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Absorption spectrum of bismuth in the vacuum ultraviolet. Observation of new transitions to Rydberg states
Chemical Physics 63 (1981) 157-163 North-Hoiland Publishing Company
ABSORPTION
SPECTRUM
OBSERVATEON
OF
NEW
OF BISMUTH TRANSITIONS
IN THE TO
VACUUM
RYDBERG
ULTRAVIOLET.
STATES
Nicole DAMANY Equipe de Spectroscopic dn C.N.R.S.
(L.A. 17I)f E.N.S.M.S.E..
42023 Saint Etienne Ceder, France
and Jean FIGUET
and Ari TOPOUZKHANIAN
Laboratoire de Speclrom@trie Ionique er Molkuiaire. 69622 Villeurbnnne Ceder, France
Received
Asso&
au C.N.R.S.
(L.A. 171). Unicersitt! Lyon I,
23 June 1981
Between 160 and 230 nm, five band systems, includingflvo new ones, have been recorded and classified by studying bismuth vapor produced in a heat pipe oven. They have been assigned to transitioos from the X ‘1; ground state nf Bi2 to high-lying Rydberg states converging to the ‘lL.3,2 and ‘FI,.,,z states of BiS at 7.53 and 8.91 eV. Some vibrational constants have been observed, by operating the heat pipe under unusual conditions.
1. Introduction Electronic transitions in the absorption spectrum of Bir were observed as early as in 1933 [l]. However, the extensive study of this molecule has been carried out very recently, particularly because of the high output continuous laser oscillations performed [Z, 31. Visible or near ultraviolet absorption [l-7], emission [8], and photoluminescence [6,7, g-111 studies, dealing with bismuth in the gas phase as weil as in inert gas matrices, have removed the uncertainty concerning the identification of the Biz ground state [7,10-K?]. It is confirmed that the state with W, = 172.98 cm-’ and r, = 2.6597 (see fig. 4), is the ground state, whereas the X’ state, which has been located 1500 cm-’ below the X state [13] appears to be the ground state of a larger molecule, most likely the tetramer B&. From the photoelectron spectrum of Biz 1141, + UniversitesLyon I and Saint Etienne. T 0301-0104/81/0000-0000/$02.75
the first three ionization potentials of this molecule have been measured at 7.53, 8.94 and 9.30 eV, respectively. But to our knowledge no reliable absorption spectrum has been recorded in the vacuum ultraviolet, besides a preliminary work performed by one of us [15]. We report here the absorption spectrum of diatomic bismuth vapor between 160 and 230 nm. In the salme region we have also observed that of Bi, which was already recorded down to 125 nm in flash-pyrolysis experiments
CW. 2. Experimental procedure Bismuth vapor, containing atoms, dimers as well as Bi, clusters (with n >2) was produced in a conventional heat pipe oven [17-191, with an active zone of 50 cm. The heat pipe was constructed of 42 mm internal diameter stainless steel pipe, type 310, with 3.2 mm wall thickness. The wick was made of 2-3 layers of P75
@ 1981 North-Holland
158
N. Dontany et al. i Ahsorprion spectrum of bismurfrin r/w VUV
stainless steel screen, type 330 (wire diameter: 0.14 mm, square mesh dimension: 0.2 mm). An electric power of 600 W was necessary to raise the temperature to 1190 K. Calcium fluoride windows were used, sealed with rubber O-rings. Though a sample of spectroscopically pure bismuth was used, two lines of atomic antimony were observed on most recordings. Argon with 99.9995% purity acted as buffer gas. The temperature was measured in the central part of the heat pipe with an external thermocouple. Molecular absorption was at best performed between 1100 and 1185 K, which corresponds to a bismuth toral pressure of 0.3 to 1.3 mbar [20] in the absence of inert gas. In order to protect both windows efficiently against particle deposition, slightly higher argon pressures were utilized, typically between 0.5 and 5 mbar. Under such conditions the heat pipe operated several weeks without any attenuation in the transmission of the continuous background radiation. However, the partial and total bismuth vapor pressures are less well-defined, becailse argon is mixed with Bir_, all along the heat pipe [l&19]. Thermodynamic data [21] show that in the above-mentioned temperature range, pure bismuth vapor contains roughly equal amounts of Bi and Bi?, thar of the heavier species being less than 1%. In the presence of a bufier gas, this ratio is rather lowered. Accordingly it is not likely that higher polymers take part in the absorption observed. As for the atomic spectrum, in spite of a large concentration, about 50% [Zl], only some sharp lines appear and have been used for calibration purposes. But very intense autoionized lines, with considerable broadening, are found at the shortest wavelengths. As one might expect, the formation of molecules is hindered with increasing buffer gas pressure, whereas the atomic spectrum is enhanced. At 1130 K and 900 mbar, all molecular bands disappear in favour of the atomic lines. The Iatter are thus observed under easier experimental conditions than in flashpyrolysis [ 161.
A concave grating spectrograph, with 1 m radius of curvature and 2000 lines/mm, was utilized at normal incidence, giving a reciprocal dispersion of 0.5 nm/mm in the first order. A deuterium discharge tube, equipped with a fused silica window, provided the background continuum. An aluminized toroidal mirror, set at 20 cm from the source, received incident light at an angle of 50’ and focused it on the entrance slit, located 170 cm farther. The heat pipe was thus crossed by a nearly parallel beam, and the spectrograph well illuminated. Exposures of 15 to 30 min were sufficient with Xord Q2 plates. A copper/germanium holiow cathode lamp, filled with neon, was used for the production of wavelength standards. But atomic absorption lines from bismuth and even antimony (found at trace level in the metal sample which fi!led the heat pipe) were generally sufficient for calibration purposes_ The spectra have been recorded with the help of a modified Chalonge microphotometer. The accuracy of the measured wavelengths is roughly 0.03 nm for sharp lines without blending. However, such a precision was not reached on the recordings, owing to their complexity. 3. Results 3.1.
Ahmic
absorption
As stated above, under favourable experimental conditions (T> 1130 K, pA,>500 mbar), the atomic spectrum appears below the first ionization limit (A > 170.1 nm). It consists of very sharp lines, already classified by Joshi et al. [16] in Rydberg series converging to various ionization limits. The first one, corresponding to Bi II (?e), lies at 58790 cm-‘. The autoionized lines, situated beyond this limit, have also been observed by the same authors [16]. Centered at 164.4, 163.6 and 161.8 nm, these lines are very strong and show considerable broadening. Their absorption is several orders of magnitude greater than that of the remaining spectrum, and they appear under whatever experimental conditions, even at relatively low temperatures (1930 K).
N. Damany er al. / Ahsorpriorrspecmcnt of bisnturirin rhe VUV
The apparent half widths of the autoionized lines, which may reach 0.5 nm for the broadest one (A = 163.6 nm), seem to show no parallel variation with the atomic absorption intensity. The latter is vey sensitive to the buffer gas pressure, whose Increase hinders the formation of molecules; whereas the former is rather temperature dependent, like the total bismuth pressure which augments with increasing temperature. Joshi et al. mention the total disappearance of these autoionized lines when the capacity of their condenser bank discharging in the flash-lamp is reduced to its half va!ue (without specifying what happens to the remaining spectrum). But such a phenomenon is contradicted by our observation, if the absorption is strictly atomic. Accordingly, it is reasonable to think that some molecular contribution exists, though inseparable from the atomic absorption at the present time. In order to confirm this assumption, one should replace the deuterium discharge lamp, which emits atomic lines in this region, by a pure background continuum source (like the BRV-source). Possible molecular features may thus be observed, particularly when the experimental conditions (temperature and buffer gas pressure) are varied in a broader range. 3.2.
Molecular
absorption
The vacuum ultraviolet absorption bands of Biz cannot be easily analyzed for the following reasons: (i) Owing to the heavy weight of this molecule, its rotational levels are too close to be separated. (ii) The bonding between the two bismuth atoms is weak. Accordingly, the spacing of vibrational levels is small: 173 cm-’ for the first ones in the ground state. (iii) Because of the low bismuth vapor pressure, the heat pipe ought to be operated at high temperatures (T> 1120 K). Several vibrational levels of the ground state are thus populated, and different v” progressions are superimposed in the absorption spectrum. Hence the interest of matrix-isolated bismuth studies [6,7,10-121
159
(performed in other spectral regions) where the absorption features are reduced to transitions originating only from the L”’= 0 level. (iv) Bismuth vapor contains other species than dimers. If the atomic spectrum is easily distinguished, this is not the case for those concerning Bi, (n > 2). Under our experimental conditions, p(Bi,) stands for less than 1% of p(Bi+ Biz), and is further reduced owing to a rather high argon pressure. Accordingly its absorption spectrum may be hardly observed, whereas such clusters are detected in laser fluorescence experiments [6,9,10-121. Between 161 and 222 nm we have observed five band systems, degraded to longer wavelengths. Systems I, III and IV appear at best at roughly 1120 K, whereas for the two others, which are weaker, a higher temperature is needed (1150-1175 EC). All these transitions originate from the X ‘Z, ground state. Sq.srem I. It extends from 222 to 206.6 nm (fig. 1). A complete analysis is not possible, owing to the complexity of the spectrum. Though an error of one or even two units in the vibrational numbering cannot be dismissed, in the absence of the isotope effect (bismuth is monoisotopic), we have tentatively ascribed the value 45490 f 5 cm-’ to zjoo, and 140 f 3 cm-’ to 0,. This system corresponds to the Fc X transition [13], whose first bands at longer wavelengths, observed by Almy and Sparks [l], are mainly u&+ L’& and c: t v;_~ progressions, and have not been analyzed. A sharp breakingoff at 206.6 nm may result from a predissociation, perhaps via a state dissociating into “D 3/z+ zDs12. atomic states (fig. 4). System 1I. Labelled previously as Mc ‘7 transition [IS], this system (fig. 2) is weaker than the first one, on which it is partly superimposed at roughly 206.5 nm. System II extends to 195.5 nm, with a regular decrease in absorption intensity from 208 nm. The foliowing constants are assigned to it: v,~~= 49620 cm-‘, and w,= 158 cm-‘. System III. Its upper state was already designated as the N state 1151. It extends from 195 to 182 nm, and has approximately the same
55
I 180
50
V cm-lx IO-3 li nm
I 190
I 200
IlI
Fig. 1. Microphotometer tmce of the absorption spectrum T= 1120 K, pAz = 0.5 mbar. Molecular systems I-III.
of atomic and molecular
bismuth,
between
1SO and 220 nm, at
range is smaller (from 173 to 168 nm only). We have measured voo= >7980 cm-’ with o, = 140 cm-‘. System V. Its extent is very limited (2.5 nm) (fig. 3), and its intensity has the same magnitude as system II. It cannot be analyzed. The value of ~00 = 59700 cm-’ is measured at the center of the broad band. To our knowledge, systems Ii through V have been observed for the first time in full. We label their upper electronic states as Q, R, S, T.
intensity as system I (fig. 1). wr has roughIy the same value 140 cm-‘, whereas vO,,= 53200 cm-‘. System 1V. This system (fig. 3) has a similar intensity as transitions I and III, but its
wavelength
Fig. 4 and table 1 summarize the equilibrium
our results, where
internuclear distance r, is
derived from the empirical relation r&,
= constant, the v&e
of this constant being
1224 as caIcuIated for the ground state.
4. Discussion
I
8
Fig. 2. hlicrophotometer trace of tbe absorption spectrum of atomic and molecuiar bismuth, between 195 and 210 m-n, at T = 1 I75 K, pxr = 0.5 mbar. Molecular system II.
On account of the vicinity of the first ionization limit, it is reasonable to think that the systems observed belong in most cases to Rydberg transitions. The two spin-orbit partners lie at 7.53 and 8.94 eV respectively [14], which cor-
N. Dammty er al. / Absorption specrrilm of bismuth in the VUV
Fig. 3. Microphotometer trace of the absorption spectrum of atomic and molecular bismuth between 160 and 180 nm, at T= 1135 K, pAr= 3 mbar. Molecular systems IV and V. and two autoionized lines of Bi.
-d
respond to 60730 and 72100 cm-‘. If the systems recorded are Rydberg transitions, the voo values must satisfy the formula: Vaa= IP-
109730/(n
-a)‘,
(1)
where IP hoIds for the ionization potential, n an integer corresponding to the principal quantum number and 6 the molecular quantum defect. Table 1 shows that for the FtX, RtX and S+X transitions, the latter is equal to 3.22, 3.17 and 3.22 respectively, whereas for Q&X and Tt X, 6 =4.85 and 5 respectively. These values are very comparable with the mean value of the atomic quantum defects relative to a nd (S = 3.2) and a rzs (6 = 5) orbital cf Bi. On the other hand, neither approximates to the one which may be calculated [23] for a np orbital (6 = 4.4). Bi2 and Nz are isoelectronic molecules. Accordingly, the ground-state electronic configurations are similar, save the order of the We and rr, molecular orbitals, which is reversed
:
2.5
3
3.5
l-r*
Fig. 3. Roush scheme of energy potentkd curves for Bi, Rydberg siates. Excepred for X state [13], the equilibrium internuclear distances r, are estimated from r$, = constant.
as for As2 and Sbz [24]:
. . . (096s)2(~“6s)z(cTg6p)1(5iu6p)4 ix;. The Rydberg transitions associated with the promotion of an electron from the x,,6p orbital give rise to a,ns and (mgs.Z~ or 8,) nd Rydberg states, the ionization leading to the two spinorbit partners alI U.3,2 and 211u.1,2 of Bi;. The bismuth molecule being weakly bound, the separation between these ‘II states is close to the one concerning the correlated atomic states ‘PO and ‘Pr. The same holds true for the
N. Darnan~ et al. ! Absorprionspectrum of bismuth in the VUV
162
Table 1 Observed
state
Rydbrrg I’&.
(cm-r)
x
0
C F Q R S T
36117 46390 49620 53200 57980 59700
Stares of w.2
Biz
lcrn-‘!
L,
173 157 l-l0 1% 140 140
2.66 2.79 2.96 7.7% 2.96 2.96
Kydbrrg
orbital
quant. Atomic orbital defect
Atomic quant. defect
Dissociation products
D (cm-‘)
4.87 3.22 1.85 3.17 3.22 5
4.95 3.29 4.90 3.24 3.26 5
6p34S.oibp 3 %” 6p3’S0i75’P,,z 6p” %?+6d ‘D3,: 6p3 “S”+8s’PlIZ 6p3’S”i7d’D,,Z 6p3’S”t6d’D 6p3 ‘S”t8s ‘P3/r
16500 12640 13920 14250 1422U 15100 17600
* Mol.
ionization potentials (7.53 and 8.94 eV for the molecule, 7.29 and S.94 eV for the atom [14]). Concerning the states which converge to the first ionization potential (607302 100 cm-‘), we have to notice thar the C state, observed by Almy and Sparks [l J folIows we11 the Rydberg formuIa with S = 4.87. it should proceed from the transfer of an electron from a 7;;6p to a ~~7s orbital. The next term of this series is the Q state (6 =3.85) resulting from the promotion io a a;& orbitaL The 8 values found for F and R states (3.22 and.3.17 respectively) clearly establish that the excited moIecular orbitals attained are correlated to rrd atomic orbitals (&, = 3.29 and 3.24); hence they are (crag.i~g or S,) 6d and 7d molecular orbitak. The S and T states lead similarly to the second ionization potential (7213Ok 100 cm-‘). The T state, with a quantum defect of 5 for n =S, has undoubtedly a . . . (~,2s)“(~,2s)‘(~,2p!‘~~~2p)~(u~ss) configuration. For the S state, 6 has the value corresponding to a 6d eIectron. Due to the large spin-orbit interaction, the Bi- molecule belongs mainly to Hund’s case (c). The ground state being a ‘X: state, transitions originating from it end at 0: and 1: states, which result only from doublet or quartet states of Bi. Thus the C, Q, and T states dissociate into a %a and another (7s or 8s) 4P atom. Assuming for D, the ground state dissociation ener,yr, 16500 cm-’ as a mean value [21,25,26] and taking into account the atomic excitation energy, one can calculate the dissociation ener-
7s 6d s5 7d 6d 8s
gies of these three states. They are equal to 12600, 14280 and 17600 cm-’ respectively, of the same magnitude as the ground state value. This is well understandable by the transfer of the electron from a bonding orbital rr,, to another bonding one, us_ When calculating the dissociation energies, it is supposed obviously that no perturbations affect these states and the potential curves are not deformed (non-crossing rule). When studying the C state, Almy and Sparks [I] have observed that the corresponding we-r=does not lead to a reasonable dissociation energy, even when assuming that this state is a valence state dissociating into zD3,a f ‘D5,?_ From our spectra it is not possible to derive any value of ~~.r,. Therefore no further information can be provided at present. One of the dissociation products of the F, R, and S states is a (6d or 7d) ‘D atom. The other may be the quadruplet %$a or a doublet, the lowest in energy being ‘D&? (11420 cm-‘). According to the first hypothesis, the calculated dissociation energies are roughly 13900, 14200 and 15100 cm-‘, very close to the one concerning the ground state. On the contrary, if dissociation products were nd ‘D and 6p3 ‘D$z, the correspcnding D values would be 25300, 25700 and 26500 cm-’ respectively, much higher than the ground state dissociation energy. This involves that the promoted electron moves on a more bonding orbital, which seems quite improbable as we have calculated for these three states r, = 2.96 A, a little larger than the ground state value. So we retain rather the first hypothesis. In this case, the orbital occupied by
N. Damany
et al. / Absorption spectrum of bismuth in the VUV
the promoted electron may be a ugnd, which has the same bonding character as 7ru6p [27]. Here aIso the dissociation energies proposed do not take into account a possible deformation of the potential energy curves. We have in particular mentioned an abrupt breaking-off in system I, which may infer a predissociation in the F state. Finally, it is noteworthy that F and S whose configuration was assumed to be 6d, fit well the Rydberg formula. This fact is not surprising because a 6d orbital, though having the same principal quantum number as the initial 6p orbital, behaves somewhat like a Rydberg orbital, owing to its largeness_
Acknowledgement WC would like to thank Professor Jean D’Incan for his interest and useful discussions. Dr. Gustav Gerber and Mr. Joachim Janes, of the University of Freiburg, are also gratefully acknowledged for technical assistance during the building of our heat pipe.
References [l] G.M. Almy and F.M. Sparks, Phys. Rev. 44 (1933) 365. [2] W-P. West and H.P. Broida, Chem. Phys. Letters 56 (1978) 283. [3] B. Wellegehausen, D. Friede and G. Steger, Optics Commun. 26 (1978) 391. [4] G. Nakamura and T. Shidei, Japan J. Phys. 10 (1934) 11. [S] N. Aslund, R.F. Barrow, W.G. Richards and D.N. Travis, Ark. Fys. 30 (1965) 171.
163
[6] R. A. Teichman III and E. R. Nixon, J. Chem. 67 (1977) 2470. [73 V.E. Bondybey, G. P. Schwartz, J. E. Grifiiths and J.H. English, Chem. Phys. Letters 76 (1980) 30. [S] S.P. Reddy and M.K. Ali, J. Mol. Specrry. 35 (1970) 285. [9] G. Gerber, K. Sakurai and H.P. Broida, J. Chem. Phys. 64 (1976) 3410. [lo] V-E. Bondybey and J.H. English, J. Chem. Phys. 73 ( 1980) 42. [ll] F. Ahmed and E.R. Nixon, J. Chem. Phys. 74 (1981) 2156. [12] M. Manzel, U. Engelhardt, H. Abe, W. Schulze and F.W. Froben, Chem. Phys. Letters 77 (1981) 514. [13] G. Gerber and H.P. B&da, J. Chem. Phys. 64 (1976) 3423. [14] S. Siizer, S.T. Lee and D.A. Shirley, J. Chem. Phys. 65 (1976) 412. [15] A. Topouzkhanian, A.M. Sibai and .I. D’Incan, 2. Natmforsch. 29a (1974) 436. [16] Y.N. Joshi and R.P. Srivastava, Can. J. Phys. 56 (1978) 1157. (171 C.R. Vidal and J. Cooper, J. Appl. Phys. 40 (1969) 3370. 1183 C.R. Vidal and F.B. Hailer, Rev. Sci. Instr. 32 (1971) 1779. [19] LA. Melton and P.H. Wine, J. Appl. Phys. 51 (1980) 4059. [20] R.E. Honig and D.A. Kramer, RCA Rev. 30 (1969) 285. [21] A.K. Fischer, J. Chem. Phys. 45 (1966) 375. [22] K.P. Huber and G. Herzberg, Constants of diatomic molecules (Van Nostrand, Princeton, 1979) p. 92. [23] C.E. Moore, Atomic energy levels, Circular No. 467 (National Bureau of Stand.. Washington, 1949). [24] R.J. Donovan and P. Strachan, Trans. Faraday Sot. 67 (1971) 3407. [25] L. Rovner, A. Drowart and J. Drowart. Trans. Faraday Sac. 63 (1967) 2906. [26] F.J. Kohl, O.M. Uy and K.D. Carlson, J. Chem. Phys. 47 (1967) 2667. 1271 G. Herzberg, MoIecular spectra and moiecutar structure. Vol. 1. Spectra of diatomic molecules, 2nd Ed. (Van Nostrand. New York, 1963) p_ 329.
ABSORPTION
SPECTRUM
OBSERVATEON
OF
NEW
OF BISMUTH TRANSITIONS
IN THE TO
VACUUM
RYDBERG
ULTRAVIOLET.
STATES
Nicole DAMANY Equipe de Spectroscopic dn C.N.R.S.
(L.A. 17I)f E.N.S.M.S.E..
42023 Saint Etienne Ceder, France
and Jean FIGUET
and Ari TOPOUZKHANIAN
Laboratoire de Speclrom@trie Ionique er Molkuiaire. 69622 Villeurbnnne Ceder, France
Received
Asso&
au C.N.R.S.
(L.A. 171). Unicersitt! Lyon I,
23 June 1981
Between 160 and 230 nm, five band systems, includingflvo new ones, have been recorded and classified by studying bismuth vapor produced in a heat pipe oven. They have been assigned to transitioos from the X ‘1; ground state nf Bi2 to high-lying Rydberg states converging to the ‘lL.3,2 and ‘FI,.,,z states of BiS at 7.53 and 8.91 eV. Some vibrational constants have been observed, by operating the heat pipe under unusual conditions.
1. Introduction Electronic transitions in the absorption spectrum of Bir were observed as early as in 1933 [l]. However, the extensive study of this molecule has been carried out very recently, particularly because of the high output continuous laser oscillations performed [Z, 31. Visible or near ultraviolet absorption [l-7], emission [8], and photoluminescence [6,7, g-111 studies, dealing with bismuth in the gas phase as weil as in inert gas matrices, have removed the uncertainty concerning the identification of the Biz ground state [7,10-K?]. It is confirmed that the state with W, = 172.98 cm-’ and r, = 2.6597 (see fig. 4), is the ground state, whereas the X’ state, which has been located 1500 cm-’ below the X state [13] appears to be the ground state of a larger molecule, most likely the tetramer B&. From the photoelectron spectrum of Biz 1141, + UniversitesLyon I and Saint Etienne. T 0301-0104/81/0000-0000/$02.75
the first three ionization potentials of this molecule have been measured at 7.53, 8.94 and 9.30 eV, respectively. But to our knowledge no reliable absorption spectrum has been recorded in the vacuum ultraviolet, besides a preliminary work performed by one of us [15]. We report here the absorption spectrum of diatomic bismuth vapor between 160 and 230 nm. In the salme region we have also observed that of Bi, which was already recorded down to 125 nm in flash-pyrolysis experiments
CW. 2. Experimental procedure Bismuth vapor, containing atoms, dimers as well as Bi, clusters (with n >2) was produced in a conventional heat pipe oven [17-191, with an active zone of 50 cm. The heat pipe was constructed of 42 mm internal diameter stainless steel pipe, type 310, with 3.2 mm wall thickness. The wick was made of 2-3 layers of P75
@ 1981 North-Holland
158
N. Dontany et al. i Ahsorprion spectrum of bismurfrin r/w VUV
stainless steel screen, type 330 (wire diameter: 0.14 mm, square mesh dimension: 0.2 mm). An electric power of 600 W was necessary to raise the temperature to 1190 K. Calcium fluoride windows were used, sealed with rubber O-rings. Though a sample of spectroscopically pure bismuth was used, two lines of atomic antimony were observed on most recordings. Argon with 99.9995% purity acted as buffer gas. The temperature was measured in the central part of the heat pipe with an external thermocouple. Molecular absorption was at best performed between 1100 and 1185 K, which corresponds to a bismuth toral pressure of 0.3 to 1.3 mbar [20] in the absence of inert gas. In order to protect both windows efficiently against particle deposition, slightly higher argon pressures were utilized, typically between 0.5 and 5 mbar. Under such conditions the heat pipe operated several weeks without any attenuation in the transmission of the continuous background radiation. However, the partial and total bismuth vapor pressures are less well-defined, becailse argon is mixed with Bir_, all along the heat pipe [l&19]. Thermodynamic data [21] show that in the above-mentioned temperature range, pure bismuth vapor contains roughly equal amounts of Bi and Bi?, thar of the heavier species being less than 1%. In the presence of a bufier gas, this ratio is rather lowered. Accordingly it is not likely that higher polymers take part in the absorption observed. As for the atomic spectrum, in spite of a large concentration, about 50% [Zl], only some sharp lines appear and have been used for calibration purposes. But very intense autoionized lines, with considerable broadening, are found at the shortest wavelengths. As one might expect, the formation of molecules is hindered with increasing buffer gas pressure, whereas the atomic spectrum is enhanced. At 1130 K and 900 mbar, all molecular bands disappear in favour of the atomic lines. The Iatter are thus observed under easier experimental conditions than in flashpyrolysis [ 161.
A concave grating spectrograph, with 1 m radius of curvature and 2000 lines/mm, was utilized at normal incidence, giving a reciprocal dispersion of 0.5 nm/mm in the first order. A deuterium discharge tube, equipped with a fused silica window, provided the background continuum. An aluminized toroidal mirror, set at 20 cm from the source, received incident light at an angle of 50’ and focused it on the entrance slit, located 170 cm farther. The heat pipe was thus crossed by a nearly parallel beam, and the spectrograph well illuminated. Exposures of 15 to 30 min were sufficient with Xord Q2 plates. A copper/germanium holiow cathode lamp, filled with neon, was used for the production of wavelength standards. But atomic absorption lines from bismuth and even antimony (found at trace level in the metal sample which fi!led the heat pipe) were generally sufficient for calibration purposes_ The spectra have been recorded with the help of a modified Chalonge microphotometer. The accuracy of the measured wavelengths is roughly 0.03 nm for sharp lines without blending. However, such a precision was not reached on the recordings, owing to their complexity. 3. Results 3.1.
Ahmic
absorption
As stated above, under favourable experimental conditions (T> 1130 K, pA,>500 mbar), the atomic spectrum appears below the first ionization limit (A > 170.1 nm). It consists of very sharp lines, already classified by Joshi et al. [16] in Rydberg series converging to various ionization limits. The first one, corresponding to Bi II (?e), lies at 58790 cm-‘. The autoionized lines, situated beyond this limit, have also been observed by the same authors [16]. Centered at 164.4, 163.6 and 161.8 nm, these lines are very strong and show considerable broadening. Their absorption is several orders of magnitude greater than that of the remaining spectrum, and they appear under whatever experimental conditions, even at relatively low temperatures (1930 K).
N. Damany er al. / Ahsorpriorrspecmcnt of bisnturirin rhe VUV
The apparent half widths of the autoionized lines, which may reach 0.5 nm for the broadest one (A = 163.6 nm), seem to show no parallel variation with the atomic absorption intensity. The latter is vey sensitive to the buffer gas pressure, whose Increase hinders the formation of molecules; whereas the former is rather temperature dependent, like the total bismuth pressure which augments with increasing temperature. Joshi et al. mention the total disappearance of these autoionized lines when the capacity of their condenser bank discharging in the flash-lamp is reduced to its half va!ue (without specifying what happens to the remaining spectrum). But such a phenomenon is contradicted by our observation, if the absorption is strictly atomic. Accordingly, it is reasonable to think that some molecular contribution exists, though inseparable from the atomic absorption at the present time. In order to confirm this assumption, one should replace the deuterium discharge lamp, which emits atomic lines in this region, by a pure background continuum source (like the BRV-source). Possible molecular features may thus be observed, particularly when the experimental conditions (temperature and buffer gas pressure) are varied in a broader range. 3.2.
Molecular
absorption
The vacuum ultraviolet absorption bands of Biz cannot be easily analyzed for the following reasons: (i) Owing to the heavy weight of this molecule, its rotational levels are too close to be separated. (ii) The bonding between the two bismuth atoms is weak. Accordingly, the spacing of vibrational levels is small: 173 cm-’ for the first ones in the ground state. (iii) Because of the low bismuth vapor pressure, the heat pipe ought to be operated at high temperatures (T> 1120 K). Several vibrational levels of the ground state are thus populated, and different v” progressions are superimposed in the absorption spectrum. Hence the interest of matrix-isolated bismuth studies [6,7,10-121
159
(performed in other spectral regions) where the absorption features are reduced to transitions originating only from the L”’= 0 level. (iv) Bismuth vapor contains other species than dimers. If the atomic spectrum is easily distinguished, this is not the case for those concerning Bi, (n > 2). Under our experimental conditions, p(Bi,) stands for less than 1% of p(Bi+ Biz), and is further reduced owing to a rather high argon pressure. Accordingly its absorption spectrum may be hardly observed, whereas such clusters are detected in laser fluorescence experiments [6,9,10-121. Between 161 and 222 nm we have observed five band systems, degraded to longer wavelengths. Systems I, III and IV appear at best at roughly 1120 K, whereas for the two others, which are weaker, a higher temperature is needed (1150-1175 EC). All these transitions originate from the X ‘Z, ground state. Sq.srem I. It extends from 222 to 206.6 nm (fig. 1). A complete analysis is not possible, owing to the complexity of the spectrum. Though an error of one or even two units in the vibrational numbering cannot be dismissed, in the absence of the isotope effect (bismuth is monoisotopic), we have tentatively ascribed the value 45490 f 5 cm-’ to zjoo, and 140 f 3 cm-’ to 0,. This system corresponds to the Fc X transition [13], whose first bands at longer wavelengths, observed by Almy and Sparks [l], are mainly u&+ L’& and c: t v;_~ progressions, and have not been analyzed. A sharp breakingoff at 206.6 nm may result from a predissociation, perhaps via a state dissociating into “D 3/z+ zDs12. atomic states (fig. 4). System 1I. Labelled previously as Mc ‘7 transition [IS], this system (fig. 2) is weaker than the first one, on which it is partly superimposed at roughly 206.5 nm. System II extends to 195.5 nm, with a regular decrease in absorption intensity from 208 nm. The foliowing constants are assigned to it: v,~~= 49620 cm-‘, and w,= 158 cm-‘. System III. Its upper state was already designated as the N state 1151. It extends from 195 to 182 nm, and has approximately the same
55
I 180
50
V cm-lx IO-3 li nm
I 190
I 200
IlI
Fig. 1. Microphotometer tmce of the absorption spectrum T= 1120 K, pAz = 0.5 mbar. Molecular systems I-III.
of atomic and molecular
bismuth,
between
1SO and 220 nm, at
range is smaller (from 173 to 168 nm only). We have measured voo= >7980 cm-’ with o, = 140 cm-‘. System V. Its extent is very limited (2.5 nm) (fig. 3), and its intensity has the same magnitude as system II. It cannot be analyzed. The value of ~00 = 59700 cm-’ is measured at the center of the broad band. To our knowledge, systems Ii through V have been observed for the first time in full. We label their upper electronic states as Q, R, S, T.
intensity as system I (fig. 1). wr has roughIy the same value 140 cm-‘, whereas vO,,= 53200 cm-‘. System 1V. This system (fig. 3) has a similar intensity as transitions I and III, but its
wavelength
Fig. 4 and table 1 summarize the equilibrium
our results, where
internuclear distance r, is
derived from the empirical relation r&,
= constant, the v&e
of this constant being
1224 as caIcuIated for the ground state.
4. Discussion
I
8
Fig. 2. hlicrophotometer trace of tbe absorption spectrum of atomic and molecuiar bismuth, between 195 and 210 m-n, at T = 1 I75 K, pxr = 0.5 mbar. Molecular system II.
On account of the vicinity of the first ionization limit, it is reasonable to think that the systems observed belong in most cases to Rydberg transitions. The two spin-orbit partners lie at 7.53 and 8.94 eV respectively [14], which cor-
N. Dammty er al. / Absorption specrrilm of bismuth in the VUV
Fig. 3. Microphotometer trace of the absorption spectrum of atomic and molecular bismuth between 160 and 180 nm, at T= 1135 K, pAr= 3 mbar. Molecular systems IV and V. and two autoionized lines of Bi.
-d
respond to 60730 and 72100 cm-‘. If the systems recorded are Rydberg transitions, the voo values must satisfy the formula: Vaa= IP-
109730/(n
-a)‘,
(1)
where IP hoIds for the ionization potential, n an integer corresponding to the principal quantum number and 6 the molecular quantum defect. Table 1 shows that for the FtX, RtX and S+X transitions, the latter is equal to 3.22, 3.17 and 3.22 respectively, whereas for Q&X and Tt X, 6 =4.85 and 5 respectively. These values are very comparable with the mean value of the atomic quantum defects relative to a nd (S = 3.2) and a rzs (6 = 5) orbital cf Bi. On the other hand, neither approximates to the one which may be calculated [23] for a np orbital (6 = 4.4). Bi2 and Nz are isoelectronic molecules. Accordingly, the ground-state electronic configurations are similar, save the order of the We and rr, molecular orbitals, which is reversed
:
2.5
3
3.5
l-r*
Fig. 3. Roush scheme of energy potentkd curves for Bi, Rydberg siates. Excepred for X state [13], the equilibrium internuclear distances r, are estimated from r$, = constant.
as for As2 and Sbz [24]:
. . . (096s)2(~“6s)z(cTg6p)1(5iu6p)4 ix;. The Rydberg transitions associated with the promotion of an electron from the x,,6p orbital give rise to a,ns and (mgs.Z~ or 8,) nd Rydberg states, the ionization leading to the two spinorbit partners alI U.3,2 and 211u.1,2 of Bi;. The bismuth molecule being weakly bound, the separation between these ‘II states is close to the one concerning the correlated atomic states ‘PO and ‘Pr. The same holds true for the
N. Darnan~ et al. ! Absorprionspectrum of bismuth in the VUV
162
Table 1 Observed
state
Rydbrrg I’&.
(cm-r)
x
0
C F Q R S T
36117 46390 49620 53200 57980 59700
Stares of w.2
Biz
lcrn-‘!
L,
173 157 l-l0 1% 140 140
2.66 2.79 2.96 7.7% 2.96 2.96
Kydbrrg
orbital
quant. Atomic orbital defect
Atomic quant. defect
Dissociation products
D (cm-‘)
4.87 3.22 1.85 3.17 3.22 5
4.95 3.29 4.90 3.24 3.26 5
6p34S.oibp 3 %” 6p3’S0i75’P,,z 6p” %?+6d ‘D3,: 6p3 “S”+8s’PlIZ 6p3’S”i7d’D,,Z 6p3’S”t6d’D 6p3 ‘S”t8s ‘P3/r
16500 12640 13920 14250 1422U 15100 17600
* Mol.
ionization potentials (7.53 and 8.94 eV for the molecule, 7.29 and S.94 eV for the atom [14]). Concerning the states which converge to the first ionization potential (607302 100 cm-‘), we have to notice thar the C state, observed by Almy and Sparks [l J folIows we11 the Rydberg formuIa with S = 4.87. it should proceed from the transfer of an electron from a 7;;6p to a ~~7s orbital. The next term of this series is the Q state (6 =3.85) resulting from the promotion io a a;& orbitaL The 8 values found for F and R states (3.22 and.3.17 respectively) clearly establish that the excited moIecular orbitals attained are correlated to rrd atomic orbitals (&, = 3.29 and 3.24); hence they are (crag.i~g or S,) 6d and 7d molecular orbitak. The S and T states lead similarly to the second ionization potential (7213Ok 100 cm-‘). The T state, with a quantum defect of 5 for n =S, has undoubtedly a . . . (~,2s)“(~,2s)‘(~,2p!‘~~~2p)~(u~ss) configuration. For the S state, 6 has the value corresponding to a 6d eIectron. Due to the large spin-orbit interaction, the Bi- molecule belongs mainly to Hund’s case (c). The ground state being a ‘X: state, transitions originating from it end at 0: and 1: states, which result only from doublet or quartet states of Bi. Thus the C, Q, and T states dissociate into a %a and another (7s or 8s) 4P atom. Assuming for D, the ground state dissociation ener,yr, 16500 cm-’ as a mean value [21,25,26] and taking into account the atomic excitation energy, one can calculate the dissociation ener-
7s 6d s5 7d 6d 8s
gies of these three states. They are equal to 12600, 14280 and 17600 cm-’ respectively, of the same magnitude as the ground state value. This is well understandable by the transfer of the electron from a bonding orbital rr,, to another bonding one, us_ When calculating the dissociation energies, it is supposed obviously that no perturbations affect these states and the potential curves are not deformed (non-crossing rule). When studying the C state, Almy and Sparks [I] have observed that the corresponding we-r=does not lead to a reasonable dissociation energy, even when assuming that this state is a valence state dissociating into zD3,a f ‘D5,?_ From our spectra it is not possible to derive any value of ~~.r,. Therefore no further information can be provided at present. One of the dissociation products of the F, R, and S states is a (6d or 7d) ‘D atom. The other may be the quadruplet %$a or a doublet, the lowest in energy being ‘D&? (11420 cm-‘). According to the first hypothesis, the calculated dissociation energies are roughly 13900, 14200 and 15100 cm-‘, very close to the one concerning the ground state. On the contrary, if dissociation products were nd ‘D and 6p3 ‘D$z, the correspcnding D values would be 25300, 25700 and 26500 cm-’ respectively, much higher than the ground state dissociation energy. This involves that the promoted electron moves on a more bonding orbital, which seems quite improbable as we have calculated for these three states r, = 2.96 A, a little larger than the ground state value. So we retain rather the first hypothesis. In this case, the orbital occupied by
N. Damany
et al. / Absorption spectrum of bismuth in the VUV
the promoted electron may be a ugnd, which has the same bonding character as 7ru6p [27]. Here aIso the dissociation energies proposed do not take into account a possible deformation of the potential energy curves. We have in particular mentioned an abrupt breaking-off in system I, which may infer a predissociation in the F state. Finally, it is noteworthy that F and S whose configuration was assumed to be 6d, fit well the Rydberg formula. This fact is not surprising because a 6d orbital, though having the same principal quantum number as the initial 6p orbital, behaves somewhat like a Rydberg orbital, owing to its largeness_
Acknowledgement WC would like to thank Professor Jean D’Incan for his interest and useful discussions. Dr. Gustav Gerber and Mr. Joachim Janes, of the University of Freiburg, are also gratefully acknowledged for technical assistance during the building of our heat pipe.
References [l] G.M. Almy and F.M. Sparks, Phys. Rev. 44 (1933) 365. [2] W-P. West and H.P. Broida, Chem. Phys. Letters 56 (1978) 283. [3] B. Wellegehausen, D. Friede and G. Steger, Optics Commun. 26 (1978) 391. [4] G. Nakamura and T. Shidei, Japan J. Phys. 10 (1934) 11. [S] N. Aslund, R.F. Barrow, W.G. Richards and D.N. Travis, Ark. Fys. 30 (1965) 171.
163
[6] R. A. Teichman III and E. R. Nixon, J. Chem. 67 (1977) 2470. [73 V.E. Bondybey, G. P. Schwartz, J. E. Grifiiths and J.H. English, Chem. Phys. Letters 76 (1980) 30. [S] S.P. Reddy and M.K. Ali, J. Mol. Specrry. 35 (1970) 285. [9] G. Gerber, K. Sakurai and H.P. Broida, J. Chem. Phys. 64 (1976) 3410. [lo] V-E. Bondybey and J.H. English, J. Chem. Phys. 73 ( 1980) 42. [ll] F. Ahmed and E.R. Nixon, J. Chem. Phys. 74 (1981) 2156. [12] M. Manzel, U. Engelhardt, H. Abe, W. Schulze and F.W. Froben, Chem. Phys. Letters 77 (1981) 514. [13] G. Gerber and H.P. B&da, J. Chem. Phys. 64 (1976) 3423. [14] S. Siizer, S.T. Lee and D.A. Shirley, J. Chem. Phys. 65 (1976) 412. [15] A. Topouzkhanian, A.M. Sibai and .I. D’Incan, 2. Natmforsch. 29a (1974) 436. [16] Y.N. Joshi and R.P. Srivastava, Can. J. Phys. 56 (1978) 1157. (171 C.R. Vidal and J. Cooper, J. Appl. Phys. 40 (1969) 3370. 1183 C.R. Vidal and F.B. Hailer, Rev. Sci. Instr. 32 (1971) 1779. [19] LA. Melton and P.H. Wine, J. Appl. Phys. 51 (1980) 4059. [20] R.E. Honig and D.A. Kramer, RCA Rev. 30 (1969) 285. [21] A.K. Fischer, J. Chem. Phys. 45 (1966) 375. [22] K.P. Huber and G. Herzberg, Constants of diatomic molecules (Van Nostrand, Princeton, 1979) p. 92. [23] C.E. Moore, Atomic energy levels, Circular No. 467 (National Bureau of Stand.. Washington, 1949). [24] R.J. Donovan and P. Strachan, Trans. Faraday Sot. 67 (1971) 3407. [25] L. Rovner, A. Drowart and J. Drowart. Trans. Faraday Sac. 63 (1967) 2906. [26] F.J. Kohl, O.M. Uy and K.D. Carlson, J. Chem. Phys. 47 (1967) 2667. 1271 G. Herzberg, MoIecular spectra and moiecutar structure. Vol. 1. Spectra of diatomic molecules, 2nd Ed. (Van Nostrand. New York, 1963) p_ 329.