5554
J. Phys. Chem. 1988,92, 5554-5555
Intermediate Case Behavior on Fluorescence Depolarization by an External Magnetic Field in Pyrimidine Vapor Nobuhiro Ohta,* Takeshi Takemura, and Hiroaki Baba Division of Chemistry, Research Institute of Applied Electricity, Hokkaido University, Sapporo 060, Japan (Received: June 21, 1988)
Fluorescence of pyrimidine vapor on excitation into high rotational levels belonging to the vibrationless level in SI is depolarized by an external magnetic field with a behavior that is characteristic of the intermediate case molecules. The fast decaying portion of fluorescence is not affectedby the magnetic field with a strength of 0-150 G, whereas the slow component is effectively depolarized by the magnetic field. The field strength required to depolarize the slow fluorescence becomes larger with shortening the lifetime of the slow fluorescence, as expected theoretically for excitation into a single rotation-vibration level.
Introduction Magnetic depolarization of resonance fluorescence (Hanle effect) has attracted much attention as a powerful tool for measuring the radiative lifetime and the magnetic moment of the excited The experiments have so far been reported in atoms and small molecules composed of two or three atoms in which the individual rotation-vibration levels are regarded as On optical excitation of a polyatomic separately molecule in which intramolecular dynamics plays an important role, however, a number of mixed levels, each of which is composed of plural zeroth-order states, are regarded as coherently excited. The question then arises, How do external magnetic fields affect the fluorescence polarization of such a large molecule? The mechanism of the magnetic depolarization of fluorescence in large molecules may differ from the one in small molecules. In this Letter, we report the preliminary results of the magnetic depolarization of fluorescence of pyrimidine vapor. Pyrimidine is well-known to belong to the intermediate case molecule which exhibits at low pressures a biexponential decay of fluorescence composed of fast and slowly decaying portion^.^,' It is shown how each of the fast and slow components of fluorescence is affected by an external magnetic field.
Experimental Section Pyrimidine (Aldrich Chemical Co.) was purified by repeated vacuum sublimation. The sample pressures were determined by using a capacitance manometer (MKS Baratron-type 170). Optical measurements were carried out with the same apparatus and the same orthogonal geometry as reported in previous The frequency of a dye laser (Lambda Physik FL2002) pumped by a XeCl excimer laser (Lambda Physik EMG 103 MSC) was doubled by a KDP crystal. The generated UV light, which is linearly polarized, was used for excitation. The exciting light has a line width of -0.2 cm-' and a duration of -10 ns. The fluorescence at 340 nm was viewed at right angles both to the direction of propagation and to the polarization direction of the exciting light. External magnetic fields, denoted by H , were applied along the propagation direction of the fluorescence observed. The time-dependent intensities of the fluorescence polarized parallel and perpendicular to the polarization direction of the exciting light are denoted by Zll(t) and I l ( t ) , respectively. Then, the time-dependent degree of polarization, P(t ) , is defined as P ( t ) = (Zll(t) - Z L ( t ) ) / { Z l 1 ( t ) + Zl(t)). The time-independent f 1) Hanle. W. 2.Phvs. 1924. 30. 93. (2j See, for a review: Zare, R.N. Arc. Chem. Res. 1971, 4, 361. (3) Kroll, M. J. Chem. Phys. 1975,63, 1803. (4) Silvers, S. J.; McKeever, M. R. Chem. Phys. 1978, 27, 27. (5) Weber, H. J.; Bylicki, F. Chem. Phys. 1987, 116, 133. (6) Uchida, K.; Yamazaki, I.; Baba, H. Chem. Phys. Left. 1976,38, 133; Chem. Phys. 1978, 35, 91. (7) Spears, K. G.; El-Manguch, M. Chem. Phys. 1977, 24, 6 5 . (8) Ohta, N.; Fujita, M.; Baba, H. Chem. Phys. Lett. 1987, 135, 330. (9) Ohta, N.; Takemura, T.; Fujita, M.; Baba, H. J . Chem. Phys. 1988, 88, 4197.
degree of polarization, P , is similarly defined by using the integrated intensities of Illand I,, which were obtained by integrating the time-dependent intensity with respect to time ( t ) .
Results and Discussion Figure 1 shows the fluorescence decays of Zll(t)and Zl(t) of pyrimidine vapor at 0.03 Torr, together with P(t),obtained at room temperature on excitation at the Q-branch peak of the 0-0 band belonging to the So S1 transition. The decays shown in parts a and b of Figure 1 were observed in the presence of H of 100 G and in a zero field, respectively. As is seen in Figure 1b, decay shapes of Ill(')and I L ( t ) in a zero field are identical with each other, and the value of P ( t ) is 0.24 during the decay of fluorescence. In the presence of H, however, Zll(t) and Zl(t) give different shapes from each other, and P ( t ) varies according to t . At 100 G, for example, P(t) is about 0.17 at t = 0 and suddenly decreases to zero with increasing t (see Figure la). By considering a large overlap between the fast and slow components at the initial stage of time, it is khown that the slow component is almost completely depolarized by H of 100 G and that the fast component is not affected by H . Under the present experimental conditions, the intensity of the scattered light was confirmed to be negligibly low. Notice that the intensity ratio between the fast and slow components at 100 G differs from the corresponding one in a zero field since only the slow component is effectively quenched by H , as mentioned in a previous paper.g Figure 2 shows plots of P vs H at various pressures, where P is defined by the sum of the integrated intensities of the fast and slow components. As is seen in Figure 2, P gradually decreases with increasing H a n d it approaches a nearly constant value. The constant value of P at high fields is about 0.08 at 0.03 Torr, and it increases monotonically with increasing pressure. These results, combined with the fact that the intensity ratio of the fast component to the slow one increases with increasing pressure,6g7 also indicate that the degree of polarization of the fast component is not affected by the magnetic field employed. It is also known from Figure 2 that the field strength required to depolarize the slow fluorescence becomes larger with increasing pressure. The time-independent degree of polarization of only the slow component, denoted by P', was evaluated by combining the decays of ZII(t) and Z,(t) with the total integrated intensities of Il and l I,. Figure 3 shows plots of P' against H at different pressures. The field strength with which P'is reduced to one-half its initial value, H I / * ,is evaluated to be about 25, 45, and 100 G at 0.03, 0.07, and 0.14 Torr, respectively. The lifetime of the slow fluorescence (7s)also depends on and the f S values at 0.03, 0.07, and 0.14 Torr are obtained to be 300, 150, and 80 ns, respectively. A theoretical treatment of the magnetic depolarization of resonance fluorescence was reported by ZareIo and Loge and Parmenter" for excitation into a single rotation-vibration level. -+
~
(10)
~~
Zare, R. N. J Chem. Phys. 1966, 45,4510.
0022-3654/88/2092-5554$01.50/00 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 5555
Letters
.
I
I I
I\
I 1
.[1 VI---Y
0.5
0
-0.1L
’
7 n
’
(a)
Time (,us) Time (,us) Figure 1. Fluorescence decays of pyrimidine vapor at 0.03 Torr for excitation at the Q-branch peak of the 0-0 band belonging to the So St transition, observed in the presence of H of 100 G (a) and in a zero field (b). Lower solid and dotted lines show the decays of I,,(t) and I l ( t ) , respectively. Upper solid lines give P ( t ) derived from I , ( t ) and I L ( t ) , and broken lines represent the averaged P ( t ) . The maximum of I l l ( t )is normalized to 2.
-
0.25
-
yLA
*-*-*-.-
0
O.*O-
+
0.44
- h i &
Torr
0.14
-A-A--b-
+ ‘\
Pyrimidine
0
*-*-
A‘ & . A ‘
\\+ O\
t
50
( 0-Oexcitation)
100
150
H ( Gauss 1
Figure 2. Plots of P vs H in pyrimidine vapor at different pressures: (0), 0.03 Torr; (+), 0.07 Torr; (A),0.14 Torr; ( O ) , 0.44 Torr. Excitation position is the same as in Figure 1.
They showed that a plot of P as a function of H gives a Lorentzian shape with
Here, g and 7 are the Land6 g factor and the radiative lifetime of the excited level, respectively, and h and p are the Planck constant and the Bohr magneton, respectively. By comparing the present values of Hllzwith 7s, H1pof the slow fluorescence is known to be approximately in inverse proportion to 7s. Thus, it is suggested that a linear relation between H1,zand holds for the slow component, as is expected from eq 1, even when the fluorescence property is characterized by the intermediate case. Further, the g factors of the S1-TI mixed levels excited simultaneously are considered to be different from one another since the plots of P’vs H shown in Figure 3 deviate from a Lorentzian shape. By assuming that eq 1 holds for the slow fluorescence with 7s in the place of 7 , the average g factor is evaluated to be (7.8 on the present excitation. f 0.7) X
7c1
H ( Gauss 1 Figure 3. Plots of P’vs H in pyrimidine vapor at different pressures: (0), 0.03 Torr; (+), 0.07 Torr; (A),0.14 Torr. Excitation position is the same as in Figure 1. The lifetime of the slow fluorescence was varied by changing a pressure. If a collision-induced relaxation occurs from initially prepared levels to other fluorescent levels, the resulting fluorescence will be depolarized by a collision. As reported in previous pap e r ~ , ~however, & ’ ~ a collisional relaxation in pyrimidine vapor leads predominantly to quenching of the slow component of fluorescence. Under the present experimental conditions, therefore, it seems unnecessary to consider the collisional effects on fluorescence depolarization. The results that both values of P and P’in a zero field are 0.24 at every pressure below 0.44 Torr also indicate that the collision-induced depolarization is negligibly small under the present experimental conditions. Fluorescence of pyrimidine vapor is known to be depolarized by an external magnetic field with a behavior which is characteristic of the intermediate case molecules; i.e., the fast component is not affected by H with a strength of 0-150 G, whereas the slow component is effectively depolarized by H . With respect to the slow component, H1lZseems to be in inverse proportion to 7s,as expected theoretically for excitation into a single rotation-vibration level (see eq 1). As mentioned previously, present results were obtained on excitation at the Q-branch peak in the bulk gas at room temperature. The average J value of the excited levels is considered to be as large as -30 ( J i s the rotational quantum number of the total angular momentum excluding the nuclear spin).g As far as the vibrationless level is concerned, a difference of the magnetic depolarization between fast and slow components could be examined only in the bulk gas experiments since a biexponential decay of fluorescence could be observed only on excitation into high rotational levels. Fluorescence following excitation into low rotational levels with J of less than 5, which could be observed in a supersonic jet, is composed of only the slow component and exhibits a single-exponential d e ~ a y . ~As J ~is seen in our previous paper,8 P of the fluorescence following excitation at the R(0) line of the 0-0 band monotonically decreases with increasing H a n d becomes zero at H above 100 G. Those results correspond well to the fact that the fast component is nonexistent for excitation at the R(0) line. Thus, the slow fluorescence of pyrimidine vapor is known to be effectively depolarized by H , regardless of the rotational quantum number of the excited levels. Acknowledgment. N.O. thanks t h e Ministry of Education, Science and Culture, for Grant-in-Aid for Scientific Research on Priority Areas. (12) Baba, H.; Ohta, N.; Sekiguchi, 0.;Fujita, M.; Uchida, K. J . Phys. Chem. 1983, 87, 943. (13) Knight, A. E. W.; Jones, J. T.; Parmenter, C. S . J . Phys. Chem. 1983,
~.
R7 9 1 2_. - I ,
(11) Loge, G. W.; Parmenter, C. S. J. Chem. Phys. 1981, 74, 29.
(14) Saigusa, H.; Lim, E. C. J . Chem. Phys. 1983, 78,91.
