Hochstrasser, Robin M.

Wessel, John E.

Zewail, Ahmed H.

Caltech Authors - Main

Zeeman Effect Studies of the Triplet States of Benzene
Robin M. Hochstrasser, John E. Wessel, and Ahmed H. Zewail 
 
Citation: The Journal of Chemical Physics 55, 3596 (1971); doi: 10.1063/1.1676621 
View online: http://dx.doi.org/10.1063/1.1676621 
View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/55/7?ver=pdfcov 
Published by the AIP Publishing 
 
Articles you may be interested in 
Triplet States of Liquid Benzene and Toluene 
J. Chem. Phys. 45, 1522 (1966); 10.1063/1.1727795 
 
Triplet State of Benzene 
J. Chem. Phys. 38, 2541 (1963); 10.1063/1.1733538 
 
Photosensitization by Benzene Vapor: Biacetyl. The Triplet State of Benzene 
J. Chem. Phys. 37, 583 (1962); 10.1063/1.1701379 
 
On the Assignment of the Lowest Triplet State in Benzene 
J. Chem. Phys. 33, 937 (1960); 10.1063/1.1731299 
 
On the Lifetime of the Lower Triplet States of Benzene 
J. Chem. Phys. 20, 765 (1952); 10.1063/1.1700564 
 
 
 Reuse of AIP Publishing content is subject to the terms: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to  IP:  131.215.225.131 On:
Mon, 29 Aug 2016 23:24:52
3596 LETTERS TO THE EDITOR 
If the principal limitation on the coherence lifetime 
of the triplet exciton is trapping rather than phonon 
scattering, then the effective trapping rate constant is 
taken to be proportional to the product of the group 
velocity of the excitons perturbed by the microwave 
field VG(w) and the density of trapping sites DT • 
Since VG(w) =p(W)-1, Eq. (2) becomes 
~Nt(w)""""Drlexp[ -E(w) /kT]}(Ki-1+Kr1). (3) 
The essential feature of Eq. (3) is that the change in 
trap spin alignment as a function of microwave fre-
quency depends only upon the Boltzmann distribution 
of the population within the exciton spin manifolds. 
This dependence is illustrated by the trap-monitored 
exciton-electron spin transition in Fig. 1 and will be 
referred to as the concentrated trap limit. 
In another limit/ the dilute trap limit, when exciton-
phonon scattering is faster than trapping, the effective 
trapping rate constant K,....,.,VGDT , where Vy is the aver-
age group velocity taken over the distribution within 
the exciton spin manifold. In this limit, ~Nt(w) 
depends upon the Boltzmann population distribution 
and the density of states p(w). Consequently, the 
trap-monitored exciton-electron spin transitions exhibit 
THE JOURNAL OF CHEMICAL PHYSICS 
a central band minimum associated with the minimum 
in p(w) at k=±7r/2.4 
Experiments within the above limits should be ex-
tremely valuable for investigation of the dynamics of 
exciton migration insomuch as intrinsic phosphores-
cence from exciton states of molecular crystals has been 
observed in only a few cases. 
This work was supported by the Inorganic Materials 
Research Division of the Lawrence Berkeley Laboratory 
under the auspices of the U.S. Atomic Energy Com-
mlsslon. 
* Alfred P. Sloan Fellow. 
1 A. H. Francis and C. B. Harris, Chern. Phys. Letters 9, 188 
(1971) . 
2 G. A. George and G. C. Morris, unpublished results kindly 
provided to us in preprint form. G. C. Morris, Fifth Molecular 
Crystals Symposium, University of Pennsylvania, Philadelphia, 
Pa., June 1970. 
3 The exciton spin manifolds are defined as the crystal k mani-
folds associated with the three zero-field triplet spin sublevels. 
4 A. H. Francis and C. B. Harris, Chern. Phys. Letters 9, 181 
(1971) . 
5 C. Dean, M. Pollak, B. M. Craven, and G. A. Jeffrey, Acta 
Cryst. 11, 710 (1958); A. Monfils, Compt. Rend. 241, 561 (1955); 
F. H. Herbstein, Acta Cryst. 19,997 (1965). 
6 R. M. Hochstrasser and J. D. Whiteman, Organic Scintillators 
and Liquid Scintillation Counting, edited by D. L. Horrocks 
and C. T. Peng (Academic, New York, 1971), p. 511. 
7 A. H. Francis and C. B. Harris (unpublished results). 
VOLUME 55, NUMBER 7 1 OCTOBER 1971 
Zeeman Effect Studies of the Triplet States of Benzene* 
ROBIN M. HOCHSTRASSER, JOHN E. WESSEL, AND AHMED H. ZEWAIL 
Department of Chemistry and Laboratory for Research on the Structure of Malter, University of Pennsylvania, 
Philadelphia, Pennsylvania 19104 
(Received 9 July 1971) 
In this Communication we present the results of 
high field Zeeman effect studies of the 3Bluf-lAlo 
transition of solid perdeuterobenzene, and preliminary 
results on magnetic field effects on the 2700 X triplet 
state of benzene. We present data on the polarization 
of these transitions. The results are relevant to the 
vibronic coupling problem in benzene1- 3 ; to experiments 
on the distortion from D6h symmetry of phosphorescing 
benzene4- 6 ; and to recent absorption spectral analysis 
of vibronic states of 3B1u benzene.7 
The spectroscopic measurements were made in the 
manner of Burland, Castro, and Robinson7 but with 
oriented crystals of benzene-d6 in a Bitter solenoid that 
maintained a magnetic field of 105 kG. 
Benzene crystals are orthorhombic, Pbca, with four 
molecules per unit cell.8 One of the three possible car-
bon-carbon 1,4 directions is nearly parallel to the b 
axis; the corresponding bond-bisecting axis (y) and 
principal inertial axis (z) project substantially only 
onto a and c. 
The prominent absorption bands at 30422, 30070, 
and 30087 cm-1 (the latter two we subsequently refer 
to as the 30 078 cm-1 doublet) display 3 times stronger 
absorption for light polarized along the a axis than for 
light polarized along the b axis. These transitions are 
therefore out-of-plane (z) polarized. Vibronically in-
duced transitions do not usually manifest the crystal 
field induced intensitv.9 ,lO 
Figure 1 shows the spectrum of the 30 078 cm-1 
doublet at zero field, and the spectrum obtained with 
H = 105 kG directed along the b axis. The ms = 0 spin 
substate of the lower energy component, and the m.= 
± 1 substates of the higher energy component are the 
radiatively active states. If the vibronic symmetry of 
an upper level were E 1u (such as from c200B1u ), then 
only the m,,=±l states would be radiatively active for 
fields along the b axis. We observed a severely broadened 
spectrum for fields along the a axis, implying a more 
complex Zeeman pattern, but resulting in a much 
poorer signal-to-noise ratio. 
 Reuse of AIP Publishing content is subject to the terms: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to  IP:  131.215.225.131 On:
Mon, 29 Aug 2016 23:24:52
LETTERS TO THE EDITOR 3597 
The above results prove that the two components of 
the 30078 cm-l doublet have different vibronic sym-
metry. In DSh and in all its subgroups containing a three-
fold axis (only centrosymmetric subgroups included), 
it is impossible to find two vibronic states from the same 
nondegenerate electronic level that have the same (z) 
polarization yet different Zeeman effects. Therefore 
the unique symmetry group for 3BIu benzene-ds ap-
propriate to the dominant features of the vibronic 
wavefunctions and spin-vibronic coupling selection 
rules is D 2h • 
The distinctive intensity distribution of the b-axis 
Zeeman pattern indicates that the b axis is not sig-
nificantly far removed from a principal (in-plane) axis 
of a molecule: the 1,4 axis (x) oriented nearly parallel 
(x makes an angle of 16° with b) 8 to the b axis is thereby 
identified as being approximately aligned with a prin-
cipal direction,u This axis is likely to be the same as the 
x fine-structure axis for the phosphorescent state of 
benzene in benzene-ds at 1.95°K reported by de Groot, 
Hesselmann, and van der Waals to make an angle of 
16.6° to the b axis.12 
Conventional spin-orbit coupling rules applied to the 
b-axis field data yield the vibronic symmetries (in D2h ) 
of the two components of the 30078 cm-l doublet as 
30087 em-I, B 3·u CY) and 30070 em-I, B2u(X), The cal-
culated results pertaining to these assignments are 
sketched in Fig. 1 using the direction cosines for our 
choice of x and y, and the crystallographic data.s 
The unique explanation for in-plane spin activity and 
out-of-plane polarization is that the state involves e2g 
vibronic coupling. We therefore conclude that the 
30078 cm-l doublet arises because of the splitting of the 
vibronic level 3E1u (in D6h ) into 3B2u and 3B3u (in D2h) 
vibronic levels of a statically distorted form of benzene. 
>-
I-
00 
Z 
w 
I-
Z 
30121 
WAVENUMBER (eM-I) 
30036 
FIG. 1. Singlet-triplet absorption of solid benzene-d6 recorded 
in the manner of Ref. 7 using phosphorescence excitation at 
4.2°K. The dashed line refers to the 30078 cm-1 doublet (0+231 
em-I), and the solid line is the same system in a 105 kG magnetic 
field directed along the b axis of the crystal. The solid vertical 
bars represent the expected pattern from two states, one (B2u ) 
having spin quantization in a plane perpendicular to the b axis 
and the other (Bau) in a plane containing the b axis. 
This splitting is caused by an asymmetric (in D6h ) 
crystal field perturbation. 
Preliminary results on the level at 30422 cm-l 
indicate that the vibronic symmetry is EIu , and there-
fore that the vibration involved is e2g. The assignments 
of Burland, Castro, and Robinson7 based on an elec-
tronic origin at 29847 cm- l that associate this mode 
with V6' (575 em-I), and the 30 078 cm-l doublet with a 
transition involving vs' (,....,231 em-I), are consistent 
with the present study which definitively characterizes 
both these active modes as e2g. The proximity of V6' 
to V6" (579 em-I) and the large reduction of vs" (1558 
em-I) on excitation confirm the predictions of vibronic 
theory. 
The Colson and Bernstein systeml3 at 2700 A was also 
studied: the polarization measurements favor an in-
plane transition moment, and the b-axis magnetic field 
line broadening (thereby verifying the triplet nature of the 
upper state) is larger than for the a-axis broadening, 
indicating that the upper state symmetry is 3EIu as 
suggested.13 We will present an analysis of the spin and 
orbital momentum properties of this state in a forth-
coming paper. 
Note added in proof: Recent work by V. L. Broude 
[Opt. Spectrosc. 30, 46 (1971)J has shown that a 
molecular principal axis of the IB2u state is nearly 
parallel to the b-crystallographic axis of benzene. 
We wish to thank Dr. A. R. McGhie for invaluable 
assistance in preparing the benzene crystals, and the 
staff of the L.R.S.M. high magnetic field facility at the 
University of Pennsylvania. 
* This research was supported by a National Institutes of 
Health Grant GM 12592 and in part by the Laboratory for 
Research on the Structure of Matter, the University of 
Pennsylvania. 
1 W. Moffitt and A. D. Liehr, Phys. Rev. 106, 1195 (1957). 
2A. D. Liehr, Z. Naturforsch. 16a, 641 (1961). 
3 J. H. van der Waals, A. M. D. Berghuis, and M. S. De Groot, 
Mol. Phys. 13, 301 (1967). 
4 A. M. Ponte Goncalves and C. A. Hutchison Jr., J. Chern. 
Phys. 49,4235 (1968). 
• M. S. de Groot and J. H. van der Waals, Mol. Phys. 6, 545 
(1963) . 
6 G. C. Nieman and D. S. Tinti, J. Chern. Phys. 46, 1432 
(1967). 
7 D. M. Burland, G. Castro, and G. W. Robinson, J. Chern. 
Phys. 52, 4100 (1970). 
8 E. G. Cox, D. W. J. Cruickshank, and J. A. S. Smith, Proc. 
Roy. Soc. (London) A247, 1 (1958). 
• R. M. Hochstrasser, J. Wessel, J. D. Whiteman, and A. H. 
Zewail, Chern. Phys. Letters, "Characterization of Triplet States 
of Axially Symmetric Benzenes Using the Zeeman Effect," (to be 
published) . 
10 G. Castro and R. M. Hochstrasser, Chern. Phys. Letters 6, 
180 (1970). 
11 Such a Zeeman pattern could arise if the doublet were factor 
group splitting [see R. M. Hochstrasser, J. Chern. Phys. 47, 
1015 (1967)J, but the spectrum (Ref. 7) indicates that the 
stronger lines are vibrationally induced, and therefore that 16 
cm-1 is too large for factor group splitting: Nevertheless, we cannot 
rule out this possibility. 
12 M. S. de Groot, r. A. M. Hesselmann, and J. H. van der 
Waals, Mol. Phys. 16,45 (969). 
13 S. D. Colson and E. R. Bernstein, J. Chern. Phys. 43, 2661 
(1965) . 
 Reuse of AIP Publishing content is subject to the terms: https://publishing.aip.org/authors/rights-and-permissions. Downloaded to  IP:  131.215.225.131 On:
Mon, 29 Aug 2016 23:24:52


Zeeman Effect Studies of the Triplet States of Benzene

https://authors.library.caltech.edu/70664/1/1.1676621.pdf

Zeeman Effect Studies of the Triplet States of Benzene

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main