- Email: [email protected]
MARY-detected ESR spectra of radical ions in liquid solutions for systems with crossing Zeeman levels
4 October 1996
CHEMICAL PHYSICS
LETTERS ELSEVIER
Chemical Physics Letters 260 (1996) 529-532
MARY-detected ESR spectra of radical ions in liquid solutions for systems with crossing Zeeman levels B . M . T a d j i k o v , D.V. Stass, Yu.N. M o l i n 1 Institute of Chemical Kinetics and Combustion, Novosibirsk 630090, Russia
Received 15 July 1996
Abstract The effect of Zeeman levels crossing in spin-correlated radical ion pairs (naphthalene)+/(hexafluorobenzene)- was monitored as the influence of an external magnetic field on the solution fluorescence under X-irradiation (MARY spectrum). The spectra obtained exhibit a line in the field, equal to triple the hfi coupling constant of hexafluorobenzene radical anion, its linewidth being determined by the unresolved hyperfine structure of the naphthalene cation. The theory predicts that the presence of weak hfi in the partner cation splits this line by the projections of the cation total nuclear momentum Mz according to nuclear statistics. Thus, a direct correspondence between the MARY and ESR spectra of radical cations allows registration of the ESR spectrum without microwave pumping.
1. Introduction The technique of Zeeman level crossing is widely used in atomic spectroscopy. Level crossings at zero (the Hanle effect [ 1 ] ) and nonzero fields allow one to study the fine structure of the spectra of atoms and simple molecules in vacuum and rarefied gases [2]. The effect originates from the coherent excitation of transitions between fine structure multiplet levels of atom ensembles by polarized light and is monitored as changes in the spatial and polarization distribution of emitted light with external an magnetic field. The effect of level crossing in spin-correlated radical ion pairs forming under X-irradiation has also been experimentally observed in liquid solutions [ 3 ] when spin relaxation times are long enough to preserve coherence. Here the effect is due to the hyperfine interactions (hfi) in radical ions and is registered as the I Corresponding author.
influence of an external magnetic field on the fluorescence yield of recombination products (MARY spectrum), the narrow MARY lines arising in the fields where the pair of Zeeman levels crosses. The sample fluorescence is isotropic, and there is no need for polarizers. The MARY signals are most prominent for pairs having no hfi in one of the partners (e.g. a radical cation) and an even ( n ) 4) number of equivalent protons (fluorines) in the counterion. In this case the spectrum shows an intense line at zero field and a weaker satellite in the field equal to triple the hyperfine coupling constant H* = 3A. In this Letter we report that the presence of a weak (a << A) hfi in the partner radical cation should additionally split this line by projections of the cation total nuclear spin Mz. This opens the possibilities of observing the hyperfine structure of a counterion in short-lived radical ion pairs using the level crossing technique. The direct correspondence between the radical cation MARY and ESR spectra exists, thus mak-
0009-2614/96/$12.00 Copyright (~) 1996 Published by Elsevier Science B.V. All rights reserved. PHS0009-2614(96)00929-3
530
B.M. Tadjikov et aL/Chemical Physics Letters 260 (1996) 529-532
ing it possible to take ESR spectra without microwave pumping. This conclusion is verified experimentally by the MARY and optically detected ESR (OD ESR) spectra of the cation-radicals of naphthalene-h8 and naphthalene-d8, taken in liquid squalane at room temperature.
MARY
ESR
2. Experimental The MARY experimental apparatus has been described elsewhere [ 3 ]. The sample, containing about 1 cm 3 of solution in a quartz cuvette, was put into the magnetic field of a BRUKER ER-200D ESR spectrometer equipped with an X-ray tube (Mo, 40 kV×40 mA) for sample irradiation and a PMT with a quartz light guide for fluorescence detection. Fluorescence was monitored through a bandpass light filter (250-320 nm). For attaining better resolution the external magnetic field was modulated at 12.5 kHz, being the lock-in amplifier reference frequency. Hence the experimental curves are the first derivatives of the corresponding field dependencies. Neither microwave pumping nor a light polarizer were used. The OD ESR spectra were taken using the same apparatus, as described in Ref. [4]. The applied microwave power of 200 mW corresponds to H1 = 0.45 G in the spectrometer cavity. Naphthalenehs, naphthalene-d8 and hexafluorobenzene, whose hyperfine parameters are known from the literature, were chosen as hole and electron acceptors. Squalane was used as an inert solvent. Prior to the experiment the samples were degassed to ,~ 10 -3 Torr. All the spectra were taken at room temperature.
3. Results and discussion Taking advantage of the fact that electrons and solvent holes are captured rather quickly, the formation of radical ion pairs N+/C6F6 under X-irradiation of the solutions and the subsequent recombination giving excited products can be described by a simplified scheme [ 5 ] N + C6F 6
Y ~N +
N + + C6F 6
> N*
+ C6F6,
(I)
+ C6F6,
(2)
-,'5o'-,'oo'
Ao'
~ ' ~o ' ,6° ' ,~o - ~ ' "
'-'~g ' " ~
....
2'~ . . . .
go
field offset AH, G
Fig. 1. The correspondence between MARY (a-e) and ESR (a~-e t) spectra for radical ions with different hfi substructure (theory). hfi parameters of the examined ion: (a,a t) no hfi; (b,b ~) 20 G(1H); (c,c t) 20 G(2H); (d J ) 20 G(3H); (e,e ~) /2 = 20 G. Recombination linewidth (hwhm) 2 G. MARY spectra centered at H* = 3000 G, hfi of probing counterion (MARY) A = 1000 G(4H). Field sweep shrunk three times for MARY spectra to get visual coincidence with corresponding ESR spectra. Note that no microwave field has been applied in the case of MARY.
N being the hole acceptor (naphthalene). The pair N +/C6F 6 is formed in the spin-correlated singlet state (reaction (1)). The yield of fluorescing products is determined by the singlet-triplet evolution in the pair, since the multiplicity (S, T) of the resulting excited molecules l'3N* corresponds to the pair multiplicity at the moment of recombination. In the experiment the light filter left only fluorescence from the singlet excited state. Time evolution of the spin state of the system is described using the density matrix formalism [6]. The curves in Figs. la-d give the calculated MARY spectrum in the vicinity of the satellite line at H* = 3A for radical ion pairs with different sets of magnetic nuclei in the cation. The spin Hamiltonian involved included the Zeeman interaction of unpaired electrons with the external magnetic field and hyperfine interactions with magnetic nuclei in radical ions. To stress the correspondence between the MARY and ESR spectra and to diminish the low field effects in MARY spectra, the hyperfine couplings for this model pair were taken to be A = 1000 G (4H) and a = 20 G (a <<
B.M. Tadjikov et al./Chemical Physics Letters 260 (1996) 529-532
A) for anion and cation, respectively. Thus, the line at H* = 3A is centered at 3000 G. In the absence of hfi in the counterion (cation) the spectrum is just a singlet (Fig. la) with the width determined by the pair recombination lifetime. Introducing magnetic nuclei in the cation leads to a line splitting by projections of the cation total nuclear spin Mz according to nuclear statistics (Figs. l b - d ) . Thus, one proton in the cation gives a doublet (Fig. lb), two equivalent protons-triplet (Fig. lc), etc., all splittings in the spectrum being equal to triple the hfi constant in the cation. Such a "tripling" for n = 4 comes from the specific slope of the energy levels near the crossing point H* = 3A. A large number of nuclei with tiny hfi couplings in the cation gives, as could be expected, an inhomogeneously broadened line of gaussian shape (Fig. le). For reference, Figs. la/-d ~ show the ESR spectra of corresponding radical cations, calculated in the strong field approximation. The ESR spectra field sweep is expanded three times for visual correspondence. The comparison of calculated MARY and ESR spectra proves them to be identical. If the conditions of the strong field for the MARY spectrum H* >> ,(2 (f2 ~ V / 2 ~ a 2 1 i ( l i + 1)) are met, the MARY spectrum of the counterion would reproduce its ESR one for an arbitrary set of nuclei, which makes it possible to take ESR spectra without a microwave field. Experimental results for naphthalene-hs(-ds) are presented in Fig. 2. Figs. 2a and b show MARY spectra of the (naphthalene) +/(hexafluorobenzene)pair in the region of the hexafluorobenzene satellite line. The pair levels crossing is determined by large hfi with six equivalent fluorines A = 135 G (6F) [7], the subensemble with total nuclear spin I = 2 playing the first fiddle [3]. Hence, the line is centered at H* -- 405 G, and its splittings should correspond to the tripled counterion hyperfine couplings. As could be seen from Fig. 2a, the naphthalene-hs radical cation MARY spectrum constitutes a broad line with unresolved hyperfine structure from protons, and the theory (bold line) fits well the lineshape with ,(2 = 11 G taken in accordance with the (naphthalenehs) + hfi data [ 8]. The line narrowing upon transition to deuterated naphthalene (Fig. 2b) supports this as-
531
MARY
OD ESR a'
¢~'"'~0' '~;'"'""2'~'"70""¢~
.'~
' ' ' ~ ....
~'5
field offset AH, G
Fig. 2. The correspondence between MARY (a, b) and OD ESR (a t, b') spectra of naphthalene radical cations (experiment). (a,b) MARY spectra of 10 - 2 M hexafluorobenzene solutions in squalane with added: (a) 10 - 2 M naphthalene-hs, (b) 10 - 2 M naphthalene-ds. The spectra centered at the hexafluorobenzene radical anion satellite line at H* = 405 G. No microwave pumping applied. Bold lines: calculations with hfi parameter S'2 = [ I G for (naphthalene-h8) + and 2.75 G for (naphthalene-riB) +. (a~,b t ) OD ESR spectra of 10 - I M hexafluorobenzene solutions in squalane with added: (a) 10 - 2 M naphthalene-hs, (b) 10 - 2 M naphthalene-ds. The OD ESR spectra center field H = 3400 G correspond to clystron operation frequency 9.5 GHz (X-band). For MARY spectra the magnetic field sweep is shrunk three times to achieve visual correspondence with OD ESR spectra.
signment. Figs. 2a r and b / show the OD ESR spectra of protonated and deuterated naphthalene cations, superimposed on the C6F 6 counterion unresolved line, smashed by ion-molecular charge transfer. Evidently, the MARY and OD ESR spectra correspond to each other quite well. The broad wings in the MARY spectra, as compared to the ESR ones, are due to the low field effects in taking MARY spectra and to the nonzero slope of the fluorescence background in stationary magnetic effect experiments. Increasing the technique sensitivity would open new prospects for employing the level crossing phenomenon in studying hfi in short-lived radical ion pairs.
Acknowledgements The authors would like to express deep gratitude to Professor A.B. Doktorov for valuable remarks made during the discussion of this work. The work was performed under the auspices of INTAS, grant 96-1626, and the Russian Foundation for Basic Research, grant 96-03-33694a.
532
B.M. Tadjikov et al./Chemical Physics Letters 260 (1996) 529-532
References [l] W. Hanle, Z. Phys. 30 (1924) 93. [2] R.N. Zare, J. Chem. Phys. 45 (1966) 4510. 131 D.V. Stass, B.M. Tadjikov and Yu.N. Molin, Chem. Phys. Lett. 235 (1995) 511, [4] O.A. Anisimov, V.M. Grigoryantz, V.I. Melekhov, V.I. Korsunsky and Yu.N. Molin, DAN SSSR (in Russian) 260 (1981) 1151,
[5] B. Brocklehurst, J. Chem. Soc. Faraday Trans. I1 72 (1976) 1869. [6] D.V. Stass, N.N. Lukzen, B.M. Tadjikov, V.M. Grigoryantz and Yu.N. Molin, Chem. Phys. Lett. 243 (1995) 533. [7] Landolt-B6rnstein, Numerical Data and Functional Relationship in Science and Technology, ed. Fischer, in: Magnetic Properties of Free Radicals, 1I/17f (Springer, New York, 1990) p. 185. [8] E Gerson and Xue-Zhi Qin, Chem. Phys. Lett. 153 (1988) 546.
CHEMICAL PHYSICS
LETTERS ELSEVIER
Chemical Physics Letters 260 (1996) 529-532
MARY-detected ESR spectra of radical ions in liquid solutions for systems with crossing Zeeman levels B . M . T a d j i k o v , D.V. Stass, Yu.N. M o l i n 1 Institute of Chemical Kinetics and Combustion, Novosibirsk 630090, Russia
Received 15 July 1996
Abstract The effect of Zeeman levels crossing in spin-correlated radical ion pairs (naphthalene)+/(hexafluorobenzene)- was monitored as the influence of an external magnetic field on the solution fluorescence under X-irradiation (MARY spectrum). The spectra obtained exhibit a line in the field, equal to triple the hfi coupling constant of hexafluorobenzene radical anion, its linewidth being determined by the unresolved hyperfine structure of the naphthalene cation. The theory predicts that the presence of weak hfi in the partner cation splits this line by the projections of the cation total nuclear momentum Mz according to nuclear statistics. Thus, a direct correspondence between the MARY and ESR spectra of radical cations allows registration of the ESR spectrum without microwave pumping.
1. Introduction The technique of Zeeman level crossing is widely used in atomic spectroscopy. Level crossings at zero (the Hanle effect [ 1 ] ) and nonzero fields allow one to study the fine structure of the spectra of atoms and simple molecules in vacuum and rarefied gases [2]. The effect originates from the coherent excitation of transitions between fine structure multiplet levels of atom ensembles by polarized light and is monitored as changes in the spatial and polarization distribution of emitted light with external an magnetic field. The effect of level crossing in spin-correlated radical ion pairs forming under X-irradiation has also been experimentally observed in liquid solutions [ 3 ] when spin relaxation times are long enough to preserve coherence. Here the effect is due to the hyperfine interactions (hfi) in radical ions and is registered as the I Corresponding author.
influence of an external magnetic field on the fluorescence yield of recombination products (MARY spectrum), the narrow MARY lines arising in the fields where the pair of Zeeman levels crosses. The sample fluorescence is isotropic, and there is no need for polarizers. The MARY signals are most prominent for pairs having no hfi in one of the partners (e.g. a radical cation) and an even ( n ) 4) number of equivalent protons (fluorines) in the counterion. In this case the spectrum shows an intense line at zero field and a weaker satellite in the field equal to triple the hyperfine coupling constant H* = 3A. In this Letter we report that the presence of a weak (a << A) hfi in the partner radical cation should additionally split this line by projections of the cation total nuclear spin Mz. This opens the possibilities of observing the hyperfine structure of a counterion in short-lived radical ion pairs using the level crossing technique. The direct correspondence between the radical cation MARY and ESR spectra exists, thus mak-
0009-2614/96/$12.00 Copyright (~) 1996 Published by Elsevier Science B.V. All rights reserved. PHS0009-2614(96)00929-3
530
B.M. Tadjikov et aL/Chemical Physics Letters 260 (1996) 529-532
ing it possible to take ESR spectra without microwave pumping. This conclusion is verified experimentally by the MARY and optically detected ESR (OD ESR) spectra of the cation-radicals of naphthalene-h8 and naphthalene-d8, taken in liquid squalane at room temperature.
MARY
ESR
2. Experimental The MARY experimental apparatus has been described elsewhere [ 3 ]. The sample, containing about 1 cm 3 of solution in a quartz cuvette, was put into the magnetic field of a BRUKER ER-200D ESR spectrometer equipped with an X-ray tube (Mo, 40 kV×40 mA) for sample irradiation and a PMT with a quartz light guide for fluorescence detection. Fluorescence was monitored through a bandpass light filter (250-320 nm). For attaining better resolution the external magnetic field was modulated at 12.5 kHz, being the lock-in amplifier reference frequency. Hence the experimental curves are the first derivatives of the corresponding field dependencies. Neither microwave pumping nor a light polarizer were used. The OD ESR spectra were taken using the same apparatus, as described in Ref. [4]. The applied microwave power of 200 mW corresponds to H1 = 0.45 G in the spectrometer cavity. Naphthalenehs, naphthalene-d8 and hexafluorobenzene, whose hyperfine parameters are known from the literature, were chosen as hole and electron acceptors. Squalane was used as an inert solvent. Prior to the experiment the samples were degassed to ,~ 10 -3 Torr. All the spectra were taken at room temperature.
3. Results and discussion Taking advantage of the fact that electrons and solvent holes are captured rather quickly, the formation of radical ion pairs N+/C6F6 under X-irradiation of the solutions and the subsequent recombination giving excited products can be described by a simplified scheme [ 5 ] N + C6F 6
Y ~N +
N + + C6F 6
> N*
+ C6F6,
(I)
+ C6F6,
(2)
-,'5o'-,'oo'
Ao'
~ ' ~o ' ,6° ' ,~o - ~ ' "
'-'~g ' " ~
....
2'~ . . . .
go
field offset AH, G
Fig. 1. The correspondence between MARY (a-e) and ESR (a~-e t) spectra for radical ions with different hfi substructure (theory). hfi parameters of the examined ion: (a,a t) no hfi; (b,b ~) 20 G(1H); (c,c t) 20 G(2H); (d J ) 20 G(3H); (e,e ~) /2 = 20 G. Recombination linewidth (hwhm) 2 G. MARY spectra centered at H* = 3000 G, hfi of probing counterion (MARY) A = 1000 G(4H). Field sweep shrunk three times for MARY spectra to get visual coincidence with corresponding ESR spectra. Note that no microwave field has been applied in the case of MARY.
N being the hole acceptor (naphthalene). The pair N +/C6F 6 is formed in the spin-correlated singlet state (reaction (1)). The yield of fluorescing products is determined by the singlet-triplet evolution in the pair, since the multiplicity (S, T) of the resulting excited molecules l'3N* corresponds to the pair multiplicity at the moment of recombination. In the experiment the light filter left only fluorescence from the singlet excited state. Time evolution of the spin state of the system is described using the density matrix formalism [6]. The curves in Figs. la-d give the calculated MARY spectrum in the vicinity of the satellite line at H* = 3A for radical ion pairs with different sets of magnetic nuclei in the cation. The spin Hamiltonian involved included the Zeeman interaction of unpaired electrons with the external magnetic field and hyperfine interactions with magnetic nuclei in radical ions. To stress the correspondence between the MARY and ESR spectra and to diminish the low field effects in MARY spectra, the hyperfine couplings for this model pair were taken to be A = 1000 G (4H) and a = 20 G (a <<
B.M. Tadjikov et al./Chemical Physics Letters 260 (1996) 529-532
A) for anion and cation, respectively. Thus, the line at H* = 3A is centered at 3000 G. In the absence of hfi in the counterion (cation) the spectrum is just a singlet (Fig. la) with the width determined by the pair recombination lifetime. Introducing magnetic nuclei in the cation leads to a line splitting by projections of the cation total nuclear spin Mz according to nuclear statistics (Figs. l b - d ) . Thus, one proton in the cation gives a doublet (Fig. lb), two equivalent protons-triplet (Fig. lc), etc., all splittings in the spectrum being equal to triple the hfi constant in the cation. Such a "tripling" for n = 4 comes from the specific slope of the energy levels near the crossing point H* = 3A. A large number of nuclei with tiny hfi couplings in the cation gives, as could be expected, an inhomogeneously broadened line of gaussian shape (Fig. le). For reference, Figs. la/-d ~ show the ESR spectra of corresponding radical cations, calculated in the strong field approximation. The ESR spectra field sweep is expanded three times for visual correspondence. The comparison of calculated MARY and ESR spectra proves them to be identical. If the conditions of the strong field for the MARY spectrum H* >> ,(2 (f2 ~ V / 2 ~ a 2 1 i ( l i + 1)) are met, the MARY spectrum of the counterion would reproduce its ESR one for an arbitrary set of nuclei, which makes it possible to take ESR spectra without a microwave field. Experimental results for naphthalene-hs(-ds) are presented in Fig. 2. Figs. 2a and b show MARY spectra of the (naphthalene) +/(hexafluorobenzene)pair in the region of the hexafluorobenzene satellite line. The pair levels crossing is determined by large hfi with six equivalent fluorines A = 135 G (6F) [7], the subensemble with total nuclear spin I = 2 playing the first fiddle [3]. Hence, the line is centered at H* -- 405 G, and its splittings should correspond to the tripled counterion hyperfine couplings. As could be seen from Fig. 2a, the naphthalene-hs radical cation MARY spectrum constitutes a broad line with unresolved hyperfine structure from protons, and the theory (bold line) fits well the lineshape with ,(2 = 11 G taken in accordance with the (naphthalenehs) + hfi data [ 8]. The line narrowing upon transition to deuterated naphthalene (Fig. 2b) supports this as-
531
MARY
OD ESR a'
¢~'"'~0' '~;'"'""2'~'"70""¢~
.'~
' ' ' ~ ....
~'5
field offset AH, G
Fig. 2. The correspondence between MARY (a, b) and OD ESR (a t, b') spectra of naphthalene radical cations (experiment). (a,b) MARY spectra of 10 - 2 M hexafluorobenzene solutions in squalane with added: (a) 10 - 2 M naphthalene-hs, (b) 10 - 2 M naphthalene-ds. The spectra centered at the hexafluorobenzene radical anion satellite line at H* = 405 G. No microwave pumping applied. Bold lines: calculations with hfi parameter S'2 = [ I G for (naphthalene-h8) + and 2.75 G for (naphthalene-riB) +. (a~,b t ) OD ESR spectra of 10 - I M hexafluorobenzene solutions in squalane with added: (a) 10 - 2 M naphthalene-hs, (b) 10 - 2 M naphthalene-ds. The OD ESR spectra center field H = 3400 G correspond to clystron operation frequency 9.5 GHz (X-band). For MARY spectra the magnetic field sweep is shrunk three times to achieve visual correspondence with OD ESR spectra.
signment. Figs. 2a r and b / show the OD ESR spectra of protonated and deuterated naphthalene cations, superimposed on the C6F 6 counterion unresolved line, smashed by ion-molecular charge transfer. Evidently, the MARY and OD ESR spectra correspond to each other quite well. The broad wings in the MARY spectra, as compared to the ESR ones, are due to the low field effects in taking MARY spectra and to the nonzero slope of the fluorescence background in stationary magnetic effect experiments. Increasing the technique sensitivity would open new prospects for employing the level crossing phenomenon in studying hfi in short-lived radical ion pairs.
Acknowledgements The authors would like to express deep gratitude to Professor A.B. Doktorov for valuable remarks made during the discussion of this work. The work was performed under the auspices of INTAS, grant 96-1626, and the Russian Foundation for Basic Research, grant 96-03-33694a.
532
B.M. Tadjikov et al./Chemical Physics Letters 260 (1996) 529-532
References [l] W. Hanle, Z. Phys. 30 (1924) 93. [2] R.N. Zare, J. Chem. Phys. 45 (1966) 4510. 131 D.V. Stass, B.M. Tadjikov and Yu.N. Molin, Chem. Phys. Lett. 235 (1995) 511, [4] O.A. Anisimov, V.M. Grigoryantz, V.I. Melekhov, V.I. Korsunsky and Yu.N. Molin, DAN SSSR (in Russian) 260 (1981) 1151,
[5] B. Brocklehurst, J. Chem. Soc. Faraday Trans. I1 72 (1976) 1869. [6] D.V. Stass, N.N. Lukzen, B.M. Tadjikov, V.M. Grigoryantz and Yu.N. Molin, Chem. Phys. Lett. 243 (1995) 533. [7] Landolt-B6rnstein, Numerical Data and Functional Relationship in Science and Technology, ed. Fischer, in: Magnetic Properties of Free Radicals, 1I/17f (Springer, New York, 1990) p. 185. [8] E Gerson and Xue-Zhi Qin, Chem. Phys. Lett. 153 (1988) 546.