Research Reports of Nagaoka National College of Technology Vol.46Ȫ2010ȫőŢűŦųLASER-INDUCED FLUORESCENCE STUDY ONTHE QUNECHING RATES OF ELECTRONICALLYEXCITED SULFUR DIOXIDE WITH ALCHOLSYoshinori MURAKAMI1, Katsuhiko SUZUKI2, Yusuke ASAKAWA2
and Nobuyuki FUJII2
1Department of Materials Engineering, Nagaoka National College of Technology
2Department of Chemistry, Nagaoka University of Technology

The relaxation kinetics for the electronically excited SO2(1A2) was investigated by measuring the time
profiles for the laser-induced fluorescence intensities after the excitation of Clements F band of SO2molecules at around 303 nm. The semi-log plots for these time profiles were bent at 150 Ǵs after theUV-laser excitations and thus co-existence of the fast and slow relaxation mechanisms was suggested forthe relaxation of the electronically excited SO2(1A2). The quenching rate constants of the electronicallyexcited SO2(1A2) with several kinds of alcohols were also investigated and it was found that the collisional
quenching cross sections ıq were well correlated with the well-depth İ of the interactions between the bathgases.
Key Words: laser-induced fluorescence, quenching, sulfur dioxide, alcohol

1. INTRODUCTION

C1B240000

30000
Energy / cm -1

Since sulfur dioxide (SO2) is known as one of themost widely recognized atmospheric pollutants, anumber of authors have investigated the reactions of
SO2 in air. Although the photochemical reactions
between the electronically excited SO2 andhydrocarbons are known to produce the unvolatile
products,1), 2) the reaction mechanism was complex
and still remained unclear. This is partly due to thecomplicated interactions among the excited states of
SO2 as well as the unresolved relaxation kinetics ofthe electronically excited SO2 formed by the UV
light illuminations.
Fig. 1 is the schematic diagram of the electronicstructures of SO2. As shown in Fig. 1, the 1A2 state,
the 1B1 state and the 3B1 state overlap with each other.
Especially the vibronic couplings between the 1A2
state and 1B1 state are so strong that the internalconversions from 1A2 level to 1B1 level occurred very
efficiently,
SO2(1A2) ψ SO2(1B1)
followed by the subsequent relaxations from the 1B1state to the 3B1 state via the intersystem crossings.
Furthermore the relaxation processes from the 1A2
state to the 1A1 ground state also occurred simultaneously via the resonant or non-resonant fluorescence.3)
In the presence of the hydrocarbons these quenchi-B1B1
A1A2

a3B1

20000

10000

X1A1

0 diagrams for the ground and excited states
Fig. 1 Energyof SO2.

ng processes were influenced by collisional
quenching between the electronically excited SO2
with hydrocarbons and therefore the detailed
understanding of such quenching processes is very
important.
In the present work we have attempted to monitorthe relaxation kinetics after the UV-excitation of SO2
using the laser-induced fluorescence technique andto find the relationships between the collisional
quenching cross sections and molecular properties.

― 37 ―

Zptijopsj!NVSBLBNJ-!Lbutvijlp!TV[VLJ-!Zvtvlf!BTBLBXB!boe!Opcvzvlj!GVKJJthe 4th harmonic of Nd:YAG laser. The semilog
plots for the dispersed fluorescence of SO2 weregiven in Fig. 4. When the wavelength of thefluorescence detection became longer than 350 nm,
the semilog plots of the fluorescence decays wasalmost straight, but the wavelength of thefluorescence detection became as short as 280 nm,
the bent structures for the semilog plots of thefluorescence intensities became prominent (see Fig.
4). This could be explained when the dispersed
fluorescence longer than 350 nm was originated only
from 1A2 state, but the dispersed fluorescenceobserved at 280 nm was combination of thefluorescence for the pure 1A2 state and for the mixed

2. EXPERIMENTAL
The arrangement of the experimental setup havebeen described elsewhere.4) Briefly, the glass cell
was evacuated up to 10-4 Torr before the experimentand then filled with about 1 Torr of SO2. For thequenching measurements the mixed gas ofalcohol/SO2 were introduced in the glass cell andflowed slowly to avoid contaminations inside theglass cell.
The probe laser, which was generated by thefrequency doubling of the dye laser pumped by theNd:YAG laser, was irradiated to the glass cell and thefluorescence was collected by the photomultipliertube which was placed perpendicular to the probe
laser beam. The time-resolved fluorescence decays
were monitored by the digitizing oscilloscope withthe averaging of about 3000 measurements.

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3. RESULTS AND DISCUSSIONS㩿㪹㪀㪞㫃㫅㩷㩿㪠㫅㫋㪼㫅㫊㫀㫋㫐㪀

Fig. 2 shows a typical example for the laser-inducedfluorescence (LIF) spectra obtained when 1 Torr ofSO2 was filled in the glass cell. As shown in Fig. 2,the Clements E, F and G bands of SO2 were observed
as the fluorescence peaks.5) By fixing the wavelengthof the dye laser at the Clements F band, the time
profiles for the laser-induced fluorescence intensities
were measured. Typical examples of thefluorescence decays and the semilog plots were givenin Fig. 3(a) and Fig. 3(b). As shown in Fig. 3(b) thesemi-log plot of the SO2 LIF intensities was bent ataround 150 ȝs and therefore they had two decay
components, indicating that the radiative decay process for the electronically excited SO2 was not a singleprocess.

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plots of SO2-LIF intensities.

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Fig. 4306

Semilog plots of the dispersed fluorescence ofSO2 with excitation of 266 nm laser light.

Fig. 2 Laser-induced fluorescence spectra of SO2.

states of the 1A2 and 1B1 states. This is consistent withthe energy diagram of Fig. 1 that shows the energy
level of the 1B1 state is higher than that of the 1A2state.
Bae et al.6) also investigated the relaxation kinetics
on the first singlet excited state of SO2 from thetime-resolved fluorescence intensities in the 300-450

To confirm such multiple decay processes for theSO2-LIF intensities, we have measured the time
profiles of the dispersed fluorescence through amonochromator. For these measurements theexcitation wavelength was changed from that of theClements F band to 266 nm, which was generated by― 38 ―

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FYDJUFE!TVMGVS!EJPYJEF!XJUI!BMDIPMTnm region. They found the semi-log plot of time
profiles of the SO2 LIF intensities had fast and slow
decay components. They further analyzed the time
resolved emission spectra with different delay times.
They found many sharp fluorescence peaks appeared
just after the excitation of SO2 and then broad
continuum emission appeared at 100 - 300 ns after
the excitation of SO2. Finally the phosphorescence
between 400-450 nm appeared after 1 ȝs of theexcitation. They assigned the sharp fluorescencepeaks and the broad continuum as the 1A1 state andthe strong mixing state of 1A1 and 1B1, respectively.
Their conclusions are consistent with our previous
conclusions. Thus the different decay rates observedin Fig. 2(b) were probably due to the different
radiative lifetimes for these excited states between
the 1A1 and the mixing state of 1A1 and 1B1.
Next we have tried to measure the quenching rate
constants for these excited states by adding severalC  20.0W㪄㪈㩷㪆㩷㪈㪇㪍㩷㫊㪄㪈

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fitting these decay profiles with linear regressions,the quenching rate constant of SO2 with alcohol foreach of the fast and slow components was obtained.
The results were summarized in Table 1. Since thequenching rate constant is dependent of the colliding
velocities, the quenching rate constants kq wereconverted to the quenching cross section ıq via thefollowing equation

ethanepropanemethanolethanol
2-propanol
1-propanol
tert-butyl alcohol

12.0ıq = kq / v

(1)

8.0

4.0
0.0

0.5

1.0

1.5

2.0

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where v is the thermal velocity of colliding gases.
As you can see in Table 1, the quenching crosssection became larger when the size of alcohols
became larger. Since Parmenter and co-workers7)
have found the correlations between the quenching
cross section ıq and well-depth İ for the interactions
between the electronically excited molecules and thequenching gas, we have attempted to adopt their
theory to our experimental results.
According to their previous reports in Ref. 7, forthe reaction of2.5

ethanepropanemethanolethanol
2-propanol
1-propanol
tert-butyl alcohol

1.8

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0.5

1.0

1.5

2.0

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A* + M ĺ A + M

2.5



(2)

where A* is the electronically excited species and M
is the molecules or atoms that quench A*. For suchreaction the quenching cross section ıq can be
expressed as follows,Fig. 5 Stern-Volmer plot for the decay rates of SO2fluorescence. (a) Faster component and (b) slower
component. ٨(solid line): ethane, ‫(ع‬solid line):propane, ً (solid line): methanol, ٟ (solid line):

ıq = C exp(İA*M / kT)

ethanol, ٨ (dashed line): 2-propanol, ‫( ع‬dashedline): 1-propanol, ً(dashed line): t-buthyl alcohol.

(3)

where k and T are the Bolzmann constant and thetemperature, respectively. İA*M is the well-depth forthe interactions between A* and M. C is a constantobtained with empirical fit to the experimental
expressed as the combination relationship values.
Assuming the well-depth İA*M can be İA*M=(İA*A*kinds of alcohols. Fig. 5(a) and Fig. 5(b) shows theStern-Volmer plot for the fast and slower
components of the SO2 LIF decays. As seen in thesefigures, relatively good linearities were obtained. By― 39 ―

Zptijopsj!NVSBLBNJ-!Lbutvijlp!TV[VLJ-!Zvtvlf!BTBLBXB!boe!Opcvzvlj!GVKJJİA*M)1/2, the above equation can be transformed tobecome

work is still needed for clarifying the differences.

   ln (ıq) = ln(C) + ȕ (İMM / k)1/2

4. CONCLUSIONS(4)Temporal decays for the SO2-LIF intensities
were found to have fast and slow components.
Based on the dispersed fluorescence analysis, it
was suggested that the fast component was thefluorescence from 1A2 and the slower component
was that from the mixing state of 1A2 and 1B1states.
The quenching cross sections of theelectronically excited SO2 with alcohols weremeasured and it was found that the good linear
relationships were obtained in the the plots for thelogarithm of ıq versus (İMM / kT2)1/2 . However theslope of the plots for ln(ıq) versus (İMM / kT2)1/2 in
the SO2/alcohol system was about two times
higher than that in the SO2/alkane system andtherefore the roles of the hydrogen bondings to thequenching processes in the SO2/alcohol system
was suggested.

2 1/2

where ȕ = (İA*A* / kT ) and İMM is the well-depth forthe interactions between M themselves. Thus theplots for the logarithm of ıq versus (İMM / kT2)1/2
should be linear. Since the well-depths İMM are often
not available for a substantial number of gases,
therefore we have adopted the following empiricalrulesİMM / k § 1.5 Tb(5)where Tb is the boiling point of gases in K. 8)
The results of the plots for the logarithm of ıqversus (İMM / kT2)1/2 in the present SO2/alcohol
system are shown in Fig. 6.
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REFERENCES
1)C.C.Badcock, H.W.Sidebottom, J.G.Calvert,G.W.Reinhardt and E.K. Damon, Mechanism of
the Photolysis of Sulfur Dioxide-Paraffin
Hydrocarbon Mixtures. J.Am.Chem.Soc. 93,
pp. 3115-3121 , 1971.
2)H.W.Sidebottom, C.C. Badcock, J.G. Calvert,
B.R.Rabe and E.K. Damon : Mechanism of theP h o t o l y s i s o f M i x t u re s o f S u l f u r D i o x i d e w i t hOlefin and Aromatic Hydrocarbons. J.Am.Chem.Soc. 93,
pp. 3115-3121, 1971.
3) E.Hegazi, F.Al-Adel, A. Hamdan, A. Dastageer,Fluorescence of Indirectly Excited Low Vibrational Levelsof the 1A2 State of Sulfur Dioxide. J. Phys. Chem. 98,
pp.12169-12175, 1994.
4)Y.Murakami, Y.Sugatani and Y.Nosaka, Laser- induced
Incandescence study on the metal aerosol particles as theeffect of the surrounding gas medium. J.Phys.Chem.A. 109,
pp.8994-9000, 2005.
5) J.H.Clements, On the absorption Spectrum of Sulfur
Dioxide. Physical Review. 47, pp.224-232, 1934.
6) S.C.Bae, G.H.Kim and J.K.Ku, Relaxation Kinetics
on the First Excited Singlet State od SO 2 from Time
Resolved Emission Spectra, Chem. Phys. Lett. 265,
pp. 385-391, 1997.
7) M.H.Lin, M.Seaver, K.Y.Tang, A.E.W.Knight and C.S.
Parmenter, The roles of intermolecular potential welldepths in collision-induced state change. J. Chem. Phys.
70, pp.5442-5457.
8) J.O.Herschfelder, C.F.Curtiss and R.F.Bird,
Molecular Theory of Liquids and Gases, Wiley,New York, 1954.

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㫄㪼㫋㪿㪸㫅㫆㫃

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14

16

18

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22

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Fig. 6 The semilog plots for the quenching cross sections
ıq versus (İMM / kT2)1/2 in the present SO2/alcoholsystem As shown in Fig. 6, good correlations were obtainedin the the plots for the logarithm of ıq versus (İMM /kT2)1/2 in the present SO2/alcohol system (both for (a)fast and (b) slow components). However the slope in
the present SO2/alcohol systems was two time higher
than those of the SO2/alkane system. The reason is
not clear, but such difference might suggest somepossibilities that alcohol/SO2 system has different
quenching mechanism because of the existence ofhydrogen bondings between alcohols. Including theaccurate measurements of the well-depth İMM, further(Received September 29, 2010)

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