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The excitation of iodine by singlet molecular oxygen
Volume 6. number 2
THE
CHEMICAL
EXCITATION
OF
PHYSICS
IODINE
BY
LETTERS
SINGLET
July 19 70
MOLECULAR
OXYGEN
R. G. DERWENT, D. R. KEARNSS and B. A THRUSH of Cambridge, Department of Pilysical Chemistry,
University
Lensfield Road,
Camlwidge,
UK
Received 12 May 1970
Singlet molecular oxygen excites iodine to aLmost every vibrational !e;rel of the B 31TO+, state. It is deduced that the first step involves excitation to the A 3111, state of 12 by 02 1,X; with subsequent excitation by 02 hg or 02 1~ ‘g.
Arnold et al. [l J observed a yellow-green chemiluminescence when iodine was added tg a stream of molecular oxygen containing singlet metastable oxygen molecules (lO2). This emission comes from the B3&+u state of I and extends irom its convergence limit to I(5% P3/2) + r*(5 2P& at 5000%1down info the infrared. The suggested mechanism was 12 + 02(lZi) I+lO2+
or P
I+I*+M-
2[O#Ag)]
- I+ 1 + 02 I (1)
+02,
(2)
12(B3no+u) + M,
(3)
reaction (2) being nearly resonant for 02(lAg) (see table 1). We have investigated this reaction using a conventional flow system in which singlet molecular oxygen in the products of a microwave rlischarge through highly purified oxygen was passed over mercuric oxide to remove atomic axyge= These experiments show that (1) and (2) are important processes, but we deduce that (3) can make only a minor contribution to the emission. Two alternative excitation mechanisms have to be considered, a direct multistep excitation of 12 (i.e. energy p00iing)
I@-$)
102 102
-
--
I2(B 3”o+u)
(4)
or ‘the excitation by singlet oxygen of an excited 12 molecule (den&d by I& and presumably in the $ Permalient address: Department of Chemistry. University of California, Riverside, California 92502; USA.
ASIA;, or high vibrational
levels of X1\” or in states) forme 3 by recombination of ground state iodine atoms.
the postulated
3111g, 3112g
1+1+&f-&M. I; + ‘02
(5)
- ~(B3”o+u)
i- 02.
(‘3)
Evidence supporting collisional excitation mechanisms (4) or (6) comes from the vibrational distribution of the emission spectrum which was photographed with a Bass-KessIer spectrograph (kindly loaned by Dr. Clyne). This shows a fairly even population of vibrational levels from the limit down to ZI’= 3. By analogy with other reTable I Energy levels To (C&l)
Species 02 ‘A,
7882
02 12;
13121
Iz A*lu
11803
I2 B3”o+u
15725
Br2 A3111u
13737
Brg B311,-,+u
lssie
Dissociation energies Species
D, (cr&)
Ref.
T2 (x+x)
12441
E91
12 wx*)
20044
t91
Br2 F+W
15900
[$I
Br2 F+x*)
19585
171
115
V&me 6. number 2‘
CHEMICALPHYSICSLETTERS
.combination p~octsses. reaction (3) would populate levels within 1 - 2 .kcal/.aote of the dissociation limit, i.e. down to v’ x 40, During the radiative lifetime, relaxation would remove up to 10 quanta of vibrational enerip at the pressures &xi [2,3]. Further the inductsdpredissociations ’ at%vels v’ = 22, 29 and 39 [4], which have unit ~~cbllisionsefficiency would prevent T2molecules @cim I + I* reaching the low Pveb of the B3FiO+u state. _ At higher pressures (> 5 ‘.arn Hg) we find that tbe chemilumineacence is !.argeliyconflned to the region within a few millimetres of the surface of ‘the flow tube which has &diameter of 25 mm. . Since iodine atoms recombine very rapidly on surfaces, their concentration would be lowest in the region of strongest d:emiIuminescence and highest in the centre of the flow tube. This is a strongargument against the importance of-iprocesses (3) or (5) plus (6) and for the participation of a species formed on the sidrface. There is no evidence of surface effects w&h 02flA ) tid the mg is re02(lC*) formed from it by energy poo f” move B on the surface with y = 30-2 [S] (we:$so find y = 10-2 for vt = Oandr=5x30-2foru’= 1 in the absence of 12). With higher iodine concentrations the glow exte,ndsfurther towards the centre of the flow tube, suggesting that iodine molecules formed by surface recombination are excited and dissociated . as th,eydiffuse towards the centre of the flow lube by a rapid reaction with a species formed or present in the homogeneous gas phase. The form of the emission around the inlet jet supports this view. The mechanism implies that iodine repeats the cycle; dissociation, diffusion of atoms to wall, wall recombination, clesorption as it is pumped along the flow tube. lf diffusion is rate determining, twenty cycles would occur in our flow tube. For the iodine concentrations used, there is adequate O&As) present to induce the required number of dl&ciations directly or via 02&Z+). Singe the glow is quenched by added water in a simihr manner to the emission from 02(1C+) at low iodine c~ncent~~ions but is less quenc ded at high iddine concentrator, O&Z;) must be involved in the =citai%on being removed more rapidly by I2 than by ‘water at high iodine concentrations. Although 02 !Zg] 2~ [bgl~gl being about 0.01% and 5d respectively, adequate 02@Z$ is produced by the energy pooling reaction
‘15. JuIy 2970
O,tl+ +q&$f
-(
02P22 ;+..02t3 5,
m
io account for the observed e&&on. This formation process is supplemented by the spinallowed pumping mechanism Rnggested by OgryzIO.
oz+Ag)
+ 17 og(3E;)
c I?,
03
02(TAg) c I* --+O&Z;’ + I,
(9) which +;Peconfirm and find that K& in 3 x X023 cm6mole-%ec-2 with presumably &$j> kg as it is more nearly resonant. The kinetic behaviour observed is best explained by assuming that 02(lZ”) dissociates the 12 and urcites to the A3llru stag, which has a long radiative lifetime, from which it is excited to the B31’& state by 02(h,) or 02(1X’). We find virtually no emission or evidence of %-issociation when Br2 is added to the system. Br2fB 3 nh) emission extending to the 2P,,z + 2Pa/a limit is however observed in systems; containirg bromine atoms and singlet oxygen molecules 161. With bromine and iodine the B3Dot, state covers an almost identical energy range but for Br2 the A31flu state and the dissociation limit to 2&/z + 2P,/z are above the energy of 02(lZ$); the former could be reached by energy pooling of two 02(lA& molecules, but this requires the intermedigte formation of a vibrationally exc%ed Br2 molecule which we consider implausible. R. G. r). thanks the Sims Empire Fo~dation for a Schoiarship.
[I]
S. J. Arnold,
N. Finlayson and %. A.Ogryzlo, J. Chem.Phys. 44 (l366) 2529. [2] J. I. Steinfeld and W. Klemperer, J. Chem. Phys. 42 (lQ65) 3475. [3] A. Chutjian, J. K. Link and L. Brewer, J. Chem. Phys. 46 (1367) 2666. [4] V. Konciratjew and L. PO& Fiz.Zh. 4 gk933)764. [S] II.P, J, IzucIantiR. P. Wayne, Pro& Roy.Sot. A306 Is] $9?C~kse, J. A. Coxon and M.A.A. Ctyne, Chem. Phys. Letters 6 (1970) 57. f7] 9. Herzberg, Molecular spectra and molecular 1 structure, I. Spectra of @atomic moIecule8 (Van ErJosfr&gd,Princeton, 1950). [S] J. I. StewIfeld. J. 3%CampbellandkA1 yies. J. Mol.Spectry. 29 (1969) 204. fSj XI.3. Le,_oy, J. Chom.‘$hys. 52 (1970) 26%.
THE
CHEMICAL
EXCITATION
OF
PHYSICS
IODINE
BY
LETTERS
SINGLET
July 19 70
MOLECULAR
OXYGEN
R. G. DERWENT, D. R. KEARNSS and B. A THRUSH of Cambridge, Department of Pilysical Chemistry,
University
Lensfield Road,
Camlwidge,
UK
Received 12 May 1970
Singlet molecular oxygen excites iodine to aLmost every vibrational !e;rel of the B 31TO+, state. It is deduced that the first step involves excitation to the A 3111, state of 12 by 02 1,X; with subsequent excitation by 02 hg or 02 1~ ‘g.
Arnold et al. [l J observed a yellow-green chemiluminescence when iodine was added tg a stream of molecular oxygen containing singlet metastable oxygen molecules (lO2). This emission comes from the B3&+u state of I and extends irom its convergence limit to I(5% P3/2) + r*(5 2P& at 5000%1down info the infrared. The suggested mechanism was 12 + 02(lZi) I+lO2+
or P
I+I*+M-
2[O#Ag)]
- I+ 1 + 02 I (1)
+02,
(2)
12(B3no+u) + M,
(3)
reaction (2) being nearly resonant for 02(lAg) (see table 1). We have investigated this reaction using a conventional flow system in which singlet molecular oxygen in the products of a microwave rlischarge through highly purified oxygen was passed over mercuric oxide to remove atomic axyge= These experiments show that (1) and (2) are important processes, but we deduce that (3) can make only a minor contribution to the emission. Two alternative excitation mechanisms have to be considered, a direct multistep excitation of 12 (i.e. energy p00iing)
I@-$)
102 102
-
--
I2(B 3”o+u)
(4)
or ‘the excitation by singlet oxygen of an excited 12 molecule (den&d by I& and presumably in the $ Permalient address: Department of Chemistry. University of California, Riverside, California 92502; USA.
ASIA;, or high vibrational
levels of X1\” or in states) forme 3 by recombination of ground state iodine atoms.
the postulated
3111g, 3112g
1+1+&f-&M. I; + ‘02
(5)
- ~(B3”o+u)
i- 02.
(‘3)
Evidence supporting collisional excitation mechanisms (4) or (6) comes from the vibrational distribution of the emission spectrum which was photographed with a Bass-KessIer spectrograph (kindly loaned by Dr. Clyne). This shows a fairly even population of vibrational levels from the limit down to ZI’= 3. By analogy with other reTable I Energy levels To (C&l)
Species 02 ‘A,
7882
02 12;
13121
Iz A*lu
11803
I2 B3”o+u
15725
Br2 A3111u
13737
Brg B311,-,+u
lssie
Dissociation energies Species
D, (cr&)
Ref.
T2 (x+x)
12441
E91
12 wx*)
20044
t91
Br2 F+W
15900
[$I
Br2 F+x*)
19585
171
115
V&me 6. number 2‘
CHEMICALPHYSICSLETTERS
.combination p~octsses. reaction (3) would populate levels within 1 - 2 .kcal/.aote of the dissociation limit, i.e. down to v’ x 40, During the radiative lifetime, relaxation would remove up to 10 quanta of vibrational enerip at the pressures &xi [2,3]. Further the inductsdpredissociations ’ at%vels v’ = 22, 29 and 39 [4], which have unit ~~cbllisionsefficiency would prevent T2molecules @cim I + I* reaching the low Pveb of the B3FiO+u state. _ At higher pressures (> 5 ‘.arn Hg) we find that tbe chemilumineacence is !.argeliyconflned to the region within a few millimetres of the surface of ‘the flow tube which has &diameter of 25 mm. . Since iodine atoms recombine very rapidly on surfaces, their concentration would be lowest in the region of strongest d:emiIuminescence and highest in the centre of the flow tube. This is a strongargument against the importance of-iprocesses (3) or (5) plus (6) and for the participation of a species formed on the sidrface. There is no evidence of surface effects w&h 02flA ) tid the mg is re02(lC*) formed from it by energy poo f” move B on the surface with y = 30-2 [S] (we:$so find y = 10-2 for vt = Oandr=5x30-2foru’= 1 in the absence of 12). With higher iodine concentrations the glow exte,ndsfurther towards the centre of the flow tube, suggesting that iodine molecules formed by surface recombination are excited and dissociated . as th,eydiffuse towards the centre of the flow lube by a rapid reaction with a species formed or present in the homogeneous gas phase. The form of the emission around the inlet jet supports this view. The mechanism implies that iodine repeats the cycle; dissociation, diffusion of atoms to wall, wall recombination, clesorption as it is pumped along the flow tube. lf diffusion is rate determining, twenty cycles would occur in our flow tube. For the iodine concentrations used, there is adequate O&As) present to induce the required number of dl&ciations directly or via 02&Z+). Singe the glow is quenched by added water in a simihr manner to the emission from 02(1C+) at low iodine c~ncent~~ions but is less quenc ded at high iddine concentrator, O&Z;) must be involved in the =citai%on being removed more rapidly by I2 than by ‘water at high iodine concentrations. Although 02 !Zg] 2~ [bgl~gl being about 0.01% and 5d respectively, adequate 02@Z$ is produced by the energy pooling reaction
‘15. JuIy 2970
O,tl+ +q&$f
-(
02P22 ;+..02t3 5,
m
io account for the observed e&&on. This formation process is supplemented by the spinallowed pumping mechanism Rnggested by OgryzIO.
oz+Ag)
+ 17 og(3E;)
c I?,
03
02(TAg) c I* --+O&Z;’ + I,
(9) which +;Peconfirm and find that K& in 3 x X023 cm6mole-%ec-2 with presumably &$j> kg as it is more nearly resonant. The kinetic behaviour observed is best explained by assuming that 02(lZ”) dissociates the 12 and urcites to the A3llru stag, which has a long radiative lifetime, from which it is excited to the B31’& state by 02(h,) or 02(1X’). We find virtually no emission or evidence of %-issociation when Br2 is added to the system. Br2fB 3 nh) emission extending to the 2P,,z + 2Pa/a limit is however observed in systems; containirg bromine atoms and singlet oxygen molecules 161. With bromine and iodine the B3Dot, state covers an almost identical energy range but for Br2 the A31flu state and the dissociation limit to 2&/z + 2P,/z are above the energy of 02(lZ$); the former could be reached by energy pooling of two 02(lA& molecules, but this requires the intermedigte formation of a vibrationally exc%ed Br2 molecule which we consider implausible. R. G. r). thanks the Sims Empire Fo~dation for a Schoiarship.
[I]
S. J. Arnold,
N. Finlayson and %. A.Ogryzlo, J. Chem.Phys. 44 (l366) 2529. [2] J. I. Steinfeld and W. Klemperer, J. Chem. Phys. 42 (lQ65) 3475. [3] A. Chutjian, J. K. Link and L. Brewer, J. Chem. Phys. 46 (1367) 2666. [4] V. Konciratjew and L. PO& Fiz.Zh. 4 gk933)764. [S] II.P, J, IzucIantiR. P. Wayne, Pro& Roy.Sot. A306 Is] $9?C~kse, J. A. Coxon and M.A.A. Ctyne, Chem. Phys. Letters 6 (1970) 57. f7] 9. Herzberg, Molecular spectra and molecular 1 structure, I. Spectra of @atomic moIecule8 (Van ErJosfr&gd,Princeton, 1950). [S] J. I. StewIfeld. J. 3%CampbellandkA1 yies. J. Mol.Spectry. 29 (1969) 204. fSj XI.3. Le,_oy, J. Chom.‘$hys. 52 (1970) 26%.