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Electron spin echoes in photo-excited triplet states of organic molecules in zero magnetic field
Volume
CHEMICAL
14. number 4
PHYSICS
LETTERS
15 .lune 1972
SPIN ECHOES IN PHOTO-EXCITED TRIPLET STATES OF ORGANIC MOLECULES IN ZERO MAGNETIC FIELD
ELECTRON
J. SCHXlIDT
We have observed electron tield. Thus for the first
spin cchws
in the photo-exited
triplet state of orgnic rr~olccul~s in zero nlafnctir: loss in cohcrencc bctwcen photo-cscired
time one is able to IIIL’LISU~C the rate of irreversible
triplet state molecules.
1. Introduction
In previous publications [ 1, 21 we have shown how a strong microwave field, resonant with the 100 1.O MHz zero-field transition of the phosphorescent triplet state of quinoline-d;, in durene-d14, may induce h modulation of the phosphorescence intensity. The frequency of this modulation - the analoyue of the nutstion ot‘ the I-component of the magnetization in a classical magnetic resonance experiment - is equal to yH, (-f is the magneto-yyric ratio and H, the amplitude of the microwave field). This modulation is a manifestation of the coherent interaction between the photo-excited triplet state and the resonant microwave field as predicted by Harris [3] . When thinking about the possibility of an electron spin-echo e.uperiment in a triplet state in Zero magnetic field one is faced with the apparent paradox that no magnetic moment is associated with any of the spin statzs. However, lvith a model given by Feynman et al. [4] one can show that the behaviour of a pair of leveis when connected by coherent, resonant radiation can be described by an equation analogous to that desoribing the precession of a gyromagnet in a magnetic field. It is then relatively simple to show that the application of a strong pulsed microwave field resonant with the transition between two of the spin states results in a time dependent linear combination of the two states that causes a magnetic moment to appear [ 1, 31. This magnetic moment is of a transient nature becatise
of the loss of phase coherence in the ensemble. The formation of the echo then proceeds in the same way as in magnetic resonance: a second microwave pulse restores the lost phase coherence and the transient magnetic moment reappears which generates a pulse of microwave power at the resonance frequency. The experiment thus is somewhat similar to that by Bloom et al. in which they observed echoes of the nuclear quadrupole transitions of the 35C1 and 81Br / =4 nuclei in Zero field [5]. In this note we present the first results of such an electron spin-echo esperiment on the photo-exited triplet state ofquinoline-1z7 and -t2’, in durene-/zlG and il,, in ~9-0 twgfzeric jkki. With the help of the spinecho phenomenon we have been able to measure the “genuine” relasation time Z’, associated with the irreversible loss in coherencehet~veen the triplet spin systems. It will appear that this memory time is strongly dependent on the nuclear spins in the crystal. If the following sections we first describe the quinoline-durene system and the experimental arrangement. Then we give the preliminary results of some T2 measurements and finally discuss some characteristic features of this type of spin-echo experiment. 2. The experiment The systems chosen are quinoline-tl, in durene-cl14, quinoline-d, in durene-/z 14 and quinoline-1z7 in
Volume
14, number 4
I’IF. I. The spin Icwl\ 01’the phosphorescent
lSJune1972
CHEMICAL PHYSICS LETTERS
triplet state of
durcnc-lr I4. By absorption of the pumping, light ~nolcculcs are raised into the first excited singlet state, irw which they rapidly cross over to the metastable tripict state by ;f radiationless process (see Eg. 1). Bccsuse of the spin szlcctivity of this intersystem crossing, the T, level carries a higher population than the T_,, level when the system is being pumped. (For instance, for quinoline-d7 in durene-d14 under continuous illumination at l.?‘K, where relaxation between the triplet spin leveis is slow, we have found A-_:!\;,, = 1 :O.GS [2] .) The energies of the three spin levels of quinoline-d, are also indicated in fig. 1. In the other in dureneil14 two systems the energy differences of the spin levels are the same to within a few MHz [2]. We have studied the electron spin echoes produced by a pulsed microwave field resonant with ihe transition between the T2 and TY levels. In the experiment the crystal is placed in a tunable re-entrant cavity immersed in liquid helium at a temperature of 12°K. The crystal, mounted against a quartz light pipe, is irradiated through ;I hole in the bottom of the cavity. Two microwave pulses of equal duration and a peak power of about 1 W are generated by an assembly consisting of a HP 869 1 I3 microwave oscillator, a Varian VZi-6941 A 1 travelling wave tube amplifier and a HP 873 1 B PIN diode modulator. These high power pulses are fed through a circulator
f;iua. 2. The electron spin-echo sipnal of quino1inc-d~ in durene-d,q produced by 2 microwave fie!d resonant with the TZ-T), transition at 1001.0 XlHz. Continuous illumination nt T= l.?‘K. Horizontal 2 p.wc per division. Pulse length 0.4 JJSCC.The upper trace represents the results of an esperimcnt with the csciting light shut off. to the cavity.
The echo
signal
passes
from
the cavity
to a ba!anced mixer and is further amplified by a super heterodyne system with an intermediate frequency of 30 MHz and a bandwidth of 8 MHz. In order to minimize the recovery time of the system after overloading by the two driving pulses, the Q of the cavity has been reduced to 1750 by introducing a microwave absorber into a region of high electric field. In this way a recovery time of about 1 ,mec is obtained. The echo sign4 resulting from two microwave pulses of equal duration rp is largest for -yHl rp = 120” [6]. In our case the corresponding pulse lengt!l r = P
0.4 psec
and the amplitude
N,
= 0.3 G.
The electron spin-echo signal of quinoline-d7 in durenedi14 is shown in the lower trace of fig. 2; it results from two 120’ pulses resonant with the TZ-Ty transition at 100 1 .O MHz. The tails behind the two driving pulses are caused by the free decay of the induced magnetic moment. The echo is formed at a time after the second pulse equal to the interval between the two driving pulses. Its structure is due IO the splitting of the T,--T.,. zero-Geld transition into two components. The upper trace of fig. 2 shows the result of a similar experiment with the exciting light shut off. It can be seen that during thz microwave pulses the receiver is completely overloaded and recovers in about 1 ysec. In fig. 3 we present the results of two echo experi-
Voiume 14, number 4
CHEMICAL PHYSKS LETTERS
15 June 1972
Boltzmann distribution. Thus in the system quinnlineat 42°K we again found T, = 38 @sec. d, in durened,,
3. Discussion The application of the Feynman et al. model lo the electron spin-who phenomenon in zero fkld has been presented in our previous papers [ 1I 2f and here we shall merely outline a few aspects pertinent to the present esperiments, The detection of the pulse of microwave power
Fig. 3. Electron spin-echo signals of quinolin~7 in durene-tile at 1001.0 MHz. Horizontal S ~STCper division_ Pulse length 0.4 psec, ?- = l.YK.
men& with different time intervals between the driving pulses. The decrease in signal height with delay time reflects the irreversible loss in coherence between the triplet spins. By plotting the echo height as a function of delay time on a logarithmic scale we find an rrlmost straight line. The resulting decay time for quinolined, in dureneit 14 is T, = 38 2 2 ,mx. This particular experiment was done at the maximum corlcentration obtainable of about 5 X lOI photo-excited triplet molecules per crn3. We have measured T, also at reduced concentrations but did not find a &ange in the
memory time. In the other two systems
the echo decay times are noticeably shorter. In quinolineli7 in durene-it 14 at 12°K we find T1 = 13.1 f I ,usec and in quinoline-lr7 in durene-Al4 Tz = 2.5 2 0.2 pseo. hloreover in the
latter two cases we did not observe a simple exponential decay. The si&nals have a tendency to fall off more rapidly with increasing delay times. We have been able to measure T, also at higher temperatures where relaxation is fast as compared with the decay rates of the individual spin levels and where a Boltzmann distribution exists in steady state. This experiment has been done by using ~1flash with a duration of2 msec as the excitation source. Then,
immediately after this flash a large population difference exists due to the selective populating of the top level TZ [2}. By performing the T2 measurement quickly one takes advantage of this large population difference before spin-lattice rekation establishes a
generated by the magnetic moment during the echo, at First sight, may appear a diflkutt experimental problem. First the concentration of p~~~~to-e~cited triplet state molecules is tow and second the sensitivity of a spectrometer that has to operate at 1000 MHz compares unfavourably with spectrometers at the usual X-band frequency of IO 000 MHz. However two important advantages exist in our GXX: (I) In zero field the line-width AY of the zero-field transition is very small because hyperfine interaction is a second order effect [7, S] _This makes it relatively easy to fuIfi1 the condition YHi > lnhv and one “hits” aII moIccu!es by the microwave pulses. (2) As mentioned already in the preceding section continuous iliumination at 1.2”K usually leads to large differences in the populations of the spin states. This spin alignment is caust?d by the very selective populating and depopulating processes and the slow spin-
lattice relaxation in the triplet states of these molecules. The population differences tend to be even larger if one excites with a tlash. Then during a short time a large alignment exists before relaxation establishes a steady state situation. These two advantages more than compensate for
the low concentration
of quinoline triplet molecules
and unfavourable resonance frequency, and make the detection of the echo feasible. Thus, with our spec-
trometer operating at 1000 MHz and 1.2% we are able to detect 5 X 1011 pltotoescitcd quinoline molecules with a signal-to-noise ratio of 1 in a single
shot spin-echo experiment. This sensitivity is in the same order of magnitude as a conventional X-band ESR spectrometer. The experimental results of the T2 measurements clearly show that the nuclear spins predominantly
15 June 1972
CHEMICAL PHYSICS LETTERS
Volume 14, number 4 determine
couragexnent
and interest
in this work.
Further
to
the triplet hpin memory time. The results of section 2 further show that even in quinoline-d7 in durene-d14 (with the longest memory time of ,t.he three systems studied) this T2 is independent of the concentration of excited triplet moIecules. Hence even in this system at the masimum concentration of about
Mr. J. van den Berg who constructed an important part of the experimental arrangement. This work was performed as part of the research programme of the “Stichting voor Fundamenteel Onderzoek der Materie” (F-0-M.) with financial support from the “Nederlandse
5 X lOI triplet spins per cm3, the influence of hutual interaction of photo-excited triplet molecules to T7
Organisatie voor Zuiver Wetenschappelijk (Z.W.O.).
Onderzoek”
is negligible.
we may say that we have proved
Concluding
perimentally
es-
that electron spin-echo experiments
photo-excited
triplet
state molecules
in
irz zero nzugmtic
are feasible. Moreover we have shown that one can measure the phase memory of the triplet mole-
References
field
cules as a function of temperature by taking advantage of the selective populating and depopulating rates in the triplet state. We think that this technique may
prove valuable for studying those processes that destroy the phase relation between photo-excited triplet state molecules, such as energy transfer in molecular crystals.
and spin diffusion
Acknowledgement
The author wishes to express his gratitude to Professor J.H. van der Weals for his continuous en-
-!!-I
[ : ] J. Schmidt, W.G. van Dorp and J.fI. van der Wsals, Chem. Phys. Letters S (1971) 345. (21 J. Schmidt. Thesis. [Jniversity of Leiden (1971); I. Scflmidt, D.A. .4ntheunis and J.H. vun der Waals, Mol. Phys. 22 (1971) 1. [ 31 C.B. Harris, J. Chcm. Phys. 54 (1971) 972. [4] R.1’. Fcynmun. F.L. Vernon and R.W. Hellwarth, J. Appl. Phys. 28 (1957) 49. [5] SI. Bloom, E.L. Hahn and B. Herzog, Phys. Rev. 97 (1955) 1699. [6] W.B. Mims, Rev. Sci. Instr. 36 (196.5) 1472. [ 71 J. Schmidt and J.H. van der Wusls, Chem. Phys. Letters 3 (1969) 546. [S] CA. Hutchison, J.V. Nicholas and G.W. Scott, J. Chem. Phys. 53 (1970) 1906.
CHEMICAL
14. number 4
PHYSICS
LETTERS
15 .lune 1972
SPIN ECHOES IN PHOTO-EXCITED TRIPLET STATES OF ORGANIC MOLECULES IN ZERO MAGNETIC FIELD
ELECTRON
J. SCHXlIDT
We have observed electron tield. Thus for the first
spin cchws
in the photo-exited
triplet state of orgnic rr~olccul~s in zero nlafnctir: loss in cohcrencc bctwcen photo-cscired
time one is able to IIIL’LISU~C the rate of irreversible
triplet state molecules.
1. Introduction
In previous publications [ 1, 21 we have shown how a strong microwave field, resonant with the 100 1.O MHz zero-field transition of the phosphorescent triplet state of quinoline-d;, in durene-d14, may induce h modulation of the phosphorescence intensity. The frequency of this modulation - the analoyue of the nutstion ot‘ the I-component of the magnetization in a classical magnetic resonance experiment - is equal to yH, (-f is the magneto-yyric ratio and H, the amplitude of the microwave field). This modulation is a manifestation of the coherent interaction between the photo-excited triplet state and the resonant microwave field as predicted by Harris [3] . When thinking about the possibility of an electron spin-echo e.uperiment in a triplet state in Zero magnetic field one is faced with the apparent paradox that no magnetic moment is associated with any of the spin statzs. However, lvith a model given by Feynman et al. [4] one can show that the behaviour of a pair of leveis when connected by coherent, resonant radiation can be described by an equation analogous to that desoribing the precession of a gyromagnet in a magnetic field. It is then relatively simple to show that the application of a strong pulsed microwave field resonant with the transition between two of the spin states results in a time dependent linear combination of the two states that causes a magnetic moment to appear [ 1, 31. This magnetic moment is of a transient nature becatise
of the loss of phase coherence in the ensemble. The formation of the echo then proceeds in the same way as in magnetic resonance: a second microwave pulse restores the lost phase coherence and the transient magnetic moment reappears which generates a pulse of microwave power at the resonance frequency. The experiment thus is somewhat similar to that by Bloom et al. in which they observed echoes of the nuclear quadrupole transitions of the 35C1 and 81Br / =4 nuclei in Zero field [5]. In this note we present the first results of such an electron spin-echo esperiment on the photo-exited triplet state ofquinoline-1z7 and -t2’, in durene-/zlG and il,, in ~9-0 twgfzeric jkki. With the help of the spinecho phenomenon we have been able to measure the “genuine” relasation time Z’, associated with the irreversible loss in coherencehet~veen the triplet spin systems. It will appear that this memory time is strongly dependent on the nuclear spins in the crystal. If the following sections we first describe the quinoline-durene system and the experimental arrangement. Then we give the preliminary results of some T2 measurements and finally discuss some characteristic features of this type of spin-echo experiment. 2. The experiment The systems chosen are quinoline-tl, in durene-cl14, quinoline-d, in durene-/z 14 and quinoline-1z7 in
Volume
14, number 4
I’IF. I. The spin Icwl\ 01’the phosphorescent
lSJune1972
CHEMICAL PHYSICS LETTERS
triplet state of
durcnc-lr I4. By absorption of the pumping, light ~nolcculcs are raised into the first excited singlet state, irw which they rapidly cross over to the metastable tripict state by ;f radiationless process (see Eg. 1). Bccsuse of the spin szlcctivity of this intersystem crossing, the T, level carries a higher population than the T_,, level when the system is being pumped. (For instance, for quinoline-d7 in durene-d14 under continuous illumination at l.?‘K, where relaxation between the triplet spin leveis is slow, we have found A-_:!\;,, = 1 :O.GS [2] .) The energies of the three spin levels of quinoline-d, are also indicated in fig. 1. In the other in dureneil14 two systems the energy differences of the spin levels are the same to within a few MHz [2]. We have studied the electron spin echoes produced by a pulsed microwave field resonant with ihe transition between the T2 and TY levels. In the experiment the crystal is placed in a tunable re-entrant cavity immersed in liquid helium at a temperature of 12°K. The crystal, mounted against a quartz light pipe, is irradiated through ;I hole in the bottom of the cavity. Two microwave pulses of equal duration and a peak power of about 1 W are generated by an assembly consisting of a HP 869 1 I3 microwave oscillator, a Varian VZi-6941 A 1 travelling wave tube amplifier and a HP 873 1 B PIN diode modulator. These high power pulses are fed through a circulator
f;iua. 2. The electron spin-echo sipnal of quino1inc-d~ in durene-d,q produced by 2 microwave fie!d resonant with the TZ-T), transition at 1001.0 XlHz. Continuous illumination nt T= l.?‘K. Horizontal 2 p.wc per division. Pulse length 0.4 JJSCC.The upper trace represents the results of an esperimcnt with the csciting light shut off. to the cavity.
The echo
signal
passes
from
the cavity
to a ba!anced mixer and is further amplified by a super heterodyne system with an intermediate frequency of 30 MHz and a bandwidth of 8 MHz. In order to minimize the recovery time of the system after overloading by the two driving pulses, the Q of the cavity has been reduced to 1750 by introducing a microwave absorber into a region of high electric field. In this way a recovery time of about 1 ,mec is obtained. The echo sign4 resulting from two microwave pulses of equal duration rp is largest for -yHl rp = 120” [6]. In our case the corresponding pulse lengt!l r = P
0.4 psec
and the amplitude
N,
= 0.3 G.
The electron spin-echo signal of quinoline-d7 in durenedi14 is shown in the lower trace of fig. 2; it results from two 120’ pulses resonant with the TZ-Ty transition at 100 1 .O MHz. The tails behind the two driving pulses are caused by the free decay of the induced magnetic moment. The echo is formed at a time after the second pulse equal to the interval between the two driving pulses. Its structure is due IO the splitting of the T,--T.,. zero-Geld transition into two components. The upper trace of fig. 2 shows the result of a similar experiment with the exciting light shut off. It can be seen that during thz microwave pulses the receiver is completely overloaded and recovers in about 1 ysec. In fig. 3 we present the results of two echo experi-
Voiume 14, number 4
CHEMICAL PHYSKS LETTERS
15 June 1972
Boltzmann distribution. Thus in the system quinnlineat 42°K we again found T, = 38 @sec. d, in durened,,
3. Discussion The application of the Feynman et al. model lo the electron spin-who phenomenon in zero fkld has been presented in our previous papers [ 1I 2f and here we shall merely outline a few aspects pertinent to the present esperiments, The detection of the pulse of microwave power
Fig. 3. Electron spin-echo signals of quinolin~7 in durene-tile at 1001.0 MHz. Horizontal S ~STCper division_ Pulse length 0.4 psec, ?- = l.YK.
men& with different time intervals between the driving pulses. The decrease in signal height with delay time reflects the irreversible loss in coherence between the triplet spins. By plotting the echo height as a function of delay time on a logarithmic scale we find an rrlmost straight line. The resulting decay time for quinolined, in dureneit 14 is T, = 38 2 2 ,mx. This particular experiment was done at the maximum corlcentration obtainable of about 5 X lOI photo-excited triplet molecules per crn3. We have measured T, also at reduced concentrations but did not find a &ange in the
memory time. In the other two systems
the echo decay times are noticeably shorter. In quinolineli7 in durene-it 14 at 12°K we find T1 = 13.1 f I ,usec and in quinoline-lr7 in durene-Al4 Tz = 2.5 2 0.2 pseo. hloreover in the
latter two cases we did not observe a simple exponential decay. The si&nals have a tendency to fall off more rapidly with increasing delay times. We have been able to measure T, also at higher temperatures where relaxation is fast as compared with the decay rates of the individual spin levels and where a Boltzmann distribution exists in steady state. This experiment has been done by using ~1flash with a duration of2 msec as the excitation source. Then,
immediately after this flash a large population difference exists due to the selective populating of the top level TZ [2}. By performing the T2 measurement quickly one takes advantage of this large population difference before spin-lattice rekation establishes a
generated by the magnetic moment during the echo, at First sight, may appear a diflkutt experimental problem. First the concentration of p~~~~to-e~cited triplet state molecules is tow and second the sensitivity of a spectrometer that has to operate at 1000 MHz compares unfavourably with spectrometers at the usual X-band frequency of IO 000 MHz. However two important advantages exist in our GXX: (I) In zero field the line-width AY of the zero-field transition is very small because hyperfine interaction is a second order effect [7, S] _This makes it relatively easy to fuIfi1 the condition YHi > lnhv and one “hits” aII moIccu!es by the microwave pulses. (2) As mentioned already in the preceding section continuous iliumination at 1.2”K usually leads to large differences in the populations of the spin states. This spin alignment is caust?d by the very selective populating and depopulating processes and the slow spin-
lattice relaxation in the triplet states of these molecules. The population differences tend to be even larger if one excites with a tlash. Then during a short time a large alignment exists before relaxation establishes a steady state situation. These two advantages more than compensate for
the low concentration
of quinoline triplet molecules
and unfavourable resonance frequency, and make the detection of the echo feasible. Thus, with our spec-
trometer operating at 1000 MHz and 1.2% we are able to detect 5 X 1011 pltotoescitcd quinoline molecules with a signal-to-noise ratio of 1 in a single
shot spin-echo experiment. This sensitivity is in the same order of magnitude as a conventional X-band ESR spectrometer. The experimental results of the T2 measurements clearly show that the nuclear spins predominantly
15 June 1972
CHEMICAL PHYSICS LETTERS
Volume 14, number 4 determine
couragexnent
and interest
in this work.
Further
to
the triplet hpin memory time. The results of section 2 further show that even in quinoline-d7 in durene-d14 (with the longest memory time of ,t.he three systems studied) this T2 is independent of the concentration of excited triplet moIecules. Hence even in this system at the masimum concentration of about
Mr. J. van den Berg who constructed an important part of the experimental arrangement. This work was performed as part of the research programme of the “Stichting voor Fundamenteel Onderzoek der Materie” (F-0-M.) with financial support from the “Nederlandse
5 X lOI triplet spins per cm3, the influence of hutual interaction of photo-excited triplet molecules to T7
Organisatie voor Zuiver Wetenschappelijk (Z.W.O.).
Onderzoek”
is negligible.
we may say that we have proved
Concluding
perimentally
es-
that electron spin-echo experiments
photo-excited
triplet
state molecules
in
irz zero nzugmtic
are feasible. Moreover we have shown that one can measure the phase memory of the triplet mole-
References
field
cules as a function of temperature by taking advantage of the selective populating and depopulating rates in the triplet state. We think that this technique may
prove valuable for studying those processes that destroy the phase relation between photo-excited triplet state molecules, such as energy transfer in molecular crystals.
and spin diffusion
Acknowledgement
The author wishes to express his gratitude to Professor J.H. van der Weals for his continuous en-
-!!-I
[ : ] J. Schmidt, W.G. van Dorp and J.fI. van der Wsals, Chem. Phys. Letters S (1971) 345. (21 J. Schmidt. Thesis. [Jniversity of Leiden (1971); I. Scflmidt, D.A. .4ntheunis and J.H. vun der Waals, Mol. Phys. 22 (1971) 1. [ 31 C.B. Harris, J. Chcm. Phys. 54 (1971) 972. [4] R.1’. Fcynmun. F.L. Vernon and R.W. Hellwarth, J. Appl. Phys. 28 (1957) 49. [5] SI. Bloom, E.L. Hahn and B. Herzog, Phys. Rev. 97 (1955) 1699. [6] W.B. Mims, Rev. Sci. Instr. 36 (196.5) 1472. [ 71 J. Schmidt and J.H. van der Wusls, Chem. Phys. Letters 3 (1969) 546. [S] CA. Hutchison, J.V. Nicholas and G.W. Scott, J. Chem. Phys. 53 (1970) 1906.