2
586
M.S. Gudipati et al. / Spectrochimica Acta Part A 56 (2000) 2581–2588
Thus, both nonradiative relaxation and perma-
ishes (Fig. 3). At the same time, we observe
nent photolysis of O increase with increasing O2
increasing permanent photolysis of O with in-
2
2
concentration in Ar matrices. The photophysical
and photochemical pathways that may be of im-
creasing O concentration. These observations re-
veal two important aspects of photochemistry of
2
portance in O aggregates are sketched in Fig. 5.
O aggregates in Rg matrices. Firstly, at higher
2
2
Due to the fact that we do not observe any
significant changes in the spectral profiles of the
excitation spectra or the line widths and positions
of the A%X vibronic bands in the emission
concentrations, permanent photolysis of O oc-
curs not only in the defect sites, but also in perfect
sites of the Ar lattice. However, after photolysis
2
of O the crystallinity of the Ar lattice gets de-
2
spectra at different O concentrations, we con-
stroyed (diminution of the Ar exciton bands at
2
clude that the electronic interactions between O2
molecules both in the ground and electronically
excited states in the aggregates are weak, in agree-
ment with the experimental [18–20] and theoreti-
cal [21,22] data on (O ) .
105 nm after photolysis of O in Fig. 3). Secondly,
2
the photolysis of O does not give rise to O atoms
2
or even if generated, these O atoms undergo
nonradiative relaxation upon excitation due to the
presence of O in their vicinity, one of the possi-
2
2
3
bilities being the formation of O [24], as shown in
4
4.2. Excitation spectra of O atoms in Ar matrices
Fig. 5.
If the photolysis of O in the aggregates indeed
2
The excitation spectra of O shown in Fig. 2
produces ozone, then there remains an important
question, why ozone itself is not further pho-
tolyzed to O2 and O, subsequently generating
2
indicate that at lower concentrations (0.1 and
.2%) permanent photolysis of O is negligible.
0
2
Thus, the excitation spectra of O atoms in Ar
shown in Fig. 3 should be due to a minor fraction
back the O aggregates. In the VUV region be-
tween 6 and 9 eV (200–138 nm) the absorption
2
(
ꢀ10%) of photolyzed O at lower concentra-
cross-section of O is much weaker than the ab-
2
3
tions and we observe a linear increase in the
intensity of the excitation spectra indicating con-
centration dependent production of atomic oxy-
sorption cross-section of O between 180 and 138
2
nm (Schumann–Runge continuum) [25,26]. Thus,
when irradiated at longer wavelengths than 138
nm, though both O and O will be photolyzed,
gen at 0.1 and 0.2% initial concentrations of O in
2
3
2
Ar. In the literature, it is reported that the perma-
the equilibrium of photolysis lies more towards
the formation of ozone (Fig. 5). The spectral
region between 9 and 12 eV (138–100 nm) is
dominated by Rydberg transitions with significant
absorption cross-sections in both O and O in the
gas-phase [25]. However, in Rg matrices, it is well
known that the Rydberg transitions are energeti-
cally blue shifted compared with the gas-phase
[27] and sometimes even strongly suppressed. We
could not detect so far the Rydberg transitions in
nent photolysis of O occurs at defect sites of Rg
2
lattice and intracage recombination of geminate O
atom-pair occurs in perfect sites [23]. At 0.5% O2
concentration, however, we observe that only 47%
of the expected isolated O atoms are generated. It
should be kept in mind that in order to spectro-
scopically detect O atoms, photo-dissociation of
3
2
O must result in permanent cage exit of at least
2
one O atom (otherwise recombination of geminate
O atom-pair takes place). Accordingly, permanent
the excitation spectra of O by monitoring the
2
photo-dissociation of O followed by cage exit of
A%X or aX emission of O in Rg matrices,
2
2
at least one of the O atoms, results in the loss of
every second O atom generated in matrices con-
probably either due to the suppression of the
Rydberg transitions or due to other relaxation
taining 0.5% O initial concentration. This is only
pathways than dissociation of O after excitation
2
2
possible, if there exists another O molecule in the
into the Rydberg transitions. Hence, we assume
2
same Ar cage or in the neighboring unit cells of
the Ar lattice, which reacts with O generating O3.
that in O too, the Rydberg transitions do not
play significant role in the photodissociation of O3
in the Rg matrices. Further VUV spectroscopic
3
At higher O concentrations (1 and 2%) genera-
2
tion of the isolated atomic oxygen further dimin-
studies on O in Rg matrices are necessary to get
3