.46 932.55 928.38 924.51
6 7 8 9
"117 614.77 -117 618.81 117 622.57 117 626.61 -117 630.91 117 635.06 117 639.00 8'21-0 R8+P5 117 647.52
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
P
8'17-0
R8
B'17-0 " " " " " "
R7 P4 R6 P3 R5 P2 Pl
0, v”)
J
606.57 602.67 598.70 594.91 591.15 587.43 583.75
0 1 2 3 4 5 b 7
117 580.03 117 576.24 117 572.56 -117 568.94 -117 565.08 B'21-0 R13 117 557.64 117 553.93 117 549.85
8 9 10 11 12 13 14 1s lb
117 -117 -117 117 117 -117 -117
o-3
o-2 R
P
R
10 11 12 13 14 15 1.4
J
=
o-1
o-o .I
(v’
8'17-0 P7 115 301.56 115 297.81 115 294.03 115 290.60 B'17-0 P8 115 283.38 115 279.88 115 276.58 115 273.21 B'17-0 P9 115 266.62 115 263.35 8'17-0 R12
P
R 113 040.77 113 044.40 113 048.92 113 053.25 113 057.70 c3 4-o PlO 113 067.41 113 076.50
113 113 113 113 113 113 113 113 113
033.07 028.98 025.48 021.75 018.26 014.96 011.79 008.46 005.55
J 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Note. - indicates that the line is weakened by absorption by one or several lines from another band. For some missing lines the wavenumber is replaced by the name of the absorbing line(s). Very few lines are observed for the (0,O) band because of self-reversal.
393
VUV SPECTRA OF N2
b’ ‘r; -X ‘Xi (21-O) P P
&o,,, ,,,I:....
10 , , 151 .
,‘.
absorption N2 emission
FIG. 3. Absorption and emission spectra of 14N2,around 85.0 nm, showing coincidences between absorption lines and apparently anomalously weak emission lines.
being blurred by a stronger band. Therefore we can say that the excited state has V’ = 0 and is the first member, with n = 3, of a Rydberg series converging toward the A211, state of the ion, as all Rydberg series in N2 start with n = 3 (9). In support of this hypothesis we find a quantum defect of -0.1079 in good agreement with the ab initiu value -0.0854 found by Raoult et al. (IO), for the Rydberg series ‘2,’ (n&r, Rydberg electron) converging toward the A211, state of the ion. Moreover the weakness of the absorption band is in good agreement with the weak value predicted for the corresponding transition moment (10). However, we could not find any emission structure in 15N2corresponding to the (0,l) emission band even with very long exposure time, indicating that the excited state is more predissociated in “N2. Four bands whose origins lie at 83.3747, 85.0264, 86.7232, and 88.4667 nm are readily identified (3)) respectively, as the bands (0, 0), (0, 1), (0, 2)) and (0, 3 ) of the system cb ‘2,’ + X1X: the (0, 0) band of which was analyzed earlier from absorption data (II). ’ The wavenumbers of the lines arc listed in Table II. As shown in Fig. 3, the (0- 1) band exhibits many apparent irregularities in intensity. As already pointed out (3), each of these irregularities is actually explained by coincidence of the emission line with one or several absorption lines pertaining to the band b’ ‘8: + X’Z: (21, 0) as can be clearly seen in Fig. 3. ’ In Table A4 of Ref. ( I I ) the rotational assignments of the R lines have to be shifted downward by one unit in J.
394
RONCIN,
LAUNAY,
AND YOSHINO
It has to be pointed out that the new state as well as the state cb lie above the limit of dissociation of Nz into N(*D) + N(2D) whereas all other states lying above this limit are partly or completely predissociated (2). Various channels for predissociation can be invoked but at this stage it is not possible to say exactly why these two particular states are not fully predissociated. However, one can notice that all possible ‘II, states converging toward the A 211Uof the ion are fully predissociated like most of ‘IIU states converging toward the X22,+ state of the ion. ACKNOWLEDGMENTS We thank Helene Lefebvre-Brian for critical reading of the manuscript. Technical assistance of M. Benharrous and J. R. Esmond is gratefully acknowledged. The present work benefits from a NATO collaborative research grant, Nh 86/0086. Some of the calculations have heen performed at the Centre Inter&ional de Calcul Electronique (CIRCE), Orsay, France. RECEIVED:
November 16, 1988 REFERENCES
I. S. G. TILFORD AND P. G. WILKINSON, J. Mol. Spectrosc. 15231-288 (1964). 2. J.-Y. RONCIN,F. LAUNAY, AND M. LARZILLIERE,Phys. Rev. LQU.53, 159-162 (1984). 3. J.-Y. RONCIN,F. LAUNAY,AND K. YOSHINO,Planer. Space Sci. 35,267-269 (1987).
4. K. P. HUBERAND G. HERZBERG,“Molecular Spectra and Molecular Structure,” Vol. IV, “Constants of Diatomic Molecules,” Van Nostrand-Reinhold, New York, 1979. 5. V. L. CARTER, J. Chem. Phys. 56,4195-4205 (1972). 6. P. GORTLER, V. SAILE,AND E. E. KOCH, Chem. Phys. Lett. 48,245-250 (1977). 7. J. W. C. JOHNSAND D. W. LEPARD, J. Mol. Spectrosc. 55,374-406 (1975). 8. E. S. CHANG AND K. YOSHINO, J. Phys. B 16, L581-585 (1983). 9. A. ~F~HUS AND P. H. KRUPENIE,J. Phys. Chem. Ref: Data 6, 113-307 (1977). ID. M. RAOULT, H. LE Rouzo, G. RASEEV,AND H. LEFEBVRE-BRION, J. Phys. B 16,4601-4617 11. P. K. CARROLLAND K. YOSHINO, J. Phys. B 5, 1614-1633 (1972).
(1983).
(1989)
Vacuum Ultraviolet Emission from Highly Excited States of Molecular Nitrogen JEAN-YVESRONCIN Equipe de Spectroscopic (C.N.R.S.. U.A. 171), Vniversitb de Saint Etienne et Lyon I, E.M.S.E., 42023 Saint Etienne Cedex, France FRANCOISE LAUNAY Obsewatoire de Paris, Section de Meudon [C. N.R.S., U.A. 812), 92195 Meudon Principaf Cedex, France AND KOUICHI YOSHINO Harvard-Smithsonian Centerfor Astrophysics, Cambridge, Massachusetts 02138 Five new emission bands have been analyzed at high resolution in the short wavelength range of the vacuum ultraviolet emission spectrum of molecular nitrogen. Four of them are assigned to the band system cb ‘2: (o’ = 0) + X’Z: (u” = O-3). The fifth band, at 82.6562 nm, corresponds to a transition from a highly excited state which was not known before but which is presently detected also in absorption at high resolution in 14N2and “N2. The new state is of ‘Z : symmetry and is identified as the first term of a Rydberg series converging toward the A ‘II, state of Nl. Both excited electronic states lie above the limit of dissociation of N2 into N(20) +N(20). They arc the only states in this energy range which are not totally predissociated. 0 1989 Academic Press, Inc.
The first extended investigation of the vacuum ultraviolet (WV) high resolution emission spectrum of N2 was done by Tilford and Wilkinson in 1964 (I). Recently, using a low pressure discharge which reduces self-absorption at short wavelengths, Roncin et al. extended the investigation down to 82 nm and reported tens of new bands (2,3), all of them except the one at 82.6562 nm being identified. The spectrum was obtained on the 10-m spectrograph of the Meudon Observatory. The puzzling emission band at 82.6562 nm exhibits clear-cut R and P branches with straightforward J numbering, as shown in Fig. 1. The wavenumbers of the lines are listed in Table I. The deduced value of the rotational constant BO of the excited state is found to be 1.786 cm-‘, very close to the BO value 1.735 cm-’ for the A%, state of N: (4). As all bands that correspond to transitions ending at II” = 0 are selfabsorbed it was reasonable to assume that the observed band corresponds to a transition ending at # = 1, i.e., (u’, 1). In the medium resolution absorption spectra reported by Carter (5) and Giirtler et al. (6) no feature can be ascribed to the expected (v’, 0) band. The absorption spectrum of Nz had been partly analyzed at high resolution in the spectral range around 8 1 nm but many features were lefi unassigned ( 7,8). Looking 0022-2852189 $3.00 Cowi@
0 1989 by Academic Press, Inc.
All rights of reproduction in any form reserved.
390
391
VUV SPECTRA OF Nz
Ar 1
82.53460 I
82.63649 82.6
I
82.7
I
nm
I
I
FIG, 1. Emission spectrum of 14N2around 82.6 nm.
carefully at an absorption spectrum of 14N2,obtained with the 6.65-m spectrograph at Cambridge, Massachusetts, we have sorted out R and P branches, also listed in Table I, of an extremely weak band, on the long wavelength side of the 7f + Xband, at 81.0942 nm. The spacing between the absorption band and the emission band is exactly equal to the first vibrational spacing of the ground state. A densitometer trace of the absorption band is shown in Fig. 2. In the absorption spectrum of “Nz a single peak at 8 1.0804 nm can be ascribed to the R head of the isotope band, the P branch
I
I
81.07
1
1
I
1
81.12
I
c
nm FIG. 2. Absorption spectrum of “N2 around 8 1.O nm.
392
RONCIN, LAUNAY, AND YOSHINO TABLE I Wavenumbers (cm-‘) and Line Assignments for the Bands ‘Z: t-t X’Z: (v’ = 0, v”) of i4N2 absorption J
(0.0)
R
123 123 123 123 123 123 123
emission
P
316.49 319.55 322.48 325.02 326.67 328.70 328.70
123 123 123 123 123 123
(0,)
R
309.46 304.71 299.66 294.67 289.20 282.89
120 120 120 120 120
) P
986.71 990.16 992.69 995.12 997.17
120 120 120 120 120 120 120 120
979.09 974.81 970.11 965.06 959.69 953.71 947.33 940.58
J
0 1 2 3 4 5 b 7 8
TABLE II Wavenumbers (cm-‘) and Line Assignments for the Bands cb ‘2: + X’S; Observed in the Emission Spectrum of N2
0 1
P
R
119 944.32 119 119 119 119
2 3 4 5
936.46 932.55 928.38 924.51
6 7 8 9
"117 614.77 -117 618.81 117 622.57 117 626.61 -117 630.91 117 635.06 117 639.00 8'21-0 R8+P5 117 647.52
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
P
8'17-0
R8
B'17-0 " " " " " "
R7 P4 R6 P3 R5 P2 Pl
0, v”)
J
606.57 602.67 598.70 594.91 591.15 587.43 583.75
0 1 2 3 4 5 b 7
117 580.03 117 576.24 117 572.56 -117 568.94 -117 565.08 B'21-0 R13 117 557.64 117 553.93 117 549.85
8 9 10 11 12 13 14 1s lb
117 -117 -117 117 117 -117 -117
o-3
o-2 R
P
R
10 11 12 13 14 15 1.4
J
=
o-1
o-o .I
(v’
8'17-0 P7 115 301.56 115 297.81 115 294.03 115 290.60 B'17-0 P8 115 283.38 115 279.88 115 276.58 115 273.21 B'17-0 P9 115 266.62 115 263.35 8'17-0 R12
P
R 113 040.77 113 044.40 113 048.92 113 053.25 113 057.70 c3 4-o PlO 113 067.41 113 076.50
113 113 113 113 113 113 113 113 113
033.07 028.98 025.48 021.75 018.26 014.96 011.79 008.46 005.55
J 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Note. - indicates that the line is weakened by absorption by one or several lines from another band. For some missing lines the wavenumber is replaced by the name of the absorbing line(s). Very few lines are observed for the (0,O) band because of self-reversal.
393
VUV SPECTRA OF N2
b’ ‘r; -X ‘Xi (21-O) P P
&o,,, ,,,I:....
10 , , 151 .
,‘.
absorption N2 emission
FIG. 3. Absorption and emission spectra of 14N2,around 85.0 nm, showing coincidences between absorption lines and apparently anomalously weak emission lines.
being blurred by a stronger band. Therefore we can say that the excited state has V’ = 0 and is the first member, with n = 3, of a Rydberg series converging toward the A211, state of the ion, as all Rydberg series in N2 start with n = 3 (9). In support of this hypothesis we find a quantum defect of -0.1079 in good agreement with the ab initiu value -0.0854 found by Raoult et al. (IO), for the Rydberg series ‘2,’ (n&r, Rydberg electron) converging toward the A211, state of the ion. Moreover the weakness of the absorption band is in good agreement with the weak value predicted for the corresponding transition moment (10). However, we could not find any emission structure in 15N2corresponding to the (0,l) emission band even with very long exposure time, indicating that the excited state is more predissociated in “N2. Four bands whose origins lie at 83.3747, 85.0264, 86.7232, and 88.4667 nm are readily identified (3)) respectively, as the bands (0, 0), (0, 1), (0, 2)) and (0, 3 ) of the system cb ‘2,’ + X1X: the (0, 0) band of which was analyzed earlier from absorption data (II). ’ The wavenumbers of the lines arc listed in Table II. As shown in Fig. 3, the (0- 1) band exhibits many apparent irregularities in intensity. As already pointed out (3), each of these irregularities is actually explained by coincidence of the emission line with one or several absorption lines pertaining to the band b’ ‘8: + X’Z: (21, 0) as can be clearly seen in Fig. 3. ’ In Table A4 of Ref. ( I I ) the rotational assignments of the R lines have to be shifted downward by one unit in J.
394
RONCIN,
LAUNAY,
AND YOSHINO
It has to be pointed out that the new state as well as the state cb lie above the limit of dissociation of Nz into N(*D) + N(2D) whereas all other states lying above this limit are partly or completely predissociated (2). Various channels for predissociation can be invoked but at this stage it is not possible to say exactly why these two particular states are not fully predissociated. However, one can notice that all possible ‘II, states converging toward the A 211Uof the ion are fully predissociated like most of ‘IIU states converging toward the X22,+ state of the ion. ACKNOWLEDGMENTS We thank Helene Lefebvre-Brian for critical reading of the manuscript. Technical assistance of M. Benharrous and J. R. Esmond is gratefully acknowledged. The present work benefits from a NATO collaborative research grant, Nh 86/0086. Some of the calculations have heen performed at the Centre Inter&ional de Calcul Electronique (CIRCE), Orsay, France. RECEIVED:
November 16, 1988 REFERENCES
I. S. G. TILFORD AND P. G. WILKINSON, J. Mol. Spectrosc. 15231-288 (1964). 2. J.-Y. RONCIN,F. LAUNAY, AND M. LARZILLIERE,Phys. Rev. LQU.53, 159-162 (1984). 3. J.-Y. RONCIN,F. LAUNAY,AND K. YOSHINO,Planer. Space Sci. 35,267-269 (1987).
4. K. P. HUBERAND G. HERZBERG,“Molecular Spectra and Molecular Structure,” Vol. IV, “Constants of Diatomic Molecules,” Van Nostrand-Reinhold, New York, 1979. 5. V. L. CARTER, J. Chem. Phys. 56,4195-4205 (1972). 6. P. GORTLER, V. SAILE,AND E. E. KOCH, Chem. Phys. Lett. 48,245-250 (1977). 7. J. W. C. JOHNSAND D. W. LEPARD, J. Mol. Spectrosc. 55,374-406 (1975). 8. E. S. CHANG AND K. YOSHINO, J. Phys. B 16, L581-585 (1983). 9. A. ~F~HUS AND P. H. KRUPENIE,J. Phys. Chem. Ref: Data 6, 113-307 (1977). ID. M. RAOULT, H. LE Rouzo, G. RASEEV,AND H. LEFEBVRE-BRION, J. Phys. B 16,4601-4617 11. P. K. CARROLLAND K. YOSHINO, J. Phys. B 5, 1614-1633 (1972).
(1983).