Infrared spectra of the novel Ge2H2 and Ge2H4 species and the reactive GeH1,2,3 intermediates in solid neon, deuterium and argon

Article abstract of DOI:10.1021/jp020219v

Laser-ablated Ge atoms and H₂ molecules in excess neon and argon react during condensation to produce GeH_1,2,3,4 as identified from infrared spectra with D₂ and HD substitution, and from agreement with frequencies determined in recent gas-phase GeH and GeH₂ fluorescence spectra and DFT calculations. The novel digermanium species Ge₂H₂ and Ge₂H₄ are formed in further reactions. Identification of Ge₂H₂ with the dibridged structure and Ge₂H₄ in the trans-bent C_2h form is made possible by isotopic substitution and quantum chemical frequency calculations, which demonstrate the value of a close working relationship between experiment and theory. The photosensitive GeH₃^- anion is formed through electron capture by a GeH₃ radical. All four germanium deuterides are observed in pure deuterium, which shows that further GeD and GeD₂ reactions with D₂ may require activation energy.

Full text of DOI:10.1021/jp020219v

J. Phys. Chem. A 2002, 106, 5809-5816
5809
Infrared Spectra of the Novel Ge2H2 and Ge2H4 Species and the Reactive GeH1,2,3
Intermediates in Solid Neon, Deuterium and Argon
Xuefeng Wang, Lester Andrews,* and Gary P. Kushto†
Department of Chemistry, P. O. Box 400319, UniVersity of Virginia, CharlottesVille, Virginia 22904-4319
ReceiVed: January 23, 2002; In Final Form: April 15, 2002
Laser-ablated Ge atoms and H2 molecules in excess neon and argon react during condensation to produce
GeH1,2,3,4 as identified from infrared spectra with D2 and HD substitution, and from agreement with frequencies
determined in recent gas-phase GeH and GeH2 fluorescence spectra and DFT calculations. The novel
digermanium species Ge2H2 and Ge2H4 are formed in further reactions. Identification of Ge2H2 with the
dibridged structure and Ge2H4 in the trans-bent C2h form is made possible by isotopic substitution and quantum
chemical frequency calculations, which demonstrate the value of a close working relationship between
experiment and theory. The photosensitive GeH3- anion is formed through electron capture by a GeH3 radical.
All four germanium deuterides are observed in pure deuterium, which shows that further GeD and GeD2
reactions with D2 may require activation energy.
Introduction
ments where diffusion and dimerization of small transients
during 3.5 K deposition are favorable reactions. This work
Germanium hydride transient species are of interest, in part,
owing to their role in the growth of germanium semiconductor
films in the chemical vapor decomposition process involving
germane.1-3 The most important of these, GeH2, germylene,
was produced as a vacuum-ultraviolet photolysis or discharge
product of germane in matrix isolation experiments,4,5 but the
infrared absorptions were not assigned correctly. Later work
identified GeH2 from laser-induced fluorescence spectra (LIF)
in the gas phase6-9 and investigated the kinetics of germylene
reactions.10-12 The pyramidal GeH3 radical was observed by
ESR in solid krypton13,14 and xenon,15 infrared absorptions were
attributed to GeH3 in solid argon,4,5 and multiphoton ionization
spectra of gaseous GeH3 have been reported.16 The kinetics of
GeH3 radical reactions have been monitored through a transient
absorption17 at 2049.8 cm-1, which is considerably higher than
the reported argon matrix fundamentals.4,5 Comparative theoreti-
cal studies of GeH3 radicals and GeH2 structures have been
performed.18-21 Last, but not least, the diatomic transient GeH
and GeD radicals have been thoroughly investigated under high-
resolution conditions.22,23
reassigns the matrix infrared spectra of GeH1,2,3 and provides
the first experimental evidence for the unsaturated Ge2H2 and
Ge2H4 species.
Experimental and Theoretical Methods
Laser-ablated germanium (Alfa, 99.999%) atoms were reacted
with H2, D2, and HD (0.2-10%) in excess argon and neon and
with pure deuterium during condensation at 3.5 K using a
Sumitomo Heavy Industries RDK-205D Cryocooler and meth-
ods described previously.33,34 Infrared spectra were recorded,
samples were annealed and irradiated, and more spectra were
recorded. Complementary density functional theory DFT cal-
culations were performed using the Gaussian 98 program,
BPW91 and B3LYP density functionals, 6-311++G(d,p) basis,
and SDD pseudopotential.35-39 Geometries were fully optimized,
and the vibrational frequencies were computed analytically from
second derivatives.
Results
The unsaturated digermanium species have been explored by
theoretical calculations and found to have unusual structures,
unlike their hydrocarbon analogues. The ground state of Ge2H2
is a C2V “butterfly” structure,24-27 like Si2H2, and Ge2H4 has a
nonplanar C2h geometry, like Si2H4.27-31 However, the saturated
Ge2H6 molecule has the ethane structure.27,30 Finally, the Ge2H4
species has been proposed as a reaction product in the
decomposition of GeH4.32
We report here a combined neon, deuterium, and argon matrix
infrared and density functional theoretical investigation of the
above germanium hydride transient species. The new matrix
identifications are greatly facilitated by recent gas-phase
measurements of GeH and GeH2 fundamentals,6-9,22,23 by
quantum chemical calculations,25 and by neon matrix experi-
Infrared spectra from neon, deuterium, and argon matrix
experiments with Ge and H2 and DFT calculations on expected
products will be presented. Table 1 lists the observed product
absorptions.
Neon Matrix. Infrared spectra of solid neon films containing
reaction products of laser-ablated Ge and 5% H2 are shown in
Figure 1 and D2 counterparts in Figure 2. The strongest
absorption is at 1859.8 (1340.9) cm-1 for H2 (D2) reactions.
The weak 1925.5 cm-1 absorption is due to GeCO, recently
identified at 1918.9 cm-1 in solid argon.40 The weak 1757 cm-1
band that grows on 8 K annealing is unshifted with HD and
D2. A very weak GeO absorption41,42 appears at 977.0 cm-1
just above new bands at 972.2, 921.3, and 903.3 cm-1. Note
the much larger yield of GeH4 than GeD4 at 2114.3 (1522.0)
cm-1 43
annealing doubles the latter absorptions and sharpens
;
* Author for correspondence: lsa@virginia.edu.
† Present address: Optical Sciences Branch, United States Naval Research
Laboratory, Washington, DC 20375.
the former bands into doublets 1862.4, 1859.8 (1342.6, 1337.3)
cm-1. The HD experiment gives slightly different sharp bands
10.1021/jp020219v CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/24/2002

5810 J. Phys. Chem. A, Vol. 106, No. 24, 2002
Wang et al.
TABLE 1: Infrared Absorptions (cm-1) Observed for
Reactions of Laser-Ablated Germanium Atoms with
Hydrogen and Deuterium in Solid Matrices
matrix
Ne
H2
D2
identification
1834.2
1832.2
1321.2
1319.5
1316.1
1313.6
1305.3
1303.8
GeH, GeD
GeH, GeD site
GeD site
D2
Ar
Ne
GeD
1813.5
GeH, GeD
GeH, GeD site
GeH2 site
GeH2, GeD2
GeH2, GeD2
GeH2 (agg)
GeH2, GeD2
GeH2, GeD2 (agg)
GeD2
1867.6
1862.5
1860.4
1855.7
921.5
1342.6
1337.3
660.2
648.5
1338.4
1335.7
1332.7
1330.4
646.5
903.5
D2
Ar
Figure 1. Infrared spectra of neon matrix formed by co-depositing
hydrogen and laser-ablated germanium: (a) 5% H2 in neon deposited
for 60 min at 3.5 K, (b) after annealing to 7 K, (c) after λ > 240 nm
photolysis for 15 min, (d) after annealing to 8 K, (e) after annealing to
10 K, and (f) after annealing to 12 K.
site
GeD2
site
GeD2
1839.2
1835.3
1830.5
913.6
2089.4
857.2
1325.4
1322.0
GeH2, GeD2
GeH2, GeD2 (agg)
GeH2, GeD2 (agg)
GeH2, GeD2
GeH3, GeD3 (ν3)
GeH3, GeD3 (ν4)
GeH3, GeD3 (ν2)
GeD3
654.4
1504.7
611.3
489.5
1500.9
610.6
Ne
D2
Ar
667.8
GeD3
GeD3
488.0
1496.5
2074.1
852.4
667.0
2114.3
822.3
GeH3, GeD3
GeH3, GeD3
GeH3
GeH4, GeD4
GeH4, GeD4
GeD4
Ne
D2
Ar
1522.0
594.5
1516.8
590.9
GeD4
2106.3
2098.7a
814.5a
1352.3
972.2
1340.0
964.3
(2053)
(2035)
789.1
1516
GeH4, GeD4 site
GeH4, GeD4
GeH4, GeD4
Ge2H2
Ge2H2, Ge2D2
Ge2H2
Ge2H2, Ge2D2
Ge2H4, Ge2D4
Ge2H4, Ge2D4
Ge2H4, Ge2D4
Ge2D4
1513.2a
591.0a
Figure 2. Infrared spectra of neon matrix formed by co-depositing
deuterium and laser-ablated germanium: (a) 5% D2 in neon deposited
for 60 min at 3.5 K, (b) after annealing to 7 K, (c) after λ > 240 nm
photolysis for 15 min, (d) after annealing to 8 K, (e) after annealing to
10 K, and (f) after annealing to 12 K.
Ne
Ar
Ne
715.1
709.3
1479.0
1465.7
569.5
1476
Ar
787
881.6
752.8
568
Ge2H4, Ge2D4
Ge2H6, Ge2D6
Ge2H6, Ge2D6
Ge2D6
Ne
631.5
542.5
542.0
1234
1208
1219.9
1202.4
D2
Ne
1700
1670
GeH3-, GeD3- (ν3, e)
-
GeH2-, GeD2
-
D2
GeD3
-
GeD2
a Band positions appear subject to effects of aggregation; compare
ref 4.
at 1858.4 and 1338.4 cm-1. Broad-band photolysis increases
2053, 2035 (1479.0, 1465.7) cm-1 bands, destroys 1704, 1670
(1234, 1208) cm-1 peaks, and decreases 972.2 (715.1) cm-1
features. Figure 3 shows the evolution of site structure on the
major product band and the relative intensities of different
product species using 1% and 0.2% H2 with different laser
energies. Under more dilute 0.2% H2 conditions, only the 1670
cm-1 intermediate band is observed, and λ > 380 nm irradiation
destroys the 1670 (1208) cm-1 bands. The 972.2 (715.1) cm-1
absorptions are compared in Figure 4 with HD and mixed H2
+ D2 product spectra.
Figure 3. Infrared spectra of neon matrix formed by co-depositing
hydrogen and laser-ablated germanium: (a) 1% H2 in neon deposited
for 60 min, (b) after annealing to 13 K, (c) 1% H2 in neon with reduced
laser energy deposited for 60 min, (d) after annealing to 9 K, (e) 0.2%
H2 in neon with reduced laser energy deposited for 60 min, and (f)
after annealing to 11 K.
GeD4 bands at 1516.8 and 590.9 cm-1 on deposition that
increase on ultraviolet photolysis and important new absorptions
at 1500.9, 1338.4, 1332.7, 1313.6, 1219.9, and 1202.4 cm-1
that show different photolysis and annealing behaviors. Another
Deuterium Matrix. Infrared spectra of pure D2 co-deposited
with Ge atoms at 3.5 K are illustrated in Figure 5. Note weak

IR Spectra of GeH1,2,3 Species
J. Phys. Chem. A, Vol. 106, No. 24, 2002 5811
Figure 6. Infrared spectra of argon matrix formed by co-depositing
hydrogen and laser-ablated germanium: (a) 2% H2 in argon deposited
for 60 min at 7 K, (b) after annealing to 18 K, (c) after λ > 240 nm
photolysis for 15 min, (d) after annealing to 28 K, and (e) after annealing
to 38 K.
Figure 4. Infrared spectra of germanium-hydrogen reaction prod-
ucts: (a) 5% H2 in neon, (b) 6% HD in neon, (c) 3% H2 + 3% D2 in
neon, and (d) 5% D2 in neon.
1 collects the H2 and D2 reaction product bands for the three
different matrix environments.
Calculations. DFT calculations were done for the subject
species using B3LYP and BPW91 functionals to predict a
consistent set of germanium hydride product vibrational fre-
quencies for H, D, and mixed isotopic molecules to aid in
making vibrational assignments. Table 2 lists the H isotopic
frequencies and Table 3 gives the D isotopic counterparts.
Our B3LYP calculations of the novel Ge2H2 and Ge2H4
structures are in excellent agreement with previous results. In
the case of Ge2H2, MP2 calculations find slightly shorter bonds
but nearly the same angles, and CISD predicts Ge-H bonds
0.03 Å shorter, a Ge-Ge bond 0.08 Å shorter, nearly the same
angles, and slightly higher frequencies.25,26 In the case of Ge2H4,
SCF finds a 0.1 Å shorter Ge-Ge bond, 0.03 Å shorter Ge-H
bonds, and similar angles, and other calculations are in these
ranges.30 Analogous B3LYP calculations using the larger
6-31++G(2d,2p) basis set gave slightly longer Ge-H but
shorter Ge-Ge bonds for Ge2H2 and both shorter bonds for
Ge2H4 again with similar angles.27
Figure 5. Infrared spectra of pure deuterium co-deposited with laser-
ablated Ge atoms: (a) 25 min deposit, (b) after annealing to 7 K, (c)
after λ > 380 nm photolysis for 15 min, (d) after 240-380 nm
photolysis for 15 min, and (e) after annealing to 8 K.
experiment with higher laser energy gave increased product band
intensities except for the latter photosensitive bands, which were
not observed.
Discussion
The new product absorptions will be assigned to germanium
hydride species on the basis of isotopic substitution, comparison
of neon, argon, and deuterium matrix results, and agreement
with DFT frequency calculations.
Argon Matrix. Infrared spectra for Ge and 2% H2 reaction
products in solid argon at 7 K are shown in Figure 6 where the
product distribution is different from that in neon owing to the
faster condensation rate of argon under the condition of these
experiments. A weak GeO band is observed at 975 cm-1, and
weak HO2 bands appear at 1389 and 1101 cm-1 on annealing
and attest the production of H atoms.44 The broad 2106.3 cm-1
GeH4 band dominates the sharp 2098.8 cm-1 absorption on
deposition, but annealing favors the latter, while the GeH4 band
at 816.8 cm-1 shifts to 814.3 cm-1, and a weak 964.3 cm-1
absorption increases. Likewise satellite features at 1889.2,
1868.2, and 1848.1 cm-1 give way on annealing to the dominant
1839.2 and 1813.5 cm-1 bands. The HD experiment gives a
slightly different 1835.8 cm-1 former band but the same 1813.5
cm-1 latter band. Argon matrix experiments using a 3.5 K
substrate and 10% H2 trap a smaller yield of the same product
bands owing to the faster condensation rate, but these include
a new 1675 cm-1 absorption. At the higher H2 concentration,
the 748.5 cm-1 band grows more on annealing and the 964.3
cm-1 band grows less, relative to the 1839.2 cm-1 feature. Table
GeH. The neon matrix absorptions at 1834.2, 1832.2 (1321.2,
1319.5) cm-1 can be assigned to the GeH (GeD) diatomics
(1.388 H/D ratio) on the basis of the agreement with the gas-
phase fundamentals at 1833.8 and 1319.8 cm-1, respectively,
from electronic band spacings.22,23 The neon matrix bands shift
to 1813.5 and 1305.3 cm-1 in solid argon, which are in line
with other such comparisons.46 A sharp band at 1313.6 cm-1
in pure deuterium (Figure 5) is also assigned to GeD. Smith
and Guillory4 assigned the 1305 cm-1 GeD band in solid argon
but misassigned the 1813 cm-1 absorption to GeH3. Our B3LYP
and BPW91 calculations (Table 2) and others come in slightly
low for the GeH fundamental, but CCSD(T) calculations give
higher frequency values.47,48
GeH2. The strongest product absorptions at 1859.8 cm-1 with
H2 and at 1340.9 cm-1 with D2 in neon (Figures 1 and 2) are
candidates for GeH2 and GeD2. Annealing sharpens these
absorptions to 1862.5, 1860.4, and 1855.7 cm-1 and a resolved

5812 J. Phys. Chem. A, Vol. 106, No. 24, 2002
Wang et al.
TABLE 2: Frequencies (cm-1), Infrared Intensities (km/mol), Bond Lengths, and Angles Calculated at the DFT/
6-311++G(d,p)/SDD Level for Germanium Hydrides
B3LYPa
BPW91a
other
experimental
GeH
2Π
1818.0 (533)
1.626 Å
1772.4 (476)
1.642 Å
1844
[1833.8, 1.587 Å]c
1813.5, 1833.8d,e
[1856, 916, 1.588 Å]f
1839.2, 1862.4d,e
1.608 Åb
GeH2
1851.5 (a1, 450)
1848.4 (b1, 514)
936.9 (a1, 76)
1.617 Å, 90.7°
2081.7 (e, 249 × 2)
2051.9 (a1, 23)
847.8 (e, 62 × 2)
718.9 (a1, 76)
1800.5 (b1, 460)
1795.4 (a1, 414)
898.6 (a1, 63)
1.624 Å, 89.9°
2031.5 (e, 224 × 2)
1986.6 (a1, 22)
820.3 (e, 52 × 2)
686.3 (a1, 58)
1.566 Å, 110.5°
2070.3 (a1, 0)
2067.1 (t2, 187 × 3)
884.3 (e, 0)
1840, 913, 1840f
1.613 Å, 93.2°
2059, 1036, 2057g
1.575 Å, 93.0°
2227h
2212
937
782
1A1
(C2V)
GeH3
2A1
(C3V)
[663 (10)]i
2074.2, 852.4, 667.6d
2089.4, 857.2, 667.8e
1.554 Å, 110.5°
2139.3 (a1, 0)
1.539 Å, 110.8°
GeH4
1A1
(Td)
2279 (0)g
[2114, 819, 1.516 Å]j
2098.7, 814.5d
2118.3 (t2, 205 × 3)
2259 (122)
1009 (0)
912.0 (e, 0)
2114.3, 822.3e
821.7 (t2, 136 × 3)
1.547 Å
1415.5 (a1, 36)
1332.6 (b1, 100)
1004.2 (b2, 342)
874.7 (a2, 0)
785.6 (t2, 107 × 3)
920 (127)
1.535 Å
1.558 Å
Ge2H2
1A1
(C2V)
1389.9 (a1, 32)
1326.8 (b1, 84)
1019.7 (b2, 302)
930.1 (a2, 0)
1534 (12)l
1446 (51)
1054 (469)
931 (0)
964.3, 972.2d,e
815.7 (a1, 83)
265.8 (a1, 1)
787.9 (a1, 67)
262.3 (a1, 1)
863 (42)
304 (0)
Ge-H: 1.802 Å
Ge-Ge: 2.459 Å
Ge-H-Ge: 86.1°
H-Ge-H: 71.4°
HGeGeH: 105.9°
2057.9 (au, 443)
2038.4 (bg, 0)
Ge-H: 1.813
Ge-Ge: 2.470
Ge-H-Ge: 86.1°
H-Ge-H: 71.1°
Ge-H: 1.771
Ge-Ge: 2.377
HGeGeH: 105.5°
Ge2H4
1Ag
(C2h)
2002.0 (au, 367)
1982.1 (bg, 0)
1968.5 (bu, 381)
1963.9 (ag, 0)
(2053), (2035)
789e
2033.0 (bu, 430)
2031.4 (ag, 0)
879.2 (bu, 240)
Ge-H: 1.562 Å
Ge-Ge: 2.372 Å
H-Ge-H: 105.8°
Ge-Ge-H: 114.6°
2102.4 (eu, 381 × 2)
2097.4, (a2u, 245)
878.6 (eu, 86 × 2)
750.8 (a2u, 610)
Ge-H: 1.550 Å
Ge-Ge: 2.484 Å
H-Ge-H: 108.5°
1537.2 (1308)
851.4 (bu, 209)
Ge-H: 1.576
Ge-H: 1.528k
Ge-Ge: 2.373
H-Ge-H: 106.0°
Ge-Ge-H: 114.5°
2051.0 (eu, 343 × 2)
2034.1 (a2u, 244)
849.5 (eu, 69 × 2)
722.1 (a2u, 524)
Ge-H: 1.562
Ge-Ge: 2.270
H-Ge-H: 109.8°
Ge-Ge-H2: 145.4
Ge2H6
1A1g
(D3d)
[2093, 2077, 879, 756]j
878.2, 748.7d
881.6, 752.8e
Ge-H: 1.531k
Ge-Ge: 2.477
H-Ge-H: 108.7°
Ge-Ge: 2.487
H-Ge-H: 108.4°
GeH-
1Π
(C∞h
Ge-H: 1.685 Å
)
GeH2
1587.1 (a1, 1145)
1576.3 (b2, 1318)
861.5 (a1, 81)
Ge-H: 1.670 Å
H-Ge-H: 90.5°
1650.9 (a1, 823)
1638.3 (e, 1193 × 2)
877.3 (e, 34 × 2)
831.6 (a1, 112)
1550.6 (a1, 1045)
1550.6 (b2, 1184)
824.1 (a1, 67)
Ge-H: 1.685 Å
H-Ge-H: 89.9°
1605.5 (e, 1074 × 2)
1600.2 (a1, 780)
842.6 (e, 28 × 2)
799.2 (a1, 92)
1670e
1700e
-
2B1
(C2V)
-
GeH3
1A1
(C3V)
Ge-H: 1.652 Å
H-Ge-H: 92.2°
Ge-H: 1.668 Å
H-Ge-H: 91.5°
Ge-H: 1.623m
H-Ge-H: 93.2°
a This work, infrared intensities in parentheses. b CCSD(T)/ECP/SDB, ref 48. c Gas phase, ref 23. d This work: argon matrix, cm-1
.
e This work:
neon matrix, cm-1
.
f Gas phase, refs 8 and 9. g DHF, ref 49. h Reference 20. i Reference 16. j Gas phase, refs 43 and 45. k Reference 30. l Reference
25. m Reference 54.
doublet at 1342.6, 1337.3 cm-1. The 1862.5 cm-1 band is much
sharper than the 1860.4, 1855.7 cm-1 absorptions; so they are
probably due to different matrix sites. The blue site at 1867.6
cm-1 (Figure 3) disappears first on annealing and photolysis
while GeH4 increases; therefore, it is presumed to be due to a
(H2)GeH2 complex. The sharp 1862.5 and 1860.4 cm-1 bands
appear to track with the sharp 921.5 cm-1 feature, whereas the
broader 1855.7 cm-1 peak and red shoulders track with the broad
903.5 cm-1 band, which shifts to 901.4 cm-1 on annealing.
The gas-phase fluorescence spectra of GeH2 and GeD2
revealed symmetric vibrational fundamentals at 1856, 916, and
1335, 657 cm-1, respectively.8,9 Supporting B3LYP frequency
calculations (Table 2) and other calculations8,49 predict the
antisymmetric and symmetric stretching fundamentals for GeH2
to be within a few cm-1 of each other and the former slightly
more intense. Hence, the sharp resolved 1342.6, 1337.3 cm-1
neon matrix bands are appropriate for ν3 (b1) and ν1 (a1) of
GeD2, but matrix sites contribute to the band contour for

IR Spectra of GeH1,2,3 Species
J. Phys. Chem. A, Vol. 106, No. 24, 2002 5813
TABLE 3. Important Frequencies (cm-1) Calculated at the
survival of GeD2 in pure D2 and the trend of deuterium matrix
absorptions appearing between neon and argon matrix values.
The present argon matrix 1839.2 and 913.6 cm-1 assignments
to GeH2 may be compared with the spectrum of GeH2 in the
H2O-GeH2 complex for which Ge-H stretching modes at
1813.6 and 1777.2 cm-1 and bending mode at 897.8 cm-1 were
identified.50 Although the symmetric Ge-H stretching mode
was complicated by a Fermi resonance with the bending mode
overtone, it appears that the two Ge-H bands are nonequivalent
in the H2O-GeH2 complex.
DFT/6-311++G(d,p)/SDD Level for Germanium Deuteridesa
B3LYP
BPW91
GeD
GeD2
1294.9
1318.2
1316.6
667.4
1262.4
1282.4
1278.5
639.9
GeD3
GeD4
Ge2D2
1485.5
1453.4
604.2
1449.8
1407.2
584.7
517.7
494.5
1513.3
1510.4
645.1
591.7
1003.1
946.8
1474.0
1464.5
625.6
565.7
985.5
GeH3. DFT calculations (Table 2) using two functionals
predict the strongest GeH3 fundamental, the ν3 (e) mode, 36
cm-1 below the strongest GeH4 absorption. Previous HF
calculations find a 13 cm-1 difference.20 Hence, the new 2089.4
and 1504.7 cm-1 neon matrix absorptions, 25 and 17 cm-1
,
942.6
717.3
621.9
728.4
661.3
respectively, below GeH4 and GeD4, are appropriate for GeH3
and GeD3. In pure deuterium, a stronger new 1500.9 cm-1
absorption is 16.0 cm-1 below GeD4 at 1516.9 cm-1. Sharp,
new argon matrix counterparts were observed slightly lower at
2074.1 and 1496.5 cm-1. In addition, the GeH4 discharge work5
shows a product band near 2073 cm-1 that can now be assigned
to GeH3. This new assignment for ν3 (e) of GeH3 is compatible
586.7
264.5
566.6
260.9
Ge2D4
1469.2
1454.8
1445.6
1445.1
627.5
1428.4
1410.4
1398.9
1398.4
607.6
with diode laser monitoring of GeH3 radicals at 2049.8 cm-1
,
Ge2D6
GeD-
1499.8
1486.5
625.6
1463.0
1441.6
604.9
but not with earlier matrix assignments4,5 of a band near 1839
cm-1, which is assigned above to GeH2.
540.7
520.0
Support is also found for this new GeH3 assignment in the
HD experiment, which gives new bands for GeH2D and GeHD2.
Three bands are observed at 1505, 1479, and 2071 cm-1 for
GeHD2 and at 2088, 2062, and 1493 cm-1 for GeH2D, which
agree with the separations predicted by our DFT calculations.
In addition, the ν4 and ν2 fundamentals for the GeH3 radical
are observed here at 857.2 and 667.8 cm-1 in solid neon with
deuterium counterparts in solid neon and deuterium (Table 1).
The matrix ν2 frequency is in excellent agreement with recent
MPI work.16 These assignments are substantiated by DFT
frequency calculations (Table 2).
1094.9
1129.9
1122.8
613.7
1173.9
1167.0
622.9
-
GeD2
1104.4
1104.1
587.0
1143.6
1138.1
598.2
-
GeD3
596.9
573.5
a Frequencies in same order and relative intensities as hydrogen
counterparts in Table 2.
GeH2 and specific modes are not resolved. Our HD experiment
gave single sharp bands at 1857.2 and 1338.3 cm-1 in each
region, although a very sharp 1858.9 cm-1 satellite evolved with
the former band on annealing. The ν1 mode of GeD2 in solid
neon is within 2 cm-1 of the gas-phase value.9 These bands
show H/D ratios 1860.4/1342.6 ) 1.386 that are typical of
Ge-H stretching vibrations. Germane itself reveals the 2114.3/
1522.0 ) 1.389 ratio. In addition, we observe the isolated GeH2
and GeD2 bending modes at 921.5 and 660.2 cm-1, which are
blue shifted 5 and 3 cm-1 by the neon matrix, and our GeHD
value is 802.2 cm-1. We note that the bending mode is
sufficiently low that the overtone does not reach the stretching
mode region. Finally the broader red-shifted features are due
to GeH2 aggregated with other species in the neon matrix.
Our 1839.2 and 1325.4 cm-1 argon matrix bands appear to
be counterparts of the above neon matrix absorptions (H/D )
1.388). Bands in the latter region (1329 cm-1) were assigned
by Smith and Guillory to GeD2, but the 1887, 1864 cm-1 and
1890, 1870 cm-1 bands assigned to GeH2 in solid argon4,5 are
not compatible with the neon matrix spectrum (nor are the 1887
and 1329 cm-1 bands4 compatible with each other, as their 1.420
ratio is too high for a Ge-H mode). The former bands are
probably due to an aggregate species, possibly a GeH2 complex
with H2, but we cannot be certain. However, the 1839 cm-1
argon matrix band first assigned4 to GeH3 is here reassigned to
GeH2 whereas the 1325 cm-1 band is correctly assigned to
GeD2.
GeH4 and Ge2H6. The observation of germane in these
experiments supports the above assignments, as further reactions
of GeH1,2,3 should produce GeH4. The present argon matrix
experiments produce the same sharp GeH4 and GeD4 absorption
observed with the authentic material, but the extra higher
frequency absorption45,50 is much weaker here. This latter peak
might be due to a germane aggregate species. In the pure
deuterium experiment, the GeD4 absorption at 1516.9 cm-1 is
much weaker than GeD3 at 1500.9 cm-1 on sample deposition,
but λ > 240 nm photolysis reduces GeD3 and increases GeD4
and Ge2D6.
Likewise, the observation of digermane is also expected in
these reactions from the dimerization of GeH3. The two strong
(ν6 and ν8) bands of Ge2H6 (Ge2D6) are observed here.4,45 The
strong Ge-H stretching modes of digermane are difficult to
resolve, but the weak 2096.5 and 2077.0 cm-1 bands are
probably due to Ge2H6. In our pure deuterium experiment,
Ge2D6 increases on photolysis at the expense of GeD3, and a
sharp 1510.4 cm-1 band appears. The 1510.4 cm-1 band is
probably due to Ge2D6 in solid deuterium.
Ge2H2. The neon matrix experiments reveal a new band at
972.2 cm-1, which decreases slightly on λ > 240 nm irradiation
and annealing. In contrast, the Ge2H6 absorptions double on
photolysis and quadruple on annealing. The 972.2 cm-1 absorp-
tion increases relative to the GeH2 bands on increasing the
relative Ge/H2 reagent ratio through the use of higher laser
energy (Figure 3). This band shifts to 715.1 cm-1 with D2 and
gives a 1.360 ratio, which is lower (more anharmonic) than
The sharp 1338.4 and 1332.7 cm-1 bands in pure deuterium
are counterparts to the above GeD2 absorptions. Note the

5814 J. Phys. Chem. A, Vol. 106, No. 24, 2002
Wang et al.
found for terminal Ge-H stretching modes. In addition, one
HD counterpart was observed at 934.5 cm-1, substantially above
the median band position. For an uncoupled symmetric mode,
with the stronger D2 products. B3LYP calculations (Table S3)
predict Ge2D4 to have two strong absorptions 17 and 40 cm-1
below the strongest GeD3 radical absorptions, and the 1479.0
and 1465.5 cm-1 absorptions 26 and 39 cm-1 lower fit nicely.
Our HD experiment gives a weak 1471 cm-1 absorption
intermediate between the above bands, as is predicted by
calculations. We would be more confident about assigning these
bands to Ge2H4 and Ge2D4 if stronger hydrogen counterparts
were observed. However, it is probable that Ge2D4 is made in
much higher yield in these experiments than Ge2H4. The GeH4
band is 10 times stronger than the GeD4 band in Figures 1 and
2, and accounting for the relative infrared intensities, the yield
of GeH4 is 5 times greater. This shows that more GeH2 reacts
with H2 during condensation in neon and suggests that less GeH2
would be available for dimerization. A strong 569.5 cm-1 band
with D2 in neon tracks with the 1479.0, 1465.5 cm-1 doublet
on photolysis and annealing. The weaker H2 counterpart at 789.1
cm-1 defines a 1.386 H/D ratio, which is appropriate for a
bending mode. Our DFT calculation predicts a very strong
bending mode for Ge2H4 at 879.3 cm-1, which is slightly higher
than the observed band, but reasonable for this unusual molecule.
like the bending mode of GeH2, the GeHD value (802.2 cm-1
)
is just above the mean of GeH2 and GeD2 values (921.5 and
660.2 cm-1 average to 790.0 cm-1). The single unique HD
counterpart and the observation of no new bands with the H2
+ D2 mixture demonstrate that a single H2 molecule is involved
here. The 972.2 cm-1 band, then, is due to the antisymmetric
motion of two H atoms where symmetry lowering on HD
substitution results in a different mode (the strongest band of
Ge2HD has more H than D motion) since the Ge2HD counterpart
at 934.5 cm-1 is substantially higher than the Ge2H2-Ge2D2
median (843.6 cm-1).
This new 972.2 cm-1 absorption is in the region predicted
earlier25 for the strongest antisymmetric (b2) absorption of C2V
“butterfly” dibridged Ge2H2. Our B3LYP calculations give
1004.2, 950.9, and 717.3 cm-1 for this strong mode of Ge2H2,
Ge2HD, and Ge2D2, respectively, and the solid neon bands at
972.2, 934.5, and 715.1 cm-1 fit this pattern very well except
for the lack of anharmonicity in the calculated frequencies. This
match of an unusual isotopic frequency pattern confirms the
matrix identification of Ge2H2, which is isostructural with Si2H2,
and both are nonplanar dibridged analogues of linear HC-
CH.24,25,51,52 Other modes of Ge2H2 are much weaker: although
DFT predicts the b1 stretching mode to be 29% as intense, the
The HD experiment gives a single counterpart at 703.0 cm-1
.
In solid argon, the product yield was very low and annealing
to 35 K produced new 1476 and 568 cm-1 bands, which are
argon matrix counterparts of Ge2D4. Only the 787 cm-1 bending
mode was observed in argon for Ge2H4.
higher level calculations find it to be only 10% as strong.25
A
Accordingly, the above absorptions are assigned to Ge2H4
and Ge2D4, which are novel nonplanar analogues of ethylene.27-31
The only previously observed GedGe moiety is the substituted
Ge2R4 species, which has the trans-folded framework.53
1352.3 cm-1 band 7% as strong as 972.2 cm-1 follows on
annealing and photolysis and exhibits an identical 1.1 cm-1
higher splitting on 9 K annealing. The 1352.3 cm-1 absorption
is assigned to the antisymmetric (b1) mode calculated near 1330
cm-1 by DFT and 1420 cm-1 by CCSD;25 the much weaker
Ge2D2 counterpart is not observed. Our DFT structure for Ge2H2
is illustrated below.
GeH3- and GeH2-. Two bands at 1700 and 1670 cm-1 are
unique in that broadband irradiation eliminates and further
annealing does not restore them. Furthermore, a similar 5% H2
experiment doped with 0.1% CCl4 gave the other product bands
with 50-70% of their former absorbances, but the 1700 and
1670 cm-1 bands were not observed. Such treatment has been
found to prevent product anion formation owing to preferential
electron capture by CCl4.54-56 Hence, the latter absorptions are
appropriate for anions, and DFT calculations were performed
-
for possible anion species. Table 2 shows that GeH3- and GeH2
absorptions fall in this region, and the above bands must be
considered accordingly.
These absorptions shift to 1234 and 1208 cm-1 with D2 in
neon, giving 1.378 and 1.382 H/D frequency ratios, which is
appropriate for a terminal Ge-H stretching frequency. Broad-
band photolysis destroyed both of these absorptions, but in a
second experiment irradiation with a λ > 380 nm filter destroyed
the 1208 cm-1 band without affecting the 1234 cm-1 absorption;
the 1670 cm-1 band was also destroyed by λ > 380 nm but not
affected by λ > 470 nm radiation, whereas the 1700 cm-1 band
was not affected at λ > 380 nm but diminished at λ > 290 nm.
It is clear that the lower band is more photosensitive than the
upper band. The pure deuterium experiment gave split coun-
terpart bands at 1219.9, 1216.7 cm-1 and at 1202.4, 1198.6
cm-1, with the latter more photosensitive (Figure 5). We are
inclined to assign the upper 1700 and 1234 cm-1 bands to
The argon matrix b2 mode isotopic counterparts were
observed at 964.3, 927.8, and 709.3 cm-1. These bands increase
more on annealing in solid argon as higher temperatures can
be attained, and they exhibit matching satellites 3.4 cm-1 lower.
The 964.3 cm-1 band increases at the expense of GeH2 on final
38 K annealing (Figure 6). In addition, the 964.3 cm-1 band
grows more on annealing than Ge2H6 in 2% H2 argon matrix
experiments, but Ge2H6 increases substantially more than Ge2H2
on annealing with 10% H2 in argon. Finally, Ge2D2 was not
observed in pure deuterium.
Smith and Guillory4 show a weak absorption near 960 cm-1
in a Ge2H6 experiment before photolysis; this band is more than
an order of magnitude weaker than the 878 cm-1 Ge2H6 band.
Hence, their 964 cm-1 product band in GeH4 experiments, which
is comparable to the 878 cm-1 band in intensity, is probably
due to Ge2H2, as we assign above, and not to Ge2H6. We note
-
-
GeH3 and GeD3 and the lower 1670 and 1208 cm-1 bands
-
to GeH2 and GeD2-. This is in accord with observation of
only the 1670 cm-1 feature at lower H2 concentration.
that Ge2H6 vapor has no absorption near 960 cm-1 45
.
Ge2H4. Two new bands appear at 1479.0 and 1465.5 cm-1
below GeD3 in neon at 1504.7 cm-1: these bands increase
substantially on broad-band photolysis (Figure 2). Very weak
2053 and 2035 cm-1 bands with H2 exhibit similar behavior:
these weak bands exhibit appropriate 1.388, 1.389 H/D ratios
Our B3LYP calculation predicts the symmetric Ge-H
-
stretching mode for GeH3 slightly higher than the antisym-
metric stretching mode, but the BPW91 functional finds the
reverse relationship. We believe that the 1700 and 1234 cm-1
bands are due to the strong antisymmetric stretching modes of

IR Spectra of GeH1,2,3 Species
J. Phys. Chem. A, Vol. 106, No. 24, 2002 5815
GeH3- and GeD3-, and the 1670 and 1208 bands are unresolved
formation of Ge2H4 involves the dimerization of GeH2, reaction
10.
-
stretching modes for GeH2 and GeD2-. The calculated
frequencies are low for both anion species, but they correctly
GeH2 + Ge f Ge2H2 (1A1)
2GeH (2Π) f Ge2H2 (1A1)
2GeH2 f Ge2H4 (1Ag)
(8)
(9)
-
predict the order GeH3 > GeH2-. A similar relationship has
-
- 60
been found for SiH3 > SiH2
.
-
Density functional calculations find the GeH3 anion more
stable than the GeH3 radical (35.3 kcal/mol, B3LYP; 34.5 kcal/
-
mol, BPW91), and GeH2 more stable than GeH2 by a lesser
(10)
amount (21.0 kcal/mol, B3LYP; 22.5 kcal/mol, BPW91), which
is consistent with the latter anion being more photosensitive.
These are near other theoretical electron affinity values (35.4,
36.8, and 38.0 kcal/mol) and the 37 ( 2 kcal/mol experimental
measurement.57-59 Photodestruction with λ > 380 nm is overkill,
but such is necessary for the matrix-isolated species and lamp
sources.54 Similar studies with SiH3- in solid neon find a higher
energy λ > 290 nm requirement for photodestruction of the
product absorption.60
Reaction Mechanisms. The first reaction observed here is
the endothermic (28 kcal/mol)61 formation of GeH, reaction 1,
during deposition with energetic laser-ablated Ge atoms.62 The
exothermic (B3LYP, -25.6 kcal/mol) reaction of Ge atoms with
H2, reaction 2, is expected to be favorable. We note that 8 K
annealing increases GeH2 by 50% (Figure 1), but a similar
treatment only sharpens GeD2 absorptions (Figure 2). Subse-
quent reactions of GeH2 with H atoms and molecules, respec-
tively, are also exothermic (B3LYP, -54.6 and -30.1 kcal/
mol). Higher level calculations find reaction 4 to be even more
exothermic (-36 to -41 kcal/mol).49 The reaction of GeH with
H2 is also exothermic (B3LYP, -17.2 kcal/mol). However,
the observation of GeD and GeD2 in pure solid deuterium
suggests that deuterium reactions may require activation energy,
and accordingly, GeD2 increases on photolysis. In contrast, Si
atoms in pure solid deuterium give mostly SiD4, with no
evidence for SiD2, suggesting that SiD2 reacts straightaway with
D2 to form SiD4.60 It appears that GeH2 is more reactive than
GeD2: photolysis decreases GeH2 by 35% and leaves GeD2
unchanged in solid neon with 5% H2 and D2. Finally, recent
flash photolysis experiments show that GeH2 is less reactive
than SiH2.63
The germyl anion is formed by the capture of ablated
electrons during condensation. This electron capture reaction
is exothermic by the computed electron affinity (B3LYP, 35.3
kcal/mol) of the GeH3 radical. The slight growth of GeH3 radical
-
absorptions on ultraviolet irradiation that destroys GeH3
absorptions is probably due to photodetachment of GeH3-, the
-
reverse of reaction 11. The GeH2 anion formed by electron
capture is less stable (electron affinity, B3LYP, 21.0 kcal/mol)
and subject to photobleaching at longer wavelengths than
-
-
GeH3-. The reversal in relative yields of GeD3 and GeD2
from solid neon to solid deuterium follows the increased yield
of GeD3 radical in pure deuterium.
GeH3 + e- f GeH3- (1A1)
GeH2 + e- f GeH2- (2B1)
(11)
(12)
Conclusions
Laser-ablated Ge atoms react with H2 in excess neon and
argon during condensation to produce GeH2 as the major product
and GeH, GeH3, and GeH4 as minor products. The digermanium
Ge2H2, Ge2H4, and Ge2H6 species are also formed. The
-
photosensitive GeH3 anion results from the capture of an
ablated electron by GeH3 radical. Identification of these products
is based on experiments with different reagent concentrations,
HD and D2 substitution, and agreement with isotopic frequencies
from DFT calculations. The novel dibridged Ge2H2 species is
characterized by a Ge-H2-Ge stretching mode at 972.2 cm-1
,
with HD and D2 counterparts at 934.5 and 715.1 cm-1. The
Ge2H4 moiety is characterized by a 789.1 cm-1 GeH2 bending
mode between values for GeH4 and Ge2H6. The yield of GeH4
is substantially higher than the yield of GeD4 in these experi-
ments. All four germanium deuterides are observed in pure
deuterium, which shows that further GeD and GeD2 reactions
with D2 may require activation energy.
+
Ge (3P) + H2 (1Σg ) f GeH (2Π) + H (2S)
(1)
(2)
(3)
Ge (3P) + H2 (1Σg ) f GeH2 (1A1)
+
GeH2 (1A1) + H (2S) f GeH3 (2A1)
Acknowledgment. We gratefully acknowledge support for
this work from NSF Grant CHE 00-78836.
GeH2 (1A1) + H2 (1Σg ) f GeH4 (1A1)
(4)
(5)
(6)
(7)
+
References and Notes
GeH + H2 f GeH3
GeH3 + GeH3 f Ge2H6
GeH2 + GeH4 f Ge2H6
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