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Far infrared spectroscopy of praseodymium magnesium nitrate using a polarizing interferometer
FAR INFRARED SPECTROSCOPY OF PRASEODYMIUM MAGNESlUM NITRATE USING A POLARIZING INTERFEROMETER D. BLOOR, V. E. SELLS* and J. R. DEAN? Department
of Physics,
Queen
(Rewired
Mary
College,
1 October
London
El 4NS, U.K
1975)
Abstract---A number of contradictions exist in the published far i.r. spectra of praseodymium magnesium nitrate. Spectra recorded with improved spectrophotometric accuracy with a polarizing interferometer have enabled these to be resolved. Transitions from the ground doublet to the first excited singlet state of the Pr 3+ ion showed no evidence of the small ground state splitting. The Zeeman splitting of this absorption is as predicted from the ground state g-value but the components are of unequal width. Other features of the spectrum are described.
1. INTRODUCTION
The far i.r. spectrum of praseodymium magnesium nitrate at low temperatures has been the electronic absorption at reported by several authors. (lm3) The results concerning 3X.2 cn- I, due to transitions between the ground doublet and the first excited singlet states of the Pr3+ ions, are contradictory. Fine structure, due to the zero field splitting of the ground doublet. has been reported (2) but has not been seen by later workers.‘3’ However, the Zeeman splitting of this absorption, due to the ground state Zeeman effect. requires a lower g-value than that measured by ESR on the praseodymium ion in lanthanum magnesium nitrate and in praseodymium magnesium nitrate.‘3p6’ Since these far i.r. measurements were made the development of a Michelson interferthe signal to noise ometer using a polarizing beam splitter (7) has greatly improved ratio and reproducibility of the spectra. These improvements result mainly from the ability to work about a mean signal level of zero in the wings of the interferogram. Fluctuations in the source, detector or electronics are then much less likely to contribute noise and spurious features to the transformed spectrum. Such interferometers have been used for far infrared astronomy where the requirements for high spectrophotometric accuracy are multiplied by the weak source intensitites. (8p9) Similar accuracy in the laboratory enables reliable spectra to be obtained quickly so that weak features or fine structure can be unambiguously identified. We have, therefore, made further measurements of the far infrared spectrum of praseodymium magnesium nitrate using a polarizing interferometer. 2. EXPERIMENTAL
TECHNIQUES
AND
RESULTS
Single crystals of praseodymium and lanthanum magnesium nitrate (PrMN and LaMN) were grown from aqueous solution at 0°C using hydrated rare-earth nitrates of 99.97; purity and Analar grade magnesium nitrate. The crystals grew as flat hexagons 1 cm across by 1-2 mm thick which were polished to a thickness of 0.5 mm or less. The spectra were recorded using a RIIC interferometer modified to work a@a polarizing interferometer. The spectral range covered was l&100 cm-’ with a maximum resoluin liquid tion of 0.1 cm-‘. Zero field spectra were obtained with the crystals mounted helium at 1.7”K directly above a germanium bolometer. The Zeeman spectra were obtained with the sample mounted in liquid helium at 4.2”K in the bore of a superconducting magnet with a maximum field of 5 T and a homogeneity of 1 part in lo4 * Present t Present
address: address:
Department Department
of Physics, University of Health, Derbyshire 425
of Western Ontario, London 72, Canada. County Council, Matlock, Derbyshire, U.K.
I
I 36
I
I
1
WA~ENUMEER
L
40
38 cm -’
Fig. 1. Absorption due to the transition from the ground doublet to the first excited singlet in PrMN in zero magnetic field at 1.7 K. sin& crystal sample 0.5 mm thick. The circles represent the uncertainty in the measured iranvnission at a resolution of 0.1 cm- ‘.
over the sample volume. A germanium bolometer was mounted below the magnet in a separate helium bath at 1.2 K. there were no significant changes in detector sensitivity due to stray fields from the magnet. In zero magnetic field at 1.7 ‘K the spectrum of an 0.5 mm thick crystal of PrMN has a smooth featureless ~~bsorption at 3X.15 cm _ ’ as shown in Fig. i. This spectrum was obtained from a single interferogram and the open circles represent the uncertainty of the calculated transmiss~oll. The absence of fine structure is not surprising since the ESR results of Sells and Bloor (“’ show that the zero field splittings of the hype&e components are comparable to their energy width. The energy width of the excited state will obscure such weak features completely. The Zeeman splitting was studied using a sample about 2 mm thick so that the Zeeman components were 60”,, absorbing when separated. Spectra obtained with fields of from 0.5 to 4.5 T applied along the z-axis of the Pr3+ ions are shown in Fig. 2. The noise in the flat portions of the spectra taken from single interferograms can be clearly seen and is significantly larger than that shown in Fig. 1. This is mainly due to the smaller area of the samples in the magnet reducing the signal Ievel at the detector to about 25”,, of that in the zero field experiment. Despite this there is a clear disparity in ~inew~dtl~ of the Zeeman components. The centres of the absorptions fit a zero field energy of 38.15 cm- 1 and a ground state y-value of 1.58, identical with the ESR vfalue.‘3-“’ The previous anomalous result’“’ was a result of the poorer spectrophotometric accuracy of the measurement and the failure to detect the unexpected asymmetry of the Zeeman components. There is no obvious explanation for this behaviour. However, the apparent structure just visible in the broad component at 4.5 T in Fig. 2 appears to be real. When several spectra are averaged the noise level in the wings of the absorption is reduced but three distinct components remain at the centre, shifted by 1-0.4, 0.0 and -04 cm- ’ from the calculated line centre. The reality of these features remains questionable, particularly in view of the history of PrMN far i.r. spectra, as they are at the limit of our technique. A number of weak absorptions not preuiously reported are clearly visible in the zero field spectra obtained with the polarizing interferometer at 0.1 cm-’ resolution. Two absorptions with half widths at half height of 0.4 cm- ’ occur at 72.1 and 74-6 cm _ ’ in LaMN and at 74.1 and 76.6 cm- ’ in PrMN. These are clearly phonons since they occur in both salts and the change in frequency between the salts is the same as that of the intense 61.5 cm- ’ E, mode in LaMN.‘“’ The broad asymmetric absorptions reported by Bloor and Campbell’“’ at 52, 70 and 85 cm ’ are resolved into two components of unequal widths separated by 2 cm ‘. A very weak and narrow absorption occurs in PrMN at 48.2 cm r. It is less than 0.1 cm-’ in half width since spectra
Far infrared spectroscopy
Fig. 2. Zeeman sphtting of the 38”15 cm-’ absorption of PrMN at 42% for an applied field parallel to the crystal hexagonal axis. The applied field increases from the bottom from O-5 to 4.5 T in steps of I 7; the spectra are offset vertically for clarity. Spectra recorded at a resolution of @l cm I.
recorded at this resolution have only one point displaced from the mean transm~ssjon level, see Fig. 3. However, this line is clearly reporduced in separately recorded spectra and its detection without recourse to averaging of interferograms or spectra is a measure of the photometric accuracy possible with the polarizing interferometer. The line is not present in the spectrum of LaMN and shifts to 48.3 cm-’ in a magnetic field of 45 T but is not split. This suggests that it is either due to a resonant mode of an impurity ion or a tunneling mode of the distorted Pr3’ site.‘“’
Ii
I
46
I WAVENUMBER
Fig. 3. Absorption spectra of PrMN and from single interferograms recorded at a PrMN; 0, I mm LaMN; the spectra are throughout the spectra
I
48
50 cm. -$
LaMN between 46 and SOcm-’ at 1,7”K, computed resolution of 0.1 cm-‘. a, 0.5 mm PrMN; c), 2mm offset vertically for clarity, Regular features extending are standing waves in the samples.
3. (.~~cL~.:sl~~s
These ~n~~~sllrelil~nts show that previous ~o~ltr~~di~tor~ results of far i.r. studies of PrMN \+rre due to inadcquatc s~?~~trophotometri~ accuraq. The greater accuracy possible with ;I polarizing intcrferometcr has enabled these problems to be resolved. Two unusual features have been revealed in the spectrum. The sharp line at 48.2 mm ’ has tbvo possible causes. Hocvever. there appears to bc no simple explanation for the asqmmctry of the Zoeman components of the 3X.15 cm- ’ absorption. since the absorption between the levels of the ground doublet is not unusual’“’ and relaxation within the ground doublet would broaden the lower absorption in the far infrared. The Pr”’ ion site is distorted to C, symmetry and this renders ;I theoretical calculation of the bchaviour of the electronic states ditkult if not impossible.
of Physics,
Queen
(Rewired
Mary
College,
1 October
London
El 4NS, U.K
1975)
Abstract---A number of contradictions exist in the published far i.r. spectra of praseodymium magnesium nitrate. Spectra recorded with improved spectrophotometric accuracy with a polarizing interferometer have enabled these to be resolved. Transitions from the ground doublet to the first excited singlet state of the Pr 3+ ion showed no evidence of the small ground state splitting. The Zeeman splitting of this absorption is as predicted from the ground state g-value but the components are of unequal width. Other features of the spectrum are described.
1. INTRODUCTION
The far i.r. spectrum of praseodymium magnesium nitrate at low temperatures has been the electronic absorption at reported by several authors. (lm3) The results concerning 3X.2 cn- I, due to transitions between the ground doublet and the first excited singlet states of the Pr3+ ions, are contradictory. Fine structure, due to the zero field splitting of the ground doublet. has been reported (2) but has not been seen by later workers.‘3’ However, the Zeeman splitting of this absorption, due to the ground state Zeeman effect. requires a lower g-value than that measured by ESR on the praseodymium ion in lanthanum magnesium nitrate and in praseodymium magnesium nitrate.‘3p6’ Since these far i.r. measurements were made the development of a Michelson interferthe signal to noise ometer using a polarizing beam splitter (7) has greatly improved ratio and reproducibility of the spectra. These improvements result mainly from the ability to work about a mean signal level of zero in the wings of the interferogram. Fluctuations in the source, detector or electronics are then much less likely to contribute noise and spurious features to the transformed spectrum. Such interferometers have been used for far infrared astronomy where the requirements for high spectrophotometric accuracy are multiplied by the weak source intensitites. (8p9) Similar accuracy in the laboratory enables reliable spectra to be obtained quickly so that weak features or fine structure can be unambiguously identified. We have, therefore, made further measurements of the far infrared spectrum of praseodymium magnesium nitrate using a polarizing interferometer. 2. EXPERIMENTAL
TECHNIQUES
AND
RESULTS
Single crystals of praseodymium and lanthanum magnesium nitrate (PrMN and LaMN) were grown from aqueous solution at 0°C using hydrated rare-earth nitrates of 99.97; purity and Analar grade magnesium nitrate. The crystals grew as flat hexagons 1 cm across by 1-2 mm thick which were polished to a thickness of 0.5 mm or less. The spectra were recorded using a RIIC interferometer modified to work a@a polarizing interferometer. The spectral range covered was l&100 cm-’ with a maximum resoluin liquid tion of 0.1 cm-‘. Zero field spectra were obtained with the crystals mounted helium at 1.7”K directly above a germanium bolometer. The Zeeman spectra were obtained with the sample mounted in liquid helium at 4.2”K in the bore of a superconducting magnet with a maximum field of 5 T and a homogeneity of 1 part in lo4 * Present t Present
address: address:
Department Department
of Physics, University of Health, Derbyshire 425
of Western Ontario, London 72, Canada. County Council, Matlock, Derbyshire, U.K.
I
I 36
I
I
1
WA~ENUMEER
L
40
38 cm -’
Fig. 1. Absorption due to the transition from the ground doublet to the first excited singlet in PrMN in zero magnetic field at 1.7 K. sin& crystal sample 0.5 mm thick. The circles represent the uncertainty in the measured iranvnission at a resolution of 0.1 cm- ‘.
over the sample volume. A germanium bolometer was mounted below the magnet in a separate helium bath at 1.2 K. there were no significant changes in detector sensitivity due to stray fields from the magnet. In zero magnetic field at 1.7 ‘K the spectrum of an 0.5 mm thick crystal of PrMN has a smooth featureless ~~bsorption at 3X.15 cm _ ’ as shown in Fig. i. This spectrum was obtained from a single interferogram and the open circles represent the uncertainty of the calculated transmiss~oll. The absence of fine structure is not surprising since the ESR results of Sells and Bloor (“’ show that the zero field splittings of the hype&e components are comparable to their energy width. The energy width of the excited state will obscure such weak features completely. The Zeeman splitting was studied using a sample about 2 mm thick so that the Zeeman components were 60”,, absorbing when separated. Spectra obtained with fields of from 0.5 to 4.5 T applied along the z-axis of the Pr3+ ions are shown in Fig. 2. The noise in the flat portions of the spectra taken from single interferograms can be clearly seen and is significantly larger than that shown in Fig. 1. This is mainly due to the smaller area of the samples in the magnet reducing the signal Ievel at the detector to about 25”,, of that in the zero field experiment. Despite this there is a clear disparity in ~inew~dtl~ of the Zeeman components. The centres of the absorptions fit a zero field energy of 38.15 cm- 1 and a ground state y-value of 1.58, identical with the ESR vfalue.‘3-“’ The previous anomalous result’“’ was a result of the poorer spectrophotometric accuracy of the measurement and the failure to detect the unexpected asymmetry of the Zeeman components. There is no obvious explanation for this behaviour. However, the apparent structure just visible in the broad component at 4.5 T in Fig. 2 appears to be real. When several spectra are averaged the noise level in the wings of the absorption is reduced but three distinct components remain at the centre, shifted by 1-0.4, 0.0 and -04 cm- ’ from the calculated line centre. The reality of these features remains questionable, particularly in view of the history of PrMN far i.r. spectra, as they are at the limit of our technique. A number of weak absorptions not preuiously reported are clearly visible in the zero field spectra obtained with the polarizing interferometer at 0.1 cm-’ resolution. Two absorptions with half widths at half height of 0.4 cm- ’ occur at 72.1 and 74-6 cm _ ’ in LaMN and at 74.1 and 76.6 cm- ’ in PrMN. These are clearly phonons since they occur in both salts and the change in frequency between the salts is the same as that of the intense 61.5 cm- ’ E, mode in LaMN.‘“’ The broad asymmetric absorptions reported by Bloor and Campbell’“’ at 52, 70 and 85 cm ’ are resolved into two components of unequal widths separated by 2 cm ‘. A very weak and narrow absorption occurs in PrMN at 48.2 cm r. It is less than 0.1 cm-’ in half width since spectra
Far infrared spectroscopy
Fig. 2. Zeeman sphtting of the 38”15 cm-’ absorption of PrMN at 42% for an applied field parallel to the crystal hexagonal axis. The applied field increases from the bottom from O-5 to 4.5 T in steps of I 7; the spectra are offset vertically for clarity. Spectra recorded at a resolution of @l cm I.
recorded at this resolution have only one point displaced from the mean transm~ssjon level, see Fig. 3. However, this line is clearly reporduced in separately recorded spectra and its detection without recourse to averaging of interferograms or spectra is a measure of the photometric accuracy possible with the polarizing interferometer. The line is not present in the spectrum of LaMN and shifts to 48.3 cm-’ in a magnetic field of 45 T but is not split. This suggests that it is either due to a resonant mode of an impurity ion or a tunneling mode of the distorted Pr3’ site.‘“’
Ii
I
46
I WAVENUMBER
Fig. 3. Absorption spectra of PrMN and from single interferograms recorded at a PrMN; 0, I mm LaMN; the spectra are throughout the spectra
I
48
50 cm. -$
LaMN between 46 and SOcm-’ at 1,7”K, computed resolution of 0.1 cm-‘. a, 0.5 mm PrMN; c), 2mm offset vertically for clarity, Regular features extending are standing waves in the samples.
3. (.~~cL~.:sl~~s
These ~n~~~sllrelil~nts show that previous ~o~ltr~~di~tor~ results of far i.r. studies of PrMN \+rre due to inadcquatc s~?~~trophotometri~ accuraq. The greater accuracy possible with ;I polarizing intcrferometcr has enabled these problems to be resolved. Two unusual features have been revealed in the spectrum. The sharp line at 48.2 mm ’ has tbvo possible causes. Hocvever. there appears to bc no simple explanation for the asqmmctry of the Zoeman components of the 3X.15 cm- ’ absorption. since the absorption between the levels of the ground doublet is not unusual’“’ and relaxation within the ground doublet would broaden the lower absorption in the far infrared. The Pr”’ ion site is distorted to C, symmetry and this renders ;I theoretical calculation of the bchaviour of the electronic states ditkult if not impossible.