Infrared multiphoton dissociation of tetramethoxygermane

Article abstract of DOI:10.1134/S0018143916060084

Infrared multiphoton dissociation of Ge(OCH₃)₄ molecules by irradiation with a pulsed TEA CO₂ laser has been studied. The basic characteristics of the process have been experimentally investigated. The spectral characterization of the dissociation has been made. The product composition of the dissociation has been analyzed, and possible dissociation pathways have been proposed.

Full text of DOI:10.1134/S0018143916060084

ISSN 0018-1439, High Energy Chemistry, 2017, Vol. 51, No. 1, pp. 24–28. © Pleiades Publishing, Ltd., 2017.
Original Russian Text © P.V. Koshlyakov, E.N. Chesnokov, L.V. Kuibida, 2017, published in Khimiya Vysokikh Energii, 2017, Vol. 51, No. 1, pp. 27–31.
PHOTOCHEMISTRY
Infrared Multiphoton Dissociation of Tetramethoxygermane
P. V. Koshlyakov*, E. N. Chesnokov, and L. V. Kuibida
Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences,
Novosibirsk, 630090 Russia
*
e-mail: pvk@kinetics.nsc.ru
Received May 25, 2016
Abstract⎯ Infrared multiphoton dissociation of Ge(OCH ) molecules by irradiation with a pulsed TEA CO
2
3
4
laser has been studied. The basic characteristics of the process have been experimentally investigated. The
spectral characterization of the dissociation has been made. The product composition of the dissociation has
been analyzed, and possible dissociation pathways have been proposed.
DOI: 10.1134/S0018143916060084
INTRODUCTION
Ge(OCH ) . The composition of gaseous products
3 4
was studied, as well as the elemental composition and
structure of nonvolatile products deposited on the
walls of the reaction vessel. The assignment of absorp-
tion bands in the IR spectrum showed that the attribu-
Germanium is known to have five stable isotopes:
7
0
72
73
74
76
Ge, 20.3, Ge, 27.3; Ge, 7.7; Ge, 36.7; and Ge,
7
.8%. Pure germanium isotopes are used in funda-
mental physics research. In particular, the isotope
−1
tion of the 1050-cm band to Ge–O vibrations is
incorrect. Quantum-chemical calculations of the
pathways of monomolecular decay of Ge(OCH )
76
Ge is needed in experiments on searching for neutri-
noless double beta-decay (GERDA experiment [1]).
3
4
76
The future demand for enriched Ge for this experi-
ment is about one ton.
were performed.
Recent years have seen significant progress in
improving the IR laser separation of silicon isotopes
using multiphoton dissociation (MPD) of organome-
tallic compounds. High isotopic selectivity parameters
were obtained for the IR MPD of 1,2-dichloroethyltri-
fluorosilane [2] and 2-chloroethyltrifluorosilane [3].
It seems proper to study the IR MPD processes of
organoelemental germanium compounds with the aim
to find the compounds that hold promise for laser iso-
tope separation.
EXPERIMENTAL
Tetramethoxygermane Ge(OCH ) was available
3
4
from Sigma-Aldrich. IR spectra were recorded on a
Bruker Vector 22 FTIR spectrometer with a resolution
−1
of 1 cm . The spectra were taken in a gas-measure-
ment glass cell with KBr windows. The cell length was
1
80 mm; the diameter, 30 mm.
All operations with tetramethoxygermane—stor-
Tetramethoxygermane Ge(OCH ) is one of the age, purification, reaction cell loading, sampling for
3
4
IR spectroscopy—were carried out in a glass vacuum
system. The pressure of the sample during these oper-
ations was measured with an MKS Baratron 626A
capacitance manometer designed for measurements in
the pressure range of 0–75 torr. To simplify the analy-
sis of complex mixtures, a procedure of low-tempera-
ture separation of their components was used. The
procedure consisted in preliminary freezing of the gas
mixture into a thermostated glass “finger” coupled
few volatile organogermanium compounds. At room
temperature, its saturated vapor pressure is 3.5 torr.
The IR MPD of tetramethoxygermane (TMOS) using
pulsed CO laser radiation was studied by Okada et al.
2
[
4]. The IR spectrum of Ge(OCH ) contains a strong
3 4
−1
band at 1050 cm . It was assumed in [4] that this band
is due to stretching vibrations of the Ge–O bond. The
isotopic frequency shift for such vibrations is estimated
at a value of up to 5 cm . Based on these estimates, the
cited authors studied the isotopic selectivity of MPD
with IR excitation in the region around 1050 cm .
However, the MPD selectivity in this region turned
out to be low.
−1
3
with a 250-cm tube to which an MKS Baratron 227A
pressure sensor (measuring pressure in the range of 0–
−1
1
torr) was connected. Then, the “finger” temperature
was slowly (over ~10 min) elevated. The pressure sen-
sor during the elevation fixed the characteristic
In this work, we conducted a more extensive inves- “steps” corresponding to the evaporation of individual
tigation of the multiphoton dissociation of components of the mixture.
24

INFRARED MULTIPHOTON DISSOCIATION
25
IR SPECTRUM
OF TETRAMETHOXYGERMANE
10
The molecular structure of Ge(OCH ) was stud-
3
4
C–O
ied in [5]. The central atom is surrounded by four oxy-
gen atoms, which occupy the vertices of a slightly dis-
torted tetrahedron. The tetrahedron is compressed by
1
.2% in the direction of one of the S axes. The C–O
4
5
0
bonds form an angle of 122.7° with the Ge–O bonds
and turned by 72° relative to the O–Ge–O plane.
Ge–O
Although the tetramethoxygermane molecule no
longer has tetrahedral symmetry, its IR spectrum is
very simple and has the features typical of the spectra
of highly symmetric molecules. Figure 1 shows the
most intense IR absorption bands. The wave numbers
of C–H stretching vibrations are in the range of the
6
00
800
1000 1200 2800 3000
Wave number, cm
–1
−1
−1
2
800–3100 cm . Bands at 670 and 1052 cm are of
the greatest interest for laser excitation. Osada et al. [4]
Fig. 1. Experimentally obtained IR spectrum of tetrame-
thoxygermane (solid curve); a pressure of 2 torr. The verti-
cal lines refer to the calculated Ge–O stretching vibration
lines.
−1
attributed the 1052-cm band to stretching vibrations
of the Ge–O bond. However, quantum-chemical cal-
culations show that this oscillation is mainly localized
in the CO groups. The results of these calculations on
the C–O and Ge–O stretching vibrations are given in
the table. Rows 22–25 refer to the stretching vibrations
Tetramethoxysilane samples were irradiated with a
pulsed IR laser in a glass cell of a 300 mm length and a of the C–O groups. In the case of tetrahedral symme-
2
2 mm diameter with NaCl windows glued at the ends. try, one inactive symmetric mode (line 25) in the IR
The laser used for irradiation was a pulsed TEA CO2 region and triply degenerated vibration of T symme-
2
try would be expected. Due to the distortion of the tet-
rahedral symmetry, the triple degeneracy is partially
laser. The homogeneous part of the laser beam was
isolated with an aperture of 1 cm in diameter. The laser
beam was not focused. The initial Ge(OCH ) pres-
−1
lifted: the vibration frequency of 1056.1861 cm (line
3
4
−
1
2
2) is slightly less than 1060.8165 cm (line 23) and
sure in the experiments was 250–420 mtorr.
−1
1
060.8194 cm (line 24). The vibrations (lines 22–24)
The chemical composition of the photolysis prod- in the experimentally measured IR spectrum have a
ucts was determined using an SRSRGA 300 quadru- frequency of 8 cm− below the calculated values. The
1
−1
pole mass spectrometer. The mass spectrometer bandwidth of the experimental spectrum is ~22 cm ,
−1
recorded ion current with a Faraday cup. The mass so that the 5-cm splitting predicted by calculations is
spectrometer was connected directly to the reaction not observed in the spectrum.
cell through a 10-μm hole. Quantum-chemical calcu-
A similar pattern is observed for the Ge–O stretch-
−1
lations were performed using the software package ing vibrations at 650 cm . There are one inactive sym-
Gaussian 98.
metric mode (line 18) and three active modes (lines
76
−1
Calculated vibration frequencies of the Ge(OCH ) molecule in the range of 500–1100 cm . DFT B3lyb method
3
4
with the 6-311++g(d, p) basis set
Frequency, cm−1
Sequential number
IR intensity
Experiment
…………
1
8
570.8020
628.8789
642.6053
642.6072
1056.1861
1060.8165
1060.8194
1082.8805
0.0000
70.0224
107.5045
107.5056
299.3204
321.1414
321.1443
0.0000
6
69 cm−1
1
9
2
2
0
1
1
052 cm−1
22
23
24
25
………….
HIGH ENERGY CHEMISTRY Vol. 51 No. 1 2017

2
6
KOSHLYAKOV et al.
β
β
0
.010
0
.006
0.008
0
.005
0.006
0
.004
0.004
0
.003
.002
ln(β) = –6.932 + 7.257Φ
0
0.002
0.001
0
1
000
1020
1040
1060
1080
1100
–1
Wave number, cm
0
0.05 0.10 0.15 0.20 0.25 0.30 0.35
2
Energy density, J/cm
Fig. 3. Spectrum of multiphoton dissociation of tetrame-
2
thoxygermane. Fluence, 200 mJ/cm . The solid curve
refers to the linear absorption spectrum of tetramethoxy-
germane.
Fig. 2. Plot of the dissociation probability versus laser radi-
ation fluence. Pressure, 35 mtorr; the 9P18 line.
1
9–21). The triple degeneracy is partially removed by of the number of absorbed photons to the photon flu-
2
distortion of the tetrahedral symmetry.
ence. Conversion of the factor of 134 cm /J gives the
−18
2
The assignment of these bands is confirmed by cal- value of σ = 2.7 × 10 cm . The cross section of the
eff
culating the isotopic shifts for germanium isotopes. linear absorption spectrum (Fig. 1) at this frequency is
76
72
–18
2
The isotopic shift due to the Ge– Ge replacement is σlin(1048.7) = 5 × 10 cm , that is, only twice as high.
−1
−1
0
.77–0.83 cm for vibrations at 1052 cm (lines 22– This means that the process of multiphoton excitation
2
4), but it is 5.9 cm−1 for vibrations at 669 cm−1
pertaining to C–O vibrations of the methoxy group is
(lines 19–21).
very efficient and is characterized by an absorption
The small isotopic shifts for C–O vibrations cross section comparable with the cross section in the
−1
(
1052 cm ) suggest that no dissociation selectivity for linear spectrum.
germanium isotopes can be achieved with the use of a
Figure 2 shows the dissociation probability for
CO laser. The germanium isotope-selective IR MPD
requires a radiation source in the frequency range of
2
Ge(OCH ) molecules as a function of fluence. At low
3 4
−1
2
6
50–680 cm . In this work, we studied the general IR energy densities of Φ < 0.35 J/cm , the experimental
MPD characteristics and dissociation products of
Ge(OCH ) for the case of excitation of C–O vibra-
data points fit well the exponential relation
β = 0.00098exp 7.26Φ
. A low dissociation yield is
associated with a large number of vibrational degrees
of freedom of the molecule. Indeed, although the
average number of absorbed photons is a few tens,
there is less than one absorbed quantum per each of
the 57 vibration modes.
3
4
(
)
−1
tions (1052 cm ) in the lasing range of the CO laser.
2
FEATURES OF MULTIPHOTON
DISSOCIATION OF Ge(OCH )
We measured absorbed energy as a function of laser
3
4
−
1
radiation fluence Φ at a frequency of 1050 cm . The
These results are consistent with the measurements
measurements were made in the range of Φ = 0–
2
by Laptev et al. [6] at an energy density of 0.38 J/cm .
2
0
.32 J/cm . The measurement results are described
Note that a sharp increase, most likely due to thermal
effects, in the yield of tetramethoxygermane dissocia-
tion with an increase in pressure above 0.5 torr was
observed in [6].
well by the formula
( )
n = 11.1Φ 1 +12Φ) (Φ + 0.039 ,
where 〈n〉 is the number of absorbed photons per mol-
ecule. The average number of absorbed photons 〈n〉
reaches 50 under the given conditions. The formula
describes the dependence close to the linear relation-
The dependence of the tetramethoxygermane dis-
sociation probability on the laser frequency (multi-
photon dissociation spectrum) is shown in Fig. 3. The
2
ship 〈n〉 = 133Φ [J/cm ], except for a small initial por-
2
maximum of the dissociation spectrum is shifted by
tion, where 〈n〉 = 280Φ [J/cm ]. The linear nature of
the multiphoton absorption allows us to introduce an ~7 cm− relative to the maximum in the linear spec-
1
effective absorption cross section defined as the ratio trum.
HIGH ENERGY CHEMISTRY
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2017

INFRARED MULTIPHOTON DISSOCIATION
27
simplify the analysis of the mixture, a procedure of the
low-temperature separation of the components was
used. The dissociation products were frozen into the
thermostated “finger”, whose temperature was slowly
increased. The corresponding pressure–temperature
plot exhibited separate steps due to the evaporation of
different fractions of the mixture.
1
10
100
Ge(OCH3)4
9
0
0
0
In a typical experiment in which 300 mtorr
Ge(OCH ) was irradiated with 230 laser pulses, the
8
7
3
4
products were separated into the following fractions.
The first fraction having a pressure of about 30 mtorr
included components that are not frozen at the liquid
nitrogen temperature. Mass spectrometric analysis
5
4
3
2
1
H CO
2
showed that the fraction is composed of hydrogen H
2
with admixtures of CO and CH . The next fraction
4
evaporated at a temperature of −110°C. This was
CH OH
3
mainly formaldehyde H CO with an admixture of
2
acetylene C H . At a temperature of −55°C, methanol
2
2
CH OH with a small amount of propane evaporated.
3
The last fraction of the mixture evaporated at about
0
°C, which was represented by the unreacted tetrame-
0
100
200
300
400
500
thoxygermane.
1
10
Thus, the main gaseous products are formaldehyde
C H
CH4
CO
H2
2
2
and methanol. There are also H , C H , CH , CO,
2
2
2
4
and C H , which are present in small quantities. After
100
3
8
determining the qualitative product composition, a
procedure for the mass spectroscopic analysis of the
gas mixture without fractionation was developed. A
mass spectrum was recorded during the irradiation of
the sample, using continuous sampling of the reaction
cell. The amount of Ge(OCH ) was determined from
C H
9
0
0
0
.7
.6
.5
.4
.3
.2
3
8
8
7
0
0
0
0
0
0
3
4
the intensity of the group of lines at m/e 105, and the
C H and CH OH contents were found by referring to
3
8
3
the lines at m/e 44 and 32, respectively. The H CO
2
concentration was determined according to the line at
m/e 30 after subtracting the contribution of CH OH.
3
The CO concentration was determined from the line
intensity at m/e 28 after subtracting the contributions
of CH OH, H CO, and C H . The concentration of
3
2
3
8
C H was determined according to the line at m/e 25.
2
2
0
.1
Figure 4 shows the rate curves for Ge(OCH ) con-
3
4
sumption and the buildup of the main gaseous prod-
0
100
200
300
400
500
ucts (left) and minor products (right). By the time t =
Time, s
3
50 s, when about 20% Ge(OCH ) is consumed, the
3 4
amounts of CH O and CH OH are about 2.5 and 5%,
2
3
Fig. 4. Dependence of the concentration of the dissocia-
tion products on the 9R24 line of CO laser upon the initial
respectively. The amount of the other products CH4,
CO, C H , H , and C H is less than 1%. No germa-
2
reactant concentration. The initial pressure of the reactant
is 388 mtorr.
2
2
2
3
8
nium-containing products were found in the gas
phase. During the dissociation of methoxygermane, a
film is formed on the walls of the reaction cell. Gas
chromatographic–mass spectrometric analysis of a
STUDY OF THE GASEOUS PRODUCTS
The composition of the gaseous products of tetra- wipe sample of this deposit washed off with various
methoxygermane MPD was found to be complex. To hydrocarbon solvents gave no results.
HIGH ENERGY CHEMISTRY Vol. 51 No. 1 2017

2
8
KOSHLYAKOV et al.
POSSIBLE FORMATION PATHWAYS
the CO laser covers the vibration bands of the CO
2
OF IR MPD PRODUCTS
group, for which the isotope shift is negligible.
The Ge(OCH ) molecule very efficiently absorbs
3
4
The composition of the gaseous products of tetra-
IR laser light. The absorption cross section in quasi-
methoxygermane MPD is similar to that observed in
the MPD of the analogous silicon compound
Si(OCH ) [6]. The primary process of Si(OCH )
continuum in the case of pumping with a CO laser is
2
–18
2
greater than 2 × 10 cm . This feature makes the two-
frequency scheme promising for selective processes.
In this case, the selective stage of vibrational excitation
will be at 15 μm and the subsequent excitation with the
3
4
3 4
dissociation is believed to be the elimination of the
methanol molecule. Similarly, we suggest that the
elimination of methanol is the primary process of
tetramethoxygermane MPD.
CO laser will be at 9.6 μm. Germanium-containing
2
dissociation products are deposited as a nonvolatile
residue on the walls of the reaction vessel. The compo-
sition of the nonvolatile products calls for further
investigation.
*
( ( )
OCH3 )4 + nhν → Ge OCH3 4 ,
Ge
*
OCH3 4
)
→ Ge(OCH ) OCH + CH OH;
3 2 2 3
Ge
(
ΔH = 55 kcal/mol.
ACKNOWLEDGMENTS
The work was supported by the Russian Founda-
tion for Basic Research, project no. 14-50-00080.
The heat of the reaction was found by quantum-
chemical calculations using the DFT B3lyb 6–311++
g (d, p) method. Calculations in terms of the RRKM
theory showed that a sufficiently rapid degradation of
Ge(OCH ) molecules is impossible unless the energy
3
4
REFERENCES
is 1.5–2 times above the activation barrier. This means
that the dissociation products remain excited and can
degrade as well. The decomposition of
Ge(OCH ) OCH yields a formaldehyde molecule:
1
. Ackermann, K.-H., Agostini, M., Allardt, M., Alt-
mann, M., Andreotti, E., Bakalyarov, A.M.,
Balata, M., and Barabanov, I., Eur. Phys. J. C, 2013,
vol. 73, p. 2330.
3
2
2
2
. Koshlyakov, P.V., Dementyev, P.S., Gorelik, S.R.,
Ge(OCH ) OCH → Ge(OCH ) + H CO;
3
2
2
3 2
2
Chesnokov, E.N., and Petrov, A.K., Appl. Phys. B,
ΔH = −25.5 kcal/mol.
2009, vol. 97, issue 3, p. 625.
3
. Dementyev,
P.S.,
Nizovtsev,
A.S.,
and
This sequence explains the formation of the main
products.
Chesnokov, E.N., J. Photochem. Photobiol., A, 2011,
vol. 222, p. 77.
4
. Okada, Y., Kato, S., Satooka, S., and Takeuchi, K.,
Spectrochim. Acta, Part A, 1990, vol. 46, p. 46.
CONCLUSIONS
5
. Aarset, K., Brady, F.J., Page, E.M., and Rice, D.A., J.
The isotopic shift of germanium isotopes for Ge–
Mol. Struct., 2000, vol. 522, p. 125.
−1
O stretching vibrations is on the order of 6 cm , which
may be sufficient for isotope-selective multiphoton
dissociation. However, the Ge–O stretching vibration
band lies in the region of 15 μm. The tuning range of
6. Laptev, V.B., Tumanova, L.M., and Ryabov, E.A.,
High Energy Chem., 1998, vol. 32, no. 2, p. 108.
Translated by S. Zatonsky
HIGH ENERGY CHEMISTRY
Vol. 51
No. 1
2017