The laser induced fluorescence spectrum of scandium monobromide (ScBr) between 795 and 845 nm has been recorded and analyzed. ScBr was produced by reacting laser vaporized Sc atoms with ethyl bromide (C 2H 5Br). Spectra of six vibrational bands of both Sc 79Br and Sc 81Br isotopomers of the C 1Σ +-X 1Σ + transition were observed. A least-squares fit of the measured line positions yielded accurate molecular constants for the v = 0-3 levels of the C 1Σ + state and the v = 0-2 levels of the X 1Σ + state. The equilibrium bond length of the C 1Σ + state has been determined to be 2.4776 . © 2012 Elsevier B.V. All rights reserved.postprin

Chan, MC

Cheung, ASC

Liao, Z

Xia, Y

Yang, M

Chemical Physics Letters

HKU Scholars Hub

Title Laser spectroscopy of the C1Σ+-X1Σ+ transition of ScBrAuthor(s) Xia, Y; Liao, Z; Yang, M; Chan, MC; Cheung, ASCCitation Chemical Physics Letters, 2012, v. 527, p. 7-10Issued Date 2012URL http://hdl.handle.net/10722/146856RightsNOTICE: this is the author’s version of a work that was acceptedfor publication in Chemical Physics Letters. Changes resultingfrom the publishing process, such as peer review, editing,corrections, structural formatting, and other quality controlmechanisms may not be reflected in this document. Changesmay have been made to this work since it was submitted forpublication. A definitive version was subsequently published inChemical Physics Letters, 2012, v. 527, p. 7-10. DOI:10.1016/j.cplett.2012.01.017 1 Laser spectroscopy of the C1Σ+ – X1Σ+ transition of ScBr    Y. Xia1, Zhenwu Liao2, Mei Yang2, Man-Chor Chan2 and A.S.C. Cheung1*  1Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong. 2Department of Chemistry, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong     Proposed running head: LIF spectrum of ScBr   *Correspondence to:  Prof. A.S.C. Cheung Department of Chemistry The University of Hong Kong Pokfulam Road Hong Kong  Fax : (852) 2857 1586 Tel : (852) 2859 2155 Email : hrsccsc@hku.hk    Text : 10 Tables : 3 Figures : 1       2 Abstract The laser induced fluorescence spectrum of scandium monobromide (ScBr) between 795 and 845 nm has been recorded and analyzed.  ScBr was produced by reacting laser vaporized Sc atoms with ethyl bromide (C2H5Br). Spectra of six vibrational bands of both Sc79Br and Sc81Br isotopomers of the C1Σ+ – X1Σ+ transition were observed. A least-squares fit of the measured line positions yielded accurate molecular constants for the v = 0 – 3 levels of the C1Σ+ state and the v = 0 – 2 levels of the X1Σ+ state. The equilibrium bond length of the C1Σ+ state has been determined to be 2.4776 Å  3 I. Introduction  Electronic transitions of scandium (Sc) monohalide molecules have been the subject of studies since the sixties [1]. This is because the scandium metal is particularly simple with only one electron in the d shell; the monohalides are model systems for understanding the role played by d electrons in chemical bonding [2]. As far as the studies of molecular spectra of scandium monohaildes are concerned, ScF [3-9], ScCl [9-17] and ScI [18-24] have received considerable attention, however, only very limited work was performed on the ScBr molecule [25-27].  Experimental observation of scandium monobromide was first reported by Fischell et al. [18] using laser induced fluorescence (LIF) spectroscopy and two electronic transition systems were recorded with low resolution between 410 and 430 nm. Using Fourier transform microwave spectroscopy, Lin et al. [25] measured the pure rotational spectrum of ScBr and reported ground state molecular constants with high precision. Using the complete active space self-consistent field (CASSCF) method and multireference configuration interaction (MRCI) calculations, Langhoff et al. [26] obtained spectroscopic constants for the X1Σ+ and a3∆ states. Using a similar method of calculation, Korek and Hamdan [27] predicted molecular properties of 23 low-lying electronic states of ScBr.   In this work, we report a high resolution laser induced fluorescence (LIF) study of the C1Σ+ - X1Σ system of ScBr in the near infrared region. Six vibrational bands of both Sc79Br and Sc81Br isotopomers were observed and analyzed. Line positions of all the vibrational bands of the individual molecules were fit simultaneously to retrieve molecular constants for both the C1Σ+ and X1Σ+ states.  4   II. Experiment  The experimental setup for studying transition metal halides using LIF spectroscopy has been discussed in detail in a previous publication [28], therefore only a brief description of the experimental conditions will be provided here. ScBr molecules were produced by the reaction of laser vaporized scandium atoms with ethyl bromide (C2H5Br) under supersonic free jet conditions. Pulses from a Nd:YAG laser at a wavelength of 1064 nm and a laser power of 10 mJ were focused onto the surface of a scandium rod to produce the Sc atoms in the gas-phase. A pulsed valve, synchronized with an appropriate delay, released a gas mixture of 3% C2H5Br in argon to react with Sc atoms. The jet-cooled ScBr molecules were excited by a continuous wave ring Ti:sapphire laser that was pumped by a diode pump solid state (DPSS) laser with the wavelength set at 532 nm. A lens system collected the LIF signal which was then sent through a monochromator, and the signal was eventually detected by a photomultiplier tube (PMT). Two functions were performed by the monochromator: it was used to record the resolved fluorescence spectrum of the ScBr molecule, and it also acted as an optical filter for removing unwanted laser scatter. The wavelength of the dye laser was measured using a wavemeter with an accuracy of one part in 107. The absolute accuracy of a measured line position is generally about 0.002 cm-1. Many individual scans were made and then connected together using the wavemeter readings.  III. Results and Discussion The laser induced fluorescence spectrum of ScBr in the near infrared region between 798 and 866 nm was recorded and analyzed. Six vibrational bands: (0, 0), (1, 0), (2, 0), (3, 0), (2, 1) and (2, 2) were studied. Each vibrational band shows resolved P and R branches only, which  5 confirms the nature of a 1Σ - 1Σ transition. Following the labeling of electronic states in scandium monohalides in the near infrared region, we label the observed transition as the C1Σ+ - X1Σ+ transition. Figure 1 shows the (3, 0) band in which the R heads of both the Sc79Br and Sc81Br isotopomers, and a portion of their P branches are clearly displayed.  The observed line positions were fit to a standard formula [29]: νo = To + B’ J’ (J’ + 1) - D’ [J’ (J’ + 1)] 2  - {B” J” (J” + 1) – D” [J”(J” + 1)] 2} (1) where the ’ and ” refer to the upper and the lower states respectively. We initially preformed least squares fit to individual vibrational bands, but, in the final fit, all bands were merged together to yield only one set of molecular constants for each vibrational level and each isotopic molecule. Since the microwave work of Lin et al. [25] reported rotational constants of the v = 0 and 1 levels with high precision, these constants were fixed in our fit.  Molecular constants determined for individual vibrational levels are reported in Table 1; the overall RMS error of the merged fit was about 0.002 cm-1. The molecular constants for the C1Σ+ state are obtained for the first time and the equilibrium bond length, re, was determined to be 2.4776 Å. The vibrational separations for the C1Σ+ state of the Sc79Br isotope are respectively ∆G1/2 = 286.95, ∆G3/2 = 285.46 and ∆G5/2 = 283.98 cm-1. For the X1Σ+ state, the ωe was determined to be 339.21 and 337.69 cm-1 for the Sc79Br and Sc81Br isotopic molecules respectively, and the B and D values for the v = 2 level were also determined in this work. A list of the measured line positions of the observed bands of the C1Σ+ - X1Σ+ transition is available from the Journal archive.  Molecular parameters for the Sc79Br and Sc81Br isotopomers are related by different powers of the mass dependence parameter ρ = (μ/μi) where μ and μi are the reduced mass of an isotope and its more abundant isotope Sc79Br [29]. The measured equilibrium molecular  6 parameters for the two isotopomers are listed in Table 2.  The agreement between the calculated and measured molecular constants for Sc81Br is excellent.  Since the scandium monohalides are considered as simple model systems, it is important to compare results from earlier calculations to establish the credibility of the theoretical computational treatment.  Both Langhoff et al. [26] and Korek et al. [27] performed CASSCF and MRCI calculations on the ScBr molecule, a comparison of their predictions and our equilibrium molecular constants are given in Table 2. The agreement between experimental and calculated value is generally very good.  Langhoff et al. [26] discussed the electronic configurations and electronic states of scandium monohalides using the molecular orbitals (MO) formed from atomic orbtials (AO) of the scandium and the halide elements.  The ground and low-lying electronic states for ScBr are similar to those for other monohalides. The electronic configurations giving rise to the ground X1Σ+ and other low-lying electronic states are as follows:  1σ2 1π4 2σ2    X1Σ+     (2)  1σ2 1π4 2σ1 1δ1   A1∆, a3∆    (3)  1σ2 1π4 2σ1 2π1   B1Π, b3Π     (4)  1σ2 1π4 2σ1 3σ1   C1Σ+, c3Σ+     (5)  1σ2 1π4 2σ1 3π1   D1Π, d3Π    (6)  The molecular orbital (MO) energy level diagram formed from the Sc atom’s 4s, 3d and 4p atomic orbitals and the halogen np orbitals would be similar to those discussed in earlier publications [18, 26]. The observed C1Σ+- X1Σ+ transition in this study corresponds to the promotion of an electron from the 2σ MO to the 3σ MO as indicated in (5). The dominant  7 configuration for the C1Σ+ state has one electron in predominantly the (2pσ+3dσ) orbital and another one in either a 4sσ or 2pσ orbital [18].   Table 3 lists equilibrium molecular constants for the ground and low-lying singlet electronic states of the four scandium monohalides. We can easily notice that the energy of the corresponding electronic state decreases from ScF to ScI. For the C1Σ+-X1Σ+ transition in scandium monohalides, the electron was taken from a closed 2σ shell and placed in the slightly anti-bonding 3σ MO. The bond length and vibrational frequency of the C1Σ+ state are respectively longer and weaker than that of the X1Σ+ state. In addition, similar trends in increasing bond length, re, and decreasing vibrational frequency, ωe, can also be found in the other electronic states of the scandium monohalides, which imply a weakening of the chemical bond between the scandium metal and the halogen elements going down the group. As far as the electronic configurations mentioned in expressions (3) to (6) are concerned, triplet states are always accompanying the singlet states.  We have work in progress to study the triplet states of ScBr, as it would be interesting to find out whether the trend that shows in the singlet states will also be true for the triplet states in ScBr.   Acknowledgements  We would like to thank Mr. P. M. Yeung and Dr. H. F. Pang for technical assistance. 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).    8 References 1. K. P. Huber and G. Herzberg, Molecular spectra and Molecular Structure IV. Constants of Diatomic Molecules Van Norstrand-Reinhold, New York.  2. S. R. Langhoff and C. W. Bauschlicher, Jr., Annu. Rev. Phys. Chem., 39 (1988) 181   3. E.A. Shenyavskaya, M.A. Lebeault-Dorget, C. Effantin, J. d'Incan, A. Bernard and J. Verges, J. Mol. Spectrosc., 171 (1995) 309.  4. L.A. Kaledin, J.E. McCord and M.C. Heaven, J. Mol. Spectrosc., 171 (1995) 569  5. M.A. Lebeault-Dorget, C. Effantin, J. d'Incan, A. Bernard, E.A. Shenyavskaya and J. Verges, J. Chem. Phys., 102 (1995) 708.  6. E.A. Shenyavskaya, J. Verges, A. Topouzkhanian, M.A. Lebeault-Dorget, J. d'Incan, C. Effantin and A. Bernard, J. Mol. Spectrosc., 164  (1994) 129.  7. M.A. Lebeault-Dorget, C. Effantin, A. Bernard, J. d'Incan, J. Chevaleyre and E.A. Shenyavskaya, J. Mol. Spectrosc., 163  (1994) 276.  8. E.A. Shenyavskaya, A.J. Ross, A. Topouzkhanian and G. Wannous, J. Mol. Spectrosc., 162 (1993) 327.  9. W. Lin, S.A. Beaton, C. J. Evans and Michael C.L. Gerry, J. Mol. Spectrosc., 199 (2000) 275.  10. A.G. Adam and J.R.D. Peers, J. Mol. Spectrosc., 182 (1997) 215.  11. F. Taher, C. Effantin, A. Bernard, J. d'Incan, J. Vergès and E.A. Shenyavskaya, J. Mol. Spectrosc., 184 (1997) 88.  12. M. Bencheikh, J. Phy. B: At., Mol. Opt. Phys., 30 (1997) L137.  13. N. Boutassetta, A.R. Allouche, and M. Aubert-Frecon, J. Phy. B: At., Mol. Opt. Phys., 29 (1996) 1637.  14. F. Taher, C. Effantin, A. Bernard, J. d'Incan, E.A. Shenyavskaya and J. Verges, J. Mol. Spectrosc., 179 (1996) 223.  15. F. Taher, A. Bernard, C. Effantin, J. d'Incan, E.A. Shenyavskaya and J. Verges, J. Mol. Spectrosc., 179 (1996) 229.  16. F. Taher, C. Effantin, J. d'Incan, A. Bernard, J. Verges and E.A. Shenyavskaya, J. Phy. B: At., Mol. Opt. Phys., 28 (1995) L181.   9 17. F. Taher, C. Effantin, J. d'Incan, A. Bernard, J. Verges and E.A. Shenyavskaya, J. Mol. Spectrosc., 173 (1995) 62.  18. D.R. Fischell, H.C. Brayman and T.A. Cool, J. Chem. Phys. 73 (1980) 4260.  19. C. Effantin, E.A. Shenyavskaya, J. d'Incan, A. Bernard, A. Topouzkhanian and G. Wannous, J. Mol. Spectrosc., 185 (1997) 249.  20. E. A. Shenyavskaya, C. Effantin, A. Bernard, J. Chevaleyre, J. d'Incan, A. Topouzkhanian, J. Vergès and G. Wannous, J. Phys. B: At., Mol. Opt. Phys., 29 (1996) L511.  21. F. Taher, C. Effantin, A. Bernard, J. d'Incan and J. Vergès, J. Mol. Spectrosc., 189 (1998) 220.  22. R.R. Reddy, Y.N. Ahammed, B.S. Devi, P.A. Azeem, K.R. Gopal and T.V.R. Rao, J Quant Spectrosc & Radia Tran, 74 (2002) 125.  23. M. Korek, S. Kontar, F. Taher-Mansour and A.R. Allouche, Int. J. Quant. Chem., 109 (2009) 236.  24. Z.H. Xia, Y. Xia, M-C. Chan and A. S-C. Cheung, J. Mol. Spectrosc. 268, (2011) 3.  25. Wei Lin, C. J. Evans and Michael C. L. Gerry,  Phys. Chem. Chem. Phys., 2 (2000) 43.  26. S.R. Langhoff, C.W. Bauschlicher and Harry Partridge, J. Chem. Phys., 89 (1988) 396.  27. M. Korek and A. Hamdan, Int. J. Quant. Chem., 108 (2008) 456.  28. Qin Ran, W.S. Tam, C. Ma and A.S.-C. Cheung, J. Mol. Spectrosc., 198 (1999) 175.  29. G. Herzberg, Spectra of Diatomic Molecules, Van Nostrand, New York, 1950.   10  Figure captions:  Figure 1.  The (3, 0) band of the C1Σ+- X 1Σ+ transition of ScBr      Table legends:  Table 1.  Molecular constants for the C1Σ+ and X1Σ+ states of ScBr (cm-1)†   † Numbers in parentheses are one standard deviation (in unit of the last figure)   * Rotational constants for the v = 0 and 1 levels of the X1Σ+ state are from Ref. 25   Table 2.  Equilibrium molecular constants for the C1Σ+ and the X1Σ+ states of ScBr(cm-1)†  † Numbers in parentheses are one standard deviation (in unit of the last figure)   Table 3.  Molecular constants for the singlet states of scandium monohalides (cm-1)   11 Table 1. Molecular constants for the C1Σ+ and the X1Σ+ states of ScBr (cm-1)†.  State v Sc79Br Sc81Br   To B 107D To B 107D  X1Σ+ 0 0.0 0.10362137* 0.390* 0 0.10269389* 0.386* 1 337.135(3) 0.10319109* 0.381* 335.631(3) 0.10226940* 0.389* 2 672.196(4) 0.102767(3) 0.49(6) 669.207(5) 0. 101840(4) 0.42(6)          C1Σ+ 0 11667.4421(3) 0.095678(1) 0.387(8) 11667.5515(3) 0.094825(1) 0.414(9) 1 11955.6839(2) 0.095296(1) 0.45(1) 11954.5069(3) 0.094441(2) 0.42(3) 2 12242.4269(3) 0.094900(1) 0.43(1) 12239.9754(3) 0.094055(1) 0.44(1) 3 12527.6710(3) 0.094509(1) 0.42(1) 12523.9583(4) 0.093668(2) 0.41(2) No. of lines                                   363 RMS error                                   0.0025 361 0.0021 † Numbers in parentheses are one standard deviation (in unit of the last figure)  * Rotational constants for the v = 0 and 1 levels of the X1Σ+ state are from Ref. 25  12 Table 2. Equilibrium molecular constants for the C1Σ+ and the X1Σ+ states of ScBr† State Param. Sc79Br Sc81Br Theoretical Expt.    Observed Calculated KH[27] LBP[26] LEG [25] X1Σ+ Te 0.0 0.0 0.0 0.0 0.0 0.0  ωe 339.209(4) 337.686(5) 337.686 343.7 329 338.8  ωeχe 1.037(2) 1.028(2) 1.028   1.099  Be 0.10385(1) 0.10295(1) 0.10292 0.10186  0.10384  104αe 4.50(1) 4.48(1) 4.46   4.30  re (Å) 2.3806   2.3985 2.430 2.3806         C1Σ+ Te 11692.105(1) 11692.104(1)  12113    ωe 289.740(1) 288.439(3) 288.438 297.7    ωeχe 0.750(2) 0.743(1) 0.7425     Be 0.09588(1) 0.09502(1) 0.9502 0.09392    104αe 3.91(2) 3.87(2) 3.87     re (Å) 2.4776   2.4979           † Numbers in parentheses are one standard deviation (in unit of the last figure)   13 Table 3. Molecular constants for the singlet states of scandium monohalides (cm-1).  State  E.C.* Parameter ScFa Sc35Clb Sc79Br ScIc X1Σ+  2σ2 ωe 735.3 447.5 339.2 277.1 re (Å) 1.7874 2.3303 2.3806 2.608 A1∆  2σ1δ Te 4587.1 3555.7  2868.5 ωe 645.2 388.1  250.0 re (Å) 1.860 2.337  2.718 B1Π  2σ2π Te 9555.6 6020.5   ωe 593.0 381.0   re (Å) 1.907 2.351   C1Σ+  2σ3σ Te [16091.9]† 12427.4 11692.1  ωe  377 289.7  re (Å) 1.907 2.3305 2.4776  D1Π  2σ3π Te 20383.378  [17390.06]†  15630.1d ωe 622.506 374.3  237.9 re (Å) 1.864 2.348  2.715 †Numbers in parenthesis are To values *E.C. – electronic configuration aRef. [6] bRef. [11] cRef. [21] dRef. [24]      14 Figure 1.  12520 125251Sc79Br01035 30201515203020155P(J)R(J)P(J) Wavenumber(cm-1)R(J)5105105Sc81Br   

Laser spectroscopy of the C1Σ+-X1Σ+ transition of ScBr

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Laser spectroscopy of the C1Σ+-X1Σ+ transition of ScBr

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