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Magnetic field effect on ion-pair transfer in ruby. Cross relaxations and resonant transfer
768
J ourii
I!
I UI11Il~SLCI~ I & (I 984) 768 770 5.trtsterdarn Nortl IoIIind
MAGNETIC FIELD EFFECT ON ION—PAIR TRANSFER IN RUBY. CROSS RELAXATIONS AND RESONANT TRANSFER M.MONTAGNA, L.GONZO, O.PILLA, G.VILIANI Dipartimento di Fisica, Universita di Trento, 38050 Povo, Trento, Italy and Gruppo Nazionale di Struttura della Materia E.DUVAL, A.MONTEIL Groupe de Spectroscopie des Solides, U.A.442 du CNRS, Université Claude Bernard, Lyon I, 69622 Villeurbanne, France The ion-pair transfer in ruby was studied under magnetic field. Nl and N2 transferred intensities behave differently. For N2 quasi-degeneracy seems to be very important in enhancing the zero—field transfer rate. Ruby has been proposed as a possible candidate for finding experimental evi dence of Anderson localisation by observation of a mobility edge through the inhomogeneous Ri line. Since the direct experimental observation of ion ion transfer is rather complex, Koo et al .~ and Chu et al ~2 utilised the emission from 4th neighbour pairs (N2 line) as a measure of the ion-ion transfer rate. In fact, localisation of the excitation in the ion system would result in a sharp decrease of the ion—pair transfer. The existence or not of Anderson loca— lisation in ruby is still an open question; it seems therefore important to stu dy the mechanisms which lead to ion-pair transfer, partly in order to ascertain if and how the latter is connected to the ion—ion transfer. The application of an external magnetic field is expected to affect strongly the transfer both by reducing the homogeneous widths and by splitting the levels. In Figure 1 the transferred component of the Nl and N2 lines for a 1800 ppm ruby sample are shown as a function of the field which was applied parallel to 0 the C 3 axis of the crystal . The excitation was by the 5145 A line of an Argon laser, 4Twhich excites at the same time both the4Asingle ions and the pairs in the 4T broad
2 band of single ions and the broad ( 2, 2) band of the pairs; since
the directly excited pair emission deacays with a time constant which is about 10 times smaller than transferred component beam and by detecting As can be seen, Nl
that of the transferred emission,the separation of the of the pair emission was obtained by chopping the laser only the slow component of the emission. and N2 behave quite differently: Nl shows two main peaks
on a roughly constant background, while N? has several peaks which are superimposed on a slowly varying background; moreover, a faster descent is exhibited 0022 2313/84/$03.000 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M Montagna eta!. / Magnetic field eff~cton ion-pair transfer in ruby
769
(a)
-~
0123456 H (TESLA) FIGURE 1
Transferred intensity as a function of the magnetic field at 4.2K. (a): Ni; (b): N2 in the first few kilogauss. By increasing the temperature, in Ni the field-independent part increases, while the resonances smooth out; in the case of N2 the steep initial descent and the resonances tend to disappear, the transferred intensity is increased at all 3 field values and the broad descent persists. We performed excitation spectra under magnetic field with different delays after the excitation pulse, and verified that the most intense peaks of figure 1
ENERGY SHIFT (om~1) FIGURE 2 Fast (a) and slow (b) emission lineshape of N2 at 1.8K (see text for details)
770
~l has,tagna
r t at
hugs, Pu field r ffr I
as,
ion iiair trails (I
I ii, its/ri
correspond to resonances between transitions of single ions and pairs; in particular, in the case of N2 the pair states which are involved are the G and H le 3’4 In this framework the initial descent of N2 could be attributed to the vels. I level which at zero field is practically resonant with the T level of single ions. In figure 2 we report two emission spectra of N2 at 1.8K and at zero field, taken with different delays after the beginning of a 0.5 as excitation pulse: (a): fast (0-1 ms); (b): slow (5-40 ns). The quadruplet structure5 is almost resolved; in the slow emission shape (deriving from transfer) the components of the N2 line due to transitions from the a 5 0 excited sublevel are enhanced. This selective population further indicates that the pair acceptor level is the I level (m5 0), since relaxation between excited states with no n15 change is in ge neral more probable. The observation of this spin-memory effect is made possible by the long spin-lattice relaxation time in the level from which N? originates; as temperature is increased from 1 .8K the effect decreases and above about 10K it is no longer observed. We note also that the width of the transferred emission is slightly but significantly larger than the direct one, which suggests that distorted pairs are preferential acceptors. The above analysis of the data reported in figures 1 and 2
shows that for
N?, the zero-field transfer is, to a large extent, resonant. As regards Ni, no excited levels exist which are so close to ~ as to give resonant contribution to the transfer at zero field; this explains the different behaviour of the two lines at low magnetic fields. REFERENCES 1) J.Koo, L.R. Walker and S. Geschwind, Phys. Rev. Lett. 35 (1975) 1669. 2) 5. Chu, H.M. Gibbs and A. Passner, Phys. Rev. B24 (1981) 7162. 3) M. Ferrari, L. Gonzo, M. Montagna, 0. Pilla and G. Viliani, in: Energy Transfer Processes in Condensed Matter, ed. B. Di Bartolo (Plenum, New York) in print. 4) P. Kisliuk, NC. Chang, P.L. Scott and M.H.L. Pryce, Phys. Rev. 184 (1969) 367.
5) M.d. Berggren, G.E. Imbusch and P.L. Scott, Phys. Rev. 188 (1969) 675.
J ourii
I!
I UI11Il~SLCI~ I & (I 984) 768 770 5.trtsterdarn Nortl IoIIind
MAGNETIC FIELD EFFECT ON ION—PAIR TRANSFER IN RUBY. CROSS RELAXATIONS AND RESONANT TRANSFER M.MONTAGNA, L.GONZO, O.PILLA, G.VILIANI Dipartimento di Fisica, Universita di Trento, 38050 Povo, Trento, Italy and Gruppo Nazionale di Struttura della Materia E.DUVAL, A.MONTEIL Groupe de Spectroscopie des Solides, U.A.442 du CNRS, Université Claude Bernard, Lyon I, 69622 Villeurbanne, France The ion-pair transfer in ruby was studied under magnetic field. Nl and N2 transferred intensities behave differently. For N2 quasi-degeneracy seems to be very important in enhancing the zero—field transfer rate. Ruby has been proposed as a possible candidate for finding experimental evi dence of Anderson localisation by observation of a mobility edge through the inhomogeneous Ri line. Since the direct experimental observation of ion ion transfer is rather complex, Koo et al .~ and Chu et al ~2 utilised the emission from 4th neighbour pairs (N2 line) as a measure of the ion-ion transfer rate. In fact, localisation of the excitation in the ion system would result in a sharp decrease of the ion—pair transfer. The existence or not of Anderson loca— lisation in ruby is still an open question; it seems therefore important to stu dy the mechanisms which lead to ion-pair transfer, partly in order to ascertain if and how the latter is connected to the ion—ion transfer. The application of an external magnetic field is expected to affect strongly the transfer both by reducing the homogeneous widths and by splitting the levels. In Figure 1 the transferred component of the Nl and N2 lines for a 1800 ppm ruby sample are shown as a function of the field which was applied parallel to 0 the C 3 axis of the crystal . The excitation was by the 5145 A line of an Argon laser, 4Twhich excites at the same time both the4Asingle ions and the pairs in the 4T broad
2 band of single ions and the broad ( 2, 2) band of the pairs; since
the directly excited pair emission deacays with a time constant which is about 10 times smaller than transferred component beam and by detecting As can be seen, Nl
that of the transferred emission,the separation of the of the pair emission was obtained by chopping the laser only the slow component of the emission. and N2 behave quite differently: Nl shows two main peaks
on a roughly constant background, while N? has several peaks which are superimposed on a slowly varying background; moreover, a faster descent is exhibited 0022 2313/84/$03.000 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M Montagna eta!. / Magnetic field eff~cton ion-pair transfer in ruby
769
(a)
-~
0123456 H (TESLA) FIGURE 1
Transferred intensity as a function of the magnetic field at 4.2K. (a): Ni; (b): N2 in the first few kilogauss. By increasing the temperature, in Ni the field-independent part increases, while the resonances smooth out; in the case of N2 the steep initial descent and the resonances tend to disappear, the transferred intensity is increased at all 3 field values and the broad descent persists. We performed excitation spectra under magnetic field with different delays after the excitation pulse, and verified that the most intense peaks of figure 1
ENERGY SHIFT (om~1) FIGURE 2 Fast (a) and slow (b) emission lineshape of N2 at 1.8K (see text for details)
770
~l has,tagna
r t at
hugs, Pu field r ffr I
as,
ion iiair trails (I
I ii, its/ri
correspond to resonances between transitions of single ions and pairs; in particular, in the case of N2 the pair states which are involved are the G and H le 3’4 In this framework the initial descent of N2 could be attributed to the vels. I level which at zero field is practically resonant with the T level of single ions. In figure 2 we report two emission spectra of N2 at 1.8K and at zero field, taken with different delays after the beginning of a 0.5 as excitation pulse: (a): fast (0-1 ms); (b): slow (5-40 ns). The quadruplet structure5 is almost resolved; in the slow emission shape (deriving from transfer) the components of the N2 line due to transitions from the a 5 0 excited sublevel are enhanced. This selective population further indicates that the pair acceptor level is the I level (m5 0), since relaxation between excited states with no n15 change is in ge neral more probable. The observation of this spin-memory effect is made possible by the long spin-lattice relaxation time in the level from which N? originates; as temperature is increased from 1 .8K the effect decreases and above about 10K it is no longer observed. We note also that the width of the transferred emission is slightly but significantly larger than the direct one, which suggests that distorted pairs are preferential acceptors. The above analysis of the data reported in figures 1 and 2
shows that for
N?, the zero-field transfer is, to a large extent, resonant. As regards Ni, no excited levels exist which are so close to ~ as to give resonant contribution to the transfer at zero field; this explains the different behaviour of the two lines at low magnetic fields. REFERENCES 1) J.Koo, L.R. Walker and S. Geschwind, Phys. Rev. Lett. 35 (1975) 1669. 2) 5. Chu, H.M. Gibbs and A. Passner, Phys. Rev. B24 (1981) 7162. 3) M. Ferrari, L. Gonzo, M. Montagna, 0. Pilla and G. Viliani, in: Energy Transfer Processes in Condensed Matter, ed. B. Di Bartolo (Plenum, New York) in print. 4) P. Kisliuk, NC. Chang, P.L. Scott and M.H.L. Pryce, Phys. Rev. 184 (1969) 367.
5) M.d. Berggren, G.E. Imbusch and P.L. Scott, Phys. Rev. 188 (1969) 675.