2215
J. Phys. Chem. 1994,98, 2275-2277
Infrared Spectra of InP, I d s , and InSb Molecules in Rare-Gas Matrices at 4 K Magnetic Effects S.Li, R. J. Van Zee, and W. Weltner, Jr.' Department of Chemistry and Center for Chemical Physics, University of Florida, Gainesville, Florida 3261 1-2046 Received: November 5, 1993; In Final Form: January 2, 1994'
InP, InAs, and InSb diatomics have been prepared by laser vaporization of the corresponding crystals into neon, argon, and/or krypton gases and condensed on a gold surface at 4 K. Their infrared spectra (10-4800 cm-l) were observed in absorption. The zero-field splittings [32-( lX~)-~2-(0+Xl)]in the ground states of InAs and InSb were observed and verified by the broadening effects of applied magnetic fields of up to 4 T. For InAs, a Franck-Condon envelope with (0,O)at 3600 cm-1 was also observed and assigned to the vibrational progression of a low-lying A311excited state. A strong absorption in these spectra was attributed to the v3 stretching mode of the triatomic InzX (X = P, As, Sb) molecules. Trends in the spectra are quite similar to those observed earlier in the GaX series.
Introduction
.4 i
For molecules containing more than one unpaired electron, spectroscopicobservationsin the infrared may include transitions corresponding to so-called zero-field splitting (zfs) along with the usual vibrational spectra. Allowed transitions to very lowlying electronic states may also be observable in this region. In linear molecules the zero-field splitting [synonymous with the parameter D (or b2) in electron spin resonance studies or analogous to X in gas-phase studies] arises from spin-spin and second-order spin-orbit interactions between the unpaired electrons.' For molecules containingheavy atoms these zfs transitions, which are normally weak low-frequency magnetic dipole transitions, can lie among the observed vibrational bands and even be of comparable strength. For example, for Se2 and Te2 with 32, ground states, these transitions occur strongly at 51 1 and 1980 cm-1, respectively.24 They can be distinguished from vibrational bands by their sensitivity to a strong magnetic field. Here, homogeneous fields up to 5 T are employed. Considering the importance of 111-V semiconductorsand their formation by chemical vapor deposition methods, it is perhaps surprising that the properties of the gaseous molecular species are not well-known. Among the indium series, there appears to be no information except for a theoretical paper by Balasubramanian on diatomic InSb and its ions.* We have therefore extended our earlier gallium studies6 to the phosphides, arsenides, and antimonides of In. Besides vibrational information, the zfs ( l a + ) transition in the 3Z ground states of InAs and InSb have been identified. Also, transition to a low-lying electronic state of InAs has been observed. Larger clusters are also evident in the spectra, but a tentative band assignment could only be made for the InzX species. Experimental Section A slightly focused Nd:YAG laser operating at 532 nm was used to vaporize indium phosphide (Johnson-Matthey, 99.999% purity), arsenide (high-purity semiconductor crystal, provided by Dr. David Vanderwaterof Hewlett Packard Co.), or antimonide (Firebird Semiconductors, Ltd., >99.999% purity) crystals while argon (Airco, >99.999% purity), neon (Matheson, 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-plated copper surface 0
Abstract published in Advance ACS Abstracts, February 1, 1994.
Q022-3654/94/2098-2275$04.50/0
160
140
120
Wavenumbers ( c m - 1 )
Figure 1. Infrared spectra of indium antimonide molecules in an argon matrix at 4 K: (A) as originally trapped; (B) after annealing to 32 K.
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- 1 13V) equipped with a liquid helium (pumped on to -1.6 K) cooled silicon bolometer for the region -10-400 cm-' and a liquid nitrogen cooled MCT detector for 4004800 cm-la4s6' A Mylar beam splitter with a specified thickness of 6 pm was used for the frequency range of 50-375 cm-1. The spectra were recorded with a resolution of 0.5 and 2 cm-' for far-IR and mid-IR, respectively, and a scan number of 100. Results Since the indium antimonide is the only molecule studied theoretically in this series, we will start our discussion with InSb and then consider the InAs and InF' diatomic molecules. Higher clusters such as triatomics are clearly present in our spectra, but assignments are difficult since isotopic shifts are not as observable here as in the gallium molecule series.6 However, comparisons with the gallium series will be used to aid in understanding the observed spectra and to make assignments of In2X bands. InSb. Figure 1 shows the absorption spectra in the range of 100-170 cm-1 which resulted from trapping the gas mixture of argon and laser-vaporized indium antimonide at 4 K. Trace A is the spectrum of the original matrix, and trace B is the spectrum after annealing at 35 K for 30 min and again cooling to 4 K. In 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vol. 98, No. 9, 1994
2276
Li et al.
TABLE 1: Infrared Transitions and Deduced Parameters. of InP, I d s , and InSb Molecules in Argon (Neon) Matrices at 4
K
InP (argon) (cm-I) k(argon) (104 dyn/cm) A311 '2, U ~ , O(cm-I)
257.9 9.57
AGl/2
InAs
In W b
180.6 8.72
145.5 7.35
3650.8 (3645.0) 3832.5 (3827.2) 4013.3 (4008.0) 3650.0 (3644.6) 182.7 (182.2) 0.5 (0.7) 119.0
473.4
InP
InzAs
In2I2lSb
249.3
176.9
143.9
+
v"0
v'= 1 u"2
Te (cm-l) wc) (cm-1) w,'x,' 1
+
(cm-1) O+ (cm-1) (32;
-
ground state, u3 (cm-1)
32;)
Masses are as follows: I%, 114.9041; 121Sb,120.9038; I2%b, 122.9041. (I
-7
150 Wavenumbers (cm-1)
260
30.9938; 7SAs,74.90457;
r
I
k
100
Figure 3. Infrared spectra of indium arsenide molecules in an argon matrix at 4 K (A) as originally trapped; (B) after annealing to 32 K.
2.5
1
7 500
400
I
Wavenumbers ( c m - 1 )
4000
3600
Figure 2. Infrared spectra of an argon matrix containinglaser-vaporized InSb showing (A) in a magnetic field of 0 T, (B) in a magnetic field of 2 T, and (C) in a magnetic field of 4 T.
Figure 4. Infrared spectrum of indium arsenide in an argon matrix at 4 K (after annealing to 32 K).
traceA,a doublet at 143.9 and 145.1 cm-Idominatesthespectrum, along with some other weak features as shown in Figure 1A. The two components of this doublet behave rather differently upon annealing; the one at 145.1 cm-I decreases its intensity and splits into a new doublet at 144.9/ 145.5 cm-l, with a relative intensity ratio of about 60:40. (Natural abundances are 'ISIn, 95.72%, W b , 57.25%, and 123Sb,42.75%) The band at 143.9 cm-I, on the other hand, increases in intensity after annealing as seen in Figure 1B. Some other weaker bands also grew in during annealing. The initial strength of the band at 145.5 cm-', its decrease upon annealing, and the relative intensity ratio of the 144.9/145.5-cm-l doublet suggest that this doublet is due to the Inl2lJ23Sb diatomic molecules. The isotopic shift is exactly that expected for the diatomic, and the derived harmonic force constant is 7.35 X lo4 dyn/cm (see Table 1). A similar doublet at 141.8 and 142.4 cm-1 was observed in krypton matrices and showed the same behavior on annealing. Attempts to trap this diatomic in neon matrices were not successful. Nevertheless, the matrix shifts from argon to krypton indicate that the gas-phase frequency probably lies between 145 and 150 cm-I, compared with the theoretically calculated value at 121 cm-I by Balasubramanian.3 In thecalculation by Balasubramanianusing CASSCF/FOCI/ SOCI/RCI, including spin-orbit interaction, the ground state of InSb was found to be X32;+, with the 32;- 32;+splitting calculated as 492 cm-*. This predicted zero-field splitting is confirmed from this present study. In Ar matrices, as shown in Figure 2, a weak absorption has been observed as a doublet at 470.6 and 473.4 cm-I. Unlike the other absorptions observed in the spectrum, this band clearly showed a magnetic field dependence, becoming weaker and broader as the field was increased
to 4 T. Similar experiments were repeated in krypton matrices, and a smaller doublet at 468.2 and 473.0 cm-I was observed, which also showed magnetic field dependence. Then this absorption, having magnetic dipole character, is assigned to the 1 O+ splitting of InSb in its ground state. The doublet character of this transition in zero magnetic field is presumably due to two matrix sites. A transition to the predicted low-lying excited 3 I I state, calculated to be 2500 cm-l higher than the X32-state,5 was not observed in these matrices, indicating that it may lie above 4800 cm-I, out of the frequency region of this study. The presence of the two prominent bands near 144 cm-1, one the doublet assigned to InSb, is familiar in this series as it was also in the Ga series.6 In Figure 1B the strong absorption at 143.9 cm-l is tentatively assigned to u3 of the In2Sb molecule. Witha resolutionof0.5 cm-',onewouldnot beexpected toresolve the two isotopomers 11sIn2121Sb and 11sIn2123Sb.There are some other weak bands in that figure, undoubtedly due to polyatomics. Ids. Figures 3 and 4 show the absorption spectra of laservaporized InAs condensed with argon at 4 K. The spectrum in Figure 3 resembles that discussed above for InSb and those for the GaX (X = P, As, Sb)6 molecules. Thus, the identification of the spectrum of InAs and In2As follows the same logic as for the InSb and In2Sbmolecules,the 180.6-cm-' band being assigned to diatomic InAs and the 176.9-cm-1 absorption to u3 of In2As. The harmonic force constant for InAs is calculated to be 8.72 X lo4 dyn/cm (see Table 1). Figure 4 shows the higher frequency region of the InAs spectrum, where two progressionsof bands are clearly seen. These
Werenumbers (cm-1)
-
The Journal of Physical Chemistry, Vol. 98, NO. 9, 1994 2277
InP, InAs, and InSb Diatomics I
=
u
:: .5
E8
+
4
molecules (GaAs, Te = 1830 cm-l and InSb, T, = 2500 cm-I), so that such a state at Td 3650 cm-l for InAs is reasonable. Besides the ground-state vibrational frequency and the A3nX3Z transition, a very weak absorption at 119.0 cm-1 in argon matrices was repeatedly observed. This weak band is unique in the sense of its magnetic field dependence. Again, there are no data available about the spin-orbit splitting of the ground state for the InAs diatomic molecule. This value of 119 cm-' is reasonablewhen compared with the zfs of 473 cm-1 for the heavier InSb molecule and the (tentative) value of 43 cm-I for the lighter GaAs mole~ule.~ InP. The spectrum of an argon matrix at 4 K formed by laser vaporization of an InP crystal is shown in Figure 5. It is similar to Figures 1 and 3 in showing two prominent bands. Phosphorus, like arsenic, has only one isotope, 31P, and the logic was the same for assigning these bands as for InAs and InSb. The sharp 257.9cm-1 absorption is assigned to diatomic InP, yielding a harmonic force constant of 9.57X 104dyn/cm. The broader band at 249.3 cm-I is then assigned to the u3 mode of In2P. Apparently, there are no other spectroscopicor theoretical data on these molecules.
0-
2so
200
Wmvenumbera ( c m - 1)
Figure 5. Infrared spectrum of indium phosphide molecules in an argon matrix at 4 K (after annealing to 32 K).
two sets of bands, both with a spacing of about 180 cm-', are attributed to two sites in the argon matrix with (0,O)bands at 3542.8and 3650.8 cm-I. Warming the matrix sample quickly to 47 K and then cooling it immediately to 4 K made the weaker series (indicated by an asterisk in Figure 4) disappear. This assignment is supported by the neon matrix spectrum which showed only one set of bands with (0,O)at 3645.0 cm-I. The intensity variation in the Franck-Condon envelopeindicates that the ground-state interatomic distance is not changed very much in the excited electronic state. This is also indicated by the small change in the vibrational frequency in the ground and excited states: argon:
w,' = 182.7, u,'x,' = 0.5 cm-'
neon:
w,' = 182.2, w,'x,'
= 0.7cm-'
as compared to AG$ = 180.6cm-1 in argon. There appear to be no other experimental, or any theoretical, studies of InAs at the present time. However, calculations by Balasubramanian on GaAs* and InSbS suggest a low-lying excited 311state for both
Acknowledgment. This research was supported by the National Science Foundation (CHE-9114387).Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We thank Dr. David Vanderwater of Hewlett Packard Co. (San Jose) for his generous gifts of semiconductor crystals. References and Notes (1) Langhoff, S. R.; Kern, C. W. In App/icurions ofElecrronicSrrucrure Theory; Schaefer 111, H. F., Ed.; Plenum: New York, 1977; Chapter 10. (2) Brabson, G. D.; Andrews, L. J. Phys. Chem. 1992,96, 9172. (3) Hassanzadeh, P.; Thompson, C.; Andrews, L. J. Phys. Chem. 1992, 96, 8246. (4) Li, S.; Van Zee, R. J.; Weltner, W., Jr. J . Chem. Phys. 1993,99,762. ( 5 ) Balasubramanian, K. J . Chem. Phys. 1990, 93, 507. (6) Li, S.; Van Zce, R. J.; Weltner, W., Jr. J. Phys. Chem. 1993, 97, 11393. (7) Li, S.; Hamrick, Y. M.; Van Zee,R. J.; Weltner, W., Jr. J. Am. Chem. Soc. 1992,114,4433. (8) Balasubramanian, K. J . Chem. Phvs. 1987, 86, 3410. (9) Lemire, G. W.; Bishea, G. A.; Hcidecke, S. A.; Morse, M. D. J . Chem. Phys. 1990, 92, 121.
