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The vacuum uv emission spectrum of the 15N22+ molecular ion
JOURNAL
OF MOLECULAR
SPECTROSCOPY
113, l59- 166 ( 1985)
The Vacuum UV Emission Spectrum of the 15Ny Molecular Ion DANIEL COSSART Laboratoire de PhotophysiqueMolhculairedu CNRS, Bit. 213, Universithde Paris&d, 91405 Orsay Cedex? France AND
FRANCOISELAUNAY Observatoirede Paris, Dkpartementd’AstrophysiqueFondamentale. U.A. 812 du CNRS, 92195 Meudon PrincipalCedex, France The vacuum uv emission of the “N:+ ion has been recorded for the first time. Rotational analysis of two bands, analogous to those already observed in the case=of the natural isotope, confirm their assignment to the D’Z&!?Z~ (0, 0) and (1, 1) bands. Moreprecisedata are also
obtainedfor the ‘2; statewhichperturbsgroundstatevibrationallevels. o 1985 Academic 1.
press, IX.
INTRODUCTION
In a previous paper (Z), we presented the rotational analysis of two emission bands obtained in the vacuum uv region from an electric discharge through natural nitrogen. The origins of these bands were determined to be at 1589.7 and 159 1.5 A. The 1589.7A band was first observed by Carroll (2), who tentatively assigned it to a D’Z:X’Z: (0, 0) transition of the doubly charged molecular nitrogen ion. The observation of the ( 1, 1) band of the same system, associated with experimental data from photofragment spectroscopy concerning the ground state of Np, by Cosby et al. (3), and ab-initio calculation results (I), removed any doubt which might exist about the assignment of these bands to the N$+ ion. Moreover, the appearance of breaking-off points in the (0,O) and ( 1, 1) bands was interpreted as resulting from perturbations of vibrational levels of the ground state by those of a neighboring 32; state. This state has a larger equilibrium internuclear distance than that of the ground state and its vibronic levels are probably efficiently predissociated through the Coulomb potential barrier. Rotational lines of the D-X transition are thus expected to be weakened and broadened in the region of maximum perturbation. We previously showed (I), on the basis of calculated 32; state constants, that the X’Z,‘(v = 0 and 1) levels are perturbed respectively by two successive (uOand u. + 1) levels of a 32; state, but it was not possible to determine the vibrational numbering in the latter, i.e., the value of uo. Analysis of corresponding emission bands of the isotopic “N$+ species was thought to be of help to solve this problem. Another problem that we hoped to solve with data obtainable from the “N:+ spectrum was the confirmation of the assignment to the D-X (2, 3) transition of a very 159
0022-2852185 $3.00 Copyright 0 1985 by Academic Press, Inc. All rights of reproduction in any form reserved.
COSSART
160
AND LAUNAY
faint and diffuse feature near 1635 A. If this assignment is correct, one would obtain a vibrational spacing in the ground state AG2,5N 1600 cm-‘, much smaller than AG1,S = 19 16 cm-’ (3), confirming the electronic configuration change which is calculated to occur in the ground state near its equilibrium internuclear distance. It should be noted that this effect is detectable from the rotational constants obtained by Cosby et al. (3) for the first three vibrational levels of the ground state. Unfortunately, in the case of the 15N2isotope, the limited exposure time did not allow us to obtain the band analogous to that observed at 1635 A in the case of the 14N2. II. EXPERIMENTAL
DETAILS
The experimental conditions were identical to those given in Ref. (I) for natural 14N2nitrogen. Our emission source [negative glow of a dc discharge to which a 3-kG transverse magnetic field (4) is applied] allowed us to obtain the spectrum given in Fig. 1. This spectrum was recorded with the Meudon Observatory 10-m vacuum uv spectrograph. About 41 hr of exposure time was necessary during which 1 liter (at atmospheric pressure) of isotopic nitrogen gas was spent. Measurements and absolute calibration of the rotational lines [using the fourth positive emission system of CO (5) as a reference spectrum] were made in a way similar to that given in Ref. (I). III. DESCRIPTION
AND ANALYSIS
OF THE
SPECTRUM
Figure 1 shows rotational line assignments of the two observed bands analogous to those of the 14N$’ ion which have been attributed to the D’I;:-X*Z,+ (0,O) and (1, 1) transitions. The appearance and position of these bands are very similar to those of the natural nitrogen spectrum. However, two main differences with respect to the spectrum given in Ref. (1) are obvious. (i) The intensity alternation is opposite to that of the 14Np spectrum, i.e., the odd lines are observed to be strong, the even lines weak; the latter being missing for the highest J values [J = 34, 36, 38 in the (0, 0) band]. This is perfectly consistent with the nuclear spins I = 1 and l/2 for the 14N and 15N isotopes, respectively (6). The intensity ratio between strong and weak lines is calculated to be 3, instead of 2 in the case of the natural isotope, thus enhancing the apparent intensity alternation. (ii) In the “N$+ spectrum, the (2,4) band of neutral nitrogen Lyman-Birge-Hopfield system no longer overlaps the R branch of the D’Zz-X’Z,+ (0,O) band, as it was the case in the 14N$+spectrum. Sudden intensity weakening is thus clearly visible here at J = 31, whereas lines disappear at J = 39. Broadening of the weak R lines in the J = 30-39 interval is detectable on the plate. However, in the right-hand side of Fig. 1, the P-branch lines with high J values are contaminated by emissions of the b” 2:-X1X,’ (1,2 1) band of neutral 15N2.This band is not presently analyzed. Its head (X = 1591.5 A) was calculated with the classical isotopic relations, from the emission data of Tschulanowsky (X-head 14N2= 1623.85
VACUUM
UV SPECTRUM
OF 15N:
.
-
1 .
e 4 . .--
._
iv) . . .
;” .-
,*o f -. . lSC? . *
:0 . . ..
:e I f
ia .:fg LL
c
M. N.
*I
m.
161
162
COSSART AND LAUNAY
A) quoted by Lofthus and Krupenie (7). A number of irregularities in the line positions and intensities mentioned in Ref. (7) are also visible here. Nevertheless, it is clear that breaking-off points occur in both R and P branches of the Nz+ spectrum, at J = 39 and 25 in the (0, 0) and (1, 1) bands, respectively. The measured wavenumbers (cm-‘) are given in Table I for the two bands analyzed. Fitting them to the quadratic expansion, T,,J = T, + B,J(J + I) - D,[J(J + 1)12. for representing the term values of the two ‘Z+ states involved in the transition, gave the vibronic molecular parameters listed in Table II. For comparison, we give also in the same table the corresponding values obtained for 14N:+ (1). TABLE I Measured Wavenumbers (cm-‘) of the D'Z:- X'Z;(O-O) and (l-l) Bands of 15N: (1-l)
(0-O) N
R
0
62907. 74. 62911. 17. 62914.62 62918.01 62921.36 62924. 70 62926.02 6293 1.26 62934.52 62937. 73 62940.91 62944.04 62947. 15 62950.22 62953.29 62956.26 62959.27 62962.25 62965. 16 62968.06 62970.95 62973.74 62976.55 62979.26 6298 1.98 62964. 74 62967.29 62989.97 62992.55 62995. 11 62997.58 63000. 10
8 9 IO 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
= blended
63004.90 63009.
6 I
P
R
P
62900. 72 62897.25’ 62893.61 62690.01 62886.36’ 62862. 73. 62879.06 62675.40 62871.67 62867.6462064.02
62841.84 62845. 10. 62840.56= 62851.76 62855.06 62858.34 62861.57
6283 1.63 62828. 59. 62824.53. 62620.44. 62817.37 62813. 77 62810. 12 62606.42 62802.70 62799.06 62795.27. 62791.01’ 62787.51 62783.70 62779. 76 62775.66 62771.90 62767.95 62763.86 62759.60 62755.74
62860. 62856.31 62852.47 62846.56 62644.63 62640.66 62836.66 62832.67 62628. 62624. 62620.44’ 62616.33 62612.09 62807.99 62603.04 62799. 62795.27’ 62791.01’ 62766. 62782.
73 35
62773.
73
62764.
63
63014.27
62756.36
63019.21
62747.35
line
19
59* 53.
57
62867.84. 6267 1.32 62874.06 62677.02 62880. 10 62882. 73’ 62885.90 62888.91 62891. 77 62894.50 62897.25. 62899.77 62902.86 62905.54 62908. 16 62911. 17” 62913. 43
62747.45 62743.29 62739. 62734.78
14
VACUUM
UV SPECTRUM
163
OF “N:+
TABLE II Rovibronic Molecular Parameters for “*‘sN:+ X’Zi-D’Z:(v 14
15
++
B
0
62903.30
(‘2+) 9
(2)
1.8784 5.7
lo600q B
(‘I+) 0
++ N2
N2
v(O,O)
= 0 and 1)
2904.30
1.7565
(6)
7.3
(4)
1.8624
(2) (6)
(3)
1.7414
(6)
(6)
”
106Do(‘X~) 5.6 (4) _-__------_ ____--__--__-__ v(l,l)
62831.77
8 (lx+) 1
106D B
(7)
1.8534
7.1
(3)
--_-____--___--_ 62835.04
(30)
(7)
1.7316
(20)
9
1
(‘I+)
7.4
(30)
5.2
(20)
9
(‘I+) 1
1.8356
(30)
1.7152
(20)
”
106Dl('Q
All
data
in cm
6.6
(30)
4.6
(20)
-1
Uncertainties are given of the last digits.
in parentheses
in units
IV. DISCUSSION
The isotopic analysis is based on vibrational and rotational energy expressions expanded in terms as defined by Dunham (8). Limitation to the second-order terms of the developments considered is valid only for potential energy expressions near Morse functions. The results of this isotopic analysis lead to the following two conclusions: (a) The assignment of the observed bands, as well as the vibrational numbering in the D'Z; and X’Z,f states, are fully confirmed by the comparisons given in Table III. The equilibrium molecular constants of the “N:+ ion given in the last column are compared to the corresponding ones obtained from those of the natural isotope, using the isotopic formulae available (6) for anharmonic oscillators, and assuming that the observed bands are the (0, 0) and ( 1, 1) bands of the electronic transition considered. The agreement is satisfactory between calculated and observed rotational constants of the “N:+ ion, if we take into account the unusual shape of the potential energy curve due to configuration change as discussed in Ref. (1). In particular, the disagreement between fitted and calculated centrifugal distorsion constants for “N:+ (Table II) may be due to this effect. It should be noted that similar “anomalous” D, values have been found by Cosby et al. (3) for the ground state of the 14N$+ion. D, value behavior differences in 14N$+and “N$’ is further indication that Morse potential hypothesis is correct only to first order. The isotopic relations are also verified for the equilibrium vibrational constants, w,. This can be seen only by comparing the (0, 0) and (1, 1) band origin differences,
e
From
From
From
a -
b -
c -
v(O,O)
+2wx
et al
Pekeris
formula
y+
(1)
+ 2[wexe(D)
- "(0,OP
on
(6)
work
(3)
= w,(X)
b
- w,(D)
+ v(l,l)
ee
e
b
our previous
Cosby
- v(l,l)
*I e = AG0.5
= AGoa5(X)
=e*+1/2a 0
AGO.5
e
=
5 + 2 wdxeC
Bob+1/2a
= AGO
OeXe
B
%?
we
AGo.5
OeXe
Be =
%
- w&(X)]
N2
++
24.5a
71.5Y
1941.6
1894.17
23.7c
1.8774
o.orP
2007.6
1965.6a
21,0c,
1.8934
0.030=
14
data
in cm
N2
++
-1
59.26
1.7545
0.026
1.7690
0.025
exp
(26)
(2)
(26)
(2)
(9)
___---
Uncertainties are given in parentheses in units of the last digits.
ALL
68.9
1875.6
22.1
1.7523
0.024
1939.5
19.6
15
reLations
1.7672
0.027
_______--___ talc from isotonic
Equilibrium Molecular Constants for the D’Z: and X’Z: States of the 14N:f and “NY Ions
TABLE III
VACUUM
165
UV SPECTRUM OF "N:
because we did not observe vibrational progressions in the spectra. For both the X and D states of the 14N:+ ion we used wan values calculated with the Pekeris formula (6). It seemed reasonable to do this, on the basis of the hypothesis of anharmonic oscillators which is implicit in the isotopic relations used here. It should be remarked that the w& ground state parameter determined from the experimental data of Cosby et al. (3) (w& = 24.5 cm-‘) is somewhat larger than the calculated one (0~~ = 21 cm-‘). One can attribute this small discrepancy to the beginning of electronic configuration change mentioned above. Conclusions which can be drawn from Table III are, on the one hand, the consistency of isotopic analysis results with the vibrational assignments that we have made for the two bands observed and, on the other hand, the validity of the anharmonic oscillator approximation (except for centrifugal distortion constants), at least for the u = 0 and 1 levels of the two D and X electronic states considered. This situation is expected to be very different for vibrational levels of the ground state higher than n = 2, where the configurational change becomes important. (b) Breaking-off points appear at J = 39 and 25, respectively, in the (0, 0) and ( 1, 1) bands of the D-X transition for the “N:+ isotope ion. Corresponding J values were J = 37 and 23 in the natural isotope (1). At first sight, as these two pairs of breaking-off J values (37 and 23 for 14N:+ and 39 and 25 for “Nz+) are not very different, one can deduce that the vibrational quantum number vo, corresponding to the level of the 3E; state perturbing the ground state 2, = 0 level, is close to zero. It should be noted that these breaking-off J values are difficult to determine within an uncertainty of two units, taking into account the different intensity alternations in the two isotopic spectra. However, one can roughly estimate the breaking-off J’-value in the “N$+ isotope spectrum corresponding to coincidence of the deperturbed rotational levels. Neglecting the isotope effects on the anharmonicity and on the centrifugal distortion, one obtains a relation between the breaking-off J’-value and the corresponding J in the case of the 14N$+spectrum:
Ji(Ji + 1) = ~J(J + 1) + ~(1 - P)(~0 P2
+
P2
1/2)wd3z-)
B(Y)
-
-
0*5wA’Z+)
B(‘Z+)
for the pair ‘Zl(v = 0) - 3Z;(vo), and, similarly, J’(J’+
I)=-
J(J + 1) + (1 - p) (u. + 3/2)wA3zl-) - 1.5wX’2+) -
P2
P2
B(%)
-
B(‘z+)
for the pair ‘Z:(2) = 1) - 3Z;(vo + I), where p = [/114/~15]“2. These expressions lead to the estimated breaking-off J’-values in the spectrum of the “N:+ (0, 0) and (1, 1) bands for different quantum numbers, tie, in the 3Z; state. They are given in Table IV. Although these estimations involve many approximations (more complicated calculations including energy variation terms involving p2wde, p3+, p4D, lead almost to the same results), one can be confident that the most convenient vibrational assignment corresponds to u. = 0, the ground state of the Nz+ ion remaining in the ‘2: state, as suggested in Ref. (1). However, the difference between the equilibrium energies of the two X’Z: and 32; states is shown to be less than 1000 cm-’ (I).
166
COSSART AND LAUNAY TABLE IV
Estimated Breaking-Off J’ Values in the Spectrum of the “N:+ (0,O) and (1, 1) Bands for Different Vibrational Numbering Schemes of the ‘Zg State
.(xy)
vo=o
J,
vo=
1
v.
=2
0
37
Ji
39
37
36
1
23
Ji
25
23
21
V. CONCLUSION
Results of isotopic analysis on the “N:+ ion confirm the analyses previously carried out for the natural species. Vibrational numbering for the two rotationally analyzed levels in the X and D states are established unambiguously, whereas more precise indication is obtained for the vibrational numbering scheme of the 32, state which perturbs the ground state. ACKNOWLEDGMENTS We thank Dr. S. Leach for suggesting valuable modifications of the manuscript, and Mr. M. Benharrous for his technical assistance during the experiments. RECEIVED:
April
4, 1985 REFERENCES
1. 2. 3. 4. 5.
D. COSSART,F. LAUNAY, J. M. ROBBE,AND G. GANDARA, J. Mol. Spectrosc. 113, 142-158. P. K. CARROLL,Cunad. J. Phys. 36, 1585-1587 (1958). P. C. COSBY,R. MOLLER,AND H. HELM, Phys. Rev. A 28,766-772 (1983). D. COSSART,J. Chim. Phys. 76, 1045-1050 (1979). A. LE FLOCH,F. LAUNAY,J. ROSTAS,R. W. FIELD, K. YOSHINO,C. BROWN,AND E. CHIPMAN,to be
published. 6. G. HERZBERG,“Molecular Spectra and Molecular Structure,” Vol. 1, p. 46 1, Van Nostrand, New York, 1950. 7. A. LOFTHUSAND P. H. KRUPENIE,J. Phys. Chem. Ref Data 6, 113-307 (1977). 8. L. L. DUNHAM,Phys. Rev. 34,438-452 (1929).
OF MOLECULAR
SPECTROSCOPY
113, l59- 166 ( 1985)
The Vacuum UV Emission Spectrum of the 15Ny Molecular Ion DANIEL COSSART Laboratoire de PhotophysiqueMolhculairedu CNRS, Bit. 213, Universithde Paris&d, 91405 Orsay Cedex? France AND
FRANCOISELAUNAY Observatoirede Paris, Dkpartementd’AstrophysiqueFondamentale. U.A. 812 du CNRS, 92195 Meudon PrincipalCedex, France The vacuum uv emission of the “N:+ ion has been recorded for the first time. Rotational analysis of two bands, analogous to those already observed in the case=of the natural isotope, confirm their assignment to the D’Z&!?Z~ (0, 0) and (1, 1) bands. Moreprecisedata are also
obtainedfor the ‘2; statewhichperturbsgroundstatevibrationallevels. o 1985 Academic 1.
press, IX.
INTRODUCTION
In a previous paper (Z), we presented the rotational analysis of two emission bands obtained in the vacuum uv region from an electric discharge through natural nitrogen. The origins of these bands were determined to be at 1589.7 and 159 1.5 A. The 1589.7A band was first observed by Carroll (2), who tentatively assigned it to a D’Z:X’Z: (0, 0) transition of the doubly charged molecular nitrogen ion. The observation of the ( 1, 1) band of the same system, associated with experimental data from photofragment spectroscopy concerning the ground state of Np, by Cosby et al. (3), and ab-initio calculation results (I), removed any doubt which might exist about the assignment of these bands to the N$+ ion. Moreover, the appearance of breaking-off points in the (0,O) and ( 1, 1) bands was interpreted as resulting from perturbations of vibrational levels of the ground state by those of a neighboring 32; state. This state has a larger equilibrium internuclear distance than that of the ground state and its vibronic levels are probably efficiently predissociated through the Coulomb potential barrier. Rotational lines of the D-X transition are thus expected to be weakened and broadened in the region of maximum perturbation. We previously showed (I), on the basis of calculated 32; state constants, that the X’Z,‘(v = 0 and 1) levels are perturbed respectively by two successive (uOand u. + 1) levels of a 32; state, but it was not possible to determine the vibrational numbering in the latter, i.e., the value of uo. Analysis of corresponding emission bands of the isotopic “N$+ species was thought to be of help to solve this problem. Another problem that we hoped to solve with data obtainable from the “N:+ spectrum was the confirmation of the assignment to the D-X (2, 3) transition of a very 159
0022-2852185 $3.00 Copyright 0 1985 by Academic Press, Inc. All rights of reproduction in any form reserved.
COSSART
160
AND LAUNAY
faint and diffuse feature near 1635 A. If this assignment is correct, one would obtain a vibrational spacing in the ground state AG2,5N 1600 cm-‘, much smaller than AG1,S = 19 16 cm-’ (3), confirming the electronic configuration change which is calculated to occur in the ground state near its equilibrium internuclear distance. It should be noted that this effect is detectable from the rotational constants obtained by Cosby et al. (3) for the first three vibrational levels of the ground state. Unfortunately, in the case of the 15N2isotope, the limited exposure time did not allow us to obtain the band analogous to that observed at 1635 A in the case of the 14N2. II. EXPERIMENTAL
DETAILS
The experimental conditions were identical to those given in Ref. (I) for natural 14N2nitrogen. Our emission source [negative glow of a dc discharge to which a 3-kG transverse magnetic field (4) is applied] allowed us to obtain the spectrum given in Fig. 1. This spectrum was recorded with the Meudon Observatory 10-m vacuum uv spectrograph. About 41 hr of exposure time was necessary during which 1 liter (at atmospheric pressure) of isotopic nitrogen gas was spent. Measurements and absolute calibration of the rotational lines [using the fourth positive emission system of CO (5) as a reference spectrum] were made in a way similar to that given in Ref. (I). III. DESCRIPTION
AND ANALYSIS
OF THE
SPECTRUM
Figure 1 shows rotational line assignments of the two observed bands analogous to those of the 14N$’ ion which have been attributed to the D’I;:-X*Z,+ (0,O) and (1, 1) transitions. The appearance and position of these bands are very similar to those of the natural nitrogen spectrum. However, two main differences with respect to the spectrum given in Ref. (1) are obvious. (i) The intensity alternation is opposite to that of the 14Np spectrum, i.e., the odd lines are observed to be strong, the even lines weak; the latter being missing for the highest J values [J = 34, 36, 38 in the (0, 0) band]. This is perfectly consistent with the nuclear spins I = 1 and l/2 for the 14N and 15N isotopes, respectively (6). The intensity ratio between strong and weak lines is calculated to be 3, instead of 2 in the case of the natural isotope, thus enhancing the apparent intensity alternation. (ii) In the “N$+ spectrum, the (2,4) band of neutral nitrogen Lyman-Birge-Hopfield system no longer overlaps the R branch of the D’Zz-X’Z,+ (0,O) band, as it was the case in the 14N$+spectrum. Sudden intensity weakening is thus clearly visible here at J = 31, whereas lines disappear at J = 39. Broadening of the weak R lines in the J = 30-39 interval is detectable on the plate. However, in the right-hand side of Fig. 1, the P-branch lines with high J values are contaminated by emissions of the b” 2:-X1X,’ (1,2 1) band of neutral 15N2.This band is not presently analyzed. Its head (X = 1591.5 A) was calculated with the classical isotopic relations, from the emission data of Tschulanowsky (X-head 14N2= 1623.85
VACUUM
UV SPECTRUM
OF 15N:
.
-
1 .
e 4 . .--
._
iv) . . .
;” .-
,*o f -. . lSC? . *
:0 . . ..
:e I f
ia .:fg LL
c
M. N.
*I
m.
161
162
COSSART AND LAUNAY
A) quoted by Lofthus and Krupenie (7). A number of irregularities in the line positions and intensities mentioned in Ref. (7) are also visible here. Nevertheless, it is clear that breaking-off points occur in both R and P branches of the Nz+ spectrum, at J = 39 and 25 in the (0, 0) and (1, 1) bands, respectively. The measured wavenumbers (cm-‘) are given in Table I for the two bands analyzed. Fitting them to the quadratic expansion, T,,J = T, + B,J(J + I) - D,[J(J + 1)12. for representing the term values of the two ‘Z+ states involved in the transition, gave the vibronic molecular parameters listed in Table II. For comparison, we give also in the same table the corresponding values obtained for 14N:+ (1). TABLE I Measured Wavenumbers (cm-‘) of the D'Z:- X'Z;(O-O) and (l-l) Bands of 15N: (1-l)
(0-O) N
R
0
62907. 74. 62911. 17. 62914.62 62918.01 62921.36 62924. 70 62926.02 6293 1.26 62934.52 62937. 73 62940.91 62944.04 62947. 15 62950.22 62953.29 62956.26 62959.27 62962.25 62965. 16 62968.06 62970.95 62973.74 62976.55 62979.26 6298 1.98 62964. 74 62967.29 62989.97 62992.55 62995. 11 62997.58 63000. 10
8 9 IO 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
= blended
63004.90 63009.
6 I
P
R
P
62900. 72 62897.25’ 62893.61 62690.01 62886.36’ 62862. 73. 62879.06 62675.40 62871.67 62867.6462064.02
62841.84 62845. 10. 62840.56= 62851.76 62855.06 62858.34 62861.57
6283 1.63 62828. 59. 62824.53. 62620.44. 62817.37 62813. 77 62810. 12 62606.42 62802.70 62799.06 62795.27. 62791.01’ 62787.51 62783.70 62779. 76 62775.66 62771.90 62767.95 62763.86 62759.60 62755.74
62860. 62856.31 62852.47 62846.56 62644.63 62640.66 62836.66 62832.67 62628. 62624. 62620.44’ 62616.33 62612.09 62807.99 62603.04 62799. 62795.27’ 62791.01’ 62766. 62782.
73 35
62773.
73
62764.
63
63014.27
62756.36
63019.21
62747.35
line
19
59* 53.
57
62867.84. 6267 1.32 62874.06 62677.02 62880. 10 62882. 73’ 62885.90 62888.91 62891. 77 62894.50 62897.25. 62899.77 62902.86 62905.54 62908. 16 62911. 17” 62913. 43
62747.45 62743.29 62739. 62734.78
14
VACUUM
UV SPECTRUM
163
OF “N:+
TABLE II Rovibronic Molecular Parameters for “*‘sN:+ X’Zi-D’Z:(v 14
15
++
B
0
62903.30
(‘2+) 9
(2)
1.8784 5.7
lo600q B
(‘I+) 0
++ N2
N2
v(O,O)
= 0 and 1)
2904.30
1.7565
(6)
7.3
(4)
1.8624
(2) (6)
(3)
1.7414
(6)
(6)
”
106Do(‘X~) 5.6 (4) _-__------_ ____--__--__-__ v(l,l)
62831.77
8 (lx+) 1
106D B
(7)
1.8534
7.1
(3)
--_-____--___--_ 62835.04
(30)
(7)
1.7316
(20)
9
1
(‘I+)
7.4
(30)
5.2
(20)
9
(‘I+) 1
1.8356
(30)
1.7152
(20)
”
106Dl('Q
All
data
in cm
6.6
(30)
4.6
(20)
-1
Uncertainties are given of the last digits.
in parentheses
in units
IV. DISCUSSION
The isotopic analysis is based on vibrational and rotational energy expressions expanded in terms as defined by Dunham (8). Limitation to the second-order terms of the developments considered is valid only for potential energy expressions near Morse functions. The results of this isotopic analysis lead to the following two conclusions: (a) The assignment of the observed bands, as well as the vibrational numbering in the D'Z; and X’Z,f states, are fully confirmed by the comparisons given in Table III. The equilibrium molecular constants of the “N:+ ion given in the last column are compared to the corresponding ones obtained from those of the natural isotope, using the isotopic formulae available (6) for anharmonic oscillators, and assuming that the observed bands are the (0, 0) and ( 1, 1) bands of the electronic transition considered. The agreement is satisfactory between calculated and observed rotational constants of the “N:+ ion, if we take into account the unusual shape of the potential energy curve due to configuration change as discussed in Ref. (1). In particular, the disagreement between fitted and calculated centrifugal distorsion constants for “N:+ (Table II) may be due to this effect. It should be noted that similar “anomalous” D, values have been found by Cosby et al. (3) for the ground state of the 14N$+ion. D, value behavior differences in 14N$+and “N$’ is further indication that Morse potential hypothesis is correct only to first order. The isotopic relations are also verified for the equilibrium vibrational constants, w,. This can be seen only by comparing the (0, 0) and (1, 1) band origin differences,
e
From
From
From
a -
b -
c -
v(O,O)
+2wx
et al
Pekeris
formula
y+
(1)
+ 2[wexe(D)
- "(0,OP
on
(6)
work
(3)
= w,(X)
b
- w,(D)
+ v(l,l)
ee
e
b
our previous
Cosby
- v(l,l)
*I e = AG0.5
= AGoa5(X)
=e*+1/2a 0
AGO.5
e
=
5 + 2 wdxeC
Bob+1/2a
= AGO
OeXe
B
%?
we
AGo.5
OeXe
Be =
%
- w&(X)]
N2
++
24.5a
71.5Y
1941.6
1894.17
23.7c
1.8774
o.orP
2007.6
1965.6a
21,0c,
1.8934
0.030=
14
data
in cm
N2
++
-1
59.26
1.7545
0.026
1.7690
0.025
exp
(26)
(2)
(26)
(2)
(9)
___---
Uncertainties are given in parentheses in units of the last digits.
ALL
68.9
1875.6
22.1
1.7523
0.024
1939.5
19.6
15
reLations
1.7672
0.027
_______--___ talc from isotonic
Equilibrium Molecular Constants for the D’Z: and X’Z: States of the 14N:f and “NY Ions
TABLE III
VACUUM
165
UV SPECTRUM OF "N:
because we did not observe vibrational progressions in the spectra. For both the X and D states of the 14N:+ ion we used wan values calculated with the Pekeris formula (6). It seemed reasonable to do this, on the basis of the hypothesis of anharmonic oscillators which is implicit in the isotopic relations used here. It should be remarked that the w& ground state parameter determined from the experimental data of Cosby et al. (3) (w& = 24.5 cm-‘) is somewhat larger than the calculated one (0~~ = 21 cm-‘). One can attribute this small discrepancy to the beginning of electronic configuration change mentioned above. Conclusions which can be drawn from Table III are, on the one hand, the consistency of isotopic analysis results with the vibrational assignments that we have made for the two bands observed and, on the other hand, the validity of the anharmonic oscillator approximation (except for centrifugal distortion constants), at least for the u = 0 and 1 levels of the two D and X electronic states considered. This situation is expected to be very different for vibrational levels of the ground state higher than n = 2, where the configurational change becomes important. (b) Breaking-off points appear at J = 39 and 25, respectively, in the (0, 0) and ( 1, 1) bands of the D-X transition for the “N:+ isotope ion. Corresponding J values were J = 37 and 23 in the natural isotope (1). At first sight, as these two pairs of breaking-off J values (37 and 23 for 14N:+ and 39 and 25 for “Nz+) are not very different, one can deduce that the vibrational quantum number vo, corresponding to the level of the 3E; state perturbing the ground state 2, = 0 level, is close to zero. It should be noted that these breaking-off J values are difficult to determine within an uncertainty of two units, taking into account the different intensity alternations in the two isotopic spectra. However, one can roughly estimate the breaking-off J’-value in the “N$+ isotope spectrum corresponding to coincidence of the deperturbed rotational levels. Neglecting the isotope effects on the anharmonicity and on the centrifugal distortion, one obtains a relation between the breaking-off J’-value and the corresponding J in the case of the 14N$+spectrum:
Ji(Ji + 1) = ~J(J + 1) + ~(1 - P)(~0 P2
+
P2
1/2)wd3z-)
B(Y)
-
-
0*5wA’Z+)
B(‘Z+)
for the pair ‘Zl(v = 0) - 3Z;(vo), and, similarly, J’(J’+
I)=-
J(J + 1) + (1 - p) (u. + 3/2)wA3zl-) - 1.5wX’2+) -
P2
P2
B(%)
-
B(‘z+)
for the pair ‘Z:(2) = 1) - 3Z;(vo + I), where p = [/114/~15]“2. These expressions lead to the estimated breaking-off J’-values in the spectrum of the “N:+ (0, 0) and (1, 1) bands for different quantum numbers, tie, in the 3Z; state. They are given in Table IV. Although these estimations involve many approximations (more complicated calculations including energy variation terms involving p2wde, p3+, p4D, lead almost to the same results), one can be confident that the most convenient vibrational assignment corresponds to u. = 0, the ground state of the Nz+ ion remaining in the ‘2: state, as suggested in Ref. (1). However, the difference between the equilibrium energies of the two X’Z: and 32; states is shown to be less than 1000 cm-’ (I).
166
COSSART AND LAUNAY TABLE IV
Estimated Breaking-Off J’ Values in the Spectrum of the “N:+ (0,O) and (1, 1) Bands for Different Vibrational Numbering Schemes of the ‘Zg State
.(xy)
vo=o
J,
vo=
1
v.
=2
0
37
Ji
39
37
36
1
23
Ji
25
23
21
V. CONCLUSION
Results of isotopic analysis on the “N:+ ion confirm the analyses previously carried out for the natural species. Vibrational numbering for the two rotationally analyzed levels in the X and D states are established unambiguously, whereas more precise indication is obtained for the vibrational numbering scheme of the 32, state which perturbs the ground state. ACKNOWLEDGMENTS We thank Dr. S. Leach for suggesting valuable modifications of the manuscript, and Mr. M. Benharrous for his technical assistance during the experiments. RECEIVED:
April
4, 1985 REFERENCES
1. 2. 3. 4. 5.
D. COSSART,F. LAUNAY, J. M. ROBBE,AND G. GANDARA, J. Mol. Spectrosc. 113, 142-158. P. K. CARROLL,Cunad. J. Phys. 36, 1585-1587 (1958). P. C. COSBY,R. MOLLER,AND H. HELM, Phys. Rev. A 28,766-772 (1983). D. COSSART,J. Chim. Phys. 76, 1045-1050 (1979). A. LE FLOCH,F. LAUNAY,J. ROSTAS,R. W. FIELD, K. YOSHINO,C. BROWN,AND E. CHIPMAN,to be
published. 6. G. HERZBERG,“Molecular Spectra and Molecular Structure,” Vol. 1, p. 46 1, Van Nostrand, New York, 1950. 7. A. LOFTHUSAND P. H. KRUPENIE,J. Phys. Chem. Ref Data 6, 113-307 (1977). 8. L. L. DUNHAM,Phys. Rev. 34,438-452 (1929).