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On the mechanism of magnetic quenching of fluorescence in the gaseous state
Journal of Luminescence 18119 (1979) 115—1 19 © North-Holland Publishing Company
ON THE MECHANISM OF MAGNETIC QUENCHING OF FLUORESCENCE IN THE GASEOUS STATE A. MATSUZAKI* and S. NAGAKURA Institute for Solid State Physics, University of Tokyo, Roppongi, Tokyo. Japan
Magnetic quenching of fluorescence in the gaseous state was studied experimentally and theoretically with glyoxal, CS 2, and NO2, and was found to be explained by mechanism I and mechanism II, which are due to interaction through the Zeeman hamiltonian and the shift and broadening of appropriate rovibronic levels by the Zeeman effect, respectively.
1. Introduction Magnetic quenching of emission 2B from gaseous molecules has been observed with the ~ state in ‘2 [1], the 2 state in NO2 [2, 3], the ‘A2 State in CS2 [4], and the ‘A~state in glyoxal [5]. The spin-multiplicities of quenched states are triplet for 12, doublet for NO2, and singlet for CS2 and glyoxal. A magnetic quenching constant krnag increases for ‘2 in proportion with the square of the field strength. For CS2, kmag increases steeply in a low field and in proportion to the square of the field strength in a high field. For glyoxal, kmag increases steeply in a low field and is saturated to a constant value. Recently, Atkins and Stannard [6,7] proposed a theory to explain these phenomena by introducing the direct mechanism and the indirect mechanism. We have developed the theory and have proposed mechanism I and mechanism II. The first-order term of mechanism I is the same as the direct mechanism. Mechanism II is based on the shift and broadening of appropriate rovibronic levels by the Zeeman effect and is different from the indirect mechanism in which a magnetic field enhances radiationless decay of a primary state into a quasi-continuum through the mixing with another discrete state. The magnetic field dependence of the collision-free quenching constant observed by the present authors [5] and KUttner et al. [8] can be well explained by mechanism II.
2. Experimental Glyoxal and NO2 molecules were excited by the use of a Molectron DL-200 dye laser pumped by a Molectron UVI000 nitrogen laser. The band width of the *
Present address: Institute of Space and Aeronautical Science, University of Tokyo. 115
116
A. Matsuzaki, S. Nagakura/Magnetic quenching of fluorescence
laser was 0.02 nm on tuning at 455.0 nm. Emissions from gaseous glyoxal and NO, were monochromatized by a Spex model 1702 spectrometer and were detected by a combination of Hamamatsu TV Rl06 photomultiplier shielded by it-metal and a PAR model 160 boxcar integrator gated by magnetic field [9].
3. Results and discussion 3.1. Mechanism of magnetic quenching We consider the two types of mechanisms for the magnetic enhancement of the intramolecular nonradiative process from a primary state, ç1’~,,to secondary states, ‘(J~ mechanism I and mechanism II. Mechanism I Mechanism I is due to the coupling of states by the Zeeman Hamiltonian H~. The nonradiative decay constant for this case is represented by the following equation [101. 2
knr = 2lThpI(tfIPIHvIh + H 60 + HZI~I1S~I = 27rhp[~/Ip~HVjh+ HSOII/JS)12 + (cIJPIHV~b+H
60f ~P6X~P~IH~It/16) +(~1PIHZjc1i,)(~fPIHVh+ H60I~/i.) 2I + =
k
k~IH~k/i6)J
0+ k
2)
kniag(H) + kmag(H
0+kmag.
(1)
Here HV~b and ~ are the Hamiltonians of vibronic and spin—orbit couplings. respectively. 2 is shown in fig. plot reasonable of kmag versus the squarethat of the magnetic field strength H Ia,The on the assumption kmag(H) value is zero. The selection rule for this mechanism is as follows [7, 9]. 1. z~J=0,±l 2. z~M= 0 3. z~N=0, ±1 4. z~K=0,±1 5. L~S0 6. F X FR >< F’ = Ftotaiiy symmetric 7. Either of the primary state or the secondary state has a magnetic moment. Here, FR is the irreducible representation of axial vector. Mechanism II Mechanism II is based on the shift of the rovibronic levels belonging to secondary states by the Zeeman effect. This mechanism becomes effective in the cases where the level densities of the secondary states are sparse at zero
A. Matsuzaki, S. Nagakural Magnetic quenching of fluorescence
kmag
2
~
kmagJ~~
H
117
J~c~
Fig. 1. The dependence of magnetic quenching constant k,ecg on the square of magnetic field strength H’: (a) for case 1; (b) for case 3; (c) for case 4.
magnetic field. In these cases the wave-function of a primary state is represented by eq. (2). =
~
+~
?,~IJsIHVib+ H 50I~i~)~Js5/(E~ E5) —
(2) The decay constant of a primary state into secondary states is proportional to ~ c~.The vibronic levels of the secondary states are shifted and broadened by the Zeeman effect, and come close to those of the primary state: That is, the energy difference IE, E~Iin eq. (2) decreases and c~increases rapidly with increasing magnetic field. When the rovibronic levels are broadened and become diffuse at high field, the magnetic field effect on c5 may be saturated. Therefore, in mechanism II, kmag increases rapidly with increasing field strength at low magnetic fields, and is saturated at high field, as is shown in fig. lb. We can classify the magnetic quenching of molecular fluorescence in gaseous state into the following four cases. Case 1. When secondary states are quasi-continuum and (~i~IH~Içfr5) is nonzero, the magnetic quenching occurs by mechanism I. A magnetic quenching constant kmag varies with H as is shown in fig. la. Case 2. When the secondary states are quasi-continuum and (ç&~~H~Iy5) is zero, the magnetic quenching is not observed. Case 3. When secondary states are sparse and (~i~IH~Ii~i5) are zero, the magnetic —
2.7 IC
T~
2.5
~
2.3(
~
(a)
~
2.1
H / kG
Fig. 2. Magnetic field effects upon dynamical behavior of the tA5(O) state of glyoxal: (a) plot of k versus H; (b) plot of hr0 versus H.
118
A. Matsuzaki, S. Nagakura/Magnetic quenching of fluorescence
quenching occurs by mechanism II. kmag varies with H as is shown in fig. lb. Case 4. When secondary states are sparse and ~i/i0IH~Ii~i6) is nonzero, a magnetic quenching occurs by mixing of mechanisms I and II. At lower field, kmag increases rapidly with increasing field strength. At higher field, kmag increases quadratically with magnetic field strength. It is shown in fig. Ic. 3.2. Applications of the theory to CS,, glyoxal, and NO,
CS2. Since the matrix element (‘A2IH~I’~) is nonzero, mechanism I occurs. The magnetic moment of ‘A2 state is due to the molecular rotation, the orbital degeneracy (‘& ‘A2 + ‘B2) and the interaction with the triplet state. Mechanism II is due to3B,, the3A,, enhancement of the spin—orbit interactions of ‘A, and states by vibronic Zeeman and effect. Therefore, CS state with 2 belongs to case 4. 3AU) and (‘Au~HzJ’Ag)are zero, there Glyoxal. Sincechannel the matrix elements (‘AUIHZI is no possible of mechanism I. However, mechanism II occurs through the interaction of ~‘A~IH 3A~). 60+H~~~I Lifetimes (T) of fluorescence from the ‘A 0(0) state of gaseous glyoxal under various pressures were observed in the presence of magnetic field of 0, 1.02. 1.99, 3.92, and 8.04kG. The plot of lIT values against glyoxal pressures results in a straight line. From its slope and interception with the ordinate, the collisional quenching constants, k, and the collision-free lifetimes, r0, are obtained for the respective field strength. The k and I/To values are plotted as a function of field strength H. The results are shown in fig. 2. The k value increases with increasing field strength at low field, but decreases at the fields higher than 2 kG. The l/T( value increases with increasing field strength below 5 kG and is saturated to a constant value of 4.9 x iO~s~’at 5kG. This shows that, although at lower fields the level density of the secondary state is sparse and the energy is lost through collision, at higher magnetic fields and magnetic quenching of the fluorescence of glyoxal due to enhancement of intramolecular processes turns out to be explicitly observable because of the increase in the state density of a secondary state due to the broadening of its rovibronic levels and also due to coupling with the higher rovibronic levels of the ground state. These experimental results as well as the theoretical consideration support that glyoxal belongs to case 3. NO2. The fluorescence excitation spectra of NO2 in the regions of 470—490 nm and 390—4 10 nm in the presence of magnetic field below 5 kG shows that the magnetic quenching depends upon the excitation energy. Furthermore, the fluorescence decay curve was found to be nonexponential in the presence of magnetic field. These facts indicate that the magnetic field brings about the mixing of states. The magnetic quenching constant increases with increasing magnetic field in the region of 0—10kG with small but reproducible fluctuations. Therefore, mechanism II may contribute to the magnetic quenching in NO2. —*
‘~
A. Matsuzaki, S. Nagakura/ Magnetic quenching of fluorescence
119
References L.A. Turner, Z. Physik 65 (1930) 464. R. Solarz, S. Butler, and D.H. Levy, J. Chem. Phys. 58 (1973) 5172. 5. Butler, C. Kahler and D.H. Levy, J. Chem. Phys. 62 (1975) 815. A. Matsuzaki and S. Nagakura, Chem. Phys. Letters 7 (1974); Bull. Chem. Soc. Japan 49 (1976) 359. [5] A. Matsuzaki and S. Nagakura, Z. Phys. Chem. (N.F.) 101 (1976) 283. [6] P.W. Atkins and P.R. Stannard, Chem. Phys. Letters 47 (1977) 113. [7] P.R. Stannard, private communication. [8] HG. Kuttner, H.L. SeIzIe and E.W. Schlag, Chem. Phys. Letters 48 (1977) 207. [9] A. Matsuzaki and S. Nagakura, Helv. Chim. Acta 61(1978) 695. [10] M. Bixon and J. Jortner, J. Chem. Phys. 50 (1969) 3284; G.W. Robinson and C.A. Langhoff, Chem. Phys. 5 (1974)1. LI] [2] [3] [4]
ON THE MECHANISM OF MAGNETIC QUENCHING OF FLUORESCENCE IN THE GASEOUS STATE A. MATSUZAKI* and S. NAGAKURA Institute for Solid State Physics, University of Tokyo, Roppongi, Tokyo. Japan
Magnetic quenching of fluorescence in the gaseous state was studied experimentally and theoretically with glyoxal, CS 2, and NO2, and was found to be explained by mechanism I and mechanism II, which are due to interaction through the Zeeman hamiltonian and the shift and broadening of appropriate rovibronic levels by the Zeeman effect, respectively.
1. Introduction Magnetic quenching of emission 2B from gaseous molecules has been observed with the ~ state in ‘2 [1], the 2 state in NO2 [2, 3], the ‘A2 State in CS2 [4], and the ‘A~state in glyoxal [5]. The spin-multiplicities of quenched states are triplet for 12, doublet for NO2, and singlet for CS2 and glyoxal. A magnetic quenching constant krnag increases for ‘2 in proportion with the square of the field strength. For CS2, kmag increases steeply in a low field and in proportion to the square of the field strength in a high field. For glyoxal, kmag increases steeply in a low field and is saturated to a constant value. Recently, Atkins and Stannard [6,7] proposed a theory to explain these phenomena by introducing the direct mechanism and the indirect mechanism. We have developed the theory and have proposed mechanism I and mechanism II. The first-order term of mechanism I is the same as the direct mechanism. Mechanism II is based on the shift and broadening of appropriate rovibronic levels by the Zeeman effect and is different from the indirect mechanism in which a magnetic field enhances radiationless decay of a primary state into a quasi-continuum through the mixing with another discrete state. The magnetic field dependence of the collision-free quenching constant observed by the present authors [5] and KUttner et al. [8] can be well explained by mechanism II.
2. Experimental Glyoxal and NO2 molecules were excited by the use of a Molectron DL-200 dye laser pumped by a Molectron UVI000 nitrogen laser. The band width of the *
Present address: Institute of Space and Aeronautical Science, University of Tokyo. 115
116
A. Matsuzaki, S. Nagakura/Magnetic quenching of fluorescence
laser was 0.02 nm on tuning at 455.0 nm. Emissions from gaseous glyoxal and NO, were monochromatized by a Spex model 1702 spectrometer and were detected by a combination of Hamamatsu TV Rl06 photomultiplier shielded by it-metal and a PAR model 160 boxcar integrator gated by magnetic field [9].
3. Results and discussion 3.1. Mechanism of magnetic quenching We consider the two types of mechanisms for the magnetic enhancement of the intramolecular nonradiative process from a primary state, ç1’~,,to secondary states, ‘(J~ mechanism I and mechanism II. Mechanism I Mechanism I is due to the coupling of states by the Zeeman Hamiltonian H~. The nonradiative decay constant for this case is represented by the following equation [101. 2
knr = 2lThpI(tfIPIHvIh + H 60 + HZI~I1S~I = 27rhp[~/Ip~HVjh+ HSOII/JS)12 + (cIJPIHV~b+H
60f ~P6X~P~IH~It/16) +(~1PIHZjc1i,)(~fPIHVh+ H60I~/i.) 2I + =
k
k~IH~k/i6)J
0+ k
2)
kniag(H) + kmag(H
0+kmag.
(1)
Here HV~b and ~ are the Hamiltonians of vibronic and spin—orbit couplings. respectively. 2 is shown in fig. plot reasonable of kmag versus the squarethat of the magnetic field strength H Ia,The on the assumption kmag(H) value is zero. The selection rule for this mechanism is as follows [7, 9]. 1. z~J=0,±l 2. z~M= 0 3. z~N=0, ±1 4. z~K=0,±1 5. L~S0 6. F X FR >< F’ = Ftotaiiy symmetric 7. Either of the primary state or the secondary state has a magnetic moment. Here, FR is the irreducible representation of axial vector. Mechanism II Mechanism II is based on the shift of the rovibronic levels belonging to secondary states by the Zeeman effect. This mechanism becomes effective in the cases where the level densities of the secondary states are sparse at zero
A. Matsuzaki, S. Nagakural Magnetic quenching of fluorescence
kmag
2
~
kmagJ~~
H
117
J~c~
Fig. 1. The dependence of magnetic quenching constant k,ecg on the square of magnetic field strength H’: (a) for case 1; (b) for case 3; (c) for case 4.
magnetic field. In these cases the wave-function of a primary state is represented by eq. (2). =
~
+~
?,~IJsIHVib+ H 50I~i~)~Js5/(E~ E5) —
(2) The decay constant of a primary state into secondary states is proportional to ~ c~.The vibronic levels of the secondary states are shifted and broadened by the Zeeman effect, and come close to those of the primary state: That is, the energy difference IE, E~Iin eq. (2) decreases and c~increases rapidly with increasing magnetic field. When the rovibronic levels are broadened and become diffuse at high field, the magnetic field effect on c5 may be saturated. Therefore, in mechanism II, kmag increases rapidly with increasing field strength at low magnetic fields, and is saturated at high field, as is shown in fig. lb. We can classify the magnetic quenching of molecular fluorescence in gaseous state into the following four cases. Case 1. When secondary states are quasi-continuum and (~i~IH~Içfr5) is nonzero, the magnetic quenching occurs by mechanism I. A magnetic quenching constant kmag varies with H as is shown in fig. la. Case 2. When the secondary states are quasi-continuum and (ç&~~H~Iy5) is zero, the magnetic quenching is not observed. Case 3. When secondary states are sparse and (~i~IH~Ii~i5) are zero, the magnetic —
2.7 IC
T~
2.5
~
2.3(
~
(a)
~
2.1
H / kG
Fig. 2. Magnetic field effects upon dynamical behavior of the tA5(O) state of glyoxal: (a) plot of k versus H; (b) plot of hr0 versus H.
118
A. Matsuzaki, S. Nagakura/Magnetic quenching of fluorescence
quenching occurs by mechanism II. kmag varies with H as is shown in fig. lb. Case 4. When secondary states are sparse and ~i/i0IH~Ii~i6) is nonzero, a magnetic quenching occurs by mixing of mechanisms I and II. At lower field, kmag increases rapidly with increasing field strength. At higher field, kmag increases quadratically with magnetic field strength. It is shown in fig. Ic. 3.2. Applications of the theory to CS,, glyoxal, and NO,
CS2. Since the matrix element (‘A2IH~I’~) is nonzero, mechanism I occurs. The magnetic moment of ‘A2 state is due to the molecular rotation, the orbital degeneracy (‘& ‘A2 + ‘B2) and the interaction with the triplet state. Mechanism II is due to3B,, the3A,, enhancement of the spin—orbit interactions of ‘A, and states by vibronic Zeeman and effect. Therefore, CS state with 2 belongs to case 4. 3AU) and (‘Au~HzJ’Ag)are zero, there Glyoxal. Sincechannel the matrix elements (‘AUIHZI is no possible of mechanism I. However, mechanism II occurs through the interaction of ~‘A~IH 3A~). 60+H~~~I Lifetimes (T) of fluorescence from the ‘A 0(0) state of gaseous glyoxal under various pressures were observed in the presence of magnetic field of 0, 1.02. 1.99, 3.92, and 8.04kG. The plot of lIT values against glyoxal pressures results in a straight line. From its slope and interception with the ordinate, the collisional quenching constants, k, and the collision-free lifetimes, r0, are obtained for the respective field strength. The k and I/To values are plotted as a function of field strength H. The results are shown in fig. 2. The k value increases with increasing field strength at low field, but decreases at the fields higher than 2 kG. The l/T( value increases with increasing field strength below 5 kG and is saturated to a constant value of 4.9 x iO~s~’at 5kG. This shows that, although at lower fields the level density of the secondary state is sparse and the energy is lost through collision, at higher magnetic fields and magnetic quenching of the fluorescence of glyoxal due to enhancement of intramolecular processes turns out to be explicitly observable because of the increase in the state density of a secondary state due to the broadening of its rovibronic levels and also due to coupling with the higher rovibronic levels of the ground state. These experimental results as well as the theoretical consideration support that glyoxal belongs to case 3. NO2. The fluorescence excitation spectra of NO2 in the regions of 470—490 nm and 390—4 10 nm in the presence of magnetic field below 5 kG shows that the magnetic quenching depends upon the excitation energy. Furthermore, the fluorescence decay curve was found to be nonexponential in the presence of magnetic field. These facts indicate that the magnetic field brings about the mixing of states. The magnetic quenching constant increases with increasing magnetic field in the region of 0—10kG with small but reproducible fluctuations. Therefore, mechanism II may contribute to the magnetic quenching in NO2. —*
‘~
A. Matsuzaki, S. Nagakura/ Magnetic quenching of fluorescence
119
References L.A. Turner, Z. Physik 65 (1930) 464. R. Solarz, S. Butler, and D.H. Levy, J. Chem. Phys. 58 (1973) 5172. 5. Butler, C. Kahler and D.H. Levy, J. Chem. Phys. 62 (1975) 815. A. Matsuzaki and S. Nagakura, Chem. Phys. Letters 7 (1974); Bull. Chem. Soc. Japan 49 (1976) 359. [5] A. Matsuzaki and S. Nagakura, Z. Phys. Chem. (N.F.) 101 (1976) 283. [6] P.W. Atkins and P.R. Stannard, Chem. Phys. Letters 47 (1977) 113. [7] P.R. Stannard, private communication. [8] HG. Kuttner, H.L. SeIzIe and E.W. Schlag, Chem. Phys. Letters 48 (1977) 207. [9] A. Matsuzaki and S. Nagakura, Helv. Chim. Acta 61(1978) 695. [10] M. Bixon and J. Jortner, J. Chem. Phys. 50 (1969) 3284; G.W. Robinson and C.A. Langhoff, Chem. Phys. 5 (1974)1. LI] [2] [3] [4]