Spectroscopic properties of gallium arsenide tetramers: Ga2As2, Ga2As2+ and Ga2As2−

Spectroscopic properties of gallium arsenide tetramers: Ga2As2, Ga2As2+ and Ga2As2−

Spectrochimica Acta Part A 61 (2005) 2730–2736 Spectroscopic properties of gallium arsenide tetramers: Ga2As2, Ga2As2+ and Ga2As2− Xiaolei Zhua,b b ...

88KB Sizes 0 Downloads 21 Views

Spectrochimica Acta Part A 61 (2005) 2730–2736

Spectroscopic properties of gallium arsenide tetramers: Ga2As2, Ga2As2+ and Ga2As2− Xiaolei Zhua,b b

a Department of Chemistry, Nanjing Normal University, Nanjing 210097, PR China Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China

Received 16 July 2004; accepted 12 October 2004

Abstract Spectroscopic properties of the low-lying electronic states of Ga2 As2 and its ions are studied using the complete active-space self-consistent field (CASSCF) and density function theory (DFT) followed by the coupled-cluster single and double substitutions (including triple excitations) (CCSD(T)) calculations. The stability of low-lying electronic states is examined by computing vibrational frequency. The energies of the ground states and a number of excited electronic states have been computed to predict the spectra of Ga2 As2 and its ions. The ionization energy, electronic affinity, and atomization energy are estimated at the CCSD(T)/6-311+G(d) level and compared with the available experimental results. © 2004 Elsevier B.V. All rights reserved. Keywords: Density function theory; Electronic state; Semiconductor

1. Introduction Spectroscopic properties of semiconductor clusters containing the elements of groups III–V and group IV have been the topics of many experimental and theoretical studies in recent years [1–28]. Detailed studies of the properties of such clusters as a function of their sizes could provide significant insight into the evolution from molecular to bulk-like properties. The early experimental studies on the Gax Asy clusters have been finished by Smalley and co-workers [1–6]. They [1] found that the relative abundance of the small clusters deviated strongly from the expected binomial distribution although the larger ones smoothly converged into a binomial distribution. Smalley and et al. [2,3] measured the ultraviolet photoelectron spectra (UPS) of mass-selected negative gallium arsenide cluster ions in the (2–50) atom size range with photon energy of 7.9 eV. The measured results demonstrate the even/odd alternation in the vertical electron affinity E-mail address: [email protected]. 1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.10.018

is strong. Neumark et al. have employed a number of experimental techniques [9–16] to study the low-lying electronic states of the groups III–V and group IV clusters. They have harnessed the anion photoelectron spectroscopic technique to probe the electronic states of groups III–V clusters such as Inx Py and Gax Py clusters [16]. Computational studies of the electronic states of groups III–V clusters have been inspired by experimental spectroscopic studies. Balasubramanian and co-workers [17–20,22,23] have used the complete active-space selfconsistent field (CASSCF) and multi-reference singles plus doubles configuration interaction (MRSDCI) techniques to investigate the low-lying electronic states of many groups III–V clusters. Electronic structure computations on Al4 P4 , Ga4 As4 , Al3 P3 , and Ga3 As3 have been performed by Al-Laham and Raghavachari [24,25]. Recently, the present authors [23,26,27] have studies the electronic states of several groups III–V clusters at the CASSCF/MRSDCI or CASSCF/DFT(B3LYP) levels. These studies would be very valuable in aiding the assignments of the observed spectra and in the predictions.

X. Zhu / Spectrochimica Acta Part A 61 (2005) 2730–2736

The equilibrium geometry of the ground state of Ga2 As2 has been found to be a rhombus structure with D2h symmetry by Balasubramanian [18] based on the CASSCF/MRSDCI technique. Meier and Peyerimhoff [28] have employed ab initio MRD-CI technique to study the ground state 1 Ag and a few excited electronic states of Ga2 As2 . However, they have not confirmed these electronic states are stable at the MRD-CI level. There is little known on the low-lying electronic states of Ga2 As2 + and Ga2 As2 − . In current study, we have systematically performed a study on possible low-lying electronic states of Ga2 As2 and its ions using the CASSCF/DFT/CCSD(T) method. Both ground states and several low-lying excited states of these species are optimized, and their energy separations, ionization energy, electronic affinity, atomization energy, and other properties for the electronic states of Ga2 As2 are computed.

2731

3. Results and discussions Table 1 displays the optimized geometries for electronic states of Ga2 As2 and its ions at the DFT and CASSCF levels. Clearly, there is a good agreement between the DFT and CASSCF equilibrium geometries with bond lengths within ˚ and bond angles within 7◦ . As shown in Table 2, no 0.17 A imaginary frequencies are found for most of low-lying electronic states of Ga2 As2 and its ions at the DFT level, which confirms that these states are stable at the DFT geometries presented in Table 1. In other words, vibrational frequency analysis indicates that the ground states and most excited states for Ga2 As2 and its ions have the rhombus geometry with D2h symmetry at the DFT potential surface. 3.1. The electronic states of Ga2 As2

2. Method of investigation Since the equilibrium geometries of the ground states of Ga2 As2 [18] and Al2 As2 [27] have been found to be the rhombus D2h structure, the initial structures for the lowlying electronic states of Ga2 As2 and its ions are set at the rhombus D2h structure. Then, these initial structures are optimized in D2h symmetry at the CASSCF and DFT(B3LYP) levels. The CCSD(T) single point energy calculation is performed based on the DFT geometries. Harmonic vibrational frequency analysis is carried out to examine whether the optimized geometries are stable or not. In the CASSCF calculations on the Ga2 As2 and its ions, we have employed relativistic effective core potentials [29] (RECPs) for Ga and As atoms with the outer 4s2 4p1 and 4s2 4p3 shells kept in the valence space, respectively. The RECPs together with valence Gaussian basis sets are taken from Refs [29,30]. These basis sets are augmented with an additional set of diffuse s and p functions and two sets of six-component 3d functions with αd1 = 0.2910 and αd2 = 0.09 for Ga and αd1 = 0.3880, αd2 = 0.1185 for As, respectively, resulting in a (4s4p2d) basis set for both Ga and As atoms. For the CASSCF computations, the 4s and 4p orbitals of both Ga and As are included in active space spanning three a1 , two b2u , three b1u , one b3g , two b3u , one b1g , and one b2g . All 16 electrons of Ga2 As2 − (15 for Ga2 As2 + and 17 for Ga2 As2 − ) are distributed in all possible ways among these active orbitals at the CASSCF level. The CASSCF method in the GAMESS package of molecular computational code [31] is used to optimize the structures of low-lying electronic states of Ga2 As2 and its ions. On the other hand, the geometry structures of the electronic states are optimized at the DFT level followed by the CCSD(T) [32] energy calibration and vibrational frequency analysis. We have also performed all-electron computations at the CCSD(T)/6311+G(d) level to estimate the ionization energy, electron affinity, and atomization energy of Ga2 As2 .

In Table 1, the Ga–Ga distance is the longer diagonal of the rhombus, As–As distances is the shorter diagonal of the rhombus, and the As–Ga–As bond angle is the very acute bond angle of the rhombus. The four equal sides of the rhombus correspond to the As–Ga distances in Table 1. The ground state of Ga2 As2 is a closed shell 1 Ag in the D2h symmetry, analogous to the In2 P2 ,In2 As2 , and Al2 As2 [17,19,27] that have been considered before. For the ground state 1 Ag , ˚ at the DFT level, is longer the Ga–Ga distance of 4.860 A, ˚ of Al2 As2 while the As–As than Al–Al distance of 4797 A ˚ is very close to that (2.294 A) ˚ of bond length of 2.293 A Al2 As2 [27] as expected. For the low-lying electronic states ˚ are shorter of Ga2 As2 , the As–As distances (2.293–3.042 A) ˚ resulting in acute than the Ga–Ga distances (4.036–4.995 A), As–Ga–As bond angles (50.5–77.7◦ ) for rhombus structures, which demonstrates that the As–As bonding interaction plays an important role in the determination of the stability for the electronic states of Ga2 As2 . On the other hand, the As–As distance in As2 and Ga–Ga distance in Ga2 are 2.094 and ˚ respectively, which are shorter than the correspond2.483 A, ˚ in the 1 Ag state of Ga2 As2 , ing values (2.293 and 4.860 A) namely, As–As and Ga–Ga interactions in Ga2 As2 are greatly weakened by the formation of As–Ga bonds. This feature is consistent with the earlier studies on In2 P2 , In2 As2 , and Al2 As2 [17,19,27]. Table 3 shows energy separations of electronic states of Ga2 As2 and its ions at the DFT and CCSD(T) levels. We note that the DFT energy order is consistent with the CCSD(T) energy order for most low-lying electronic states of Ga2 As2 and its ions. The dipole-allowed electronic transition from ground state 1 Ag is to the excited state 1 B1u . As shown in Table 3, this state lies 3.98 eV above the ground state at the CCSD(T) level. The atomization energy for Ga2 As2 is computed as a supermolecular computation.As listed in Table 3, the energy required to break the Ga–As bonds  in Ga2 As2 cluster leading to two Ga atoms (2 P) and As2 (1 g + ) dimer is 2.64 eV at the CCSD(T)/6-311+G(d) level. The atomization energy to completely separate Ga2 As2 into two Ga (2 P) and two As (4 S) is

2732

X. Zhu / Spectrochimica Acta Part A 61 (2005) 2730–2736

Table 1 Geometrical parameters for the electronic states of Ga2 As2 and its ions at the DFT(B3LYP) and CASSCF levels Cluster

Ga2 As2

State (D2h ) −

2B

lg

4B

2g

2B

2g

2B

2u

2B

3u

2B

3g

2B

1u

2A u 4A g 4B

Ga2 As2

Ga2 As2 +

1g

1A g 1B

2g

3B

2u

3B

3u

3B

3g

1B

3g

1B

1g

3B

1u

1B

1u

2 B3g 2B

1u

2B

1g

2B

3u

2A g 4B

2u

Geometrical parameters ˚ Ga–Ga (A)

˚ Ga–As (A)

˚ As–As (A)

˚ As–Ga–As (A)

4.552 (4.463 3.722 (3.689 3.732 (3.712 5.058 (4.997 5.041 (4.864 4.646 (4.562 4.922 (4.955 5.213 (5.299 5.082 (5.020 5.074 (4.880

2.589 2.564 2.505 2.509 2.503 2.501 2.776 2.746 2.765 2.698 2.648 2.629 2.714 2.743 2.859 2.888 2.848 2.812 2.843 2.772

2.467 2.524 3.352 3.400 3.336 3.352 2.287 2.280 2.272 2.335 2.541 2.615 2.290 2.355 2.347 2.298 2.575 2.535 2.565 2.631

56.9 59.0) 84.0 85.3) 83.6 84.1) 48.6 49.1) 48.5 51.3) 57.3 59.6) 49.9 50.8) 48.5 46.9) 53.7 53.6) 53.6 56.7)

4.860 (4.836 4.995 (4.966 4.869 (4.852 4.549 (4.530 4.039 (3.983 4.036 (3.985 4.790 (4.860 4.775 (4.832 4.707 (4.712

2.687 2.684 2.759 2.756 2.757 2.741 2.560 2.564 2.528 2.546 2.626 2.558 2.706 2.729 2.679 2.711 2.643 2.655

2.293 2.331 2.344 2.393 2.587 2.554 2.347 2.403 3.042 3.173 3.039 3.210 2.518 2.487 2.429 2.462 2.403 2.447

50.5 51.5) 50.3 51.5) 56.0 55.5) 54.6 55.9) 74.0 77.1) 70.7 77.7) 55.5 54.0) 53.9 54.0) 54.1 54.9)

5.570 (5.691 4.598 (4.603 5.912 (6.121 5.369 (5.453 5.330 (5.433 5.325 (5.486

2.994 3.057 2.588 2.603 3.161 3.262 2.945 2.987 2.902 2.957 2.988 3.051

2.200 2.233 2.377 2.431 2.242 2.261 2.419 2.441 2.299 2.336 2.712 2.673

43.1 42.8) 54.7 55.7) 41.5 40.6) 48.5 48.2) 46.7 46.5) 54.0 52.0)

The values in the parentheses are the CASSCF geometry parameters.

calculated to be 6.39 eV at the same level. This implies that the As–As bonding interaction is considerably stronger than the Ga–Ga interaction in Ga2 As2 and plays a determinative role in the properties for the electronic states of Ga2 As2 . The nature of bonding in the electronic states can be comprehended via the leading configurations, compositions of orbitals and Mulliken populations. Table 1 displays that

the gross populations of Ga range from 2.695 to 3.043 for the electronic states of Ga2 As2 , while the total As populations are between 4.957 and 5.305. The excess populations on As arise from charge transfer from Ga to As, leading to Ga+ As− polarities of bonds for most electronic states, except 3 B3u state, of Ga2 As2 . Table 4 presents the leading configurations for the electronic states of Ga2 As2 . As

X. Zhu / Spectrochimica Acta Part A 61 (2005) 2730–2736

2733

Table 2 Vibrational frequenciesa and IR intensities of the electronic states of Ga2 As2 and its ions System

State (D2h )

Vibrational frequencies (IR intensities)

Ga2 As2 −

2B 1g 4B 2g 2B 2g 2B 2u 2B 3u 2B 3g 2B 1u 2A u 4A g 4B 1g

95 (1.5) 47 (0.1) 71 (0.5) 39 (0.4) 36 (0.4) 21 (0.03) 31 (0.3) 48 (0.7) 27 (142.5) 42 (0.1)

222 (57.4) 157 (0.3) 116 (0.1) 59 (3.9) 60 (3.9) 87 (1.4) 67 (3.6) 53 (3.8) 32 (0.7) 67 (1.8)

1A g 1B 2g 3B 2u 3B 3u 3B 3g 1B 3g 1B 1g 1B 1g 1B 1u

53 (0.1) 61 (0.4) 88 (4.3) 60 (4.1) 154 (2.6) 152 (2.1) 93 (0.9) 55 (4.0) 99 (1.0)

90 (1.8) 83 (2.1) 179 (62.1) 74 (22.7)

2B 3g 2B 1u 2B 1g 2B 3u 2A g 4B 2u

21 (2.0) 71 (2.3) 16 (2.7) 33 (0.5) 12 (1.8) 20 (0.9)

31 (2.8) 114 (1.4) 41 (4.0) 62 (2.1) 20 (1.0) 76 (5.3)

Ga2 As2

Ga2 As2 +

a

186 (70.2) 93 (19.8) 208 (69.4)

165 (2.1) 159 (0.1) 158 (51.7) 162 (51.9) 200 (56.9) 133 (0.7) 133 (48.9) 79 (0.7) 72 (105.1)

164 (1.9)

179 (54.2)

194 (71.2) 170 (72.0) 105 (1.6)

114 (0.8) 123 (94.6) 126 (10.1) 87 (95.2) 137 (79.5) 144 (90.1)

The vibrational frequencies which IR intensities are less than 0.05 cm−1 are not listed in this table.

Table 3 Energy separations and Mul liken populations for the electronic states of Ga2 As2 and its ions System

State (D2h )

Ga2 As2

2B

1g

4B

2g

2B

2g

2B

2u

2B

3u

2B

3g

2B

1u 2A u 4A g 4B 1g 1A

Ga2 As2

Ga2 As2

+

g

1B

2g

3B

2u

3B

3u

3B

3g

1B

3g

1B

1g

3B

1u

1B

1u

2B

3g

2B

1u

2B

1g

2B

3u 2A g 4B 2u

E (eV)

Mulliken population

DFT(B3LYP)

CCSD(T)

Ga

As

−1.74 −0.67 −0.45 −0.13 0.06 0.27 0.44 0.91 1.40 1.52

−1.44 −0.37 −0.05 0.13 0.32 0.66 0.70 1.26 1.82 1.98

3.110 3.152 3.147 3.385 3.386 3.002 3.144 3.259 3.348 3.343

5.390 5.348 5.353 5.115 5.114 5.498 5.356 5.241 5.152 5.157

0.00 1.28 1.59 2.22 2.24 2.30 2.65 3.47 3.86

0.00 1.47 1.74 2.11 2.21 2.78 2.88 3.66 3.98

2.843 2.759 2.817 3.043 2.917 2.695 2.812 2.960 2.780

5.157 5.241 5.183 4.957 5.083 5.305 5.188 5.040 5.220

7.02 7.30 8.45 8.62 9.08 10.10

6.85 7.09 8.36 8.65 8.95 10.02

2.498 2.595 2.402 2.572 2.557 2.548

5.002 4.905 5.098 4.928 4.943 4.952

2734

X. Zhu / Spectrochimica Acta Part A 61 (2005) 2730–2736

Table 4 Leading configurations for the electronic states of Ga2 As2 and its ions Cluster

Ga2 As2

State (D2h ) −

2B

1g

4B

2g

2B

2g

2B

2u

2B

3u

2B

3g

2B

1u 2A u 4A g 4B 1g

Ga2 As2

Ga2 As2 +

1A

g

1B

2g

3B

2u

3B

3u

3B

3g

1B

3g

1B

1g

3B

1u

1B

1u

2B

3g

2B

1u

2B

1g

2B

3u 2A g 4B 2u

Weight

Leading configuration 1ag

2ag

3ag

1b1u

2b1u

3b1u

1b2u

2b2u

1b3u

2b3u

1b3g

1b1g

1b2g

0.96 0.91 0.91 0.96 0.94 0.96 0.85 0.97 0.99 0.97

2 2 2 2 2 2 2 2 2 2

2 2 2 2 2 2 2 2 2 2

2 2 2 2 2 2 2 2 2 2

2 2 2 2 2 2 2 2 2 2

2 1 1 2 2 2 2 2 2 2

0 0 0 0 0 0 1 0 0 0

2 2 2 2 2 2 2 2 2 2

0 1 1 1 0 0 0 1 1 0

2 2 2 2 2 2 2 2 1 1

0 0 0 0 1 0 0 0 0 1

2 2 2 2 2 1 2 1 2 2

1 1 1 0 0 2 0 1 1 1

0 0 0 0 0 0 0 0 0 0

0.96 0.98 0.99 0.93 0.90 0.87 0.96 0.80 0.88

2 2 2 2 2 2 2 2 2

2 2 2 2 2 2 2 2 2

2 2 2 2 2 2 1 2 2

2 2 2 2 2 2 2 2 2

2 2 2 1 1 1 2 1 1

0 0 0 0 0 0 0 0 0

2 2 2 2 2 2 2 2 2

0 0 0 0 1 1 0 0 0

2 2 1 2 2 2 2 2 2

0 0 0 0 0 0 0 0 0

2 1 2 2 2 2 2 1 1

0 1 1 0 0 0 1 1 1

0 0 0 1 0 0 0 1 1

0.95 0.94 0.95 0.96 0.95 0.93

2 2 2 2 2 2

2 2 2 2 2 2

2 2 2 2 1 1

2 2 2 2 2 2

2 1 2 2 2 2

0 0 0 0 0 0

2 2 2 2 2 2

0 0 0 0 0 0

2 2 2 1 2 1

0 0 0 0 0 0

1 2 0 2 2 2

0 0 1 0 0 1

0 0 0 0 0 0

seen from Table 4, the 1ag 2 2ag 2 1b2u 2 1b1u 2 portion of electronic configuration is common for all electronic states of Ga2 As2 considered. The difference in the properties among the electronic states is caused by the different occupancies for the 3ag , 2b2u , 2b1u , 1b3g , 1b3u , 2b3u , 1b1g and lb2g orbitals. Consequently, analysis of compositions of these orbitals could provide insight into the nature of the low-lying electronic states. The 3ag orbital is the strong bonding ␴ orbital along y-axis, which is composed of [As1 (py )-As2 (py )]. The 1b3u is the bonding ␲ orbital with [As1 (px )+As2 (px )]. The 1b1g and 1b2g are the antibonding ␲ orbitals composed of [As1 (px )−As2 (px )] and [Ga1 (px )−Ga2 (px )], respectively. The nature of bonding for 2b2u ,2b1u ,1b3g , and 2b3u orbitals is somewhat complex. The 2b2u orbital is made of [Ga1 (py )+Ga2 (py )]+[As1 (s)−As2 (s)] while the 2b1u orbital is mainly on [Ga1 (s)−Ga2 (s)] combined with [Ga1 (pz )+Ga2 (pz )−[As1 (pz )+As2 (pz )]. The 1b3g orbital is consisted of [As1 (pz )−As2 (pz )]+[Ga1 (py )−Ga2 (Py )]. The 2b3u is [Ga1 (px )+Ga2 (px )]-[As1 (px )+As2 (px )]. In the 1 Ag ground state, the 3ag , 2b1u , and 1b3u orbitals are doubly occupied, and these orbitals exhibit strong bonding interaction between As atoms. For neutral Ga2 As2 , the ground state has a much short As–As diagonal, resulting in the rhombus equilibrium structure. All the excited states arise from transfer of electrons from the bonding to the antibonding orbitals, which leads to energies above the ground state 1 Ag of Ga2 As2 . For example, the difference in the properties of the ground state 1 Ag and ex-

cited state 3 B2u is caused by different electronic occupancies for the 1 b3u and 1 b1g orbitals. The 1 b3u orbital which contains bonding ␲ interaction between two As atoms is doubly occupied in the state 1 Ag but singly occupied in 3 B2u ,while the 1 b antibonding ␲ orbital between As atoms is half-filled in 1g 3 B . It is of interest to note that the As–Ga–As bond angles 2u (74.0 and 70.7◦ ) of the electronic states 3 B3g and 1 B3g are much larger than that (50.5◦ ) of state 1 Ag of Ga2 As2 at the DFT level. It is due to the fact that promoting an electron from 2 b in the electronic state 1 A of Ga As to 2 b , which in1u g 2 2 2u cludes As–As antibonding ␴ interaction and Ga–Ga bonding ␲ interaction, leads to longer As–As bond length and larger As–Ga–As bond angle in the states 3 B3g and 1 B3g . 3.2. The electronic states of Ga2 As2 + On the basis of the leading configurations illustrated in Table 4, we could predict that the highest-occupied orbital of the ground state of Ga2 As2 + is 1 b3g . Therefore, the ground state of Ga2 As2 + arises from the removal of an electron from the closed shell 1 b3g orbital of Ga2 As2 . This would result in the ground state 2 B3g for Ga2 As2 + . The Ga–Ga and Ga–As distances are elongated relative to the neutral structure upon ionization. The ground state 2 B3g of Ga2 As2 + shows the populations of Ga (2.498) and As (5.002) while the corresponding values of neutral Ga2 As2 are Ga (2.843) and As (5.157). These populations illustrate that the ionization appears mostly on Ga sites. The first adiabatic ionization

X. Zhu / Spectrochimica Acta Part A 61 (2005) 2730–2736

energy of Ga2 As2 is computed as 6.85 and 6.20 eV at the CCSD(T)/4s4p2d and CCSD(T)/6-311+G(d) levels, respectively, which will be useful in the photoionization spectroscopy of Ga2 As2 .The first excited state of Ga2 As2 + is 2 B1u , which is obtained from the neutral ground state 1 A by the removal of an electron from the 2 b g 1u orbital, and the ionization energy is computed to be 7.09 eV at the CCSD(T) level. As a result, the state 2 B1u is only 0.24 eV above the ground state at the CCSD(T) level. This reveals that 1 b3g and 1 b1u orbitals are competing for the ionization process. 3.3. The electronic states of Ga2 As2 − and predictions of anion photoelectronic spectra of Ga2 As2 − The Ga2 As2 − anion is formed by adding an electron to the 1 b1g LUMO of the state 1 Ag of the neutral Ga2 As2 , resulting in the state 2 B1g of Ga2 As2 − . As shown before, the 1 b1g orbital is composed of [As1 (px )-As2 (px )], which is an antibonding ␲ orbital between two As atoms. Therefore, the As–As interaction is weakened while the Ga–Ga and Ga–As interactions are enhanced during the formation of the Ga2 As2 − anion, which is consistent with the geometry change of Ga2 As2 − , namely, the Ga–Ga distance is con˚ and Ga–As bond lengths also shrink a tracted by 0.308 A bit while the As–As bond length is elongated in the 2 B1g state compared to the corresponding values of the 1 Ag state of Ga2 As2 . The ground state 2 B1g of Ga2 As2 − exhibits the populations of Ga (3.11) and As (5.39), compared to the state 1 Ag of Ga2 As2 whose populations are Ga (2.843) and As (5.157), which demonstrates both Ga and As sites are involved in the electron attachment process. The adiabatic electronic affinity of Ga2 As2 is computed as 2.33 eV at the CCSD(T)/6-311+G(d) level. The vertical electronic affinity of Ga2 As2 is found to be 2.02 eV at the same level, which is in good agreement with the experimental value (2.1 eV) reported by Smalley and co-workers [3]. Neumark and co-workers [16] have determinated the adiabatic electron affinities and energy separations of the lowlying electronic states for several groups III–V clusters using anion photoelectron spectroscopy. To the best of our knowledge, such spectroscopic studies on Ga2 As2 have not been made so far. Hence, our predictions would be very valuable. Based on our computed results and the anion photoelectron spectrum of Ga2 P2 − [16], we predict the following features in the anion photoelectron spectrum of Ga2 As2 − . The peak X of the anion Ga2 As2 − will appear near 1.44 eV. The excited state 1 B2g is 1.47 eV higher than the ground state of Ga2 As2 , and this may correspond to the peak A in the spectrum. The excited state 3 B2u lies 1.74 eV above the 1 Ag ground state. There are two electronic states (3 B3u and 3 B3g ) near 2 eV, resulting in peak B of Ga2 As2 − . The electronic states 1 B3g and 1 B1g lie 2.78 and 2.88 eV above the ground state 1 Ag of Ga2 As2 . Any of these two excited states is a candidate for peak C.

2735

4. Conclusions In summary, nine electronic states of Ga2 As2 , six states of Ga2 As2 + , and 10 states of Ga2 As2 − are studied using the CASSCF/DFT/CCSD(T) method. We have presented the optimized gometries, vibrational frequencies, and energy separations for low-lying electronic states of these species at the DFT and CCSD(T) levels. The ionization energy, electron affinity, and atomization energy of Ga2 As2 are estimated at the CCSD(T)/6-311+G(d) level. The properties of both the ground states and excited state are analyzed for Ga2 As2 and its ions. Based on our computed results, the main features of anion photoelectron spectrum of Ga2 As2 are predicted, which will be very valuable for spectroscopic studies on Ga2 As2 − .

Acknowledgement This work was supported by a grant from Personnel Bureau of Nanjing of china (Project No. 2004103TSNB443).

References [1] S.C. O’Brien, Y. Liu, Q.L. Zhang, F.K. Tittel, R.E. Sraalley, J. Chem. Phys. 84 (1986) 4074. [2] Y. Liu, Q.L. Zhang, F.K. Tittel, R.F. Curl, R.E. Smalley, J. Chem. Phys. 85 (1986) 7434. [3] C. Jin, K.J. Taylor, J. Conceicao, R.E. Smalley, Chem. Phys. Lett. 175 (1990) 17. [4] L. Wang, L.P.F. Chibante, F.K. Tittel, R.F. Curl, R.E. Smalley, Chem. Phys. Lett. 172 (1990) 335. [5] Q.-L. Zhang, Y. Liu, R.F. Curl, F.K. Tittel, R.E. Smalley, J. Chem. Phys. 88 (1988) 1670. [6] S.C. O’Brien, Y. Liu, Q. Zhang, J.R. Heath, F.K. Tittel, R.E. Smalley, J. Chem. Phys. 84 (1986) 4074. [7] (a) K.D. Kolenbrander, M.L. Mandich, J. Chem. Phys. 92 (1990) 4759; (b) K.D. Rinnen, K.D. Kolenbrander, A.M. DeSantolo, M.L. Mandich, J. Chem. Phys. 96 (1992) 4088. [8] R.J. Van Zee, S. Li, W.J. Weltner, J. Chem. Phys. 98 (1993) 4335. [9] (a) G.R. Burton, C. Xu, C.C. Arnold, D.M. Neumark, J. Chem. Phys. 104 (1996) 2757; (b) C. Xu, T.R. Taylor, G.R. Burton, D.M. Neumark, J. Chem. Phys. 108 (1998) 1395. [10] C.C. Aronold, D.M. Neumark, J. Chem. Phys. 99 (1994) 3353. [11] C.C. Aronold, D.M. Neumark, J. Chem. Phys. 100 (1994) 1797. [12] C.C. Aronold, D.M. Neumark, Can. J. Phys. 72 (1994) 1322. [13] C. Xu, E. deBeer, D.W. Arnold, C.C. Aronold, D.M. Neumark, J. Chem. Phys. 101 (1994) 5406. [14] G.R. Burton, C. Xu, C.C. Aronold, D.M. Neumark, J. Chem. Phys. 104 (1996) 2757. [15] (a) T.R. Taylor, H. Gomez, K.R. Asmis, D.M. Neumark, J. Chem. Phys. 115 (2001) 4620; (b) C. Xu, G.R. Burton, T.R. Taylor, D.M. Neumark, J. Chem. Phys. 107 (1997) 3428; (c) C. Xu, T.R. Taylor, G.R. Burton, D.M. Neumark, J. Chem. Phys. 108 (1998) 1395. [16] (a) K.R. Asmis, T.R. Taylor, D.M. Neumark, Chem. Phys. Lett. 308 (1999) 347; (b) T.R. Taylor, K.R. Asmis, C.S. Xu, D.M. Neumark, Chem. Phys. Lett. 297 (1998) 133.

2736 [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]

X. Zhu / Spectrochimica Acta Part A 61 (2005) 2730–2736 P.Y. Feng, K. Balasubramanian, Chem. Phys. Lett. 264 (1997) 449. K. Balasubramanian, Chem. Phys. Lett. 171 (1990) 58. P.Y. Feng, K. Balasubramanian, Chem. Phys. Lett. 296 (1998) 283. P.Y. Feng, K. Balasubramanian, Chem. Phys. Lett. 295 (1997) 547. J.Y. Yi, Chem. Phys. Lett. 325 (2000) 269. K. Balasubramanian, J. Phys. Chem. A104 (2000) 1969. K. Balasubramanian, X. Zhu, J. Chem. Phys. 115 (2001) 8858. M.A. Al-Laham, K. Raghavachari, Chem. Phys. Lett. 187 (1991) 13. M.A. Al-Laham, K. Raghavachari, J. Chem. Phys. 98 (1993) 8770. X. Zhu, J. Mol. Struct. (Theochem) 638 (2003) 99. X. Zhu, Z. Zhou, J. Mol. Struct. (Theochem) 671 (2004) 105. U. Meier, S.D. Peyerimhoff, E. Grein, J. Chem. Phys. 150 (1991) 331.

[29] K. Balasubramanian, Relativistive Effects in Chemistry, WileyInterscience, New York, 1997. [30] M.M. Hurley, L.F. Pacios, P.A. Christiansen, R.B. Ross, W.C. Ermler, J. Chem. Phys. 84 (1986) 6840. [31] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Gordon, J.H. Jensen, S. Koseki, N. Matsunaga, K.A. Nguyen, S.J. Su, T.L. Windus, M. Dupuis, J.A. Montgomery, J. Comput. Chem. 14 (1993) 1347. [32] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T.A. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski et al., Gaussian 94 (revision D. 1), 1995.