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High resolution spectrum of Xe2 in the vacuum ultraviolet region. Molecular systems related to the two lower resonance lines
Volume 24, number 3
1 February 1974
CHEMICAL PHYSICS LETTERS
.
$&I9 RESOLUTION SPECTRUM OF Xe, IN THE VACUUM ULTRAVIOLET REGION. .’ MOLECULAR SYSTEMS RELATED Tb THE TWO LOWER RESONANCE LINES M.C. CASI’EX and N. DAMANY Laboratoire
des Interactions
Molhda&es
et des Sautes
Pressions,
CNRS, 92190
Meudon. France
Received 23 November 1973
The absorption spectrum of Xe2 has been studied with high resolution between 1290 and 1500 18, giving further information about the lower escited moiecutar states.
1. Introduction
and N, emission lines were superimposed on this continuum, with help of a magnetic lamp [6], as wave-
This paper deals with high resolution spectroscopy investigations of the Xe, pseudo-molecule in the far W and presents new information about molecular systems after preliminary results given between 1150 and 1500 W fl]. The study of the heaviest rare gas dimer is particularly difficult: this is due to a strong overlapping of the rotation-vibration bands, resulting from the low values of the ground state vibrational and rotation& constants (ue = 17 cm-l, Be = 0.0 I3 cm-l), the well depth & being 250 to 300”K, from various experimental works [2--41_
length standards. With a 30 9 slit width, exposure times of the order
The molecular systems described here are related to the (is(3/2)7, and 6s’( 1fZ)y, atomic Ieveb.
2. Experimental Spectra were obtained with a ten-meter spectrograph* fitted with a 1200 lines/mm grating working in the first order, and giving a resolution better than 0.02 A (1.4 cm-l at 1200 ri). The backgrourid continuum was produced by the vacuum spark source previously described IS]. 02
of one hour were required, using QS #ford plates. The absorotion cell, fitted with MgFz windows pressed on indium o-rings, could be cooled by flowing nitrogen at a controlled temperature down to 77OK. Temperature effects were studied at constant densities of xenon atoms: the pressure (up to one atmosphere) was measured at room temperature and then the cell closed. The wavelength accuracy is zkO.03 A, but the diffuseness of some bands preciuded the de-
termination of their position with this precision. Nearly all the analysis was performed on the low temperature data, this for several reasons: (i) as the absorption of the dimer bands increases at low temperature, due to an’increasing number of dimers 171, some vibrational bands can emerge from the wings of the broadened resonance lines; (ii) at low temperature overlapping of the transitions is less pronounced-
3. Results
3.1. System I * This ten-meter spectrographwas buiIt conjointly by CNRS and the Observatory of Meudon under the responsi@ity Drs. S. Leach &nd P. FelenFock and hfissF. Lauwy.We thank her partic&rIy for her kind help.
.,:. -:
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of ..
This band system is related to the 5p6 ‘So + 5ps 6s(3@ resonance iine [l]. The high resolution data obtained, do i&t bring , .’
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Votume 24, number 3 Table 1
Wavelengths and wavenumben of the structures observed,& both sidesof the resonance line 6s(3/2$, 68 045 cm-’ Blue bands
Red bands
Waveh (AI number km-’ )
a (A)
1489.47 67 138 1488.14 67 200 1486.90 67 254 1485.73
Wavenumber (cm-t)
(146695) 1467.70
(68 169) 68 134
1468.35 1468.93
68 104 68 077
67 307
1484.48 67357 further i~fo~atio~ about the low energy side of the line: at a pressure of 100 torr five bands, already observed in absorption f l] and in emission spectra 181, emerge from the red wing with an apparent width of approximately 30 cm-l (table I). But the high energy side reveals an unexpected profile: at pressures of about 10 torr we observed four diffuse bands (fig. I) superposed on a weak cont~uum which extends to 3.50 cm-t from the line center. They are listed in table 1. We might suppose that these blue bands correspond to transitions from the lower u” levels to high U’ levels of the excited molecular state O& a stable state [ 1,9]
Fig. I. fIensitometer trace of the &(3/t)? (1469.60 A) resonance line (p = 19 ton, measuredat room temperature, l= 120 mm).
resulting from the combination of the two atomic terms 5p6 lS0 and 6s(3/2)7, Then it seems difficult, as has been found from calculations of the possibIe potential curves, with the choice, as a matter of convenience of Morse and LennardJones potentials, to explain the blue and the red bands by transitions to
the same excited state (fig. 2), unless to suppose a hump in the 0: state, as also suggested by Tanaka [lo]
in the case of ArZ for a similar sy!tem. An al-
ternative interpretation would be to ascribe the bIue system to transitions to the lu unstable molecular _state, derived from rhe same ii&it_
3.2. System N The very closed structure observed by us [ I]_-on .the low energy side of the 6s’(1/2$ rkonance line were classified in a system IL On the high resolution ,spectra, for preskes higher than 15 t&r, numerouk vibrational structures appear,‘& fart&r from the,tine center (up to 700 cm-l) t&e higher tile pressure:The whole aspect of this,system (fig. 3) zi absence of -. 438
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separation
li
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Fig_2.’ Pro&sed potentk curves of the2ower mofecidar ei cited states. Well p&metkqfor t$e kaund state are from .;, .. .,.. -- ,’ .‘ ref. [4& .__
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vo1sme 24; number 3
’ ..CHE~fICAL.~~YSLCS~E~~S
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tit a pressure of 2 to& (T= :.: 1; : listed in tnble 3. are visibl& The band at’ 1293.48 A is the ‘$atellite band” qi“the- .- ‘. xenon resonance line’, described in earlier papers .[I I. 121. and is due to the presence of krypton in the -. ceIl.~~iI’the observations made in other conditions show that the Xe2 bands are unaffected by krypton atoms. Among the molecular states resulting from the combination of the terms Sp6 ‘So and 6s’( I/2)?, only 0: and 1, can combine with the ground state, according to the molecular selection rules. Both are supposed
.143”K).
Five sharp bands
41/2c
-
Fig. 3. Densitometer trace of the low enerm side of the 6s’(i/2): (1295.58 +%.Iresonance line (p = 55 torr, measured at room temperature - traces of Kr, 1= i 20 mm). defined bands and apparent red convergence - may be explained by extensive overlapping of u’ and vn progressions. From Franck-Condon factor calculations, we have found relative positions of the potential curves which may explain the structures observed and the intensity distribution. The bands are listed in table 3, without vibrational assignment. As for system I, a high resolution study brings further information: at low pressure, Xe2 bands appear first on the high energy side of the 6s’( l/2)! resonance line. Fig. 4 reproduces a densitometer trace sharply
T = lL3K
Table 2 Wavelengths and wavenumbers of the structures observed on the Iow energy side of the resonance fine Ss’(1/2);,
Fig. 4. Densitometer trace of the high energy side of the 6s’(lf2): resonance line. This data, chosen for clearness, belongs to P series related to a hi& resolution study of rare gas mixtures [7] (pxe.= 2 ton and pKr = 1.5 torr. measured at room temperature. I = 120 mmi.
77 185 cm-’
1298.43 1298.92 1299.39 1300.00 -1300.58. 1301.17 1301;75 l302.38
Wavenumber (cm-‘)
A ra,
77 016 76 987 76 959 76 923 76 889 76 854 76819
1304.4076 663 1304.64 76 649 1304.94 76 632 1305.24 76614 i305.52 76 59? 1305.80 76581 1306.10 76563
76 782 76 7.51.
1396:43 1306-7 1 1307.0f 1307.i9
1?02.91-’ 1?03,50
.76716
1303.96
76659 .’
:_
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-
. .
. .
Wavenumber (cm-‘)
Table 3 \Vaveler,gths and wavenumbers of the system obseked the high energy side of the 1295.58 li line
76544 76 528 76510 ‘76494
:
1294.5 1
-
1294.65 129416
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129490 .- 1295.02
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X(A)
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Wavenumber (cm-‘) :
77249 77241 77 234 77.226 .77219q
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on
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” _.:. .: _.. $319.
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.Voluti&24,ntiber
3
CHEMICAL PHYSICS LEmERS
-. to b$:repu&ve states [!!I] at small r, but as a result of. -. dispersion fdrces, they may present at largei a shal-”
1 F&r&y :
...‘_
:’
!974”:
se&edj some b&r@related to.forbidden 4tdmk tram& tions. These wiu be the bbject of a forthcOming paper.. .. .: .: .. References
low minimum responsible foithe molecular system : .‘&se&e& : L : : We have calculated, using the Morse function, aid with the help of h: Franck-Condon .factor prograni, a possible potential c&e for the 0: state:.the well parameters (e = 1000 cm-l, r,_,,= 3.6 fi) seem consistent witbthe red system observed.- As for the molecular
[ 11 M.C. Castex and N. D&any, Cbem. F’hys. Letters 13 (1972) 1S8_ [2] J-0. Hirschfelder,CF. Curt& and R.B. Bird, The molecular theory of gases and liquids (Wiley, New York, 1954); [3] A. Saran and A.K.Barua, Can. J. Phys. 42 (1964) 2026. [4] E. Buluggiuand C. Foglia, Chem. Phys. Letters 1 (1967)
system I, it is difficult to explain the blue and the red system by the same potential curve. The possibility of transitions to a state resuRing from the combination of the terms 5p6 ISo and Sp5 $(1/2)!, perhaps might be considered (fig. 2, curve lu).
[S] :Damany, J.Y. Roncin and N. Damany, Appl. Opt. 5 (1966) 297. [6] Licence ANVAR. Manufacturedby Creusot-Loire.
[71 MC Castex, to be published. 181 P-G. Wilkinson and Y. Tanaka, J. Opt. Sot. fun. 45 (1955) 344. [9] R.S. Mull&en; J. Chem. Phys. 52 (1970) 5170. [lOI Y, Tanaka and K.Yoshino, J. Chem. F’hys.53 (1970) 2012. [Ill M.C. Castex, R. Granierand J. Remand, Compt. Rend. Acad. Sci (Paris) 268 (1969) 552. 112) M.C. Castex, Compt. Rend. Acad. Sci. (Paris) 270 (1970) 207.
c Conchlsion This experimental work brings some new information on the lowest excited moIecuIar states of the Xe, dimer, at interatomic distances corresponding to the ground state well. Numerous other molecular systems have been ob-
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1 February 1974
CHEMICAL PHYSICS LETTERS
.
$&I9 RESOLUTION SPECTRUM OF Xe, IN THE VACUUM ULTRAVIOLET REGION. .’ MOLECULAR SYSTEMS RELATED Tb THE TWO LOWER RESONANCE LINES M.C. CASI’EX and N. DAMANY Laboratoire
des Interactions
Molhda&es
et des Sautes
Pressions,
CNRS, 92190
Meudon. France
Received 23 November 1973
The absorption spectrum of Xe2 has been studied with high resolution between 1290 and 1500 18, giving further information about the lower escited moiecutar states.
1. Introduction
and N, emission lines were superimposed on this continuum, with help of a magnetic lamp [6], as wave-
This paper deals with high resolution spectroscopy investigations of the Xe, pseudo-molecule in the far W and presents new information about molecular systems after preliminary results given between 1150 and 1500 W fl]. The study of the heaviest rare gas dimer is particularly difficult: this is due to a strong overlapping of the rotation-vibration bands, resulting from the low values of the ground state vibrational and rotation& constants (ue = 17 cm-l, Be = 0.0 I3 cm-l), the well depth & being 250 to 300”K, from various experimental works [2--41_
length standards. With a 30 9 slit width, exposure times of the order
The molecular systems described here are related to the (is(3/2)7, and 6s’( 1fZ)y, atomic Ieveb.
2. Experimental Spectra were obtained with a ten-meter spectrograph* fitted with a 1200 lines/mm grating working in the first order, and giving a resolution better than 0.02 A (1.4 cm-l at 1200 ri). The backgrourid continuum was produced by the vacuum spark source previously described IS]. 02
of one hour were required, using QS #ford plates. The absorotion cell, fitted with MgFz windows pressed on indium o-rings, could be cooled by flowing nitrogen at a controlled temperature down to 77OK. Temperature effects were studied at constant densities of xenon atoms: the pressure (up to one atmosphere) was measured at room temperature and then the cell closed. The wavelength accuracy is zkO.03 A, but the diffuseness of some bands preciuded the de-
termination of their position with this precision. Nearly all the analysis was performed on the low temperature data, this for several reasons: (i) as the absorption of the dimer bands increases at low temperature, due to an’increasing number of dimers 171, some vibrational bands can emerge from the wings of the broadened resonance lines; (ii) at low temperature overlapping of the transitions is less pronounced-
3. Results
3.1. System I * This ten-meter spectrographwas buiIt conjointly by CNRS and the Observatory of Meudon under the responsi@ity Drs. S. Leach &nd P. FelenFock and hfissF. Lauwy.We thank her partic&rIy for her kind help.
.,:. -:
:
: ‘.
of ..
This band system is related to the 5p6 ‘So + 5ps 6s(3@ resonance iine [l]. The high resolution data obtained, do i&t bring , .’
^.
. ._
--.
_
..
‘I -
‘.
--
4$
Votume 24, number 3 Table 1
Wavelengths and wavenumben of the structures observed,& both sidesof the resonance line 6s(3/2$, 68 045 cm-’ Blue bands
Red bands
Waveh (AI number km-’ )
a (A)
1489.47 67 138 1488.14 67 200 1486.90 67 254 1485.73
Wavenumber (cm-t)
(146695) 1467.70
(68 169) 68 134
1468.35 1468.93
68 104 68 077
67 307
1484.48 67357 further i~fo~atio~ about the low energy side of the line: at a pressure of 100 torr five bands, already observed in absorption f l] and in emission spectra 181, emerge from the red wing with an apparent width of approximately 30 cm-l (table I). But the high energy side reveals an unexpected profile: at pressures of about 10 torr we observed four diffuse bands (fig. I) superposed on a weak cont~uum which extends to 3.50 cm-t from the line center. They are listed in table 1. We might suppose that these blue bands correspond to transitions from the lower u” levels to high U’ levels of the excited molecular state O& a stable state [ 1,9]
Fig. I. fIensitometer trace of the &(3/t)? (1469.60 A) resonance line (p = 19 ton, measuredat room temperature, l= 120 mm).
resulting from the combination of the two atomic terms 5p6 lS0 and 6s(3/2)7, Then it seems difficult, as has been found from calculations of the possibIe potential curves, with the choice, as a matter of convenience of Morse and LennardJones potentials, to explain the blue and the red bands by transitions to
the same excited state (fig. 2), unless to suppose a hump in the 0: state, as also suggested by Tanaka [lo]
in the case of ArZ for a similar sy!tem. An al-
ternative interpretation would be to ascribe the bIue system to transitions to the lu unstable molecular _state, derived from rhe same ii&it_
3.2. System N The very closed structure observed by us [ I]_-on .the low energy side of the 6s’(1/2$ rkonance line were classified in a system IL On the high resolution ,spectra, for preskes higher than 15 t&r, numerouk vibrational structures appear,‘& fart&r from the,tine center (up to 700 cm-l) t&e higher tile pressure:The whole aspect of this,system (fig. 3) zi absence of -. 438
.1
. : :_ _‘ ‘, . :‘
-,j _. :
:,
-.‘I
: ,:
.,_ ,. _, _ . ,. ,_; _’ :,
_
?.,
o;;1~5~_~._~~ -ip6,;_ I&&~~
:
separation
li
i
i
,.
Fig_2.’ Pro&sed potentk curves of the2ower mofecidar ei cited states. Well p&metkqfor t$e kaund state are from .;, .. .,.. -- ,’ .‘ ref. [4& .__
: 1 : ,. -.. .‘ ., .;. _.-
i I
._ ‘ ; :.,:..
:.
. _‘_.
.,. .,. ,:
:.:,.: : .,_.
,,..
;. . : ..,--. .’ :: -.,-.
._
,.
‘.
‘.
.,
vo1sme 24; number 3
’ ..CHE~fICAL.~~YSLCS~E~~S
_.. .
1 Fcbruik$ :9-&J .I
:.
_-
‘.
ofa sp&tru&o,bt&~ed
tit a pressure of 2 to& (T= :.: 1; : listed in tnble 3. are visibl& The band at’ 1293.48 A is the ‘$atellite band” qi“the- .- ‘. xenon resonance line’, described in earlier papers .[I I. 121. and is due to the presence of krypton in the -. ceIl.~~iI’the observations made in other conditions show that the Xe2 bands are unaffected by krypton atoms. Among the molecular states resulting from the combination of the terms Sp6 ‘So and 6s’( I/2)?, only 0: and 1, can combine with the ground state, according to the molecular selection rules. Both are supposed
.143”K).
Five sharp bands
41/2c
-
Fig. 3. Densitometer trace of the low enerm side of the 6s’(i/2): (1295.58 +%.Iresonance line (p = 55 torr, measured at room temperature - traces of Kr, 1= i 20 mm). defined bands and apparent red convergence - may be explained by extensive overlapping of u’ and vn progressions. From Franck-Condon factor calculations, we have found relative positions of the potential curves which may explain the structures observed and the intensity distribution. The bands are listed in table 3, without vibrational assignment. As for system I, a high resolution study brings further information: at low pressure, Xe2 bands appear first on the high energy side of the 6s’( l/2)! resonance line. Fig. 4 reproduces a densitometer trace sharply
T = lL3K
Table 2 Wavelengths and wavenumbers of the structures observed on the Iow energy side of the resonance fine Ss’(1/2);,
Fig. 4. Densitometer trace of the high energy side of the 6s’(lf2): resonance line. This data, chosen for clearness, belongs to P series related to a hi& resolution study of rare gas mixtures [7] (pxe.= 2 ton and pKr = 1.5 torr. measured at room temperature. I = 120 mmi.
77 185 cm-’
1298.43 1298.92 1299.39 1300.00 -1300.58. 1301.17 1301;75 l302.38
Wavenumber (cm-‘)
A ra,
77 016 76 987 76 959 76 923 76 889 76 854 76819
1304.4076 663 1304.64 76 649 1304.94 76 632 1305.24 76614 i305.52 76 59? 1305.80 76581 1306.10 76563
76 782 76 7.51.
1396:43 1306-7 1 1307.0f 1307.i9
1?02.91-’ 1?03,50
.76716
1303.96
76659 .’
:_
_,:
-
. .
. .
Wavenumber (cm-‘)
Table 3 \Vaveler,gths and wavenumbers of the system obseked the high energy side of the 1295.58 li line
76544 76 528 76510 ‘76494
:
1294.5 1
-
1294.65 129416
-
_.
_. .-
-. :
129490 .- 1295.02
:_:
‘.
‘,
.I’-.’
._
X(A)
‘.
-.
-_
.-.
-.
‘.
., _: ..
:
Wavenumber (cm-‘) :
77249 77241 77 234 77.226 .77219q
..
,,
-.
,-
.: ...’ ‘.. .. .; -.,. ,_. .: : ,.~ . .._ . . --
:
on
..’
..
.,
_.
‘.
.,
” _.:. .: _.. $319.
z .. :.; : _. -.
.Voluti&24,ntiber
3
CHEMICAL PHYSICS LEmERS
-. to b$:repu&ve states [!!I] at small r, but as a result of. -. dispersion fdrces, they may present at largei a shal-”
1 F&r&y :
...‘_
:’
!974”:
se&edj some b&r@related to.forbidden 4tdmk tram& tions. These wiu be the bbject of a forthcOming paper.. .. .: .: .. References
low minimum responsible foithe molecular system : .‘&se&e& : L : : We have calculated, using the Morse function, aid with the help of h: Franck-Condon .factor prograni, a possible potential c&e for the 0: state:.the well parameters (e = 1000 cm-l, r,_,,= 3.6 fi) seem consistent witbthe red system observed.- As for the molecular
[ 11 M.C. Castex and N. D&any, Cbem. F’hys. Letters 13 (1972) 1S8_ [2] J-0. Hirschfelder,CF. Curt& and R.B. Bird, The molecular theory of gases and liquids (Wiley, New York, 1954); [3] A. Saran and A.K.Barua, Can. J. Phys. 42 (1964) 2026. [4] E. Buluggiuand C. Foglia, Chem. Phys. Letters 1 (1967)
system I, it is difficult to explain the blue and the red system by the same potential curve. The possibility of transitions to a state resuRing from the combination of the terms 5p6 ISo and Sp5 $(1/2)!, perhaps might be considered (fig. 2, curve lu).
[S] :Damany, J.Y. Roncin and N. Damany, Appl. Opt. 5 (1966) 297. [6] Licence ANVAR. Manufacturedby Creusot-Loire.
[71 MC Castex, to be published. 181 P-G. Wilkinson and Y. Tanaka, J. Opt. Sot. fun. 45 (1955) 344. [9] R.S. Mull&en; J. Chem. Phys. 52 (1970) 5170. [lOI Y, Tanaka and K.Yoshino, J. Chem. F’hys.53 (1970) 2012. [Ill M.C. Castex, R. Granierand J. Remand, Compt. Rend. Acad. Sci (Paris) 268 (1969) 552. 112) M.C. Castex, Compt. Rend. Acad. Sci. (Paris) 270 (1970) 207.
c Conchlsion This experimental work brings some new information on the lowest excited moIecuIar states of the Xe, dimer, at interatomic distances corresponding to the ground state well. Numerous other molecular systems have been ob-
. .: .
:
..
:..
..
.
:, :
:_
:-
:
:
‘:
,’
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. .
y#o
.
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.-
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