4275
J. Phys. Chem. 1902, 8 6 , 4275-4276
SO, / Ar 500 Torr €-band
n
20 m m
0
IO
20
time I wavelength
/
nm
Figure 3. Fluorescence excitation spectra of SO2 excited by the 0.02-cm-' bandwidth laser line in the supersonic jet of Ar with P, = 700 torr (a) and 110 torr (b).
30
40
JJS
Flgure 4. Time evolution of the fluorescence from the (J',Kl,K:) = (l,l,O) level of SO2' in the Ar jet with a source pressure of 500 torr as a function of the nozzle-laser beam distance.
section of SO2* by Ar must be less than this value.
Supplementary Material Available: Four figures showing the fluorescence decay data of jet-cooled SO2 by irradiation with a 0.0'2- and 0.5-cm-' bandwidth laser (5 pages). Ordering information is given on any current masthead page.
been observed in the He jet also.4 In Figure 3 are shown the fluorescence excitation spectra of SO2 in a supersonic jet of Ar with a high and a low source pressure which have been observed by irradiation with a 0.02-cm-' bandwidth laser. The spectra suggest that the 0.1-0.2-cm-' bandwidth laser may not resolve each rotational line especially for the case of a low nozzle source pressure. Therefore, the appearance of the fast decay component of the fluorescence in Figure 1 of MacDonald and Lee's paper2 at a high nozzle source pressure, similar to our Figure 2, may be interpreted as an indication of more contributions of the fluorescence from particular single rovibronic levels in which the short-lived component predominates. It has been reported in the previous paper' that the fluorescence from a lower rotational level tends to possess a larger proportion of the short-lived component. It is desirable, however, to test the present interpretation of MacDonald and Lee's data on the basis of the fluorescence excitation spectrum observed by their apparatus as a function of the nozzle source pressure and dependence of the fluorescence decay on the nozzle-laser beam distance. We have reported the beating fluorescence decay of the 'b(0) line (no. 12 line in Figure 7 of ref l), though a simple exponential decay of this line is observed by MacDonald and Lee. In the present experiment, as shown in Figure 4, it has been found that the beating is inhibited when the nozzle-laser beam distance is less than 7 mm at a nozzle source pressure of more than 500 torr. If collisions are responsible for the disappearance of the beating fluorescence decay at a shorter nozzle-laser beam distance, the collision cross section is estimated according to eq 2 of ref 1to be around 500 A2. The fluorescence quenching cross
Sir: In the preceding comment,' the authors presented additional experimental data to show that the short-lived component of SOz* fluorescence decays with a lifetime of -5 ws, independent of Po (110-700 torr Ar) and x (4-14 mm) in Figure 1. They conclude that this value of lifetime is not collision induced, contrary to our suggestion based on our data2 that SO2* fluorescence decays with Po dependence (0.7-7 atm of He and 0.7-2.6 atm of Ar) and the extrapolated, zero-pressure (collision-free)lifetime is 10-18
(4)Results in the He jet are shown in Figure 4 of the supplementary material.
(1) H. Watanabe, S.Tsuchiya, and S. Koda, J.Phys. Chem., preceding comment in this issue. (2) B. G. MacDonald and E. K. C. Lee, J.Phys. Chem., 86,323 (1982).
Department of Pure and Applied Sciences College of General Education University of Tokyo Komaba, Meguro-ku, Tokyo 153, Japan Department of Reaction Chemistry Faculty of Engineering Universtty of Tokyo Hongo, Bunkyo-ku, Tokyo 113, Japan
Hajlme Watanabe Sojl Tsuchlya'
Selichlro Koda
Received: March 5, 1982; I n Final Form: July 8, 1982
Reply to the Comment, "Fluorescence Quenching Collisions of Rotationally Cold SO,(A 'A,) In a Supersonic Jet" by H. Watanabe, S. Tsuchiya, and S. Koda
0022-3654/82/2086-4275$0 1.25/0 0 1982 American Chemical Society
Additions and Corrections
!c) 27atm I 1 ( + 5.0)32872 5cm-’ ( - 5 0 )
‘ E ’ band f--
’laser Flgwe 1. Fluorescence excitation spectra of the SO2 “E” band as a function of the reservoir pressure (Po).
TABLE I: Some SRL Lifetime Data for t h e SO, “E” Band Given b y Watanabe et ala peak est no. intb
transition
( J ‘ , K ,K:)
10 10 3
’R,(2)‘ ‘R,(2) ‘Q,(2) ‘Q,(4) ‘R,(O)
(3,2,1)‘ 3,1,2 2,2,1 4,2,3 1.1,O
7 8 10 11 12 13 19 20 21
6
10 2 3 4 2
?
?
PQ,(3)
3,0,3 1.0.1 , . 7,0,7
PQ,(1)
pQ;j7j
TS, ps
TL, ps
Iso/ILo
22 17 23 7 3.5 14 1.7 5.0 31 15 beating decay 3.5 15 1.8 23 3 4.7 18 4 4.9 3.3 2.1 1.8 4.6 4.1
a From Table I11 and Figure 7 of ref 3. Our estimates of fluorescence excitation intensity from Figure 7 in ref 3. We assign this peak to ‘R1( 1)from Po= 1-20 atm runs in our experiment (see Figure 1). Watanabe et al.3 assigns peak no. 9 t o ‘ R l ( l ) .
‘
Therefore, they suggest that our observation of single-exponential decay a t low Po is due to a combined fluorescence from several rovibronic levels (at relatively high temperature). In our opinion, this cannot be the ps.
explanation for the following reasons. We have earlier measured fluorescence decays for three “rotational lines”, ‘R0(2),‘R0(O),and PQ1(l), with 7 = 10, 10, and 18 ps, respectively.2 Their 0.02-cm-’ data are summarized in Table I. It is possible that, with a 0.10.2-cm-’ bandwidth excitation in our experiment, we could have excited peaks 7 and 8, peaks 10-13, and peaks 14-21. Considering the fluorescence excitation intensities and the values of T ~ T, ~ and , Iso/lLolisted in Table I, however, we find it difficult to reconstruct a “single”exponential decay with a lifetime of 10 or 18 ps. The fluorescence excitation spectra taken with our apparatus as a function of Po are shown in Figure 1. The transition assignments are based on those given by Hamada and Merer4 and the computed spectra using the rotational constants.4 The important feature to note is that the ‘R1(l) and ‘R0(2) peaks (peaks 7 and 8 in ref 3) are partially resolved under our experimental condition and of near equal intensities at Po = 4-27 atm (or Tmt= 2.5-1.3 K) which supports the assignment of peak 7 to ‘RI(l) rather than ‘R1(2). We have also observed the fluorescence decay of ‘ h ( 0 ) having the Ybeatn.The appearance of the “beat” was neither prominent nor reproducible probably due to the variation in laser modes and bandwidths, and we did not pursue further to quantify it because of the collisional quenching observed at high Po. We feel that the “quenching”of the beat at Po> 500 torr and x < 7 mm in their experiment,'^^ giving the collision cross section of 500 A2 by Ar, is a significant observation, particularly in view of the very efficient collision-induced electronic relaxation of SRL’s of SO2* by a long-range interaction mechanism as observed in our experiment2 as well as in T,-relaxation and microwave line broadening experiments? These observations indicate, we believe, that there is not sufficient information for their suggestion.’ We hope to find a more satisfactory explanation and report it in the near future.
-
(3) H. Watanabe, Y. Hydo, S. Tsuchiya, and S. Koda, J. Phys. Chem., 86, 685 (1982). (4) Y. Hamada and A. J. Merer, Can. J. Phys., 53, 2555 (1975). (5) D. L. Holtermann, E. K. C. Lee, and R. Nanes. J . Chem. Phys., 76,
3341 (1982).
Department of Chemistry University of California Iwine. California 927 17
Bruce G. MacDonald Edward K. C. Lee
Received: June 3, 1982; I n Final Form: August 2, 1982
ADDITIONS AND CORRECTIONS 1982, Volume 86 J. M. Howell, A. M. Sapse,* E. Singman, and G . Snyder: Ab Initio Self-Consistent Field Calculations of NOz-(H20), and NOJHzO), Clusters. Page 2345. The correct spelling of the last author’s name is G. Snyder.
