Gas-phase electronic absorption spectroscopy of substituted bis(η6-benzene)chromium derivatives: Rydberg transitions in bis(η6-anisole)chromium and bis(η6-2,6- dimethylpyridine)chromium

Article abstract of DOI:10.1007/s11172-005-0051-8

Gas-phase electronic absorption spectra of chromium bisarene complexes with oxygen- and nitrogen-containing ligands, (η⁶-PhOMe) ₂Cr (1) and (η⁶-2,6-Me₂C₅H ₃N)₂Cr (2), were first measured. Rydberg bands disappearing on going to the condensed-phase spectra were revealed. The first ionization potentials of complexes 1 and 2 (5.30 and 5.40 eV, respectively) were determined from the Rydberg frequencies. The Rydberg transitions were assigned and the corresponding Rydberg term values and quantum defects were determined. The effect of heteroatoms on the Rydberg structure parameters was revealed by comparing the spectra of complexes 1 and 2 with those of unsubstituted analogs. The appearance, in the ligand side chain, of an oxygen atom capable of being involved in conjugation with the π-electron system of the aromatic ring results in substantial broadening of the observed Rydberg bands. This can be associated with an increased ligand contribution to the HOMO of the sandwich compound. The influence of the oxygen atom on the ionization energy of the molecule is insignificant. In contrast to this, introduction of a nitrogen atom into the carbocycle leads to a noticeable increase in the ionization potential of the molecule, while the ligand contribution to the HOMO of the complex remains practically unchanged.

Full text of DOI:10.1007/s11172-005-0051-8

1
932
Russian Chemical Bulletin, International Edition, Vol. 53, No. 9, pp. 1932—1937, September, 2004
Gasꢀphase electronic absorption spectroscopy
6
of substituted bis(η ꢀbenzene)chromium derivatives:
6
Rydberg transitions in bis(η ꢀanisole)chromium and
6
bis(η ꢀ2,6ꢀdimethylpyridine)chromium
S. Yu. Ketkov,aꢀ G. A. Domrachev, A. M. Ob´edkov, A. Yu. Vasil´kov, L. P. Yur´eva, and C. P. Mehnert
a
a
b
b
c
a
G. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
4
9 ul. Tropinina, 603950 Nizhny Novgorod, Russian Federation.
Fax: +7 (831 2) 12 7497. Eꢀmail: sketkov@imoc.sinn.ru
b
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
8 ul. Vavilova, 119991 Moscow, Russian Federation.
2
Fax: +7 (095) 135 5085. Eꢀmail: alexvas@ineos.ac.ru
Inorganic Chemistry Laboratory, University of Oxford,
OX1 3QR, Oxford, South Parks Road, UK.
Fax: +44 (01865) 27 2690
c
Gasꢀphase electronic absorption spectra of chromium bisarene complexes with oxygenꢀ
6
6
and nitrogenꢀcontaining ligands, (η ꢀPhOMe) Cr (1) and (η ꢀ2,6ꢀMe C H N) Cr (2), were
2
2
5
3
2
first measured. Rydberg bands disappearing on going to the condensedꢀphase spectra were
revealed. The first ionization potentials of complexes 1 and 2 (5.30 and 5.40 eV, respectively)
were determined from the Rydberg frequencies. The Rydberg transitions were assigned and the
corresponding Rydberg term values and quantum defects were determined. The effect of
heteroatoms on the Rydberg structure parameters was revealed by comparing the spectra of
complexes 1 and 2 with those of unsubstituted analogs. The appearance, in the ligand side
chain, of an oxygen atom capable of being involved in conjugation with the πꢀelectron system
of the aromatic ring results in substantial broadening of the observed Rydberg bands. This can
be associated with an increased ligand contribution to the HOMO of the sandwich compound.
The influence of the oxygen atom on the ionization energy of the molecule is insignificant. In
contrast to this, introduction of a nitrogen atom into the carbocycle leads to a noticeable
increase in the ionization potential of the molecule, while the ligand contribution to the
HOMO of the complex remains practically unchanged.
Key words: chromium bisarene complexes, anisole, 2,6ꢀdimethylpyridine, electronic abꢀ
sorption spectra, Rydberg transitions, ionization potential, substituent effect.
Like ferrocene derivatives, bisarene chromium comꢀ
the MO of the sandwich molecule are given in parentheꢀ
6
plexes are convenient model compounds for theoretiꢀ
cal and experimental investigations of the nature of
metal—ligand bonding in transition metal πꢀcomplexes.
ses. Specific spectral properties of (η ꢀarene) Cr are govꢀ
2
erned by the nonbonding character of the totally symmetꢀ
ric HOMO, which is nearly completely composed of the
1
2,3
This is advanced by the high symmetry (point group D )
Cr 3dz ꢀorbital. Electronic absorption spectra of these
2
6
h
and closed 18ꢀelectron shell of unsubstituted
complexes dramatically change on going from the gas
6
5—14
bis(η ꢀbenzene)chromium. The electron shell of this
phase to the condensed phase.
This is due to the
1
2—4
complex has the configuration A1g
: ...[e (Bz,
appearance, in the gasꢀphase spectra, of relatively narrow
bands corresponding to transitions of the nonbonding
2g
4
2
3
d
2
2
)] [a (3d )] ..., where the contributions of the
xy,x –y 1g z2
benzene orbitals and of the 3dꢀlevels of the metal atom to
3dz ꢀelectron to Rydberg levels of the sandwich molecule.
2
A clearly seen Rydberg structure makes the gasꢀphase
absorption spectra of bisarene chromium complexes much
more interesting and informative than the spectra obꢀ
tained in the condensed phase.
The spectra of bis(η ꢀbenzene)chromium and its
alkylꢀsubstituted and halogenꢀsubstituted derivatives exꢀ
*
Based on materials of a lecture presented at the International
Conference "Modern Trends in Organoelement and Polymer Chemꢀ
istry" dedicated to the 50th anniversary of the A. N. Nesmeyanov
Institute of Organoelement Compounds of the Russian Acadꢀ
emy of Sciences (Moscow, May 30—June 4, 2004).
6
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1854—1859, September, 2004.
066ꢀ5285/04/5309ꢀ1932 © 2004 Springer Science+Business Media, Inc.
1

6
Spectra of substituted bis(η ꢀarene)chromium
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 9, September, 2004
1933
6
hibit series of bands that are described by the Rydberg
formula
spectra of gaseous bis(η ꢀethylbenzene)chromium (3) and
6
bis(η ꢀmꢀxylene)chromium (4), which are unsubstituted
analogs of the complexes 1 and 2, respectively. Analysis
of the Rydberg structures in the spectra of compounds 3
and 4 was reported earlier.
2
2
ν = I – R/(n – δ) = I – R/(n*) = I – T,
(1)
n
7
,11,13
where ν is the wavenumber of a series member, I is the
n
ionization threshold equal to the energy of detachment of
an electron from the 3d ꢀorbital, R is the Rydberg conꢀ
2
z
–
1
stant (109737 cm ), n is the principal quantum number,
δ is the quantum defect, n* is the effective principal quanꢀ
tum number, and T is the term value (energy of interacꢀ
1
5
tion of the Rydberg electron with the cation core).
Knowing the convergence limit of the Rydberg series, the
6
first ionization potentials (IP) of (η ꢀarene) Cr molecules
2
7
,11—13,16,17
can be determined with high accuracy.
Parameters of Rydberg transitions contain informaꢀ
tion on the geometry of free sandwich molecules, deloꢀ
Experimental
calization degree of the dz
2
ꢀorbital, configuration interacꢀ
tions, and other features of the electronic structure of
Metal complexes 1—4 were synthesized by lowꢀtemperature
metal complexes.1
1—14,18—22
Therefore, analysis of the gasꢀ
cocondensation of Cr atoms and an organic ligand, which alꢀ
2
6—29
phase electronic absorption spectra of bisarene chromium
compounds provides a unique possibility of studying the
influence of the nature and structure of the organic ligand
on the electronic structure of sandwich compounds. Earꢀ
lows individual bisarene compounds to be obtained.
The
structures of the compounds under study were confirmed by
1
elemental analysis and H NMR spectroscopy data, which are
2
6
consistent with the results obtained earlier for complexes 2
and 1, 3, and 4.30 The complexes were purified by recrystallizaꢀ
tion followed by double vacuum sublimation.
2
3,24
lier,
we have shown that replacement of hydrogen
atoms by fluorine or chlorine atoms in the benzene rings
Absorption spectra of gaseous compounds were recorded on
a Specord UV—VIS spectrometer (Carl Zeiss, Germany) using
an evacuated heated quartz cell at 80—140 °C. The best resoluꢀ
6
of (η ꢀPhH) Cr causes a large shift and substantial broadꢀ
2
ening of the Rydberg bands. Broadening of a Rydberg
transition can be due to an increase in the ligand conꢀ
tribution to the HOMO of the complex or to configuꢀ
ration interactions.18—24 To analyze these effects in
more detail, in this work we first studied the spectra of
chromium complexes with two oxygenꢀ and nitrogenꢀ
–1
tion of the spectrometer was 30 cm . To reveal the Rydberg
bands, the gasꢀphase spectra were compared with the absorption
spectra of 1 and 2 in nꢀheptane, which were recorded on the
same spectrometer in an evacuated quartz cell at room temperaꢀ
ture. The gasꢀphase electronic spectra of complexes 1 and 2
were compared with the spectra of compounds 3 and 4, which
were also recorded on the same instrument. Analysis of the
6
6
containing ligands, (η ꢀPhOMe) Cr (1) and (η ꢀ2,6ꢀ
2
Me C H N) Cr (2).
2
5
3
2
31
Rydberg series was performed by nonlinear regression methods.
Results and Discussion
The gasꢀphase absorption spectrum of complex 1 exꢀ
hibits five relatively broad bands, among which four disꢀ
appear on going to the spectrum of a solution (Fig. 1).
Disappearance of the absorption bands on going from the
gas phase to the condensed phase is indicative of Rydberg
1
4,15
nature of the corresponding transitions.
The waveꢀ
Anisole was used as the oxygenꢀcontaining ligand. In
this compound the lone electron pair (LEP) of the oxygen
atom is involved in conjugation with the aromatic πꢀsysꢀ
tem.2 p—πꢀConjugation can result in a change in the
numbers of the Rydberg bands observed in the spectrum
of complex 1 are listed in Table 1. Analysis of the Rydberg
frequencies shows that three bands form a series that conꢀ
5
–1
verges to an ionization threshold at 42770 cm (5.30 eV).
character of the interaction between the Cr 3dz
2
ꢀorbital
Information on the photoelectron spectrum of comꢀ
and the anisole MO. In contrast to this, the unpaired
electron of the nitrogen atom in the 2,6ꢀdimethylpyridine
molecule is involved in the ring πꢀsystem. It was of
particular interest to analyze the changes in the gasꢀ
phase spectrum of the sandwich complex due to this
type of substitution. For comparison we recorded the
plex 1 is unavailable but the spectrum of complex
6
6
32
(η ꢀPhOMe)(η ꢀPhCN)Cr was reported. The first IP of
6
6
this compound is very similar to the IP of (η ꢀPhMe)(η ꢀ
PhCN)Cr (5.75 and 5.73 eV, respectively), which allows
one to expect similar values of the first IPs for complexes
6
6
(η ꢀPhOMe) Cr and (η ꢀPhMe) Cr. This assumption is
2
2

1934
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 9, September, 2004
Ketkov et al.
log(I /I )
The quantum defect δ 1.30 characterizing the series
0
R5px,y
members with n = 5 and n = 6 is typical of transitions to
the Rydberg pꢀlevels,5
—14
being intermediate between the
1
0
.0
corresponding values for the Rnpx,yꢀ and Rnp ꢀseries in
z
6
the spectra of alkylꢀsubstituted bis(η ꢀbenzene)chromium
This allows the bands at 34750 and
7800 cm (see Table 1) to be assigned as transitions
1
7,10,11,13
derivatives.
2
–1
3
R6px,y
.5
R4px,y
from the HOMO to the R5p and R6p levels, respectively.
The first member of the pꢀseries (ν = 26500 cm– ) is
characterized by a characteristic increase in the quantum
1
defect (see Table 1) due to increased penetration of the
R4s
22 ν•10–3/cm–1
lowest Rydberg orbital into the cation core.9
—12,35
Taking
into account that the lowest px,yꢀbands in the spectra of
3
8
30
all sandwich complexes with monoꢀsubstituted ligands
6
Fig. 1. Electronic absorption spectrum of bis(η ꢀanisole)chroꢀ
mium in the gas phase (1) and in nꢀheptane (2).
7,11,13,14,20
studied earlier
are much more intense than the
pzꢀpeaks, the Rydberg pꢀbands in the spectrum of comꢀ
plex 1 should be assigned to the 3dz2 → Rnpx,y transiꢀ
tions. The longꢀwavelength shoulder (ν = 23300 cm–
1
)
in good agreement with the convergence limit value obꢀ
tained for the series in the absorption spectrum of 1 (the
corresponds to transition to the lowest Rydberg sꢀorbital.
6
13
–1
first IP of (η ꢀPhMe) Cr is 5.29 eV ). Noteworthy is that
The corresponding term value (19470 cm ) is very simiꢀ
2
lar to the T(R4s) value for complex 3 (19390 cm– , see
1
the ionization energy of the sandwich complex somewhat
increases on going from system 3 to 1 (see Table 1).
A reverse trend is observed for free ligands (the vertical
Table 1).
Comparison of the spectra of complexes 1 and 3 (Figs 1
and 2) shows that introduction of an oxygen atom to the
side chain of the aromatic ligand causes a substantial
broadening of all Rydberg bands. A similar effect was
33
34
IPs of ethylbenzene and anisole are 8.77 and 8.39 eV,
respectively). This means that the difference in the energy
shifts of the groundꢀstate energy levels of the neutral molꢀ
ecule and the cation upon introduction of an O atom
changes as a result of ligand coordination to the Cr atom.
6
observed for halogenꢀsubstituted (η ꢀPhH) Cr derivaꢀ
2
2
3
tives. The lone electron pair of the heteroatom involved
Table 1. Wavenumbers ν (cm–1), term values T (cm–1), effective principal quantum numbers n*, quantum defects δ, and assignment
–
1
–1
–1
–1
of the Rydberg bands in the spectra of molecules 1 (I = 42770 cm ), 2 (I = 43570 cm ), 3 (I = 42330 cm ), and 4 (I = 41490 cm
)
ν
T
n*
δ
Assignment
1
3
1
3
1
3
1
3
2
2
3
3
3300 sh
6500
4750
22940
25250
33900
37160
19470
16270
8020
19390
17080
8430
2.37
2.60
3.70
4.70
2.38
2.53
3.61
4.61
1.63
1.40
1.30
1.30
1.62
1.47
1.39
1.39
R4s
R4px,y
R5px,y
R6px,y
7800 sh
4970
5170
2
4
2
4
2
4
2
4
2


3200
22500
24600
24870
29100
32580 sh
32980
33870
34750 sh
36280
36730
37930
38200 sh
38940
39110 sh
39600
40000
20370

18990
16890
16620
12390
8910
8510
7620
6740
5210
4760
3560
3290
2550
2380
1890
1490
2.32


2.40
2.57
2.71
2.98
3.51
3.59
3.79
4.04
4.59
4.80
5.55
5.78
6.56
6.79
7.62
8.58
1.68


1.60
1.43
1.29
1.02
1.49
1.41
1.21
0.96
1.41
1.20
1.45
1.22
1.44
1.21
1.38
1.42
R4s
R4px,y
R4px,y
R4d
R5px,y
R5px,y
R5pz
R5d
R6px,y
R6pz
R7px,y
R7pz
R8px,y
R8pz
R9px,y
R10px,y

3
3
3
3
3
3
3
3
4
4
4
4
4
2230 sh
4870
5300 sh
5900 sh
7180 sh
8300
8830
9970
0300
1000
11340
8700
8270
7670
6390
5270
4740
3600
3270
2570
2370
1890
1500
3.11
3.55
3.64
3.78
4.14
4.56
4.81
5.52
5.79
6.53
6.80
7.62
8.55
0.89
1.45
1.36
1.22
0.86
1.44
1.19
1.48
1.21
1.47
1.20
1.38
1.45
1200
1680
2070 sh

6
Spectra of substituted bis(η ꢀarene)chromium
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 9, September, 2004
1935
log(I /I )
log(I /I )
0
0
1
0
.0
10 8 7
6
n = 5 Rnpx,y (δ = 1.45)
n = 5 Rnp (δ = 1.22)
9
8 7
6
5
n = 4
Rnpx,y
1
0 87 6
z
(
δ = 1.40)
1
0
.0
.5
R4d
.5
R4s
24 ν•10–3/cm–1
R5d
1
4
0
32
2
Fig. 2. Gasꢀphase electronic absorption spectrum of bis(η6ꢀ
R4s
ethylbenzene)chromium.
4
0
32
24 ν•10–3/cm–1
6
Fig. 4. Electronic absorption spectrum of bis(η ꢀ2,6ꢀdimethylꢀ
pyridine)chromium in the gas phase (1) and in nꢀheptane (2).
in conjugation with the πꢀsystem of the aromatic ring
changes the pattern of the interaction between the Cr
3
dz
2
ꢀorbital and the ligand MO. In the unsubstituted comꢀ
surface intersects the totally symmetꢀ
plex the nodal 3dz
2
solution. Using regression analysis methods, the followꢀ
ing two series were revealed:
ric benzene πꢀMO in the region of maximum electron
density.3 This is, in particular, the reason for the nonꢀ
6
6
νn(см–1) = 43570 – 109737/(n – 1.45) ;
2
bonding character of the HOMO of (η ꢀPhH) Cr. Violaꢀ
2
n = 5 – 10, r = 0.99995, s = 30 см–1,
(2)
tion of symmetry of the πꢀelectron system of the ligand as
a result of conjugation with the electron pair of the hetꢀ
eroatom (Fig. 3) causes strengthening of the interaction
νn(см–1) = 43600 – 109737/(n – 1.22) ;
2
n = 5 – 8, r = 0.99990, s = 30 см–
1
,
(3)
between the 3dz
2
ꢀorbital and the MO of the organic fragꢀ
ment, which affects the Rydberg structure. Even greater
broadening of the Rydberg bands in the spectra of sandꢀ
wich complexes was observed on the opening of the
carbocycle;21 this was associated with delocalization of
where r is the correlation coefficient and s is the standard
deviation. Both series converge to the same ionization
threshold corresponding to the first IP of the sandwich
molecule. The ionization energy calculated from the
Rydberg frequencies (5.40 eV) is in good agreement with
the 3dz ꢀelectrons.
2
In contrast to compound 1, almost no symmetry vioꢀ
lation of the ligand πꢀsystem occurs in molecule 2 (see
Fig. 3). Therefore, one can expect that the gasꢀphase
absorption spectrum of 2 will exhibit narrow Rydberg
bands. Indeed, a resolved Rydberg structure is observed
in the shortꢀwavelength region of the spectrum (Fig. 4).
This feature disappears on going to the spectrum of a
37
the photoelectron spectroscopy data (5.43 eV).
The quantum defects of the series in the spectrum of
complex 2 (see Eqns (2) and (3)) are nearly equal to the
δ value for the unsubstituted analog, 4 (Fig. 5). The series
in the spectrum of 2 should therefore be assigned unamꢀ
biguously to transitions to the Rnpx,y and Rnp levels,
z
respectively. Indeed, in the spectra of molecules 2 and 4
the peaks corresponding to the R5px,y transition are split
into p ꢀ and p ꢀcomponents (see Figs 4 and 5, respecꢀ
x
y
tively). In addition to the Rnp bands two clearly defined
C
C
O
C
N
–
1
shoulders at 32230 and 37180 cm are observed in the
shortꢀwavelength region of the spectrum of 2. Compariꢀ
son of the corresponding term values with the data obꢀ
tained for complex 4 (see Table 1) shows that the shoulꢀ
ders correspond to transitions to the R4dꢀ and R5dꢀorbitꢀ
als, respectively.
Cr(dz
2
)
2
Cr(dz )
1
2
At n > 5, the term values for the Rydberg pꢀbands in
the spectra of complexes 2 and 4 are very similar (see
Table 1). Therefore, introduction of a nitrogen atom into
the ring has a little effect on the character of interaction
between the Rydberg electron and the cation core. Howꢀ
ever, replacement of mꢀxylene by 2,6ꢀdimethylpyridine
Fig. 3. Mutual arrangement of the Cr 3dz
pzꢀorbitals of carbon atom and heteroatom for complexes 1 and 2
shown is the section by the plane passing through the nuclei of
the ring atoms in positions 1 and 4 and the nucleus of the Cr
atom; dashed lines denote the 3dz nodal surface).
2
ꢀorbital and the
(
2

1
936
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 9, September, 2004
Ketkov et al.
log(I /I )
The results obtained demonstrate high sensitivity of
the Rydberg transitions in the gasꢀphase absorption specꢀ
tra of sandwich compounds to electronic structure disturꢀ
bances induced by heteroatoms in the organic fragment of
metal complexes. The Rydberg series permits determinaꢀ
0
9
8 7
6
5
n = 4 Rnpx,y (δ = 1.42)
9
8 7 6 n = 5 Rnp (δ = 1.22)
z
6
tion of the first IPs of (η ꢀarene) Cr using conventional
2
1
0
.0
.5
UV spectrometers. Analysis of changes in the Rydberg
parameters upon replacement of one ligand by another
allows one to reveal the influence of the structure of the
organic fragment on the extent of involvement of the Cr
R4d
R4s
3
2
dz ꢀorbital in chemical bonding, on the energy characꢀ
teristics of the Rydberg states of the sandwich molecule,
and on the configuration interactions. This information is
necessary to gain a better insight into the electronic strucꢀ
ture of sandwich complexes and the mechanisms of
photophysical and photochemical processes involving
sandwich molecules.
R5d
4
0
32
24 ν•10–3/cm–1
Fig. 5. Gasꢀphase electronic absorption spectrum of
bis(η ꢀmꢀxylene)chromium.
6
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 03ꢀ03ꢀ
3
2944), the Foundation of President of the Russian Fedꢀ
in the sandwich compound causes an appreciable increase
eration (Program for Support of the Leading Scientific
Schools, Grant NShꢀ1652.2003.3), and the Foundation
for Support of National Science (grant in the framework
of the Program "Young Ph.D. and D.Sc. researchers").
–
1
(
by more than 2000 cm ) in the IP due to the high
electronegativity of nitrogen. A similar effect was also
observed upon introduction of a nitrogen atom in the free
ligand (IPs of mꢀxylene and 2,6ꢀdimethylpyridine are
3
4
38
8
.56 and 8.87 eV, respectively). As a result, all Rydberg
References
bands are shifted toward the shortꢀwavelength region on
going from complex 4 to 2. This changes the character of
exhibition of the lowest Rydberg excitations in the specꢀ
trum of 2. The shift of the R4p and R4s transitions leads
to strengthening of the interaction between these excitaꢀ
tions and higher valence excitations that contribute to the
broad intense band with a maximum near 30000 cm–1
observed both in the gas phase and the condensed phase
1. A. Haaland, Acta Chem. Scand., 1965, 19, 41.
2. J. Weber, M. Geoffroy, A. Goursot, and E. Penigault, J. Am.
Chem. Soc., 1978, 100, 3995.
3
4
5
. D. W. Clack and K. D. Warren, Struct. Bonding (Berlin),
980, 39, 1.
. J. C. Green, Struct. Bonding (Berlin), 1981, 41, 37 (and
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. S. Yu. Ketkov, G. A. Domrachev, and G. A. Razuvaev,
Dokl. Akad. Nauk SSSR, 1987, 292, 890 [Dokl. Chem. USSR,
1
(
see Fig. 4). The R4s band in the spectrum of complex 2 is
much broader than in the spectrum of complex 4 (see
1
987 (Engl. Transl.)].
–
1
Figs 4 and 5). The T(R4s) values increases by ~1400 cm
6. S. Yu. Ketkov, G. A. Domrachev, and G. A. Razuvaev, Opt.
Spektrosk., 1987, 62, 227 [Opt. Spectrosc. USSR, 1987, 62
on going from complex 4 to 2 (see Table 1) due to the
mutual repulsion between the R4s state and an upper
valence excited level in molecule 2. As to the R4p states,
configuration interactions cause so large broadening that
the corresponding Rydberg bands are not observed in the
gasꢀphase absorption spectrum (see Fig. 4).
(
Engl. Transl.)].
7
8
. S. Yu. Ketkov, G. A. Domrachev, and G. A. Razuvaev, Opt.
Spektrosk., 1987, 63, 284 [Opt. Spectrosc. USSR, 1987, 63
(
Engl. Transl.)].
. S. Yu. Ketkov, G. A. Domrachev, and G. A. Razuvaev,
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Thus, different reasons are responsible for broadening
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Received July 6, 2004;
in revised form August 30, 2004