Electronic Transitions of Iridium Monoxide: Ground and Low-Lying

Sep 10, 2012 - *Tel: (852) 2859 2155 Fax: (852) 2857 1586. E-mail: [email protected]. Abstract. Abstract Image. The electronic transition spectrum of IrO...
0 downloads 0 Views 421KB Size
Download PDF
Article pubs.acs.org/JPCA

Electronic Transitions of Iridium Monoxide: Ground and Low-Lying Electronic States H. F. Pang, Y. W. Ng, and A. S-C. Cheung* Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong S Supporting Information *

ABSTRACT: The electronic transition spectrum of IrO in the spectral region between 448 and 650 nm has been recorded and analyzed using laser vaporization/reaction free jet expansion and laser induced fluorescence spectroscopy. The IrO molecule was produced by reacting laser-ablated iridium atoms with N2O seeded in argon. Five electronic transition systems, namely, the [17.6]2.5 − X2Δ5/2, [17.8]2.5 − X2Δ5/2, [21.5]2.5 − X2Δ5/2, [22.0]2.5 − X2Δ5/2, and [21.9]3.5 − Ω = 3.5 systems were identified. Transition lines of both the 191IrO and 193IrO isotopes were observed and analyzed. IrO was determined to have a X2Δ5/2 ground state. A least squares fit of the measured rotational lines yielded molecular constants for the ground and low-lying electronic states. A molecular orbital energy level diagram has been used to help with the assignment of the observed electronic states. and Scullman,14 using a hollow cathode source, performed a similar emission experiment with higher resolution. They recorded and analyzed six transition systems also in the visible region. These systems were identified to be originated from two lower states, namely A′ and A″ states; rotational and vibrational constants for the upper and lower states were reported. Jansson and Scullman15 also attempted to record the absorption spectrum using matrix isolation spectroscopy, but their experiments were unsuccessful. Using laser ablated iridium atom to react with oxygen, Citra and Andrews16 observed the infrared spectrum of IrO in solid argon and also performed density functional theory (DFT) calculations. They predicted a 4 − Σ ground state for IrO. Recently, Yao et al.7 performed DFT calculations on the entire 5d period and also predicted a 4Σ− ground state for IrO. We report here rotationally resolved spectroscopic studies of electronic transitions of IrO in the visible region using the technique of laser vaporization/reaction with free jet expansion and laser induced fluorescence (LIF) spectroscopy. In this work, five electronic transition systems have been recorded and analyzed. Spectra of both isotopes 191IrO and 193IrO were resolved and studied. Electronic configurations giving rise to the observed electronic states have been examined using a molecular orbital energy level diagram.

I. INTRODUCTION Transition metal oxides (TMO) have been studied experimentally and theoretically for a long period of time.1−8 They constitute a diverse and fascinating class of compounds whose importance ranges from astrophysics and catalysis to high temperature chemistry. Knowledge of chemical bonding in a simple transition metal−oxygen diatomic system is crucial in understanding various molecular properties2 of these compounds and their applications in the condensed phase.9 The electronic structure of TMO is of special interest because the near degeneracy of the d orbitals and the various spin configurations1,2 could give rise to many high spin multiplicity low-lying electronic states. Theoretical calculations had been systematically performed on diatomic TMO, which covered the entire 3d, 4d, and 5d periods;4,6,7 however, experimentally, even up to the present moment, a number of these oxides are still not properly understood or studied. This situation could arise from the difficulty in producing sufficient molecules in the gas phase for spectroscopic observation or, very often, even though their molecular spectra were observed, the complexity of the spectra forbids immediate analysis. In the 5d transition period, iridium monoxide (IrO) is one of the diatomic TMO molecules that has both problems, and its ground state is, at the present moment, still not clearly identified. Diatomic iridium-containing molecules formed from iridium atom and one main group element have recently attracted considerable experimental attentions, in particular, the 2p main group molecules. Among them, the IrB,9 IrC,10 IrN,11 and IrF12 are generally well understood, but not the IrO molecule. IrO was first studied by Raziunas et al.;13 the emission spectrum was recorded using a grating spectrograph, which showed four band heads in the visible region, but no analysis was made. Jansson © 2012 American Chemical Society

II. EXPERIMENT Our LIF spectrometer and the laser vaporization/reaction experimental setup were discussed in detail in earlier Received: May 29, 2012 Revised: August 24, 2012 Published: September 10, 2012 9739

dx.doi.org/10.1021/jp305197r | J. Phys. Chem. A 2012, 116, 9739−9744

The Journal of Physical Chemistry A

Article

publications;17,18 therefore, only a brief description of the relevant experimental conditions for obtaining the IrO spectrum is given here. The IrO molecule was produced by reacting laser ablated iridium atom with either oxygen or nitrous oxide (N2O) seeded in argon. Both reagents were able to produce the IrO molecule. The reaction with N2O gave the better signal-to-noise ratio spectrum, and therefore, subsequently, all our spectra were recorded with N2O seeded in argon. The IrO produced was under supersonic free jet condition and its spectrum was recorded using LIF spectroscopy. Output energy around 5 mJ/pulse of the second harmonic (532 nm) of a Nd:YAG laser was focused onto an iridium rod to generate iridium atoms. A pulsed valve, synchronized with appropriate time delay, released a gas mixture of 6% of N2O in argon to react with the iridium atom for producing IrO. Both tunable pulsed dye laser and optical parametric oscillator (OPO) laser were used in this work. Our pulsed dye laser operated with Coumarin dyes was pumped by a Nd:YAG laser with wavelength set at 355 nm, which produced tunable dye laser output between 435 and 550 nm in the visible region. Our OPO laser was pumped by an injection seeded Nd:YAG laser, the signal beam between the spectral region 550 and 650 nm was used. The energy output from the two tunable lasers was used to excite the IrO molecule. The LIF signal was collected by a lens system and fed into a 0.3 m monochromator before it was detected by a photomultiplier tube (PMT). The monochromator was used for two purposes, filtering the unwanted scattered radiation and also resolving various wavelengths intrinsic in the fluorescence signal. The PMT output was sent to a fast oscilloscope for averaging and storage. The tunable laser line width of both the dye and OPO lasers was around 0.07 cm−1 and the typical pulse energy used in our experiments was around 5 mJ. An analysis of the observed spectrum indicated that, with supersonic cooling, the temperature of the molecule produced was about 50 K.

bands. In the spectral region studied, we were able to identify and analyzed five electronic transition systems in which four of them involved the same common lower state and the transitions are with ΔΩ = 0 and Ω = 2.5, namely the [17.6]2.5 − X2Δ5/2, [17.8]2.5 − X2Δ5/2, [21.5]2.5 − X2Δ5/2, and [22.0]2.5 − X2Δ5/2 systems; the other transition belongs to a ΔΩ = 0 and Ω = 3.5, [21.9]3.5 − Ω = 3.5 system. Figure 2

Figure 2. Observed molecular transitions of IrO.

summarized the electronic transitions identified in this work. We noticed that the line width of rotational line in the recorded spectrum is generally wider than the expected Doppler line width; this is quite probably that the unresolved hyperfine structure from the Ir nucleus in the ground and excited states gives rise to the larger line width. Using the present experimental setup that has only limited resolving power, we would not be able to study any further this hyperfine interaction. Each transition system observed is discussed in detail below. A list of the measured line positions of all the recorded transitions is deposited in the Journal archive. A. [17.6]2.5 − X2Δ5/2 System. Four vibrational bands: (0, 0), (1, 0), (2, 0), and (0, 1) were recorded for the [17.6]2.5 − X2Δ5/2 system. Assignment of rotational lines was straightforward. Each band consists of P, Q, and R branches. The P and Q branch lines are generally well revolved, and the R lines form a head. Figure 3 shows the head region of the (0, 0) band. The first line of the branches are respectively P(3.5), Q(2.5), and R(2.5), which confirmed that the transition band belongs to an Ω′ = 2.5 − Ω″ = 2.5 transition. Vibrational quantum number assignment was confirmed by examining the isotopic shift between band origins of the isotopes. Rotational line positions of each band were fit to the following expression:19

III. RESULTS AND DISCUSSION The LIF spectrum of IrO in the visible region between 16 000 and 23 000 cm−1 has been examined. Figure 1 shows a portion of the low resolution spectrum with a couple of transition

ν = νo + B′J ′(J ′ + 1) − D′[J ′(J ′ + 1)]2 − {B″J ″(J ″ + 1) − D″[J ″(J ″ + 1)]2 }

(1)

where the prime and double-prime refer to the upper and lower states, respectively. νo is the band origin, and B and D are the rotational and the centrifugal distortion constants. The leastsquares fitting of the transition lines in these bands were performed in two stages using the expression in eq 1: first, a band-by-band fit was done and eventually all the bands were merged together in a single fit. Due to the fact that only low J (J < 18) lines were measured, the centrifugal distortion constant

Figure 1. Broad-band scan between 16 500 and 17 900 cm−1 of IrO. 9740

dx.doi.org/10.1021/jp305197r | J. Phys. Chem. A 2012, 116, 9739−9744

The Journal of Physical Chemistry A

Article

Figure 4. Wavelength resolved fluorescence spectrum of the (0, 0) band of the [17.6]2.5 − X2Δ5/2 transition of IrO.

Figure 3. (0, 0) band of the [17.6] 2.5 − X2Δ5/2 transition of IrO.

D was set to zero in all the least-squares fits. Molecular constants determined for the [17.6]2.5 and the X2Δ5/2 states are listed in Table 1.

of the X′ state. A reasonable explanation would be that the transition strength from the X′ state to the [17.6]2.5 state is very low. The fact that we were able to observe this unknown substrate indicated that the X′ state is only weakly connected to the X2Δ5/2 state. More work is needed to improve the low signal-to-noise ratio of the weak spectrum and to characterize this nearby state. Jansson and Scullman14 also recorded and analyzed this same [17.6]2.5 − X2Δ5/2 system and they labeled it as the D′ − A′ system. This transition system was correctly identified to have a ΔΩ = 0 but wrongly assigned with Ω″ = 3/2, and they only observed the P and R branches but no Q branch was reported. Their determined lower state molecular constants are very close to ours. B. [17.8]2.5 − X2Δ5/2 System. For this new transition, we recorded and analyzed the (0, 0), (1, 0), and (2, 0) bands. Figure 5 depicts the (2, 0) band of this system, which shows clearly the P, Q, and R branches. Again the identification of the first line of the branches, namely P(3.5), Q(2.5), and R(2.5),

Table 1. Molecular Constants for the Ground and Excited States of IrO (cm−1)a 193

191

IrO

IrO

state

v

νo

B

νo

B

[21.9]3.5

1 0 0 1 0 2 1 0 2 1 0 0 3 2 1 0

a + 22474.84 a + 21869.66 22006.05 22390.25 21552.41 19402.31 18626.70 17830.13 18946.67 18275.25 17602.75 a 2672.09 1790.72 900.02 0.0

0.3301 0.3362 0.3366 0.3516 0.3554 0.3462 0.3484 0.3517 0.3460 0.3493 0.3518 0.3850 0.3758 0.3781 0.3806 0.3831

a + 22475.12 a + 21869.66 22006.65 22390.51 21552.41 19402.95 18627.92 17831.09 18947.15 18275.45 17602.75 a 2673.12 1791.41 900.51 0.0

0.3303 0.3365 0.3369 0.3519 0.3256 0.3464 0.3489 0.3520 0.3462 0.3495 0.3520 0.3853 0.3764 0.3787 0.3807 0.3834

[22.0]2.5 [21.5]2.5 [17.8]2.5

[17.6]2.5

Ω = 3.5 X2Δ5/2

Error limits of the band origin and the B value are ±0.03 and ±0.0002 cm−1, respectively.

a

Figure 4 shows the wavelength resolved fluorescence spectrum of the (0, 0) band of the [17.6]2.5 − X2Δ5/2 system. It is obvious that besides the progression of the vibrational level of the X2Δ5/2 state, there is another progression belonging to another substate. We labeled this other substate as X′ state, which is about 249 cm −1 above the ground state. Experimentally, we did arrange our tunable dye laser with wavelength set at 17 354 cm−1 (which is 249 cm−1 lower than 17 603 cm−1) to pump molecules from the 249 cm−1 state to the same upper state, but no LIF signal was detected at the X2Δ5/2 state. However, we were able to record a weak electronic transition spectrum by pumping the v = 0 of X′ state to an upper state and detected the LIF signal at the v = 1 level

Figure 5. (2, 0) band of the [17.8]2.5 − X2Δ5/2 transition of IrO. 9741

dx.doi.org/10.1021/jp305197r | J. Phys. Chem. A 2012, 116, 9739−9744

The Journal of Physical Chemistry A

Article

confirmed our assignment. From fitting the line positions, the obtained lower state rotational constant B is identical to that of the [17.6]2.5 − X2Δ5/2 system within experimental error. Molecular constants determined for the [17.8]2.5 state are listed in Table 1. The vibrational separation ΔG1/2 and ΔG3/2 measured for the upper state are 796.57 and 775.61 cm−1, respectively. A wavelength resolved fluorescence pattern similar to that of the [17.6]2.5 − X2Δ5/2 system has also been observed in this system, which provides direct confirmation that these two systems share the same common lower state. C. [21.5]2.5 − X2Δ5/2 System. The (0, 0), (1, 1), and (1, 0) bands of this new transition system have been observed and analyzed. The measured vibrational separation of the upper state was ΔG1/2 = 837.84 cm−1. Molecular constants for the [21.5]2.5 state are also reported in Table 1. Once again, a wavelength resolved fluorescence pattern similar to that of the [17.6]2.5 − X2Δ5/2 system has also been observed in this system. D. [22.0]2.5 − X2Δ5/2 System. Four bands, namely (0, 0), (0, 1), (0, 2), and (0, 3), were observed in the [22.0]2.5 − X2Δ5/2 system. This system is generally weaker than other ones observed. The first lines of the P, Q, and R branches confirmed the assignment of a ΔΩ = 0 and Ω″ = 2.5 transition. It is unusual that transition bands with a high vibrational level of the lower state was observed but not a band with the v > 0 level of the upper state; this might be due to a peculiar combination of the Franck−Condon factor and transition dipole moment of this system. Molecular constants obtained from the leastsquares fit are also listed in Table 1. The B value for the lower state is the same as those in the [17.6]2.5 − X2Δ5/2 system within experimental error, and also with Ω = 2.5; therefore, the system is from the same X2Δ5/2 state. The isotopic separation of the band origins of the (0, 0) band is relatively large; we believe that the shift is originated from the vibrational constants for upper and lower electronic states.18 The vibrational separation ΔG1/2, ΔG3/2, and ΔG5/2 measured for the lower state are 900.02, 890.70, and 881.37 cm−1, respectively. The equilibrium molecular constants obtained are given in Table 2. Jansson and Scullman14 also recorded and analyzed this same system and they labeled it as the H′ − A′ system. They have correctly identified it as a ΔΩ = 0 transition, but the lower state of this transition was wrongly assigned to be of Ω = 1.5, which is different from our Ω = 2.5. Nevertheless, the B values of the A′ lower state are very close to our determined values. E. [21.9]3.5 − Ω = 3.5 System. This is the only Ω′ = 3.5 − Ω″ = 3.5 transition system recorded. We did examine carefully the first line of the branches and also the lower state B value, which indicates this system is different from the other ones studied. Figure 6 depicts the head region of the (0, 0) band, which shows the P(4.5), Q(3.5), and R(3.5) lines. The molecular constants obtained are also reported in Table 1. The vibrational separation ΔG1/2 measured for the upper state is 605.18 cm−1. The equilibrium molecular constants obtained are given in Table 2. Jansson and Scullman14 also recorded and analyzed this same system; they labeled it as the H″ − A″ system. The transition system was correctly identified to have a ΔΩ = 0 but wrongly assigned with Ω″ = 2.5, which is different from our assignment of Ω″ = 3.5. For all the transition systems observed, both the 193IrO and 191 IrO isotopes were analyzed. Molecular parameters for the isotopic molecules are related by different powers of the mass dependence parameter ρ = (μ/μi)1/2, where μ and μi are the

Table 2. Equilibrium Molecular Constants for the Excited and Ground States of IrO (cm−1) 191

state

parameter

[21.9]3.5

T0 ΔG1/2 Be 103αe re (Å) T0 ΔG1/2 Be 103αe re (Å) Te ωe ωeχe Be 103αe re (Å) Te ωe ωeχe Be 103αe re (Å) ωe ωeχe Be 103αe re (Å)

[21.5]2.5

[17.8]2.5

[17.6]2.5

X2Δ5/2

193

IrO

a+21869.66 605.18 0.3271 6.1 1.8678 21552.41 837.84 0.3573 3.8 1.7874 17423.99 817.53 10.48 0.3529 2.75 1.7981 17266.10 673.58 0.54 0.3534 2.90 1.7969 909.35 4.66 0.3843 2.44 1.7231

IrO

observed

calculated

a+21869.66 605.46 0.3272 6.2

605.42 0.3273 6.1

21552.41 838.10 0.3575 3.7

838.18 0.3576 3.8

17424.50 818.63 10.90 0.3533 2.80

817.86 10.49 0.3532 2.75

17266.03 673.70 0.50 0.3536 2.90

673.85 0.54 0.3537 2.9

909.83 4.70 0.3844 2.30

909.71 4.66 0.3846 2.44

Figure 6. (0, 0) band of the [21.9]3.5 − Ω = 3.5 transition of IrO.

reduced mass of an isotope and its heaviest isotope 193IrO. As it is shown in Table 2, the agreement between the molecular parameters for these isotopes is excellent.

IV. DISCUSSION With respect to the 5856, 5990, 6899, and 6972 Å bands observed in emission by Raziunas et al.,13 we were able to record and analyze the 5990 Å band, which turned out to be the 9742

dx.doi.org/10.1021/jp305197r | J. Phys. Chem. A 2012, 116, 9739−9744

The Journal of Physical Chemistry A

Article

(0, 1) band of the [17.6]2.5 − X2Δ5/2 system. For the other three wavelength regions mentioned, we could not observe any spectrum despite our effort to search for them. It is quite possible that these bands were emission bands from the upper state to higher vibrational levels of the ground state, and such transition bands would not be so readily observable in our LIF spectrum. Among the four ΔΩ = 0 and Ω = 2.5 transitions, three of them were observed earlier by Jansson and Scullman;14 however, they did not observe the Q branch in any of these transitions. This situation could be related to the way their spectra were obtained. It should be noted that their emission spectra were recorded at relatively high temperatures and with rotational lines recorded up to J > 70. The absence of the Q branch in their spectra could be due to the fact that, for a ΔΩ = 0 transition, the Q branch is weak and the high J lines of the R branch are strong and wrapping around from the band head that overlap the weak Q branch. As seen from their spectrogram, to properly record the high J lines of the R branch, their spectra could have a short exposure time and, consequently, the weak Q branch would not be able to show up. The molecular constants determined in this work agree very well with those obtained by Jansson and Scullman,14 but their Ω values for both their lower and upper states were incorrectly determined. Our spectrum benefited from the low temperature environment of the supersonic jet expansion, the temperature of the molecule was around 50 K and the first lines of the branches were observed, which unambiguously confirmed the Ω values of the transitions. One of the most urgent concerns for the IrO molecule is to characterize the ground state. As far as theoretical predictions were concerned, DFT calculations by both Citra and Andrews16 and Song et al.6 obtained 4Σ− to be the ground state of IrO. None of these results is in agreement with our experimental observation of the Ω = 2.5 ground state. The failure of the DFT predictions could be due to the calculations were nonrelativistic, where the scalar relativistic effect was not correctly treated.20 With the assumption that a single configuration gives rise to observable electronic states, we would like to try using the molecular orbital (MO) theory approach to give an explanation for our observed spectrum. Figure 7 shows qualitatively the MO energy level diagram formed from the 6p, 6s, and 5d atomic orbitals (AOs) of the iridium (Ir) atom and the 2p orbital of the oxygen atom. The lower energy 1σ and 1π MOs and the higher energy 3σ, 2π, 4σ, and 3π MOs are

formed from the 5dσ, 5dπ, 6pσ, and 6pπ AOs of the Ir atom and the 2p AO of the O atom. The 2σ MO is essentially the 6sσ AO of the Ir atom. The 1δ MO is basically the 5dδ AO of the Ir atom, because there is no other δ symmetry orbital nearby. Across the transition metal period, the metal 5d orbital drops in energy compared to the 6s, whereas oxygen 2p is lower in energy because the metal ionization potentials increase. The 2σ, 1δ, and 2π MOs are very close in energy. There are 13 electrons to be placed into the MOs, the ground state of the IrO molecule could be complicated by the near “degeneracy” of these MOs. Table 3 lists some of the electronic configurations by putting electrons to the MOs. Electronic configurations with labels A, B, and C give rise to many electronic states with relatively similar energies. For the ground state of IrO, there is no doubt that Ω = 2.5 is the lowest substate; it is quite possible that this substate is a spin−orbit component of a 2Δi. As to the question of which is the dominating electronic configuration giving rise to the observed Ω″ = 2.5 component, there is no easy answer. It is intuitive to compare the monoxides in the same group. As far as group VIIIA elements (Co, Rh, and Ir) are concerned, among those configurations listed above, labels A and B are the configurations giving rise to the ground state of CoO (X4Δi) and RhO (X4Σ−), respectively.21,22 Table 4 lists the molecular spectroscopic properties of the group VIIIA monoxides and atomic parameters. If the energy order of the MOs is similar to that of CoO and RhO, the configuration of π2δ3 with no electron in the σ orbital (configuration in label C gives rise to the 2Δi state) would not be the configuration with the lowest energy. Because the atomic spin−orbit parameter of Ir is large, it plays a significant role in giving large splitting to the spin−orbit components of electronic states in the IrO molecule. In addition, the low-lying electronic states arising from the near “degenerate” electronic configurations are close in energy and crowded. Under the above situations, not only is the Hund's case (c) coupling scheme in this molecule expected (electronic substates can only be identified by their Ω values), but also the energy order of the low-lying electronic states would be scrambled. The ground state is the one combining all effects and eventually ends up with the lowest energy. Our results indicated that the ground state is with Ω = 2.5, which is logically arises from the 2Δi state. This is an ideal case for high level theoretical calculations to be performed at the individual spin−orbit component level to provide a clear picture of the energy order of the low-lying states. Not only would such calculations give an explanation to the experimental findings in this work, but they are also critical tests of the abilities of theoretical calculations today. As far as the upper states are concerned, those observed Ω = 2.5 substates must come from various electronic states, having the same spin multiplicity as either the ground state, such as 2Φ and 2Δ, or the quartet state: 4Π, 4Δ, 4Φ, and 4Γ. It is not possible at present to make any unambiguous assignment as to which of these states corresponds to our observed upper states. A similar situation is also true for the Ω = 3.5 upper state. In summary, we studied five electronic transition systems of the IrO molecule. Four of them originated from the same X2Δ5/2 state. Accurate vibrational and rotational constants for the low-lying and ground electronic states are reported. This work provides an opportunity to experimentally examine and characterize the ground state of IrO; however, we still have difficulties explaining the observed upper and X2Δ5/2 states from the point of view of the MO theory with the single configuration approach. We strongly urge theoreticians to

Figure 7. Molecular orbital energy level diagram of IrO. 9743

dx.doi.org/10.1021/jp305197r | J. Phys. Chem. A 2012, 116, 9739−9744

The Journal of Physical Chemistry A

Article

Table 3. Electronic Configurations of Low-Lying Excited and Ground States of IrO molecular orbital occupancies label A B C D E F G

1σ 2 2 2 2 2 2 2

1π

2σ

4 4 4 4 4 4 4

1δ

2 1

2π

3 4 3 4 4 3 4

1 1

3σ

CoOc 4 Δ7/2 π2δ3 1.631 851.7 4s23d7 530

RhOd Σ σπ2 1.716 799 5s16d8 1253 4 −

2 2 4 2 3 3 1

1

1

IrO Δ5/2 δ3 1.726 900.02 6s25d7 3905 2

Electronic configuration giving rise to the ground state. bReference 23. cReference 21. dReference 22.

perform high level calculations with respect to individual spin− orbit component to rationalize these observations.

ASSOCIATED CONTENT

S Supporting Information *

Tables of assigned rotational lines. This material is available free of charge via the Internet at http://pubs.acs.org.

■

δπ σπ2 δ3 σπ2 π3 δ3σπ3 σπσ

electronic states 2 − 2 + 2

Δi, Σ , Σ , Δ, 4 − 2 − 2 + 2

2 Δi, 2Γi Σ , Σ , Σ, Δ 2 Δi 4 − 2 − 2 +,2 Σ , Σ , Σ Δ 2 Πi 4 Φi, (2)2Φi,4Πi, (2)2Π 4 Π, 2Π, 2Π 4

(10) Ma, T. M.; Leung, J. W. H.; Cheung, A. S. C. Chem. Phys. Lett. 2004, 385, 259. (11) Marr, A. J.; Flores, M. E.; Steimle, T. C. J. Chem. Phys. 1996, 104, 8183. (12) Adam, A. G.; Granger, A. D.; Downie, L. E.; Tokaryk, D. W.; Linton, C. Can. J. Phys. 2009, 87, 557. (13) Raziunas, V.; Macur, G.; Katz, S. J. Chem. Phys. 1965, 43, 1010. (14) Jansson, K.; Scullman, R. J. Mol. Spectrosc. 1972, 43, 208. (15) Jansson, K.; Scullman, R. Ber. Bunsen-Ges. Phys. Chem. Chem. Phys. 1978, 82, 92. (16) Citra, A.; Andrews, L. J. Phys. Chem. A 1999, 103, 4182. (17) Ran, Q.; Tam, W. S.; Ma, C. S.; Cheung, A. S. C. J. Mol. Spectrosc. 1999, 198, 175. (18) Pang, H. F.; Cheung, A. S. C. Chem. Phys. Lett. 2009, 471, 194. (19) Herzberg, G. Spectra of Diatomic Molecules; Van Nostrand: New York, 1950. (20) Liu, Wenjian; Franke, R. J. Comput. Chem. 2002, 23, 564. (21) Adam, A. G.; Azuma, Y.; Barry, J. A.; Huang, G. J.; Lyne, M. P. J.; Merer, A. J.; Schroder, J. O. J. Chem. Phys. 1987, 86, 5231. (22) Jensen, R. H.; Fougere, S. G.; Balfour, W. J. Chem. Phys. Lett. 2003, 370, 106. (23) Lefebvre-Brion, H.; Field, R. W. The spectra and dynamics of diatomic molecules; Elsevier Academic Press: Amsterdam, 2004.

a

■

configuration 3 2

Table 4. Comparison of Molecular Properties of the Ground State of Group VIIIA Monoxides molecule symmetry electron configurationa bond length, ro (Å) ΔG1/2 (cm−1) ground electron configuration of metal atom atomic spin−orbit parameter (cm−1)b

4σ

AUTHOR INFORMATION

Corresponding Author

*Tel: (852) 2859 2155 Fax: (852) 2857 1586. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The work described here was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. HKU 7014/06P).

■

REFERENCES

(1) Langhoff, S. R.; Bauschlicher, C. W., Jr. Annu. Rev. Phys. Chem. 1988, 39, 181. (2) Merer, A. J. Annu. Rev. Phys. Chem. 1989, 40, 407. (3) Harrison, J. F. Chem. Rev. 2000, 100, 679. (4) Bridgeman, A. J.; Rothery, J. J. Chem. Soc., Dalton Trans. 2000, 211. (5) Gutsev, G. L.; Andrews, L.; Bauschlicher, J. C. W. Theor. Chim Acta 2003, 109, 298. (6) Song, P.; Guan, W.; Yao, C.; Su, Z.; Wu, Z.; Feng, J.; Yan, L. Theor. Chim. Acta 2007, 117, 407. (7) Yao, C.; Guan, W.; Song, P.; Su, Z.; Feng, J.; Yan, L.; Wu, Z. Theoretical Chemistry Accounts: Theory, Computation, and Modeling (Theoretica Chimica Acta) 2007, 117, 115. (8) Henrich, V. E.; Cox, P. A. The surface science of metal oxides; Cambridge University Press: Cambridge, U.K., 1994. (9) Ye, J.; Pang, H. F.; Wong, A. M. Y.; Leung, J. W. H.; Cheung, A. S. C. J. Chem. Phys. 2008, 128, 154321. 9744

dx.doi.org/10.1021/jp305197r | J. Phys. Chem. A 2012, 116, 9739−9744