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High-resolution infrared absorption spectra of the ν3 and ν4 hybrid bands of hydrazoic acid (HN3)
JOURNAL
OF MOLECULAR
SPECTROSCOPY
152, 101-108
( 1992)
High-Resolution Infrared Absorption Spectra of the v3 and v4 Hybrid Bands of Hydrazoic Acid ( HN3) JIZIRGEN BENDTSEN Department of Chemistry, University of Aarhus. DK-8000 Aarhus C, Denmark
AND FLEMMING
M. NICOLAISEN
Department of Chemistry, The H. C. Orsted Institute, University of Copenhagen. DK-2100 Copenhagen, Denmark
The infrared absorption spectrum of HN, has been measured in the region 900 to 1900 cm-’ with a resolution of 0.005 cm-‘. In this region absorption is mainly due to the two bands uj and v,, centered at 1267 and 1147 cm-‘, respectively. Rotational-vibrational transitions have been assigned to AK, = 0 as well as to the hitherto unobserved AK, = f 1 transitions. The observations have been extended to K. = 9 for the excited states as compared to K, = 5 from previous measurements. The individual subbands were analyzed yielding values for the subband origins and effective rotational constants of the excited states. A crossing is observed between the high K, levels of the Yestate and levels of an unassigned vibrational state. o 1992 Academic press, inc. INTRODUCTION
Measurements of the infrared absorption spectra of the u3 and u4 bands of HN3 have been reported earlier ( 1, 2). In these studies only the parallel components of the hybrid bands were detected and analyzed. The complexity of the reported spectra demands that the highest possible resolution and detectivity over a large wavenumberrange is needed in order to be able to resolve and analyze the spectrum in detail. Although the resolution in these investigations was fairly high (0.03 cm-’ for the v4 band and Doppler limited for the tunable diode laser spectrum of the v3 band), the spectrum of v4 (2) shows additional unassigned weak and unresolved lines. The highresolution tunable diode laser spectrum of u3( 1) covers only a very limited wavenumber range. Therefore the wavenumber region 900-1900 cm-’ was recorded with 0.005 cm-’ resolution using a long path absorption cell. The absorption within the region examined is due mainly to transitions to the v3 and v4 levels, but transitions to the overtones and the combination bands involving the v5 and V6fundamentals are also situated in this region and can be detected. This work reports the observation and analysis of the strong AK, = 0 components and the hitherto unobserved weak AK, = fl components of the v3 and u4 fundamentals. EXPERIMENTAL
DETAILS
The sample of HN3 was prepared as described previously (3). The spectra were recorded on a Bruker 120 HR instrument. The measurements of the strongly absorbing AK, = 0 transitions were performed using a IO-cm cell filled to 1 Torr pressure and a 20-cm cell with 2 Torr pressure for v4 and v3, respectively. In order to obtain the 101
0022-2852192 $3.00 Copyright 0 1992 by Academic Press. Inc. All rights of repraluaion
in any form resewed.
102
BENDTSEN AND NICOLAISEN
%A branch
branch
%
'416,
&7Iv, 3 ‘R,(j)
‘R,(L)
I
c~p,(lOl
oR,-(161vx
1151.25 1151.50 II&5
115;.00 II&
cm-1
FIG. 1.Part of the region of the QQaland QQazbranches of the v4band showing the asymmetry splitting of the “Q, branch. Two QR, lines originating from an unassigned vibration V, are indicated together with QP and QR lines from the other ~4 subbands. The numbers on the uppermost axis are J-values for the initial states of the individual QQ transitions.
weakly absorbing AK, = + 1 transitions, two spectra were measured with 4 Torr pressure: one with 5 m path length and the other with 20 m path length. For all. spectra the observed resolution is better than or equal to 0.005 cm-‘. The wavenumbers of the observed spectra were measured by means of the peakfmder program given in Ref. (4). The wavenumber calibration was performed by means of
I
150;. 50
15oi.50
1504.00
cm-
FIG. 2. Trace of the RQs branch of the vg band. The numbers associated to the lines are J-values for the initial states of the individual transitions.
THE
1265.50
1266.60
1265.60
1266.90
V, AND
“4 BANDS
1265.70
1269.00
1265.80
1269.10
103
OF HNS
1269.20
1265.90
1269.30
cm-l
cm-l
FIG. 3. This figure consists of two traces in absorbance showing the opposite degradation of the QQ2 and oQ3-branches of U, due to the interaction with the levels with K, = 3 and 4 in the V~state.
the lines due to absorption in the water vapor and the CO2 which were inevitably present in the sample. Selected water vapor and COz lines were assigned to wavenumbers given in Ref. (5), and a straight calibration curve through the wavenumber origin and these selected lines was constructed. The precision of the determination of the wavenumbers of the calibration lines in the different spectra was found to be +-0.0005 cm-’ for the CO* lines and +0.0003 cm-’ for the water vapor lines. The accuracy of the HN3 lines is estimated to be within 0.001 cm-’ in accordance with the absolute accuracy of the calibration lines given in Ref. (5). The results of the peakfinding and the calibration including lines with signal to noise ratio greater than 2 are wavenumbers of 17 865 lines. Of these lines 3086 were assigned to the u3 and v4 bands. A table of the wavenumbers used in the fit together with their residuals has been deposited with the Editor of this journal. Copies may be obtained from the author. Part of the observed spectra is shown in Figs. 1 to 4. Although
104
BENDTSEN AND NICOLAISEN
20 R
(8) v3
RQ,(J) v,
I
I
I
I
I cm-l
1496.00
I pP,(6Ny
15 PP,(7rVy
PP31
)O : 1458.00
/ pP,(51vy
RQ,(J) v, H2( I
I
1459.00
I
I 1460.00
I
I 1461.00
cm-l
FIG. 4. The two traces in this figure show the opposite degradations of the RQ7and aQ8 branches of Y., due to the crossing with an unknown vibrational state.
the wavenumber range of these selected parts is very limited, it should give an idea of the huge amount of information in the spectra. ASSIGNMENTS AND ANALYSES
The assignments of the a-type transitions terminating at the levels with low K, values in the two states were taken from Refs. ( I ) and (2). It was then straightforward to assign the higher K,, AK, = 0 transitions. The b-type transitions were identified using the values for the energy levels of the u3 and u4 states (I, 2) in combination with the values for the energy levels of the ground state (6). The analysis of the spectral data was performed as described in Ref. (2). Only subband analyses were performed due to the fact that the v3 and v4 levels are perturbed. To each assigned wavenumber, the value for the ground state level (6) was
105
THE Y, AND “4 BANDS OF HNs TABLE I
Subband Origins and Effective Rotational Parameters in cm -’ of the v4 State Obtained by Analyzing the AK, = 0 and f 1 Transitions (The Errors Quoted Are Standard Errors.) m3
K, A% “0
B+C 2
0
1
0
0
“4
2 0,-l
0,-l
1147.4042(3)
1151.5476(6)
0.395539(l)
0.396169(3)
0.3954940)
0.394156(2)
0.333X2)
0.4253@)
-1.176(7)
-0.67(3)
1.64(l)
-0.83(4)
1.73(Z)
5.6(9)
9.8(3)
-63(3)
&O(6)
10’ DJ
1.98(Z)
10n HJ
6.9(4)
1OSLJ
1155.652x3)
1160.2191(3) 0.396125(Z)
-3.415)
10s SJ
%
O.-l
4
1152.3321c2)
1029
lo4 St.d.
3
28.36(6)
3.4(Z)
14
33
16
14
18
5
6
7
8
9
Oil
O,+l
+1
+l
+1
1164.756w5)
1166.9621(5)
1172.3778(3)
1174.7392(3)
1175.9110(2)
0.3956%x3)
0.395167(3)
0.3952OlW
0.391695(5)
0.396640(Z)
lo7 DJ
1.84(3)
1.89(Z)
1.79(l)
-11.3(l)
1012 H J
8.0(E)
8.3(7)
9.7(4)
--159(10)
31
23
12
AK, “0
B+C 2 10zy
1015 LJ lo* St.d.
2.41(Z)
au31 9
6
added. In this way values for the upper state levels were determined. Some of these levels were determined two or even three times through the observation of different subband transitions terminating at the same upper level. This has been indicated in Tables I and II where the number of values for AK, for a given value of K, gives the number of subbands contributing to the determination of the levels for that specific value of K,. The analyses were then carried out on the upper state values only. The results are given in Tables I and II. In Table III are listed the values for the pure K,-dependent part of the rotational energy levels of the two fundamentals. See also Fig. 5. DISCUSSION
When the results obtained in this work are compared with the values given in Refs. ( 1) and (2) for v3 and v4, it is seen that additional information has been obtained about the levels with K, = 6, 7, and 8 for the v3 state and K, = 7, 8, and 9 for the v4 state. These high K, levels (>2000 cm-‘) have been reached through the observation
106
BENDTSEN AND NICOLAISEN TABLE II
Subband Origins and Effective Rotational Parameters in cm -’ of the vg State Obtained by Analyzing the AK, = 0 and + 1 Transitions (The Errors Quoted Are Standard Errors. )
“0
B+C 2
1266632X3)
1256.3367(2)
1265.5950(3)
1269.3200(Z)
1274.3574(3)
0.3cl6963u)
0.3973370)
0.399274(2)
0.396227(l)
0.396199(3)
0.003O62m
0.00411(1)
B_C 2 lOI DJ
1.760)
2.28(l)
5.41(4)
1.93(l)
-1.09w
1013 HJ
76(4)
9x3)
974(27)
7x3)
-1346eo)
10’4 L I
1.15@1
10s 6, lo4 St.d.
-2.83(l) 11
12
12
5
6
7
O,+l
O.+l
O&l
-2.60)
3.3(3) a
1281.1770(4)
12.39.9958(41 1300.6170(4)
0.3967.3x3)
0.396782(2)
0.39665X23
lo7 D,,
1.91(3)
1.93(3)
l.%!(Z)
10’3 H J
71(10)
lX?@)
@X6)
29
22
17
% B+C 2
17
B_c 2
lo4 St.d.
TABLE III Values in cm-’ for the Pure K,-Dependent Part of the Rotational Energy in the v4and uj States
0
1147.40
1266.63
1
1171.53
1278.32
2
1232.13
1345.40
3
1334.SS
1446.46
4
1477.73
1591.66
5
1658.97
1775.39
6
1677.39
1996.38
7
2131.58
2269.62
a
242026
2557.3’
9
2742.43
Note. Value estimated from the weak RQ7branch.
THE v3 AND uq BANDS OF HN,
107
cm-l
2800-9
-8
2500 -8 -7
-7
-6
2000 -6
-5 -5 -L
1500- -L-_
--=___3
_33-__--.-p -1 -0
-2
h
VI -0 VL
Ka
K,
IOOOFIG. 5. Diagram showing the pure &-dependent
part of the rotational energy of the v3 and the v4states.
of the hitherto unobserved AK, = f 1 transitions. Despite the much higher resolution and the much broader wavenumber region covered in this work as compared to previous works, the accuracy of the molecular parameters determined is only slightly improved compared to the values given in Refs. ( 1) and (2). This may be explained by the fact that the chosen procedure implies the addition of ground state values to the observed wavenumbers in order to obtain values for the upper state levels. The values for the ground state levels given in Ref. (6) have been determined with an accuracy of kO.003 cm-‘. Thus the lower accuracy in the measurements of the ground state values determines the overall accuracy in this investigation. Using the observed wavenumbers for a ground state combination difference analysis, the ground state values may be determined with higher accuracy. We have decided, however, to measure the pure rotational spectrum in order to improve the accuracy of the ground state values. This work will be performed in the near future. Through combination of AK, = - 1, 0, and 1 transitions terminating at the same upper level, this level is threefold determined. A multiple determination normally
108
BENDTSEN AND NICOLAISEN AB.103 cm-l
0
L
9
I
I
1
I
I
I
16
25
36
L9
6.4
81
FIG. 6. Values of &-&
and &-&
Ki
for the individual subbands plotted against Ka.
improves the accuracy of the measured value. In this case the improvement may be cancelled to some extent by calibration errors, because the transitions used in the determination are observed in wavenumber regions far away from each other. From Tables I and II it is seen that there is a slight tendency for the standard deviation to be higher for the analyses involving two or three values for AK,. The crossing of levels between u4 and v3 is clearly evident from Fig. 2 where the influence on the vg subbands is shown. Another crossing is observed for the u4 state K, = 8 and 9 levels, which can be seen from Fig. 4. This indicates another vibration level which has two successive levels just above 2420 cm-’ and just below 2742 cm-‘. This crossing is also seen from Fig. 6 where the values for AB with K, = 8 and 9 deviate markedly. Throughout the region QQ-branches not belonging to the v3, u4 system are observed. Some of these bands have been assigned with respect to J and K,, but the vibrational assignment is still uncertain. In Figs. 1 and 4 some of these vibrationally unknown lines are shown indicated by vXand vY.Work is in progress to assign these bands, and the results will be reported in a future publication. RECEIVED:
October 18, 199 1 REFERENCES
1. K. YAMADA AND M. TAKAMI,J. MOI.spectr~s~. 84,431-446 ( 1980). 2. J. BENDTSEN AND F. M. NICOLAISEN, J. Mol. Spectrosc. 133, 193-200 ( 1989). 3. J. BENDTSEN AND F. M. NICOLAISEN, J. Mol. Spectrosc. 119,456-466 ( 1986). 4. P. JENSEN,The Giessen Program Package, Justus Liebig University Giessen (unpublished). 5. G. GUELACHVILI AND K. NARAHARIRAO, Handbook of InfraredStandards,pp. 366-49 1, Academic Press, Orlando, FL, 1986. 6. F. HEGELUNDANDJ. BENDTSEN, J. Mol.
Spectrosc.
124,
306-3 16 ( 1987).
OF MOLECULAR
SPECTROSCOPY
152, 101-108
( 1992)
High-Resolution Infrared Absorption Spectra of the v3 and v4 Hybrid Bands of Hydrazoic Acid ( HN3) JIZIRGEN BENDTSEN Department of Chemistry, University of Aarhus. DK-8000 Aarhus C, Denmark
AND FLEMMING
M. NICOLAISEN
Department of Chemistry, The H. C. Orsted Institute, University of Copenhagen. DK-2100 Copenhagen, Denmark
The infrared absorption spectrum of HN, has been measured in the region 900 to 1900 cm-’ with a resolution of 0.005 cm-‘. In this region absorption is mainly due to the two bands uj and v,, centered at 1267 and 1147 cm-‘, respectively. Rotational-vibrational transitions have been assigned to AK, = 0 as well as to the hitherto unobserved AK, = f 1 transitions. The observations have been extended to K. = 9 for the excited states as compared to K, = 5 from previous measurements. The individual subbands were analyzed yielding values for the subband origins and effective rotational constants of the excited states. A crossing is observed between the high K, levels of the Yestate and levels of an unassigned vibrational state. o 1992 Academic press, inc. INTRODUCTION
Measurements of the infrared absorption spectra of the u3 and u4 bands of HN3 have been reported earlier ( 1, 2). In these studies only the parallel components of the hybrid bands were detected and analyzed. The complexity of the reported spectra demands that the highest possible resolution and detectivity over a large wavenumberrange is needed in order to be able to resolve and analyze the spectrum in detail. Although the resolution in these investigations was fairly high (0.03 cm-’ for the v4 band and Doppler limited for the tunable diode laser spectrum of the v3 band), the spectrum of v4 (2) shows additional unassigned weak and unresolved lines. The highresolution tunable diode laser spectrum of u3( 1) covers only a very limited wavenumber range. Therefore the wavenumber region 900-1900 cm-’ was recorded with 0.005 cm-’ resolution using a long path absorption cell. The absorption within the region examined is due mainly to transitions to the v3 and v4 levels, but transitions to the overtones and the combination bands involving the v5 and V6fundamentals are also situated in this region and can be detected. This work reports the observation and analysis of the strong AK, = 0 components and the hitherto unobserved weak AK, = fl components of the v3 and u4 fundamentals. EXPERIMENTAL
DETAILS
The sample of HN3 was prepared as described previously (3). The spectra were recorded on a Bruker 120 HR instrument. The measurements of the strongly absorbing AK, = 0 transitions were performed using a IO-cm cell filled to 1 Torr pressure and a 20-cm cell with 2 Torr pressure for v4 and v3, respectively. In order to obtain the 101
0022-2852192 $3.00 Copyright 0 1992 by Academic Press. Inc. All rights of repraluaion
in any form resewed.
102
BENDTSEN AND NICOLAISEN
%A branch
branch
%
'416,
&7Iv, 3 ‘R,(j)
‘R,(L)
I
c~p,(lOl
oR,-(161vx
1151.25 1151.50 II&5
115;.00 II&
cm-1
FIG. 1.Part of the region of the QQaland QQazbranches of the v4band showing the asymmetry splitting of the “Q, branch. Two QR, lines originating from an unassigned vibration V, are indicated together with QP and QR lines from the other ~4 subbands. The numbers on the uppermost axis are J-values for the initial states of the individual QQ transitions.
weakly absorbing AK, = + 1 transitions, two spectra were measured with 4 Torr pressure: one with 5 m path length and the other with 20 m path length. For all. spectra the observed resolution is better than or equal to 0.005 cm-‘. The wavenumbers of the observed spectra were measured by means of the peakfmder program given in Ref. (4). The wavenumber calibration was performed by means of
I
150;. 50
15oi.50
1504.00
cm-
FIG. 2. Trace of the RQs branch of the vg band. The numbers associated to the lines are J-values for the initial states of the individual transitions.
THE
1265.50
1266.60
1265.60
1266.90
V, AND
“4 BANDS
1265.70
1269.00
1265.80
1269.10
103
OF HNS
1269.20
1265.90
1269.30
cm-l
cm-l
FIG. 3. This figure consists of two traces in absorbance showing the opposite degradation of the QQ2 and oQ3-branches of U, due to the interaction with the levels with K, = 3 and 4 in the V~state.
the lines due to absorption in the water vapor and the CO2 which were inevitably present in the sample. Selected water vapor and COz lines were assigned to wavenumbers given in Ref. (5), and a straight calibration curve through the wavenumber origin and these selected lines was constructed. The precision of the determination of the wavenumbers of the calibration lines in the different spectra was found to be +-0.0005 cm-’ for the CO* lines and +0.0003 cm-’ for the water vapor lines. The accuracy of the HN3 lines is estimated to be within 0.001 cm-’ in accordance with the absolute accuracy of the calibration lines given in Ref. (5). The results of the peakfinding and the calibration including lines with signal to noise ratio greater than 2 are wavenumbers of 17 865 lines. Of these lines 3086 were assigned to the u3 and v4 bands. A table of the wavenumbers used in the fit together with their residuals has been deposited with the Editor of this journal. Copies may be obtained from the author. Part of the observed spectra is shown in Figs. 1 to 4. Although
104
BENDTSEN AND NICOLAISEN
20 R
(8) v3
RQ,(J) v,
I
I
I
I
I cm-l
1496.00
I pP,(6Ny
15 PP,(7rVy
PP31
)O : 1458.00
/ pP,(51vy
RQ,(J) v, H2( I
I
1459.00
I
I 1460.00
I
I 1461.00
cm-l
FIG. 4. The two traces in this figure show the opposite degradations of the RQ7and aQ8 branches of Y., due to the crossing with an unknown vibrational state.
the wavenumber range of these selected parts is very limited, it should give an idea of the huge amount of information in the spectra. ASSIGNMENTS AND ANALYSES
The assignments of the a-type transitions terminating at the levels with low K, values in the two states were taken from Refs. ( I ) and (2). It was then straightforward to assign the higher K,, AK, = 0 transitions. The b-type transitions were identified using the values for the energy levels of the u3 and u4 states (I, 2) in combination with the values for the energy levels of the ground state (6). The analysis of the spectral data was performed as described in Ref. (2). Only subband analyses were performed due to the fact that the v3 and v4 levels are perturbed. To each assigned wavenumber, the value for the ground state level (6) was
105
THE Y, AND “4 BANDS OF HNs TABLE I
Subband Origins and Effective Rotational Parameters in cm -’ of the v4 State Obtained by Analyzing the AK, = 0 and f 1 Transitions (The Errors Quoted Are Standard Errors.) m3
K, A% “0
B+C 2
0
1
0
0
“4
2 0,-l
0,-l
1147.4042(3)
1151.5476(6)
0.395539(l)
0.396169(3)
0.3954940)
0.394156(2)
0.333X2)
0.4253@)
-1.176(7)
-0.67(3)
1.64(l)
-0.83(4)
1.73(Z)
5.6(9)
9.8(3)
-63(3)
&O(6)
10’ DJ
1.98(Z)
10n HJ
6.9(4)
1OSLJ
1155.652x3)
1160.2191(3) 0.396125(Z)
-3.415)
10s SJ
%
O.-l
4
1152.3321c2)
1029
lo4 St.d.
3
28.36(6)
3.4(Z)
14
33
16
14
18
5
6
7
8
9
Oil
O,+l
+1
+l
+1
1164.756w5)
1166.9621(5)
1172.3778(3)
1174.7392(3)
1175.9110(2)
0.3956%x3)
0.395167(3)
0.3952OlW
0.391695(5)
0.396640(Z)
lo7 DJ
1.84(3)
1.89(Z)
1.79(l)
-11.3(l)
1012 H J
8.0(E)
8.3(7)
9.7(4)
--159(10)
31
23
12
AK, “0
B+C 2 10zy
1015 LJ lo* St.d.
2.41(Z)
au31 9
6
added. In this way values for the upper state levels were determined. Some of these levels were determined two or even three times through the observation of different subband transitions terminating at the same upper level. This has been indicated in Tables I and II where the number of values for AK, for a given value of K, gives the number of subbands contributing to the determination of the levels for that specific value of K,. The analyses were then carried out on the upper state values only. The results are given in Tables I and II. In Table III are listed the values for the pure K,-dependent part of the rotational energy levels of the two fundamentals. See also Fig. 5. DISCUSSION
When the results obtained in this work are compared with the values given in Refs. ( 1) and (2) for v3 and v4, it is seen that additional information has been obtained about the levels with K, = 6, 7, and 8 for the v3 state and K, = 7, 8, and 9 for the v4 state. These high K, levels (>2000 cm-‘) have been reached through the observation
106
BENDTSEN AND NICOLAISEN TABLE II
Subband Origins and Effective Rotational Parameters in cm -’ of the vg State Obtained by Analyzing the AK, = 0 and + 1 Transitions (The Errors Quoted Are Standard Errors. )
“0
B+C 2
1266632X3)
1256.3367(2)
1265.5950(3)
1269.3200(Z)
1274.3574(3)
0.3cl6963u)
0.3973370)
0.399274(2)
0.396227(l)
0.396199(3)
0.003O62m
0.00411(1)
B_C 2 lOI DJ
1.760)
2.28(l)
5.41(4)
1.93(l)
-1.09w
1013 HJ
76(4)
9x3)
974(27)
7x3)
-1346eo)
10’4 L I
1.15@1
10s 6, lo4 St.d.
-2.83(l) 11
12
12
5
6
7
O,+l
O.+l
O&l
-2.60)
3.3(3) a
1281.1770(4)
12.39.9958(41 1300.6170(4)
0.3967.3x3)
0.396782(2)
0.39665X23
lo7 D,,
1.91(3)
1.93(3)
l.%!(Z)
10’3 H J
71(10)
lX?@)
@X6)
29
22
17
% B+C 2
17
B_c 2
lo4 St.d.
TABLE III Values in cm-’ for the Pure K,-Dependent Part of the Rotational Energy in the v4and uj States
0
1147.40
1266.63
1
1171.53
1278.32
2
1232.13
1345.40
3
1334.SS
1446.46
4
1477.73
1591.66
5
1658.97
1775.39
6
1677.39
1996.38
7
2131.58
2269.62
a
242026
2557.3’
9
2742.43
Note. Value estimated from the weak RQ7branch.
THE v3 AND uq BANDS OF HN,
107
cm-l
2800-9
-8
2500 -8 -7
-7
-6
2000 -6
-5 -5 -L
1500- -L-_
--=___3
_33-__--.-p -1 -0
-2
h
VI -0 VL
Ka
K,
IOOOFIG. 5. Diagram showing the pure &-dependent
part of the rotational energy of the v3 and the v4states.
of the hitherto unobserved AK, = f 1 transitions. Despite the much higher resolution and the much broader wavenumber region covered in this work as compared to previous works, the accuracy of the molecular parameters determined is only slightly improved compared to the values given in Refs. ( 1) and (2). This may be explained by the fact that the chosen procedure implies the addition of ground state values to the observed wavenumbers in order to obtain values for the upper state levels. The values for the ground state levels given in Ref. (6) have been determined with an accuracy of kO.003 cm-‘. Thus the lower accuracy in the measurements of the ground state values determines the overall accuracy in this investigation. Using the observed wavenumbers for a ground state combination difference analysis, the ground state values may be determined with higher accuracy. We have decided, however, to measure the pure rotational spectrum in order to improve the accuracy of the ground state values. This work will be performed in the near future. Through combination of AK, = - 1, 0, and 1 transitions terminating at the same upper level, this level is threefold determined. A multiple determination normally
108
BENDTSEN AND NICOLAISEN AB.103 cm-l
0
L
9
I
I
1
I
I
I
16
25
36
L9
6.4
81
FIG. 6. Values of &-&
and &-&
Ki
for the individual subbands plotted against Ka.
improves the accuracy of the measured value. In this case the improvement may be cancelled to some extent by calibration errors, because the transitions used in the determination are observed in wavenumber regions far away from each other. From Tables I and II it is seen that there is a slight tendency for the standard deviation to be higher for the analyses involving two or three values for AK,. The crossing of levels between u4 and v3 is clearly evident from Fig. 2 where the influence on the vg subbands is shown. Another crossing is observed for the u4 state K, = 8 and 9 levels, which can be seen from Fig. 4. This indicates another vibration level which has two successive levels just above 2420 cm-’ and just below 2742 cm-‘. This crossing is also seen from Fig. 6 where the values for AB with K, = 8 and 9 deviate markedly. Throughout the region QQ-branches not belonging to the v3, u4 system are observed. Some of these bands have been assigned with respect to J and K,, but the vibrational assignment is still uncertain. In Figs. 1 and 4 some of these vibrationally unknown lines are shown indicated by vXand vY.Work is in progress to assign these bands, and the results will be reported in a future publication. RECEIVED:
October 18, 199 1 REFERENCES
1. K. YAMADA AND M. TAKAMI,J. MOI.spectr~s~. 84,431-446 ( 1980). 2. J. BENDTSEN AND F. M. NICOLAISEN, J. Mol. Spectrosc. 133, 193-200 ( 1989). 3. J. BENDTSEN AND F. M. NICOLAISEN, J. Mol. Spectrosc. 119,456-466 ( 1986). 4. P. JENSEN,The Giessen Program Package, Justus Liebig University Giessen (unpublished). 5. G. GUELACHVILI AND K. NARAHARIRAO, Handbook of InfraredStandards,pp. 366-49 1, Academic Press, Orlando, FL, 1986. 6. F. HEGELUNDANDJ. BENDTSEN, J. Mol.
Spectrosc.
124,
306-3 16 ( 1987).