Crystal Spectroscopy at the Johns Hopkins University, Lecture notes of Chemistry

Download Crystal Spectroscopy at the Johns Hopkins University and more Lecture notes Chemistry in PDF only on Docsity! 1 . CRYSTAL SPECTROSCOPY AT THE JOHNS HOPKINS UNIVERSITY * H. M. Crosswhite and H. W. Moos ** The Johns Hopkins University, Baltimore, Maryland ABSTRACT N Comparison of the f r e e ion and crystal energy 4f levels is given for SmIII, HoIII, PrIV and E r I V . general depression of some 12,000 cm-' for the 5d c rys ta l bands relative to the f r ee ion levels can also be f o r the trivalent ions a r e not yet very clear . Fo r the divalent spectra a seen. The relationships Spectrum lines of Ho3+ in Lac13 show a wide range of sharpness; for some, which require resolving powers in excess of 600,000, de- tailed hyperfine structure studies a r e being made, The trichloride lattice vibronic sidebands associated with Pr3+ and Nd3' in NdC13 have been studied in absorption. that 1 ) two ions, 2 ) number of vibrational modes. They show vibronics of the same type a r e identical in frequency for the the number of vibronics is very much greater than the In pure NdC13 and NdBr3 crystals , line splittings of as little -as . 1 cm-' have been observed which can be attributed to neares t and next- neare s t neighbor antiferromagnetic and ferromagnetic inter - actions between the Nd3' ions. Zeeman studies of GdC13:Er3' and :Tb3+ were made both above and below the Curie temperature. high temperatures and field strengths follows an expression derivable f rom a molecular field model. Below 2 . 2 a spon- taneous splitting occurs due partly, but not entirely, to the magnetic dipole field in the ferromagnetic domain. The magnetic permeability at 0 % \ % I O t u r c \ I do rs( Trivalent r a r e ear th spectra in CaF2 a r e complicated by the presence of s i tes of different symmetry, By rotating the c rys ta l s , U b t ' - 2 i n fields of up to 38,000 gauss and studying the variation of the Zeeman spectra , it is possible to deduce s i te symmetries. ear ths have been studied in type I1 CaF2, with spectral lines f rom ions in tetragonal s i tes being most frequently observed. Most of the r a r e When EuZt i s placed in trivalent LaC13, color centers can be produced by uv radiation. The appearance of these color centers coincides with the ionization of Eu2+ to Eu3'. Further , the color centers show polarization with respect to the C3 axis indicating a site of high symmetry. Crystal field parameters a r e given for Er3' i n GdC13, LaC13, LaBr3, YGaG, LuGaG, and YC13. Energy t ransfer from S to R and C to B have been studied in Phonon-induced relaxation ra tes nave been determined LaC13:Nd3'. f rom measurements on the radiative and pair processes. t ransfer shows a strong temperature dependence. C to B The absorption spectra after f lash excitation of transition metal ions in crystals a r e being studied. metastable excited states and other processes probably associated with charge transfer phenomena are observed. Both absorption f rom * This research was partially supported by the U. S. Atomic Energy Commission; National Aeronautics and Space Administration under Grant NsG 361; U. S. Army Research Office, Durham; and the A i r Force Office of Scientific Research, Office of Aerospace Research, United States Air Force, under AFOSR contract number AF 49 (638)-1497. ** Alfred P. Sloan Foundation Fellow . J . # i i 'I . . I . . , . . . . . , . . , . _ . . ~. . . i ' 1 - 2 1 , VI 4 w . . +, *P I k I . . . . I ' .. , . . ... . - . . . . + # . . . . . , . 6 ! w > w 4 w H w 4 4 b a * . r 3 d w rl z w ? w & w - bN Rl U rl r)(u(rwmw 4 7 I U I I 1 I mlnlnlnu . I =to (urn rlN N ! I I I I I I I wma mma mmc ? YU a l n E s i N I I I I I I I I ' FI ...... , '-. '1'99 m mpr 0W r l r l r i N - Y? w m lnW r l r l N 99'9 00- -+ In00 rlrld N ? A =r C Id k . I 4 c1 ! - 9 2 *f ICm N 9 * . 10 Some HoIII levels a r e a lso knownJ8 but the 41 :5/2 - 4113/2 separation in CaFz (- 5100 cm-’) is the only divaient c rys ta l information available9; i t is 5438 cm-l in the f r ee ion. The 4fn-’ bands a r e so broad in crystals that it i s not easy to make the E. Loh” has measured the absorption i n correlation with the f r ee ion levels. the ultraviolet of CaFz doped with trivalent r a r e ear ths and finds relatively narrow bands for Ce3’ and Pr3’ at 32,5CiJ and 45y 600 cm-’ respectively. The corresponding lowest f r ee ion levels a r e at P97373 arid 61,17!4y5 giving a shift of about 16, 000 cm-’. The lowest absorption band for Er3’ reported by Loh is a t 64,200 cm-’; the lowest f r ee ion level is actually lower: 52,48: cm-l. sextet and transitions to the ground state levels f rom which transitions of consequence occur begin at 68,352 cm about 4150 cm-’ higher tnan the GaFz position. The right side elemer,ts present more of a problem. However, this is a a r e not observed. The lowest -1 “1 5 /2 , only If one keeps the rule that the spin seiection rule is obeyed, the resul ts for the divalent ions, however, a r e reasonably consistent, as shown in Table IV. (All energies a r e in c m ” ) TABLE IV Comparison of Divalent Crystal 5d Bands with F r e e Ion Levels Tree Ion R2+ c r y s tal9 Permitted Lowest Ce -- 3277 3277’’ Pr 3000 i2847 12847’2 Sm 14109 278&6 26990’ Ho 10800 25699 18033* Yb 28000 39721 3338613 -1 The average difference between columns 2 and 3 is about 12,000 cm . 11 ' . . 111. HIGH RESOLUTION CRYSTAL STUDIES H o in Lac13 Ho3+ in LaC13, but in the same crystal some extremely broad ones also occur, as shown in Figure 314. charge indicate a resolving power in the blue of about 600, 00015, o r .04 cm-'. Figure 4 (top curve) shows a densitometer trace of the hyperfine structure of the Jl line, which is a t least a s sharp a s the instrument can resolve. A similar tes t on the D, line i n the red, where the resolution is closer to the theoretical limit of . 0 2 4 cm-', shows an estimated half width of .03 cm-'. Some of the sharpest lines yet observed have been in . 1 percent I Tests on the five-meter spectrograph with a mercury dis- I Two of the hyperfine components of the J1 line show an unsymmetric broadening a t zero magnetic field which Dieke took a s an indication of the presence of the hitherto missing M = 0 line to which he gave the label J014. field is applied, this coincidence no longer occurs and all lines become equally sharp. However, a t a field of 11,600 gauss, the pattern becomes complicated again, as shown in the lower curve of Figure 4. sharp a t this field, this cannot be an effect of the ground state, but must be that of an interference in the A levels. I If a magnetic As other spectrum lines remain excited This has not yet been cleared up. Line Broadening in R. E. /La Mixtures When a r a r e earth is mixed into a Lac13 lattice in very low concentrations, the spectrum lines a r e generally sharp, but a s the concentration i s increased, the constitution of the surrounding ions becomes statistically uncertain, and a s the ion s izes differ, the resulting crystal pertur- bation will vary. Figure 5 shows such a line of Nd3'.l6 Fo,~ 1pO percent NdC13 the lines again a r e sharp, but other effects appear (see below). dependence of the Pr3' spectrum in mixtures with LaC13. studies of line width show that the lines have a Gaussian shape rather than Lorentzian and E. S. Dorman has developed a statistical theory which can account for this." The pure PrC13 crystals also show extensive pair spectra. Transit ion probabilities have also been determingd by numerical integration of high resolution photoelectric absorption curves a This will be greatest a t about 50 percent concentration. Figure 6 shows the Concentration Careful high resolution 18 n 17 12 Vibronic Side Bands of more or less diffuse lines a s the concentration is increased. Figure 6 also shows the development of an extensive se r i e s A similar effect i s seen for pure NdC13 in Figure 7. the strong electronic r a r e earth spectral lines give information on the phonon These vibronic side bands which accompany I I I I I 1 frequencies of the crystal and their interactions with different electronic levels I of the r a r e earth ion. Because of the relatively high symmetry of the Lac13 type lattices (p63/m) and the low number of atoms per unit cell ( 8 ) , this is a particu- larly attractive system to study. 19' 'O In trichloride lattices it is found that I i I j ! Nd and Pr give relatively strong vibronics. i I 34- 3+ The system chosen for discussion here is 1 percent Pr3' in NdC13, wherein t i t is possible to compare impurity vibronic frequencies with those of the main constituent. The transitions from 4 I of Nd" and 'H4 ( p f 2 ) to 3 P ~ and 3 P 2 of Pr3' were studied in absorption at 4.2 K using magnetic fields up to 30,000 gauss. and shows the same Zeeman splitting a s the main line. ranging from 1 to 10 cm-'. I , 3/2 9/2 'D and 'P ( p = f 5/2) to 'PlI2, 5/2 0 The vibronic spectrum is polarized The lines a r e quite narrow Figure 7 shows the 'I ( p = f 5/2) to 'P transition 9 /2 1 /2 of NdC13, Of the vibronic lines which a re common to both Nd3' and Pr3', there are no observable shifts in the vibronic frequencies to within P em-', indicating both the Nd" and Pr3' ions a r e perturbed by phonons of the same frequency. , Table V shows the phonon frequencies obtained from the vibronics accompanying the electronic transition 4 I (cc= f 5/2) to 'P of NdC13 and 3H4 (p= f 2 ) to . 3 P ~ of 1 percent Pr3' in NdCls.'l All measurements a r e a t zero magnetic field and have an accuracy of better than one cm 9/2 1 /2 -1 . The frequencies of the observed Raman active phonons a r e also given.'I The number of frequencies listed is much larger than that predicted by assuming interaction only with k = 0 phonons (16). points of high symmetry in the Brillonin zone must be considered in a further 3+ analysis. The agreement of the phonon frequency of many of the Pr3+ and Nd vibronics gives a basis for using a single ion analysis for Nd3'. Using this ap- proximation, the polarization selection rules, for Nd3+ ,were derived and are being used for analyzing the spectra. 4 Both multiphonon processes and other J 15 n ti N v 2 3 I= a b a d s % r i m 0 0 0 0 0 0 ? " 1 ? ? a a r t r i v m v m 7 : 9 9 , s 9 o o o o q o cr) o m - 9 0 0 * $ I 0 0 a3 0 $I 9 N m 0 co 0 $I m m 0 9 12 0 -H CD 0 9 0: N 0 $I 0 l- a 0 9 N 0 $I 9 2 0 cv 0 $I co 0 9 ? N 0 4i N 0 9 "1 m' cv 9 9 0 $I m 12 a 0 N 0 -H N 0 9 9 N 0 $I CD l- a 0 9 -3 0 -ti ti F a 0 9 0 $I co u) a 0 ? 0 $I u) 0 9 N 0 $I N 0 9 '0. P I 0 $I d F a 0 9 9 0 $I N l- a 0 9 * 0 $I 9 m m a 0 v) 0 4i 9 Ln "1 0 N 0 $I a) 0 9 00. . . . . !- i ! i I , I 16 - n NF U w a w a ? 4 0 ul .4 0 I= a b a N W O N m 0 0 0 0 0 9 . 9 ; 9 9 , a O N r n 0 0 0 0 9 ; 9 9 W m N N 0 0 0 0 9 9 9 9 r l m q 4 0 0 v) * * e 0 0 0 0 17 success . 0 Crystals of GdC13 with 1 percent E r and Tb have been studied above and below the Curie temperature (2.2OK) at fields of up to 36 kilogauss in order to measure the internal magnetic fields for various conditionsz5. temperatures, the line splitting is consistent with a permeability and magnetization given by the expressions A t higher , u = 1 + 4 n M / H z z Z M = . l 9 2 5 B 1 (H t 16.21 M ) p g / k T ] Z 7/2 z z for magnetic fields parallel to the crystal axis. function B (x) is the Brillouin 7 /2 B (x) = 4 coth (4x) - 1/2 coth (x/2) 7/2 20 Charge Compensation of Rare Earth Ions ions in alkaline earth fluorides is complicated by the fact that the trivalent ion replaces a M2' ion. the local environment of some of the r a r e earth ions, modifying the si te sym- metry. In type I1 CaF2 (grown in an excess of fluorine), the predominate s i te symmetr ies a r e cubic, tetragonal and trigonal. measurements on type II CaFz:Er3+ indicate a relative abundance of l5:20:l, respectively. ac l o o > direction. The spectra of trivalent r a r e earth The charge compensation defects introduced influence Electron paramagnetic resonance The tetragonal site i s thought to be due to an intersti t ial F- along The spectra due to a l l of their si tes a r e superimposed, s o that i t is not possible to say without further investigation which spectral line a r i s e s f rom which site. (This, of cours'e, must be decided before any further analysis can .be made) . Also, due to the existence of equivalent but magnetically distinguishable s i tes , there is no zero field polarization to aid in the interpretation of the spectra. The problem of paramagnetic trivalent r a r e earth ions in nonequivalent s i tes has been studied by means of epr techniques. splitting factor a s the magnetic field i s rotated about a given axis, i t is possible to deduce the si te symmetry of the ion. C. W. Rector, using the 21-foot Paschen magnetic fields up to 38,000 gauss, has been able to develop a s imilar technique f o r the optical spectra. 2 7 D 2 8 photographed every 5 . with rotational angle of the Zeeman effect, composite for type I1 CaF2:Nd with an axis along the [ 100 J dire'ction; the left hand side is due to a trigonal site with a [ 11 1 ] direction axis. The angular variations a r e quite different. Note particularly the 70 - 110 split of the turning points in the trigonal case com- pared with the 90" - 90 split of the tetragonal coalescence points. tional patterns, i t is possible to assign representations to the levels. F rom the variation of the The crystals a r e rotated and the Zeeman spectra 0 A composite is then assembled showing the variation Figure 12 shows a tracing of such a The right hand side is due to a tetragonal si te 3+ . 1 0 0 0 split in the tetragonal case. Also, note the 70° - l l O o Using selection rules and the rota- At present , Pr3', Nd3: 21 varying degrees. si tes. Most of the lines studied in type I1 CaF2 indicate tetragonal It is interesting to note that non-Kramers sal ts can also be studied. At sufficiently large magnetic fields, a c rys ta l Paschen-Bach effect occurs, decoupling cer ta in pairs of levels from the component of the c rys ta l field splitting them. These pseudo-Kramers' doublets then show a rotational variation much like that of a Kramers ' doublet. value, such effects a r e particularly striking in Ho3', as shown in Figure 13. A related problem is that of putting a divalent ion, Eu2', i n a Because of the high J trivalent lattice, LaCl?'. to the d levels a r e observed, no sharp lines appear. indicate hexagonal symmetry. light, darkening due to color centers appears. produced a t room or lower temperatures by a source such as a mercury pen lamp and removed by mild heating. with the presence of EuZS since they do not appear i n other doped Lac13 crystals . 'Fo to 5D2, p= 2 , absorption line of Eu3' as the c rys ta l becomes more colored, indicating a relationship between the ionization of E u i and the production of the color centers (Figure 14) . Although broad bands corresponding to transitions Rotational epr studies When the c rys ta l s a r e exposed to ultraviolet The color centers can be The color centers a r e clearly connected Plates taken on the 21-foot Paschen show the appearance of the 2+ Four absorption bands have been measured, two in 'IT and two in o polarization with respect to the optic axis of the c rys ta l (Figure 15) . The severa l bands keep a constant ra t io as the c rys ta l i s bleached indicating a single type of center. Fur ther , the polarization of the bands with respect to the C3 axis indicates an unusual symmetry for the electron trap. IV. CRYSTAL FIELD CALCULATIONS 22 Erbium-Doped Crystals of the Er3' crystals for which the energy levels a r e given in Table 11. a r e given in Table VIII. and Varsanyi3', except that J-mixing has been taken into account. Crystal field calculations have been made for several These The LaC13 values a r e similar to those found by Dieke The two se ts of garnet parameters a lso include J-mixing effects. A comparison of the computed and experimental levels fo r YGaG case is given in Figure 16. The parameter adjustment was made using the Z and Y groups only; the A group correlations a r e independent. YAG and LuA garnets but so f a r the agreement is not nearly so good. details can be found in Dieke' s monograph31. Experimental data a lso exists for the Further Figure 17 shows some resul ts of Rakestraw' s3', in which a c rys ta l f ield of very low symmetry, in this case Cz, can accidently be approximated by a field of much higher symmetry, the octahedral field parameters for which a r e given in the last column of Table VIII. TABLE VI11 3+ Crystal Field Parameters for Er K Q GdC13 LaC13 LaBr3 YGaG LuGaG Y C13 2 0 112 96 117 -14 32 2 2 89 58 4 0 -45 .5 -34.9 -39.6 -238 -245 175 4 2 255 175 4 4 920 948 t 8 7 5 6 0 -25 .0 -25 .6 -19.2 33 .0 338 10.4 - 6 2 58 -83 6 4 645 664 -2 1 8 . 4 6 6 246 2 49 -70 -48 2 12 25 I . Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7 . Figure 8. Figure 9. FIGURE CAPTIONS Five-meter spectrograph with plane grating predisperser in a typical setup for metal spark spectra. F r e e ion spark spectra of erbium at various peak currents, taken on 5-meter plane grating spectrograph in 26th to 33rd order . Equivalent first order wavelengths are indicated below. Different types of HoJt absorption lines. (1-4) sharp D3' line, and D1, D5 and J3 hyperfine lines, all on 5-meter; (5-10) J3, J5', J5, Jg, I1 and &, all on Paschen. Densitometer t races of hyperfine structure of 4180 A Ho3' J1 line in LaC13. Top: zero field; bottom: short wavelength set of Zeeman components with 11,600 gauss field. Effect on concentration on line width of Nd" C group in Nd/La mixtures . Effect of concentration on Pr3+ 4883 A 3 P o absorption line for Pr/La mixtures. The concentrations are, from top to bottom, 0.25$, 2 % 15$ , 50$, 85$, and 100%. The effect. Some of the strong phonon lines a r e also over-absorbed. C Group of Nd3' in Lac13 and pure Nd crystals. structure is due to Nd-ion exchange interactions. ( p = t 5/2) to 'P transition of NdCl3 and its Zeeman 1 /2 - The electronic line a t the right i s highly over-absorbed. The triple ' Parallel Zeeman splitting of I1 (n) in NdBr3. 1.5 K. (30 cm-' ) wide. Bath temperature is 0 Region shown is centered at A = 4241 1% and is 5 vac Figure 10. Para l le l Zeeman effect of Gd3* A group near 31 15 1% in pure GdCl3 at 4.2OK (n polarization). Figure 11. Zeeman effect of 9683 1 Yb 3+ line in yttrium aluminum garnet showing superimposed patterns from magnetically inequivalent si tes. Figure 12. Composite showing the variation of Zeeman absorption spectra of type I1 CaFz:Nd3’ for rotation about the [ 1101 axis. The ternpera- ture is near 4 .2 K. 0 Figure 14. Absorption spectrum of a LaCl3:Eu ” crystal at 4650 1 in0 polari- zation. a. No prior irradiation, 77OK. b. 5-minute irradiation prior to exposure at 77 K. c. 120-minute irradiation prior to exposure a t 77 K. , d. Crystal deeply colored prior to exposure a t 4.2 K. 0 0 0 e. LaC13 crystal grown to contain predominantly Eu 3’ . Exposure at 4.2OK. 0 Figure 15. Absorption spectra a t 77 K in LaC13:Eu” crystals. induced by ultraviolet radiation. Color centers Figure 16. Observed and calculated Stark shifts of three lowest groups of Er3’ in yttrium gallium garnet. Figure 17. Observed and calculated Stark shifts of four lowest groups of Er3’ in anhydrous yttrium chloride (octahedral approximation). Figure 18. The ’E 4 ’Tzg absorption band a t 3840 A in n polarization as a function of temperature, 26 27 REFERENCES .. 1. 2. W. J. Car te r , Ph. D. Thesis, Johns Hopkins University (1966) K. H. Hellwege and S. Hiifner, Fifth R a r e Ear th Research Conference, Amee, Iowa (1965) R. Lang, Canada Journ. Res. A13, l ( 1 9 3 5 ) ; A14, 127 (1936) 3. 4. J. Sugar, J. Opt. SOC. Am. 55, 1058 (1965) 5. - - - H. M. Crosswhite, G. H. Dieke, and W. J. Car te r , J. Chem. Phye. 43, 2047 (1965) D. J. G. Irwin, unpublished A. Dupont, Ph. D. Thesis, Johns Hopkins University (1966) J. H. McElaney, Ph. D. Thesis, Johns Hopkins University (1964) D. S. McClure and Z. Kiss, J. Chem. Phys. - 39, 3251 (1963) E. Loh, Phys. Rev. 147, 332 (1966) J. Sugar, J. Opt. SOC. Am. - 55, 33 (1965) J. Sugar, J. Opt. SOC. Am. - 53, 831 (1963) - 6. 7. 8. 9. 10. 11. 12. - 13. 14. 15. 16. 17. 18. * 19. 20. 21. 22. 23. 24. 25. 26. B., W. Bryant, J. Opt. SOC. Am. - 55, 771 (1965) G. H. Dieke and B. Pandey, J. Chem. Phys. - 41, 1952 (1964) G. H. Dieke and D. F. Heath, Japan J. Appl. Phys. - 4, Suppl. 1, 455 (1965) A. H. Piksis , unpublished E. Dorman, J. Chem. Phys. _. 44, 2910 (1966) G. H. Dieke and E. Dorman, Phys. Rev. Le t te rs - 11, 17 (1963) I. Richman, R. A. Satten and E. Y. Wong, J. Chem. Phys. _. 39, 1833 (1963) R. A. Satten, J. Chem. Phys. 40, - 1200 (1964) E. Cohen, unpublished J. T. Hougen and S. Singh, Proc. Roy. SOC. - A277, 193 (1964) G. A. Pr ina , Physics Let te rs - 20, 323 (1966); Phys. Rev.'(accepted for , publica tion ) J. S. Marsh, Ph. D. Thesis, Johns Hopkins University (1966); Physics L e t t e r s 20, 355 (1966) D. J. Randazao, Ph. D. Thesis, Johns Hopkins University (1966) W. P. Wolf, M. Ball, M. T. Hutchinge, M. J. M. L e a e k a n d A . F. G. Wyatt , J. Phye. Sac, Japan - 17, Suppl. B-1, 443 (1962) -.. ,- * - I , . . 1 I 1 1 1 1 1 - --"- Le-. I = - _ - -- . k- I I-- 3 - 1c t - - I -- I I 1 1 1 1 1 1 0 0 In c\1 a 0 E 0 0 0 - I E 0 F I. I Ib . I % HO,+ in LOCI , JI tine 23918 cm-' I a2 261 zai 2st at 261 a2 SIOPN , b Q ' 3 0 (3 L 0 ' ' I . + fo n 7 , . ' / b . , I . . 36 32 28 '24 20 8 4 Figure 9. Parallel Zeeman s litting of 11 (n) in NdBrj. temperature i s 1.5 K. Region shown is centered at h a c = 4241%. and is 51 (30 cm") wide. Bath 8 . . 5 2 2 7 1 Line of E group - [ l lO]- 145' -[111]-110° - [112] - 90' - [OOIJ- 55. 6298 A Lino of c group i i i ' i i i :i 1 . f I - 0 f ld - I Tr igon o D A- t o t r agonal Figure 12. Composite showing the variation of eeman ab- rorption r ectra of type I1 CaF2: Nd' 4 for rotation abogt the pilo] axir. The temperature ir near 4.2 K and the field ir 36,650 gaurs. . - . Ca 5 :Ho3+ F group (5F3) 35,130 gauss -[ I I T ] -[ I I O ] - [ I I I ] -[ 1 1 2 1 - [TTZ ] - [ T T I ] -0 f ld '. L 1 \ . . ... A C E I . i 4 B 4 I,,, F 54 3 2 L Y , 4 I,,&? f - 4 - 32 - = - E II c , It6 1.4 ItC ,6 I I 1 I / I I \ \ \ 100.K \ I 1 I \ I \ 1 3400 3800 4200