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Chemiluminescent reaction between I2(D0u+) and Xe to yield XeI(B2Σ12 )
Volume.122, number 5
CHEMICAL
CHEMILUMINESCENT B-V. O’GRADY Deprxrmrenr
Received
REACHON
PHYSICS
BETWEEN
.20 December 1985
LETTERS
I,(D 0,‘) AND Xe TO YIELD XeI(B ‘Z,,),
’ and R.J. DONOVAN
of ChemiIlT.
23 September
UniuersiI_r
of Edinburgh.
1Ven Mains
Road
Edinburgh
E:/19 3Jf.
UK
1985
A new slaw-selecwd chemiluminescenl reaction between 12(DO: ) and Xe, yielding Xel(B’\‘1,2). is reported. I2 wab opkally excilcd to the DO: SIBI~ and Ihe threshold for reaction determined as 192+2 nm. The rate constant for removal (all channels) of I,(DO,+ ) was determined as k = (2.2iOJ)~lO-‘~ cm’ molecule-’ s-l_
1. Introduction It is now well established that absorption by I,, in the region 175-210 nm, populates the I,(D 0:) ionpair state. Fluorescence and collisional quenching of this state has been studied in some detail [l-3] : of particular importance to the present work is the quenching behaviour observed with the rare gases. Quenching by Ar has been studied in most detail [2-51 and it is known that a complex collisional cascade takes place, through several intermediate ionpair states, leading eventually to the lowest ion-pair state, the Iz(D’ 3112g) state. This state then gives rise to intense narrow-band emission at 340 run (D’ 3112g + A’ 3B2u): for rare gases, other than Xe, the D’ State is extremely resistant to quenching and this property has been exploited in the construction of an efficient optically pumped iodine laser [6] for which high pressures of Ar were used to transfer rapidly population from the D 0: state to the D’ 3B2 “reservoir” state. However, as noted previously B31, Xe unlike Ar is quite efficient at quenching 12(D’ 3112,) and fluore+ cence from this state passes through a maximum as
progressively higher pressures of Xe are mixed with 1,. In tire work reported here we show that i2(D Oi), selectively excited by.ultraviolet light at X G 192 nm, reacts rapidly withXe to yield the electronically excited product, XeI(B 2E1,2). We also show that phys
’ Permanent Tasmania;
address: Chemistry Department, Universily of P-0. Box 252C. Hobart; Tasmania 7001. Australia.
0 009-2614/85/S 0330 0 EIseyier~Science Publishers (North-Holland PhysiCs Publishing Division)
B-V_
ical quenching of I,(D 0:) to the lowest ion-pair state, 12(D’ 3 I12s), competes with reaction. The formation of electronically excited XeI(B 2Er& has been observed following pulse radiolysis of gaseous xenon-iodine mixtures [7]. The source of XeI(B 2ZZl,2) was attributed in part to reactions involving electronically excited iodine molecules and ground-state xenon atoms, but the identity of the excited molecular state or states was not established. Indeed, under the conditions used for pulse radiolysis, a large number of excited.and ionic species are generally formed and these could also produce XeI(B * C1j2).
2. Experimental The experiments reported here employed a PerkinElmer 65040 spectrofluorimeter. Excitation wavelengths below 200.run were obtained by using the excitation monochromator in second order: this produces a large scattered light signal in the 400 nm region when scannin g the fluorescence monochromator, but this is not important for.the present work. It is also important to note that I2 does not fluoresce when excited by light in the region of 400 nm: fluorescence is only observed for X > 500 run and X =G2 10 run. The fluorescence monochromator was also used in second order to observe XeI(B) emission, as better signal/noise ratios were obtained in this way: firstand second-order spectra could be readily distinguished
503
Volume 122, number 5
CHEMICAL PHYSICS
20 December 1985
LErrERS
using suitable filters. None of the reported spectra have been corrected for monochromator throughput or photomultiplier response. Samples of I, and Xe were contained in a Spectrosil fluorescence cell (1 cm* cross section), fitted with a greaseless tap to allow gas handling via a conventional greaseless vacuum line. Analar grade (BDH) iodine was degassed by prolonged pumping and a few crystals were sublimed into the fluorimeter cell to maintain a constant vapour pressure throughout the experiments. B.O.C., spectroscopic grade, xenon was used as supplied. The pressure of xenon added to the iodine was recorded on a Baratron pressure gauge.
3. Results and discussion Three sets of experiments were performed. In the first emission spectra, between 220 and 360 run, were recorded as a function of excitation wavelength: the results are shown in fig. 1. Three main bands are observed at 253,325 and 340 run with minor bads presen t between 265 and 3 15 nm. Of the three main bands, the band at 253 nm increases in intensity with decreasing wavelength, whereas the remaining bands all decrease in intensity. The emission at 253 nm is readily assigned [8] to the B 1Z1,2 -+ X 22,,2 transition of XeI and that at 340 nm to the well characterised D’3l-l zg -‘A’%u transition of iodine [6]. The bands between 265 and 325 nm in the main are from the D 0: + X 0; transition in iodine. The small peak which appears at 288 nm is in approximately the correct position for the C + A transition of XeI but it must be precluded on energetic grounds. It is possibly part of the I, system around 288 nm reported by TelIinghuisen [5]. If this is the case the results suggest that there is also an energy threshold for its formation. The bands at 325 nm may also contain a contribution from XeI(B + A) but it is difKcult to draw deftite conclusions from the present work. The second set of experiments involved the measurement of excitation spectra for both the 340 run Ix(D’ + A’) and the 253 nm XeI(B +X) bands. The onset of XeI(B) formation was found to lie at shorter wavelengths than that for formation of $(D’): the onset for XeI(B) formation was I92 -+ 2 nm, while that for Iz(D’) was 210 nm (i.e. the wavelength at which absorption to the D state starts). The minimum 504
I
250
1
300
I
350
x/m -
Fig. 1. Fluorescence spectra observed following excitation of mixtures of I2 and Xc (p - 0 3 Torr, PXe = 10 Torr) at the wavelengths indicated: (aIf- 186 -run. (b) 188 nm, (c) 190 nm. (d) 193 run, (e) 195 run. Spectra were recorded using both excitation and fluorescence monochromators in second order. The spectra have not been corrected for variations in the wavclengtb response of the fluorimcter.
energy required to form XeI(B) can be estimated as the sum of the bond dissociation energy in 12 plus the B-X separation for XeI, i.e. 52300 cm-l, which corresponds to an onset wavelength of 191 run. The uncertainty in the onset wavelength given here is due to the band pass of the excitation monochromator (2 nm), the appreciable population of I,Or) in u” = 1 and 2, and to the distortion of the excitation spectra by the rapid falloff in the exciting radiation below 200 run. The third set of experiments were directed at the pressure dependence of the emission spectra excited at 188 run. Plots of I,,, and Ix3 versus Xe pressure are shown in fig. 2. In both cases the intensity of the peaks reach a maximum and then decrease, with Pmax being at approximately 25 and 50 Torr (1 Torr = 133 Pa) for the 340 and the 253 nm bands respectively.
Volume 122;number
5
CHEMICAL
PHYSICS
LITTERS
20 Doember
be ruled out on.thermo&emical
0
50
100 PI.... -
Fig. 2. Variation of fluorescence and l,(D’)
(croses)
intensity of XeI(B) with pressure of Xe.
The main steps can therefore
(circles)
be summarised as follows:
12(X) + hu +12(D), I,(D)
+ Xe + XeI(B)
+ I ,
12(D)+ Xe + Iz(D’) + Xe I,(D)
+ 12(x)
proceed
through
lower
unbound
+ 12~ ,
XeI(B) + XeI(x)
+ hv(253
nm) ,
XeI(B) + Xe + I + 2Xe , 12(D’)+Xe+12(orI+r)+Xe
_
Stern-Volmer plots of the reciprocal of the fluorescence intensity for 12(D), or XeI(B), against the pressure of xenon, together with the known radiative lifetime of 12(D) (15.4 ns), yield a rate constant for removal (all channels) of 12(D) by Xe of k = (2.2 + OS) X lo-lo cm3 molecule-1 s-l. This value is in line with values obtained [4] for the other rare gases for the quenching of I,(D). The decrease in XeI(B) emission at higher pressures may in part be due to formation of Xe21L. Preliminary experiments using synchrotron radiation at X < 190 M-I and pressures 6f 500 Tqrr of Xe have produced a broad emission at 380 run. This emission has been attributed
but qu&ch-
states of XeI. The reaction of Iz(D 0:) with Xe can be compared with other recent reports of reaction between Cll(l C,) and several of the rare gases [9,10], and reaction between IBr(D) and Xe [ 111. We have also recently observed [12] reaction between ICI(E) and Xe to yield both XeI(B) and XeCl(B). It is clear therefore that a whole class of interesting reactions between excited halogen molecules and rare gas atoms, awaits further detailed study. in conclusion the present experiments have shown that, (a) XeI(B) can be formed via single photon excitation of 12@ 03 followed by reaction with Xe, (b) that the threshold energy for the appearance of X51(B) is close to the the%nochemical threshold (52300 cm-l) and that (c) the overall quenching cross section for 12(D) by Xe is large. Experiments are in progress, using synchroton radiation, to further elucidate the relative importance of the two removal channels for 12(D) as a function of excitation wavelength and Xe pressure. tig could
.o
grounds
1385
by others
[S] as being due to Xe21*.
The quenching of I,@‘) by high pressures of xenon has been reported earlier. This behaviour is quite different from that observed with argon when there is no appreciable quenching [4] even at pressures of 7 X 10’? Torr. The formation of XeI(B) from I,@‘) can
Acknowledgement The support of BVO’G by a grant from the Royal Society of Edinburgh is gratefully acknowledged_ We also thank the SERC for the use of the fluorimeter at its Daresbury Laboratory and Dr. J.P.T. Wilkinson for helpful discussions_
References [l] [2] [3]
R-J. Donovan. D-V. O’Crady. L Lain and C. Fotakis, J. Chem. Phys. 78 (1983) 3727. H. Hemmati and G.J. Collins, Chem. Phys Letters 75 (1980) 488. M. Martin, C Fotakis, RJ. Donovan and M.J. Shaw, Nuovo Cimento 63B (1981) 300.
[4] R-J. Donovan, B.V. O’Grady, H. MacKenzie and L. Lain, to be published_ (51 A-L Guy, KS. Viswanathan, k Sur and J. Tellinghuken, Chem. Phys. Letters 73 (1980)
582.
[6]
M-J. Shaw, CB. Edwards, F. O’NeiU, C Fotakis and RJ. Donovan, Appl Phyr Letters 37 (1980) 346.
[7]
R Cooper, F- Griever and M.C. Sauer Jr., J. Phyr Chem. 81 (1977) 1689.
505
Volume 122. number 5
CHEMICti
[8]
J.J. Ewing and CA Brau, Phys Rev. Al2 (1975) 129. [9] T. MiXer, B. Jordan. P. Giirther. G. Zimmerer. D. H&s.
[lo]
J. Le Calve and MC Caste& Cbem. Phys 76 (198% 295. T. Ishiwata, A. Tokunaga and L Tanaka, Chem. Phys. Letters 112 (1984)
506
356.
PHYSICS LETTERS Ill] [12]
20 December
-
1985
M. MacDonald and RJ:Donovan, Chem. Phyr Letters IO1 (1983) 284. EA. Kerr, RJ. Donovan, J.P.T_ W-son, D. Shawand 1-H. hlunro. in preparation
J.P.Z Wilkin~n.
CHEMICAL
CHEMILUMINESCENT B-V. O’GRADY Deprxrmrenr
Received
REACHON
PHYSICS
BETWEEN
.20 December 1985
LETTERS
I,(D 0,‘) AND Xe TO YIELD XeI(B ‘Z,,),
’ and R.J. DONOVAN
of ChemiIlT.
23 September
UniuersiI_r
of Edinburgh.
1Ven Mains
Road
Edinburgh
E:/19 3Jf.
UK
1985
A new slaw-selecwd chemiluminescenl reaction between 12(DO: ) and Xe, yielding Xel(B’\‘1,2). is reported. I2 wab opkally excilcd to the DO: SIBI~ and Ihe threshold for reaction determined as 192+2 nm. The rate constant for removal (all channels) of I,(DO,+ ) was determined as k = (2.2iOJ)~lO-‘~ cm’ molecule-’ s-l_
1. Introduction It is now well established that absorption by I,, in the region 175-210 nm, populates the I,(D 0:) ionpair state. Fluorescence and collisional quenching of this state has been studied in some detail [l-3] : of particular importance to the present work is the quenching behaviour observed with the rare gases. Quenching by Ar has been studied in most detail [2-51 and it is known that a complex collisional cascade takes place, through several intermediate ionpair states, leading eventually to the lowest ion-pair state, the Iz(D’ 3112g) state. This state then gives rise to intense narrow-band emission at 340 run (D’ 3112g + A’ 3B2u): for rare gases, other than Xe, the D’ State is extremely resistant to quenching and this property has been exploited in the construction of an efficient optically pumped iodine laser [6] for which high pressures of Ar were used to transfer rapidly population from the D 0: state to the D’ 3B2 “reservoir” state. However, as noted previously B31, Xe unlike Ar is quite efficient at quenching 12(D’ 3112,) and fluore+ cence from this state passes through a maximum as
progressively higher pressures of Xe are mixed with 1,. In tire work reported here we show that i2(D Oi), selectively excited by.ultraviolet light at X G 192 nm, reacts rapidly withXe to yield the electronically excited product, XeI(B 2E1,2). We also show that phys
’ Permanent Tasmania;
address: Chemistry Department, Universily of P-0. Box 252C. Hobart; Tasmania 7001. Australia.
0 009-2614/85/S 0330 0 EIseyier~Science Publishers (North-Holland PhysiCs Publishing Division)
B-V_
ical quenching of I,(D 0:) to the lowest ion-pair state, 12(D’ 3 I12s), competes with reaction. The formation of electronically excited XeI(B 2Er& has been observed following pulse radiolysis of gaseous xenon-iodine mixtures [7]. The source of XeI(B 2ZZl,2) was attributed in part to reactions involving electronically excited iodine molecules and ground-state xenon atoms, but the identity of the excited molecular state or states was not established. Indeed, under the conditions used for pulse radiolysis, a large number of excited.and ionic species are generally formed and these could also produce XeI(B * C1j2).
2. Experimental The experiments reported here employed a PerkinElmer 65040 spectrofluorimeter. Excitation wavelengths below 200.run were obtained by using the excitation monochromator in second order: this produces a large scattered light signal in the 400 nm region when scannin g the fluorescence monochromator, but this is not important for.the present work. It is also important to note that I2 does not fluoresce when excited by light in the region of 400 nm: fluorescence is only observed for X > 500 run and X =G2 10 run. The fluorescence monochromator was also used in second order to observe XeI(B) emission, as better signal/noise ratios were obtained in this way: firstand second-order spectra could be readily distinguished
503
Volume 122, number 5
CHEMICAL PHYSICS
20 December 1985
LErrERS
using suitable filters. None of the reported spectra have been corrected for monochromator throughput or photomultiplier response. Samples of I, and Xe were contained in a Spectrosil fluorescence cell (1 cm* cross section), fitted with a greaseless tap to allow gas handling via a conventional greaseless vacuum line. Analar grade (BDH) iodine was degassed by prolonged pumping and a few crystals were sublimed into the fluorimeter cell to maintain a constant vapour pressure throughout the experiments. B.O.C., spectroscopic grade, xenon was used as supplied. The pressure of xenon added to the iodine was recorded on a Baratron pressure gauge.
3. Results and discussion Three sets of experiments were performed. In the first emission spectra, between 220 and 360 run, were recorded as a function of excitation wavelength: the results are shown in fig. 1. Three main bands are observed at 253,325 and 340 run with minor bads presen t between 265 and 3 15 nm. Of the three main bands, the band at 253 nm increases in intensity with decreasing wavelength, whereas the remaining bands all decrease in intensity. The emission at 253 nm is readily assigned [8] to the B 1Z1,2 -+ X 22,,2 transition of XeI and that at 340 nm to the well characterised D’3l-l zg -‘A’%u transition of iodine [6]. The bands between 265 and 325 nm in the main are from the D 0: + X 0; transition in iodine. The small peak which appears at 288 nm is in approximately the correct position for the C + A transition of XeI but it must be precluded on energetic grounds. It is possibly part of the I, system around 288 nm reported by TelIinghuisen [5]. If this is the case the results suggest that there is also an energy threshold for its formation. The bands at 325 nm may also contain a contribution from XeI(B + A) but it is difKcult to draw deftite conclusions from the present work. The second set of experiments involved the measurement of excitation spectra for both the 340 run Ix(D’ + A’) and the 253 nm XeI(B +X) bands. The onset of XeI(B) formation was found to lie at shorter wavelengths than that for formation of $(D’): the onset for XeI(B) formation was I92 -+ 2 nm, while that for Iz(D’) was 210 nm (i.e. the wavelength at which absorption to the D state starts). The minimum 504
I
250
1
300
I
350
x/m -
Fig. 1. Fluorescence spectra observed following excitation of mixtures of I2 and Xc (p - 0 3 Torr, PXe = 10 Torr) at the wavelengths indicated: (aIf- 186 -run. (b) 188 nm, (c) 190 nm. (d) 193 run, (e) 195 run. Spectra were recorded using both excitation and fluorescence monochromators in second order. The spectra have not been corrected for variations in the wavclengtb response of the fluorimcter.
energy required to form XeI(B) can be estimated as the sum of the bond dissociation energy in 12 plus the B-X separation for XeI, i.e. 52300 cm-l, which corresponds to an onset wavelength of 191 run. The uncertainty in the onset wavelength given here is due to the band pass of the excitation monochromator (2 nm), the appreciable population of I,Or) in u” = 1 and 2, and to the distortion of the excitation spectra by the rapid falloff in the exciting radiation below 200 run. The third set of experiments were directed at the pressure dependence of the emission spectra excited at 188 run. Plots of I,,, and Ix3 versus Xe pressure are shown in fig. 2. In both cases the intensity of the peaks reach a maximum and then decrease, with Pmax being at approximately 25 and 50 Torr (1 Torr = 133 Pa) for the 340 and the 253 nm bands respectively.
Volume 122;number
5
CHEMICAL
PHYSICS
LITTERS
20 Doember
be ruled out on.thermo&emical
0
50
100 PI.... -
Fig. 2. Variation of fluorescence and l,(D’)
(croses)
intensity of XeI(B) with pressure of Xe.
The main steps can therefore
(circles)
be summarised as follows:
12(X) + hu +12(D), I,(D)
+ Xe + XeI(B)
+ I ,
12(D)+ Xe + Iz(D’) + Xe I,(D)
+ 12(x)
proceed
through
lower
unbound
+ 12~ ,
XeI(B) + XeI(x)
+ hv(253
nm) ,
XeI(B) + Xe + I + 2Xe , 12(D’)+Xe+12(orI+r)+Xe
_
Stern-Volmer plots of the reciprocal of the fluorescence intensity for 12(D), or XeI(B), against the pressure of xenon, together with the known radiative lifetime of 12(D) (15.4 ns), yield a rate constant for removal (all channels) of 12(D) by Xe of k = (2.2 + OS) X lo-lo cm3 molecule-1 s-l. This value is in line with values obtained [4] for the other rare gases for the quenching of I,(D). The decrease in XeI(B) emission at higher pressures may in part be due to formation of Xe21L. Preliminary experiments using synchrotron radiation at X < 190 M-I and pressures 6f 500 Tqrr of Xe have produced a broad emission at 380 run. This emission has been attributed
but qu&ch-
states of XeI. The reaction of Iz(D 0:) with Xe can be compared with other recent reports of reaction between Cll(l C,) and several of the rare gases [9,10], and reaction between IBr(D) and Xe [ 111. We have also recently observed [12] reaction between ICI(E) and Xe to yield both XeI(B) and XeCl(B). It is clear therefore that a whole class of interesting reactions between excited halogen molecules and rare gas atoms, awaits further detailed study. in conclusion the present experiments have shown that, (a) XeI(B) can be formed via single photon excitation of 12@ 03 followed by reaction with Xe, (b) that the threshold energy for the appearance of X51(B) is close to the the%nochemical threshold (52300 cm-l) and that (c) the overall quenching cross section for 12(D) by Xe is large. Experiments are in progress, using synchroton radiation, to further elucidate the relative importance of the two removal channels for 12(D) as a function of excitation wavelength and Xe pressure. tig could
.o
grounds
1385
by others
[S] as being due to Xe21*.
The quenching of I,@‘) by high pressures of xenon has been reported earlier. This behaviour is quite different from that observed with argon when there is no appreciable quenching [4] even at pressures of 7 X 10’? Torr. The formation of XeI(B) from I,@‘) can
Acknowledgement The support of BVO’G by a grant from the Royal Society of Edinburgh is gratefully acknowledged_ We also thank the SERC for the use of the fluorimeter at its Daresbury Laboratory and Dr. J.P.T. Wilkinson for helpful discussions_
References [l] [2] [3]
R-J. Donovan. D-V. O’Crady. L Lain and C. Fotakis, J. Chem. Phys. 78 (1983) 3727. H. Hemmati and G.J. Collins, Chem. Phys Letters 75 (1980) 488. M. Martin, C Fotakis, RJ. Donovan and M.J. Shaw, Nuovo Cimento 63B (1981) 300.
[4] R-J. Donovan, B.V. O’Grady, H. MacKenzie and L. Lain, to be published_ (51 A-L Guy, KS. Viswanathan, k Sur and J. Tellinghuken, Chem. Phys. Letters 73 (1980)
582.
[6]
M-J. Shaw, CB. Edwards, F. O’NeiU, C Fotakis and RJ. Donovan, Appl Phyr Letters 37 (1980) 346.
[7]
R Cooper, F- Griever and M.C. Sauer Jr., J. Phyr Chem. 81 (1977) 1689.
505
Volume 122. number 5
CHEMICti
[8]
J.J. Ewing and CA Brau, Phys Rev. Al2 (1975) 129. [9] T. MiXer, B. Jordan. P. Giirther. G. Zimmerer. D. H&s.
[lo]
J. Le Calve and MC Caste& Cbem. Phys 76 (198% 295. T. Ishiwata, A. Tokunaga and L Tanaka, Chem. Phys. Letters 112 (1984)
506
356.
PHYSICS LETTERS Ill] [12]
20 December
-
1985
M. MacDonald and RJ:Donovan, Chem. Phyr Letters IO1 (1983) 284. EA. Kerr, RJ. Donovan, J.P.T_ W-son, D. Shawand 1-H. hlunro. in preparation
J.P.Z Wilkin~n.