Electronic Transitions of Ruthenium Monoxide - The Journal of

Publication Date (Web): July 5, 2013 ... transition systems, namely, the [18.1]Ω = 4–X5Δ4, [16.0]Ω = 5–X5Δ4, [18.1]Ω = 3–X5Δ3, and [15.8]Î...
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Electronic Transitions of Ruthenium Monoxide Na Wang, 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 ruthenium monoxide (RuO) molecule in the spectral region between 545 nm to 640 nm has been recorded and analyzed using laser ablation/reaction free-jet expansion and laser induced fluorescence spectroscopy. The RuO molecule was produced by reacting laser-ablated ruthenium atoms with N2O seeded in argon. Nine vibrational bands were recorded, and they are identified to belong to four electronic transition systems, namely, the [18.1]Ω = 4−X5Δ4, [16.0]Ω = 5−X5Δ4, [18.1]Ω = 3−X5Δ3, and [15.8]Ω = 4−X5Δ3 systems. RuO was determined to have a X5Δ4 ground state. A least-squares fit of the measured rotational lines yielded molecular constants for the ground and the low-lying electronic states. A molecular orbital energy level diagram has been used to help with the assignment of the observed electronic states.

I. INTRODUCTION Transition metal oxides (TMOs) have been studied for many years.1−5 TMO molecules are interesting to scientists because of their importance in astrophysics,6−9 catalysis,10 and high temperature chemistry.9,10 Diatomic TMO molecules are model compounds for the understanding of chemical bonds involving electrons in the d orbitals; however, the degeneracy of the d orbitals could give rise to many low-lying electronic states with high spin multiplicity.8,9,11 High multiplicity states are often complex, and the analysis of their electronic transition spectrum is by no mean trivial. RuO is one of these molecules, which has a complex electronic structure and its ground state has still not properly been identified up to the present. Raziunas et al.12 studied the emission spectrum of the RuO molecule using a low direct-current arc discharge light source with ruthenium oxide and reported the ground state to be a 3Σ+ state with the bond length, ro = 1.70 Å. Scullman and Thelin13 also studied the same emission bands with a hollow cathode lamp as the molecular source. The ground state was also reported to be of 3Σ+ symmetry, and the bond length obtained was slightly longer, ro = 1.72 Å, than that of Raziunas et al.12 Using multiconfiguration self-consistent-field (MC-SCF) calculations, Krauss and Stevens examined the electronic structure and predicted the ground state to be a 5Δ state.14 Recently, Zhou and co-workers15,16 using matrix isolation infrared absorption spectroscopy and density functional theory calculations obtained a 5Δ ground state with an experimental and calculated vibrational frequency of 834 and 860 cm−1, respectively, and also a calculated bond length of 1.714 Å. In this paper, we reported rotationally resolved spectroscopic studies of the electronic transitions of RuO in the visible region using the laser ablation/reaction free-jet expansion technique and laser induced fluorescence (LIF) spectroscopy. Four electronic transition systems have been identified, which involved the Ω = 3 and Ω = 4 components of the X5Δi state of RuO. © 2013 American Chemical Society

II. EXPERIMENT The apparatus and detailed procedures for producing metal containing molecules using laser ablation/reaction free-jet expansion source and LIF spectroscopy have been described in our earlier publications.17,18 Only a brief description of the relevant experimental conditions for obtaining RuO spectrum is given here. Pulses from a Nd:YAG laser with wavelength of 532 nm and energy of 5−6 mJ were focused onto the surface of the ruthenium rod to generate Ru atoms. The RuO molecules were formed from the reaction of the Ru atoms with 6% N2O seeded in argon. A pulsed optical parametric oscillator (OPO) laser, pumped by another Nd:YAG laser with wavelength set at 355 nm, produced tunable laser output at the visible regions, which was used to excite the jet-cooled RuO molecules. The energy output from the OPO laser was typically about 10 mJ per pulse, its wavelength was measured by a wavemeter with accuracy about ±0.02 cm−1, and the line width was estimated to be around 0.07 cm−1. The fluorescent signal from the RuO molecule was directed into a 0.25 m monochromator, and subsequently detected by a photomultiplier tube (PMT). The monochromator was used for dual purposes: it recorded the wavelength resolved fluorescence spectrum and also acted as an optical filter for removing scattered light from the background. The PMT output was fed into a fast oscilloscope for averaging and storage. Because our LIF spectrometer has only limited resolution, which was insufficient to resolve isotopic transition lines from the Ru atom, the observed lines were often isotopic bands and the line width observed was usually much worse than 0.07 cm−1. Special Issue: Terry A. Miller Festschrift Received: May 9, 2013 Revised: July 4, 2013 Published: July 5, 2013 13279

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III. RESULTS AND DISCUSSION A. Low-Resolution Broadband Spectrum. The electronic transition spectrum of RuO in the visible regions between 545 and 640 nm was recorded. Nine vibrational bands were recorded and identified to be either ΔΩ = 0 or ΔΩ = 1 transitions with Ω″ = 3 or Ω″ = 4. These bands have been identified to be from four electronic transition systems, namely, [18.1]Ω = 4−X5Δ4, [16.0]Ω = 5−X5Δ4, [18.1]Ω = 3−X5Δ3, and [15.8]Ω = 4−X5Δ3 systems. Figure 1 summarizes these

Figure 2. (0, 0) Band of the [18.1]Ω = 4 −X5Δ4 transition of RuO.

all the bands was performed in two stages using expression (1). A band-by-band fit was done initially and, subsequently, all the bands were merged together in one single final fit to retrieve one set of molecular constants for each vibrational level. The molecular constants determined are listed in Table 1. The vibrational separation, ΔG1/2, measured for the X5Δ4 is 855.82 cm−1. Table 1. Molecular Constants for [18.1]Ω = 3, [18.1]Ω = 4, [15.8]Ω = 4, [16.0]Ω = 5, X5Δ3, and the X5Δ4 states of RuO(cm−1)a

Figure 1. Observed electronic transitions of RuO.

RuO

transitions pictorially. It is because ruthenium has seven isotopes and five of them have relatively high abundance [99Ru (12.7%), 100Ru (12.6%), 101Ru (17.0%), 102Ru (31.6%), 104 Ru (18.7%)] that these mass differences cause the vibrational and rotational constants of each isotope to differ very slightly, and hence the rotational energy pattern and also the band origin of the isotopic molecules. Consequently the observed spectrum shows much wider line width than the Doppler line width. Transition systems observed are discussed in detail separately. A list of the measured line positions of all bands is deposited in the Supporting Information. B. [18.1]Ω = 4−X5Δ4 Transition System. The (0, 0), (1, 1), and (0, 1) bands of the [18.1]Ω = 4−X5Δ4 transition system were recorded and analyzed. Each band shows resolved P, Q, and R branches. The assignment of rotational lines was straightforward. The band head region of the (0, 0) band of this transition is shown in Figure 2. The observation of the first lines of each branch, namely, P(5), Q(4), and R(4), indicated that this band has Ω′ = Ω″ = 4 value; hence, it has been assigned as the [18.1]Ω = 4−X5Δ4 transition system. The observed line positions of the each bands were fit to the following expression:19

ν

T

Beff

1 0 1 0 1 0 0 1 0 1 0

a + 18849.86(2) a + 18064.99(1) 18881.18(1) 18085.99(2) a + 16771.94(2) a + 15788.25(2) 16045.68(8) a + 856.27(2) a 855.82(2) 0

0.3792(3) 0.3806(1) 0.3806(1) 0.3817(4) 0.3808(5) 0.3859(5) 0.3830(6) 0.4106(2) 0.4128(5) 0.4102(5) 0.4134(4)

[18.1]Ω = 4 [15.8]Ω = 4 [16.0]Ω = 5 X5Δ3 X5Δ4 a

Errors quoted in parentheses are one standard deviation (in units of the last significant figure quoted). bThe energy of the Ω = 3 component is given as a (see Text).

Scullman and Thelin13 have also recorded and analyzed this same system, and they labeled it as the “5526 Å” system. This transition system was correctly identified to be a ΔΩ = 0 transition. However, only P and R branches were observed, no Q branch was reported, and it was incorrectly assigned as a 3 Σ−3Σ transition. Their determined upper and lower state molecular constants are very similar to those in Table 1. C. [16.0]Ω = 5−X5Δ4 Transition System. The (0, 0) band of this transition was recorded and analyzed. The first line of each branch, namely, P(6), Q(5), and R(4), shows that this band has Ω′ = 5 and Ω″ = 4 value. In addition, the R and Q branches have higher intensity than the P branch, which is consistent with a ΔΛ = +1 transition. Hence, it was assigned as the [16.0]Ω = 5−X5Δ4 transition system. We did search in the vicinity of this band for other vibrational bands but, unfortunately, no other bands that could be assigned comfortably to this system. The observed (0, 0) band has

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

state [18.1]Ω = 3

b

(1)

where the ′ and ″ superscripts refer to the upper and the lower states respectively. The νo is the band origin, and B and D are the rotational and centrifugal distortion constants respectively. With our relatively low temperature molecular source, the highest J value observed was 18, the centrifugal distortion constant, D, could not be properly determined from rotational levels with relatively low J quantum number, and was therefore set to zero in the fit. The least-squares fitting of line position of 13280

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relatively narrow line width because of a smaller spread of rotational lines of the isotopic molecules. The molecular constants obtained are also listed in Table 1. D. [18.1]Ω = 3−X5Δ3 Transition System. For this transition, we recorded and analyzed the (0, 0), (1, 0), and (0, 1) bands. The branch structure is quite similar to that of the [18.1]Ω = 4−X5Δ4 transition. The corresponding first line of the branches are P(4), Q(3), and R(3) lines, which basically confirm that these bands are of Ω′ = Ω″ = 3 value, and it has been assigned as the [18.1]Ω = 3−X5Δ3 transition system. The molecular constants determined are also given in Table 1. The vibrational separation, ΔG1/2, of the [18.1]Ω = 3 state is measured to be 784.87 cm−1, and the ΔG1/2 of the X5Δ3 state is 856.27 cm−1. Scullman and Thelin13 also reported the analysis of the same system and they labeled it as the “5532 Å” system. This transition system was correctly identified to be a ΔΩ = 0, but was mistakenly grouped as one of the vibrational bands of the “5526 Å” system and wrongly assigned as a 3Σ−3Σ transition. However, the molecular constants reported by Scullman and Thelin13 for the [18.1]Ω = 3 state and X5Δ3 state are very close to our determined values. E. [15.8]Ω = 4−X5Δ3 Transition System. The (0, 0) and (1, 0) bands of this transition have been recorded and analyzed. Figure 3 shows the band head region of the (0, 0) band. The

Figure 4. Molecular orbital energy level diagram of RuO.

atom. The lowest energy 11σ and 5π MOs and the higher energy 13σ and 6π MOs are formed from the main group 2p AO of O atom and Ru 4dσ and 4dπ AOs. The 12σ MO is essentially the Ru 5s AO. The 2δ MO is the Ru 4dδ AO, because there is no other δ symmetry orbital available. We have studied earlier transition metal monoxides in group VIIIA, and the FeO molecule has a 5Δi ground state from the electronic configuration is δ3σ1π2.20 Since RuO is iso-electronic with FeO, it is reasonable to examine a similar electronic configuration for RuO, which is the following: (11σ )2 (5π )4 (2δ)3 (12σ )1(6π )2 → X5Δi

(2)

The δ σ π configuration gives rise to 12 electronic states,19 and the state with the highest spin multiplicity is a 5Δ state. Since the number of electrons in the δ MO is more than halffilled, the 5Δ state arises from the configuration in (2) is an inverted 5Δ state. For a 5Δi state, among the five spin components the Ω = 0 component has the highest energy, and the Ω = 4 component has the lowest energy. From the analysis of the transition bands, we concluded that the lower states have Ω = 3 and Ω = 4 components, which fit very well with a 5Δi state as the ground state. Moreover, the transitions with Ω = 4 component are generally stronger in intensity than the Ω = 3 component, which is consistent with the fact that the Ω = 4 component is lower in energy than the Ω = 3 component, and hence with higher population. The equilibrium molecular constants of the state studied are listed in Table 2. As far as the upper states are concerned, it is not possible at this point to assign unambiguously the Λ value; however, a reasonable assignment can be made from considering the arrangement of electrons in the MO diagram. We have partial information concerning the labels of the upper state, from the stronger intensity of the R and Q branches in the [16.0]Ω = 5−X5Δ4 and [15.8]Ω = 4−X5Δ3 the transitions, the upper state could possibly have a Λ value one unit larger than the 5Δ state, making them the [16.0]5Φ5 and [15.8]5Φ4 states. It is plausible that the excited 5Φ state arises from an electronic configuration of δ3σ1π1σ1, which is the result of promoting an electron from the 6π to the 13σ MO. For the slightly higher energy [18.1]Ω = 4 state, it is likely to be the 5Δ4 component of a 5Δ state with an electronic configuration of δ3π2σ1 arising from the promotion of an electron from the 12σ to the 13σ MO. Krauss and Stevens,14 using multiconfiguration self-consistent-field (MC-SCF) calculations, examined the electronic structure of RuO and predicted a 5Δ ground state with bond length and vibrational frequency of 1.74 Å and 814 cm−1, respectively. Their predicted vibrational separation is in 3 1 2

Figure 3. (0, 0) Band of the [15.8]Ω = 4 −X5Δ3 transition of RuO.

observation of the P(5), Q(4), and R(3) first lines from their corresponding branches confirms the bands are of Ω′ = 4 and Ω″ = 3 value. The R and Q branches are of higher intensity than the P branch, which is consistent with a ΔΛ = +1 transition. Figure 3 depicts the band-head region, and the first line of each branch is clearly shown. These two bands have been assigned to be the [15.8]Ω = 4−X5Δ3 transition system. The molecular constants obtained are also reported in Table 1. The vibrational separation, ΔG1/2 = 983.69 cm−1 and the rotational constant, B1 = 0.3808 cm−1 for this level are respectively larger and smaller than expected, which indicate this level is probably subjected to perturbation. F. Discussion. Using a molecular orbital (MO) energy level diagram, we examine the observed molecular transitions in this work. Following the notations and labels commonly used for the MOs formed from second row (4d) transition metal and main group (2p) element,14 Figure 4 shows qualitatively the relative energy order of the MOs formed from the 4d and 5s atomic orbitals (AOs) of the Ru atom and the 2p AO of the O 13281

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Table 2. Equilibrium Molecular Constants for RuO (cm−1)a

a

parameter

[18.1]Ω = 3

[18.1]Ω = 4

[15.8]Ω = 4

X5Δ3

X5Δ4

To ΔG1/2 Be re (Å) αe

a + 18064.99(2) 784.87 0.3813 1.787 0.0014

18881.18(1) 795.19 0.3822 1.785 0.0011

a + 15788.25(2) 983.69 0.3884 1.771 0.0051

a 856.27(2) 0.4139 1.715 0.0022

0 855.82(2) 0.4148 1.714 0.0035

Errors quoted in parentheses are one standard deviation (in units of the last significant figure quoted).

■

reasonable good agreement with our determined value, ΔG1/2 = 855.82 cm−1, but their bond length was large when compared to our determined value of 1.715 Å. The agreement of our results in this work with those of Zhou and co-workers15,16 is excellent. The 5Δ ground state was calculated to have a spin− orbit coupling constant, A = −254 cm−1, which indicated the different spin components are separated by over 500 cm−1. This large separation is generally consistent with the much lower intensity observed for the transitions involving the Ω = 3 component in this work. We have not been able to measure the energy separation between the Ω = 3 and Ω = 4 components. We have used the parameter a to designate the term energy of the ground state Ω = 3 component. It would be interesting to compare the spectroscopic properties of RuO with ruthenium diatomic molecules formed from other main group 2p elements. Table 3 compares the

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

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a

molecule

RuB

electronic config. symmetry Be (cm−1) re (Å) ΔG1/2 (cm−1)

δ3 2 Δ5/2 0.5834 1.706 911.02

RuC

b

δ4 1 + Σ 0.6072 1.608 1029.6

c

*E-mail: [email protected]. Phone: (852) 2859 2155. Fax: (852) 2857 1586. 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 701008).

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d

RuN

RuO

RuF

δ4σ1 2 + Σ 0.5545 1.571 1108.3

δ3σ1π2 5 Δ4 0.4148 1.714 855.82

δ3σ1π3 4 Φ9/2 0.2866 1.916 534

AUTHOR INFORMATION

Corresponding Author

Table 3. Ground State Symmetry, Bond Length, and Vibrational Frequency of Diatomic Ruthenium Molecules a

ASSOCIATED CONTENT

S Supporting Information *

REFERENCES

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