Collisional quenching of excited iodine atoms I(5p 5 2 P 1/2) by selected moleculesD. H. Burde and R. A. McFarlane Citation: The Journal of Chemical Physics 64, 1850 (1976); doi: 10.1063/1.432323 View online: http://dx.doi.org/10.1063/1.432323 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/64/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Conservation of the Kr+(2 P 1/2) state in the reactive quenching of Kr(5s′[1/2]0) atoms by halogencontainingmolecules J. Chem. Phys. 105, 5020 (1996); 10.1063/1.472348 Collisional quenching of excited iodine atoms (5p 5 2 P 1/2) by Cl2 in a flow system J. Chem. Phys. 82, 2590 (1985); 10.1063/1.448309 Kinetics of the deactivation of I(52 P 1/2) by Br2. I. Quenching of excited iodine atoms J. Chem. Phys. 69, 1797 (1978); 10.1063/1.436839 Quenching and reactions of laserexcited I(52 P 1/2) atoms with halogen and interhalogen molecules J. Chem. Phys. 69, 641 (1978); 10.1063/1.436629 Reaction of Electronically Excited Iodine Atoms, I(52P1/2), with Methyl Iodide J. Chem. Phys. 49, 953 (1968); 10.1063/1.1670167

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

131.193.242.21 On: Thu, 27 Nov 2014 16:03:38

NOTES

Collisional quenching of excited iodine atoms 1(5p 5 2P1/2)

by selected molecules O. H. Burde

Department of Applied Physics. Cornell University. Ithaca. New York 14853

R. A. McFarlane

School of Electrical Engineering and Materials Science Center. Cornell University. Ithaca. New York 14853 (Received 22 October 1975)

The iodine laser employs the transition I(5 p5 Z P1/Z) - 1(5p5 Z P3/Z) + hll(l. 315 J.l) in atomic iodine. The rates of quenching of the upper state 1* (5p5 Z P1/Z) by collisions with species in the laser medium are needed for computer mod­elling of the kinetics. Of particular interestis the quench­ing rate oU* by COz which is frequently used to lower the gain by pressure broadening. A cw iodine laser system under development1 involves production of 1* by reso­nant energy transfer from OZ(l.o.) to ground state atomic iodine 1(5p5 Z P3/Z ). The Oze.o.) molecules are produced in the reaction of calcium hypochlorite with hydrogen peroxide. Species present in the reaction products in­clude HzO, Oz, and Hz0z' A study was undertaken to measure the second order quenching rate constants for these and related molecules.

The experimental apparatus has been described in de­tail previously. Z.3 A nitrogen-laser-pumped tunable dye laser was used to photolyze Iz producing 1* and I by dis­sociation from the B(37TO .) state. Because of the very

u efficient quenching of 1* by Iz, 3 n-C3F71 was used as the source of I* in studies of less efficient quenchers. The n-C3F71 was dissociated directly by the nitrogen laser as its absorption band lies in the ultraviolet. Pressure readings were made by an MKS Baratron gauge and by a Robinson-Halpern transducer attached directly to the absorption cell.

The concentration of 1* was monitored photoelectri­cally by time-resolved atomic absorption. Atomic res­onance radiation from a microwave-powered discharge lamp passed through the absorption cell. A McPherson model 225 vacuum ultraviolet monochromator was used to isolate the 2062 A line 1(6sZP3/Z)-1(5p5ZP1/Z). Ab­sorption at this wavelength provided a direct measure­ment of [1*] versus time following each laser pulse. Several thousand laser pulses were averaged using a

TABLE I .. Second order quenching rate constant for the deac­tivation of I 2P1l2'

Quenching Literature molecule This work value Ref.

12 3.6±0.3xlo-11 3.0xIO-11 6

°2 2. 5±0. 3xIO-11 2.5 XIO-l1 8 H20 (12 source) 2.3±0.3xIO-12 7.2 xIO-13 11

(n-CaF 71 source) 2.7±0.3xIO-12

D20 4.3±0.6 x IO-14 6.2x10-14 12

H20 2 1. 0 ±O. 2 xlO-l1

CO2 1.3±0.2x10-16 1.3xIO-16 12

Biomation model 8100 digitizer and a Northern Scientif­ic model 575 averager. The digitized data was fit to an exponential decay function using an iterative least squares curve fitting routine utilizing typically 500 pOints per decay curve.

The kinetics of the 1* concentration are described by

- d[I*l/dt = (Anm +kv +kQ1 [ Ql] +kQz[ Qz])[I*] =k'[I*], (1)

The spontaneous emission term Anm and the diffusion term kv were negligible under the experimental condi­tions. The third term represents quenching due to the molecule used as the source of 1*, that is Iz or n -C 3F 71. The last term represents quenching by the gas under study. Equation (1) may be integrated to give [1*] = [I*]to exp( - k' t). All decays observed were single ex­ponential.

In previous experiments employing resonance absorp­tion techniques, 4 it has been found that the transmitted light intensity can be described by a modified Beer­Lambert law given by Itr = Ioexp[ - (t:cW]. The factor y is an empirically determined parameter which accounts for line reversal and broadening. It can be shown5 that for small absorptions y approaches unity. In this ex­periment, absorptions were never larger than 10 per­cent and y was measured to be O. 89 ± 0.07. Hence, only a slight modification of the Beer-Lambert law was nec­essary in this experiment. The decay constant k' is

35

• • + 15

N

.£. N

10 0

~

5 • I I !

0.1 0.2 0.3 0.4

P02 (TORR)

FIG. 1. Quenching of 1* (5p5 2~/2) due to collisions with 02 as a function of 02 pressure. 12 pressure constant at 0.030 torr.

1850 The Journal of Chemical Physics. Vol. 64, No.4. 15 February 1976 Copyright © 1976 American Institute of Physics

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

131.193.242.21 On: Thu, 27 Nov 2014 16:03:38

Letters to the Editor 1851

6 ./ .tI

./ 'c&~ I '" '----' U

~N Cl.~ 32

/

:.::

+ 1/ d" 2S • __ ~I __ ~I __ ~I __ ~I __ ~I __ ~I __

0'" 1.0 2.0 3.0 4.0 5.0 6.0 L PHf) + PHA (TORR)

FIG. 2. Quenching of 1* (5p 5 2PI /2) due to collisions with H20 and H20 2 as a function of (PH20 + P H202), where P H20 /PH202 =

0.65/0.35.

given by kobS/Y' where kobS is derived from the curve fitting routine.

All gases used were Linde research grade. Individ­ual batch analysis showed impurities to be negligible.

Table I summarizes the experimental results. The most effective quencher studied is ~. This rate3 which we found to be substantially faster than the previously accepted value has recently been confirmed by two other groups. 6,7 The second order quenching rates of O2 (see Fig. 1) and CO2 are in good agreement with the litera­ture values. The H20 quenching rate was measured us­ing both 1.! and n-C3F71 as the source of 1*. The mea­sured second order quenching rate constants agree with­in the estimated error. The D20 rate is some fifty times slower. Similar large isotope effects in the quenching of I * have been seen for Hz and Dz, 8 CH31 and CD31, 9 and recently for HD. 10

To measure a quenching rate of 1* by H20 a , a few microliters of a 50 percent solution by weighf3 was in­jected into the system. This completely vaporized

yielding a vapor of 35 percent mole fraction H20 2 and 65 percent mole fraction H20. After a data run was taken, the H20 2 and n-C3F71 were frozen out into a cold finger in the absorption cell. The cell was then pumped to remove any traces of 02 which might have formed by dissociation of HzOz. After warming to room tempera­ture, a second run was taken. In all cases, there was little difference in the observed decay times, indicating that the H20z was stable and that contributions to the ob­served rate due to 0z were negligible. This sequence was repeated with four fresh samples of the 50 percent solution (Fig. 2). By using our measured value of kHZO ' a value for kHZ02 was calculated.

Support for this research was obtained from the Na­tional Science Foundation through the Materials Science Center, Cornell University, and from the Air Force Office of Scientific Research (AFSC) under Grant 75-2781.

I A• K. MacKnight and P. J. Modreski, Air Force Weapons Lab. Report AFWL-TR-74-344 (1974).

2D. H. Burde, R. A. McFarlane, and J. R. Wiesenfeld, Phys. Rev. A10, 1917 (1974).

3D. H. Burde, R. A. McFarlane, and J. R. Wiesenfeld, Chern. Phys. Lett. 32, 296 (1975).

4J. J. Deakin, D. Husain, and J. R. Wiesenfeld, Chern. Phys. Lett. 10, 146 (1971).

5C. C. Davis and R. A. McFarlane (in preparation). 61. Arnold, F. J. Comes, and S. Pionteck, Chern. Phys. 9,

237 (1975). 7F. Wodarczyk (private communication). 8J . J. Deakin and D. Husain, J. Chern. Soc. Faraday II 68,

1603 (1972). 9R. J. Donovan and C. Fotakis, J. Chern. Phys. 61, 2159

(1974). lOR. J. Butcher, R. J. Donovan, and R. H. Strain, J. Chern.

Soc. Faraday II 70, 1837 (1974). l1 J • J. Deakin and D. Husain, J. Chern. Soc. Faraday II 68,

41 (1972). 12R. J. Donovan and D. Husain, Trans. Faraday Soc. 62, 2023

(1966). 13The solution of 50 percent H20 2 by weight corresponds to a

solution containing 35 percent mole fraction H20 2 and 65 per­cent mole fraction H20. This solution was supplied by the FMC Corporation, Inorganic Chemicals DiviSion, New York, NY.

J. Chern. Phys., Vol. 64, No.4, 15 February 1976

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

131.193.242.21 On: Thu, 27 Nov 2014 16:03:38