J. Phys. Chem. 1991,95,2113-2115
2113
Resonance-Enhanced Multiphoton Ionization Measurement of Ci(2P3/2and 2P,,,) Produced in the Photolysis of OClO from 355 to 370 nm Elizabeth Bisbenden, Jennifer Haddock, and D. J. Donaldson*-t Scarborough College and Department of Chemistry, University of Toronto, Toronto, Ontario, Canada MSS 1A1 (Received: November 12, 1990; In Final Form: December 19, 1990)
Chlorine atoms are detected in their ground and spin-orbit excited states as photodissociationproducts of room-temperature OClO excited at wavelengths 356-363 nm. A tunable dye laser is used to excite OClO in the wavelength range 355-370 nm, and a second tunable dye laser is used to probe for C1 (and C1+) atoms via their (2 + 1) REMPI spectrum near 235 nm. The quantum yield for CI formation near 362 nm is 0.15 f 0.10, with about 5 times more ground-state than spin-orbit excited CI produced.
Introduction Recently, Vaida and co-workers'*2have reported an enhancement of the C1+ signal in the resonance-enhanced multiphoton ionization (REMPI) spectrum of jet-cooled chlorine dioxide, OCIO, in a narrow wavelength region between 360 and 365 nm. They proposed that this enhancement could be due to a rearrangement3 to the unstable isomer, CIOO, followed by its uniThe C1 atoms are then nomwnantly moleculcr decay to CI + 02. ionized by the -360-nm laser radiation, giving rise to the enhanced CI+ signal. If this isomerization channel were to occur, it could be of great significance to our understanding of stratospheric ozone destruction. Current thinking'*' is that OClO is a sink for active chlorine in the stratosphere; the relevant chemistry is thought to be
-
Br
+ O3
CIO
+ BrO
BrO
+ O2
OClO
+ Br
OCI + o ( 3 ~ )
ocio O(3P) + O2
+M
-
0,+ M
which involves no net ozone loss.s The photoproduction of CI atoms from OClO would increase the concentration of active chlorine, and thus the ozone destruction rate, over that presently accounted for in atmospheric modeling. The one-color though suggestive of CI atom production, could not unambiguously establish the source of the C1+ signal. In that experiment, CI+ not only could be due to nonresonant ionization of C1 atoms but also could arise from other sources, such as dissociation of C10+ or OCIO+, photodissociation of CIO, followed by nonresonant ionization of CI atoms, etc. We have performed experiments to probe for the formation of chlorine atoms immediately following the near-UV photolysis of OC10. The experiments use one tunable laser to photodissociate OClO and a second to probe for CI atoms, via their REMPI spectrum. Our results demonstrate that atomic C1 is indeed formed upon excitation of OClO by one photon at wavelengths between 356 and 363 nm. The energetics of this process are
-+ -+ -+
OClO
362 nm
CI
02('2;)
W * -314 kJ mol-'
CI
02('A8)
A P = -218 kJ mol-'
C1
02('2:)
AHo = -155 kJ mol-'
Our determination of the quantum yield for CI production at the peak of the CI action spectrum is 0.15 f 0.1, with the atoms predominantly formed in their ground spin-orbit state (2Py2). *To whom correspondenceshould be addrased. NSERC University Research Fellow. 0022-3654/91/2095-2113%02.50/0
Experimental Seetion The experiments are performed at room temperature in a low-pressure flow cell outfitted with two Cu electrodes. The pressure is maintained at 0.5-2.0 Torr by adjusting the flow rate of C12 through a column of technical grade NaC1OP6 The presence and concentration of OClO in the flow cell under experimental conditions are ascertained by performing an absorption experiment in the same flow cell using a diode array spectrophotometer. The near-UV absorption spectrum of the yellow effluent gas is identical with the room-temperature spectrum of OCIO.' No other absorbers (except C12) are observed in the spectrum. Two laser beams are counterpropagated through the cell between the electrodes. One, the pump laser, is not focused and photoexcites the OClO to its predissociative excited A (2AJ state? in the near-UV region of the spectrum. Either the frequencydoubled output of a Nd:YAG-pumped dye laser or the fundamental of an excimer-pumped dye laser is used to excite the OCIO the results are independent of which laser system is employed. Laser pulse energies in the wavelength range 355-370 nm of 100-300 IrJ for the Nd:YAG-pumped system and 500-2000 pJ for the excimer-pumped laser are used. The probe laser is the frequency-doubled output of a second Nd:YAG-pumped dye laser, whose wavelength is chosen to be resonant with a two-photon transition9 of ground-state chlorine atoms ('P3,2) at 235.336 nm or spin-orbit excited atoms (CI', 2P1,2)at 237.808 nm. A third photon ionizes the chlorine in both cases, resulting in a (2 + 1) REMPI signal. The probe laser is focused into the flow cell with a 7.54111 f/l Suprasil lens; typical pulse energies at 235 nm are 50-300 pJ. The pulse and probe lasers are externally triggered such that the probe laser follows the pump, typically by I00 f 20 ns. In later experiments, delays of 50 f 20 ns are used. These short delay times ensure that the newborn photolysis products suffer I 1 collision prior to detection. One of the copper electrodes is held at +350 V and the other at ground potential. Positive ions formed in the probe laser pulse are repelled onto the ground plate. The resulting transient ion current pulse is detected as the voltage across a 1-Mil resistor and sent to a boxcar integrator averaging over 10-30 laser shots. The boxcar output is sent to a laboratory computer, where typically 2-4 spectra are averaged. 1M9; 312, 405. (2) Ruhl, E.; Jefferson, A.; Vaida, V. J . Phys. Chem. 1990, 94, 2990. (3) Gole, J. L. J. Phys. Chem. 1980, 84, 1333. (4) Solomon, S.;Sandcrs, R. W.; Miller, H. L., Jr. J. Geophys.Res. 1990, 950.13ao7. (5) Cox, R. A.; Hayman, G.D. Natwe 1988,332, 796. (6) Derby, R. I.; Hutchineon, W. S. Inorg. S y n d 1953, 4, 152. (7) Wahner, A,; Tyndall, G.S.;Ravishankam, A. R. J. Phys. Chem. 1981, 91..~ 2734. (8) Richard, E. C.; Wickham-Jones,C. T.; Vaida, V. J. Phys. Ch" 1989, 93, 6346. Richard, E. C.; Vaida, V. J. Chem. Phys. 1990, 94, 153, 163. (9) Arepalli, S.; Presser, N.; Robie, D.; Gordon, R. J. Chem. Phys. Lrrr. 1 ~ 5 , 1 1 8aa. ,
0 1991 American Chemical Society
2114 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991
I
I
I
I
I
356
358
360
362
364
Letters
I
366 Wavelength of Photodissociation Laser /nm
Figure 1. REMPI action spectra of ground-stateC1(2P3,2)formed in the photodissociation of (a) OClO and (b) Cl,. Here the probe laser is fixed at 235.336 nm, and the pump laser is scanned from 370 to 355 nm. The
235.38
235.36
235.34
235.32
Wavelength 01 Probe Laser / nm
Figure 2. REMPI spectra of C1(2P312)from OClO photolysis under different pump laser conditions: (a) pump laser blockad; (b) pump laser wavelength = 355 nm; (c) pump laser at 362.5 nm.
spectra are not normalized for the pump laser energy.
Results and Discussion Two types of experiment are carried out. In the first, the probe laser wavelength is held fixed, at the maximum of the C1 (or CP) REMPI peak, and the pump laser is scanned from 355 to 370 nm. The resulting action spectrum of C1 atom signal vs OClO excitation wavelength is shown in Figure la. Here, 0.5 Torr of OClO is dissociated, using -2 mJ/pulse of excimer-pumped dye laser radiation, and ground-state CI is detected, using 300 pJ/pulse of 235-nm radiation. Four features can be resolved in this spectrum, the most intense falling at -362 nm and the signal diminishing at wavelengths longer than -363 nm and shorter than -356 nm. No structure is observed between 364 and 370 nm; the signal remains weak and featureless. The action spectrum shown in Figure la is qualitatively very similar to the CI+ spectrum reported by Ruhl et a1.,2though its exact shape is somewhat different. It is, however, essentially identical with the absorption spectrum of OClO reported by Richard and Vaida? in the region between 364 and 356 nm. At longer wavelengths, between 364 and 370 nm, the absorption spectrum contains another cluster of four bandsE which are not seen here. For comparison, Figure 1b displays the ground-state CI atom action spectrum when 0.5 Torr of neat C12is flowed through the cell, bypassing the OClO generator. No structure is evident in this spectrum,wen though there is signal due to C1 atoms (formed through dissociation of the Clz) present. Since Clz is the only other CI atom generating species present in the experiment, the signal shown in Figure l a is clearly the result of OClO photolysis. A very similar, though less intense, action spectrum is obtained by measuring the C1* with the probe laser fixed at 237.808 nm. The second type of experiment measures the C1 atom (2 + 1) REMPI spectrum at a fixed pump laser wavelength, by tuning the probe laser wavelength. Figure 2 shows the spectrum of C1(2P3 2) observed with 1.0 Torr of OClO sample and (a) the pump iaser blocked, (b) the pump laser tuned to 355 nm, and (c) the pump laser tuned to 362.5 nm. A small background is observed with only the probe laser present; this signal is 4-5 times smaller than that measured with both lasers. Similar results are obtained scanning the probe laser over the C P resonance at 238 nm. These spectra show unambiguously that it is CI atoms which are detected in this experiment. The dependence of the observed CI atom REMPI signal on the power of the pump laser is determined over the range of pulse energies 100-1 500 rJ/pulse. The slope of the linear 1n (signal) vs In (laser energy) plot is 0.9 f 0.2, as required for a onephoton dependence. The corresponding slope for the probe laser is 3.3 f 0.6, consistent with the three-photon nature of the REMPI
-
12.2 0 4
30
2 8
2 6
In (probe laser energy)
I
1 5
.1
0
3 2
1
I
I
-0 5
00
0 5
In (pump laser energy)
I
Figure 3. Laser power dependences of C1 REMPI signal. (A) Dependence of signal on probe laser (235 nm). The slope of this plot is 3.3 0.6. (e) Dependence on pump laser (362nm). The slope is 0.9 f 0.2.
process. Figure 3 illustrates these power studies for the probe laser (A) and for the pump laser (B). Another source of C1 atom signal in our experiment that could contribute to the results is chemical reactions of the initial photoproducts, OCI + 0, with unphotolyzed OCIO. Energetically possible reactions which yield C1 as a product are OC1 + OClO -* OC1 + C1 + O2 0
+ OClO
-
O3+ C1
AHo = +17 kJ mol-'
AHo = -92 kJ mol-'
neither of which has been reported.l0 We check for the possibility of chemical reaction causing our signal by scanning the delay between the pump and probe lasers from 0 to lo00 m. Here we measure a monotonic decrease in the CI atom REMPI signal as the delay time is increased, indicating that the CI atoms are all produced at t = 0 ns. In another experiment, we measure the (IO) Watson, R. T.J . Phys. Chem. Ref, Doto 1977, 6,871.
The Journal of Physical Chemistry, Vol. 95, No. 6,I991 2115
Letters REMPI signal as a function of OClO pressure (as determined from the absorbance). In the low-pressure regime of these results, the CI signal is linear in OClO concentration. These two results mitigate against chlorine atom production by chemical reaction, since a reaction would show an induction time for CI buildup and also demonstrate a quadratic dependence of signal on pressure. As well, photolysis of OClO at wavelengths between 364 and 370 nm will produce OC1and 0 atom produw, if bimolecular reaction were responsible for the observed signal, we would observe CI in that wavelength range. Thus, we may rule out chemical reactions as the source of our signal. Photolysis of the OC1 product by either the pump or the probe laser is a more insidious artifact. Our laser power dependences indicate a one-photon dependence on the pump laser and a three-photon dependence on the probe. These dependences are consistent with, and strongly suggestive of, the mechanism proposed by Vaida et al.:’S2 I
OClO
X
355-370 nm
c1+0 2
followed by 2 X 235 (238) nm
c1(2p3/2*(2pl/2))
-
C1(2D3/2)
However, other possibilities for CI atom formation are OClO
355-370 nm
OCl(u’)
OCl(u’)
+0
AHo = -84 kJ mol-’
355-370 nm
OCI(A;211)
-+ prcdirsociation
OC1(A;211) and
OClO OCl(U’)
355-370 nm
235 (238) MI
C1
0
OCl(U’)
c1+0
Neither of these will give rise to our observed photon dependences unless one or more of the steps are saturated. The exact energetics of these reactions will depend on the energy disposal into the OC1 product in each case. In the first case, (a), one-photon photolysis of OClO produces vibrationally excited OCI, which could absorb an additional pump laser photon to access low vibrational levels in the predissociative A state. In this instance, the C1 atom signal would be a convolution of the absorption spectra of OClO and OCI in the -360-nm spectral region, with one of the transitions saturated. We do observe peaks in the C1 action spectrum that correspond to the OCIO absorption, but only for one clujtei of absorption bands, companding to q‘ -- 10 in the OClO (A-X) spectrum. A second such cluster, at somewhat longer wavelengths, is observed strongly in absorption7.* but does nor give rise to CI atom signal. If photodissociation of OCI following its production via OClO photolysis was responsible for the CI signal, that signal should be observed in the 364-370-nm wavelength region, oorresponding to the uI’ = 9 band in the OClO spectrum. Therefore, we believe that OCI photolysis by the pump laser cannot be responsible for the results reported here. Situation (b), above, involves production of OC1 by the pump laser, its excitation into the dissociation continuum of the A state
by the probe laser, and then the generation of signal via the (2 1) REMPI of CI atoms, as before. This mechanism is also entirely ruled out by the fact that we see no signal between 370 and 364 nm, even though OClO dissociation takes place in that range.8 An additional argument is that the A state of OCI has as its dissociation limit O(ID) + Cl(2P3/2).” If there are no perturbations, excitation into the A-state dissociation continuum will produce CI atoms in their ground spin-orbit states only. As noted above, we observe significant signal from CI(2Pl/2),with the same action spectrum as that of the *P3/2 state, implying a common source. Since this should not arise from OCI A-state dissociation, we conclude that process (b) is also not the source of the observed C1 atom signal. The evidence all points toward the formation of C1 atoms as a primary product of one-photon absorption of OClO at wavelengths near 360 nm. The sensitivity of the experiment is calibrated by generating a known density of ground-stateI2CI atoms via 355-nm dissociation of C12. We estimate a detection limit of -10” C1 atoms for the present system. As noted above, we have also measured the CI REMPI signal arising from 361-nm photolysis of C12and OClO as a function of pressure. In the low-pressure (10.5 Torr) regime, this signal is linear with precursor pressure for both precursors. Using the absorption cross sectionloof C1, at 361 nm, we can relate a given signal level to a concentration of C1 atoms. The same signal level is measured for a lower pressure of OCIO, all other experimental variables being held constant. From the concentration of C1 atoms, the pressure of OC10, and its a b sorption coefficient’ at 361 nm, we determine a quantum yield for C1 formation at this wavelength to be 0.15 f 0.10, Preliminary measurements of the Cl*(2Pl/2)indicate that it is formed in much smaller quantities than the ground state, with an estimated branching ratio Cl/Cl* of 1/0.2. These results are in conflict with a very recent report,13 which sets an upper limit of 5 X lo4 to the quantum yield of CI atoms from OClO photolysis between 359 and 368 nm. However, those measurements were made using very high pressure conditions (many atmospheres) and without a delay time between the photodissociation and probe lasers for the OClO studies. As well, the dependence of the signal on the laser pulse energies is not reported. We are confident that the results reported here represent the “true” quantum yield for C1 production. A large quantum yield for C1 atom formation could have significant implications to the understanding of ozone depletion mechanisms. Current thinking is that photolysis of OClO results in products C10 + 0 1 v 4 this has no net effect on O3concentration. If C1 is also formed in significant amounts, the ozone loss rate could increase substantially? due to ozone-depleting reactions of C1 atoms. Work is presently under way to establish absolute cross sections for Cl production over a wider wavelength range than is reported here and to measure the complementary O2product of this dissociation.
+
Acknowledgment. This work was supported by NSERC Canada and by the Ontario Laser and Lightwave Research Centre. We are grateful to Professor V. Vaida for many stimulating discussions about OClO spectroscopy and photochemistry. (11) Durie, R. A.; Ramsay, D. A. Can. J. Phys. 1958, 36, 35. (12) Matsumi, Y.; Kawasaki, M.; Sato, T.; Kinugawa, T.; Arikawa, T. Chem. Phys. Lett. 1989,155,486. (13) Lawrence, W. G.; Clemitshaw, K. C.; Apkarian, V. A. J. Geophys. Res. 1990, 950, 18591.
