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Reactions of chemically produced O2(1Δ) with Pb atoms
Volume 96. number 5
REACTIONS
CHEMICAL
OF CHEMICALLY
PHYSICS
PRODUCED
LETTERS
22 April 1983
O,(’ A) WITH Pb ATOMS
J. BACHAR and S. ROSENWAKS
Rccwed
21 October
1962: in final form 4 February
1983
The potcnri.d of the chemical Ol(‘a) generator as a means of electronic excitation of metal atoms and oxides is demon3trJtcd m J %tud\ of the reactions of 02(' A) with Pb. The resonant energy trnnsfer process, Pb(3Pc,) + O,(‘A) 4 Pb(3Pr) +
O2 (” x). ir found tu be the lie> for producinp excited Pb rend PbO.
I. Introduction
02
The os~~en molecule in its lowest electronically esclrcd stiitc. ‘A,. has an extremely long radiative life( I], and is quenched with a very 111111’.2 b5 minutes lo\\ efficiency upon collisions with various types of uslls (e.g. ==10-s with Pyrex [?I)_ Also, 02(1A) can he produced in high concentrations. = 10% of the total O2 at aO.3 Torr. via microwave (MN’) discharge in osyse11 [3] snd in very high concentrations, up to XSOF. via reaction of Cl, with basic hydrogn peroxide solution I-1]_ These properties make O#Ll)a useful source of clectroruc erler_ry that can be formed in one location and exploited &ev.herc. O,(‘Zt) Ius successfully
Indeed.
chemically used to pump
-
,
E‘I&---
-la a‘r--
‘p,-
-
prcsenr work we have combined an efficient chentical Oz(lJl generator with a metal evaporation furnace and studied the interaction of singlet osygen with lead atoms. Pb was chosen because its lowest excited state, 3P,. is expected to be formed by the resonant eners transfer process [13]
526
“p,-
‘_Y-
‘PO---
+ Oz(‘A)
-. Pb(jP,)
Fig. 1. The lov.est
+ O#A)
-+ O#Z)
.
(3
energy Ievels of 02. Pb and PbO.
Also, some of the reactions
t O2(3C) + 0.01 eV.(l)
-I-O#Z)
X-
between Pb and 02, when
at least one of the species is excited, are exoergic enough to produce PbO_ The lowest energy levels of 02, Pb and
Higher states of Pb can be formed by further reactions of the excited Pb with O,( ‘A) or 02( 1Z): the latter is produced by the eners pooling reaction [9] O,(Q)
‘1 -
the
Pb(3P0)
BA-
been the chernicai iodine atom laser [5--S]; MW generated O,(la) has bern reacted with a number bf molecules and atoms [9 -- 121 and the escitation states of Ihe ensuing species III
PbO
o-
‘S,-
generated
have been studied.
Pb
PbO are depicted in fig. 1. Both excited Pb(Pb*) and excited PbO(PbO*) have been observed in our system and possible excitation mechanisms have been investigated. The chemical production
0 0092614/83/0000--0000/S
of 02(‘A)
is more involv-
03.00 0 1983 North-Holland
Volume 96, number 5
CHEMICAL PHYSICS LETTERS
ed than its production
by MW discharge. ‘iiowever, the former method has the advantage of producing high concentration of 02(lA) which is free of ions, electrons, atoms and of highly excited 0, [except Oz(l Z)] _ By alternatively using the MW and the chemical generator we were able to study some kinetic details of the O?/Pb system with the convenient MW source, to check them when necessary
with the pure chemical source,
and to investigate
application
the special features that result from of high concentration of O#A).
2. Experimental The experimental apparatus is depicted in fig. 2. Its main parts are the 02(lA) generating systems and the metal evaporation furnace. The construction and operation of the 02(1A) chemical generator has previously been described in detail [S]. Briefly, a stream of Cl, gas is bubbled through lo-15 I_rmpores of sintered glass into basic hydrogen peroxide solution. The effluent from the solution [mainly 02(32)/02(tA)] is passed through a cryogenic trap for water vapor removal and mixed with the Pb vapor emerging from the furnace_ Alternatively, 02(lA) is produced by a 2450 MHz MW discharge in pure oxygen_ Downstream of the discharge the gas is passed through a tube coated with HgO for 0 atoms removal. Typical 02 pressure in either generators is 0.15 Torr. The operation of the MW generator is optimized for maximum O,(LA) signals; it is then assumed that [02(tA)] is 10% [3]. TypieaEy
THERM!STOR
22 April 1983
[02(lA)] from the chemical generator is three times higher than that from the MW_ The metal evaporation furnace is of the type described in refs. [ 14,l S] _ The Pb is evaporated from a resistively heated alumina crucible, entrained in argon stream (~0.18 Torr) and carried to the Pb/O2 mixing zone. The temperature is monitored by a thermocouple near the crucible and by a thermistor in the mixing zone. Typical working temperature of the crucible is ~700 K (at temperatures X50 K clogging of the furnace precluded its stable operation); the working temperature in the mixing zone is then ~400 K. The concentration of 02(LA) is monitored both upstream and downstream of the mixing zone by observing the 0,(1A-3Z) emission at 1270 nm with cooled. intrinsic Ge detectors [8] _ The emission of 02( 1 Z) and PbO* emerging from the mixing zone is measured by a monochromator/photomultiplier/picoammeter/recorder system_ Ground-state and metastable Pb atoms are monitored by atomic absorption spectroscopy using a Pb hollow cathode lamp. The transitions used for monitoring the Pb states were 3PO-3Py, 3P -3Dp, 3P2-3Fz, 1D2-3P;, r D2-‘Py and ‘S0-3P~_1
3_ Rest&s Mixing of ground state 02 with Pb atoms did not resu!t in any visible reaction or measurable amount of excited species. On the other hand. the reaction of 02(L A) with Pb resulted in a bluish, diffuse flame, and
II,
THERMO-
Fig. 2. Schematic of the experimentai setup_ All measures are in cm.
527
Volum+ 96, number 5
CHEMICAL PHYSICS LETTERS
both Pb* and PbO* were observed_ The O,(lA) concentration downsrream of the mising zone decreased rapidly as the furnace temperature, i.e. [Pb], was increased and no 02(IA) could be observed when the furnace temperature was above ~600 K. The detailed btha~~or of rhe various species is summarized below.
In n search for the Pb states present in the flame \~ht’n operating the MW generator. the ground state, ‘P,. and the lowest escited states, SP, and 3P2. were Jeteacd: their concentration ratio. under the typical e\~rimcntai conditions (section 2). was 12/4/l _The
22 April 1983
since for reliable absorption measurements the O,fiA) has had to be switched on and off without changing the flow conditions_ In some Previous studies of chemiluminescence of PbO it was suspected that the Pb atoms evaporated from the furnace are either thermally [17] or electrically [ 1S] (by discharge in the heating wire) excited_ In our experintents thermal excitation could not have been significant since Pb* atoms disappeared when the O,(t A) generator was swjtched off_ Electrical escitation could not have been significant since no change in [Pb”] was observed when the electrical current to the heating wire was momentarily stopped.
ntXf hirriler stdles, t D2 and 1So_ were not observed. T.kin~kto account the transitiorl probabilities [ 161 md the responsivity of our detection sysreni we estimaw hat [ID,]. [‘So] <$ [‘PI]. The absolute con-
s~n~rarior~ofPbatomswasevaluated tobe~lOtocm_3_ The cwcen~ration of excited Pb was equal. within espct’ttttmtal error. IO the reduction in i3P0] wvhert the M\\‘generrl~or was switched on. Monitoring [Pb*] while usmg fhc chemical generator has not been convenient.
300
‘400
The emission spectra of PbO* resulting front the reaction of Pb atoms with 10% and with 30% 02(lA) are presented in fig_ 3. A list of the identified transitions, including their relative intensities, is given in table I_ The identification of the emission bands was carried out using the spectroscopic constants given in ref. (191 for X. a and A. in ref. [17] for b, in ref. [ 181 for B and in
500 WAVELENGTH
600
fnm)
I’IE_ 3. Emission spwfrd of PbO* resulting froze reaction of Pb + 10% &(‘A) (upper spectrum) and of Pb + 30% O,(‘A) (lower r;ps
CHEMKAL
Volume 96, number S
72 April 1983
PHYSICS LETiERS
Table 1 Identified emissions of PbO*. AU the transitions are to the ground electronic state. Conditions are as in fig. 3. The intensities of the emissions, in arbitrary units, uncorrected for the instrument wavelength response, are given in column ‘P’ for the Pb f 30% @('A) reaction, and in “Il” for Pb + 10% O,(‘A). In cases where several transitions overlap, only those which appear to be the strongest ones under our experimental conditions are listed. The possitife cases of overlap can be traced in fig. 3 and are presented in detail in ref. [21] Transition
I
D(3,W D(2,0) D(l,O) D(O,O) D(1.1) D@, 1) C’(9,O) f)(W) C’(7.0) D(0,3) 00.4) C&O) D(0.4) C’(4,O) D(OSI D(1,6) C’(3.0) C(6,1) C’(2,O)
3.6 7.2 9.6 10.8 10.8 21.6 7.8 19.8 8.4 12.0 9.6 12.0 13.8 10.2 9.6 7.8 Ii.4 9.0 12.0 12.0 12.6 13.8 16.8 20.4 14.4 13.8 33.0 16.2 9.0 4.8 8.4 8.4 3.6 4.8 -7.2 7.8 7.8 4.8 42 5.4 4.8 6.6 4.8 1.4
W,~) C(3,O) C(4,l) C’(0J-J) B(%Q C(4,2) (x.0) B(4,O) B(S, 1) B(O,S) a(9.2) B(l,6) A(O.2) a(8,2) B(L7) A@,3) b(3.1) af2,O) aG2) N4,7) A(O,S) a&l) AU ,6) a(3,7,) A@&
II
I/II -
1.6 2.6 2.6 2.4 4.0 3.2 4.4 2.2 3.6 2.6 4.0 3.8 3.4 3.4 2.8 1.6 3.2 4.8 5.0 6.0 6.0 7.8 11.2 6.2 6.4 13.4 6.8 4.4 2.8 4.0 4.2 1.8 3.0 4.0 3.6 33 2.2 2.8 1.8 2.4 2.0 2.4
4.5 3.7 4.1 4.5 5.4 2.4 4.5 3.8 3-3 3.7 3.0 3.6 3.0 2.8 2.8 2.4 2.8 2.5 2.4 2.1 2.3 2.1 1.8 2.3 2.1 2.4 2.3 2.0 l-7 2.1 2.0 2.0 1.6 1.8 2.1 2.4 2.1 1.5 3.0 2.0 3.3 2.0 -
- Transition B(3,O) b(18,l) B(4,l) B(2,0) a(18.1) b(f8,2) B&O) B(2.1) s(l8,2) BfO.0) B(3.1) a(l8,31 B~O0,3~ A(.%11 Af3,O) a(18,4) B(OJ) A(2,O) B(2,4) aWM B(O,3) aC8,O) AtI,]) B(O,4) BWSI a(8.1) A(l,z>
I
11
I/II
24.6 39.6 19.8 29.4 21.6 36.0 33.0 16.8 31.2 23-4 30.0 32.4 24.0 13.8 10.2 31.6 22.8 15.0 6.6 18.0 19.8 9.6 8.4 13.8 9.6 4.2 6.0
14.8 14.8 8.8 14.6 10.0 13.8 14.4 8.0 13.2 13.0 KS 13.6 13-z 8.7 6-2 10.6 12.8 5.2 4-4
1.6 2.6 2.2 2.0 2.1 2.6 2.3 2.1 2.3 1-s 1.9 2.3 1.8 1.6 1.6 3-O l-7 2.8 1.5 7’ w-3
7.8 12.6 3.8 4.0 5.2 s-4 2.4 4.0
1.5 2.5 2.1 2.6 1.7 1.7 1-S
Volume 96. number 5
ref. 1201 for the C. C’ and D states. For several transitions originating from hi_eh-lying vibrational levels of the a and (particularly) b states of PbO*, the available spectroscopic data permit only tentative assignment. These transitions are marked by dashed lines in fig. 3. As is obvious from table 1 and from fig. 3. the intensity of the PbO’ emission increases with O,(I A) concentration. Note that the enhaIIcement of the emission intensity from rile high-lying D state is about twice of that I‘rorn tllc other states. It is \\ortlI noting that prelimina~ studies of 02(1~)/S~i mixrures reveal that in this txsc 3s \tcll_ the c11us51011 from high SnO states is e~di.uicclt11i01c r1i.mIii31 from the low 011es whell (ol(l.q
IS IIlLTeased
22 April 1983
CHEMICAL PHYSICS LETTERS
(21
I_
3.1. Mxhattisttzs
for Pb* production
The data presented in section 3.1 indicate that the main effect of the reaction between excited O2 and Pb atoms in the mixing zone is excitation of the latter, production of PbO being only a minor process_ This is hardly surprising since most of the excited O2 is in the IA state and the reaction between Pb(3P0) and 02(‘4) to produce Pb(;P,) [reaction (I)] is highly resonant whereas the reaction between the same species to produce PbO is endoergic by 0.3 eV. It is therefore concluded that reaction (1) is the Inain route for Pb(3Po) removal and for Pb(jPI) production in our system_ Production of Pb(3P2) can be accomplished by the reaction Pb(3P,)+
02(1A)-+Pb(3P2)+
O#Z)
_
(3)
The contribution of tile reactions of Pb(jP,+I) with 02(‘S) to the production of Pb(‘P?) is expected to be relatively small since [02(IZ)]/[02(1A)] < 0.01 under our experimental conditions [32]. An additional. possible esit channel for reaction (3) is discussed below. It is noteworthy that chemi-excitation of Pb(3P,Yz) has been observed during the combustion of acetylene aIId o~ygcn containing traces of tetramethyllead 1231, but the mechanism for this process has not been investigJted_ 4.2. Possible tttccftattisttts for PbO’ prodwtiotr
4_
Discussiorr
Production ofPbO* by reactions of Pb(3Po_I_2) with 02(‘S, 1A. *S) is tltermocheIIlicall_v precluded (since the Pb atoms and the 02 molecules in the mising zone dre translationally cold, the contribution to PbO’ production by reactions of fast species is negligible). On the other hand, PbO can be formed by reactions of Pb(jP2) with 02(- ‘C. *4, IS). of Pb(3PI) with 01(14, lx) and ofPo(3Pu) with O,(‘Z). The formation of PbO* can thus proceed via excitation of PbO by energy transfer from excited 0, or Pb. Since the observed PbO* levels lie>2 eV above PbO(X, u = 0) whereas the energy transfer processes can provide d 1_G eV (see fig. I), the PbO precursor must be vibrationally excited to 20.4 eV. Taking into account both the concentrations of the excited species and the exoergicity of the possible reactions between them, the main route for PbO* production is expected to follow the sequence Pb(jP2)
530
+ 0#4)
+ PbO(X, u) + O(jP)
,
(4)
CHEMICAL
Volume 96. number 5
PbO(X, V) + O#A)
+ PbO(a, b) f- 0~(3Z),
PHYSICS
(5)
states being formed via further excitation of the Ivng-Gved a and b states (&O p [24]) by O#A), The abvyementioned behavior vf PbO(D) supports this mechanism_ An additional observation which is possibly related to resonant energy transfer from Oz(lA) is the strong emission from the a, u = 18 and b, u = 18 states of PbO. These two vibrational states lie =7900 cm-l above a, u = 0 and b, u = 0, respectively, and can thus be fvrmed by promotion of the latter states via energy transfer from O&A). higher
Since [O#A)] decreases with [Pb], formation of O,(lZ) via the energy pooling reaction (2) decreases as well with [Pb]. The observation that [O,(tC)] increases with [Pb] (at high concentrations of the latter) invokes addition& mechanisms for O&iZ) formation. Assuming that Oz(‘A) is converted mainly to Pb(3PI) by reaction (I). one possible mechanism is Pb(3Pt)
i- O#A)
* Pb(3P0)
+ O,(l I;) e
(Sa)
Note that reaction (3a) is expected to be more efficient than the very inefficient,spin forbidden,energy pooling reaction (3) [Z]. Also, reaction @a) is similar to the ~ve~~stablis~~ed mechanism for O,(lE) production via energy transfer from excited I atoms {Zj IC’P,12) + OzCtA)-+
I(‘P$
+ O&)
-
(61
5 _Conclusions The cvmb~nat~on of the metal evaporation furnace with the chemical O,(lA) generator produces a unique device for studying the interaction of singlet oxygen with metal atoms. The present study demonstrates that intense emission from PbO*, including emission bands not observed by other methods, can thus be obtained. The extra enhancement of the emission intensity from high lying states of PbO, when [O-&A)] is increased, points out that it may be possible to achieve preferential population of these states using the efficient chemicd generators available tvday.
LETTERS
22 April 1983
Acknowledgement This research was supported by a grant from the United States--Israel Binativmd Science Fvundation (BSF), Jerusalem, Israel..
References [ X1 R.M. Badger. A-C. Wright and R-F.. Wbitlock, J. Citrxn. Phys. 43 fl96S) 4345. 121 R-F_ Heidner III and C_E. Gardner. Report SD-TR-796,
The Aerospace Coloration (1979). [3j DJ. Benard and N.R. Pchelkin, Rev_ Sci_ Instr_ 49 (1978) 794. [4] RJ. Richardson. C.E. Wswti, P.A.G. Carr, F-E, Hovis and W.V. Lilenfeld, 5. Appl. Phys. 52 (1981) 4962. [S] W-E. McDermott, N.R. Pchelkin. D.J. Bznard and R.R. Bousek, Appl. Phys. tetters 32 (1978) 469. [6f D-3. Benard, WE. McDermott, N-R. Pchelkii and R.R.
Bousek, Apple Phys. Letters 34 (1979) 4% f7 j R-J. Richardson and C-E. Wiswdi, Appl. Phys. Letters 35 (1979) 138_ [ 8 f J_ Bachrtrand S. Rosenwaks, Appf. Fhys. Let fess 4 1 (1982)
16_
(91 E.A. OgryzIo. in: Sin&t osysen, eds. H-H. \Vasserrn.tn and R.\V_Murray (Academic Press. New York. 1979) pp- 35-58. [ 101 R. Winter, I. Barnes, E.H. Fink, J. Wildt and F. Z-Jbel, f il]
Chem. Phys. Letters 73 (1380) 297. W Hack and O_ Horie, Chem. Phys. Letters SZ (19SIj
327_ [ 121 IV-E. McDermott and D.J. Benard, Cbem. Phys. Letters 64 (1979) 60_ [ 13f IX Busaln and 1,G.E Littler, Combust. Flame 32 (1974) 295. [ 141 J.B. West, R-S. Bradford Jr., J.D. EversoIeand CR. Jones.
Rev. Sci. Instr. 46 (1975) 164. [ 151 L NadIer and S_ Rosensaks, Chem. Phys. Letters 69 (19SO) X6_ { 161 J. Lot&m, Y_ Guern. J_ Cariou and A. Jnhmnin-Giies, J. Quant. Spectry. Radi;ttive Transfer 21 f1979) 143. f 171 M-J.. I(uryfo, W_ Braun. S_ Abramowitz and hi. Krxss, J. Res, Narl. Bur. Std. US 80.4 (1976) 167. [ 181 R-C. Oldenborg, CR. Dickson and R.N. Zare, J. Mol.
Spectry, 58 (1975) 283. [ 191 C. Linton and H-P. Broida,J. hial. Spectry. 62 (1976) 396. [20] 3. Rosen, Selected constants relative to diatomic molecules (Pergamon Press, Osford. 1970). I21 ] J. Bachar, M. SC. Thesis, Ben-Gurion University, BeerShew f1982)_ [33] RF- Heidner III. CE. Gardner, T_M. El-Sayed. GL Se?& and V-V_ Rasper. J. Chem. Phys. 73 (I9813 5618. [ 231 A. Gabay. hf. Rokni, J. Shmtdovich and S. Yatsiv. J. Citem. Phys. 67 (1977) 2284. [ 241 B.C. Wicke. S-P. Tang and J.F. Friichtenicht, Chem. Phys. Letters 53 (1978) 304.
531
REACTIONS
CHEMICAL
OF CHEMICALLY
PHYSICS
PRODUCED
LETTERS
22 April 1983
O,(’ A) WITH Pb ATOMS
J. BACHAR and S. ROSENWAKS
Rccwed
21 October
1962: in final form 4 February
1983
The potcnri.d of the chemical Ol(‘a) generator as a means of electronic excitation of metal atoms and oxides is demon3trJtcd m J %tud\ of the reactions of 02(' A) with Pb. The resonant energy trnnsfer process, Pb(3Pc,) + O,(‘A) 4 Pb(3Pr) +
O2 (” x). ir found tu be the lie> for producinp excited Pb rend PbO.
I. Introduction
02
The os~~en molecule in its lowest electronically esclrcd stiitc. ‘A,. has an extremely long radiative life( I], and is quenched with a very 111111’.2 b5 minutes lo\\ efficiency upon collisions with various types of uslls (e.g. ==10-s with Pyrex [?I)_ Also, 02(1A) can he produced in high concentrations. = 10% of the total O2 at aO.3 Torr. via microwave (MN’) discharge in osyse11 [3] snd in very high concentrations, up to XSOF. via reaction of Cl, with basic hydrogn peroxide solution I-1]_ These properties make O#Ll)a useful source of clectroruc erler_ry that can be formed in one location and exploited &ev.herc. O,(‘Zt) Ius successfully
Indeed.
chemically used to pump
-
,
E‘I&---
-la a‘r--
‘p,-
-
prcsenr work we have combined an efficient chentical Oz(lJl generator with a metal evaporation furnace and studied the interaction of singlet osygen with lead atoms. Pb was chosen because its lowest excited state, 3P,. is expected to be formed by the resonant eners transfer process [13]
526
“p,-
‘_Y-
‘PO---
+ Oz(‘A)
-. Pb(jP,)
Fig. 1. The lov.est
+ O#A)
-+ O#Z)
.
(3
energy Ievels of 02. Pb and PbO.
Also, some of the reactions
t O2(3C) + 0.01 eV.(l)
-I-O#Z)
X-
between Pb and 02, when
at least one of the species is excited, are exoergic enough to produce PbO_ The lowest energy levels of 02, Pb and
Higher states of Pb can be formed by further reactions of the excited Pb with O,( ‘A) or 02( 1Z): the latter is produced by the eners pooling reaction [9] O,(Q)
‘1 -
the
Pb(3P0)
BA-
been the chernicai iodine atom laser [5--S]; MW generated O,(la) has bern reacted with a number bf molecules and atoms [9 -- 121 and the escitation states of Ihe ensuing species III
PbO
o-
‘S,-
generated
have been studied.
Pb
PbO are depicted in fig. 1. Both excited Pb(Pb*) and excited PbO(PbO*) have been observed in our system and possible excitation mechanisms have been investigated. The chemical production
0 0092614/83/0000--0000/S
of 02(‘A)
is more involv-
03.00 0 1983 North-Holland
Volume 96, number 5
CHEMICAL PHYSICS LETTERS
ed than its production
by MW discharge. ‘iiowever, the former method has the advantage of producing high concentration of 02(lA) which is free of ions, electrons, atoms and of highly excited 0, [except Oz(l Z)] _ By alternatively using the MW and the chemical generator we were able to study some kinetic details of the O?/Pb system with the convenient MW source, to check them when necessary
with the pure chemical source,
and to investigate
application
the special features that result from of high concentration of O#A).
2. Experimental The experimental apparatus is depicted in fig. 2. Its main parts are the 02(lA) generating systems and the metal evaporation furnace. The construction and operation of the 02(1A) chemical generator has previously been described in detail [S]. Briefly, a stream of Cl, gas is bubbled through lo-15 I_rmpores of sintered glass into basic hydrogen peroxide solution. The effluent from the solution [mainly 02(32)/02(tA)] is passed through a cryogenic trap for water vapor removal and mixed with the Pb vapor emerging from the furnace_ Alternatively, 02(lA) is produced by a 2450 MHz MW discharge in pure oxygen_ Downstream of the discharge the gas is passed through a tube coated with HgO for 0 atoms removal. Typical 02 pressure in either generators is 0.15 Torr. The operation of the MW generator is optimized for maximum O,(LA) signals; it is then assumed that [02(tA)] is 10% [3]. TypieaEy
THERM!STOR
22 April 1983
[02(lA)] from the chemical generator is three times higher than that from the MW_ The metal evaporation furnace is of the type described in refs. [ 14,l S] _ The Pb is evaporated from a resistively heated alumina crucible, entrained in argon stream (~0.18 Torr) and carried to the Pb/O2 mixing zone. The temperature is monitored by a thermocouple near the crucible and by a thermistor in the mixing zone. Typical working temperature of the crucible is ~700 K (at temperatures X50 K clogging of the furnace precluded its stable operation); the working temperature in the mixing zone is then ~400 K. The concentration of 02(LA) is monitored both upstream and downstream of the mixing zone by observing the 0,(1A-3Z) emission at 1270 nm with cooled. intrinsic Ge detectors [8] _ The emission of 02( 1 Z) and PbO* emerging from the mixing zone is measured by a monochromator/photomultiplier/picoammeter/recorder system_ Ground-state and metastable Pb atoms are monitored by atomic absorption spectroscopy using a Pb hollow cathode lamp. The transitions used for monitoring the Pb states were 3PO-3Py, 3P -3Dp, 3P2-3Fz, 1D2-3P;, r D2-‘Py and ‘S0-3P~_1
3_ Rest&s Mixing of ground state 02 with Pb atoms did not resu!t in any visible reaction or measurable amount of excited species. On the other hand. the reaction of 02(L A) with Pb resulted in a bluish, diffuse flame, and
II,
THERMO-
Fig. 2. Schematic of the experimentai setup_ All measures are in cm.
527
Volum+ 96, number 5
CHEMICAL PHYSICS LETTERS
both Pb* and PbO* were observed_ The O,(lA) concentration downsrream of the mising zone decreased rapidly as the furnace temperature, i.e. [Pb], was increased and no 02(IA) could be observed when the furnace temperature was above ~600 K. The detailed btha~~or of rhe various species is summarized below.
In n search for the Pb states present in the flame \~ht’n operating the MW generator. the ground state, ‘P,. and the lowest escited states, SP, and 3P2. were Jeteacd: their concentration ratio. under the typical e\~rimcntai conditions (section 2). was 12/4/l _The
22 April 1983
since for reliable absorption measurements the O,fiA) has had to be switched on and off without changing the flow conditions_ In some Previous studies of chemiluminescence of PbO it was suspected that the Pb atoms evaporated from the furnace are either thermally [17] or electrically [ 1S] (by discharge in the heating wire) excited_ In our experintents thermal excitation could not have been significant since Pb* atoms disappeared when the O,(t A) generator was swjtched off_ Electrical escitation could not have been significant since no change in [Pb”] was observed when the electrical current to the heating wire was momentarily stopped.
ntXf hirriler stdles, t D2 and 1So_ were not observed. T.kin~kto account the transitiorl probabilities [ 161 md the responsivity of our detection sysreni we estimaw hat [ID,]. [‘So] <$ [‘PI]. The absolute con-
s~n~rarior~ofPbatomswasevaluated tobe~lOtocm_3_ The cwcen~ration of excited Pb was equal. within espct’ttttmtal error. IO the reduction in i3P0] wvhert the M\\‘generrl~or was switched on. Monitoring [Pb*] while usmg fhc chemical generator has not been convenient.
300
‘400
The emission spectra of PbO* resulting front the reaction of Pb atoms with 10% and with 30% 02(lA) are presented in fig_ 3. A list of the identified transitions, including their relative intensities, is given in table I_ The identification of the emission bands was carried out using the spectroscopic constants given in ref. (191 for X. a and A. in ref. [17] for b, in ref. [ 181 for B and in
500 WAVELENGTH
600
fnm)
I’IE_ 3. Emission spwfrd of PbO* resulting froze reaction of Pb + 10% &(‘A) (upper spectrum) and of Pb + 30% O,(‘A) (lower r;ps
CHEMKAL
Volume 96, number S
72 April 1983
PHYSICS LETiERS
Table 1 Identified emissions of PbO*. AU the transitions are to the ground electronic state. Conditions are as in fig. 3. The intensities of the emissions, in arbitrary units, uncorrected for the instrument wavelength response, are given in column ‘P’ for the Pb f 30% @('A) reaction, and in “Il” for Pb + 10% O,(‘A). In cases where several transitions overlap, only those which appear to be the strongest ones under our experimental conditions are listed. The possitife cases of overlap can be traced in fig. 3 and are presented in detail in ref. [21] Transition
I
D(3,W D(2,0) D(l,O) D(O,O) D(1.1) D@, 1) C’(9,O) f)(W) C’(7.0) D(0,3) 00.4) C&O) D(0.4) C’(4,O) D(OSI D(1,6) C’(3.0) C(6,1) C’(2,O)
3.6 7.2 9.6 10.8 10.8 21.6 7.8 19.8 8.4 12.0 9.6 12.0 13.8 10.2 9.6 7.8 Ii.4 9.0 12.0 12.0 12.6 13.8 16.8 20.4 14.4 13.8 33.0 16.2 9.0 4.8 8.4 8.4 3.6 4.8 -7.2 7.8 7.8 4.8 42 5.4 4.8 6.6 4.8 1.4
W,~) C(3,O) C(4,l) C’(0J-J) B(%Q C(4,2) (x.0) B(4,O) B(S, 1) B(O,S) a(9.2) B(l,6) A(O.2) a(8,2) B(L7) A@,3) b(3.1) af2,O) aG2) N4,7) A(O,S) a&l) AU ,6) a(3,7,) A@&
II
I/II -
1.6 2.6 2.6 2.4 4.0 3.2 4.4 2.2 3.6 2.6 4.0 3.8 3.4 3.4 2.8 1.6 3.2 4.8 5.0 6.0 6.0 7.8 11.2 6.2 6.4 13.4 6.8 4.4 2.8 4.0 4.2 1.8 3.0 4.0 3.6 33 2.2 2.8 1.8 2.4 2.0 2.4
4.5 3.7 4.1 4.5 5.4 2.4 4.5 3.8 3-3 3.7 3.0 3.6 3.0 2.8 2.8 2.4 2.8 2.5 2.4 2.1 2.3 2.1 1.8 2.3 2.1 2.4 2.3 2.0 l-7 2.1 2.0 2.0 1.6 1.8 2.1 2.4 2.1 1.5 3.0 2.0 3.3 2.0 -
- Transition B(3,O) b(18,l) B(4,l) B(2,0) a(18.1) b(f8,2) B&O) B(2.1) s(l8,2) BfO.0) B(3.1) a(l8,31 B~O0,3~ A(.%11 Af3,O) a(18,4) B(OJ) A(2,O) B(2,4) aWM B(O,3) aC8,O) AtI,]) B(O,4) BWSI a(8.1) A(l,z>
I
11
I/II
24.6 39.6 19.8 29.4 21.6 36.0 33.0 16.8 31.2 23-4 30.0 32.4 24.0 13.8 10.2 31.6 22.8 15.0 6.6 18.0 19.8 9.6 8.4 13.8 9.6 4.2 6.0
14.8 14.8 8.8 14.6 10.0 13.8 14.4 8.0 13.2 13.0 KS 13.6 13-z 8.7 6-2 10.6 12.8 5.2 4-4
1.6 2.6 2.2 2.0 2.1 2.6 2.3 2.1 2.3 1-s 1.9 2.3 1.8 1.6 1.6 3-O l-7 2.8 1.5 7’ w-3
7.8 12.6 3.8 4.0 5.2 s-4 2.4 4.0
1.5 2.5 2.1 2.6 1.7 1.7 1-S
Volume 96. number 5
ref. 1201 for the C. C’ and D states. For several transitions originating from hi_eh-lying vibrational levels of the a and (particularly) b states of PbO*, the available spectroscopic data permit only tentative assignment. These transitions are marked by dashed lines in fig. 3. As is obvious from table 1 and from fig. 3. the intensity of the PbO’ emission increases with O,(I A) concentration. Note that the enhaIIcement of the emission intensity from rile high-lying D state is about twice of that I‘rorn tllc other states. It is \\ortlI noting that prelimina~ studies of 02(1~)/S~i mixrures reveal that in this txsc 3s \tcll_ the c11us51011 from high SnO states is e~di.uicclt11i01c r1i.mIii31 from the low 011es whell (ol(l.q
IS IIlLTeased
22 April 1983
CHEMICAL PHYSICS LETTERS
(21
I_
3.1. Mxhattisttzs
for Pb* production
The data presented in section 3.1 indicate that the main effect of the reaction between excited O2 and Pb atoms in the mixing zone is excitation of the latter, production of PbO being only a minor process_ This is hardly surprising since most of the excited O2 is in the IA state and the reaction between Pb(3P0) and 02(‘4) to produce Pb(;P,) [reaction (I)] is highly resonant whereas the reaction between the same species to produce PbO is endoergic by 0.3 eV. It is therefore concluded that reaction (1) is the Inain route for Pb(3Po) removal and for Pb(jPI) production in our system_ Production of Pb(3P2) can be accomplished by the reaction Pb(3P,)+
02(1A)-+Pb(3P2)+
O#Z)
_
(3)
The contribution of tile reactions of Pb(jP,+I) with 02(‘S) to the production of Pb(‘P?) is expected to be relatively small since [02(IZ)]/[02(1A)] < 0.01 under our experimental conditions [32]. An additional. possible esit channel for reaction (3) is discussed below. It is noteworthy that chemi-excitation of Pb(3P,Yz) has been observed during the combustion of acetylene aIId o~ygcn containing traces of tetramethyllead 1231, but the mechanism for this process has not been investigJted_ 4.2. Possible tttccftattisttts for PbO’ prodwtiotr
4_
Discussiorr
Production ofPbO* by reactions of Pb(3Po_I_2) with 02(‘S, 1A. *S) is tltermocheIIlicall_v precluded (since the Pb atoms and the 02 molecules in the mising zone dre translationally cold, the contribution to PbO’ production by reactions of fast species is negligible). On the other hand, PbO can be formed by reactions of Pb(jP2) with 02(- ‘C. *4, IS). of Pb(3PI) with 01(14, lx) and ofPo(3Pu) with O,(‘Z). The formation of PbO* can thus proceed via excitation of PbO by energy transfer from excited 0, or Pb. Since the observed PbO* levels lie>2 eV above PbO(X, u = 0) whereas the energy transfer processes can provide d 1_G eV (see fig. I), the PbO precursor must be vibrationally excited to 20.4 eV. Taking into account both the concentrations of the excited species and the exoergicity of the possible reactions between them, the main route for PbO* production is expected to follow the sequence Pb(jP2)
530
+ 0#4)
+ PbO(X, u) + O(jP)
,
(4)
CHEMICAL
Volume 96. number 5
PbO(X, V) + O#A)
+ PbO(a, b) f- 0~(3Z),
PHYSICS
(5)
states being formed via further excitation of the Ivng-Gved a and b states (&O p [24]) by O#A), The abvyementioned behavior vf PbO(D) supports this mechanism_ An additional observation which is possibly related to resonant energy transfer from Oz(lA) is the strong emission from the a, u = 18 and b, u = 18 states of PbO. These two vibrational states lie =7900 cm-l above a, u = 0 and b, u = 0, respectively, and can thus be fvrmed by promotion of the latter states via energy transfer from O&A). higher
Since [O#A)] decreases with [Pb], formation of O,(lZ) via the energy pooling reaction (2) decreases as well with [Pb]. The observation that [O,(tC)] increases with [Pb] (at high concentrations of the latter) invokes addition& mechanisms for O&iZ) formation. Assuming that Oz(‘A) is converted mainly to Pb(3PI) by reaction (I). one possible mechanism is Pb(3Pt)
i- O#A)
* Pb(3P0)
+ O,(l I;) e
(Sa)
Note that reaction (3a) is expected to be more efficient than the very inefficient,spin forbidden,energy pooling reaction (3) [Z]. Also, reaction @a) is similar to the ~ve~~stablis~~ed mechanism for O,(lE) production via energy transfer from excited I atoms {Zj IC’P,12) + OzCtA)-+
I(‘P$
+ O&)
-
(61
5 _Conclusions The cvmb~nat~on of the metal evaporation furnace with the chemical O,(lA) generator produces a unique device for studying the interaction of singlet oxygen with metal atoms. The present study demonstrates that intense emission from PbO*, including emission bands not observed by other methods, can thus be obtained. The extra enhancement of the emission intensity from high lying states of PbO, when [O-&A)] is increased, points out that it may be possible to achieve preferential population of these states using the efficient chemicd generators available tvday.
LETTERS
22 April 1983
Acknowledgement This research was supported by a grant from the United States--Israel Binativmd Science Fvundation (BSF), Jerusalem, Israel..
References [ X1 R.M. Badger. A-C. Wright and R-F.. Wbitlock, J. Citrxn. Phys. 43 fl96S) 4345. 121 R-F_ Heidner III and C_E. Gardner. Report SD-TR-796,
The Aerospace Coloration (1979). [3j DJ. Benard and N.R. Pchelkin, Rev_ Sci_ Instr_ 49 (1978) 794. [4] RJ. Richardson. C.E. Wswti, P.A.G. Carr, F-E, Hovis and W.V. Lilenfeld, 5. Appl. Phys. 52 (1981) 4962. [S] W-E. McDermott, N.R. Pchelkin. D.J. Bznard and R.R. Bousek, Appl. Phys. tetters 32 (1978) 469. [6f D-3. Benard, WE. McDermott, N-R. Pchelkii and R.R.
Bousek, Apple Phys. Letters 34 (1979) 4% f7 j R-J. Richardson and C-E. Wiswdi, Appl. Phys. Letters 35 (1979) 138_ [ 8 f J_ Bachrtrand S. Rosenwaks, Appf. Fhys. Let fess 4 1 (1982)
16_
(91 E.A. OgryzIo. in: Sin&t osysen, eds. H-H. \Vasserrn.tn and R.\V_Murray (Academic Press. New York. 1979) pp- 35-58. [ 101 R. Winter, I. Barnes, E.H. Fink, J. Wildt and F. Z-Jbel, f il]
Chem. Phys. Letters 73 (1380) 297. W Hack and O_ Horie, Chem. Phys. Letters SZ (19SIj
327_ [ 121 IV-E. McDermott and D.J. Benard, Cbem. Phys. Letters 64 (1979) 60_ [ 13f IX Busaln and 1,G.E Littler, Combust. Flame 32 (1974) 295. [ 141 J.B. West, R-S. Bradford Jr., J.D. EversoIeand CR. Jones.
Rev. Sci. Instr. 46 (1975) 164. [ 151 L NadIer and S_ Rosensaks, Chem. Phys. Letters 69 (19SO) X6_ { 161 J. Lot&m, Y_ Guern. J_ Cariou and A. Jnhmnin-Giies, J. Quant. Spectry. Radi;ttive Transfer 21 f1979) 143. f 171 M-J.. I(uryfo, W_ Braun. S_ Abramowitz and hi. Krxss, J. Res, Narl. Bur. Std. US 80.4 (1976) 167. [ 181 R-C. Oldenborg, CR. Dickson and R.N. Zare, J. Mol.
Spectry, 58 (1975) 283. [ 191 C. Linton and H-P. Broida,J. hial. Spectry. 62 (1976) 396. [20] 3. Rosen, Selected constants relative to diatomic molecules (Pergamon Press, Osford. 1970). I21 ] J. Bachar, M. SC. Thesis, Ben-Gurion University, BeerShew f1982)_ [33] RF- Heidner III. CE. Gardner, T_M. El-Sayed. GL Se?& and V-V_ Rasper. J. Chem. Phys. 73 (I9813 5618. [ 231 A. Gabay. hf. Rokni, J. Shmtdovich and S. Yatsiv. J. Citem. Phys. 67 (1977) 2284. [ 241 B.C. Wicke. S-P. Tang and J.F. Friichtenicht, Chem. Phys. Letters 53 (1978) 304.
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