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Perturbation on intensity profiles of the CO 4pσ K 1Σ+3sσ B 1Σ+ (00) and 4pπ L 1Π3sσ B 1Σ+ (00) bands
Volume 212, number 1,2
CHEMICAL PHYSICS LETTERS
3 September 1993
Perturbation on intensity profiles of the CO 4~0 K ‘X+-3x1 B ‘Z+ (O-O) and 4p~ L ‘II-3x5 B ‘Z+(O-0) bands Shigeyuki Sekine ’ and Chiaki Hirose Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259, Nagatsuta, Mtdori, Yokohama 227, Japan
Received 25 June 1993; in final form 2 July 1993
The 4po K ‘E+-~so B %+(0-O) and 4pn L ‘Il-3so B ‘X+(O-O) bands of carbon monoxide were re-cxamined by the optogalvanic method. We found that the intensity ofthe rotational lines was proportional to the laser power up to about lo9 W cm2 and that the band profiles were affected by the Cuncoupling interaction among the rotational levels of the 4pa L ‘II( u=O), the 4po K’X+(v=O), and an unidentified ‘Z+ state. The 3pn C ‘C+(v=5 or 6) state which is higher than the 4pr L ‘TI(v=O) state by less than 200 cm-’ wasproposed as a likely candidate for the unidentified state.
1. Introduction The occurrence of an l-uncoupling interaction between the rotational levels of npo ‘C* and npn ‘l-I states in a singlet np complex is manifested in n-type doubling - the splitting of e( ‘II+) and f(‘II-) symmetry levels. The interaction also leads to an anomalous intensity distribution of the rotational lines of the npo ‘C+-n’so ‘C+ and npx ‘II-n’so ‘C+ transitions [l-4]. In the singlet 4p-complex( v=O ) levels of CO, the value of the n-type doubling constant q was obtained as 0.0213+0.0012 cm-’ for the 4prtL’II(v=O) state from the analysis of the optogalvanic (OG) spectrum of the 4px L ‘II-3~s B ‘C+ (O-O) band [ 5 1. The value of 0.07 cm-’ calculated by the pure precession model [ 6,7] is approximately three times larger than the experimentally derived value, and perturbation by nearby E states was suggested [ 51. Two other electronic bands, namely the ( 1~~3~0) W ‘II3so B ‘E+ (O-O) [ 81 and the L’ ‘II-3so B ‘C+ (u’ unspecified-u” =O) bands [ 91, have also been identified on the optogalvanic spectrum. Although the precise value of the n-type doubling constant for the (17?3so)W ‘lI(v=O) state was not derived in.our previous analysis, the value was predicted to be small ’ Present address: Electrotechnical Laboratory, l- 1-4, Umezono, Tsukuba, Ibaraki 305, Japan. Elsevier Science Publishers B.V.
from consideration of the dominant electron configuration of the ( 1x33so)W ‘II(u=O) state: the Rydberg electron is in the 2~0 orbital. On the other hand, the L’ ‘II( u unspecified) state was found to be in appreciable interaction with an unidentified *C+ state: we identified only the rotational lines belonging to the Q branch, but did not observe any lines in the R branch, even its head (see fig. 1 of ref. [ 91). The present paper focuses on the observation of the OG spectrum and the analysis of the perturbed intensity distribution of rotational lines of the 4~0 K ‘C+-3so B ‘X+(0-0) and the 4pz L ‘II-3~0 B ‘Z+ (O-O) bands. The analysis of the intensity protiles supports our previous suggestion that an unidentified ‘Z+ state is present and perturbs the rotational levels of the 4prc L ‘IT( u=O) state [ 51. The significant magnitude of the interaction in the 4pcomplex ( V= 0) suggests that the unidentified ‘C+ state is related to the electron configuration with an electron excited to a p Rydberg orbital, and the 3px C ‘C+ (v= 5 or 6 ) state is proposed as a candidate.
2. Experimental The OG spectrum of CO was observed using a DC hollow cathode discharge tube (see fig. 1 of ref. [ 5 ] 1. The discharge current was approximately 5 mA and a 1: 1 mixture of CO and oxygen flowed in the tube 129
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at approximately 100 Pa. A frequency-tunable dye laser operating with rhodamine 6G dye was pumped by the all-line output of a cw argon ion laser (5 W type ). The laser beam was collinearly focused on the inside of the cylindrical hollow cathode by a lens cf=250 mm). The maximum output power of the dye laser was 0.3 W at 16200 cm-’ with a linewidth of approximately 1 cm-’ and the power density was estimated to be IO9 W cm-2 at the focal point. The OG signal was lock-in amplified at the modulation frequency (1.3 kHz) of the laser beam. The output power of the dye laser was monitored by a photodiode and was recorded along with the OG signals to normalize the intensity of each rotational line.
3. Results and discussion To examine the dependence of the signal intensity on the laser power, the band-head intensity of the 4~0 K ‘Et-3so B ‘E+ (O-O) band was measured while the output power of the dye laser was varied by using neutral density filters. The linewidth of the dye laser was much wider than the spectral linewidth which is dominated by the Doppler width, and the intensity of the observed rotational lines as plotted in fig. 1 was derived from the peak height. It is seen that the signal intensity is linearly proportional to the laser power up to x lo9 W cm-‘.
0
100
50 LASER
POWER
‘/,
Fig. I. The dependence of the peak intensity of optogalvanic signal on laser power. The peak intensity of the R-branch bandhead of the CO 4~0 K IX+-3~0 B ‘X+ (O-O) band is plotted by open circle against the output power of the dye laser. The laser power at 100% is approximately lo9 W cm-‘. The laser beam was focused on the inside of the hollow cathode by a lens (f= 250 mm ).
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1993
The observed spectrum of the 4po K ‘X+-3~0 B ‘C+ (O-O) and 4pn: L ‘II-3so B ‘C+ (O-O) bands is shown in figs. 2a and 3a, respectively. As seen in fig. 3a the Q-branch lines of the 4pn L *l-I-3so B ‘Zf (O0) band, a higher-resolution spectrum of which has been already reported [51,were not resolved by the present resolution. The anomaly in the intensity distribution of the 4~0 K ‘C+-3~0 B ‘C+ (O-O) band was revealed in that different values, 410 + 30 K and 500 f 30 K, for the rotational temperatures were derived by a conventional analysis of the P and R branches, respectively. The difference arises from the f-uncoupling interaction, and an improved analysis was carried out by taking account of the interaction as described below. The formulas for the relative intensity of the rotational lines of perturbed npo ‘E’-n’so ‘E+ and npn: ‘Il-n’so ‘E+ bands have been given by Lefebvre-Brion and Field [2] (see eqs. (5.3.29a), (5.3.29b), and (5.3.40a) of ref. [2]). In a quantitative analysis of the intensity interference effect in an npo ‘Cc- n’so ‘E+ band, it is convenient to use the parameter e(J) defined by (see eq. (5.3.33) of ref. [ 2 I ) I(R(J-I))-I(P(J+l)) e(J)=I(R(J-l))tZ(P(Jtl))’
(1)
where Z(P(J+ 1)) and I(R(J-1)) are the intensities of the rotational lines of the P and R branches, respectively. The values of 0(J) for the observed 4po K ‘Z+-3so B ‘C+ (O-O) band are plotted by open circles in fig. 4. The results of theoretical calculations made with and without the inclusion of the effect of the interaction between the 4po K ‘Z’(u=O) and 4pa L ‘II (v=O) states are shown by curves (a) and (b), respectively. The assumed rotational temperature T,,was 450 K, which is the average of the values derived from the previously mentioned preliminary analysis. The experimental points are seen to lie between the two theoretical curves, but to be much closer to curve (a). The calculated spectral feature of the 4po K ‘C+-3so B ‘E+ (O-O) band, which is shown in fig. 2b, was seen to reproduce the observed feature (fig. 2a) well, and we thus concluded that the observed intensity anomaly of the rotational lines of the 4~0 K ‘E+-3~0 B ‘C+ (O-O) band is primarily caused by the l-uncoupling interaction in the singlet 4p-complex levels.
Volume 2 12, number I,2
CHEMICAL PHYSICS LETTERS
(b) simulated
3 September 1993
T,,,=450K
16100
16150
wavenumber / cm-l Fig. 2. (a) Observed and (b) calculated rotational profile of the CO 4pa K ‘I+-3s~ B ‘Z+ (O-O) band. In (b), the I-uncoupling interaction between the levels of only the 4pa K ‘Z+( ~0) and 4px L ‘lI(v=O) states was taken into account and a rotational temperature of 450 K was assumed. The intensity of each rotational line in (b) is weighted by the laser power at its peak wavelength.
250
q 200 =! S v9 150 x .5 c 100 Y c .50
(b) simulated
T,,,=450K
0 16300
16350
wavenumber / cm
16400 -1
Fig. 3. (a) Observed and (b) calculated rotational profile of the CO 4pn L *II-3sa B ‘Z+ (O-O) band. The Q branch was not resolved at the resolution of the present experiment. The same calculation as that which gave fig. 2b was made. The intensity of each rotational line in (b) is weighted by the laser power at its peak wavelength.
The I-uncoupling interaction is also expected to affeet the intensity distribution of rotational lines of the P and R branches of the 4prc L ‘II-3~0 B ‘C+ (O0) band. The observed intensity of the rotational lines
was anomalously weaker than expected and the presence of an additional perturbation in the singlet 4pcomplex states was suggested. We note that the calculated intensity of the P- and R-branch lines (fig. 131
Volume 2 12, number I ,2
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Table 1 Rotational constants for the CO singlet 4p-complex( u=O) and theC0’ X%+(v=O) state
Trot =450K
15
Levels [symmetry]
Rotational constants (cm-‘)
Ref.
4pnL’fI+(v=O)
[e] B[‘II+]
1.998*0.014 1.9812f0.0001
151 1101
4pnL’l--(v=O)
[f] B[‘II-]
1.95971kO.00024
J
-0.3 Fig. 4. Observed and calculated values of e(J) plotted against the rotational quantum number J. The experimental values derived from eq. (1) are plotted by open circles. Curve (a) is the result calculated by assuming the I-uncoupling interaction based on the pure precession model, and curve (b ) is the result calculated by assuming no perturbation. The rotational temperature was set to 450 Kin calculating (a) and (b).
3b, calculated with the same units as in fig. 2b) is much stronger than that of the 4~0 K ‘C+3sa B ‘C+ (O-O) band (fig. 2b) because of higher laser power in this region. The Q-branch lines of the 4pn: L ‘fI-3so B ‘I+ (O-O) band have higher intensity than the P- and R-branch lines as seen in fig. 3a, indicating that the upper 4px L ‘II- (v=O) levels f symmetry) are free from the additional perturbation. In that case, the rotational levels of the perturbing state are all of e symmetry and thus ‘Z+ symmetry is assigned to the state. The transition from the 3~0 B ‘C+ (u= 0) level to the perturbing level is expected to show up through the intensity borrowing from the 4prt L ‘II-3~0 B ix+ (O-O) band, but no such signal was present on the QG spectrum, possible due to an unspecified mechanism of the OG signal; the transition does not affect the characteristics of the discharge plasma to give the signal. This feature is quite similar to the case observed for the L”lI-3so B ‘Z+(r+O) band [9]. The effect of the additional perturbation also appeared in fig. 4 as a slight, but systematic departure of the observed values from the theoretical values (curve (a) ). The departure was not improved by simply varying the value of T,,. Table 1 lists the previously reported values of the rotational constants. The slight difference between the values of B(4px L’II-(u=O)) [ 51 and B(CO+ X *C+ (v= 0) ) [ 121 also indicates that there is no heterogeneous perturbation in the 4px L ‘II- (v= 0) 132
3 September 1993
CHEMICAL PHYSICS LETTERS
I51
4pcrK’E+(u=O) [e] B[‘Z+]
1.900f 0.002 1.9189+_0.0001
Ittl 1’01
CO+ X *Z+(Y=O)[e] B
1.967465
1121
rotational levels. The differences B(4prc L ‘II+ (v=O))-B(4prt L’n-(v=O))2:0.02 cm-’ and B(4plc L’I--(v=O))-B(4po K’E+(v=O)) ~00.04 cm-’ are equal to the A-type doubling constant in the lowest order of approximation, but both values are significantly smaller than the value of qcalc= 0.07 cm- ’ [ 51 estimated from the pure precession model. The discrepancy between the two observed values and that between the observed and calculated q values is ascribed to the f-uncoupling interaction of the 4pn L’II(v=O) state with the unidentified ‘C+ state which is located higher than the 4p7cL ‘Il( v= 0) state. For the singlet 4p-complex (v= 0) to undergo such a strong interaction, the ‘E+ state must be associated with the electron configuration in which the excited electron is in an np(n=3-5) Rydberg orbital. But, we expect that the magnitude of the matrix element between a rotational level of the 4p7r L ‘II( ~0) state and that of the unidentified ‘C+ states is smaller than that between the level of the 4prt L ‘l-I+ (v= 0) state and the level of the 4po K ‘Z’ (v= 0) state, and the unidentified ‘C+ level must then be located at less than 200 cm-’ above the L ‘II( v=O) state. The 3pn C ‘E+ (v=5 or 6) level seems to be a likely candidate, although there is no report which identified the state. In conclusion, we have shown that the intensity profile of the 4pcr K’C+-3s~ B ‘C+(O-0) band is primarily affected by the /-uncoupling perturbation of the rotational levels of the 4po K ‘C+ (v= 0) state by those of the 4~71L ‘fI( ~0) state. For the P- and R-branch lines of the 4pn. L ‘II-so B ‘C+ (O-O) band an additional interaction of the 4prr L ‘fI( v=O) state
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CHEMICAL PHYSICS LETTERS
with a ‘C+ state, possibly the 3px C ‘Z+( U= 5 or 6) state, was suggested.
References [ 1 ] 1. Kovacs, Rotational structure in the spectra of diatomic molecules (Adam Hilger, Bristol, 1969). [2] H. Lefebvre-Btion and R.W. Field, Perturbations in the spectra of diatomic molecules (Academic Press, New York, 1986). [ 31J.T. Hougen, NBS Monogr. I 15( k970) 1. [ 41 G. He&erg, Molecular spectra and molecular structure, Vol. 1, Spectra of diatomic molecules, 2nd Ed. (Krieger, New York, 1989).
3 September 1993
[5] S. Sekine, T. Masaki, Y. Adachi and C. Hirose, J. Chem. Phys. 89 (1988) 3951. [ 61 R.S. Mulliken, J. Am. Chem. Sot. 86 ( 1964) 3 183. [ 71 J.H. Van Vleck, Phys. Rev. 23 (1929) 467. [S] S. Sekine, Y. Adachi and C. Hirose, J. Chem. Phys. 90 (1989) 5346. [ 91 S. Sekine, S. Iwata and C. Hirose, Chem. Phys. Letters 180 (1991) 173. [ 101M. Ogawa and S. Ogawa, J. Mol. Spectry. 41 (1972) 393. [ I1 ] T. Masaki, Y. Adachi and C. Hirose, Chem. Phys. Letters 139 (1987) 62. ( 121T.A. Dixon and R.C. Woods, Phys. Rev. Letters 34 (1975) 61.
133
CHEMICAL PHYSICS LETTERS
3 September 1993
Perturbation on intensity profiles of the CO 4~0 K ‘X+-3x1 B ‘Z+ (O-O) and 4p~ L ‘II-3x5 B ‘Z+(O-0) bands Shigeyuki Sekine ’ and Chiaki Hirose Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259, Nagatsuta, Mtdori, Yokohama 227, Japan
Received 25 June 1993; in final form 2 July 1993
The 4po K ‘E+-~so B %+(0-O) and 4pn L ‘Il-3so B ‘X+(O-O) bands of carbon monoxide were re-cxamined by the optogalvanic method. We found that the intensity ofthe rotational lines was proportional to the laser power up to about lo9 W cm2 and that the band profiles were affected by the Cuncoupling interaction among the rotational levels of the 4pa L ‘II( u=O), the 4po K’X+(v=O), and an unidentified ‘Z+ state. The 3pn C ‘C+(v=5 or 6) state which is higher than the 4pr L ‘TI(v=O) state by less than 200 cm-’ wasproposed as a likely candidate for the unidentified state.
1. Introduction The occurrence of an l-uncoupling interaction between the rotational levels of npo ‘C* and npn ‘l-I states in a singlet np complex is manifested in n-type doubling - the splitting of e( ‘II+) and f(‘II-) symmetry levels. The interaction also leads to an anomalous intensity distribution of the rotational lines of the npo ‘C+-n’so ‘C+ and npx ‘II-n’so ‘C+ transitions [l-4]. In the singlet 4p-complex( v=O ) levels of CO, the value of the n-type doubling constant q was obtained as 0.0213+0.0012 cm-’ for the 4prtL’II(v=O) state from the analysis of the optogalvanic (OG) spectrum of the 4px L ‘II-3~s B ‘C+ (O-O) band [ 5 1. The value of 0.07 cm-’ calculated by the pure precession model [ 6,7] is approximately three times larger than the experimentally derived value, and perturbation by nearby E states was suggested [ 51. Two other electronic bands, namely the ( 1~~3~0) W ‘II3so B ‘E+ (O-O) [ 81 and the L’ ‘II-3so B ‘C+ (u’ unspecified-u” =O) bands [ 91, have also been identified on the optogalvanic spectrum. Although the precise value of the n-type doubling constant for the (17?3so)W ‘lI(v=O) state was not derived in.our previous analysis, the value was predicted to be small ’ Present address: Electrotechnical Laboratory, l- 1-4, Umezono, Tsukuba, Ibaraki 305, Japan. Elsevier Science Publishers B.V.
from consideration of the dominant electron configuration of the ( 1x33so)W ‘II(u=O) state: the Rydberg electron is in the 2~0 orbital. On the other hand, the L’ ‘II( u unspecified) state was found to be in appreciable interaction with an unidentified *C+ state: we identified only the rotational lines belonging to the Q branch, but did not observe any lines in the R branch, even its head (see fig. 1 of ref. [ 91). The present paper focuses on the observation of the OG spectrum and the analysis of the perturbed intensity distribution of rotational lines of the 4~0 K ‘C+-3so B ‘X+(0-0) and the 4pz L ‘II-3~0 B ‘Z+ (O-O) bands. The analysis of the intensity protiles supports our previous suggestion that an unidentified ‘Z+ state is present and perturbs the rotational levels of the 4prc L ‘IT( u=O) state [ 51. The significant magnitude of the interaction in the 4pcomplex ( V= 0) suggests that the unidentified ‘C+ state is related to the electron configuration with an electron excited to a p Rydberg orbital, and the 3px C ‘C+ (v= 5 or 6 ) state is proposed as a candidate.
2. Experimental The OG spectrum of CO was observed using a DC hollow cathode discharge tube (see fig. 1 of ref. [ 5 ] 1. The discharge current was approximately 5 mA and a 1: 1 mixture of CO and oxygen flowed in the tube 129
Volume 212, number
CHEMICAL PHYSICS LETTERS
I,2
at approximately 100 Pa. A frequency-tunable dye laser operating with rhodamine 6G dye was pumped by the all-line output of a cw argon ion laser (5 W type ). The laser beam was collinearly focused on the inside of the cylindrical hollow cathode by a lens cf=250 mm). The maximum output power of the dye laser was 0.3 W at 16200 cm-’ with a linewidth of approximately 1 cm-’ and the power density was estimated to be IO9 W cm-2 at the focal point. The OG signal was lock-in amplified at the modulation frequency (1.3 kHz) of the laser beam. The output power of the dye laser was monitored by a photodiode and was recorded along with the OG signals to normalize the intensity of each rotational line.
3. Results and discussion To examine the dependence of the signal intensity on the laser power, the band-head intensity of the 4~0 K ‘Et-3so B ‘E+ (O-O) band was measured while the output power of the dye laser was varied by using neutral density filters. The linewidth of the dye laser was much wider than the spectral linewidth which is dominated by the Doppler width, and the intensity of the observed rotational lines as plotted in fig. 1 was derived from the peak height. It is seen that the signal intensity is linearly proportional to the laser power up to x lo9 W cm-‘.
0
100
50 LASER
POWER
‘/,
Fig. I. The dependence of the peak intensity of optogalvanic signal on laser power. The peak intensity of the R-branch bandhead of the CO 4~0 K IX+-3~0 B ‘X+ (O-O) band is plotted by open circle against the output power of the dye laser. The laser power at 100% is approximately lo9 W cm-‘. The laser beam was focused on the inside of the hollow cathode by a lens (f= 250 mm ).
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1993
The observed spectrum of the 4po K ‘X+-3~0 B ‘C+ (O-O) and 4pn: L ‘II-3so B ‘C+ (O-O) bands is shown in figs. 2a and 3a, respectively. As seen in fig. 3a the Q-branch lines of the 4pn L *l-I-3so B ‘Zf (O0) band, a higher-resolution spectrum of which has been already reported [51,were not resolved by the present resolution. The anomaly in the intensity distribution of the 4~0 K ‘C+-3~0 B ‘C+ (O-O) band was revealed in that different values, 410 + 30 K and 500 f 30 K, for the rotational temperatures were derived by a conventional analysis of the P and R branches, respectively. The difference arises from the f-uncoupling interaction, and an improved analysis was carried out by taking account of the interaction as described below. The formulas for the relative intensity of the rotational lines of perturbed npo ‘E’-n’so ‘E+ and npn: ‘Il-n’so ‘E+ bands have been given by Lefebvre-Brion and Field [2] (see eqs. (5.3.29a), (5.3.29b), and (5.3.40a) of ref. [2]). In a quantitative analysis of the intensity interference effect in an npo ‘Cc- n’so ‘E+ band, it is convenient to use the parameter e(J) defined by (see eq. (5.3.33) of ref. [ 2 I ) I(R(J-I))-I(P(J+l)) e(J)=I(R(J-l))tZ(P(Jtl))’
(1)
where Z(P(J+ 1)) and I(R(J-1)) are the intensities of the rotational lines of the P and R branches, respectively. The values of 0(J) for the observed 4po K ‘Z+-3so B ‘C+ (O-O) band are plotted by open circles in fig. 4. The results of theoretical calculations made with and without the inclusion of the effect of the interaction between the 4po K ‘Z’(u=O) and 4pa L ‘II (v=O) states are shown by curves (a) and (b), respectively. The assumed rotational temperature T,,was 450 K, which is the average of the values derived from the previously mentioned preliminary analysis. The experimental points are seen to lie between the two theoretical curves, but to be much closer to curve (a). The calculated spectral feature of the 4po K ‘C+-3so B ‘E+ (O-O) band, which is shown in fig. 2b, was seen to reproduce the observed feature (fig. 2a) well, and we thus concluded that the observed intensity anomaly of the rotational lines of the 4~0 K ‘E+-3~0 B ‘C+ (O-O) band is primarily caused by the l-uncoupling interaction in the singlet 4p-complex levels.
Volume 2 12, number I,2
CHEMICAL PHYSICS LETTERS
(b) simulated
3 September 1993
T,,,=450K
16100
16150
wavenumber / cm-l Fig. 2. (a) Observed and (b) calculated rotational profile of the CO 4pa K ‘I+-3s~ B ‘Z+ (O-O) band. In (b), the I-uncoupling interaction between the levels of only the 4pa K ‘Z+( ~0) and 4px L ‘lI(v=O) states was taken into account and a rotational temperature of 450 K was assumed. The intensity of each rotational line in (b) is weighted by the laser power at its peak wavelength.
250
q 200 =! S v9 150 x .5 c 100 Y c .50
(b) simulated
T,,,=450K
0 16300
16350
wavenumber / cm
16400 -1
Fig. 3. (a) Observed and (b) calculated rotational profile of the CO 4pn L *II-3sa B ‘Z+ (O-O) band. The Q branch was not resolved at the resolution of the present experiment. The same calculation as that which gave fig. 2b was made. The intensity of each rotational line in (b) is weighted by the laser power at its peak wavelength.
The I-uncoupling interaction is also expected to affeet the intensity distribution of rotational lines of the P and R branches of the 4prc L ‘II-3~0 B ‘C+ (O0) band. The observed intensity of the rotational lines
was anomalously weaker than expected and the presence of an additional perturbation in the singlet 4pcomplex states was suggested. We note that the calculated intensity of the P- and R-branch lines (fig. 131
Volume 2 12, number I ,2
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Table 1 Rotational constants for the CO singlet 4p-complex( u=O) and theC0’ X%+(v=O) state
Trot =450K
15
Levels [symmetry]
Rotational constants (cm-‘)
Ref.
4pnL’fI+(v=O)
[e] B[‘II+]
1.998*0.014 1.9812f0.0001
151 1101
4pnL’l--(v=O)
[f] B[‘II-]
1.95971kO.00024
J
-0.3 Fig. 4. Observed and calculated values of e(J) plotted against the rotational quantum number J. The experimental values derived from eq. (1) are plotted by open circles. Curve (a) is the result calculated by assuming the I-uncoupling interaction based on the pure precession model, and curve (b ) is the result calculated by assuming no perturbation. The rotational temperature was set to 450 Kin calculating (a) and (b).
3b, calculated with the same units as in fig. 2b) is much stronger than that of the 4~0 K ‘C+3sa B ‘C+ (O-O) band (fig. 2b) because of higher laser power in this region. The Q-branch lines of the 4pn: L ‘fI-3so B ‘I+ (O-O) band have higher intensity than the P- and R-branch lines as seen in fig. 3a, indicating that the upper 4px L ‘II- (v=O) levels f symmetry) are free from the additional perturbation. In that case, the rotational levels of the perturbing state are all of e symmetry and thus ‘Z+ symmetry is assigned to the state. The transition from the 3~0 B ‘C+ (u= 0) level to the perturbing level is expected to show up through the intensity borrowing from the 4prt L ‘II-3~0 B ix+ (O-O) band, but no such signal was present on the QG spectrum, possible due to an unspecified mechanism of the OG signal; the transition does not affect the characteristics of the discharge plasma to give the signal. This feature is quite similar to the case observed for the L”lI-3so B ‘Z+(r+O) band [9]. The effect of the additional perturbation also appeared in fig. 4 as a slight, but systematic departure of the observed values from the theoretical values (curve (a) ). The departure was not improved by simply varying the value of T,,. Table 1 lists the previously reported values of the rotational constants. The slight difference between the values of B(4px L’II-(u=O)) [ 51 and B(CO+ X *C+ (v= 0) ) [ 121 also indicates that there is no heterogeneous perturbation in the 4px L ‘II- (v= 0) 132
3 September 1993
CHEMICAL PHYSICS LETTERS
I51
4pcrK’E+(u=O) [e] B[‘Z+]
1.900f 0.002 1.9189+_0.0001
Ittl 1’01
CO+ X *Z+(Y=O)[e] B
1.967465
1121
rotational levels. The differences B(4prc L ‘II+ (v=O))-B(4prt L’n-(v=O))2:0.02 cm-’ and B(4plc L’I--(v=O))-B(4po K’E+(v=O)) ~00.04 cm-’ are equal to the A-type doubling constant in the lowest order of approximation, but both values are significantly smaller than the value of qcalc= 0.07 cm- ’ [ 51 estimated from the pure precession model. The discrepancy between the two observed values and that between the observed and calculated q values is ascribed to the f-uncoupling interaction of the 4pn L’II(v=O) state with the unidentified ‘C+ state which is located higher than the 4p7cL ‘Il( v= 0) state. For the singlet 4p-complex (v= 0) to undergo such a strong interaction, the ‘E+ state must be associated with the electron configuration in which the excited electron is in an np(n=3-5) Rydberg orbital. But, we expect that the magnitude of the matrix element between a rotational level of the 4p7r L ‘II( ~0) state and that of the unidentified ‘C+ states is smaller than that between the level of the 4prt L ‘l-I+ (v= 0) state and the level of the 4po K ‘Z’ (v= 0) state, and the unidentified ‘C+ level must then be located at less than 200 cm-’ above the L ‘II( v=O) state. The 3pn C ‘E+ (v=5 or 6) level seems to be a likely candidate, although there is no report which identified the state. In conclusion, we have shown that the intensity profile of the 4pcr K’C+-3s~ B ‘C+(O-0) band is primarily affected by the /-uncoupling perturbation of the rotational levels of the 4po K ‘C+ (v= 0) state by those of the 4~71L ‘fI( ~0) state. For the P- and R-branch lines of the 4pn. L ‘II-so B ‘C+ (O-O) band an additional interaction of the 4prr L ‘fI( v=O) state
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with a ‘C+ state, possibly the 3px C ‘Z+( U= 5 or 6) state, was suggested.
References [ 1 ] 1. Kovacs, Rotational structure in the spectra of diatomic molecules (Adam Hilger, Bristol, 1969). [2] H. Lefebvre-Btion and R.W. Field, Perturbations in the spectra of diatomic molecules (Academic Press, New York, 1986). [ 31J.T. Hougen, NBS Monogr. I 15( k970) 1. [ 41 G. He&erg, Molecular spectra and molecular structure, Vol. 1, Spectra of diatomic molecules, 2nd Ed. (Krieger, New York, 1989).
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[5] S. Sekine, T. Masaki, Y. Adachi and C. Hirose, J. Chem. Phys. 89 (1988) 3951. [ 61 R.S. Mulliken, J. Am. Chem. Sot. 86 ( 1964) 3 183. [ 71 J.H. Van Vleck, Phys. Rev. 23 (1929) 467. [S] S. Sekine, Y. Adachi and C. Hirose, J. Chem. Phys. 90 (1989) 5346. [ 91 S. Sekine, S. Iwata and C. Hirose, Chem. Phys. Letters 180 (1991) 173. [ 101M. Ogawa and S. Ogawa, J. Mol. Spectry. 41 (1972) 393. [ I1 ] T. Masaki, Y. Adachi and C. Hirose, Chem. Phys. Letters 139 (1987) 62. ( 121T.A. Dixon and R.C. Woods, Phys. Rev. Letters 34 (1975) 61.
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