J . Phys. Chem. 1993,97, 11393-1 1396
11393
Far-Infrared Spectra of Small Gallium Phosphide, Arsenide, and Antimonide Molecules in Rare-Gas Matrices at 4 K S. Li, R. J. Van Zee, and W. Weltner, Jr.' Department of Chemistry and Center for Chemical Physics, University of Florida, Gainesville. Florida 3261 1 Received: June 15, 1993; In Final Form: August 17, 1993'
GaX, GazX, and GaX2 molecules, where X = P, As, and Sb, have been prepared by laser vaporization of the corresponding GaX crystals into argon and krypton gases and condensed on a gold surface at 4 K. Their far-infrared spectra (50-350 cm-l) were observed in absorption and interpreted via the natural abundance of isotopes of Ga and Sb. Trends in the spectra, in the derived force constants, and in the structural information are evident in the series. The analyses for Ga2As and GaAs2 indicated Ca symmetries but with angles near looo and 40°, respectively, in accord with earlier theoretical calculations.
I. Introduction In the course of studies of high-spin molecules in the infrared spectral region,lJ our interest was directed to the triplet groundstate GaAs molecule3 and then to all of the absorption bands in the region 150-250 cm-1 produced by the trapping of gallium arsenide molecules in rare-gas matrices at 4 K. As shown here, these bands could beassigned to thediatomic and triatomicGa,As, molecules. These molecules had been detected by Smalley's group among gallium arsenide clusters formed in supersonic beam studies! Lou et al.5 in that group also carried out local-spindensity calculations on these clusters containing up to ten atoms. Earlier ab initio calculationshave been made by Balasubramanian and by Meier, Peyerimhoff, Bruna, and Grein on GaAs6~7and GaAs2*v9and by Graves and Scuseria on the small closed-shell Ga,As, molecules with x = y.Io Our concern here is specifically with GaAs, GazAs, and GaAsz. There are not experimental data on the triatomics, but there appears to be essential agreement in the results of the various theoretical calculationsas to their ground electronic states and structures. Also, there are apparently no experimental or theoretical data on the corresponding phosphide and antimonide molecules, so that the infrared studies were extended to them. 11. Experimental Section
A slightly focused Nd:YAG laser operating at 532 nm was used to vaporize gallium phosphide (Johnson-Matthey, 99.999% pure), arsenide (Johnson-Matthey,99.9999% pure), or antimonide (Johnson-Matthey, 99% pure) crystals while argon (Airco, >99.999% purity) or krypton (Matheson, >99.999% purity) gas was admitted at a rate of about 10 mmol/h over a period of about 3 h. The matrices were condensed on a gold-platedcopper surface cooled to 4 K by a continuous flow of liquid helium. The IR spectra were measured in reflection with a vacuum Fouriertransform infrared spectrometer (Bruker IFS-113V) equipped with a liquid helium (pumped on to 1.6 K) cooled silicon bolometer.' A mylar beam splitter with a specified thickness of 6 pm was used for the frequency range 50-375 cm-I. The spectra were recorded with a resolution of 0.5 cm-' and a scan number of 100.
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111. Results
Since more is known, experimentally and theoretically, about the gallium arsenide molecules, our discussion will begin with them and then consider the lighter phosphide and heavier antimonide species. *Abstract published in Aduance ACS Absrrucrs, October 1, 1993.
Gallium Arsenide Molecules. Figure 1 shows the absorption spectrum in the range 150-250 cm-' which resulted from trapping the gas mixture of argon and laser-vaporized gallium arsenide at 4 K. Trace A is the spectrum of the original matrix, and trace B is the spectrum after annealing at 30 K for 10 min and again cooling to 4 K. Three sets of bands were initially observed in trace A: a doublet at -208 cm-I, a triplet at -205 cm-1, and a doublet at 174 cm-I. Annealing of the matrix decreased the intensity of the doublet at 208 cm-I relative to the other two sets of bands, as seen in Figure 1. Some other weaker absorption also grew in during the annealing. Attempts were made to test the dependence of the relative intensities of the bands on the laser power, but altering the rate of vaporization did not have an appreciable effect. GaAs. The ratio of the intensities of the two bands of 208.5 and 207.0 cm-I in argon matrices, shown more clearly in Figure 2,is measured to be about 59:41,which is very close to the natural isotopic abundance of gallium atoms (69Ga, 60.4%, and 71Ga, 39.6%; arsenic is 100% 75As). Thus the initial strength of this doublet, its decrease upon annealing, and the gallium isotopic shift suggest that this doublet is due to the 69q7IGaAs diatomic molecules. A similardoublet at 204.9and 203.4cm-1 was observed in a krypton matrix and showed the same behavior on annealing. This vibrational frequency is in good agreement with AGl/z = 207(3) cm-I observed in the gas phase for 69GaAsby Morse's group.3 (Our trend of matrix shifts indicates that the true gasphase value probably lies above that in argon and therefore near the upper error limit, Le., near 210 cm-I.) The isotopic shift is exactly that expected for the diatomic, and the derived harmonic forceconstant is9.20X 104dyn/cm (seeTable1). Thisassignment is further confirmed in the case of gallium antimonide discussed later in this paper where four diatomic isotopomers due to both 69971Gaand 1z1J23Sbare clearly observed and shown to have a similar spectral pattern. GatAs. The triplet structure at 205.4,204.7,and 204.0 cm-' in argon matrices, shown clearly in Figure 2,has relative intensities in the ratio 36.1:47.2:16.7.The natural abundance of the two gallium isotopes would produce three isotopomers of Ga-As-Ga in essentially that relative abundance (see Table I). Theory*.g indicates that this molecule has Cb symmetry with a bond angle of about 98O. Then these bands are assignable to an expected strong asymmetric stretching frequency, v3. One can obtain the angle 2a and the force constant kll - kl2 from the frequencies for the symmetric isotopomers 69-75-69 (205.4 cm-l) and 7175-71 (204.0cm-1).11 These are found to be 99.7' and 8.26 X 104dyn/cm, respectively. Using these parameters in formulas11V12 for the mixed isotopomer 69-75-71 yields v3 = 204.7 cm-', in exact agreement with the observed value. There are weaker bands
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0022-3654/93/209711393$04.00/0 0 1993 American Chemical Society
Li et al.
11394 The Journal of Physical Chemistry, Vol. 97, No. 44, 1993
.4
0
200 Wavenumbers ( c m - 1 )
250
150
Figure 1. Infrared spectra of gallium arsenide molecules in an argon matrix at 4 K: (A) as originally trapped; (B) after annealing to 32 K.
.4
0)
f,
f
2
.2
U
A
B
0
c
205 Wavenumbers (em-1)
I
Figure 2. Expanded infrared spectra of gallium arsenidemolecules in an argon matrix at 4 K: (A) as originally trapped; (B) after annealing to 32 K.
TABLE k Infrared Transitions of Ga,As,. Molecules in Argon Matrices at 4 K nat
Vob
60.4 39.6
208.5 207.0
k = 9.20 X 104 dyn/cm
36.5 47.8 15.7
205.4 204.7 204.0
kll
60.4 39.6 60.4 39.6
231.0 230.2 174.1 172.7
abund (W) (cm-I) GaAs 69-75 71-75
derived parametes'
Ga2s (Cd u3 69-75-69
69-75-71 71-75-71
GaAs2 (Cd 75-69-75 75-71-75 v2 75-69-75 75-71-75 VI
- kl2 = 8.26 X 104 dyn/cm*
Ga-As-Ga angle = 99.7' kll+ kl2 = 7.64 X 104 dyn/cmc k33 = 7.25 X 104 dyn/cm As-Ga-As angle = 38.3'
a Atomic masses are 69Ga,68.9257; "Ga, 10.9248; and l5As,74.9216. Obtained from bands of symmetric isotopomersusing the foirmulas in ref 11. C Calculated from formulas in ref 11 but neglecting kl,. See text.
in Figure 1 remaining to be assigned. A possible candidate for V I of Ga2As is the weak band at 160 cm-1. GaAs2. The remaining strong doublet at 174 cm-1 is then assigned to the 69.71GaAs~ molecules. This is supported by the intensityratioofthe 174.1-cm-1 band to that at 172.7cm-1, which is approximately 60:40, implying that there is just one Ga atom in the molecule. There is another weaker band at 230 cm-l (see Figure 1B)with intensity behavior parallelingthat of the 174-cm-I bands, so that it can also be assigned to GaAs2. Clearly, these two vibrational frequencies for GaAs2 are not those expected for
300
200
250 Wavenumbers ( c m - 1 )
Figure 3. Infrared spectra of gallium phosphide molecules in an argon matrix at 4 K: (A) as originally trapped; (B) after annealing to 32 K.
an obtuse-angled C, molecule such as Ga2As where the asymmetric stretching frequency v3 dominates and is at higher frequency than the v2 bending mode. However, these observations are in accord with a cyclic model in which there is As-As bonding and an acute As-Ga-As angle. In the extreme there is an analogy with Ga+O2-, (and alkali-metal superoxides)13J4where the most intense band at 382 cm-1 is assigned as the vz(A1) mode and the weak one near 1090 cm-1 is vl(Al), characteristic of an 0 2 anion.14-15 In GaAs2 the ionicity is reduced, since the electron affinity of Asz, measured as 0.10 f 0.18 eV16 (but Meier et a1.9 calculate a considerably higher value, 0.48 eV), is presumably not as large as that of O2 (0.44 eV).I7 However, the lack of an intense v 3 band is indicative of an acute-angled C, structure for GaAs2. This is in accord with the theoretical calculations on GaAs2 by Balasubramanian? by Meier, Peyerimhoff, and Grein? and by Lou, Nordlander, and S m a l l e ~ .The ~ first two were ab initio (MRSD-CI), and the latter was a local-spin-density calculation. They are in essential agreement and yield a 2B2 ground state with e = 46.5 f l o , and with Ga-As and As-As bond lengths of 2.8 and 2.2 A, respectively. The higher frequency band at VI = 23 1 cm-I has some underlying structure, but again its two strongest features at 231.2 and 230.2 cm-1 have an approximate intensity ratio of 60:40. The Ga isotopic splitting is somewhat smaller than that for the 174-cm-1 band. With the four bands assigned for u1 and vz for the two isotopomers, the Teller-Redlich product ratio YIVZ for 69GaAs2and 7*GaAs2is found to be 1.0125. This diverges slightly, and in the expected direction due to anharmonicity,18from the theoretical value of 1.0098. Using the observed frequencies in equations for XI X2 and X l X 2 for a CZVXY2 molecule, neglecting the interaction force constant kl3," yields kll k12= 7.642 X lo4,k33 = 7.252 X 104, and 2a = 38.3O. Thus, this assignment provides evidence of a small As-Ga-As angle, in agreement with theory. However, the derived parameters are sensitive to variations in the angle. If only the two frequencies u1 and v2 observed for 69GaAszare used, along with the theoreticallyderivedangle 2a = 47S0,*oneobtains the quite different force constants kll + k12 = 5.629 X 104 and k33 = 1.055 X 105. Gallium Phosphide Molecules. Figures 3 and 4 contain the infrared spectra of the Ga,Py molecules in an argon matrix at 4 K. One notices the expected similarity to Figures 1 and 2 but shifted by about 80 cm-1 to higher frequencies. As for arsenic, phosphorus has only one isotope, 3lP. The identification of the spectrum of each species follows the same logic as that for the GaxAsymolecules. The spectrum in Kr at 4 K in Figure 5 also has a doublet at about 220 cm-l, but the bands at 280 cm-1 in argon appear to have completely overlapped. Gap. The bands for 69-31 and 71-31 again appear on the
+
+
The Journal of Physical Chemistry, Vol. 97, No. 44, 1993 11395
Gallium Phosphide, Arsenide, and Antimonide
.1
285 280 Wavenumbers ( c m - 1 )
290
275
-
180 160 Wavenumbers ( c m - 1 )
Figure 4. Expanded infrared spectra of gallium phosphide moleculesin an argon matrix at 4 K: (A) as originally trapped; (B) after annealing to 32 K.
Figure 6. Infrared spectra of gallium antimonidemoleculesin an argon matrix at 4 K (A) as originally trapped; (B) after annealing to 32 K.
,
Ga2P. The distinct triplet at 281 cm-1 can again be assigned to this presumed CZ,molecule as 9 , the asymmetric stretching frequency. Then from the two symmetric isotopomer frequencies a t 281.2 and 279.9 cm-l in Ar, one obtains kll - k12 = 1.051 X los dyn/cm and a Ga-P-Ga angle of 85.7O from Herzberg's formula (neglecting k13).11 The forceconstant is somewhatlarger and the bond angle somewhat smaller than the corresponding GazAs parameters (see Table I). Using these Ga2P parameters in formulas for the mixed Ga isotope molecule yields exact agreement with the observed 69-31-71 frequency. GaP2. Again the unsymmetrical doublet at 220 cm-l in Ar in Figure 3 (and at 216 cm-l in Kr in Figure 5 ) is the exact counterpart of the 174-cm-l doublet in Figure 1 and is therefore identified as vz of the presumed C, molecule. v1 is clearly seen in the Kr spectrum in Figure 5 a t 322 cm-1, but from the width of the line the isotopic splitting is judged to be less than or equal to about 0.7 cm-l. This vibrational mode was not observed in the less intense Ar spectrum. Using the Kr frequencies with the isotopic splitting of 0.7 cm-1 for the v1 band, one calculates the Teller-Redlich ratio to be vlu2 as 1.0087 as compared to 1.0067 theoretical. Then in the same procedure as that for GaAsz, neglecting k13, one finds kll + k12 = 9.4 X lo4 dyn/cm, kj3 = 5.7 X lo4,and 2a = 52O (seeTableI1). Thesemust beconsidered particularly uncertain because of the estimated splitting in the v 1 frequency. Gallium Antimonide Molecules. Antimony has two isotopes, with natural abundance lZ1Sb57.3%and lZ3Sb42.8%,so that one expects every band in the spectra of the phosphide and arsenide molecules to be a t least doubled, leading to more complex band shapes. However, the diatomic spectrum is quite distinct. GaSb. The two doublets at 173 cm-l in Figure 6 are clearly assignable to the four diatomic isotopomers. Another significance of definitely assigning these four isotopomers for a9,71Ga121J23Sb is that it provides further support to the spectral patternsobserved for the diatomic gallium compounds. The calculated force constant is k = 7.817 X 104dyn/cm (seeTableIII),demonstrating a continuing decrease in bond strength as one proceeds down the group V elements. G a S b . Similar to the case of the phosphide and arsenide, the bands assigned to this CZ, molecule occur as an approximate triplet (see Figures 6 and 7) just below the diatomic bands. However, there are now six isotopomers with considerable variation in natural abundance, as indicated in Table 111. If this triplet is assigned as v3, the asymmetric stretching frequency, with the peaks a t 167.4 cm-1 corresponding to 69-121-69 and a t 166.0 cm-l to 71-121-71, one calculates kll - k12 = 6.69 X lO4dyn/cm and the G a S b - G a angle as 103.4O. The calculated frequencies for the other symmetric isotopomers are then 166.8 cm-1 for 69-123-69 and 165.4 cm-1 for 71-123-71. These
350
300 250 Wavenumbers ( c m - 1 )
Figure 5. Infrared spectra of gallium phosphide molecules in a krypton matrix at 4 K (A) as originally trapped; (B) after annealing to 40 K.
TABLE Ik Infrared Transitions of Ga,P, Molecules in Argon Matrices at 4 K nat VOb molecule abund (W) (cm-l) derived parameters" GaP69-31 60.4 283.6' k = 1.013 X 105dyn/cm 71-31 39.6 282.5' Ga2P v3 69-31-69 36.5 281.2 kll - klz = 1.051 X los dyn/cmc 69-31-71 47.8 280.5 71-31-71 15.7 279.9 Ga-P-Ga angle = 85.7' Gap2 v1 31-69-31 60.4 322d kll+ kl2 = 9.4 X lo4 dyn/cmc 31-71-31 39.6 k33 = 5.7 X 104 dyn/cm vz 31-69-31 60.4 220.9 P-Ga-P angle = 52O 31-71-31 39.6 219.6 a Atomi~massof~~Pis30.993 80. b Thetwomajorpeaksinthatregion in Figure 4. Obtained from bands of symmetric isotopomersusing the formulas in ref 11. d From krypton matrix spectra (Figure 5) assuming isotopicsplitting = 0.7 cm-l, within the line width. e Calculated from the formulas in ref 11 by neglecting kl3. See text. high-frequency side of the strong triplet a t 281 cm-l, but as can be seen from Figure 4, there is some overlap with another doublet; perhaps the diatomic occupies two sites in the Ar matrix. Apparently these bands have shifted in Kr, so that they now lie a t the same frequency as the strong triplet. In Ar, the two isotopomer bands lie a t 283.6 and 282.5 cm-1, yielding a force constant k = 1.013 X lo5 dyn/cm (see Table 11), slightly larger than that for GaAs.
11396 The Journal of Physical Chemistry, Vol. 97, No. 44, 1993
TABLE III Infrared Transitions of Gasby Molecules in Argon Matrices at 4 K molecule
nat vot4 abund (W) (cm-l)
TABLE IV: Vibrational and Structural Properties of Gallium Phosphide, Arsenic, and Antimonide Molecules Deduced from Infrared Spectra in Argon Matrices at 4 K
derived parameters'
GaSb 69-121 69-1 23 71-121 71-123
34 26 23 17
173.8 173.3 172.8 171.8
Ga2Sb ~3 69-121-69 69-123-69 69-1 2 1-7 1 69-123-71 71-121-71 71-123-7 1
21 15 27.5 20.5 9 7
167.4
k l l - kl2
166.7
G a S b - G a angle = 102.4'
GaSb2 V I 121-69-121 121-69-1 23 123-69-1253 12 1-7 1-1 2 1 121-7 1-1 23 ~2 123-71-1253
Li et al.
k = 7.83
X
AG1/2(argon) (cm-I) AG~p(gas)(cm-9 k(argon) (lo4 dyn/cm)
104dyn/cm
= 6.69 X lo4 dyn/cmb
(cm-1) k l l - k12 (lo4 dyn/cm) Ga-X-Ga angle (deg) v3
166.0 VI v2
19.5 29 11 13 19.5 7
(cm-I) (cm-I)
kll + k12 (lo4 dyn/cm)b k3sb
183
Y-Ga-Y angleb (deg)
'From ref 3. S b - G a S b angle I34.8" 158 or 148
'Atomic masses are W b , 120.9038 and L23Sb, 122.9041. bTaking = 167.4 cm-l for 69-121-69 and v3 = 166.0 cm-l for 71-121-71 and using the formulas in ref 11. See text. c Taking V I = 183 cm-1 and v3 = 158 cm-I for 121-69-121 and using the formulas in ref 11, assuming kl3 = 0, to calculate the maximum value of 2a. See text. v3
283.6 10.14
208.5 207(3)' 9.20
69Ga2P
69Ga2As
281.2 10.52 85.8
205.4 8.26 99.7
173.8 7.82 69Ga2121Sb 167.4 6.69 103.4
69GaP2
69GaAs2
69Ga121Sb2
322 217.3 9.4 5.7 52
23 1 174.1 7.64 7.25 38.2
183' 158c
