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Rydberg states of SiBr
JOURNAL OF MOLECULAR SPECTROSCOPY 106,72-76
(1984)
Rydberg States of SiBr G. BOSSER Laboratoire de Spectroscopic mokklaire, FacultC des Sciences, Universitk de Tours, Part de Grandmont, 37 Tours, France
AND H. BREDOHL AND I. DUBOIS Institut d’AstrophysiqueS Universiti de Li;ge, B-4200 Ougrke-Liege, Belgium New uv absorption spectra have been observed for SiBr. Five Rydberg states are identified to the states (4x7) ‘Z+, (Go) *Z+, (4pr) *II, (3d?r) *II, and (3d6) ‘A by comparison with SiF and SiCI. The ionization potentials of Sic1 and SiBr have been determined for the first time, and were 6.82 and 6.67 eV, respectively.
INTRODUCTION
Nineteen Rydberg states have been observed and analyzed for the SiF radical (I, 2). Most of them have been classified into Rydberg series; the ionization potential and quantum defects have been obtained (3). For the Sic1 radical, five Rydberg states are known, but they have not been classified into Rydberg series except for the first B %+(4~a) state (4-6). In the case of the SiBr radical, beside the B2Z+, state which has been analyzed by Bosser and Lebreton (7), only low-resolution ultraviolet absorption spectra have been observed by Oldershaw and Robinson (8). New absorption spectra of SiBr have been obtained in the uv region, and the four systems (C, D, E, F - X) mentioned by Oldershaw and Robinson (8) have been confirmed with slightly modified vibrational attributes. By comparing with SiF, it has been possible to identify the Rydberg states of Sic1 and SiBr. The purpose of the present paper is to give a short account of these results. EXPERIMENTALDETAILS The absorption spectra have been obtained by means of the flash discharge techniques in helium with traces of SiBi-,. The capacity used was a 0.05-ELFcapacitor charged to 18 kV. The best delay was found to be about 100 +ec. High-resolution spectra taken in the second order of a 21-ft. Eagle instrument have shown that all the states above the B*Z+ are strongly predissociated. The spectra used for the vibrational analysis (see below) have been obtained on a Hilger Medium quartz spectrograph. The measurements of the heads observed were difficult on account of the diffuse character of the bands, and the accuracy is probably as bad as 5 cm-‘. 0022-2852184 $3.00 Copyright 0
1984by Academic Press, Inc.
All rights of reproduction in any form resewed.
72
RYDBERG
STATES OF SiBr
73
VIBRATIONAL ANALYSIS
Thirty-four violet-degraded bands have been observed for SiBr. They include all of the 24 bands observed by Oldershaw and Robinson (8), except the two at an energy higher than 46 500 cm-’ which have not been observed on our spectra. Twelve new bands have been observed for the first time. These 34 bands have been arranged in four different systems corresponding to those mentioned by Oldershaw and Robinson (8). The vibrational analysis has been extended to higher V’values, and they take only one hot band into account; this one has been assigned to the 0- 1 transition of the D-X system, which is by far the strongest one in this spectral region. The vibrational assignments are given in Table I for the four systems. The assignment was straightforward on account of the spin-orbit coupling constant of the X*II ground state, which was easily observed. The classification of Table I gives the constants shown in Table II for the C, D, E, and F states. The values of the vibrational frequency clearly show these states to be Rydberg states. DISCUSSION In Fig. 1, the first Rydberg states of SiF are compared to those observed for Sic1 and SiBr. The correlation seems to be fairly obvious. The B state of Sic1 and SiBr is known to be the first Rydberg state corresponding to the - . * (?r)4(a)*(4sa) - 2X+ electron configuration, as is the case for SiF. The correlation is equally obvious for the C state, which is the *IIr state coming from the - - * W4W2(4P) - *II, configuration. The evolution of the spin-orbit coupling constant of that state along the sequence is a clear confirmation of this TABLE I Vibrational Analysis (bandheads in cm-‘) -I-
c-x
“l_“-
D-X
E-X
F-X
43255 blended by O-O
O-I
o-o
40690 41105
43675 44100
44135 44555
44570 44990
1-o
41225 41640
44235 44650
44690 45105
45125 45545
2-O
41740 42160
44780 45195
45220 45640
3-O
42250 42670
45310 45730
4-o
42750 43165
45825 46255
blended by 2-O E-X 46085
blended by 3-O D-X 46150
46345
5-O -
-
-
74
BOSSER, BREDOHL, AND DUBOIS TABLE II Molecular Constants (in cm-‘) of the Excited C, D, E, and F States
TO w&?
C
D
E
F
40898
43888
44315
44780
547.5
566
572.5
570
6.25
WeXe A
5.6
IO
20
25
20
7.5 20
identification. Indeed, A = 16 cm-’ in the C’ 211 state of SiF (I) and A = 11 cm-’ in the C’II state of Sic1 (5), and this constant takes the value A N 5 cm-’ in the corresponding state of SiBr (Table II). There is a state in SiF, namely the C” 2Z+ (1) coming from the 0 . Gus X (4pu) - 2Z+ configuration, which has no corresponding state in Sic1 nor in SiBr. This situation is easily understood since the C” 2Z+ state of SiF has been discovered (1) by means of the transition to the A2Z+ state in the visible; the C”-X transition to the ground state being extremely weak. 60
55
7 E _:: 50 _z z LL w z 45 w
40 Si
F
SI Cl
Si Br
35,-
FlG. 1. Correlation diagram for the Rydberg states of SiF, SiCl, and SiBr.
75
RYDBERGSTATES OF SiBr TABLEIII IonizationPotentialand QuantumDefectfor the nsu Series SiF
Sic1
IP
58960 7.31
5 (nsa)
SiBr
I
I
I .869
cm ev
-1
I
I
55017 6.82
mlev
I .715
53785 6.67
cm
-I
ev
I .67
The D state of Sic1 has been shown to be a %Z+state (6). It therefore seems normal to identify it to the . * (T)~(cI)~(~scT)- 2Z+ configuration, observed in SiF at 47 630 cm-‘. Within this assumption, and from term values, one can calculate, for Sic1 as well as for SiBr, the ionization potential (I.P.) and the quantum defect(s) of the nsa series. The values which have been derived are given in Table III, where they are compared to those of SiF. The evolution of I.P. and G(nsa) from SiF to SiCl and SiBr is quite satisfactory. In SiF, the quantum defect is almost equal to one of the ns series of atomic silicon; in that radical, the Rydberg electrons keep a strong silicon character on account of the high ionization potential of fluorine ( 17.42 eV) compared to 8.15 eV for silicon. In Sic1 and SiBr, this character decreases since, although higher than in atomic silicon, the ionization potential of chlorine and bromine is smaller than that of fluorine (13.01 and 11.85 eV, respectively). If we now take the ionization potential obtained for SiCl and SiBr, we can calculate the quantum defect for the np?r series from the term value of the C211 state. We found 6 = 1.195 for SiCl and 6 = 1.082 for SiBr. The same arguments as those used in the preceding discussion of the nsu series could apply since, in atomic silicon, the quantum defect of the np series has a value between 1.56 and 1.21. This leaves no doubt about the identification of the C state to the 211r state corresponding to the 4p?r configuration. Two Rydberg states, namely the E and F states, remain to be interpreted for SiCl and SiBr. From the diagram of Fig. 1, these states should be, in all probability, related to states of the 3d complex. If we assume these states to be 3dr and 3d6, respectively, we found, from the ionization potentials previously determined, negative quantum defects of the order of -0.35 to -0.50. This is not really surprising since they are indeed slightly negative in SiF (3) and also in atomic silicon. This peculiarity is due to a perturbation between 3s23pnd (n 3 3) and 3s3p3 configurations, as mentioned by Radziemsky and Andrew (9). l
CONCLUSION The observation of four Rydberg states in Sic1 and in SiBr, and the comparison to SiF allow us to identify these states unambiguously to Rydberg configurations, and give the first measurement of the ionization potential of these two radicals.
76
BOSSER, BREDOHL,
AND
DUBOIS
ACKNOWLEDGMENTS We are pleased to acknowledge a grant from the Belgian FRFC (Contracts 2.4554.75 and 1.5.595.83F). Thanks are due to Dr. J. M. Robbe for helpful discussions and critical comments on the manuscript.
RECEIVED: January
3 1, 1984 REFERENCES
1. HOUBRECHTS,Y., DUBOIS, I., AND BREDOHL, H., .I. Phys. B. 15, 603-611 (1982). 2. HOUBRECHTS, Y., DUBOIS, I., AND BREWHL, H. J. Phys. B 15,4551-4560 (1982). 3. ROBBE, J. M., HOUBRECHTS, Y., DUBOIS, I., AND BREWHL, H., to be published. 4. BREDOHL,H., DEMOULIN,PH., HOUBRECHTS,Y., AND M~LEN, F., J. Phys. B 14, 177 1-1776 (198 1). 5. MBLEN, F., HOUBRECHTS,Y., DUBOIS, I., BUI, L,? HUY&N, AND BREJXIHL,H., J. Phys. B 14, 36373642 (1981). 6. BREDOHL,H., CORNET, R., DUBOIS, I., AND MBLEN, F., J. Phys. B 15, 727-730 (1982). 7. BOSSER,G., AND LEBRETON,J., J. Chim. Phys. 75,956-960 (1978). 8. OLDERSHAW,G. A., AND ROBINSON,K. Trans. Farad. Sot. 67, 1870- 1874 (197 1). 9. RADZIEMSKY,L. J., AND ANDREW, K. L., J. Opt. Sot. Amer. 55, 474-491 (1965).
(1984)
Rydberg States of SiBr G. BOSSER Laboratoire de Spectroscopic mokklaire, FacultC des Sciences, Universitk de Tours, Part de Grandmont, 37 Tours, France
AND H. BREDOHL AND I. DUBOIS Institut d’AstrophysiqueS Universiti de Li;ge, B-4200 Ougrke-Liege, Belgium New uv absorption spectra have been observed for SiBr. Five Rydberg states are identified to the states (4x7) ‘Z+, (Go) *Z+, (4pr) *II, (3d?r) *II, and (3d6) ‘A by comparison with SiF and SiCI. The ionization potentials of Sic1 and SiBr have been determined for the first time, and were 6.82 and 6.67 eV, respectively.
INTRODUCTION
Nineteen Rydberg states have been observed and analyzed for the SiF radical (I, 2). Most of them have been classified into Rydberg series; the ionization potential and quantum defects have been obtained (3). For the Sic1 radical, five Rydberg states are known, but they have not been classified into Rydberg series except for the first B %+(4~a) state (4-6). In the case of the SiBr radical, beside the B2Z+, state which has been analyzed by Bosser and Lebreton (7), only low-resolution ultraviolet absorption spectra have been observed by Oldershaw and Robinson (8). New absorption spectra of SiBr have been obtained in the uv region, and the four systems (C, D, E, F - X) mentioned by Oldershaw and Robinson (8) have been confirmed with slightly modified vibrational attributes. By comparing with SiF, it has been possible to identify the Rydberg states of Sic1 and SiBr. The purpose of the present paper is to give a short account of these results. EXPERIMENTALDETAILS The absorption spectra have been obtained by means of the flash discharge techniques in helium with traces of SiBi-,. The capacity used was a 0.05-ELFcapacitor charged to 18 kV. The best delay was found to be about 100 +ec. High-resolution spectra taken in the second order of a 21-ft. Eagle instrument have shown that all the states above the B*Z+ are strongly predissociated. The spectra used for the vibrational analysis (see below) have been obtained on a Hilger Medium quartz spectrograph. The measurements of the heads observed were difficult on account of the diffuse character of the bands, and the accuracy is probably as bad as 5 cm-‘. 0022-2852184 $3.00 Copyright 0
1984by Academic Press, Inc.
All rights of reproduction in any form resewed.
72
RYDBERG
STATES OF SiBr
73
VIBRATIONAL ANALYSIS
Thirty-four violet-degraded bands have been observed for SiBr. They include all of the 24 bands observed by Oldershaw and Robinson (8), except the two at an energy higher than 46 500 cm-’ which have not been observed on our spectra. Twelve new bands have been observed for the first time. These 34 bands have been arranged in four different systems corresponding to those mentioned by Oldershaw and Robinson (8). The vibrational analysis has been extended to higher V’values, and they take only one hot band into account; this one has been assigned to the 0- 1 transition of the D-X system, which is by far the strongest one in this spectral region. The vibrational assignments are given in Table I for the four systems. The assignment was straightforward on account of the spin-orbit coupling constant of the X*II ground state, which was easily observed. The classification of Table I gives the constants shown in Table II for the C, D, E, and F states. The values of the vibrational frequency clearly show these states to be Rydberg states. DISCUSSION In Fig. 1, the first Rydberg states of SiF are compared to those observed for Sic1 and SiBr. The correlation seems to be fairly obvious. The B state of Sic1 and SiBr is known to be the first Rydberg state corresponding to the - . * (?r)4(a)*(4sa) - 2X+ electron configuration, as is the case for SiF. The correlation is equally obvious for the C state, which is the *IIr state coming from the - - * W4W2(4P) - *II, configuration. The evolution of the spin-orbit coupling constant of that state along the sequence is a clear confirmation of this TABLE I Vibrational Analysis (bandheads in cm-‘) -I-
c-x
“l_“-
D-X
E-X
F-X
43255 blended by O-O
O-I
o-o
40690 41105
43675 44100
44135 44555
44570 44990
1-o
41225 41640
44235 44650
44690 45105
45125 45545
2-O
41740 42160
44780 45195
45220 45640
3-O
42250 42670
45310 45730
4-o
42750 43165
45825 46255
blended by 2-O E-X 46085
blended by 3-O D-X 46150
46345
5-O -
-
-
74
BOSSER, BREDOHL, AND DUBOIS TABLE II Molecular Constants (in cm-‘) of the Excited C, D, E, and F States
TO w&?
C
D
E
F
40898
43888
44315
44780
547.5
566
572.5
570
6.25
WeXe A
5.6
IO
20
25
20
7.5 20
identification. Indeed, A = 16 cm-’ in the C’ 211 state of SiF (I) and A = 11 cm-’ in the C’II state of Sic1 (5), and this constant takes the value A N 5 cm-’ in the corresponding state of SiBr (Table II). There is a state in SiF, namely the C” 2Z+ (1) coming from the 0 . Gus X (4pu) - 2Z+ configuration, which has no corresponding state in Sic1 nor in SiBr. This situation is easily understood since the C” 2Z+ state of SiF has been discovered (1) by means of the transition to the A2Z+ state in the visible; the C”-X transition to the ground state being extremely weak. 60
55
7 E _:: 50 _z z LL w z 45 w
40 Si
F
SI Cl
Si Br
35,-
FlG. 1. Correlation diagram for the Rydberg states of SiF, SiCl, and SiBr.
75
RYDBERGSTATES OF SiBr TABLEIII IonizationPotentialand QuantumDefectfor the nsu Series SiF
Sic1
IP
58960 7.31
5 (nsa)
SiBr
I
I
I .869
cm ev
-1
I
I
55017 6.82
mlev
I .715
53785 6.67
cm
-I
ev
I .67
The D state of Sic1 has been shown to be a %Z+state (6). It therefore seems normal to identify it to the . * (T)~(cI)~(~scT)- 2Z+ configuration, observed in SiF at 47 630 cm-‘. Within this assumption, and from term values, one can calculate, for Sic1 as well as for SiBr, the ionization potential (I.P.) and the quantum defect(s) of the nsa series. The values which have been derived are given in Table III, where they are compared to those of SiF. The evolution of I.P. and G(nsa) from SiF to SiCl and SiBr is quite satisfactory. In SiF, the quantum defect is almost equal to one of the ns series of atomic silicon; in that radical, the Rydberg electrons keep a strong silicon character on account of the high ionization potential of fluorine ( 17.42 eV) compared to 8.15 eV for silicon. In Sic1 and SiBr, this character decreases since, although higher than in atomic silicon, the ionization potential of chlorine and bromine is smaller than that of fluorine (13.01 and 11.85 eV, respectively). If we now take the ionization potential obtained for SiCl and SiBr, we can calculate the quantum defect for the np?r series from the term value of the C211 state. We found 6 = 1.195 for SiCl and 6 = 1.082 for SiBr. The same arguments as those used in the preceding discussion of the nsu series could apply since, in atomic silicon, the quantum defect of the np series has a value between 1.56 and 1.21. This leaves no doubt about the identification of the C state to the 211r state corresponding to the 4p?r configuration. Two Rydberg states, namely the E and F states, remain to be interpreted for SiCl and SiBr. From the diagram of Fig. 1, these states should be, in all probability, related to states of the 3d complex. If we assume these states to be 3dr and 3d6, respectively, we found, from the ionization potentials previously determined, negative quantum defects of the order of -0.35 to -0.50. This is not really surprising since they are indeed slightly negative in SiF (3) and also in atomic silicon. This peculiarity is due to a perturbation between 3s23pnd (n 3 3) and 3s3p3 configurations, as mentioned by Radziemsky and Andrew (9). l
CONCLUSION The observation of four Rydberg states in Sic1 and in SiBr, and the comparison to SiF allow us to identify these states unambiguously to Rydberg configurations, and give the first measurement of the ionization potential of these two radicals.
76
BOSSER, BREDOHL,
AND
DUBOIS
ACKNOWLEDGMENTS We are pleased to acknowledge a grant from the Belgian FRFC (Contracts 2.4554.75 and 1.5.595.83F). Thanks are due to Dr. J. M. Robbe for helpful discussions and critical comments on the manuscript.
RECEIVED: January
3 1, 1984 REFERENCES
1. HOUBRECHTS,Y., DUBOIS, I., AND BREDOHL, H., .I. Phys. B. 15, 603-611 (1982). 2. HOUBRECHTS, Y., DUBOIS, I., AND BREWHL, H. J. Phys. B 15,4551-4560 (1982). 3. ROBBE, J. M., HOUBRECHTS, Y., DUBOIS, I., AND BREWHL, H., to be published. 4. BREDOHL,H., DEMOULIN,PH., HOUBRECHTS,Y., AND M~LEN, F., J. Phys. B 14, 177 1-1776 (198 1). 5. MBLEN, F., HOUBRECHTS,Y., DUBOIS, I., BUI, L,? HUY&N, AND BREJXIHL,H., J. Phys. B 14, 36373642 (1981). 6. BREDOHL,H., CORNET, R., DUBOIS, I., AND MBLEN, F., J. Phys. B 15, 727-730 (1982). 7. BOSSER,G., AND LEBRETON,J., J. Chim. Phys. 75,956-960 (1978). 8. OLDERSHAW,G. A., AND ROBINSON,K. Trans. Farad. Sot. 67, 1870- 1874 (197 1). 9. RADZIEMSKY,L. J., AND ANDREW, K. L., J. Opt. Sot. Amer. 55, 474-491 (1965).