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Transient nutations of photoelectrons from alkali metal anions
Vohmne 58. number 3
TRANSIENT
CHEMICAL
NUTATIONS
PHYSICS
OF PHOTOELECTRONS
LE’lTERS
FROM ALKALI
i October
$978
METAL ANIONS
Soon Sam KM and S.I. WEISSMAN Department of Chemistry, Washington University. St_ Louis, Missouri 63130. USA Received 5 July 1978 Transient nut&ions of the magnetization of photoelectrons ejected from rubidium and cesium anions are descriied. The nature of the nutations requires that the ‘time between photoexcitation and app earaace of the photoelectron is less than ==lO-’ 5.
Glarum and Marshall [I] first reported that illumination of solutions of rubidium in ethereal solvents produces a pammagnetic species with inverted population among its Zeeman levels. The electron paramagnetic resonance spectrum consists of a single sharp line with magnetogyric ratio very clcse to the free electron value. The signal intensity corresponds to a spin temperature at birth of -0.7 K. They found that iilumination by a pulsed xenon lamp which dissipated about 200 1 per pulse and furnished a flash of ilhunination with rise time of about 100 w and decay time of 2 ms produced a transient emissive electron paramagnetic resonance which had about the same rise time as the light flash. Glarum and Marshall attributed the transient EPR signal to electrons ejected from rubidium anions and suggested that they were born in their upper Zeeman states owing to the combined effects of spin orbit and exchange interactions in the separating fragments_ The simplest version of their proposal leads to spin polarization if the magnetogyric ratios in the two fragments are not equal. Our experiments were carried out with solutions of alkali metals in tetrahydrofuran containing about 5 X 10m3 M of the complexing agent Krytofur-222. Use of this complex& agent ensures conversion of a substantial fraction of the dissoIved alkali metal to its anion [2,3]. We used for excitation either a pulsed xenon lamp with a red filter (Corning 2-58) or a tuneable dye laser containing nile blue A perchlorate pumped by a nitrogen laser. The dye laser yielded pukes with 15 CJ or^ energy at 690 nm and duration about 5 ns. With the pulsed xenon lamp we used a detection system with time resohrtion of about 60 fls, 326
, I
_-I
lo-
I
Fig. 1. The transient EPR signa& from the photoelectrons from Rb- in THF at room temperature. BeIow the base Line is emissivc, above the base lisle is absorptive. The lower trace rep resents the shape of the Iight pulse from the xenon Iamp.
with the pulsed laser one with resolution about 50 ns 141. AU observations were made under continuous microwave excitation in an X band spectrometer tuned to respond to the absorptive component of magnetization. A representative curve for the rise and decay of the signal from a so!ution of rubidium at time resolution of 60 cls is shown in fig_ l_ Except for the small spikes at onset and termination of the light pulse the curves are almost exponential with time constant of 5 ms. The initial spike is emissive, the steady state signal absorptive. At 50 ns resolution the transient nun&ions, as displayed in figs. 2 and 3, appear. They resemble the transient nutations of nuclear magnetization first described by Torrey [S] some three decades ago and the
Volume 58, number 3
CHE&tICAL PHYSICS LET’ERS
I October 1978
2 !i&.ec
4
mW
dEp--
m
8mw
-
_-
2 5i.tsec
A
=-
m270K
L--
N230K
#
Fig. 3. Upper three: the observed transient nutations from the photoelectrons from Cs- in THF at different microwave powers. The temperature was i+:220 K. Lower three: calculared nlitation patterns with T = 6.0 ps The wl’s we.= 0.85, 1.30 and 1.90 rad/r.rs,respectively.
ml9OK rP====--
Fig 2. The observed transient nutations from the photoelectrons from Rb- in THF. Upper three: transient nutations at different microwave powers. The estimated amplitudes of the microwave magnetic fields, HI, are, in gauss, 0.05,0.07 and 0.11 respectively. The temperature was a230 K. Middle three: A, laboratory magnetic field was set at resorance, HI = 0.05 G, B and C, off-resonance by 0.05 G, 0.07 G respectively. The temperature was J 230 K. Lower three: at temperatures, = 270 Kz = 230 K and * 190 K, and at slightly different HI, 0.06 C, 0.05 G and 0.03 G, respectively.
nutations of electronic magnetization of an*&rasemiquake radicals observed by Atkins et al. [6]_ No signal averaging was used, each curve having been obtained following a single pulse from the dye laser. Although fewer than 5 X 1013 photons are delivered to the sample at each pulse, owing to the low spin temperature at which the spins are born and ability to detect their state in time considerably shorter than the relaxation time, intense signals, albeit ones of short duration, are observed. The theory of nutations [5,6]
indicates that for a resonance characterized by Tl = T2 the time course of the absorptive component of magnetization A(t) is given by: A(t) = A0 exp($2 = [(o
t/T-,) sin S2t ,
- &Jo)2 i- cd;] 112 .
(1)
A, is a constant; wl = 74Yl, 7 is the magneto&c ratio;If, is the amplitude of the rotating field; wo is the resonant frequency and o the applied frequency, Our results are in moderately good accord -with eq. (I). Tl = Tz = 6.0 w with little variation with temperature. The steady state line shape is close to lorentzian with breadth (allowing for field inhomogeneity and the effects of the field modulation) consistent with the value of T2 deduced from the decay of the nutation. No measurable difference between the nutations of ektrons ejected from rubidium anions and cesium anions is observed. We have detected no signals evoked by the pulsed laser from sdutions of sodium or potassium. We be327
Jieve that they wiJl ultbnately be found but have not ye& been observed owing to amaJ.IpoIa5zations at birth. We have been unable to apply signnalaveraging to these systems because of inability to maintain constant field-frequency ratio. Direct measurements of the long tie ffuctuations in kIys:ron frequency demonstrate that the changes in microwave frequency &me produce a distribution of nutation frequencies [eq_ (I)] sufficient to wipe out an zveraged pattern. Attainment of nutation patterns with signal fo noise sufficient to detect deviations from eq- (1) would be valuable. Eq. (1) is derived for a model in which the spins are born instantaneously (i.e. in a small fraction of a nutation period), yet all radical pair models for production of polarized spins require a frnite time for evolution of the initially excited system to its fmal spin polarized state. For instance a scheme in which the times between initial excitation and appearance of the spins are exponentially distributed with characteristic time TF leads to a nut&ion pattern 4(t) A(r) = Ao{wlrF [exp(--t/rF)
328
1 Oc+zr
CHEMRXL PHYSICS LBTTERS
Volume 58, number 3
- exp(--t/X’.)
cos Rt]
1978
When rF * 1jwl ,T2, we recover eq. (r>- When l/w1 = rF < T2r there is a significant phase shift in :he nutations and superposition of a nonoscillatory decay. The moderate concordance between our results and eq. (I)
sugg&s that rF
= 1om7.s_
This work has been supported by the National Science Foundation.
References [I] S_H..CIarum and l-H_ Marsha& I+ Chent Phys. 52 (1970) 5555. [2If.L.Dye, J.&L Ceraso, M.T. Lok, B.L. Bamett and FJ. Teehan,S. Am. Chem. Sot. 96 (1974)608. {3] F-3. Tehan, 9-L. Barcert and J.L. Dye, I_ Am_ Chem. Sot. 96 (1974) 7203. [4]S.S.Kim and S-1. Weissman, 3. Ma81 Resort. 24 (1976) 167. [S]H.C. Toney, Phys. Rev. 76 (1949) 1059. (61P.W. Atkins, A.J. Dobbs and K-A. hfclauchlan, Chem. Phys. Letten 25 (1974)
105.
TRANSIENT
CHEMICAL
NUTATIONS
PHYSICS
OF PHOTOELECTRONS
LE’lTERS
FROM ALKALI
i October
$978
METAL ANIONS
Soon Sam KM and S.I. WEISSMAN Department of Chemistry, Washington University. St_ Louis, Missouri 63130. USA Received 5 July 1978 Transient nut&ions of the magnetization of photoelectrons ejected from rubidium and cesium anions are descriied. The nature of the nutations requires that the ‘time between photoexcitation and app earaace of the photoelectron is less than ==lO-’ 5.
Glarum and Marshall [I] first reported that illumination of solutions of rubidium in ethereal solvents produces a pammagnetic species with inverted population among its Zeeman levels. The electron paramagnetic resonance spectrum consists of a single sharp line with magnetogyric ratio very clcse to the free electron value. The signal intensity corresponds to a spin temperature at birth of -0.7 K. They found that iilumination by a pulsed xenon lamp which dissipated about 200 1 per pulse and furnished a flash of ilhunination with rise time of about 100 w and decay time of 2 ms produced a transient emissive electron paramagnetic resonance which had about the same rise time as the light flash. Glarum and Marshall attributed the transient EPR signal to electrons ejected from rubidium anions and suggested that they were born in their upper Zeeman states owing to the combined effects of spin orbit and exchange interactions in the separating fragments_ The simplest version of their proposal leads to spin polarization if the magnetogyric ratios in the two fragments are not equal. Our experiments were carried out with solutions of alkali metals in tetrahydrofuran containing about 5 X 10m3 M of the complexing agent Krytofur-222. Use of this complex& agent ensures conversion of a substantial fraction of the dissoIved alkali metal to its anion [2,3]. We used for excitation either a pulsed xenon lamp with a red filter (Corning 2-58) or a tuneable dye laser containing nile blue A perchlorate pumped by a nitrogen laser. The dye laser yielded pukes with 15 CJ or^ energy at 690 nm and duration about 5 ns. With the pulsed xenon lamp we used a detection system with time resohrtion of about 60 fls, 326
, I
_-I
lo-
I
Fig. 1. The transient EPR signa& from the photoelectrons from Rb- in THF at room temperature. BeIow the base Line is emissivc, above the base lisle is absorptive. The lower trace rep resents the shape of the Iight pulse from the xenon Iamp.
with the pulsed laser one with resolution about 50 ns 141. AU observations were made under continuous microwave excitation in an X band spectrometer tuned to respond to the absorptive component of magnetization. A representative curve for the rise and decay of the signal from a so!ution of rubidium at time resolution of 60 cls is shown in fig_ l_ Except for the small spikes at onset and termination of the light pulse the curves are almost exponential with time constant of 5 ms. The initial spike is emissive, the steady state signal absorptive. At 50 ns resolution the transient nun&ions, as displayed in figs. 2 and 3, appear. They resemble the transient nutations of nuclear magnetization first described by Torrey [S] some three decades ago and the
Volume 58, number 3
CHE&tICAL PHYSICS LET’ERS
I October 1978
2 !i&.ec
4
mW
dEp--
m
8mw
-
_-
2 5i.tsec
A
=-
m270K
L--
N230K
#
Fig. 3. Upper three: the observed transient nutations from the photoelectrons from Cs- in THF at different microwave powers. The temperature was i+:220 K. Lower three: calculared nlitation patterns with T = 6.0 ps The wl’s we.= 0.85, 1.30 and 1.90 rad/r.rs,respectively.
ml9OK rP====--
Fig 2. The observed transient nutations from the photoelectrons from Rb- in THF. Upper three: transient nutations at different microwave powers. The estimated amplitudes of the microwave magnetic fields, HI, are, in gauss, 0.05,0.07 and 0.11 respectively. The temperature was a230 K. Middle three: A, laboratory magnetic field was set at resorance, HI = 0.05 G, B and C, off-resonance by 0.05 G, 0.07 G respectively. The temperature was J 230 K. Lower three: at temperatures, = 270 Kz = 230 K and * 190 K, and at slightly different HI, 0.06 C, 0.05 G and 0.03 G, respectively.
nutations of electronic magnetization of an*&rasemiquake radicals observed by Atkins et al. [6]_ No signal averaging was used, each curve having been obtained following a single pulse from the dye laser. Although fewer than 5 X 1013 photons are delivered to the sample at each pulse, owing to the low spin temperature at which the spins are born and ability to detect their state in time considerably shorter than the relaxation time, intense signals, albeit ones of short duration, are observed. The theory of nutations [5,6]
indicates that for a resonance characterized by Tl = T2 the time course of the absorptive component of magnetization A(t) is given by: A(t) = A0 exp($2 = [(o
t/T-,) sin S2t ,
- &Jo)2 i- cd;] 112 .
(1)
A, is a constant; wl = 74Yl, 7 is the magneto&c ratio;If, is the amplitude of the rotating field; wo is the resonant frequency and o the applied frequency, Our results are in moderately good accord -with eq. (I). Tl = Tz = 6.0 w with little variation with temperature. The steady state line shape is close to lorentzian with breadth (allowing for field inhomogeneity and the effects of the field modulation) consistent with the value of T2 deduced from the decay of the nutation. No measurable difference between the nutations of ektrons ejected from rubidium anions and cesium anions is observed. We have detected no signals evoked by the pulsed laser from sdutions of sodium or potassium. We be327
Jieve that they wiJl ultbnately be found but have not ye& been observed owing to amaJ.IpoIa5zations at birth. We have been unable to apply signnalaveraging to these systems because of inability to maintain constant field-frequency ratio. Direct measurements of the long tie ffuctuations in kIys:ron frequency demonstrate that the changes in microwave frequency &me produce a distribution of nutation frequencies [eq_ (I)] sufficient to wipe out an zveraged pattern. Attainment of nutation patterns with signal fo noise sufficient to detect deviations from eq- (1) would be valuable. Eq. (1) is derived for a model in which the spins are born instantaneously (i.e. in a small fraction of a nutation period), yet all radical pair models for production of polarized spins require a frnite time for evolution of the initially excited system to its fmal spin polarized state. For instance a scheme in which the times between initial excitation and appearance of the spins are exponentially distributed with characteristic time TF leads to a nut&ion pattern 4(t) A(r) = Ao{wlrF [exp(--t/rF)
328
1 Oc+zr
CHEMRXL PHYSICS LBTTERS
Volume 58, number 3
- exp(--t/X’.)
cos Rt]
1978
When rF * 1jwl ,T2, we recover eq. (r>- When l/w1 = rF < T2r there is a significant phase shift in :he nutations and superposition of a nonoscillatory decay. The moderate concordance between our results and eq. (I)
sugg&s that rF
= 1om7.s_
This work has been supported by the National Science Foundation.
References [I] S_H..CIarum and l-H_ Marsha& I+ Chent Phys. 52 (1970) 5555. [2If.L.Dye, J.&L Ceraso, M.T. Lok, B.L. Bamett and FJ. Teehan,S. Am. Chem. Sot. 96 (1974)608. {3] F-3. Tehan, 9-L. Barcert and J.L. Dye, I_ Am_ Chem. Sot. 96 (1974) 7203. [4]S.S.Kim and S-1. Weissman, 3. Ma81 Resort. 24 (1976) 167. [S]H.C. Toney, Phys. Rev. 76 (1949) 1059. (61P.W. Atkins, A.J. Dobbs and K-A. hfclauchlan, Chem. Phys. Letten 25 (1974)
105.