- Email: [email protected]
The quenching of chlorine fluorescence in the gas phase
Volume 44, number 3
CKFMICAL PNYSJCS LE7TERS
R.E. HUIE*, N.J.T. LONG and B.A. THRUSH Physicnl Chemisrry, Vnivcrsifyof Cambridge, CombridgeCR? IEP, UK
fJepartment of
Received 13 September 1976
Laser induced fluorescence has been observed from the 0 3rT(O:) state of chlorine. Lifetimes have hem 19 to 208 Pa, but the radiative lifetime is too long te, bc extracted from the data. A rate mcMcbpnt of 3.2 x cm3 mol -’ s’-’ was measured for quenching by Cl1 or Ar, the removal process being collision indoced ~~~~~r~~ the 3n(O$ state.
I. introduction The low lying electronic states of the halogen molecules consist of a t Z, ground state, 311states, and a IIT state which is entirely repulsive. In the triplet manifold of chlorine, absorption only to the B 3 !I@:) state has been observed [ 1J , while emission from the radiative recombination of chlorine atoms and from the thermal decomposition of nitrogen trichloride shows weak emission from the A 3 !7( 1J state also [2] _ For bromine [3] and iodine [4] it has been possible to measure directly the lifetime of the B state using laser induced fluorescence, but an nt[empt to measure the lifetime of the B state of chlorine was unsuccessful [3]. Fluorescence, however, has been observed from gaseous chlorine excited by an argon ion laser [5] and from chlorine trapped in inert gas matrices using dye [6] and argon ion [?I lasers. in this paper, we report the measurement of Bifetimes for the B 3H(Oi) state of chlorine using a dye laser ten times more powerful than used in the pre vious attempt.
2. Experimental Research grade chiorine wa trap to trap distiIlation_ It was 10 cm diameter glass bulb whit using a mercury di3usim pm CelPv&x!:&?,enreevacuated and refilled to the pmmm, ~eas~~~~ using a silicone oil manometer. matt black on the outside to reduce ~~~~~~a~scztamed light. It was fitted with a 80 cm tide arm mm&n-
The fluorescence
The signal was avew 9425 boxcar system simultaneaursly 43s * Visiting Scientist. Permanent address: Institute for Materials Research, National Bureau of Standards, Washington, D.C. 2G234, USA.
of
monitored by a chart recmder.
CHEMICAL PHYSICS LE-I-IERS
Volume 44, number 3 3. Results
l/r=
The maximum fluorescence intensity was observed at a pumping wavelength of 485 nm, corresponding to a transition from the ground state to u’ = 20 of the B state 181. The fluorescence signal was weak, not much greater than required for measurement by our apparatus. The decay of the fluorescence signal was found to obey first order kinetics. About ten points were measured from each recorder trace and plotted as the logarithm of the intensity against time (fig. 1). The lifetime was derived using a weighted linear least squares program [9] with each point weighted according to its intensity. The lifetime was measured over a pressure range of I9 to 208 Pa and fitted to the Stern-Volmer equation (fig. 2):
1.5 Dccernber
l/rO+kqP,
1976
(1)
where 7. is the radiative lifetime, k, the quenching rate constant, and P the pressure. As the cil manometer loses relative precision at low pressures, the points were weighted assuming a constant reading uncertainty of 0.3 mm of oil and combining this with the standard error of the lifetime. A radiative decay rate for the B state of chlorine within the standard error of zero, l/70 = -0.06 + 0.06 ys-l, was derived. The quenchingratewask=(1.30+0.0G)X lo4 s-l Pa-l = (3.21 *0.15)X 1013 cm3 mol-l s-l,givingacalculated cross section of 8.9 K2. A measurement using a longer exciting wavelength of 498 nm (0” 13) I&], shghtly above the energy required for predissociation into two ground state atoms,gave no change in lifetime. No signal could be detected at exciting waveIengths greater than 505 run where the chlorine absorption is weaker.
The signal was too small to measure
with
reasonable precision at pressures below 19 Pa.< Lifetime measurements were also made using a mixture of 10% chIorine in argon. A radiative decay rate of l/r,-, = -0.13 f 0.10 p.s-l and a quenching rate of k, = (1.27 + 0.08) X lo4 s-l Pa-l .(3.14 + 0.19) X 1013 cm3 mol-l s-l WCI: obtained, nearly identical with the values for pure chlorine.
4. Discussion
oo_L-_Ii~ 06
09
12
15
time. 1,s Fig. 1. Decay 109 Pn.
of fluoresccnw
in pure chlorine,
total prcssurc,
32r
Fig. 2. Stern-Volmer
pure chlorine.
plot for tlic fluorescence lifetime in
The observed emission must come either from the initially populated 311(Oi) state or from the 31t(I,) or 311(2u) states which Iie below it. These ratter states would have longer radiative lines than the 3tI(Oi) state, for which Coxon [2J has calculated a radiative life of 600 ps from the intensity of the absorption spectrum. The present experiments only establish that the radiative life of the emitting state exceeds 10 ps. In a recent study of the laser induced fluorescence of chlorine in rare gas matrices, Bondybey and Fletcher [6] showed that the excitation spectrum in the region of 500 nm corresponded to population of the 3 II state but that the emission apparently came from a different electronic state lying some 650 cm-1 lower. This they identified with the 311(2,) state, which correlates with two Cl(2P3/2) atoms. Their work yields a radiative lift of 120-140 ms, extrapolated to the gas phase, for the 311(2,) state. Very low col609
Volume 44, number 3
CII~:MIChL PIIYSICS LETTERS
lisional quenching by rare gases is indicated, since emission is observed from the solid. This is in contrast with the present observations, where argon is an efticient quencher. It is therefore clear that the 311(2,,) state is not rcsponsiblc for the emission observed here In the chlorine afterglow, where the 311(01) state is strongly populated [ 10,I I], very weak emission is observed from the 311(1u) state, which lies between the 311(2u) and 311(OE) states. The emission, howcvcr, occurs at wavelengths greater than 740 nm [2] and would not be detected in the present experiments. WC therefore conclude that the ohscrvcd emission comes from the 31!(Oi) state and the emitting levels are depopulated at about one collision in three. Kinetic studies of the chlorine afterglow [IO,1 I] show that levels of the 311(Oz) state below the prcdissociation threshold are depopulntcd at about this rate by atomic chlorine, but that quenching by argon or molecular chIorme is at least 100 times slower. Since the levels populated in the present experiments lie above the predissociation limit for two C1(21a3,2) atoms, the obscrved removal process must be collision induced prcdlssociatiori. Lxdmindtmrl of rllc potential CLIIVL’Sfor chlorine [2,6] along with the high collision efficiency in&c&c that the prcdissociation occurs via the 3II( 1u) state, for which AR = 1, rather than the 311(2,) state,
where A52 = 2. The inverse of the collision induced predlssociatmn would be the direct population of CI,(31J(O:)) in a three body collision including two ground state chlorine atoms and a third body. For both of thcsc proccsscs, the transition between the 311(0z) and (1 u) states ~111occur predominantly between their inner limbs where the two states arc closest and have similar shapes. It is therefore unlikely that the predissociation rate will vary greatly between the present case where the excited chlorine mo!ecule is about 600 cm-t above the predissociation limit and the levels that would be populated by thermal chlorme atoms. WC can therefore USCequilibrium statistical mechanics [12,13] toestimate the rate ofpopulation of C12(317(Oz)) by collision induced inverse predissoaation. Taking the equilibrium distance in the molecu!e to be 2.76 A (v’ = 13) [ 141, an equilibrium constant between the 31J(Oi) state and two ground state chlorine atoms of 0.43 cm3 mol-l at 298 K is calculated. MultipIying this by the quenching rate con-
610
15 Dcccmbcr 1976
stant gives 1.4 X lOI cm6 mol-* s-1 as the rate constant for the population of the 31J(Oi) state in the three body collision of two ground state chlorine atoms with molecular chlorine or argon. This is almost a factor of ten lower than the rate constant derived by Brownc and Ogryzlo [l I] for the population of C12(311(0t)) in the reaction of Cl + Cl + Cl,. WC therefore conclede that the mechanism of population of the 3n(Oz) state in the chlorine afterglow is not collision induced inverse predissociation. The most probable mechanism is a three body reaction into bound levels of the 311(lu) state, followed by collision induced crossing into 31J(Oz) levels below the predissociation limit.
Acknowledgement WC thank the Science Rcscarch Council for equipment grants and a Studentship for N.J.T.L.
111 M.A.A. Uyne and J.A. Coxon. J. Mol. Socctrv. . - 33 (1970) 3si. PI J.A. Couon, Molecul,lr Spectroscopy.Spcclalist Pcriodlcal Rcportq. Vol. I, cds. RF. Barrow, D.A. Long .md D.J. hfdlcn (The Chernrcal Suciety, London, 1973) p 177. [31 G. Capcllc, K. Sakurai and 1I.P. Broida, J. Chcm. Phys. 54 (1971) 1728. I41 K. Sakurai, G. Capcllc and 11-P. Broida, J. Clam. Phys. 54 (1971) 1220. ISJ W. tfolzer, W.F. Murphy and H.J. Bcrpstcin, J. Chcm. Phys. 52 (1970) 469. [61 V-E. Bundybcy and C. Fletcher, J. Chcm. Phys. 64 (1976) 3515. I71 B.S. Ault, W.f . Howard and L. Andrcws, J. Mol. Spectry. 55 (1975) 217. PI AX. Douglns,C.K. M~llcr and B.P. Stoichcff, Can. J. Phys. 41 (1963) 1174. 191 V. Gcnstdt, Rothamstcd I;xpcrimental Station, private communication. 1101 M.A.A. Clync and D.H. Stcdman, Trans. I-araday Sot. 64 (1968) 1816. [111 R.J. Brownc and GA. Ogryzlo. J.Chcm. Phys. 52 (1970) 5774. [I21 T. Carrinpton, J. Chcm. Phys. 57 (1972) 2033. (131 M-F. Goldc and B.A. Thrush, Rept. Progr. Phys. 36 (1973) 1285. I141 J.A. Coxon, J. Quant. Spectry. Radiative Transfer 11 (1971) 443.
CKFMICAL PNYSJCS LE7TERS
R.E. HUIE*, N.J.T. LONG and B.A. THRUSH Physicnl Chemisrry, Vnivcrsifyof Cambridge, CombridgeCR? IEP, UK
fJepartment of
Received 13 September 1976
Laser induced fluorescence has been observed from the 0 3rT(O:) state of chlorine. Lifetimes have hem 19 to 208 Pa, but the radiative lifetime is too long te, bc extracted from the data. A rate mcMcbpnt of 3.2 x cm3 mol -’ s’-’ was measured for quenching by Cl1 or Ar, the removal process being collision indoced ~~~~~r~~ the 3n(O$ state.
I. introduction The low lying electronic states of the halogen molecules consist of a t Z, ground state, 311states, and a IIT state which is entirely repulsive. In the triplet manifold of chlorine, absorption only to the B 3 !I@:) state has been observed [ 1J , while emission from the radiative recombination of chlorine atoms and from the thermal decomposition of nitrogen trichloride shows weak emission from the A 3 !7( 1J state also [2] _ For bromine [3] and iodine [4] it has been possible to measure directly the lifetime of the B state using laser induced fluorescence, but an nt[empt to measure the lifetime of the B state of chlorine was unsuccessful [3]. Fluorescence, however, has been observed from gaseous chlorine excited by an argon ion laser [5] and from chlorine trapped in inert gas matrices using dye [6] and argon ion [?I lasers. in this paper, we report the measurement of Bifetimes for the B 3H(Oi) state of chlorine using a dye laser ten times more powerful than used in the pre vious attempt.
2. Experimental Research grade chiorine wa trap to trap distiIlation_ It was 10 cm diameter glass bulb whit using a mercury di3usim pm CelPv&x!:&?,enreevacuated and refilled to the pmmm, ~eas~~~~ using a silicone oil manometer. matt black on the outside to reduce ~~~~~~a~scztamed light. It was fitted with a 80 cm tide arm mm&n-
The fluorescence
The signal was avew 9425 boxcar system simultaneaursly 43s * Visiting Scientist. Permanent address: Institute for Materials Research, National Bureau of Standards, Washington, D.C. 2G234, USA.
of
monitored by a chart recmder.
CHEMICAL PHYSICS LE-I-IERS
Volume 44, number 3 3. Results
l/r=
The maximum fluorescence intensity was observed at a pumping wavelength of 485 nm, corresponding to a transition from the ground state to u’ = 20 of the B state 181. The fluorescence signal was weak, not much greater than required for measurement by our apparatus. The decay of the fluorescence signal was found to obey first order kinetics. About ten points were measured from each recorder trace and plotted as the logarithm of the intensity against time (fig. 1). The lifetime was derived using a weighted linear least squares program [9] with each point weighted according to its intensity. The lifetime was measured over a pressure range of I9 to 208 Pa and fitted to the Stern-Volmer equation (fig. 2):
1.5 Dccernber
l/rO+kqP,
1976
(1)
where 7. is the radiative lifetime, k, the quenching rate constant, and P the pressure. As the cil manometer loses relative precision at low pressures, the points were weighted assuming a constant reading uncertainty of 0.3 mm of oil and combining this with the standard error of the lifetime. A radiative decay rate for the B state of chlorine within the standard error of zero, l/70 = -0.06 + 0.06 ys-l, was derived. The quenchingratewask=(1.30+0.0G)X lo4 s-l Pa-l = (3.21 *0.15)X 1013 cm3 mol-l s-l,givingacalculated cross section of 8.9 K2. A measurement using a longer exciting wavelength of 498 nm (0” 13) I&], shghtly above the energy required for predissociation into two ground state atoms,gave no change in lifetime. No signal could be detected at exciting waveIengths greater than 505 run where the chlorine absorption is weaker.
The signal was too small to measure
with
reasonable precision at pressures below 19 Pa.< Lifetime measurements were also made using a mixture of 10% chIorine in argon. A radiative decay rate of l/r,-, = -0.13 f 0.10 p.s-l and a quenching rate of k, = (1.27 + 0.08) X lo4 s-l Pa-l .(3.14 + 0.19) X 1013 cm3 mol-l s-l WCI: obtained, nearly identical with the values for pure chlorine.
4. Discussion
oo_L-_Ii~ 06
09
12
15
time. 1,s Fig. 1. Decay 109 Pn.
of fluoresccnw
in pure chlorine,
total prcssurc,
32r
Fig. 2. Stern-Volmer
pure chlorine.
plot for tlic fluorescence lifetime in
The observed emission must come either from the initially populated 311(Oi) state or from the 31t(I,) or 311(2u) states which Iie below it. These ratter states would have longer radiative lines than the 3tI(Oi) state, for which Coxon [2J has calculated a radiative life of 600 ps from the intensity of the absorption spectrum. The present experiments only establish that the radiative life of the emitting state exceeds 10 ps. In a recent study of the laser induced fluorescence of chlorine in rare gas matrices, Bondybey and Fletcher [6] showed that the excitation spectrum in the region of 500 nm corresponded to population of the 3 II state but that the emission apparently came from a different electronic state lying some 650 cm-1 lower. This they identified with the 311(2,) state, which correlates with two Cl(2P3/2) atoms. Their work yields a radiative lift of 120-140 ms, extrapolated to the gas phase, for the 311(2,) state. Very low col609
Volume 44, number 3
CII~:MIChL PIIYSICS LETTERS
lisional quenching by rare gases is indicated, since emission is observed from the solid. This is in contrast with the present observations, where argon is an efticient quencher. It is therefore clear that the 311(2,,) state is not rcsponsiblc for the emission observed here In the chlorine afterglow, where the 311(01) state is strongly populated [ 10,I I], very weak emission is observed from the 311(1u) state, which lies between the 311(2u) and 311(OE) states. The emission, howcvcr, occurs at wavelengths greater than 740 nm [2] and would not be detected in the present experiments. WC therefore conclude that the ohscrvcd emission comes from the 31!(Oi) state and the emitting levels are depopulated at about one collision in three. Kinetic studies of the chlorine afterglow [IO,1 I] show that levels of the 311(Oz) state below the prcdissociation threshold are depopulntcd at about this rate by atomic chlorine, but that quenching by argon or molecular chIorme is at least 100 times slower. Since the levels populated in the present experiments lie above the predissociation limit for two C1(21a3,2) atoms, the obscrved removal process must be collision induced prcdlssociatiori. Lxdmindtmrl of rllc potential CLIIVL’Sfor chlorine [2,6] along with the high collision efficiency in&c&c that the prcdissociation occurs via the 3II( 1u) state, for which AR = 1, rather than the 311(2,) state,
where A52 = 2. The inverse of the collision induced predlssociatmn would be the direct population of CI,(31J(O:)) in a three body collision including two ground state chlorine atoms and a third body. For both of thcsc proccsscs, the transition between the 311(0z) and (1 u) states ~111occur predominantly between their inner limbs where the two states arc closest and have similar shapes. It is therefore unlikely that the predissociation rate will vary greatly between the present case where the excited chlorine mo!ecule is about 600 cm-t above the predissociation limit and the levels that would be populated by thermal chlorme atoms. WC can therefore USCequilibrium statistical mechanics [12,13] toestimate the rate ofpopulation of C12(317(Oz)) by collision induced inverse predissoaation. Taking the equilibrium distance in the molecu!e to be 2.76 A (v’ = 13) [ 141, an equilibrium constant between the 31J(Oi) state and two ground state chlorine atoms of 0.43 cm3 mol-l at 298 K is calculated. MultipIying this by the quenching rate con-
610
15 Dcccmbcr 1976
stant gives 1.4 X lOI cm6 mol-* s-1 as the rate constant for the population of the 31J(Oi) state in the three body collision of two ground state chlorine atoms with molecular chlorine or argon. This is almost a factor of ten lower than the rate constant derived by Brownc and Ogryzlo [l I] for the population of C12(311(0t)) in the reaction of Cl + Cl + Cl,. WC therefore conclede that the mechanism of population of the 3n(Oz) state in the chlorine afterglow is not collision induced inverse predissociation. The most probable mechanism is a three body reaction into bound levels of the 311(lu) state, followed by collision induced crossing into 31J(Oz) levels below the predissociation limit.
Acknowledgement WC thank the Science Rcscarch Council for equipment grants and a Studentship for N.J.T.L.
111 M.A.A. Uyne and J.A. Coxon. J. Mol. Socctrv. . - 33 (1970) 3si. PI J.A. Couon, Molecul,lr Spectroscopy.Spcclalist Pcriodlcal Rcportq. Vol. I, cds. RF. Barrow, D.A. Long .md D.J. hfdlcn (The Chernrcal Suciety, London, 1973) p 177. [31 G. Capcllc, K. Sakurai and 1I.P. Broida, J. Chcm. Phys. 54 (1971) 1728. I41 K. Sakurai, G. Capcllc and 11-P. Broida, J. Clam. Phys. 54 (1971) 1220. ISJ W. tfolzer, W.F. Murphy and H.J. Bcrpstcin, J. Chcm. Phys. 52 (1970) 469. [61 V-E. Bundybcy and C. Fletcher, J. Chcm. Phys. 64 (1976) 3515. I71 B.S. Ault, W.f . Howard and L. Andrcws, J. Mol. Spectry. 55 (1975) 217. PI AX. Douglns,C.K. M~llcr and B.P. Stoichcff, Can. J. Phys. 41 (1963) 1174. 191 V. Gcnstdt, Rothamstcd I;xpcrimental Station, private communication. 1101 M.A.A. Clync and D.H. Stcdman, Trans. I-araday Sot. 64 (1968) 1816. [111 R.J. Brownc and GA. Ogryzlo. J.Chcm. Phys. 52 (1970) 5774. [I21 T. Carrinpton, J. Chcm. Phys. 57 (1972) 2033. (131 M-F. Goldc and B.A. Thrush, Rept. Progr. Phys. 36 (1973) 1285. I141 J.A. Coxon, J. Quant. Spectry. Radiative Transfer 11 (1971) 443.