Absorption and fluorescence of OClO A2A2-X2B1 in solid Ne, Ar, and Kr. I. Vibrationally unrelaxed A→X emission

Article abstract of DOI:10.1063/1.476668

Dispersed laser-induced fluorescence of the A²A₂→X²B₁ transition of OClO in solid Ne in the spectral range 500-770 nm was recorded when the origin at 20991cm^-1 was excited. Progressions with spacings near 939 and 446cm^-1 are associated with vibrational modes ν₁ and ν₂ of the X state. A simultaneous fit of both modes yields ω₁^″=957.1±1.4, ω₂^″=452.6±0.4, x₁₁^″=4.47±0.04, x₂₂^″=0.54±0.05, and x₁₂^″=4.00±0.05cm^-1. When the 1₀¹ line of the A-X system at 21699cm^-1 was excited, vibrationally unrelaxed emission was observed in the spectral region 480-600 nm. Excitation of the 2₀¹ line at 21284cm^-1 generated weak vibrationally unrelaxed progressions. The visible absorption spectrum of OClO in solid Ne in the region 415-488 nm was recorded with a Fourier-transform spectrometer, yielding ν₀₀=20991.3, ν₁^′=707.9, ν₂^′=292.5, and 2ν₃^′=887.6cm^-1 for the A state. Simultaneous fits considering either only ν₁ and ν₂ modes or all three modes yield corresponding spectral parameters. Similar experiments were performed with OClO in solid Ar and Kr. Pronounced increases in ν₁^′ (716.0cm^-1 in Ar and 712.5cm^-1 in Kr) and ν₂^′ (302.3cm^-1 in Ar and 303.0cm^-1 in Kr) and a decrease in ν₀₀ (188cm^-1 and 331cm^-1 red-shifted, respectively) from that in the gas phase indicate substantial perturbation of the A state in solid Ar and Kr. An absorption continuum underlying the A-X system is attributed to absorption to the 1²A₁ state above the predissociation barrier. The phonon interaction increases and the threshold of the continuum decreases as the matrix host is altered from Ne to Ar to Kr.

Full text of DOI:10.1063/1.476668

Absorption and fluorescence of OClO Ã 2 A 2 –X̃ 2 B 1 in solid Ne, Ar, and Kr. I.
Vibrationally unrelaxed Ã→X emission
̃
Chin-Ping Liu, Lih-Huey Lai, Ying-Yu Lee, Shao-Ching Hung, and Yuan-Pern Lee
Citation: The Journal of Chemical Physics 109, 978 (1998); doi: 10.1063/1.476668
View online: http://dx.doi.org/10.1063/1.476668
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 109, NUMBER 3
15 JULY 1998
2
2
˜
˜
Absorption and fluorescence of OClO A A2 –X B1 in solid Ne, Ar, and Kr.
˜
˜
I. Vibrationally unrelaxed A˜X emission
Chin-Ping Liu, Lih-Huey Lai, Ying-Yu Lee, Shao-Ching Hung, and Yuan-Pern Leea)
Department of Chemistry, National Tsing Hua University, 101, Sec. 2, Kuang Fu Road, Hsinchu,
Taiwan 30043
͑Received 6 January 1998; accepted 7 April 1998͒
2
2
˜
˜
Dispersed laser-induced fluorescence of the A A2→X B1 transition of OClO in solid Ne in the
spectral range 500–770 nm was recorded when the origin at 20 991 cmϪ1 was excited. Progressions
with spacings near 939 and 446 cmϪ1 are associated with vibrational modes ␯1 and ␯2 of the X state.
˜
A simultaneous fit of both modes yields ␻1Љϭ957.1Ϯ1.4, ␻2Љϭ452.6Ϯ0.4, xЉ11ϭ4.47Ϯ0.04, x2Љ2
1
ϭ0.54Ϯ0.05, and x1Љ2ϭ4.00Ϯ0.05 cmϪ1. When the 10 line of the A–X system at 21 699 cmϪ1 was
˜ ˜
excited, vibrationally unrelaxed emission was observed in the spectral region 480–600 nm.
Excitation of the 201 line at 21 284 cmϪ1 generated weak vibrationally unrelaxed progressions. The
visible absorption spectrum of OClO in solid Ne in the region 415–488 nm was recorded with a
Fourier-transform spectrometer, yielding ␯00ϭ20 991.3, ␯1Јϭ707.9, ␯Ј2ϭ292.5, and 2␯Ј
3
ϭ887.6 cmϪ1 for the A state. Simultaneous fits considering either only ␯ and ␯ modes or all three
˜
1
2
modes yield corresponding spectral parameters. Similar experiments were performed with OClO in
solid Ar and Kr. Pronounced increases in ␯Ј (716.0 cmϪ1 in Ar and 712.5 cmϪ1 in Kr͒ and ␯Ј
1
2
͑302.3 cmϪ1 in Ar and 303.0 cmϪ1 in Kr͒ and a decrease in ␯ (188 cmϪ1 and 331 cmϪ1
00
˜
red-shifted, respectively͒ from that in the gas phase indicate substantial perturbation of the A state
in solid Ar and Kr. An absorption continuum underlying the A˜ –˜X system is attributed to absorption
to the 1 A1 state above the predissociation barrier. The phonon interaction increases and the
threshold of the continuum decreases as the matrix host is altered from Ne to Ar to Kr. © 1998
American Institute of Physics. ͓S0021-9606͑98͒00127-5͔
2
˜
Werner30,31 indicate that predissociation of the A state in-
volves subtle interplay between these states. No experimen-
tal observation of these additional states has been reported.
Infrared absorption spectra of OClO and its isomer
˜
I. INTRODUCTION
Chlorine oxide ͑ClO͒ plays an important role in atmo-
spheric chemistry. The Cl/ClO pair catalytically decomposes
ozone in the stratosphere.1–3 Recently, the role of chlorine
dioxide ͑OClO͒ as a possible additional source of depletion
of stratospheric polar ozone was investigated.4,5 OClO is also
suggested to be a product of the reaction
ClOO isolated in inert matrices32–36 and in crystalline ice37,38
are extensively studied. In the only report on the A˜ –X˜ sys-
35
¨
tem of matrix-isolated OClO, Muller and Willner recorded
the UV absorption spectrum of OClO in solid Ne; although
they stated that all line positions resemble those in the gas
phase, no values were listed. Absorption near the origin
͑450–476 nm͒ was too weak to be characterized.
ClOϩBrO→OClOϩBr,
͑1͒
and is thus an indicator of BrO chemistry.6,7 Therefore the
photochemistry of OClO has drawn the attention of several
investigators.8–20
The photoluminescence of species isolated in matrices
typically emanates from the ground vibrational level of the
lowest of a group of closely spaced electronic states. Hence,
it is interesting to perform laser excitation of OClO in ma-
Absorption bands of OClO in the region 300–480 nm
2
2
˜
˜
are ascribed to the A A2←X B1 system; spectral analysis
of this system in the gas phase is well established.9–11,21–27
˜
As the A state is highly predissociative, it has a small yield
˜
trices so as to observe emission from either the A or other
of fluorescence.24,27 The A state has a lifetime of less than
˜
states. We report here visible absorption and laser-induced
fluorescence spectra of OClO in solid Ne, Ar, and Kr. Vibra-
tionally unrelaxed emission was observed when the ␯ or ␯
100 ps, which decreases as vibrational quanta increase.26 Ex-
citation of
predissociation.11,22
Theoretical calculations predicted three additional elec-
␯
and
␯
vibrational modes enhances
2
3
1
2
2
˜
mode of A A2 was excited. There is also evidence that a
continuous background underlying the A˜ –˜X system may be
assigned to absorption to the 1 A1 state. Our spectral inves-
2
2
2
˜
˜
tronically excited states ( B2 , 1 A1 , and 2 A1͒ lying near
2
˜
28–31
2
˜
the A A2 state.
Extensive calculations by Peterson and
tigation provides basis for photoexcitation of OClO to vari-
˜
ous vibronic levels of A state which shows mode specificity,
a͒
Author to whom correspondence should be addressed; jointly appointed by
the Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei,
Taiwan. Electronic mail: yplee@net.nthu.edu.tw; FAX: 886-3-5722892.
site selectivity, and a strong dependence on the host, as re-
ported in a subsequent paper ͑Paper II͒.39
0021-9606/98/109(3)/978/10/$15.00
978
© 1998 American Institute of Physics
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J. Chem. Phys., Vol. 109, No. 3, 15 July 1998
Liu et al.
979
Ϫ1
˜ ˜
FIG. 1. Visible absorption spectra of OClO A←X in solid Ne, Ar, and Kr at 5.5 K (M/Rϭ3200, resolution 0.5 cm ͒. Lines associated with three sites M,
P, Q, are indicated for ͑2,1,0͒. The threshold of the continuum is indicated by arrows.
II. EXPERIMENT
halogen light source, a quartz beam splitter, and a Si detec-
tor. A bandpass filter ͑450Ϯ40 nm͒ served both to increase
detectivity and to avoid photodecomposition due to emission
in the shorter wavelength region. Typical resolution is
0.5 cmϪ1 and the signal was averaged for 400 scans.
OClO was synthesized by reaction of KClO3, H2SO4,
and oxalic acid as described previously.42 The gaseous prod-
uct was carried with N2 gas, dehydrated on passing a U-tube
containing P2O5, and collected in a trap at 163 K ͑slush of
n-hexane and liquid N2͒; subsequent vacuum distillation be-
tween 170 and 210 K further purified the OClO product. The
purity of OClO was verified by its IR absorption in a gas
cell. Ne ͑99.995%͒, Ar ͑99.999%͒, and Kr ͑99.99%͒ were
used without further purification.
The experimental setup is similar to that described
previously.40,41 Briefly, the matrix was formed on deposition
of a gas mixture onto either a gold-plated or a platinum-
plated copper target maintained at 5 K by means of a closed-
cycle cryogenic system with a three-stage expander ͑APD,
HC-4͒. Typically the flow rate was 2–5 mmol hϪ1 with a
deposition period of 1–3 h. The cryogenic system was de-
signed to allow recording visible and IR absorption spectra
and laser-induced fluorescence spectra with the same sample.
The matrix was irradiated with either a dye laser
͑Lambda Physik, LPD3002E͒ pumped by a XeCl excimer
laser ͑Lambda Physik, LPX220i, pulse width Ͻ17 ns, spec-
tral width ϳ0.20 cmϪ1͒ at 308 nm or, in later stages of ex-
periments, a solid state OPO system ͑Spectra Physics,
MOPO-730, 3–8 ns, spectral width Ͻ0.20 cmϪ1͒ pumped
with a Nd-YAG laser ͑GCR-270, injection-seeded͒, and
laser-induced emission was recorded during irradiation. This
emission, after passing appropriate glass filters, was spec-
trally resolved with a 0.32-m or 0.64-m monochromator
III. RESULTS AND DISCUSSION
A. Visible absorption spectra of OClO in matrices in
the region 415–488 nm
2
2
˜
˜
Visible absorption spectra of OClO A A2←X B1 in
solid Ne, Ar, and Kr were recorded with a Fourier-transform
interferometer equipped with a quartz beamsplitter and a Si
detector ͑Fig. 1͒. These spectra provide spectral parameters
͑Jobin-Yvon, model HR320 or HR640, 1200 grooves mmϪ1
,
reciprocal linear dispersion 2.5 or 1.27 nm mmϪ1, respec-
tively͒ and detected with a photomultiplier ͑Hamamatsu
R955 or R928͒. The signal was subsequently amplified
͑bandwidth 5 MHz͒ before being sent to a boxcar integrator
͑Stanford Research Systems, SR250͒. A digital delayed pulse
generator ͑Stanford Research Systems, DG535͒ served to
provide triggering sequencies for each instrument. The signal
was typically averaged over 20–30 laser pulses for each data
point. For measurements of dispersed fluorescence, the
monochromator was typically stepped at 0.05–0.10 nm in-
crements; the slit width was 0.05–0.10 mm. For the excita-
tion scan, the monochromator was set at a specific wave-
length with slit width 0.05–0.10 mm, and the laser was
stepped at 0.02-nm increments.
˜
of the A state more accurate than those from fluorescence
excitation spectra partly because of better resolution and
wavelength accuracy achievable with an interferometer, and
partly because of the relatively weak fluorescence due to
predissociation. Line positions of OClO observed in various
matrices are listed in Table I. Lines of OClO in solid Ne are
red-shifted from those reported in the gas phase by approxi-
mately 24 cmϪ1, indicating a small perturbation by the Ne
host. Lines are red-shifted ϳ178 or 331 cmϪ1 when the ma-
trix host is Ar or Kr, respectively. Vibrational wave numbers
2
˜
for the A A2 state of OClO in solid Ne ͑707.9, 292.5, and
887.6 cmϪ1 for ␯1 , ␯2 , and 2␯3 , respectively͒ are close to
10
Visible absorption spectra were recorded with a Fourier-
transform spectrometer ͑Bomem DA8͒ using a quartz-
those ͑708.6, 288.1, and 886.8 cmϪ1
͒
of gaseous OClO.
Vibrational wave numbers of ␯1 , ␯2 , and 2␯ are 716.0,
3
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980
J. Chem. Phys., Vol. 109, No. 3, 15 July 1998
TABLE I. Absorption wave numbers/cmϪ1 of the A–X system of OClO in matrices.
Liu et al.
˜ ˜
Ne matrix
20 991.3
Ar matrix
Kr matrix
Gasa
( Ј , Ј , Ј)
v1 v2 v3
͑0,0,0͒
͑0,1,0͒
20 827.8
͑20 891.4, 20 848.9͒
21 130.1
20 684.8
͑20 758.6, 20 725.9͒
20 987.8
͑21 060.2, 21 027.4͒
21 293.3
21 015.5
b
b
21 283.8
21 699.2
21 303.6
21 724.1
͑21 194.5, 21 165.5͒
͑0,2,0͒
͑1,0,0͒
21 543.8
21 397.3
͑21 606.0, 21 575.2͒
͑21 472.0, 21 437.6͒
21 581.9
͑0,0,2͒
͑1,1,0͒
21 878.9
21 987.6
21 902.3
22 007.1
21 842.6
21 696.1
͑21 907.3, 21 870.6͒
͑21 770.0, 21 734.3͒
21 875.9
͑0,1,2͒
͑1,2,0͒
͑2,0,0͒
22 160.3
22 268.1
22 397.9
22 182.3
22 423.3
22 138.2
22 253.0
21 993.3
22 102.5
͑22 315.8, 22 284.1͒
22 425.0
͑22 176.9, 22 143.5͒
22 271.4
͑1,0,2͒
͑2,1,0͒
22 567.9
22 683.1
22 596.9
22 704.1
22 547.9
22 397.6
͑22 611.3, 22 580.7͒
22 716.1
͑22 472.4, 22 437.2͒
22 561.1
͑1,1,2͒
͑2,2,0͒
͑3,0,0͒
22 848.5
22 962.5
23 091.6
22 872.6
23 116.6
22 841.6
22 955.4
22 690.4
22 800.5
͑23 017.6, 22 986.3͒
23 115.4
͑22 874.5, 22 840.3͒
22 958.4
͑2,0,2͒
͑3,1,0͒
23 249.5
23 371.9
23 279.7
23 393.6
23 245.4
23 091.4
͑23 308.9, 23 274.8͒
͑23 167.7, 23 131.4͒
23 245.0
͑2,1,2͒
͑3,2,0͒
͑4,0,0͒
23 526.1
23 647.1
23 778.7
23 551.1
23 804.2
23 376.9
23 493.0
23 649.8
͑23 712.2, 23 682.1͒
23 802.6
͑3,0,2͒
͑4,1,0͒
23 928.8
24 054.6
23 958.2
24 076.8
23 936.4
aReference 10.
bWave numbers due to two additional sites are listed in parentheses; see text.
302.3, ϳ895, and 712.4, 303.0, 897.1 cmϪ1 for the A state of
˜
The largest discrepancy between our parameters and
those derived from jet-cooled OClO is ␻Ј because xЈ33 is
OClO in solid Ar and Kr, respectively. The matrix shifts,
3
undetermined from our data. Richard and Vaida10 reported a
negative x3Ј3 and attributed it to a double-minimum potential
in the Q3 coordinate. We observed several lines involving
particularly for ␯ ͑ϳ5%͒, are relatively large with solid Ar
2
˜
and Kr, indicating substantial perturbation of the A state by
the matrix host.
Observed line positions were fitted by least squares to
the equation
2␯ which were unobserved in the supersonic jet experi-
2
ment; hence xЈ22 is determined for the first time. Values of ␻Ј
1
and ␻Ј increase substantially whereas xЈ22 decreases as the
matrix host is changed from Ne to Ar or Kr.
␯ϭA ϩ␻Ј Јϩ 1͒ϩ␻Ј Јϩ 1͒ϩ␻Ј Јϩ 1͒ϪxЈ
͑v1 ͑v2 ͑v3
Ј
2
͑v1
Ј
2
2
2
1
2
3
11
ϩ 1͒2ϪxЈ Јϩ 1͒2ϪxЈ Јϩ 1͒ Јϩ 1͒ϪxЈ
͑v2 ͑v1 ͑v2
Ј
There are two additional sites in solid Ar. Site Q with
line positions approximately 63 cmϪ1 greater than those as-
sociated with the major site (M) has features more intense
than those of site P, approximately 31 cmϪ1 greater than
those of site M, as labeled in Fig. 1. Observed line positions
of both sites are listed parenthetically in Table I. We are able
to correlate lines corresponding to these sites with those in
IR absorption by means of site-selective photolysis, to be
discussed in Paper II.39 Similarly, there are two additional
sites of OClO in solid Kr: line positions of sites Q and P are
about 74 cmϪ1 and 40 cmϪ1, greater than those of site M,
respectively, as listed parenthetically in Table I.
͑v2
2
2
2
2
22
12
23
ϩ 1͒ Јϩ 1͒ϪxЈ Јϩ 1͒ Јϩ 1͒,
͑v3 ͑v1 ͑v3
͑2͒
2
2
2
2
13
with
A ϭ␯ Ϫ 1 ␻Јϩ␻Јϩ␻Ј͒ϩ 1 xЈ ϩxЈ ϩxЈ ϩxЈ ϩxЈ ͒
͑
͑
11
Ј
00
2
4
1
2
3
22
12
23
13
͑3͒
in which ␻iЈ(iϭ1–3) is the ith harmonic vibrational wave
number of the A state, xЈ is the anharmonicity, and ␯00 is the
wave number of the origin. The term associated with xЈ33 is
omitted because only lines involving 3ϭ0, 2 are observed.
Fitted results are listed in Table II; all listed uncertainties
denote single standard deviation of the fitting. As lines in-
volving 2␯ are sparse, we also fit the data involving only ␯
˜
ij
v
Lines of O37ClO in solid Ar and Kr are resolved from
35
˜
those of O ClO for absorption levels above A(2,0,0); they
are listed in Table III. The isotopic shifts agree well with
those reported with supersonic experiments.10
3
1
and ␯ modes; the results are also listed in Table II for com-
2
parison.
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J. Chem. Phys., Vol. 109, No. 3, 15 July 1998
TABLE II. Comparison of spectral parameters/cmϪ1 of X and A states of OClO.
Liu et al.
981
˜
˜
Ne matrix
Ar matrix
Kr matrix
Gasa
2B1 state
˜
X
␻
␻
x11
x22
x12
957.1Ϯ0.4b
452.6Ϯ0.4b
4.47Ϯ0.04b
0.54Ϯ0.05b
4.00Ϯ0.05b
944.8c
958.7Ϯ0.4b
454.7Ϯ0.3b
4.40Ϯ0.03b
0.59Ϯ0.03b
3.58Ϯ0.04b
947.1c
955.8Ϯ0.4b
454.7Ϯ0.3b
4.41Ϯ0.04b
0.64Ϯ0.03b
3.94Ϯ0.04b
955.95
449.85
4.2
0.15
4.5
945.2
447.3
1110.8
1
2
␯
944.1c
—
1
␯
448.7c,d
450.7c,d
2
␯
1107.6c
1106.5c
1102.2c
3
2A2 statee
˜
A
␯
20 991.8Ϯ0.9
͑20 991.6Ϯ0.7͒
20 827.9Ϯ0.7
͑20 827.8Ϯ0.5͒
20 684.7Ϯ1.1
͑20 684.3Ϯ0.9͒
21 017.2
713.2
00
f
f
f
␻
719.5Ϯ1.1
͑715.8Ϯ0.8͒
302.1Ϯ2.2
͑300.0Ϯ1.5͒
449.6Ϯ0.8
3.4Ϯ0.2
͑3.5Ϯ0.2͒
3.0Ϯ0.6
͑3.0Ϯ0.5͒
3.8Ϯ0.3
729.4Ϯ0.9
͑725.5Ϯ0.6͒
307.0Ϯ2.0
͑305.5Ϯ1.3͒
454.3Ϯ1.1
3.6Ϯ0.2
͑3.6Ϯ0.1͒
0.8Ϯ0.5
͑0.8Ϯ0.4͒
3.7Ϯ0.3
726.3Ϯ1.6
͑722.3Ϯ1.2͒
311.2Ϯ2.9
͑309.2Ϯ2.1͒
455.2Ϯ1.1
3.3Ϯ0.3
͑3.5Ϯ0.2͒
0.9Ϯ0.8
͑0.9Ϯ0.7͒
5.1Ϯ0.4
1
␻
285.3
2
␻
397.7
2.76
3
x11
x22
x12
3.55
͑3.8Ϯ0.3͒
8.4Ϯ0.3
4.5Ϯ0.6
͑3.8Ϯ0.2͒
8.4Ϯ0.3
3.3Ϯ0.6
͑5.2Ϯ0.3͒
9.9Ϯ0.4
4.7Ϯ0.7
x13
x23
x33
5.47
2.22
Ϫ13.15
708.6
288.1
886.8
␯
707.9
292.5
887.6
716.0
302.3
712.5
303.0
897.1
1
␯
2
2␯
3
aReference 10.
bDerived from dispersed fluorescence.
cIR absorption.
dReference 35.
eFrom visible absorption.
fParameters derived by fitting progressions involving only ␯ and ␯ are listed parenthetically.
1
2
10v 20v in Fig. 2͑A͒;
and
are vibrational quantum num-
v2
B. Dispersed fluorescence of OClO in solid Ne
v1
1
2
˜
bers of the X state. Observed line positions were fitted by
Part of the dispersed fluorescence spectrum of OClO in
solid Ne recorded with laser excitation at 20 991 cmϪ1, the
origin of the A–X system, is illustrated in Fig. 2͑A͒; ob-
least squares to the equation
˜ ˜
␯ϭA Ϫ␻Љ Љϩ 1͒Ϫ␻Љ Љϩ 1͒ϩxЉ Љϩ 1͒2ϩxЉ
͑v1 ͑v2 ͑v1 ͑v2
22
Љ
Љ
2
2
2
1
2
11
served line positions are listed in Table IV. The spectrum
shows two series of progressions with spacings approxi-
mately 447 and 930 cmϪ1, corresponding to ␯ and ␯ of the
ϩ 1͒2ϩxЉ Љϩ 1͒ Љϩ 1͒,
͑v1 ͑v2
͑4͒
2
2
2
12
2
1
2
A ϭ␯ ϩ 1␻Љϩ 1␻ЉϪ 1xЉ Ϫ 1xЉ Ϫ 1xЉ ,
͑5͒
˜
X B1 state. The assignment is unambiguous, labeled as
Љ
00
2
2
4
4
4
12
1
2
11
22
in which ␻iЉ(iϭ1,2) are harmonic vibrational wave numbers
˜
35
TABLE III. Absorption wave numbers/cmϪ1 of the A–X system of O ClO
ij
˜ ˜
and xЉ are anharmonicities of the X state. The fitting yields
and O37ClO in solid Ar and Kr.
␯00ϭ20 996.5Ϯ0.7, ␻Љ1ϭ957.1Ϯ0.4, ␻Љ2ϭ452.6Ϯ0.4, x1Љ1
ϭ4.47Ϯ0.04, xЉ22ϭ0.54Ϯ0.05, x1Љ2ϭ4.00Ϯ0.05 cmϪ1. The
vibrational frequencies ␯1ϭ946.1 and ␯2ϭ449.5 cmϪ1 pre-
Ar matrix
Kr matrix
O35ClO
dicted using these parameters agree with observed IR absorp-
( Ј , Ј , Ј)
v1 v2 v3
O37ClO
O35ClO
O37ClO
35,39
tion at 944.8 and 448.7 cmϪ1
.
The derived vibrational
͑2,0,0͒
͑1,0,2͒
͑2,1,0͒
͑2,2,0͒
͑3,0,0͒
͑2,0,2͒
͑3,1,0͒
͑4,0,0͒
22 253.0
22 425.0
22 547.9
22 841.6
22 955.4
23 115.4
23 245.4
23 649.8
22 245.7
22 102.5
22 271.4
22 397.6
22 690.4
22 800.5
22 958.4
23 091.4
23 493.0
22 095.5
22 260.9
22 389.0
22 681.1
22 788.1
22 942.9
23 077.7
23 474.6
wave numbers and anharmonicities agree with those reported
for the gas phase, as compared in Table II.
22 539.4
22 943.0
When OClO in solid Ne is excited with a laser at
1
0
21 699 cmϪ1
͑corresponding to the 1 line of the A˜ –X˜ sys-
tem͒, the fluorescence spectrum differs from that observed
upon excitation at 20 991 cmϪ1 (000 line͒, as illustrated in
panels ͑A͒ and ͑C͒ of Fig. 2. Observed line positions are
23 231.4
23 631.5
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982
J. Chem. Phys., Vol. 109, No. 3, 15 July 1998
Liu et al.
˜ ˜
˜
˜
˜
FIG. 2. Dispersed fluorescence spectra of OClO A→X in solid Ne upon excitation to: ͑A͒ A(0,0,0), ͑B͒ A(0,1,0), and ͑C͒ A(1,0,0). Monochromator ͑HR
640͒ slit width 0.125 mm; R and U designate emission from vibrationally relaxed and unrelaxed levels, respectively ͑see text͒.
listed in the lower part of Table IV. Vibrational relaxation in
matrices, in particular for polyatomic molecules, is usually
so rapid that only emission from the vibrationally relaxed
state is observed; consequently the same dispersed fluores-
cence spectrum is observed regardless of vibrational excita-
tion into the upper state. Observation of a different emission
spectrum upon excitation to 110 indicates that the emitter is
either vibrationally unrelaxed or is from a different electronic
state. Assignments of vibrational numbers were achieved on
comparison of line positions relative to those obtained with
excitation at the origin. An average deviation of 707.2
Ϯ1.1 cmϪ1 is determined with the best assignment ͑listed in
bers ( 1,0,0) of each ␯ progression also support our assign-
ment and rule out possibility of alternative assignments.
Hence, the emitting state clearly lies 707Ϯ1 cmϪ1 above the
v
2
˜
A(0,0,0) state. The spacing fits well with the vibrational
wave number 707.9 cmϪ1 of the ␯ mode of the A state,
˜
1
derived from the visible absorption spectrum. This progres-
sion is likely due to emission from the vibrationally unre-
˜
laxed state A(1,0,0) of OClO. Emission from the vibra-
˜
tionally relaxed A(0,0,0) state is too weak to be identified.
Hence, in the case of OClO the electronic predissociation is
apparently so fast that it prevails over intrastate vibrational
relaxation and only vibrationally unrelaxed emission is de-
tected.
Table IV͒. Variations of the assigned Љ value by Ϫ1 and Љ
v1 v2
by ϩ1 and ϩ2 yield much poorer fits ͑224.4Ϯ6.3 and
662.8Ϯ2.4 cmϪ1, respectively͒. The intense leading mem-
When OClO in solid Ne is excited at 21 284 cmϪ1 (201 of
TABLE IV. Wave numbers/cmϪ1 of lines in dispersed fluorescence spectrum of the A–X system of OClO in
˜ ˜
solid Ne.
ϭ1
ϭ2
ϭ3
ϭ4
ϭ5
ϭ6
ϭ7
ϭ8
v
v
v
v
v
v
v
v
Excitation at 20 991 cmϪ1 (000 line͒
1v0
20 051
19 605
19 160
19 112
18 671
18 231
17 792
17 353
16 916
18 185
17 748
17 310
16 876
17 265
16 833
16 399
15 969
15 539
15 107
16 354
15 925
15 496
15 068
14 642
14 219
15 454
15 028
14 603
14 180
13 757
14 561
14 139
13 718
13 299
13 677
13 260
10v201
10v202
10v203
10v204
10v205
10v206
15 575
18 474
Excitation at 21 284 cmϪ1 (201 line͒
10v210
19 402
17 557
16 643
17 064
15 746
ϳ14 857
Excitation at 21 699 cmϪ1 (101 line͒
1v1
19 819
19 378
18 892
18 455
17 974
17 541
17 106
11v201
11v202
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J. Chem. Phys., Vol. 109, No. 3, 15 July 1998
Liu et al.
983
sponds to the 110
line of the A˜ –˜X system, indicating direct
excitation to the emitting state. Vibrationally unrelaxed
˜
emission is hence observed when the A(1,0,0) state is ex-
˜
cited, and similarly when the A(0,1,0) state is excited.
Vibrationally unrelaxed emission is reported for only a
few triatomic molecules, such as CF2 ͑Ref. 43͒, ClCF ͑Ref.
44͒, CNN ͑Ref. 45,46͒, and NCO ͑Ref. 47͒. That emission
˜
from vibrationally unrelaxed OClO A state occurs because
˜
the A state is highly predissociative with an extremely short
lifetime ͑Ͻ100 ps͒.26 Vibrational relaxation in matrices48
fails to compete with predissociation/emission. This behavior
is also consistent with a small fluorescence yield observed in
this experiment. The fluorescence yield is extremely small
when ␯Ј is excited, indicating that predissociation via the ␯Ј
2
2
mode is much more facile than when ␯Ј is excited.
1
D. Dispersed fluorescence and excitation spectrum of
OClO in solid Ar
FIG. 3. Comparison of the absorption spectrum ͑A͒ with fluorescence exci-
0
0
˜ ˜
tation spectra of OClO A←X in solid Ne, recorded by monitoring line 1221
at 535.6 nm ͑B͒ and line 112 at 504.6 nm ͑C͒. Monochromator ͑HR 320͒ slit
width 0.05 mm.
Part of the dispersed fluorescence spectrum of OClO in
solid Ar with laser excitation at 480.12 nm (20 828 cmϪ1) is
illustrated in Fig. 4͑A͒; observed wave numbers are listed in
Table V. The spectrum is similar to that of OClO in solid Ne
͑Fig. 2͒ except that lines are better resolved. After fitting the
data to Eq. ͑4͒, we obtain ␯00ϭ20 829Ϯ0.7, ␻Љ1ϭ958.7
Ϯ0.4, ␻Љ2ϭ454.7Ϯ0.3, xЉ11ϭ4.40Ϯ0.03, xЉ22ϭ0.59Ϯ0.03,
and x1Љ2ϭ3.58Ϯ0.04 cmϪ1 ͑Table II͒. Predicted vibrational
spacings of ␯ and ␯ from these parameter are 948.1 and
˜ ˜
A–X͒, dispersed fluorescence is weak, as indicated by the
poor ratio of signal-to-noise in Fig. 2͑B͒. The spectrum con-
sists mainly of a single progression in ␯Љ ͑labeled U in the
1
˜
figure͒ arising from the excited A(0,1,0) state, with a weak
progression ͑labeled R͒ of emission from the vibrationally
˜
1
2
relaxed state A(0,0,0). Observed line positions of the unre-
451.7 cmϪ1, agreeing with 947.1 and 450.7 cmϪ1 reported
from the IR absorption spectrum of OClO in solid Ar.36,39
Similar to Ne matrix, excitation of OClO at 464.17 nm
laxed emission are also listed in Table IV, with vibrational
assignments derived by comparison of line separations with
˜
those originating from A(0,0,0). The emitting state of the
1
˜ ˜
progression labeled U lies 291Ϯ3 cmϪ1 above the A(0,0,0)
͑the 10 line of A–X system͒ yields vibrationally unrelaxed
emission, illustrated in Fig. 4͑C͒. Observed line positions are
listed in the bottom part of Table V; they are 717.1
Ϯ1.4 cmϪ1 greater than the corresponding lines observed
with 000 excitation. The spacings fit well with vibrational
˜
state; the energy separation fits well with the vibrational
wave number 292.5 cmϪ1 of the ␯ mode of the A state
˜
2
obtained from the visible absorption spectrum.
wave numbers 716.0 cmϪ1 for the ␯ mode in the A state.
˜
1
C. Excitation spectrum of OClO in solid Ne
When the 201 line of OClO in solid Ar was excited at
Because photodecomposition of OClO readily occurs at
shorter wavelength, all excitation spectra were recorded with
␭Ͼ458 nm. In this case, matrix isolation is disadvantageous
because the sample may not be replenished readily as in a
gaseous flow system. The excitation spectrum of OClO in
the region 20 830–21 820 cmϪ1 ͑480–458 nm͒, recorded by
monitoring the 102210 line at 535.6Ϯ0.25 nm, is shown in Fig.
3͑B͒; the absorption spectrum is shown in Fig. 3͑A͒ for com-
parison. The most intense line lies at 20 992 cmϪ1, corre-
sponding to the origin of the transition. A weak line at
21 699 cmϪ1 corresponding to the 110 line was also observed.
The 210 line at 21 284 cmϪ1 is weak. The small intensities of
these two lines are consistent with the proposal that predis-
sociation competes successfully with vibrational relaxation;
473.26 nm, a more complex spectrum was recorded, illus-
trated in Fig. 4͑B͒. The spectrum consists of emission lines
from the vibrationally relaxed upper state ͑labeled R͒ and
lines from vibrationally unrelaxed states ͑labeled U͒ with
comparable intensities. Observed line positions of the latter,
as listed in the middle part of Table V, are 303Ϯ2 cmϪ1
greater than corresponding lines observed with 000 excitation,
consistent with 302.3 cmϪ1 observed for the ␯ mode in the
2
˜
˜
A state. Vibrational relaxation from A(0,1,0) of OClO in
solid Ar appears more facile than in solid Ne, indicating an
increased lifetime of this predissociative state. This effect is
consistent with the increase of ␯ from 292.5 to 302.3 cmϪ1
2
as the matrix host is altered from Ne to Ar, as the latter has
a deeper well and may consequently disfavor predissocia-
tion.
Excitation spectra of OClO in solid Ar when relaxed or
unrelaxed emission lines are probed are similar to those ob-
served in solid Ne except that excitation to upper vibrational
states contributes to the population of the lower vibrational
state relatively more than those in solid Ne. The pattern of
˜
hence excitation to levels with energies greater than A(0,0,0)
˜
populates inappreciably the A(0,0,0) state.
When the line at 504.6Ϯ0.25 nm ͑the unrelaxed 112 line
˜ ˜
spectrum shows a single line at 21 699 cmϪ1 in the region
of the OClO A–X system͒ was monitored, the excitation
480–458 nm, as illustrated in Fig. 3͑C͒. This line corre-
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984
J. Chem. Phys., Vol. 109, No. 3, 15 July 1998
Liu et al.
˜ ˜
˜
˜
˜
FIG. 4. Dispersed fluorescence spectra of OClO A→X in solid Ar upon excitation to: ͑A͒ A(0,0,0), ͑B͒ A(0,1,0), and ͑C͒ A(1,0,0). Monochromator ͑HR
640͒ slit width 0.125 mm; R and U designate emission from vibrationally relaxed and unrelaxed levels, respectively.
excitation spectra also supports the assignment that vibra-
tionally unrelaxed emission is observed as we excited OClO
to the ͑1,0,0͒ and ͑0,1,0͒ states.
shown in Fig. 5͑A͒; observed wave numbers are listed in
Table VI. Table II lists the fitted spectral parameters: ␯
00
ϭ20 689.6Ϯ0.7, ␻1Љϭ955.8Ϯ0.4, ␻2Љϭ454.7Ϯ0.3, x1Љ1
ϭ4.41Ϯ0.04, xЉ22ϭ0.64Ϯ0.03, and xЉ12ϭ3.94Ϯ0.04 cmϪ1
.
These parameters imply ␯1ϭ945.6 cmϪ1, consistent with the
observed IR line at 944.1 cmϪ1 for OClO in solid Kr.39
Excitation at the 201 line at 20 988 cmϪ1 and 110 line at
21 397 cmϪ1 also showed predominantly vibrationally unre-
E. Dispersed fluorescence and excitation spectrum of
OClO in solid Kr
The dispersed fluorescence spectrum of OClO in solid
Kr during laser excitation at 483.44 nm (20 685 cmϪ1) is
35
TABLE V. Wave numbers/cmϪ1 of lines in dispersed fluorescence spectrum of the A–X system of O ClO and
˜ ˜
O37ClO ͑listed parenthetically͒ in solid Ar.
Vϭ1 ϭ2 ϭ3
ϭ4
ϭ5
ϭ6
ϭ7
ϭ8
v
v
v
v
v
v
v
Excitation at 20 828 cmϪ1 (000 line͒
1v0
19 880
19 434
18 988
18 540
18 098
18 941
18 498
18 056
17 614
17 173
16 733
16 297
18 012
17 571
17 132
16 696
16 259
15 822
17 091
16 654
16 219
15 786
15 352
14 919
16 176
15 745
15 314
14 884
14 454
14 028
13 601
13 176
15 273
14 844
14 417
13 991
13 566
13 143
14 377
13 953
13 530
13 108
13 492
13 071
10v201
10v202
10v203
10v204
10v205
10v206
10v207
Excitation at 21 130 cmϪ1 (201 line͒
10v210
19 242
18 313
17 394
16 479
15 578
Excitation at 21 544 cmϪ1 (101 line͒
1v1
20 600
20 152
19 705
19 259
18 812
19 660
18 727
͑18 744͒
18 287
͑18 307͒
17 850
͑17 871͒
17 412
͑17 437͒
16 975
17 807
͑17 829͒
17 371
͑17 395͒
16 936
͑16 960͒
16 501
͑16 526͒
16 069
16 894
͑16 921͒
16 462
͑16 489͒
16 031
͑16 058͒
15 599
͑15 628͒
15 990
15 562
a
11v201
11v202
11v203
19 216
18 772
͑18 788͒
18 329
͑18 350͒
17 890
11v204
11v205
17 449
16 538
ϳ15 637
aLines of O37ClO are listed parenthetically.
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J. Chem. Phys., Vol. 109, No. 3, 15 July 1998
Liu et al.
985
˜ ˜
˜
˜
˜
FIG. 5. Dispersed fluorescence spectra of OClO A→X in solid Kr upon excitation to: ͑A͒ A(0,0,0), ͑B͒ A(0,1,0), and ͑C͒ A(1,0,0). Monochromator ͑HR
640͒ slit width 0.125 mm; R and U designate emission from vibrationally relaxed and unrelaxed levels, respectively.
35
TABLE VI. Wave numbers/cmϪ1 of lines in dispersed fluorescence spectrum of the A–X system of O ClO
˜ ˜
and O37ClO ͑listed parenthetically͒ in solid Kr.
ϭ1
ϭ2
ϭ3
ϭ4
ϭ5
ϭ6
ϭ7
ϭ8
v
v
v
v
v
v
v
v
Excitation at 20 685 cmϪ1 (000 line͒
1v0
19 743
19 298
18 850
18 406
18 809
17 881
͑17 898͒
17 441
16 962
͑16 985͒
16 527
͑16 554͒
16 092
͑16 122͒
15 658
͑15 691͒
15 226
14 797
14 368
16 053
͑16 082͒
15 621
15 152
14 724
14 297
13 873
13 448
14 259
13 835
13 414
13 376
a
10v201
10v202
10v203
18 366
͑18 379͒
17 923
͑17 943͒
17 481
͑17 461͒
17 003
͑17 026͒
16 566
͑16 591͒
16 130
15 696
15 261
14 831
15 190
14 760
10v204
10v205
10v206
10v207
17 042
16 604
14 335
13 906
13 481
Excitation at 20 988 cmϪ1 (201 line͒
10v210
19 111
18 182
17 262
16 353
15 452
14 559
Excitation at 21 397 cmϪ1 (101 line͒
1v1
19 519
18 590
͑18 608͒
18 152
͑18 172͒
17 714
͑17 737͒
17 277
͑17 303͒
16 839
͑16 868͒
16 403
17 672
͑17 697͒
17 237
͑17 264͒
16 803
16 763
16 332
15 900
15 470
15 042
15 862
15 434
a
11v201
19 074
͑19 089͒
11v202
11v203
11v204
11v205
11v206
11v207
19 560
19 116
18 672
18 229
18 633
͑18 651͒
18 192
͑18 213͒
17 753
͑17 776͒
17 313
͑17 339͒
16 873
͑16 832͒
16 368
15 936
15 506
15 076
15 970
͑16 905͒
16 438
aLines of O37ClO are listed parenthetically.
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986
J. Chem. Phys., Vol. 109, No. 3, 15 July 1998
Liu et al.
G. Effects of the matrix host
laxed emission, shown in Figs. 5͑B͒ and 5͑C͒, respectively.
Observed lines, listed in Table VI, are 300.5Ϯ0.8 and
709.8Ϯ1.1 cmϪ1 greater than corresponding lines observed
with 000 excitation. These values agree with observed vibra-
˜
Spectral parameters of the X state are perturbed only
slightly by the matrix hosts, with variations of vibrational
wave numbers less than 0.8%. However, spectral parameters
tional spacings 303.0 and 712.5 cmϪ1 for ␯ and ␯1 , respec-
2
˜
of the A state are affected more substantially. The origin of
tively, from the visible absorption spectrum.
Ϫ1
˜ ˜
the A–X system is red-shifted by 24, 188, and 331 cm
from the gaseous value for Ne, Ar, and Kr matrices, respec-
˜
tively. The vibrational wave numbers of OClO A state in
F. Possible assignment of a continuum
solid Ne behaves similarly to that of gaseous OClO, as indi-
cated by nearly identical spectral parameters. In contrast, vi-
Blank experiments were performed for matrices contain-
ing pure Ne, Ar, or Kr; the absorption spectra show only
small variation in baseline, indicating negligible effects due
to scattering. These background spectra were subtracted from
absorption spectra of matrix-isolated OClO to yield the spec-
tra shown in Fig. 1.
A continuum absorption underlies the A˜ –˜X line spectra
of OClO. This continuum is more enhanced as the matrix
host is altered from Ne to Ar or Kr. A similar continuum was
reported for OClO in the gas phase14 and in solid Ne.35 This
brational wave numbers of ␯1 –␯ varied by as much as
3
˜
1.0%, 5.2%, and 1.2%, respectively, for OClO(A) in solid
Ar and Kr. The large increase in ␯ indicates a deeper
potential-energy surface of the A state of OClO in solid Ar
and Kr, with respective to bending motion. The stabilization
is further indicated by observation of A( 1,2,0) lines in solid
2
˜
˜
v
Ar and Kr, as the ␯ mode is highly predissociative and 2␯
2
2
was unobserved in the gas phase.10 The x2Ј2 value is therefore
derived for the first time. The increased predissociative life-
time of A(0,1,0) due to stabilization is also indicated by
observation of more vibrationally relaxed fluorescence of
2
˜
continuum might be due to absorption to the 1 A1 state
˜
above its predissociative barrier resulting from avoided
2
crossing of two A1 states. Estimated thresholds of this con-
˜
OClO(A) in solid Ar.
tinuum are 462, 465, and 470 nm ͑about 0.08 eV above the
The matrix effect is also indicated by pronounced pho-
non couplings. As shown in Fig. 1, phonon wings of OClO
lines are small in solid Ne, become relatively more enhanced
in solid Ar and Kr. The threshold of the continuum tenta-
tively assigned to 1 A1 decreases as the matrix host is al-
tered from Ne to Ar to Kr, indicating increased stabilization
˜ ˜
A–X origin͒ for OClO in solid Ne, Ar, and Kr, as indicated
by arrows in Fig. 1. According to theoretical calculations,
this barrier lies ϳ2.9 eV, slightly above calculated T0 ͑2.77
2
˜
eV͒ of the A A2 state. Above the barrier, OClO dissociate to
2
˜
2
˜
ClOϩO on the 1 A1 surface via the bending mode with a
near-linear transition state.
2
˜
of the 1 A1 surface in Ar and Kr hosts.
An indirect support of this assignment comes from ob-
In summary, OClO in solid Ne behaves similarly to that
in the gaseous phase, whereas the potential-energy surface of
OClO(A) is affected substantially in solid Ar and Kr, ac-
cording to observed spectral parameters. Such differences
reflect varied rates of dissociation, product yield, and site
selectivity upon photolysis, as we discuss in Paper II.39
servation of a continuum at wave lengths ͑450–800 nm͒
much greater than those ͑400–600 nm͒ of A˜ –˜X progressions
˜
of OBrO in solid Ar.49 The 1 A1 and 2 A1 states are cor-
2
2
˜
˜
2
2
related, after avoided crossing, with the linear ⌸u and ⌬g
2
*
states, respectively. The ⌬g state is formed by exciting a ␲
u
2
*
electrons to the ␴ orbital whereas the
⌸
state is the
u
g
*
ground electronic state with three ␲ electrons. As the cen-
u
IV. CONCLUSION
tral atom X changes from Cl to Br, the separation between
2⌬g and ⌸u states of linear OXO decreases; consequently
2
2
˜
Dispersed laser-induced fluorescence of the A A2
2
˜
the barrier of avoided crossing decreases. The A A2 state is
2
˜
→X B1 transition of OClO in solid Ne, Ar, and Kr in the
connected to a linear 2⌸g state which is formed by excitation
spectral range 480–770 nm was recorded when the origin
n
g
*
of a ␲ electron to the ␲ orbital. The separation between
˜
u
was excited. Vibrational progressions in ␯ and ␯ of the X
1
2
2⌸g and ⌸u decreases less than that between ⌬g and ⌸u
2
2
2
state are active and a simultaneous fit of both modes yields
2
2
as X changes from Cl to Br, because ⌸g← ⌸u involves
values of ␻1Љ , ␻Љ2 , xЉ11 , xЉ22 , and x1Љ2 . When 110 and 201 lines
mainly only p orbitals whereas 2⌬g← ⌸u involves s orbital
2
˜ ˜
of the A–X system were excited, vibrationally unrelaxed
of the central atom. Hence, the predissociative barrier of the
emission originating from the excited levels was observed;
2
2
˜
˜
1 A1 state of OBrO is expected to lie much below the A A2
state.
˜
˜
predissociation of A(0,1,0) is more facile than A(1,0,0).
Visible absorption spectra of OClO in solid Ne, Ar, and Kr
were recorded in the range 415–488 nm, yielding spectral
A possibility remains that the continuum should be as-
signed to absorption of dimeric or polymeric OClO. We
tested samples with host/guest ratios in the range 800–3200
but observed no apparent variation of the absorption con-
tinuum. The IR absorption spectrum also indicates a negli-
gible proportion of dimers in such samples. Upon photolysis
at wavelength corresponding to the continuum, no evidence
of Cl2O4, which is a product after photodissociation of
dimeric OClO, was observed. Hence, our results indicate that
assignment of the continuum to dimer is less favored.
˜
parameters of the A state that indicate substantial stabiliza-
2
˜
tion of the A A2 OClO in solid Ar or Kr. The continuous
absorption underlying the A˜ –˜X system may be assigned to
2
˜
absorption to the 1 A1 state above the predissociative
threshold.
Note added in proof. Recently Reid et al.50 determined
resonance Raman depolarization ratios of OClO in cyclohex-
2
2
˜
˜
ane solution and found evidence of the 1 A1 –X B1 transi-
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