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Laser spectroscopy of the A9–X8 transition of holmium monochloride
Journal of Molecular Spectroscopy 217 (2003) 26–31 www.elsevier.com/locate/jms
Laser spectroscopy of the A9–X8 transition of holmium monochlorideq M.J. Dick and C. Linton* Physics Department, University of New Brunswick, P.O. Box 4400, Fredericton, NB, Canada E3B 5A3 Received 13 June 2002; in revised form 16 October 2002
Abstract Using laser excitation spectroscopy, 18 red-degraded bands belonging to a single electronic transition of holmium monochloride have been observed in the 615–670 nm region. Thirteen of the bands, with vibrational levels 0 6 v0 6 3 and 0 6 v00 6 4 have been obtained at high resolution and rotationally analyzed. Observation of the first lines in some of the bands has shown that X ¼ 8 in the ground state and X ¼ 9 in the upper state. By analogy with HoF, this transition has been labeled as A9–X8. The X ¼ 8 assignment for the X state establishes the ground state configuration of HoCl as Hoþ ð4f 10 6s2 ÞCl , in accord with predictions of Ligand Field Theory. From the rotational analysis, the main equilibrium molecular constants of xe ¼ 317:13ð15Þ cm1 , Be ¼ 0:096873ð66Þ cm1 for the upper state and xe ¼ 336:001ð37Þ cm1 , Be ¼ 0:102186ð73Þ cm1 for the ground state have been obtained. Ó 2002 Elsevier Science (USA). All rights reserved. Keywords: Laser spectroscopy; Holmium monochloride; Diatomic molecules; Electronic structure
1. Introduction As part of an ongoing investigation into the spectra and structure of molecules containing lanthanide atoms, we have recently been studying the properties of lanthanide monohalides. Lanthanide molecules are of interest because the atoms contain several unpaired felectrons which give rise to many electronic states of high angular momentum and lead to very complex spectra. We have previously been successful in using laser spectroscopy to simplify the spectra of several lanthanide oxides [1,2] and fluorides [3,4] and, by comparing our results with various theoretical calculations [5,6], we have been able to gain some insight into the structure and bonding properties of these molecules. Holmium halides are potentially particularly useful indicators of the properties of these molecules. Holmium has only one isotope which simplifies the spectroscopy. It has a nuclear spin of 7/2 and a large magnetic moment and should therefore give spectra with measurable hyq
Supplementary data for this article are available on ScienceDirect. Corresponding author. Fax: +506-453-4581. E-mail address: [email protected] (C. Linton). *
perfine structure which is an excellent diagnostic tool for determining details of electronic structure. The holmium halides are also unique in that the states from the two predicted lowest lying configurations have greatly different electronic properties and it should therefore be easy to determine the configuration of the ground state from the analysis of the spectrum. The lanthanide halides can generally be represented as ionically bonded M þ X where M represents the lanthanide metal and X the halogen ligand. The electronic properties of the molecule are primarily determined by those of the metal centered electron orbitals and it is therefore the electron configuration of the metal ion which plays the major role in determining the electronic state of the molecule. A ligand field theoretical (LFT) approach has been developed in which the electronic properties of the molecule are derived from a model in which the atomic ion states are perturbed by the axial field of the X ligand. For all the holmium halides, LFT calculations [5] have predicted that the ground state will arise from the metal ion 4f 10 6s2 configuration, for which the lowest lying state will have X ¼ 8, whereas the lowest state of the predicted first excited configuration, 4f 11 6s, should have X ¼ 0. Rotational analysis can easily distinguish
0022-2852/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII: S 0 0 2 2 - 2 8 5 2 ( 0 2 ) 0 0 0 4 2 - 5
M.J. Dick, C. Linton / Journal of Molecular Spectroscopy 217 (2003) 26–31
between these and enable the ground state configuration to be unambiguously determined. For HoF, the ground state was found to have X ¼ 8 [3,7] thus confirming the LFT prediction. Another useful diagnostic indicator of the configuration of an electronic state is its vibrational frequency. Because of the their compactness, f orbitals effectively shield the nuclear charge from the ligand whereas this is not the case for s orbitals. For configurations with more f electrons, the ligand will ‘‘see’’ less charge from the nucleus, and the electrostatic interaction will be weaker leading to a lower vibrational frequency. On this basis, electronic states resulting from f N 1 s2 will have a larger vibrational frequency than those from f N s. Although LFT does not predict them explicitly, vibrational frequencies for a particular configuration remain approximately constant across the lanthanide group. Thus, for example, the value of 500 cm1 obtained for YbF, which has a ground state configuration of 4f 14 6s will also be expected for all lanthanide fluoride states from f N s configurations whereas the 600 cm1 obtained for HoF will apply to states from f N 1 s2 configurations. For YbCl, the only lanthanide chloride so far observed [8,9], the ground configuration was found to be 4f 14 6s and the vibrational frequency was found to be 290 cm1 . Thus, lanthanide chloride states with f N s configurations are expected to have frequencies of about 290 cm1 and those from f N 1 s2 will have a significantly larger vibrational frequency. Of the holmium halides, only HoF has been previously observed. Robbins and Barrow [7] observed two transitions, A9–X8 in the 520–600 nm region and B8– X8 in the 450–500 nm region and analyzed several bands of the A–X transition and the 2–0 band of the B–X transition. On the basis of their analysis, they assigned the ground state as X ¼ 8. Using laser spectroscopy, we [3] extended the analysis of the A–X transition and confirmed the assignment of the ground state to the 4f 10 6s2 configuration of Hoþ and showed that it was consistent with LFT predictions [5]. In work currently in progress, we have shown that there are several states in the region of the B state and are currently trying to assign them. For HoCl, the 4f 10 6s2 configuration is also predicted by LFT [5] to lie lowest with the 4f 11 6s configuration about 2000 cm1 higher in energy. There have been no previous spectroscopic observations on this molecule. The aim of the present work is to observe the spectrum of HoCl at high resolution, build up an energy diagram, try and observe the hyperfine structure to determine detailed electronic structure information and test the predictions of LFT. Using a laser ablation source and pulsed dye laser, we have observed several HoCl transitions. In this paper, we report the results of the high resolution rotational analysis of the A9–X8 transition.
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2. Experimental details As there were no previous data on HoCl and we therefore did not know the wavenumber regions of the electronic transitions, we first took low resolution survey excitation spectra from 400 to 700 nm using a pulsed dye laser and an ablation source. The ablation source has been described previously [10] and will not be described in detail here. In the present experiments, a holmium rod was ablated with the ultraviolet beam from a pulsed Nd YAG laser and the resulting atoms were reacted with CHCl3 to form HoCl molecules in the gas phase. These were then sent through a pulsed nozzle where they formed a molecular beam. The beam from a pulsed dye laser was sent into the chamber, perpendicular to the molecular beam and, using a range of dyes, the laser wavelength was scanned from 400 to 700 nm. Several groups of molecular bands were observed, the most promising being what appeared to be an extensive band system in the 615–670 nm region. The above system was then examined at high resolution using a Broida oven source and a Coherent 699-29 ‘‘Autoscan’’ ring dye laser using DCM dye. In the Broida oven, holmium metal was heated in a graphite crucible and the holmium vapor was carried into the reaction region by 2 Torr of Argon gas where it reacted with CHCl3 vapor to produce a chemiluminescent flame of HoCl molecules in the gas phase. The beam from the ring laser was then directed into the flame and the resultant fluorescence was detected using a Jarrell Ash 0.5 m monochromator. High resolution excitation spectra were obtained by scanning the dye laser frequency with the monochromator wavelength fixed to detect fluorescence from the excited state of interest. Dispersed fluorescence spectra were obtained by fixing the laser frequency and scanning the monochromator wavelength.
3. Results 3.1. Survey spectra with the ring laser The first experiments with the ring laser were designed to search for band heads and determine approximate values for the vibrational spacings. The laser was first set to excite the band head of the strongest transition observed in the pulsed laser spectra at 642 nm. Dispersed fluorescence from scanning the monochromator showed a progression of RQP triplets whose intensity distribution, R > Q > P at low J and Q > R > P at higher J, indicated that the transition is DX ¼ þ1 and that X is large. The triplets are spaced at intervals of 330 cm1 and there was no fluorescence to the blue of the laser. This suggests that the lower state of the transition excited by the laser was v ¼ 0 of the ground state and that the vibrational frequency is 330 cm1 . As this was the most
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intense observed transition, it was assumed to be the 0–0 band. Excitation spectra of the 0–0 sequence were then taken with the monochromator detecting 330 cm1 to the red of the laser wavelength, i.e., in the 0–1 sequence. After the 1–1 head had been found, the laser wavenumber was set 330 cm1 to the red to search for bands in the 0–1 sequence with the monochromator detecting in the 0–0 sequence region. This process was repeated, sequence by sequence, until as many band heads as could be detected were observed. The bands are all red degraded and the 2–0, 1–0, and 0–0 sequences followed the expected pattern with the band heads increasing in wavelength with increasing vibration. However, in the 0– 1 sequence, the band heads themselves formed a ‘‘head of heads’’ and the sequence turned around. The 1–2 band is to the red of the 0–1 band as expected but the 2–3 band is 1 cm1 to the blue of 1–2 and the sequence continues towards the blue. In the 0–2 sequence, the band head wavelength decreases with increasing vibration although the bands themselves are still degraded to longer wavelengths. Finding and sorting out the pattern of heads required very careful selective detection. A schematic diagram showing the position of all the observed band heads is shown in Fig. 1 and the band head wavenumbers are listed in Table 1. The vibrational assignments were confirmed by observation of band heads of the Ho37 Cl isotopomer. The unusual distribution of band heads indicates that the vibrational constants, xe , are similar in the ground and excited states with that of the ground state being slightly higher, and that the anharmonicity is much greater in the ground state than in the excited state. Thus, as v increases, the vibrational spacings in the ground state decrease more rapidly than in the excited state.
Table 1 Observed bands in the A9–X8 transition of Ho35 Cl v0
v00
Head (cm1 )
Origin (cm1 )
2 3 1 2 3 0 1 0 1 2 3 4 5 0 1 2 3 4
0 1 0 1 2 0 1 1 2 3 4 5 6 2 3 4 5 6
16191.069 16173.866 15877.866 15862.399 15852.884 15562.697 15549.227 15234.067 15228.301 15229.282 15237.677 15252.658 15272.528 14913.250 14916.285 14926.453 14943.211 14964.881
16190.6982 16173.3633 15877.4117 15861.8615 –a 15562.0838 15548.5750 15233.2471 15227.3575 15228.1061 15236.1561 –a –a 14912.0296 14914.8196 14924.5743 –a –a
a
Observed bands that were too weak for rotational analysis.
3.2. High resolution spectra High resolution excitation spectra were obtained for 13 transitions, shown in Fig. 2, with 0 6 v0 6 3 and
Fig. 2. Energy level diagram showing all observed bands in the A9–X8 transition of Ho35 Cl.
Fig. 1. Schematic stick spectrum showing the positions of the observed band heads of the A9–X8 transition of Ho35 Cl. Note the ‘‘head of heads’’ in the 0–1 sequence.
0 6 v00 6 4. Each upper and lower state vibrational level was involved in at least two transitions so that assignments could be confirmed by comparing combination differences and the data set would be well determined. For such a heavy molecule, the B value would be very small and the rotational structure very crowded. In addition, the closeness of the band heads, especially in the vicinity of the 0–1 band, would lead to severe overlapping of the rotational lines of the different bands. In order to minimize the confusion caused by overlapping transitions, it
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was essential to use very careful selective detection. For example, the 1–1 band is weaker than and overlapped by the intense 0–0 band. However, setting the monochromator to detect the 1–0 transition will eliminate any 0–0 lines from the spectrum. Similarly, in the very crowded 1– 0 sequence where there are four bands in a 10 cm1 region, it was possible to obtain clean spectra of each of the bands by careful selection of the detection wavelength and reducing the range of J detected in each scan by narrowing the monochromator slits as far as possible. The bands, in general, consist of strong R and Q branches and very weak P branches. In the more intense bands, the first line in the R branch was usually easily identified whereas the low J Q branch lines were weaker and broadened by unresolved hyperfine structure. The P branch was much weaker and, in many cases, too weak to be measured. As in HoF [3], the hyperfine structure due to the I ¼ 3:5 nuclear spin of Ho was not resolved but manifested itself through a slight broadening of the low J lines. This limited the accuracy of the line position measurements. Because of the crowded rotational structure, several diagnostic methods were used to aid in the assignment of the lines. For more intense bands, it was possible to narrow the monochromator slits so that only a narrow range of J values was detected and, at higher J, the excitation scan showed three clearly distinct branches with a small number of lines in each. In this way, it was possible to identify lines with approximately the same J (to within 2) in each branch and make preliminary assignments. Many bands showed perturbations and these were also a very useful tool in assigning the lines. An example of this can be seen in Fig. 3 which shows the 2–1 band. An identical pattern of intensities is seen in the labeled groups of lines in the R, Q, and P branches. In each group of nine lines, the first (lowest wavenumber), seventh, and ninth lines are less intense than the others and are clearly perturbed. This identifies lines in each branch with a common
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Fig. 3. Excitation spectrum of the 2–1 band of the A9–X8 transition of Ho35 Cl. The labeled lines in the R, Q, and P branches show perturbations at J ¼ 36, 38, and 44 in the A9 state.
rotational level which serves as an excellent starting point for assigning the band. In Fig. 3, the perturbations are found to be at J ¼ 36, 38, and 44 in the upper state. The final assignments were made and checked using combination differences. Within each band, the combination relations, D1 F 00 ¼ RðJ 1Þ QðJ Þ ¼ QðJ 1Þ P ðJ Þ and D1 F 0 ðJ Þ ¼ RðJ 1Þ QðJ 1Þ ¼ QðJ Þ P ðJ Þ must hold and the combination differences for bands with a common level must also agree. This provided multiple checks on the assignments. There was a large zero gap at the origin and the first lines were identified in several bands as R(8), Q(9), P(10) which showed that the transition is X0 ¼ 9 X00 ¼ 8. This is similar to the A–X transition in HoF and has thus been assigned as A9 X 8. Because of the large spin–orbit interaction, a HundÕs case (c) coupling model is the most appropriate way to represent the electronic states of heavy molecules such as HoF and HoCl. Once assignments were made, each band was fitted separately to a polynomial in [J ðJ þ 1Þ X2 ] to obtain preliminary constants, Bv and Dv . These were then used to predict further line positions and assign some of
Table 2 Molecular constants (in cm1 ) of the A9 and X8 states of Ho35 Cla State
v
Tv
Bv
108 Dv
A9
0 1 2 3
15562.0838(18)b 15877.4117(15) 16190.6982(16) 16502.2800(25)
0.096714(12) 0.096181(12) 0.095809(12) 0.095445(12)
5.06(34) 3.20(32) 3.60(33) 3.51(35)
X8
0 1 2 3 4
Be ¼ 0:096873ð66Þ, ae ¼ 0:000418ð24Þ, xe ¼ 317:13ð15Þ xe xe ¼ 0:937ð38Þ 0 0.101720(12) 328.8367(17) 0.101081(12) 650.0542(20) 0.100332(12) 962.5921(19) 0.099509(12) 1266.1239(25) 0.098650(12)
Be ¼ 0:102186ð73Þ, ae ¼ 0:000771ð25Þ, a b
1
xe ¼ 336:001ð37Þ, xe xe ¼ 3:37ð17Þ
Standard deviation of fit ¼ 0:007 cm . Numbers in parentheses are the standard error in the last two digits.
3.14(34) 3.61(34) 3.62(34) 3.96(34) 4.81(35)
xe ye ¼ 0:116ð23Þ
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the weaker lines. After all the individual bands had been fitted, the data were put into a global least squares fit to obtain a single set of molecular parameters which fit the 1052 lines to within a standard deviation of 0:007 cm1 . The final rotational and equilibrium constants are listed in Table 2 and the band origins are listed, along with the band heads, in Table 1. The vibrational constants in Table 2 were obtained from least squares fits to the term energies, Tv , giving an rms uncertainty of 0:07 cm1 in the vibrational energies. A complete list of observed line wavenumbers and assignments has been deposited in the supplementary material archive of the journal.
4. Discussion Both the result of the rotational analysis and the ground state vibrational frequency support the assignment of the ground state to the Hoþ ð4f 10 6s2 ÞCl superconfiguration as predicted by LFT calculations [5]. The value of X ¼ 8 is also consistent with the 4f 10 6s2 configurational assignment. The 4f 10 core has a 5 I8 ground state, i.e., the core angular momentum has Jc ¼ 8. The closed 6s2 shell of Hoþ and the 3p6 shell of Cl contribute nothing to the angular momentum so the total atomic angular momentum corresponds to Ja ¼ 8. This, when projected on the internuclear axis, gives an X ¼ 8 ground state. The vibrational frequency, xe 337 cm1 , is significantly higher than the 290 cm1 found for the 4f 14 6s configuration of YbCl, which is consistent with expectations for 4f 10 6s2 . Thus, vibrational frequencies in the 330–340 cm1 range will be an indicator for the 4f N1 6s2 configuration of other lanthanide monochlorides. Another indicator of the configurational assignment and test of the LFT predictions is the pattern and energies of low lying electronic states. For a pure f 10 s2 configuration, the electronic states all arise from the unfilled f shell. The dispersed fluorescence experiments gave vibrational progressions in the ground state but did not show transitions to other low lying states. This is not surprising in this case as the upper state had X ¼ 9 and there were no allowed transitions to states with X < 8. The lowest states of f 10 s2 all arise from projections onto the internuclear axis of the Jc ¼ 8 component of the Hoþ 5 I8 state. This gives states with X ¼ 8; 7; 6; . . . 1; 0 with the X ¼ 8 state lying lowest. LFT predicts that these states are close in energy and all nine states occur within a 350 cm1 energy region. Next will be a series of states with X from 7 to 0 arising from the next spin–orbit component, 5 I7 , with Jc ¼ 7. These are predicted to be in the region about 4000 cm1 above the ground state. The other spin–orbit components with Jc ¼ 6; 5; 4 will each give series of electronic states with maximum X ¼ 6; 5; 4. Hence, the ground state is the only X ¼ 8 state arising from the f 10 s2 configuration and the only one that can be accessed
through fluorescence from the A9 state. The f 11 s configuration will give groups of states with X ¼ 0 lying lowest and ranging from 0 to 8. Only the X ¼ 8 state from this configuration could appear in the fluorescence spectrum. This is predicted to be at 2160 cm1 above the ground state and was not observed. In order to build up a picture of the low lying states with X < 8, it will be necessary to excite states with X ¼ 8 or less. There were several perturbations in the spectra, all in the upper state, but there were no obvious perturbation patterns that would yield information about the perturbing state or states. As was the case in HoF, many lines appeared to be doubled but it is not clear whether these were genuine rotational perturbations giving main + extra line pairs or whether they were hyperfine perturbations effectively splitting the hyperfine multiplets into two segments. It will be necessary to resolve the hyperfine structure in order to distinguish between the two possibilities. The upper state of the transition has been labeled A9 by analogy with HoF. The rotational structure in HoCl is a lot more dense but looks very similar to that of the A9–X8 transition in HoF. In both molecules, the hyperfine structure was not resolved but there was significant broadening of the low J lines in both molecules. Examination of the hyperfine structure would be very valuable in providing more detailed information on the electronic structure and configurations and experiments are presently underway, using the ablation source and ring laser, to resolve the hyperfine structure in both molecules.
5. Conclusions In this paper, we have described the first spectroscopic observation of holmium monochloride and the rotational analysis of the A9–X8 transition. The ground state, like that of HoF, has been shown to arise from the 4f 10 6s2 configuration of Hoþ in agreement with predictions of ligand field theory [5]. The ground state vibrational frequency of 337 cm1 is expected to be similar for all lanthanide chloride states arising from f N 1 s2 configurations of the metal ion and can be used as an aid in configurational assignment. The present experiments represent a first step in our attempt to understand the electronic structure of HoCl and test the theoretical calculations. In order to gain more insight into the electronic configurations, the degree of configurational mixing, and to make meaningful comparisons between HoF and HoCl, it will be necessary to observe the pattern of the low lying states in both molecules. Other transitions have been observed at higher frequencies and experiments are presently being conducted to obtain these at high resolution with the hope of observing more low lying states and testing and,
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if necessary amending, the ligand field theory calculations. In order to make a more detailed examination and comparison of the electronic structure of HoF and HoCl, it will be important to examine the hyperfine structure in both molecules. Experiments using the ablation source and ring laser are in the initial stages and are showing some resolved hyperfine structure in both molecules.
Acknowledgments This work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. The authors wish to thank Joyce MacGregor and Dr. Allan Adam for their help with the experiments.
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References [1] G. Bujin, C. Linton, J. Mol. Spectrosc. 147 (1991) 120–141. [2] S.A. McDonald, S.F. Rice, R.W. Field, C. Linton, J. Chem. Phys. 93 (1990) 7676–7686. [3] L.A. Kaledin, C. Linton, T.E. Clarke, R.W. Field, J. Mol. Spectrosc. 154 (1992) 417–426. [4] K.L. Dunfield, C. Linton, T.E. Clarke, J. McBride, A.G. Adam, J.R.D. Peers, J. Mol. Spectrosc. 174 (1995) 433–445. [5] A.L. Kaledin, M.C. Heaven, R.W. Field, L.A. Kaledin, J. Mol. Spectrosc. 179 (1996) 310–319. [6] M. Dolg, H. Stoll, H. Preuss, Chem. Phys. 165 (1992) 21–30. [7] D.J.W. Robbins, R.F. Barrow, J. Phys. B 7 (1975) L234– L235. [8] T.C. Melville, J.A. Coxon, C. Linton, J. Mol. Spectrosc. 200 (2000) 229–234. [9] C. Linton, A.G. Adam, J. Mol. Spectrosc. 206 (2001) 161–165. [10] A.G. Adam, L.P. Fraser, W.D. Hamilton, M.C. Steeves, Chem. Phys. Lett. 230 (1994) 82–86.
Laser spectroscopy of the A9–X8 transition of holmium monochlorideq M.J. Dick and C. Linton* Physics Department, University of New Brunswick, P.O. Box 4400, Fredericton, NB, Canada E3B 5A3 Received 13 June 2002; in revised form 16 October 2002
Abstract Using laser excitation spectroscopy, 18 red-degraded bands belonging to a single electronic transition of holmium monochloride have been observed in the 615–670 nm region. Thirteen of the bands, with vibrational levels 0 6 v0 6 3 and 0 6 v00 6 4 have been obtained at high resolution and rotationally analyzed. Observation of the first lines in some of the bands has shown that X ¼ 8 in the ground state and X ¼ 9 in the upper state. By analogy with HoF, this transition has been labeled as A9–X8. The X ¼ 8 assignment for the X state establishes the ground state configuration of HoCl as Hoþ ð4f 10 6s2 ÞCl , in accord with predictions of Ligand Field Theory. From the rotational analysis, the main equilibrium molecular constants of xe ¼ 317:13ð15Þ cm1 , Be ¼ 0:096873ð66Þ cm1 for the upper state and xe ¼ 336:001ð37Þ cm1 , Be ¼ 0:102186ð73Þ cm1 for the ground state have been obtained. Ó 2002 Elsevier Science (USA). All rights reserved. Keywords: Laser spectroscopy; Holmium monochloride; Diatomic molecules; Electronic structure
1. Introduction As part of an ongoing investigation into the spectra and structure of molecules containing lanthanide atoms, we have recently been studying the properties of lanthanide monohalides. Lanthanide molecules are of interest because the atoms contain several unpaired felectrons which give rise to many electronic states of high angular momentum and lead to very complex spectra. We have previously been successful in using laser spectroscopy to simplify the spectra of several lanthanide oxides [1,2] and fluorides [3,4] and, by comparing our results with various theoretical calculations [5,6], we have been able to gain some insight into the structure and bonding properties of these molecules. Holmium halides are potentially particularly useful indicators of the properties of these molecules. Holmium has only one isotope which simplifies the spectroscopy. It has a nuclear spin of 7/2 and a large magnetic moment and should therefore give spectra with measurable hyq
Supplementary data for this article are available on ScienceDirect. Corresponding author. Fax: +506-453-4581. E-mail address: [email protected] (C. Linton). *
perfine structure which is an excellent diagnostic tool for determining details of electronic structure. The holmium halides are also unique in that the states from the two predicted lowest lying configurations have greatly different electronic properties and it should therefore be easy to determine the configuration of the ground state from the analysis of the spectrum. The lanthanide halides can generally be represented as ionically bonded M þ X where M represents the lanthanide metal and X the halogen ligand. The electronic properties of the molecule are primarily determined by those of the metal centered electron orbitals and it is therefore the electron configuration of the metal ion which plays the major role in determining the electronic state of the molecule. A ligand field theoretical (LFT) approach has been developed in which the electronic properties of the molecule are derived from a model in which the atomic ion states are perturbed by the axial field of the X ligand. For all the holmium halides, LFT calculations [5] have predicted that the ground state will arise from the metal ion 4f 10 6s2 configuration, for which the lowest lying state will have X ¼ 8, whereas the lowest state of the predicted first excited configuration, 4f 11 6s, should have X ¼ 0. Rotational analysis can easily distinguish
0022-2852/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved. PII: S 0 0 2 2 - 2 8 5 2 ( 0 2 ) 0 0 0 4 2 - 5
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between these and enable the ground state configuration to be unambiguously determined. For HoF, the ground state was found to have X ¼ 8 [3,7] thus confirming the LFT prediction. Another useful diagnostic indicator of the configuration of an electronic state is its vibrational frequency. Because of the their compactness, f orbitals effectively shield the nuclear charge from the ligand whereas this is not the case for s orbitals. For configurations with more f electrons, the ligand will ‘‘see’’ less charge from the nucleus, and the electrostatic interaction will be weaker leading to a lower vibrational frequency. On this basis, electronic states resulting from f N 1 s2 will have a larger vibrational frequency than those from f N s. Although LFT does not predict them explicitly, vibrational frequencies for a particular configuration remain approximately constant across the lanthanide group. Thus, for example, the value of 500 cm1 obtained for YbF, which has a ground state configuration of 4f 14 6s will also be expected for all lanthanide fluoride states from f N s configurations whereas the 600 cm1 obtained for HoF will apply to states from f N 1 s2 configurations. For YbCl, the only lanthanide chloride so far observed [8,9], the ground configuration was found to be 4f 14 6s and the vibrational frequency was found to be 290 cm1 . Thus, lanthanide chloride states with f N s configurations are expected to have frequencies of about 290 cm1 and those from f N 1 s2 will have a significantly larger vibrational frequency. Of the holmium halides, only HoF has been previously observed. Robbins and Barrow [7] observed two transitions, A9–X8 in the 520–600 nm region and B8– X8 in the 450–500 nm region and analyzed several bands of the A–X transition and the 2–0 band of the B–X transition. On the basis of their analysis, they assigned the ground state as X ¼ 8. Using laser spectroscopy, we [3] extended the analysis of the A–X transition and confirmed the assignment of the ground state to the 4f 10 6s2 configuration of Hoþ and showed that it was consistent with LFT predictions [5]. In work currently in progress, we have shown that there are several states in the region of the B state and are currently trying to assign them. For HoCl, the 4f 10 6s2 configuration is also predicted by LFT [5] to lie lowest with the 4f 11 6s configuration about 2000 cm1 higher in energy. There have been no previous spectroscopic observations on this molecule. The aim of the present work is to observe the spectrum of HoCl at high resolution, build up an energy diagram, try and observe the hyperfine structure to determine detailed electronic structure information and test the predictions of LFT. Using a laser ablation source and pulsed dye laser, we have observed several HoCl transitions. In this paper, we report the results of the high resolution rotational analysis of the A9–X8 transition.
27
2. Experimental details As there were no previous data on HoCl and we therefore did not know the wavenumber regions of the electronic transitions, we first took low resolution survey excitation spectra from 400 to 700 nm using a pulsed dye laser and an ablation source. The ablation source has been described previously [10] and will not be described in detail here. In the present experiments, a holmium rod was ablated with the ultraviolet beam from a pulsed Nd YAG laser and the resulting atoms were reacted with CHCl3 to form HoCl molecules in the gas phase. These were then sent through a pulsed nozzle where they formed a molecular beam. The beam from a pulsed dye laser was sent into the chamber, perpendicular to the molecular beam and, using a range of dyes, the laser wavelength was scanned from 400 to 700 nm. Several groups of molecular bands were observed, the most promising being what appeared to be an extensive band system in the 615–670 nm region. The above system was then examined at high resolution using a Broida oven source and a Coherent 699-29 ‘‘Autoscan’’ ring dye laser using DCM dye. In the Broida oven, holmium metal was heated in a graphite crucible and the holmium vapor was carried into the reaction region by 2 Torr of Argon gas where it reacted with CHCl3 vapor to produce a chemiluminescent flame of HoCl molecules in the gas phase. The beam from the ring laser was then directed into the flame and the resultant fluorescence was detected using a Jarrell Ash 0.5 m monochromator. High resolution excitation spectra were obtained by scanning the dye laser frequency with the monochromator wavelength fixed to detect fluorescence from the excited state of interest. Dispersed fluorescence spectra were obtained by fixing the laser frequency and scanning the monochromator wavelength.
3. Results 3.1. Survey spectra with the ring laser The first experiments with the ring laser were designed to search for band heads and determine approximate values for the vibrational spacings. The laser was first set to excite the band head of the strongest transition observed in the pulsed laser spectra at 642 nm. Dispersed fluorescence from scanning the monochromator showed a progression of RQP triplets whose intensity distribution, R > Q > P at low J and Q > R > P at higher J, indicated that the transition is DX ¼ þ1 and that X is large. The triplets are spaced at intervals of 330 cm1 and there was no fluorescence to the blue of the laser. This suggests that the lower state of the transition excited by the laser was v ¼ 0 of the ground state and that the vibrational frequency is 330 cm1 . As this was the most
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intense observed transition, it was assumed to be the 0–0 band. Excitation spectra of the 0–0 sequence were then taken with the monochromator detecting 330 cm1 to the red of the laser wavelength, i.e., in the 0–1 sequence. After the 1–1 head had been found, the laser wavenumber was set 330 cm1 to the red to search for bands in the 0–1 sequence with the monochromator detecting in the 0–0 sequence region. This process was repeated, sequence by sequence, until as many band heads as could be detected were observed. The bands are all red degraded and the 2–0, 1–0, and 0–0 sequences followed the expected pattern with the band heads increasing in wavelength with increasing vibration. However, in the 0– 1 sequence, the band heads themselves formed a ‘‘head of heads’’ and the sequence turned around. The 1–2 band is to the red of the 0–1 band as expected but the 2–3 band is 1 cm1 to the blue of 1–2 and the sequence continues towards the blue. In the 0–2 sequence, the band head wavelength decreases with increasing vibration although the bands themselves are still degraded to longer wavelengths. Finding and sorting out the pattern of heads required very careful selective detection. A schematic diagram showing the position of all the observed band heads is shown in Fig. 1 and the band head wavenumbers are listed in Table 1. The vibrational assignments were confirmed by observation of band heads of the Ho37 Cl isotopomer. The unusual distribution of band heads indicates that the vibrational constants, xe , are similar in the ground and excited states with that of the ground state being slightly higher, and that the anharmonicity is much greater in the ground state than in the excited state. Thus, as v increases, the vibrational spacings in the ground state decrease more rapidly than in the excited state.
Table 1 Observed bands in the A9–X8 transition of Ho35 Cl v0
v00
Head (cm1 )
Origin (cm1 )
2 3 1 2 3 0 1 0 1 2 3 4 5 0 1 2 3 4
0 1 0 1 2 0 1 1 2 3 4 5 6 2 3 4 5 6
16191.069 16173.866 15877.866 15862.399 15852.884 15562.697 15549.227 15234.067 15228.301 15229.282 15237.677 15252.658 15272.528 14913.250 14916.285 14926.453 14943.211 14964.881
16190.6982 16173.3633 15877.4117 15861.8615 –a 15562.0838 15548.5750 15233.2471 15227.3575 15228.1061 15236.1561 –a –a 14912.0296 14914.8196 14924.5743 –a –a
a
Observed bands that were too weak for rotational analysis.
3.2. High resolution spectra High resolution excitation spectra were obtained for 13 transitions, shown in Fig. 2, with 0 6 v0 6 3 and
Fig. 2. Energy level diagram showing all observed bands in the A9–X8 transition of Ho35 Cl.
Fig. 1. Schematic stick spectrum showing the positions of the observed band heads of the A9–X8 transition of Ho35 Cl. Note the ‘‘head of heads’’ in the 0–1 sequence.
0 6 v00 6 4. Each upper and lower state vibrational level was involved in at least two transitions so that assignments could be confirmed by comparing combination differences and the data set would be well determined. For such a heavy molecule, the B value would be very small and the rotational structure very crowded. In addition, the closeness of the band heads, especially in the vicinity of the 0–1 band, would lead to severe overlapping of the rotational lines of the different bands. In order to minimize the confusion caused by overlapping transitions, it
M.J. Dick, C. Linton / Journal of Molecular Spectroscopy 217 (2003) 26–31
was essential to use very careful selective detection. For example, the 1–1 band is weaker than and overlapped by the intense 0–0 band. However, setting the monochromator to detect the 1–0 transition will eliminate any 0–0 lines from the spectrum. Similarly, in the very crowded 1– 0 sequence where there are four bands in a 10 cm1 region, it was possible to obtain clean spectra of each of the bands by careful selection of the detection wavelength and reducing the range of J detected in each scan by narrowing the monochromator slits as far as possible. The bands, in general, consist of strong R and Q branches and very weak P branches. In the more intense bands, the first line in the R branch was usually easily identified whereas the low J Q branch lines were weaker and broadened by unresolved hyperfine structure. The P branch was much weaker and, in many cases, too weak to be measured. As in HoF [3], the hyperfine structure due to the I ¼ 3:5 nuclear spin of Ho was not resolved but manifested itself through a slight broadening of the low J lines. This limited the accuracy of the line position measurements. Because of the crowded rotational structure, several diagnostic methods were used to aid in the assignment of the lines. For more intense bands, it was possible to narrow the monochromator slits so that only a narrow range of J values was detected and, at higher J, the excitation scan showed three clearly distinct branches with a small number of lines in each. In this way, it was possible to identify lines with approximately the same J (to within 2) in each branch and make preliminary assignments. Many bands showed perturbations and these were also a very useful tool in assigning the lines. An example of this can be seen in Fig. 3 which shows the 2–1 band. An identical pattern of intensities is seen in the labeled groups of lines in the R, Q, and P branches. In each group of nine lines, the first (lowest wavenumber), seventh, and ninth lines are less intense than the others and are clearly perturbed. This identifies lines in each branch with a common
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Fig. 3. Excitation spectrum of the 2–1 band of the A9–X8 transition of Ho35 Cl. The labeled lines in the R, Q, and P branches show perturbations at J ¼ 36, 38, and 44 in the A9 state.
rotational level which serves as an excellent starting point for assigning the band. In Fig. 3, the perturbations are found to be at J ¼ 36, 38, and 44 in the upper state. The final assignments were made and checked using combination differences. Within each band, the combination relations, D1 F 00 ¼ RðJ 1Þ QðJ Þ ¼ QðJ 1Þ P ðJ Þ and D1 F 0 ðJ Þ ¼ RðJ 1Þ QðJ 1Þ ¼ QðJ Þ P ðJ Þ must hold and the combination differences for bands with a common level must also agree. This provided multiple checks on the assignments. There was a large zero gap at the origin and the first lines were identified in several bands as R(8), Q(9), P(10) which showed that the transition is X0 ¼ 9 X00 ¼ 8. This is similar to the A–X transition in HoF and has thus been assigned as A9 X 8. Because of the large spin–orbit interaction, a HundÕs case (c) coupling model is the most appropriate way to represent the electronic states of heavy molecules such as HoF and HoCl. Once assignments were made, each band was fitted separately to a polynomial in [J ðJ þ 1Þ X2 ] to obtain preliminary constants, Bv and Dv . These were then used to predict further line positions and assign some of
Table 2 Molecular constants (in cm1 ) of the A9 and X8 states of Ho35 Cla State
v
Tv
Bv
108 Dv
A9
0 1 2 3
15562.0838(18)b 15877.4117(15) 16190.6982(16) 16502.2800(25)
0.096714(12) 0.096181(12) 0.095809(12) 0.095445(12)
5.06(34) 3.20(32) 3.60(33) 3.51(35)
X8
0 1 2 3 4
Be ¼ 0:096873ð66Þ, ae ¼ 0:000418ð24Þ, xe ¼ 317:13ð15Þ xe xe ¼ 0:937ð38Þ 0 0.101720(12) 328.8367(17) 0.101081(12) 650.0542(20) 0.100332(12) 962.5921(19) 0.099509(12) 1266.1239(25) 0.098650(12)
Be ¼ 0:102186ð73Þ, ae ¼ 0:000771ð25Þ, a b
1
xe ¼ 336:001ð37Þ, xe xe ¼ 3:37ð17Þ
Standard deviation of fit ¼ 0:007 cm . Numbers in parentheses are the standard error in the last two digits.
3.14(34) 3.61(34) 3.62(34) 3.96(34) 4.81(35)
xe ye ¼ 0:116ð23Þ
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the weaker lines. After all the individual bands had been fitted, the data were put into a global least squares fit to obtain a single set of molecular parameters which fit the 1052 lines to within a standard deviation of 0:007 cm1 . The final rotational and equilibrium constants are listed in Table 2 and the band origins are listed, along with the band heads, in Table 1. The vibrational constants in Table 2 were obtained from least squares fits to the term energies, Tv , giving an rms uncertainty of 0:07 cm1 in the vibrational energies. A complete list of observed line wavenumbers and assignments has been deposited in the supplementary material archive of the journal.
4. Discussion Both the result of the rotational analysis and the ground state vibrational frequency support the assignment of the ground state to the Hoþ ð4f 10 6s2 ÞCl superconfiguration as predicted by LFT calculations [5]. The value of X ¼ 8 is also consistent with the 4f 10 6s2 configurational assignment. The 4f 10 core has a 5 I8 ground state, i.e., the core angular momentum has Jc ¼ 8. The closed 6s2 shell of Hoþ and the 3p6 shell of Cl contribute nothing to the angular momentum so the total atomic angular momentum corresponds to Ja ¼ 8. This, when projected on the internuclear axis, gives an X ¼ 8 ground state. The vibrational frequency, xe 337 cm1 , is significantly higher than the 290 cm1 found for the 4f 14 6s configuration of YbCl, which is consistent with expectations for 4f 10 6s2 . Thus, vibrational frequencies in the 330–340 cm1 range will be an indicator for the 4f N1 6s2 configuration of other lanthanide monochlorides. Another indicator of the configurational assignment and test of the LFT predictions is the pattern and energies of low lying electronic states. For a pure f 10 s2 configuration, the electronic states all arise from the unfilled f shell. The dispersed fluorescence experiments gave vibrational progressions in the ground state but did not show transitions to other low lying states. This is not surprising in this case as the upper state had X ¼ 9 and there were no allowed transitions to states with X < 8. The lowest states of f 10 s2 all arise from projections onto the internuclear axis of the Jc ¼ 8 component of the Hoþ 5 I8 state. This gives states with X ¼ 8; 7; 6; . . . 1; 0 with the X ¼ 8 state lying lowest. LFT predicts that these states are close in energy and all nine states occur within a 350 cm1 energy region. Next will be a series of states with X from 7 to 0 arising from the next spin–orbit component, 5 I7 , with Jc ¼ 7. These are predicted to be in the region about 4000 cm1 above the ground state. The other spin–orbit components with Jc ¼ 6; 5; 4 will each give series of electronic states with maximum X ¼ 6; 5; 4. Hence, the ground state is the only X ¼ 8 state arising from the f 10 s2 configuration and the only one that can be accessed
through fluorescence from the A9 state. The f 11 s configuration will give groups of states with X ¼ 0 lying lowest and ranging from 0 to 8. Only the X ¼ 8 state from this configuration could appear in the fluorescence spectrum. This is predicted to be at 2160 cm1 above the ground state and was not observed. In order to build up a picture of the low lying states with X < 8, it will be necessary to excite states with X ¼ 8 or less. There were several perturbations in the spectra, all in the upper state, but there were no obvious perturbation patterns that would yield information about the perturbing state or states. As was the case in HoF, many lines appeared to be doubled but it is not clear whether these were genuine rotational perturbations giving main + extra line pairs or whether they were hyperfine perturbations effectively splitting the hyperfine multiplets into two segments. It will be necessary to resolve the hyperfine structure in order to distinguish between the two possibilities. The upper state of the transition has been labeled A9 by analogy with HoF. The rotational structure in HoCl is a lot more dense but looks very similar to that of the A9–X8 transition in HoF. In both molecules, the hyperfine structure was not resolved but there was significant broadening of the low J lines in both molecules. Examination of the hyperfine structure would be very valuable in providing more detailed information on the electronic structure and configurations and experiments are presently underway, using the ablation source and ring laser, to resolve the hyperfine structure in both molecules.
5. Conclusions In this paper, we have described the first spectroscopic observation of holmium monochloride and the rotational analysis of the A9–X8 transition. The ground state, like that of HoF, has been shown to arise from the 4f 10 6s2 configuration of Hoþ in agreement with predictions of ligand field theory [5]. The ground state vibrational frequency of 337 cm1 is expected to be similar for all lanthanide chloride states arising from f N 1 s2 configurations of the metal ion and can be used as an aid in configurational assignment. The present experiments represent a first step in our attempt to understand the electronic structure of HoCl and test the theoretical calculations. In order to gain more insight into the electronic configurations, the degree of configurational mixing, and to make meaningful comparisons between HoF and HoCl, it will be necessary to observe the pattern of the low lying states in both molecules. Other transitions have been observed at higher frequencies and experiments are presently being conducted to obtain these at high resolution with the hope of observing more low lying states and testing and,
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if necessary amending, the ligand field theory calculations. In order to make a more detailed examination and comparison of the electronic structure of HoF and HoCl, it will be important to examine the hyperfine structure in both molecules. Experiments using the ablation source and ring laser are in the initial stages and are showing some resolved hyperfine structure in both molecules.
Acknowledgments This work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. The authors wish to thank Joyce MacGregor and Dr. Allan Adam for their help with the experiments.
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References [1] G. Bujin, C. Linton, J. Mol. Spectrosc. 147 (1991) 120–141. [2] S.A. McDonald, S.F. Rice, R.W. Field, C. Linton, J. Chem. Phys. 93 (1990) 7676–7686. [3] L.A. Kaledin, C. Linton, T.E. Clarke, R.W. Field, J. Mol. Spectrosc. 154 (1992) 417–426. [4] K.L. Dunfield, C. Linton, T.E. Clarke, J. McBride, A.G. Adam, J.R.D. Peers, J. Mol. Spectrosc. 174 (1995) 433–445. [5] A.L. Kaledin, M.C. Heaven, R.W. Field, L.A. Kaledin, J. Mol. Spectrosc. 179 (1996) 310–319. [6] M. Dolg, H. Stoll, H. Preuss, Chem. Phys. 165 (1992) 21–30. [7] D.J.W. Robbins, R.F. Barrow, J. Phys. B 7 (1975) L234– L235. [8] T.C. Melville, J.A. Coxon, C. Linton, J. Mol. Spectrosc. 200 (2000) 229–234. [9] C. Linton, A.G. Adam, J. Mol. Spectrosc. 206 (2001) 161–165. [10] A.G. Adam, L.P. Fraser, W.D. Hamilton, M.C. Steeves, Chem. Phys. Lett. 230 (1994) 82–86.