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Rydberg States of SiCl
Journal of Molecular Spectroscopy 197, 28 –31 (1999) Article ID jmsp.1999.7881, available online at http://www.idealibrary.com on
Rydberg States of SiCl H. Bredohl,* J. Breton,† ,‡ I. Dubois,* J. M. Esteva,† ,§ D. Macau-Hercot,* ,1 and F. Remy* *Institut d’Astrophysique et de Ge´ophysique–Universite´ de Lie`ge, B-4000 Lie`ge, Belgium; †L.U.R.E., Baˆtiment 209 D, Universite´ de Paris Sud, F-91405, Orsay, France; ‡Equipe de Spectroscopie VUV, Universite´ P. et M. Curie, Case 81, 4, Place Jussieu, F-75252, Paris, France; and §Laboratoire de Spectroscopie Atomique et Ionique, Universite´ de Paris Sud, Baˆtiment 350, F-91405, Orsay, France Received November 11, 1998; in revised form April 29, 1999
A fluorescence spectrum, excited by the synchrotron radiation of Superaco in a microwave discharge through He 1 SiCl 4, leads to the observation of five new electronic states of SiCl, converging to the ground X 1 S 1 state of the SiCl 1 ion. The (n s s ) and (n p p ) Rydberg series are extended respectively to n 5 6 and n 5 7. A new ionization limit is observed in absorption at 11.238 eV corresponding to the a 3 P electronic state of SiCl 1. Four new states are observed which converge to that limit. © 1999 Academic Press
A 2 S 1 and the B9 2 D r states are valence states. The other five known states are Rydberg states. The first ionization potential corresponding to the X 1 S 1 ground state of the SiCl 1 ion has been given by Bosser et al. (3), who determined IP 5 6.82 eV from only the two Rydberg states B 2 S 1 (4 s s ) and E 2 S 1 (5 s s ). Of course, given the shortness of the extrapolation, this value must be considered cautiously. Weber and Armentrout (4) reported a value of 7.44 eV, based on reaction threshold energy. Two calculations by Dewar and Lie (5) and Hastie and Margrave (6) yield 7.36 eV and 7.53 eV, respectively. Recently, Marijnissen and ter Meulen (7), by mass selected
INTRODUCTION
The first observation of a SiCl spectrum traces back to 1914 (1). Since then, many studies, using different techniques, contributed to the knowledge of the SiCl radical. An extensive study has been done by Me´len (2), who gives a fairly complete view of the results concerning the electronic states of SiCl up to 1988. Table 1 gives the results obtained for the eight known electronic states of SiCl (2). Besides the ground state, only the
1
Research Associate FNRS.
FIG. 1. The fluorescence spectrum of SiCl between 3000 and 1700 Å. The bands marked in this figure appear on top of the synchrotron radiation continuum. 28 0022-2852/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
RYDBERG STATES OF SiCl
TABLE 1 Electronic States, Molecular Constants, and Electron Configuration of SiCl
photo-ionization efficiency spectroscopy, gave a very precise value of 7.3296(14) eV. Concerning SiCl 1, theoretical calculations by Nishimura et al. (8) leads to the conclusion that besides the ground X 1 S 1 state, the only stable state is the first excited a 3 P state. In the present work, we report the observation of new Rydberg states as well as the observation of a new ionization limit of SiCl, corresponding to the a 3 P state of SiCl 1, observed by Tsuji et al. (9). EXPERIMENTAL DETAILS
The experimental setup has been described elsewhere in an earlier paper concerning CCl (10). The 2450 MHz microwave cavity can be placed on the axial part of the discharge tube or at different positions on a lateral tube. The 3-m Balzers normal incidence monochromator with a 300-lines/mm grating was used on the SA 61 synchrotron beam line of Superaco to disperse the radiation entering the discharge, while a 0.5-m Seya–Namioka monochromator with a 1200-lines/mm grating was used to record the spectrum at the exit of the discharge. The 3-m monochromator with a 300lines/mm grating is a medium-resolution spectrograph (reciprocal dispersion 7.5 Å/mm). Moreover, since three toroidal mirrors and two plane mirrors are necessary to conduct the light through the absorbing medium, wide slits ('400 mm) have to be used to keep a sufficient photon flux. The role of the Seya–Namioka monochromator is mainly to filter the light emitted by the discharge. The width of the slits is the same as that of the 3-m monochromator. The resulting resolution is consequently very poor and the precision of the wavelength measurements is of course seriously affected and estimated to 4 Å.
FIG. 2. Energy levels diagram of SiCl including the new states.
Copyright © 1999 by Academic Press
29
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BREDOHL ET AL.
TABLE 2 The (n s s) and (n p p) Rydberg Series and the Results of the Fitting to the Rydberg Formula
SiCl was prepared by flowing helium in the discharge with traces of SiCl 4 (Aldrich, 99.999%) vapor at room temperature.
TABLE 3 The Rydberg Series Converging to the a 3P State of SiCl 1 and the Result of the Fitting to the Rydberg Formula
OBSERVED SPECTRUM
We first observed the direct emission spectrum with the cavity on the axial part of the tube and without synchrotron radiation passing through the discharge. We obtained, as expected, the known spectrum of SiCl: the B 3 X, B9 3 X, C 3 X, E 3 X, and D 3 X transitions, as well as the strongest SiI atomic lines at 2881 and a group of six overlapping lines (2528, 2524, 2519, 2516, 2514, 2506 Å). When the synchrotron radiation is sent through the emitting plasma, the emission spectrum is observed on top of the continuum but is strongly extended down to a shorter waveCopyright © 1999 by Academic Press
31
RYDBERG STATES OF SiCl
length very likely due to fluorescence excited by the synchrotron radiation. When the cavity is placed on the lateral tube, the emission of the microwave discharge can be suppressed in direct view of the Seya–Namioka monochromator, and some absorption due to SiCl is observed down to the O 2 threshold at 1027 Å. DISCUSSION
The fluorescence spectrum is shown on Fig. 1 from 3000 to 1700 Å. Two Rydberg series are indicated: the (n s s ) series, starting at the B 2 S 1 (4 s s ) state, and the (n p p ) series, starting at the C 2 P (4 p p ) state. The B9( 2 D) 3 X transition, the strongest one, is also visible. It is overlapped by the strong 2881 Å atomic lines of SiI. The D–X transition is also present but, being given the poor resolution used, is partly overlapped by the weak E 3 X transition. The F 3 X transition is also present. The strong emission around 2520 Å is the envelope of six strong lines of SiI, as mentioned above. The E 2 S 1 and F 2 S 1 states (2), were identified as the (5 s s ) 2 S 1 and (3 d s ) 2 S 1 Rydberg states, respectively, by comparison with SiF (11). Two new states at 48 077 and 52 219 cm 21 are in fact the (5 s s ) and (6 s s ) states, respectively. The (n p p ) series is observed from the C 2 P (4 p p ) state (2) to the (7 p p ) state. Three states at 44 444, 44 643, and 46 512 cm 21 are identified to the (3 d s ), (3 d p ), and (3 d d ) states as confirmed by their low value of the quantum defect (see Table 3). Finally, a weak band at 43 010 cm 21 cannot be explained by any Rydberg transition and is still not interpreted. It could be a valence state but the final identification would request a rotational analysis. Five new electronic states have been discovered in the present study, allowing the extension of two Rydberg series, up to n 5 6 for the n(s s ) series and up to n 5 7 for the (n p p ) series. Figure 2 gives an energy level diagram of all the known electronic states of SiCl, converging to the X 1 S 1 ground state of SiCl 1. When the Rydberg states are fitted to the usual Rydberg formula n 5 IE 2 R/n* 2 , where IE is the ionization energy, R the Rydberg constant, and n* 5 (n 2 d ), we obtain the values given in Table 2. The ionization potential is in agreement with the value obtained by Marijnissen and ter Meulen (7) within 1.4 standard deviation. The absorption spectrum observed with the discharge in the lateral tube shows an unreported ionization limit at 1103 Å, that is to say, 90 662 cm 21 or 11.24 eV corresponding closely to the a 3 P electronic state of SiCl 1 observed by Tsuji et al. (9).
At lower energy, four bands are observed at 1143, 1165, 1211, and 1311 Å. These bands seem to converge to the ionization limit at 1103 Å and must be Rydberg states of SiCl corresponding to the electron configuration KLKL ( z s ) 2 ( y s ) 2 (w p ) 4 ( x s ) (v p ) (nlm 1 ), giving rise to 2P or 2S 1 states. When these four bands are fitted to the Rydberg formula, we obtain the results given in Table 3. The difference between 59 433 cm 21 (X 1 S 1 ) and 90 662 cm 21 (a 3 P) is 31 229 cm 21 near the a 3 P 0 3 X 1 S 1 transition observed by Tsuji et al. (9) at 31 721 cm 21. CONCLUSION
The present study of the fluorescence and absorption spectrum of a microwave discharge through He 1 SiCl 4 allows the identification of five new electronic states of SiCl: two 2S 1 and three 2P states which extend the Rydberg series to n 5 6 for the (n s s ) series and to n 5 7 for the (n p p ) series. This yields a value of the ionization potential in very good agreement with the precise value determined by Marijnissen and ter Meulen (7). The identifications proposed by Me´len (2) have undergone some modifications: the D state is not attributed to the 2 S 1 (3 d s ) state, the E state to the 2 P (3 d p ) state, and the F state to the 2 D (3 d s ) state. The ionization limit corresponding to the a 3 P electronic state of SiCl 1 has been observed for the first time and four Rydberg states have been observed, converging to this ionization limit. ACKNOWLEDGMENTS We are pleased to thank the FNRS in Belgium, the CNRS in France, and the European Union through the “Training and Mobility of Researchers” (TMR) Programme for financial help and facilities.
REFERENCES 1. W. Jevons, Proc. R. Soc. London A 81, 187–193 (1914). 2. F. Me´len, Ph.D. thesis, Universite´ de Lie`ge, Belgique, 1988. 3. G. Bosser, H. Bredohl, and I. Dubois, J. Mol. Spectrosc. 106, 72–76 (1984). 4. M. E. Weber and P. B. Armentrout, J. Phys. Chem. 93, 1596 –1604 (1989). 5. J. S. Dewar and C. Lie, Organometallics 6, 1486 –1490 (1987). 6. J. W. Hastie and J. L. Margrave, J. Phys. Chem. 73, 1105–1116 (1969). 7. A. Marijnissen and J. J. ter Meulen, Chem. Phys. Lett. 263, 803– 810 (1996). 8. Y. Nishimura, T. Mizuguchi, M. Tsuji, S. Obara, and K. Morokuma, J. Chem. Phys. 78, 7260 –7264 (1983). 9. M. Tsuji, T. Mizuguchi, and Y. Nishimuro, Can. J. Phys. 59, 985–989 (1981). 10. F. Remy, D. Macau-Hercot, I. Dubois, H. Bredohl, E. Some´, and J. Breton, J. Mol. Spectrosc. 169, 440 – 444 (1995).
Copyright © 1999 by Academic Press
Rydberg States of SiCl H. Bredohl,* J. Breton,† ,‡ I. Dubois,* J. M. Esteva,† ,§ D. Macau-Hercot,* ,1 and F. Remy* *Institut d’Astrophysique et de Ge´ophysique–Universite´ de Lie`ge, B-4000 Lie`ge, Belgium; †L.U.R.E., Baˆtiment 209 D, Universite´ de Paris Sud, F-91405, Orsay, France; ‡Equipe de Spectroscopie VUV, Universite´ P. et M. Curie, Case 81, 4, Place Jussieu, F-75252, Paris, France; and §Laboratoire de Spectroscopie Atomique et Ionique, Universite´ de Paris Sud, Baˆtiment 350, F-91405, Orsay, France Received November 11, 1998; in revised form April 29, 1999
A fluorescence spectrum, excited by the synchrotron radiation of Superaco in a microwave discharge through He 1 SiCl 4, leads to the observation of five new electronic states of SiCl, converging to the ground X 1 S 1 state of the SiCl 1 ion. The (n s s ) and (n p p ) Rydberg series are extended respectively to n 5 6 and n 5 7. A new ionization limit is observed in absorption at 11.238 eV corresponding to the a 3 P electronic state of SiCl 1. Four new states are observed which converge to that limit. © 1999 Academic Press
A 2 S 1 and the B9 2 D r states are valence states. The other five known states are Rydberg states. The first ionization potential corresponding to the X 1 S 1 ground state of the SiCl 1 ion has been given by Bosser et al. (3), who determined IP 5 6.82 eV from only the two Rydberg states B 2 S 1 (4 s s ) and E 2 S 1 (5 s s ). Of course, given the shortness of the extrapolation, this value must be considered cautiously. Weber and Armentrout (4) reported a value of 7.44 eV, based on reaction threshold energy. Two calculations by Dewar and Lie (5) and Hastie and Margrave (6) yield 7.36 eV and 7.53 eV, respectively. Recently, Marijnissen and ter Meulen (7), by mass selected
INTRODUCTION
The first observation of a SiCl spectrum traces back to 1914 (1). Since then, many studies, using different techniques, contributed to the knowledge of the SiCl radical. An extensive study has been done by Me´len (2), who gives a fairly complete view of the results concerning the electronic states of SiCl up to 1988. Table 1 gives the results obtained for the eight known electronic states of SiCl (2). Besides the ground state, only the
1
Research Associate FNRS.
FIG. 1. The fluorescence spectrum of SiCl between 3000 and 1700 Å. The bands marked in this figure appear on top of the synchrotron radiation continuum. 28 0022-2852/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.
RYDBERG STATES OF SiCl
TABLE 1 Electronic States, Molecular Constants, and Electron Configuration of SiCl
photo-ionization efficiency spectroscopy, gave a very precise value of 7.3296(14) eV. Concerning SiCl 1, theoretical calculations by Nishimura et al. (8) leads to the conclusion that besides the ground X 1 S 1 state, the only stable state is the first excited a 3 P state. In the present work, we report the observation of new Rydberg states as well as the observation of a new ionization limit of SiCl, corresponding to the a 3 P state of SiCl 1, observed by Tsuji et al. (9). EXPERIMENTAL DETAILS
The experimental setup has been described elsewhere in an earlier paper concerning CCl (10). The 2450 MHz microwave cavity can be placed on the axial part of the discharge tube or at different positions on a lateral tube. The 3-m Balzers normal incidence monochromator with a 300-lines/mm grating was used on the SA 61 synchrotron beam line of Superaco to disperse the radiation entering the discharge, while a 0.5-m Seya–Namioka monochromator with a 1200-lines/mm grating was used to record the spectrum at the exit of the discharge. The 3-m monochromator with a 300lines/mm grating is a medium-resolution spectrograph (reciprocal dispersion 7.5 Å/mm). Moreover, since three toroidal mirrors and two plane mirrors are necessary to conduct the light through the absorbing medium, wide slits ('400 mm) have to be used to keep a sufficient photon flux. The role of the Seya–Namioka monochromator is mainly to filter the light emitted by the discharge. The width of the slits is the same as that of the 3-m monochromator. The resulting resolution is consequently very poor and the precision of the wavelength measurements is of course seriously affected and estimated to 4 Å.
FIG. 2. Energy levels diagram of SiCl including the new states.
Copyright © 1999 by Academic Press
29
30
BREDOHL ET AL.
TABLE 2 The (n s s) and (n p p) Rydberg Series and the Results of the Fitting to the Rydberg Formula
SiCl was prepared by flowing helium in the discharge with traces of SiCl 4 (Aldrich, 99.999%) vapor at room temperature.
TABLE 3 The Rydberg Series Converging to the a 3P State of SiCl 1 and the Result of the Fitting to the Rydberg Formula
OBSERVED SPECTRUM
We first observed the direct emission spectrum with the cavity on the axial part of the tube and without synchrotron radiation passing through the discharge. We obtained, as expected, the known spectrum of SiCl: the B 3 X, B9 3 X, C 3 X, E 3 X, and D 3 X transitions, as well as the strongest SiI atomic lines at 2881 and a group of six overlapping lines (2528, 2524, 2519, 2516, 2514, 2506 Å). When the synchrotron radiation is sent through the emitting plasma, the emission spectrum is observed on top of the continuum but is strongly extended down to a shorter waveCopyright © 1999 by Academic Press
31
RYDBERG STATES OF SiCl
length very likely due to fluorescence excited by the synchrotron radiation. When the cavity is placed on the lateral tube, the emission of the microwave discharge can be suppressed in direct view of the Seya–Namioka monochromator, and some absorption due to SiCl is observed down to the O 2 threshold at 1027 Å. DISCUSSION
The fluorescence spectrum is shown on Fig. 1 from 3000 to 1700 Å. Two Rydberg series are indicated: the (n s s ) series, starting at the B 2 S 1 (4 s s ) state, and the (n p p ) series, starting at the C 2 P (4 p p ) state. The B9( 2 D) 3 X transition, the strongest one, is also visible. It is overlapped by the strong 2881 Å atomic lines of SiI. The D–X transition is also present but, being given the poor resolution used, is partly overlapped by the weak E 3 X transition. The F 3 X transition is also present. The strong emission around 2520 Å is the envelope of six strong lines of SiI, as mentioned above. The E 2 S 1 and F 2 S 1 states (2), were identified as the (5 s s ) 2 S 1 and (3 d s ) 2 S 1 Rydberg states, respectively, by comparison with SiF (11). Two new states at 48 077 and 52 219 cm 21 are in fact the (5 s s ) and (6 s s ) states, respectively. The (n p p ) series is observed from the C 2 P (4 p p ) state (2) to the (7 p p ) state. Three states at 44 444, 44 643, and 46 512 cm 21 are identified to the (3 d s ), (3 d p ), and (3 d d ) states as confirmed by their low value of the quantum defect (see Table 3). Finally, a weak band at 43 010 cm 21 cannot be explained by any Rydberg transition and is still not interpreted. It could be a valence state but the final identification would request a rotational analysis. Five new electronic states have been discovered in the present study, allowing the extension of two Rydberg series, up to n 5 6 for the n(s s ) series and up to n 5 7 for the (n p p ) series. Figure 2 gives an energy level diagram of all the known electronic states of SiCl, converging to the X 1 S 1 ground state of SiCl 1. When the Rydberg states are fitted to the usual Rydberg formula n 5 IE 2 R/n* 2 , where IE is the ionization energy, R the Rydberg constant, and n* 5 (n 2 d ), we obtain the values given in Table 2. The ionization potential is in agreement with the value obtained by Marijnissen and ter Meulen (7) within 1.4 standard deviation. The absorption spectrum observed with the discharge in the lateral tube shows an unreported ionization limit at 1103 Å, that is to say, 90 662 cm 21 or 11.24 eV corresponding closely to the a 3 P electronic state of SiCl 1 observed by Tsuji et al. (9).
At lower energy, four bands are observed at 1143, 1165, 1211, and 1311 Å. These bands seem to converge to the ionization limit at 1103 Å and must be Rydberg states of SiCl corresponding to the electron configuration KLKL ( z s ) 2 ( y s ) 2 (w p ) 4 ( x s ) (v p ) (nlm 1 ), giving rise to 2P or 2S 1 states. When these four bands are fitted to the Rydberg formula, we obtain the results given in Table 3. The difference between 59 433 cm 21 (X 1 S 1 ) and 90 662 cm 21 (a 3 P) is 31 229 cm 21 near the a 3 P 0 3 X 1 S 1 transition observed by Tsuji et al. (9) at 31 721 cm 21. CONCLUSION
The present study of the fluorescence and absorption spectrum of a microwave discharge through He 1 SiCl 4 allows the identification of five new electronic states of SiCl: two 2S 1 and three 2P states which extend the Rydberg series to n 5 6 for the (n s s ) series and to n 5 7 for the (n p p ) series. This yields a value of the ionization potential in very good agreement with the precise value determined by Marijnissen and ter Meulen (7). The identifications proposed by Me´len (2) have undergone some modifications: the D state is not attributed to the 2 S 1 (3 d s ) state, the E state to the 2 P (3 d p ) state, and the F state to the 2 D (3 d s ) state. The ionization limit corresponding to the a 3 P electronic state of SiCl 1 has been observed for the first time and four Rydberg states have been observed, converging to this ionization limit. ACKNOWLEDGMENTS We are pleased to thank the FNRS in Belgium, the CNRS in France, and the European Union through the “Training and Mobility of Researchers” (TMR) Programme for financial help and facilities.
REFERENCES 1. W. Jevons, Proc. R. Soc. London A 81, 187–193 (1914). 2. F. Me´len, Ph.D. thesis, Universite´ de Lie`ge, Belgique, 1988. 3. G. Bosser, H. Bredohl, and I. Dubois, J. Mol. Spectrosc. 106, 72–76 (1984). 4. M. E. Weber and P. B. Armentrout, J. Phys. Chem. 93, 1596 –1604 (1989). 5. J. S. Dewar and C. Lie, Organometallics 6, 1486 –1490 (1987). 6. J. W. Hastie and J. L. Margrave, J. Phys. Chem. 73, 1105–1116 (1969). 7. A. Marijnissen and J. J. ter Meulen, Chem. Phys. Lett. 263, 803– 810 (1996). 8. Y. Nishimura, T. Mizuguchi, M. Tsuji, S. Obara, and K. Morokuma, J. Chem. Phys. 78, 7260 –7264 (1983). 9. M. Tsuji, T. Mizuguchi, and Y. Nishimuro, Can. J. Phys. 59, 985–989 (1981). 10. F. Remy, D. Macau-Hercot, I. Dubois, H. Bredohl, E. Some´, and J. Breton, J. Mol. Spectrosc. 169, 440 – 444 (1995).
Copyright © 1999 by Academic Press