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Research in progress and program of development in Liège
J. Quanf. Spcefrosc. Radiat. Transfrr. Vol. 2, pp. 321425.
Pergamon Press Ltd., Printed in Great Britain.
RESEARCH IN PROGRESS AND PROGRAM DEVELOPMENT IN L&GE
OF
A. MON~LS and B. ROSEN Astrophysical Institute of the University of Li*ge, LiBge,
Belgium I. THE main apparatus for our research in the vacuum ultraviolet consists of a 3 m focal length spectrograph in the Paschen mounting, working at normal incidence with a 15,000 In/in. grating. Spectra can be taken between 700 and 4500 A in the first order. Up to 8 spectra can be recorded on one 12 in. plate covering about 1500 A. The grating now available gives a resolving power of more than 50,000 at 3000 A, which is near the theoretical value. We have just received from BAUSCH and LOMB a new 30,000 In/in. grating, which not only will increase the resolving power of the 1st order, but which will allow us to use a second slit (situated on the other side of the used part of the Rowland circle) which has been designed in order to make the 1000-2000 A region available in the second order. 2.
The construction of a 6 m spectrograph in Eagle mounting is presently under way. The main features of this instrument are similar to those of the 10 m one mounted in Dr. Douglas’ laboratory in Ottawa. The spectrum is kept horizontal, the optics completely coplanar, and the volume is limited by the use of an oblique entry, the slit being separated from the main tank. We hope that the spectrograph will be in working order within 18 months. In order to extend the spectrum down to very’short wavelength, to 50 A if possible, we have introduced a request for financial support for the construction of a grazing incidence spectrograph. 3.
One of our main problems is the development of new sources. We have built recently a high-frequency source for electrodeless excitation, which gives pulses of 2, 1 and 05 p sec. Preliminary experiments have shown that the Nitrogen afterglow is excited with very high intensity. We intend to use it for the detailed study of various kinds of postluminescence. We expect that the very short duration of the pulses will enable us to study the first phases of the afterglow and to give thus new spectroscopic information for the theoretical interpretation of these complex phenomena. Simultaneously with the development of the high-frequency pulse generator, we are developing various kinds of nonconventional sources, including a high temperature hollow cathode for the study of the role played by the thermal conditions in discharges 321
322
A. MONFILS and B. ROSEN
and, in particular, for the study of carbon reactions at high temperatures (up to 2500°C). We hope also to complete before the end of this year an installation for flash photolysis in the vacuum ultraviolet. It seems possible that the pulse generator could be used in combination with flash photolysis and will enable the simultaneous investigation of the fluorescence and of the production of transient species. 4.
The first investigation which we have undertaken with the 3 m instrument concerned the study of the spectrum of SOs in the 2200-1750 A region. Previous investigations by Duchesne and Rosen using a quartz spectrograph have led to the arrangement of the numerous bands in this region into 4 different systems. This arrangement however has been criticized by Dr. Coon and his co-workers, who seem to assume that the whole spectrum is due to a single transition. The question is of importance first because there are some indications that the symmetry of the molecule changes by excitation and, secondly, because the existence of several transitions in this region is strongly supported by theoretical considerations (Walsh, Mulliken). The results of the new investigation in Liege by Dubois and Rosen are still preliminary; strong support has been given for the existence of at least 3 and probably 4 different transitions, and the constants given previously have been improved. The dispersion of our actual spectrograph is not sufficient, however, and the investigation has been resumed with the aid of new high dispersion spectra of SOi and SOsl* which have been obtained in Dr. Douglas’ laboratory in Ottawa with a 10 m grating which Dr. Douglas kindly put at our disposal. The first results seem to confirm our previous assumption. We hope to be able to obtain very soon more decisive results. 5. One of us (A.M.) has studied for several years the vacuum ultraviolet absorption spectrum of Hz. This work is based mainly on spectra recorded in Dr. Herzberg’s laboratories in Ottawa, but spectra of Ha, mainly in emission, have also been recorded by our 3 m spectrograph. The study may be divided into two parts: (1) BlZuf f
Xl&+,
and Cfl-Iu+
Xl&+
transitions
The C and C states had previously been studied only for the low quantum number vibrational states, because of a perturbation existing between the two excited states. The perturbations have now been studied and the analysis extended to higher vibrational states. It has been possible to approach very near to the dissociation limit, and a few states only have been left unassigned, mainly because of the high density of lines in the region. The vibrational study has been completed by an investigation of corresponding spectra of DZ and HD. The isotopic effect of the perturbation constant has been compared to Kronig’s formula and found in qualitative agreement. The deperturbation of the C state has been made, which allowed us to compute the “normal” A-doubling. The effect, as well as the isotopic relation, has been found to be in qualitative agreement with Van Vleck’s formulas. (2) Rydberg series We have increased the number of i&f (np) and lIIu (np) states from 3 previously known to seven, i.e. lZy+ (2p), llIU (2p), l&+ (3p),lII, (3p), I&+ (4&i& (4~) and’& (5~). A new interesting result of this study is that the DIIIr (3~) state, which seemed to account
Research in progress and program of development in Like
323
for the bulk of the absorption spectrum below 850 A, accounts in fact for only a very small part of it. Strongly predissociated, the lines decrease rapidly in intensity, so that the rotational constants of the c sub-states are very difficult to obtain. Only the Q branches, which are not predissociated, are easy to assign. The other systems become less and less intense, especially for the ZU+ states. The preionization found by BEUTLER (1) in the v’ = 6 level of the DrlL term could not be confirmed. A very careful study of the band shows, in the 7th order, a small blurr, but this may well be due to the superposition of the R(0) and R( 1) line of the B" lx,+ (4~) + X and D rII, (3~) + X transitions, which in Hz are almost coincident. The rotational study of these bands is now under way.
FIG. 1.
B.
324
A.Rosa~
MONFILSL&
Vibrational results related to the B, B’, B”, D and D’ terms have been published recently@). Data concerning the D” llTu (5~) are given in Table 1. TABLE1. ELECTRONIC TERMS AND VIBRATIONAL CONSTANTS
T.
(cm-l)
We
(cm-l)
OF THE D”
(5~) STATE
wxe
(cm-l)
2
121227.17 121 215.06 2011.9 2320.8
49-3 64.7
Da
12122544
32.7
1648.9
‘II,
Figure 1 shows, as an example, how the wedata found for HZ converge towards the value accepted for H,+ by HERZBERG@). Quantum defects have been computed, which have been found to be positive for the lZ, + states and negative for the llTUstates (Fig. 2).
In-n*
FIG. 2.
Research in progress and program of development in Liege
325
The rotational analysis is well under way and more than one hundred bands have been assigned. to several systems. The data are collected in order to prepare a programme for a computer. The developments reported here are part of the programme of the Institute of Astrophysics of the University of Liege in the field of space research.
REFERENCES H. REIJTLER, H. 0. JUNGER,Z. Phys. 100, 81 (1936). 2. A. MONFILS, Bull. Acad. Belg. Cl. Si. 47, 585, 599, 816 (1961). 3. G. HERZBER~, Spectra of Diatomic Molecules Van Nostrand, New York (1950). 1.
Pergamon Press Ltd., Printed in Great Britain.
RESEARCH IN PROGRESS AND PROGRAM DEVELOPMENT IN L&GE
OF
A. MON~LS and B. ROSEN Astrophysical Institute of the University of Li*ge, LiBge,
Belgium I. THE main apparatus for our research in the vacuum ultraviolet consists of a 3 m focal length spectrograph in the Paschen mounting, working at normal incidence with a 15,000 In/in. grating. Spectra can be taken between 700 and 4500 A in the first order. Up to 8 spectra can be recorded on one 12 in. plate covering about 1500 A. The grating now available gives a resolving power of more than 50,000 at 3000 A, which is near the theoretical value. We have just received from BAUSCH and LOMB a new 30,000 In/in. grating, which not only will increase the resolving power of the 1st order, but which will allow us to use a second slit (situated on the other side of the used part of the Rowland circle) which has been designed in order to make the 1000-2000 A region available in the second order. 2.
The construction of a 6 m spectrograph in Eagle mounting is presently under way. The main features of this instrument are similar to those of the 10 m one mounted in Dr. Douglas’ laboratory in Ottawa. The spectrum is kept horizontal, the optics completely coplanar, and the volume is limited by the use of an oblique entry, the slit being separated from the main tank. We hope that the spectrograph will be in working order within 18 months. In order to extend the spectrum down to very’short wavelength, to 50 A if possible, we have introduced a request for financial support for the construction of a grazing incidence spectrograph. 3.
One of our main problems is the development of new sources. We have built recently a high-frequency source for electrodeless excitation, which gives pulses of 2, 1 and 05 p sec. Preliminary experiments have shown that the Nitrogen afterglow is excited with very high intensity. We intend to use it for the detailed study of various kinds of postluminescence. We expect that the very short duration of the pulses will enable us to study the first phases of the afterglow and to give thus new spectroscopic information for the theoretical interpretation of these complex phenomena. Simultaneously with the development of the high-frequency pulse generator, we are developing various kinds of nonconventional sources, including a high temperature hollow cathode for the study of the role played by the thermal conditions in discharges 321
322
A. MONFILS and B. ROSEN
and, in particular, for the study of carbon reactions at high temperatures (up to 2500°C). We hope also to complete before the end of this year an installation for flash photolysis in the vacuum ultraviolet. It seems possible that the pulse generator could be used in combination with flash photolysis and will enable the simultaneous investigation of the fluorescence and of the production of transient species. 4.
The first investigation which we have undertaken with the 3 m instrument concerned the study of the spectrum of SOs in the 2200-1750 A region. Previous investigations by Duchesne and Rosen using a quartz spectrograph have led to the arrangement of the numerous bands in this region into 4 different systems. This arrangement however has been criticized by Dr. Coon and his co-workers, who seem to assume that the whole spectrum is due to a single transition. The question is of importance first because there are some indications that the symmetry of the molecule changes by excitation and, secondly, because the existence of several transitions in this region is strongly supported by theoretical considerations (Walsh, Mulliken). The results of the new investigation in Liege by Dubois and Rosen are still preliminary; strong support has been given for the existence of at least 3 and probably 4 different transitions, and the constants given previously have been improved. The dispersion of our actual spectrograph is not sufficient, however, and the investigation has been resumed with the aid of new high dispersion spectra of SOi and SOsl* which have been obtained in Dr. Douglas’ laboratory in Ottawa with a 10 m grating which Dr. Douglas kindly put at our disposal. The first results seem to confirm our previous assumption. We hope to be able to obtain very soon more decisive results. 5. One of us (A.M.) has studied for several years the vacuum ultraviolet absorption spectrum of Hz. This work is based mainly on spectra recorded in Dr. Herzberg’s laboratories in Ottawa, but spectra of Ha, mainly in emission, have also been recorded by our 3 m spectrograph. The study may be divided into two parts: (1) BlZuf f
Xl&+,
and Cfl-Iu+
Xl&+
transitions
The C and C states had previously been studied only for the low quantum number vibrational states, because of a perturbation existing between the two excited states. The perturbations have now been studied and the analysis extended to higher vibrational states. It has been possible to approach very near to the dissociation limit, and a few states only have been left unassigned, mainly because of the high density of lines in the region. The vibrational study has been completed by an investigation of corresponding spectra of DZ and HD. The isotopic effect of the perturbation constant has been compared to Kronig’s formula and found in qualitative agreement. The deperturbation of the C state has been made, which allowed us to compute the “normal” A-doubling. The effect, as well as the isotopic relation, has been found to be in qualitative agreement with Van Vleck’s formulas. (2) Rydberg series We have increased the number of i&f (np) and lIIu (np) states from 3 previously known to seven, i.e. lZy+ (2p), llIU (2p), l&+ (3p),lII, (3p), I&+ (4&i& (4~) and’& (5~). A new interesting result of this study is that the DIIIr (3~) state, which seemed to account
Research in progress and program of development in Like
323
for the bulk of the absorption spectrum below 850 A, accounts in fact for only a very small part of it. Strongly predissociated, the lines decrease rapidly in intensity, so that the rotational constants of the c sub-states are very difficult to obtain. Only the Q branches, which are not predissociated, are easy to assign. The other systems become less and less intense, especially for the ZU+ states. The preionization found by BEUTLER (1) in the v’ = 6 level of the DrlL term could not be confirmed. A very careful study of the band shows, in the 7th order, a small blurr, but this may well be due to the superposition of the R(0) and R( 1) line of the B" lx,+ (4~) + X and D rII, (3~) + X transitions, which in Hz are almost coincident. The rotational study of these bands is now under way.
FIG. 1.
B.
324
A.Rosa~
MONFILSL&
Vibrational results related to the B, B’, B”, D and D’ terms have been published recently@). Data concerning the D” llTu (5~) are given in Table 1. TABLE1. ELECTRONIC TERMS AND VIBRATIONAL CONSTANTS
T.
(cm-l)
We
(cm-l)
OF THE D”
(5~) STATE
wxe
(cm-l)
2
121227.17 121 215.06 2011.9 2320.8
49-3 64.7
Da
12122544
32.7
1648.9
‘II,
Figure 1 shows, as an example, how the wedata found for HZ converge towards the value accepted for H,+ by HERZBERG@). Quantum defects have been computed, which have been found to be positive for the lZ, + states and negative for the llTUstates (Fig. 2).
In-n*
FIG. 2.
Research in progress and program of development in Liege
325
The rotational analysis is well under way and more than one hundred bands have been assigned. to several systems. The data are collected in order to prepare a programme for a computer. The developments reported here are part of the programme of the Institute of Astrophysics of the University of Liege in the field of space research.
REFERENCES H. REIJTLER, H. 0. JUNGER,Z. Phys. 100, 81 (1936). 2. A. MONFILS, Bull. Acad. Belg. Cl. Si. 47, 585, 599, 816 (1961). 3. G. HERZBER~, Spectra of Diatomic Molecules Van Nostrand, New York (1950). 1.