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A spectrum of rhenium oxide
JOURNAL
OF MOLECULAR
SPECTROSCOPY
84,424-430
A Spectrum
(1980)
of Rhenium
WALTERJ.BALFOURAND
Oxide
F.B.ORTH
Department of Chemistry, University of Victoria, Victoria. B. C. V8W 2Y2, Canada
Thearc emission spectrum of the ReO molecule has been photographed in the region 590-860 nm and three bands of a single electronic transition have been rotationally analyzed. The separation of lines of the isotopic molecules iS5Re0 and la7Re0 leads to the conclusion that the vibrational assignments for these bands are l-0, O-O, and 0- 1. It is conceivable that an electronic isotope shift of -0.08 cm-’ exists. The following vibrational and rotational data (cm-l) have been determined: ~~(0-0) = 14 038.42, AG’(1/2) = 867.85, AG”(U2) = 979.14; B; = 0.3889, ti: = 0.0019, B: = 0.4257, &’ = 0.0043. It is concluded that A’ - A” = + 1 with A” 2 2. I. INTRODUCTION
There is a basic need for laboratory investigations of the spectra of transition metal oxides in order to advance our understanding of structure and bonding. We have photographed the arc emission spectrum of ReO in the region 590860 nm. This spectrum exhibits the isotope shift for 185Re0 and lE7Re0 and three bands belonging to a single system have been rotationally analyzed. Although our spectrum is essentially similar to the lower dispersion spectrum attributed to ReO and published in the Vatican atlas of metallic oxide spectra (I), no vibrational or rotational analyses have been previously reported. II. EXPERIMENTAL
DETAILS
A simple arc in air between copper electrodes with Re powder and/or Re,O, powder placed in a small hole drilled in the tip of the cathode produces the emission. We used a 150-V d.c. power supply with a ballast resistance of -42 s2 and an electrode gap of -5 mm. The spectrograms were taken in the sixth, seventh, and eighth orders off a 186 x 63 mm, 300 grooves/mm grating in a Jarrell-Ash 3.4m Ebert spectrograph. Kodak 103a-D, 103a-F, and 1-N spectroscopic plates were used for detection and a commercial iron-neon hollow cathode lamp was used for standards. The reciprocal dispersion on different plates varies from -0.068 to -0.115 run/mm but in most cases is less than 0.090 nm/mm. The D and F plates required exposure times of only 1 to 2 min whereas the (unhypersensitized) N plates required exposure times of 5 to 15 min (2 to 7 sample loads). The blackbody continuum from the hot source limits the features that can be adequately photographed. Commercial Re and Re207 powders containing the natural proportion of rhenium isotopes, 37.07% ls5Re and 62.93% ls7 Re, were used. The plates were measured at the University of British Columbia using a manually operated comparator equipped with a photoelectric setting device. For sharp lines, we estimate 0022-2852/80/120424-07$02.00/O Copyright 0 1980 by Academic AU rights of reproduction
Press, Inc.
in any form reserved.
424
SPECTRUM
OF ReO
425
that the absolute positions are accurate to - 50.04 cm-‘, while the reliability of relative positions occurring within -500 cm-’ is - kO.02 cm-‘. III. DESCRIPTION
OF SPECTRUM
In our spectrum rotational branch structure is apparent in the following wavelength regions (nm): (i) 633.5-640.0, (ii) 644.4-667.0, (iii) 670.4-690.0, (iv) 711.9735.8, (v) 765.2-784.2, and (vi) 842.2-850.0. By far the most salient features of the ReO spectrum are the bands in regions (iii), (iv), and (v) (henceforth designated the 670-, 712-, and 765-nm bands, respectively). In all regions the degrading is toward the red, however, only in regions (iv) and (v) are there clearly identifiable band heads-at 711.9033 and 765.2283 nm, respectively. The branch lines in region (iii) belong to R, Q, and P branches of a single band and the same is true for regions (iv) and (v). The relative intensity of the branches is Z(Q) > I(R) > Z(P) in each of the three bands. Two important additional facts are observed. 1. The branch lines in the 670 and 765 nm bands occur in pairs agreeing with the natural proportion of the rhenium isotopes la5Re and lX7Re. In the 670-nm band the heavier isotope lies to the red, in the 765-nm band it lies to the blue, and in the 712-nm band the absence of isotopic pairs suggests that the isotope separation is too small to be resolved. 2. There is agreement within the accuracy of measurement in the A,F”(J) values for the 670- and 7 12-nm bands and in the A, F’(J) values for the 7 12 and 765 bands. Generally there is a fair amount of blending of lines. This is due to the general richness of the spectrum, the presence of Re isotopes, and a great abundance of Re atomic lines. We have not identified any molecular impurities in the spectrum. The weaker branches corresponding to the less abundant 185Re0 isotope are particularly troubled by blending and for this reason all molecular constants (see Section IV) have been determined from lX7Re0 data. The wavenumbers of the band lines for the 187Re0 molecule are given in the Appendix. Note that blends or obscured lines are so indicated. The quality of the spectra obtained for regions (i), (ii), and (vi) is generally inferior to that for regions (iii), (iv), and (v) for a combination of reasons including lack of intensity and blackbody continuum emission from the arc source. However, some qualitative statements can be made: (a) Region (i): 633.5-640.0
nm
The most prominent feature in this region is a (ls7Re0) branch and its accompanying P5Re0) isotopic branch. The branch can be followed for some 60 members spanning the 633.5 to 640.5 wavelength interval when the 1s7Re0-*85Re0 separation increases from -0.2 cm-’ to -1 cm-‘. Thirty members of a second, weaker isotopic pair of branches are found between 634.2 and 637.2 nm. Data are lacking for any detailed analysis. The 2-O band of the system analyzed is expected to lie in this spectral region. (b) Region (ii): 644.4-677.0
nm
Regular branch structure is evident in the wavelength intervals 645-647.5 and 648-662 nm. These observations confirm the structure shown at lower resolution
BALFOLJR AND ORTH
426
in the Vatican atlas. In the shorter wavelength interval a single branch showing a 187Re0-185Re0 splitting of - -0.35 cm-’ has been picked out. An abrupt break in the regularity of the branch occurs near 647.6 nm indicating the presence of rotational perturbation or predissociation. By contrast the interval beyond the perturbed region, 648-662 nm, shows a branch, of which more than 60 members have been identified, where the 187Re0-185Re0 splitting varies regularly from +0.20 to +0.35 cm-’ with increasing wavelength. (c) Region (vi): 842.7-850
nm
This region shows a series of lines converging to a head near 842.14 nm and showing no isotopic structure. The feature is clearly visible in Vatican atlas photographs. IV. ANALYSIS
For each of the 670-, 712-, and 765nm bands the relative J numbering of the R, Q, and P branches was unambiguously determined by finding branch members such that A,F’(J) = R(J) - Q(J) = Q(J + 1) - P(J + 1) and ArF”(J) = R(J) - Q(J + 1) = Q(J) - P(J + 1). The absolute J numbering was determined in the usual way; independently from plots of A$‘(J), A$“(J), and Q(J) first differences, The consistency of these results suggests an uncertainty in the absoluteJ numbering of f one unit, however, uncertainty arising from extrapolation error is difficult to gauge. Since the ReO molecule has an odd number of electrons, the J values must be half-integers. The facts given in Section III regarding the nature of the 1*5Re0-1*7Re0 isotope separation and the nature of the agreement in AIF” and AIF’ values are consistent with an interpretation that the 670-, 712-, and 765nm bands are the I-O, O-O, and 0- 1 bands, respectively, of a single electronic transition. That these vibrational assignments are correct was verified by a comparison of experimental and calculated vibrational isotope shifts, Au. In Fig. 1, the magnitude of the isotope separation in the Q branch is illustrated as a function of J(J + 1). It can be seen that the experimental vibrational isotope shifts (i.e., Av = AQ(J = 0)) for the 670- and 765-nm bands are -0.42 and +0.35 cm-‘, respectively, and that their uncertainties might be estimated at 20.03 cm-‘. The vibrational isotope shift may be calculated within the framework of the Born-Oppenheimer approximation according to the equation: AY(u’,v”) = ~187-
11185 =
(1 - p)[o:(u’ + 112) - C&u0 + l/2)] + (1 - p2)[w&(v” + l/2)2 - W&U’ + l/2)2],
(1)
where v18, and u185are the wavenumbers of the purely vibrational transitions of 187Re0 and lS5Re0, respectively, and p2 is the ratio of reduced masses ~(‘*‘ReO)I p(ls5Re0) = 1.0008537. For the purpose of calculating Av(z)‘,u”) we estimated the
427
SPECTRUM OF ReO
AQ = Q(1*7Re0)-Q(‘8sReO)
670
nm band
* . . _.____+--=Pr
-0.6 t
,
1 2000
4000
6000
I
I
6000
10000
J(J+I) FIG. 1. Isotope shift in the Q branch of the 765- and 670-nm bands of ReO. Only data from reasonably unblended lines are plotted. A slight increase in the shift with increasing_/ is discernible. The isotopic lines of the 712-nm band are unresolved.
anharmonicity constants w& and w,x; from the Pekeris relation to be 10.5 and 3.1 cm-‘, respectively. Actually the calculated values of AVare not very sensitive to the values of w,x, chosen. Using data for the bands origins (see Table III) and choosing the 670-, 712-, and 765nm bands as l-0, O-O, and O-l bands, respectively, we find the calculated values of Av to be -0.343 5 0.008, +0.024 2 0.003, and +0.438 -+ 0.008 cm-l, respectively. The corresponding experimental values of Av are -0.42 ? 0.03, 0.0 ? O.O7l, and +0.35 + 0.03 cm-‘, respectively. No other acceptable choice of vibrational assignments yields calculated values of Av that compare as well with the experimental values. The o: and WEvalues based on the above assumptions are 874 and 1000 cm-‘, respectively. It should be noted that there is a small discrepancy between the calculated and experimental values ’ No isotopic pairs were observed in the 712-nm band. We estimate that an isotope separation exceeding 0.07 cm-’ would have been detectable. TABLE I Rotational Constants (cm-‘) from Fitting ‘“‘ReO Data u
B:
0
0.3879 ? 0.0004 0.3860 + 0.0005
1
Nore. The values corresponding
D: x 10" 0.339 r 0.050 0.250 + 0.070 to v = 0 are averages.
B:' 0.4235 -t 0.0002 0.4192 t 0.0005
D"” x lo6 0.331 + 0.010 0.318 r 0.070
428
BALFOLJR AND ORTH TABLE II Equilibrium Constants (cm-‘) Obtained from B, Values in Table I Electronic state
B,
%
rr (nm)
Upper
0.3889 0.4257
0.0019 0.0043
0.1715 0.1639
Lower
of AV of -0.08 cm-‘. It is conceivable that this discrepancy is attributable to an electronic isotope shift not taken into account by Eq. (I). The rotational constants and band origins were calculated by the least-squares method using the following relations: A,F(J)
= 2B,(J
+ 1) - 40,(5
+ 1)3,
Q(J) = v,, + (B: - B’I,)J(J + 1) - (DQ - D;)Y(J
+ l)*.
The results are given in Tables I, II, and III. V. DISCUSSION
It has been assumed in the preceding sections that the molecule under analysis is ReO. What is unambiguous is that the molecule is diatomic and includes a Re atom. However, the possibility that the molecule is ReN cannot be categorically denied here. If the p value for ReN is used in Eq. (l), then the calculated Au values for the 670- and 76%nm bands are -0.31 and +0.39 cm-‘, respectively. Nevertheless, we fully expect that the molecule is ReO. In this laboratory, under identical experimental conditions except that a praseodymium sample was used instead of rhenium, the well-known PrO spectrum was obtained. Furthermore, our spectrum corresponds to the lower dispersion spectrum obtained by Gatterer (I), and using samples under the same experimental conditions Gatterer consistently obtained oxide spectra. The possibility that the molecule is ReO+ also cannot be categorically denied here, but again the experimental conditions generally favor the production of the neutral molecule. We have no positive evidence regarding the electronic characterization of the transition although the present observations do exclude some possibilities. The coupling in ReO is expected to be intermediate between Hund’s case (a) and TABLE III Origins (cm-‘) of the Analyzed Bands of ‘*‘ReO V”
\
V’
0
1
0
1
14 038.42 14 906.27
13 059.28
Note. The uncertainty in these values is -0.10 cm-‘.
SPECTRUM TABLE
OF ReO
429
Al
case (c), tending towards case (c). fI’ and w cannot be established since the first lines of the branches could not be observed. The presence of strong Q branches and the relative intensity of the branches indicate that AA = A’ - h” = +l. Furthermore, the absence of measurable A-type doubling, even for the highest observed J values, suggests that neither state is a II state. We conclude therefore thatA’-A”=+lwithYr2. The multiplicities of the states are entirely in doubt. It has been stated on theoretical grounds (2) that the ground state of ReO is %+. If this is the case, then the transition studied here does not involve the ground state. Spin-orbit coupling
430
BALFOUR AND ORTH
in ReO is probably quite large and the bands presently analyzed probably belong to a subsystem of a multiplet transition with other components lying in regions which have’ not been deciphered. Clearly, additional experimental work on ReO is needed to establish the nature of the transition involved. Spectra of lower excitation temperatures than those produced in an arc, e.g., spectra produced by the composite-wall hollow cathode described by Bacis (3), may prove useful in this regard. Further experiments are planned. APPENDIX
Assigned
Lines of la7Re0 Bands
The lines given in Table A 1 are ‘*‘ReO data which have been assigned to a single electronic subsystem. All values are given as vacuum wavenumbers in reciprocal centimeters. The numbers in parentheses at the head of each band define the band in terms of u’ and a”, respectively. Abbreviations: *, blend; OBSD, obscured; UML, unmeasured line. ACKNOWLEDGMENT The authors are grateful to the Natural Sciences and Engineering Research Council of Canada and the University of Victoria for support funding. RECEIVED:
January 17, 1980 REFERENCES
1. A. Chem. Phys. l&481-489
3. R. BACIS,J. Phys. E. 9, 1081-1086 (1976).
(1976).
OF MOLECULAR
SPECTROSCOPY
84,424-430
A Spectrum
(1980)
of Rhenium
WALTERJ.BALFOURAND
Oxide
F.B.ORTH
Department of Chemistry, University of Victoria, Victoria. B. C. V8W 2Y2, Canada
Thearc emission spectrum of the ReO molecule has been photographed in the region 590-860 nm and three bands of a single electronic transition have been rotationally analyzed. The separation of lines of the isotopic molecules iS5Re0 and la7Re0 leads to the conclusion that the vibrational assignments for these bands are l-0, O-O, and 0- 1. It is conceivable that an electronic isotope shift of -0.08 cm-’ exists. The following vibrational and rotational data (cm-l) have been determined: ~~(0-0) = 14 038.42, AG’(1/2) = 867.85, AG”(U2) = 979.14; B; = 0.3889, ti: = 0.0019, B: = 0.4257, &’ = 0.0043. It is concluded that A’ - A” = + 1 with A” 2 2. I. INTRODUCTION
There is a basic need for laboratory investigations of the spectra of transition metal oxides in order to advance our understanding of structure and bonding. We have photographed the arc emission spectrum of ReO in the region 590860 nm. This spectrum exhibits the isotope shift for 185Re0 and lE7Re0 and three bands belonging to a single system have been rotationally analyzed. Although our spectrum is essentially similar to the lower dispersion spectrum attributed to ReO and published in the Vatican atlas of metallic oxide spectra (I), no vibrational or rotational analyses have been previously reported. II. EXPERIMENTAL
DETAILS
A simple arc in air between copper electrodes with Re powder and/or Re,O, powder placed in a small hole drilled in the tip of the cathode produces the emission. We used a 150-V d.c. power supply with a ballast resistance of -42 s2 and an electrode gap of -5 mm. The spectrograms were taken in the sixth, seventh, and eighth orders off a 186 x 63 mm, 300 grooves/mm grating in a Jarrell-Ash 3.4m Ebert spectrograph. Kodak 103a-D, 103a-F, and 1-N spectroscopic plates were used for detection and a commercial iron-neon hollow cathode lamp was used for standards. The reciprocal dispersion on different plates varies from -0.068 to -0.115 run/mm but in most cases is less than 0.090 nm/mm. The D and F plates required exposure times of only 1 to 2 min whereas the (unhypersensitized) N plates required exposure times of 5 to 15 min (2 to 7 sample loads). The blackbody continuum from the hot source limits the features that can be adequately photographed. Commercial Re and Re207 powders containing the natural proportion of rhenium isotopes, 37.07% ls5Re and 62.93% ls7 Re, were used. The plates were measured at the University of British Columbia using a manually operated comparator equipped with a photoelectric setting device. For sharp lines, we estimate 0022-2852/80/120424-07$02.00/O Copyright 0 1980 by Academic AU rights of reproduction
Press, Inc.
in any form reserved.
424
SPECTRUM
OF ReO
425
that the absolute positions are accurate to - 50.04 cm-‘, while the reliability of relative positions occurring within -500 cm-’ is - kO.02 cm-‘. III. DESCRIPTION
OF SPECTRUM
In our spectrum rotational branch structure is apparent in the following wavelength regions (nm): (i) 633.5-640.0, (ii) 644.4-667.0, (iii) 670.4-690.0, (iv) 711.9735.8, (v) 765.2-784.2, and (vi) 842.2-850.0. By far the most salient features of the ReO spectrum are the bands in regions (iii), (iv), and (v) (henceforth designated the 670-, 712-, and 765-nm bands, respectively). In all regions the degrading is toward the red, however, only in regions (iv) and (v) are there clearly identifiable band heads-at 711.9033 and 765.2283 nm, respectively. The branch lines in region (iii) belong to R, Q, and P branches of a single band and the same is true for regions (iv) and (v). The relative intensity of the branches is Z(Q) > I(R) > Z(P) in each of the three bands. Two important additional facts are observed. 1. The branch lines in the 670 and 765 nm bands occur in pairs agreeing with the natural proportion of the rhenium isotopes la5Re and lX7Re. In the 670-nm band the heavier isotope lies to the red, in the 765-nm band it lies to the blue, and in the 712-nm band the absence of isotopic pairs suggests that the isotope separation is too small to be resolved. 2. There is agreement within the accuracy of measurement in the A,F”(J) values for the 670- and 7 12-nm bands and in the A, F’(J) values for the 7 12 and 765 bands. Generally there is a fair amount of blending of lines. This is due to the general richness of the spectrum, the presence of Re isotopes, and a great abundance of Re atomic lines. We have not identified any molecular impurities in the spectrum. The weaker branches corresponding to the less abundant 185Re0 isotope are particularly troubled by blending and for this reason all molecular constants (see Section IV) have been determined from lX7Re0 data. The wavenumbers of the band lines for the 187Re0 molecule are given in the Appendix. Note that blends or obscured lines are so indicated. The quality of the spectra obtained for regions (i), (ii), and (vi) is generally inferior to that for regions (iii), (iv), and (v) for a combination of reasons including lack of intensity and blackbody continuum emission from the arc source. However, some qualitative statements can be made: (a) Region (i): 633.5-640.0
nm
The most prominent feature in this region is a (ls7Re0) branch and its accompanying P5Re0) isotopic branch. The branch can be followed for some 60 members spanning the 633.5 to 640.5 wavelength interval when the 1s7Re0-*85Re0 separation increases from -0.2 cm-’ to -1 cm-‘. Thirty members of a second, weaker isotopic pair of branches are found between 634.2 and 637.2 nm. Data are lacking for any detailed analysis. The 2-O band of the system analyzed is expected to lie in this spectral region. (b) Region (ii): 644.4-677.0
nm
Regular branch structure is evident in the wavelength intervals 645-647.5 and 648-662 nm. These observations confirm the structure shown at lower resolution
BALFOLJR AND ORTH
426
in the Vatican atlas. In the shorter wavelength interval a single branch showing a 187Re0-185Re0 splitting of - -0.35 cm-’ has been picked out. An abrupt break in the regularity of the branch occurs near 647.6 nm indicating the presence of rotational perturbation or predissociation. By contrast the interval beyond the perturbed region, 648-662 nm, shows a branch, of which more than 60 members have been identified, where the 187Re0-185Re0 splitting varies regularly from +0.20 to +0.35 cm-’ with increasing wavelength. (c) Region (vi): 842.7-850
nm
This region shows a series of lines converging to a head near 842.14 nm and showing no isotopic structure. The feature is clearly visible in Vatican atlas photographs. IV. ANALYSIS
For each of the 670-, 712-, and 765nm bands the relative J numbering of the R, Q, and P branches was unambiguously determined by finding branch members such that A,F’(J) = R(J) - Q(J) = Q(J + 1) - P(J + 1) and ArF”(J) = R(J) - Q(J + 1) = Q(J) - P(J + 1). The absolute J numbering was determined in the usual way; independently from plots of A$‘(J), A$“(J), and Q(J) first differences, The consistency of these results suggests an uncertainty in the absoluteJ numbering of f one unit, however, uncertainty arising from extrapolation error is difficult to gauge. Since the ReO molecule has an odd number of electrons, the J values must be half-integers. The facts given in Section III regarding the nature of the 1*5Re0-1*7Re0 isotope separation and the nature of the agreement in AIF” and AIF’ values are consistent with an interpretation that the 670-, 712-, and 765nm bands are the I-O, O-O, and 0- 1 bands, respectively, of a single electronic transition. That these vibrational assignments are correct was verified by a comparison of experimental and calculated vibrational isotope shifts, Au. In Fig. 1, the magnitude of the isotope separation in the Q branch is illustrated as a function of J(J + 1). It can be seen that the experimental vibrational isotope shifts (i.e., Av = AQ(J = 0)) for the 670- and 765-nm bands are -0.42 and +0.35 cm-‘, respectively, and that their uncertainties might be estimated at 20.03 cm-‘. The vibrational isotope shift may be calculated within the framework of the Born-Oppenheimer approximation according to the equation: AY(u’,v”) = ~187-
11185 =
(1 - p)[o:(u’ + 112) - C&u0 + l/2)] + (1 - p2)[w&(v” + l/2)2 - W&U’ + l/2)2],
(1)
where v18, and u185are the wavenumbers of the purely vibrational transitions of 187Re0 and lS5Re0, respectively, and p2 is the ratio of reduced masses ~(‘*‘ReO)I p(ls5Re0) = 1.0008537. For the purpose of calculating Av(z)‘,u”) we estimated the
427
SPECTRUM OF ReO
AQ = Q(1*7Re0)-Q(‘8sReO)
670
nm band
* . . _.____+--=Pr
-0.6 t
,
1 2000
4000
6000
I
I
6000
10000
J(J+I) FIG. 1. Isotope shift in the Q branch of the 765- and 670-nm bands of ReO. Only data from reasonably unblended lines are plotted. A slight increase in the shift with increasing_/ is discernible. The isotopic lines of the 712-nm band are unresolved.
anharmonicity constants w& and w,x; from the Pekeris relation to be 10.5 and 3.1 cm-‘, respectively. Actually the calculated values of AVare not very sensitive to the values of w,x, chosen. Using data for the bands origins (see Table III) and choosing the 670-, 712-, and 765nm bands as l-0, O-O, and O-l bands, respectively, we find the calculated values of Av to be -0.343 5 0.008, +0.024 2 0.003, and +0.438 -+ 0.008 cm-l, respectively. The corresponding experimental values of Av are -0.42 ? 0.03, 0.0 ? O.O7l, and +0.35 + 0.03 cm-‘, respectively. No other acceptable choice of vibrational assignments yields calculated values of Av that compare as well with the experimental values. The o: and WEvalues based on the above assumptions are 874 and 1000 cm-‘, respectively. It should be noted that there is a small discrepancy between the calculated and experimental values ’ No isotopic pairs were observed in the 712-nm band. We estimate that an isotope separation exceeding 0.07 cm-’ would have been detectable. TABLE I Rotational Constants (cm-‘) from Fitting ‘“‘ReO Data u
B:
0
0.3879 ? 0.0004 0.3860 + 0.0005
1
Nore. The values corresponding
D: x 10" 0.339 r 0.050 0.250 + 0.070 to v = 0 are averages.
B:' 0.4235 -t 0.0002 0.4192 t 0.0005
D"” x lo6 0.331 + 0.010 0.318 r 0.070
428
BALFOLJR AND ORTH TABLE II Equilibrium Constants (cm-‘) Obtained from B, Values in Table I Electronic state
B,
%
rr (nm)
Upper
0.3889 0.4257
0.0019 0.0043
0.1715 0.1639
Lower
of AV of -0.08 cm-‘. It is conceivable that this discrepancy is attributable to an electronic isotope shift not taken into account by Eq. (I). The rotational constants and band origins were calculated by the least-squares method using the following relations: A,F(J)
= 2B,(J
+ 1) - 40,(5
+ 1)3,
Q(J) = v,, + (B: - B’I,)J(J + 1) - (DQ - D;)Y(J
+ l)*.
The results are given in Tables I, II, and III. V. DISCUSSION
It has been assumed in the preceding sections that the molecule under analysis is ReO. What is unambiguous is that the molecule is diatomic and includes a Re atom. However, the possibility that the molecule is ReN cannot be categorically denied here. If the p value for ReN is used in Eq. (l), then the calculated Au values for the 670- and 76%nm bands are -0.31 and +0.39 cm-‘, respectively. Nevertheless, we fully expect that the molecule is ReO. In this laboratory, under identical experimental conditions except that a praseodymium sample was used instead of rhenium, the well-known PrO spectrum was obtained. Furthermore, our spectrum corresponds to the lower dispersion spectrum obtained by Gatterer (I), and using samples under the same experimental conditions Gatterer consistently obtained oxide spectra. The possibility that the molecule is ReO+ also cannot be categorically denied here, but again the experimental conditions generally favor the production of the neutral molecule. We have no positive evidence regarding the electronic characterization of the transition although the present observations do exclude some possibilities. The coupling in ReO is expected to be intermediate between Hund’s case (a) and TABLE III Origins (cm-‘) of the Analyzed Bands of ‘*‘ReO V”
\
V’
0
1
0
1
14 038.42 14 906.27
13 059.28
Note. The uncertainty in these values is -0.10 cm-‘.
SPECTRUM TABLE
OF ReO
429
Al
case (c), tending towards case (c). fI’ and w cannot be established since the first lines of the branches could not be observed. The presence of strong Q branches and the relative intensity of the branches indicate that AA = A’ - h” = +l. Furthermore, the absence of measurable A-type doubling, even for the highest observed J values, suggests that neither state is a II state. We conclude therefore thatA’-A”=+lwithYr2. The multiplicities of the states are entirely in doubt. It has been stated on theoretical grounds (2) that the ground state of ReO is %+. If this is the case, then the transition studied here does not involve the ground state. Spin-orbit coupling
430
BALFOUR AND ORTH
in ReO is probably quite large and the bands presently analyzed probably belong to a subsystem of a multiplet transition with other components lying in regions which have’ not been deciphered. Clearly, additional experimental work on ReO is needed to establish the nature of the transition involved. Spectra of lower excitation temperatures than those produced in an arc, e.g., spectra produced by the composite-wall hollow cathode described by Bacis (3), may prove useful in this regard. Further experiments are planned. APPENDIX
Assigned
Lines of la7Re0 Bands
The lines given in Table A 1 are ‘*‘ReO data which have been assigned to a single electronic subsystem. All values are given as vacuum wavenumbers in reciprocal centimeters. The numbers in parentheses at the head of each band define the band in terms of u’ and a”, respectively. Abbreviations: *, blend; OBSD, obscured; UML, unmeasured line. ACKNOWLEDGMENT The authors are grateful to the Natural Sciences and Engineering Research Council of Canada and the University of Victoria for support funding. RECEIVED:
January 17, 1980 REFERENCES
1. A. Chem. Phys. l&481-489
3. R. BACIS,J. Phys. E. 9, 1081-1086 (1976).
(1976).