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The vacuum UV spectrum of HgBr2
JOVRNAL
OF MOLECULAR
SPECTROSCOPY
The Vacuum
32, 287-295 (1969)
UV Spectrum of HgBrP
A. GEDANKEN, B. RAZ, U. EVEN, Department
of Chemistry,
Tel-Aviv
AND
University,
I. ELIEZER Tel-Aviv,
Israel
The band spectrum of HgBrz was investigated in the &human vacuum uv region (1800 &1900 d). A new assignment of this spectrum is given. The vibration frequencies for HgBrt were found to be: Ye’ = 18.5cm-l
ylV = 221 cm-l
~2’ = 20 cm-l
~2~= 53 cm-l
~3’ = 224 cm-l
~3” = 295 cm-’
INTRODUCTION
Mercuric halides exhibit band absorption in the 1700 8-2100 A region. Measurements were first performed on HgClz and HgBr, by Wehrli (1) who also proposed an assignment of the bands observed. According to this assignment the observed bands were due to progressions in the symmetric stretching vibration and sequences in the bending vibration of Av = 0, f2. No changes in the quantum numbers of the antisymmetric stretching vibration were considered. Intensity calculations performed by Coon (2) revealed the inadequacy of this assignment for HgCh . Using a recursion formula for Franck-Condon integrals, Coon showed that a factor of more than a hundred existed between intensity values of Av, = f2 calculated according to Wehrli’s assignment and those measured by him. A new assignment for HgCl, spectrum was then proposed, based on a sequence of Av = 0 in the antisymmetric stretching vibration. Good agreement was then obtained both for the energy values and for the intensities. Wehrli’s measurements on HgBr2 were re-examined by Bell (3)) who followed the same principles used by Coon for HgCL , but succeeded in assigning the symmetric stretching vibration only. From Raman spectroscopy a value of yin = 220 cm-’ is obtained for HgBr, and this is in good agreement with the prominent progression observed in the spectrum. The second important progression yielded a frequency of 190 cm-’ which was attributed to ~1’. In this case there exist some discrepancies between observed and assigned energy values (see Table I). In any case, some 20 bands remained unassigned. A variation on Wehrli’s interpretation was proposed by Sponer and Teller (4). These authors suggested a new set of bending vibration frequencies for 287
288
GEDANKEN,
RAZ, EVEN,
AND
ELIEZER
HgBrz ; namely, ~2~ = 41 cm-’ and ~2’ = 36 cm-‘. Using these values, Bell (3) showed, in a way similar to Coon’s treatment for HgClz , that the intensities Au expected for the transitions 2 = f2 are lower by a factor of 100 as compared with the observed ones. Bell was therefore led to reassign Sponer and Teller’s (020 -+ 000) transition and attributed it to (0, 212, 1 + 0, 02 , 1). He thus had to assign a value of 189 cm-’ to Ye’. This value does not obey the well-known relation (5) between symmetric and antisymmetric stretching vibration frequencies vg =
vl(l+ g>;,
where vs and VI are the antisymmetric and symmetric vibration frequencies respectively, m is the halogen mass and M is the mercury atomic mass. In view of the large number of unexplained bands, we felt it of interest to remeasure the spectrum of HgBr2 , and have been able to propose a new assignment which is found to be in good agreement with the energy values as well as with the observed intensities. EXPERIMENTAL
Because of the low vapor pressure of HgBr2 at room temperatures, a controlled temperature cell in the range 7O”C-200°C with an optical path of 70 mm was constructed. The cell was made of stainless steel (SS 304) which proved to be heated by a satisfactory for use with gaseous HgBr2 . The cell was uniformly Thermo-Coax wire wound on its periphery. The cell periphery was then optically insulated using an external cover in order to prevent any light emitted by the heating element from reaching the photomultiplier, and thus contributing to stray light effects. Suprasil and sapphire windows were used. The latter seems a better choice because of its enhanced transparency and its better thermal conductivity. The cell-window interface was made vacuum tight by viton “0” rings. The cell was sealed during the measurement by a valve constructed in such a way that no surface cooler than the cell’s inner surface would be exposed to the vapors thus preventing distillation. The cell windows were heated by special heating elements in order to ensure uniform temperature and to prevent deposition of material on the windows which diminishes their transparency. The mercuric bromide to be examined (C.P. grade repurified by sublimation) was introduced into a glass capillary which was glued to the inner part of the vacuum valve in a manner that assured its thermal isolation from the other parts of the cell assembly. The cell was then pumped and heated to the desired temperature. When good vacuum (better than 2 X 10d6 Torr) was obtained, the valve was closed, causing simultaneous breakage of the capillary. A McPherson 225 Spectrograph was used at normal incidence, with photoelectric detection and a resolution of 0.25 A. The light source used was a Hin-
SPECTRUM OF HgBrz
289
terreger discharge lamp operating on 1 mm of hydrogen. The continum emission of X > 1675 H was used. RESULTS
The absorption spectrum of HgBrz at (100” f 1°C) is shown in Fig. l.The transition energies are in very good correspondence with those previously reported by Bell (3). The excessive line broadening in the HgBr, spectrum (as compared to HgCh) , which is probably due to predissociation (see Discussion), introduced some uncertainty in the determination of the relative intensities. DISCUSSION
The abundance of absorption peaks in the absorption spectrum of HgBr, , relative to those which could be assigned using symmetric and antisymmetric stretching vibration only, led us to introduce the bending vibration as a possible contribution to the spectrum. Table I compares previous experimental and calculated (S) band energies to our proposed assignment. The frequencies used by us for the assignment of the HgBrz spectrum are: V1’= 185 cm-’
~1~ = 221 cm-’
v21=
~-2~=
20 em-’
%I = 224 cm-’
53 cm-’
va’ = 295 cm-’
FIG. 1. The vacuum uv absorption spectrum of gaseous HgBrp at 100°C
(53
(53
(54
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8
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290)
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TABLE
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Frequencies,
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4)
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ment.
Present
The
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0.03
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0.05
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-7
0
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0.30
0.20
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0.40
0.40
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0.20
Intensity.
Estimated
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0.05
0
present
Assignment.
to
According
-5
Assign-
Intensity
Calculated
0.16
-2
-35
ment.
Previous
to
Deviation
According
-4d
Assign-
to
Deviation
According
The
Assignments.
SPECTRUM OF HgBrz
291
(55
23
24
(d)
(c)
0-J)
(0)
(54
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22
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910)
OS in
deviationsin
underlined
The
are
deviations
3.
ore
from
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in
Numbering
Values
Reference ref.(3) deviations
deviations
are
and
from
energies
3)
ore
from
( -2)
(+I)
(-
( + 6)
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(-7)
( -11)
obtained
in
to the
(3). work.
meosuremenh.
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+3
‘he Deviation recording to ‘revious Assign ment.
present
present
ref
the
-10 -
-8 -
-10 -
The Deviation Accordin to Present w ssign ment.
Assignments.
according
values
000
000
000
010
002
000
010
energies
from
underlined
+
400
(1813.70)
(1819.77) +
+
+
+
+
310
302
200
10
And
Assignment
+
000
000
2
Present
320
+
Assignment(3)
Intensities
Continued.
300
300+
200
Previous
Frequencies,
i
(1818.58)
(1821.16)
1824.24
(54
(1824.15)
1825.74
(1825.78)
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771)
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742)
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AR
Bond
TABLE
0,03
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0,18
0.06
0.54
0.70
Slculated Intensity brccording to present ksslgnmenl
0.40
0.50
Estimated Intensity
SPECTRUM OF HgBrz
293
The experimental intensities are also compared to those calculated according to our new assignment. The intensity calculations were based on the following recursion formula for Franck-Condon integrals obtained by Ansbacher (6) : R(U), v”) =
me;(;)’ R(v’ vN
/
-
1) +
.R(v’ - 1, /
-
1) - I&
v”) = !!I&%? R(;, l+P
v”) +
1,
-R(d
R(v',
-
1,
0 v
(;):
A2
i
R(v'
A2
-
2,
v”) (2)
$
(>: (s)’
R(d, vN - 2), 21
where R( v’, v”) , is the I’ranck-Condon overlap integral for the transition V’t vN, p = OI~/~’ and ~1~= 4r’Cv/h. Aq = qn - q’ is the separation of the equilibrium positions of the two states.’ For the degenerate bending vibration, the two dimensional overlap integrals were calculated from the overlap integrals for one-dimensional oscillators, using the following sums: F(v, v) = S$ [R(s, s)R(v - s, v - s)]”
(3)
5 2[R(s, s)R(v + 2 - 8, 21- a)12, F(v + 2, v) = s=o
(4)
and
where F(v, v) and F(v2’, v) are the Franck-Condon factors for the v +- v and for the v + 2 +- v transition between two-dimensional oscillators, respectively. The calculation was based on the value 1.15 (estimated from the experimental spectrum) for the intensity ratio between the (100 +- 000) and the (000 c 000) transitions. Using this value in Eq. (2) and the relation Ar = Aq/(2m)“’ for the change in b$nd length associated with the electronic transition, we obtained Ar = 0.046 A. This result is consistent with the corresponding value for HgC12 (9) which is Ar = 0.042 A. A consistency check on our assignment was obtained from thermochemical data. The bending vibration frequencies for HgBr2 as well as HgC12 and Hg12 were calculated using the thermodynamic data of Braune and Knoke (7). In this calculation we used bond lengths which were recently obtained from electron diffraction data (8) and new vibrational frequencies of the halogen molecules (9) and mercuric halides (1,Z). The thermodynamic data used were the temperature dependence of the equilibrium constants for the reaction. 1For the non-totally symmetric vibrations, Aq = 0 was taken.
GEDANKEN,
294
RAZ, EVEN, TABLE
AND
ELIEZER
II
BENDING VIBRATION FREQUENCIES FOR HgX, Molecules
up” cm-1 from thermodynamic data Previous estimate
HgClz HgBrt HgIz
MOLECULES
Present values
71 64 50
J%Xzk) + Hdd + X&d
76 62 51
X = Cl, Br, I.
(5)
Using previous frequency values for V: and v:, the values of V: in Table II were obtained. The value of v~” = 53 cm-l obtained for HgBrz from the optical spectrum is reasonably consistent with the thermodynamic data. The sequence Av = f2 for the bending vibration which is not observed in HgClz , is detectable in HgBrz because of the large frequency difference between ground and excited states. Part of the sequences of As = 0 f 2 is identified in the spectrum. The use of our frequency ratio Y~“/vz’ = 53/20 = 2.65 in the intensity calculations (as compared with the ratio 41/36 = 1.14 proposed by Sponer and Teller, (4) and used by Bell (S)), explains why these bands are intense enough to be experimentally observed. Turning our attention to the stretching vibrations one should notice that the to excited state obeys Eq. (1) leadvalue of pa” = 224 cm-’ which we attribute ing to Q’ = 248 cm-’ which is in reasonable agreement with the proposed value for ~3’. Finally, we should test the self-consistency of the assignment, and enquire which bands of moderate and strong intensity are missing from the experimental spectrum. Bands belonging to transitions 2 +- 2 in the bending vibration and higher sequences of this vibration are not observed in the spectrum although on the basis of an intensity calculation these bands should be detectable. Thus, for is expected to be more intense than the example, the (020 +- 020) transition (000 t 000) by a factor of 1.06). We propose that these bands are excessively broadened because of an electronic predissociation, which is known to occur in HgBrz (1). The symmetry of a u2 = 2 state is 7r X K = A + c+ + c- which indicates the possible symmetry of the perturbing electronic continuum state. On the other hand, the bands 3 +- 1 and 4 t 2 belonging to the Avz = +2 sequence appear in the spectrum but they coincide with other bands. The 3 +- 1 transitions always coincides with bands of the progressions in VI’ and ~1’ and the 4 t 2 coincides with either (010 +- ~~‘10) or (~1’10 +- 010). To conclude this consistency check it should be pointed out that we do not observe in the spectrum
SPECTRUM
?95
OF IIgBrz
the transition (110 + 010) which we expect to be at 54575 cm-’ and to be intense enough to be oserved. In conclusion, we believe that the present assignment elucidates the main features of the electronic spectrum of HgBr2 .
We are
indebted to Professor J. Jortner for geuerous assistance aud helpful discussious.
RECEIVED:
April
2, 1969 BEFEKENCES
I. H. WEHRLI, HA. Phys. Acta 11, 339 (1938). R. 8. BELL, R. D. MCKENZIE, AND J. B. COON, J. Mol. Spectr!/. 20, 217 (1966). 3. S. BELL, J. Mol. Spectry. 23, 98 (1967). 4. H. SPONER AND E. TELLER, Rev. Mod. Phys. 13, 75 (1941). 5. W. PENNEY AND G. SUTHI~RUND, Proc. Roy. sot. 166, 654 (1936). 6. F. ANSBACHER, Z. Naturforsch. 14a, 889 (1959). 7. H. BRAUNE AND S. KNOKE, 2. Physik. Chem. (Leipzig) B23, 163 (1933). 8. H. GREGG, G. C. HAMPSON, G. I. JENKINS, P. L. F. JONES, AND L. E. SUTTON, Trans. Faraday Sot. 33, 852, 1937. 9. G. HERZBERG, ‘LMolecular Princeton, N. J. (1963).
Spectra
and Molecular
Structure,”
Vol.
I. Van Nostrand,
OF MOLECULAR
SPECTROSCOPY
The Vacuum
32, 287-295 (1969)
UV Spectrum of HgBrP
A. GEDANKEN, B. RAZ, U. EVEN, Department
of Chemistry,
Tel-Aviv
AND
University,
I. ELIEZER Tel-Aviv,
Israel
The band spectrum of HgBrz was investigated in the &human vacuum uv region (1800 &1900 d). A new assignment of this spectrum is given. The vibration frequencies for HgBrt were found to be: Ye’ = 18.5cm-l
ylV = 221 cm-l
~2’ = 20 cm-l
~2~= 53 cm-l
~3’ = 224 cm-l
~3” = 295 cm-’
INTRODUCTION
Mercuric halides exhibit band absorption in the 1700 8-2100 A region. Measurements were first performed on HgClz and HgBr, by Wehrli (1) who also proposed an assignment of the bands observed. According to this assignment the observed bands were due to progressions in the symmetric stretching vibration and sequences in the bending vibration of Av = 0, f2. No changes in the quantum numbers of the antisymmetric stretching vibration were considered. Intensity calculations performed by Coon (2) revealed the inadequacy of this assignment for HgCh . Using a recursion formula for Franck-Condon integrals, Coon showed that a factor of more than a hundred existed between intensity values of Av, = f2 calculated according to Wehrli’s assignment and those measured by him. A new assignment for HgCl, spectrum was then proposed, based on a sequence of Av = 0 in the antisymmetric stretching vibration. Good agreement was then obtained both for the energy values and for the intensities. Wehrli’s measurements on HgBr2 were re-examined by Bell (3)) who followed the same principles used by Coon for HgCL , but succeeded in assigning the symmetric stretching vibration only. From Raman spectroscopy a value of yin = 220 cm-’ is obtained for HgBr, and this is in good agreement with the prominent progression observed in the spectrum. The second important progression yielded a frequency of 190 cm-’ which was attributed to ~1’. In this case there exist some discrepancies between observed and assigned energy values (see Table I). In any case, some 20 bands remained unassigned. A variation on Wehrli’s interpretation was proposed by Sponer and Teller (4). These authors suggested a new set of bending vibration frequencies for 287
288
GEDANKEN,
RAZ, EVEN,
AND
ELIEZER
HgBrz ; namely, ~2~ = 41 cm-’ and ~2’ = 36 cm-‘. Using these values, Bell (3) showed, in a way similar to Coon’s treatment for HgClz , that the intensities Au expected for the transitions 2 = f2 are lower by a factor of 100 as compared with the observed ones. Bell was therefore led to reassign Sponer and Teller’s (020 -+ 000) transition and attributed it to (0, 212, 1 + 0, 02 , 1). He thus had to assign a value of 189 cm-’ to Ye’. This value does not obey the well-known relation (5) between symmetric and antisymmetric stretching vibration frequencies vg =
vl(l+ g>;,
where vs and VI are the antisymmetric and symmetric vibration frequencies respectively, m is the halogen mass and M is the mercury atomic mass. In view of the large number of unexplained bands, we felt it of interest to remeasure the spectrum of HgBr2 , and have been able to propose a new assignment which is found to be in good agreement with the energy values as well as with the observed intensities. EXPERIMENTAL
Because of the low vapor pressure of HgBr2 at room temperatures, a controlled temperature cell in the range 7O”C-200°C with an optical path of 70 mm was constructed. The cell was made of stainless steel (SS 304) which proved to be heated by a satisfactory for use with gaseous HgBr2 . The cell was uniformly Thermo-Coax wire wound on its periphery. The cell periphery was then optically insulated using an external cover in order to prevent any light emitted by the heating element from reaching the photomultiplier, and thus contributing to stray light effects. Suprasil and sapphire windows were used. The latter seems a better choice because of its enhanced transparency and its better thermal conductivity. The cell-window interface was made vacuum tight by viton “0” rings. The cell was sealed during the measurement by a valve constructed in such a way that no surface cooler than the cell’s inner surface would be exposed to the vapors thus preventing distillation. The cell windows were heated by special heating elements in order to ensure uniform temperature and to prevent deposition of material on the windows which diminishes their transparency. The mercuric bromide to be examined (C.P. grade repurified by sublimation) was introduced into a glass capillary which was glued to the inner part of the vacuum valve in a manner that assured its thermal isolation from the other parts of the cell assembly. The cell was then pumped and heated to the desired temperature. When good vacuum (better than 2 X 10d6 Torr) was obtained, the valve was closed, causing simultaneous breakage of the capillary. A McPherson 225 Spectrograph was used at normal incidence, with photoelectric detection and a resolution of 0.25 A. The light source used was a Hin-
SPECTRUM OF HgBrz
289
terreger discharge lamp operating on 1 mm of hydrogen. The continum emission of X > 1675 H was used. RESULTS
The absorption spectrum of HgBrz at (100” f 1°C) is shown in Fig. l.The transition energies are in very good correspondence with those previously reported by Bell (3). The excessive line broadening in the HgBr, spectrum (as compared to HgCh) , which is probably due to predissociation (see Discussion), introduced some uncertainty in the determination of the relative intensities. DISCUSSION
The abundance of absorption peaks in the absorption spectrum of HgBr, , relative to those which could be assigned using symmetric and antisymmetric stretching vibration only, led us to introduce the bending vibration as a possible contribution to the spectrum. Table I compares previous experimental and calculated (S) band energies to our proposed assignment. The frequencies used by us for the assignment of the HgBrz spectrum are: V1’= 185 cm-’
~1~ = 221 cm-’
v21=
~-2~=
20 em-’
%I = 224 cm-’
53 cm-’
va’ = 295 cm-’
FIG. 1. The vacuum uv absorption spectrum of gaseous HgBrp at 100°C
(53
(53
(54
-54
7
8
12
11
10
X
290)
291
54
217
(54
215)
54 -
;;;)
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(;
1841.91
(1841.W)
1844.42 -
(1844.51)
(1846.01) 1845.95
;
(;z.;)
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(;;
(1848.26)
- 1849.35
(1849.45)
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(1853.57)
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(1855.77)
(1857.08)
(1862.54)
A
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;;)
9%)
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885)
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-
(;;
073
(53
5
6
9
070)
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4
848)
(53
690)
(53
3
cm-’
Energy
2
No.
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001
0
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.
+
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211 220
+
I,IIhI
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OlO+
And
Assignment
OlOc
‘l”Z5
I.
Intensities
TABLE
’ Present
Frequencies,
AssignmenE3)
L); +j
0
Previous
Band
fp
4)
-2)
-2)
(
(
-2)
0)
( t4)
(
(+2)
(
( -3)
( -3)
( -1)
(-2)
( -4)
(+2)
(t
( -4)
( 0) (-2)
(t
ment.
Present
The
0.10
0.02
0.09
0.12
0.03
0.04 0.01
0.05
=3 -
-1
-7
-4
2
-a
-7
0
0.13
0.42
0.30
0.20
0.08 0.26
0.40
0.40
0.10
0.20
Intensity.
Estimated
0.39
0.51
0.12
0.05
0
present
Assignment.
to
According
-5
Assign-
Intensity
Calculated
0.16
-2
-35
ment.
Previous
to
Deviation
According
-4d
Assign-
to
Deviation
According
The
Assignments.
SPECTRUM OF HgBrz
291
(55
23
24
(d)
(c)
0-J)
(0)
(54
(54
22
(54
136)
988)
952)
910)
OS in
deviationsin
underlined
The
are
deviations
3.
ore
from
parentheses
parentheses
The
in
Numbering
Values
Reference ref.(3) deviations
deviations
are
and
from
energies
3)
ore
from
( -2)
(+I)
(-
( + 6)
(-13)
(-7)
( -11)
obtained
in
to the
(3). work.
meosuremenh.
-12
+3
‘he Deviation recording to ‘revious Assign ment.
present
present
ref
the
-10 -
-8 -
-10 -
The Deviation Accordin to Present w ssign ment.
Assignments.
according
values
000
000
000
010
002
000
010
energies
from
underlined
+
400
(1813.70)
(1819.77) +
+
+
+
+
310
302
200
10
And
Assignment
+
000
000
2
Present
320
+
Assignment(3)
Intensities
Continued.
300
300+
200
Previous
Frequencies,
i
(1818.58)
(1821.16)
1824.24
(54
(1824.15)
1825.74
(1825.78)
820)
772
54
1826.79
(1826.75)
y 817 --..-
771)
(54
741
54
21
20
19
742)
(54
AR
Bond
TABLE
0,03
0,14
0,18
0.06
0.54
0.70
Slculated Intensity brccording to present ksslgnmenl
0.40
0.50
Estimated Intensity
SPECTRUM OF HgBrz
293
The experimental intensities are also compared to those calculated according to our new assignment. The intensity calculations were based on the following recursion formula for Franck-Condon integrals obtained by Ansbacher (6) : R(U), v”) =
me;(;)’ R(v’ vN
/
-
1) +
.R(v’ - 1, /
-
1) - I&
v”) = !!I&%? R(;, l+P
v”) +
1,
-R(d
R(v',
-
1,
0 v
(;):
A2
i
R(v'
A2
-
2,
v”) (2)
$
(>: (s)’
R(d, vN - 2), 21
where R( v’, v”) , is the I’ranck-Condon overlap integral for the transition V’t vN, p = OI~/~’ and ~1~= 4r’Cv/h. Aq = qn - q’ is the separation of the equilibrium positions of the two states.’ For the degenerate bending vibration, the two dimensional overlap integrals were calculated from the overlap integrals for one-dimensional oscillators, using the following sums: F(v, v) = S$ [R(s, s)R(v - s, v - s)]”
(3)
5 2[R(s, s)R(v + 2 - 8, 21- a)12, F(v + 2, v) = s=o
(4)
and
where F(v, v) and F(v2’, v) are the Franck-Condon factors for the v +- v and for the v + 2 +- v transition between two-dimensional oscillators, respectively. The calculation was based on the value 1.15 (estimated from the experimental spectrum) for the intensity ratio between the (100 +- 000) and the (000 c 000) transitions. Using this value in Eq. (2) and the relation Ar = Aq/(2m)“’ for the change in b$nd length associated with the electronic transition, we obtained Ar = 0.046 A. This result is consistent with the corresponding value for HgC12 (9) which is Ar = 0.042 A. A consistency check on our assignment was obtained from thermochemical data. The bending vibration frequencies for HgBr2 as well as HgC12 and Hg12 were calculated using the thermodynamic data of Braune and Knoke (7). In this calculation we used bond lengths which were recently obtained from electron diffraction data (8) and new vibrational frequencies of the halogen molecules (9) and mercuric halides (1,Z). The thermodynamic data used were the temperature dependence of the equilibrium constants for the reaction. 1For the non-totally symmetric vibrations, Aq = 0 was taken.
GEDANKEN,
294
RAZ, EVEN, TABLE
AND
ELIEZER
II
BENDING VIBRATION FREQUENCIES FOR HgX, Molecules
up” cm-1 from thermodynamic data Previous estimate
HgClz HgBrt HgIz
MOLECULES
Present values
71 64 50
J%Xzk) + Hdd + X&d
76 62 51
X = Cl, Br, I.
(5)
Using previous frequency values for V: and v:, the values of V: in Table II were obtained. The value of v~” = 53 cm-l obtained for HgBrz from the optical spectrum is reasonably consistent with the thermodynamic data. The sequence Av = f2 for the bending vibration which is not observed in HgClz , is detectable in HgBrz because of the large frequency difference between ground and excited states. Part of the sequences of As = 0 f 2 is identified in the spectrum. The use of our frequency ratio Y~“/vz’ = 53/20 = 2.65 in the intensity calculations (as compared with the ratio 41/36 = 1.14 proposed by Sponer and Teller, (4) and used by Bell (S)), explains why these bands are intense enough to be experimentally observed. Turning our attention to the stretching vibrations one should notice that the to excited state obeys Eq. (1) leadvalue of pa” = 224 cm-’ which we attribute ing to Q’ = 248 cm-’ which is in reasonable agreement with the proposed value for ~3’. Finally, we should test the self-consistency of the assignment, and enquire which bands of moderate and strong intensity are missing from the experimental spectrum. Bands belonging to transitions 2 +- 2 in the bending vibration and higher sequences of this vibration are not observed in the spectrum although on the basis of an intensity calculation these bands should be detectable. Thus, for is expected to be more intense than the example, the (020 +- 020) transition (000 t 000) by a factor of 1.06). We propose that these bands are excessively broadened because of an electronic predissociation, which is known to occur in HgBrz (1). The symmetry of a u2 = 2 state is 7r X K = A + c+ + c- which indicates the possible symmetry of the perturbing electronic continuum state. On the other hand, the bands 3 +- 1 and 4 t 2 belonging to the Avz = +2 sequence appear in the spectrum but they coincide with other bands. The 3 +- 1 transitions always coincides with bands of the progressions in VI’ and ~1’ and the 4 t 2 coincides with either (010 +- ~~‘10) or (~1’10 +- 010). To conclude this consistency check it should be pointed out that we do not observe in the spectrum
SPECTRUM
?95
OF IIgBrz
the transition (110 + 010) which we expect to be at 54575 cm-’ and to be intense enough to be oserved. In conclusion, we believe that the present assignment elucidates the main features of the electronic spectrum of HgBr2 .
We are
indebted to Professor J. Jortner for geuerous assistance aud helpful discussious.
RECEIVED:
April
2, 1969 BEFEKENCES
I. H. WEHRLI, HA. Phys. Acta 11, 339 (1938). R. 8. BELL, R. D. MCKENZIE, AND J. B. COON, J. Mol. Spectr!/. 20, 217 (1966). 3. S. BELL, J. Mol. Spectry. 23, 98 (1967). 4. H. SPONER AND E. TELLER, Rev. Mod. Phys. 13, 75 (1941). 5. W. PENNEY AND G. SUTHI~RUND, Proc. Roy. sot. 166, 654 (1936). 6. F. ANSBACHER, Z. Naturforsch. 14a, 889 (1959). 7. H. BRAUNE AND S. KNOKE, 2. Physik. Chem. (Leipzig) B23, 163 (1933). 8. H. GREGG, G. C. HAMPSON, G. I. JENKINS, P. L. F. JONES, AND L. E. SUTTON, Trans. Faraday Sot. 33, 852, 1937. 9. G. HERZBERG, ‘LMolecular Princeton, N. J. (1963).
Spectra
and Molecular
Structure,”
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