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COMMENTS ON "LASER-EXCITED FLUO PESCENCE OF THE HYDROXYL RADICAL:

RELAXATION COEFFICIENTS AT ATMOSPHERIC PRESSURE"

Charles C. Nang

Research Staff, Ford Motor Company, Dearborn, Michigan 48121-2053

The reported results of Chan et al do not support their contention

that the quenching cross sections due to oxygen are pressure dependent.

Discrepancies among their own results are also noted.

G

c v

(bi&SA -Ci3-175442) COMMENTS UN LASER-EXCITED N84-20843

FLUORESCENCE OF THE HYDROXYL RADICAL;

RELIXATI ON COEFFICIENTS AT ATMOSPHERIC 

PRESSURE, 1PPF.NDIX 5 (Ford Motor Co.) 6 p Unclas

8C 102/61 101 CSCL 20E G3/36 18846
The lifetime of the excited state of an atom or molecule can often

be determined from the rate of fluorescence decay originating from the

excited state. By measuring this fluorescence lifetime as a function of

buffer gas pressure, an accurate determination can be made of the rates of

collision-induced transitions away from the excited state. In the case of

the hydroxyl radical (OH), direct lifetime measurements have been made over

a range of low system pressures up to about 25 torr. 1-5 However, attempts

to extend this type of measurements to atmospheric pressure have been met

with increasing difficulty since excitation and detection with s^jbnanosecond

resolution are required. Recently, Chan et al reported measurements near

atmospheric pressure using a 7-nanosecond laser for excitation, concluding

that a strong pressure dependence existed for the electronic quenching cross

section of ON due to oxygen. They also stated that this pressure dependence

would mean a factor of ten error in the OH concentrations deduced from

fluorescence mR asurements in ambient air, a topic of considerable interest

to us. The purpose of this comment is to point out that their reported

results do not support their conclusion, and that the results themselves

do not appear to be internally consistent. On the other hand, various

observations published in the last ten years by a number of other independent workers 1-4,7 all tend to support the supposition that pressure dependence

at atmospheric pressure is negligible for the quenching and relaxation cross

sections of OH. In view of these considerations, we do not believe the conclusions reached by Chan et al are valid.

The success of any measurement of fluorescence lifetime is predicated

upon the use of an exciting laser and detection elc -Aronics both of which are

fast compared to the fluorescence lifetime to be measured. This point is

brought to light in the earlier publications. For example, an accurate

determination of the fluorescence lifetime in Eq. (6) or Eq. (7) of Ref. 6

would not be possible when the fluorescence lifetime to be determined is
-2-

much shorter than the pulsewidth of the exciting laser and/or the ► -esolution time of the detection electronics. The data which led Chan et al to

claim the existence of a pressure dependence were presumably taken in

oxygen at pressures of 40 torr and higher. At 40 torr, as may be deduced

from the results in their Fig. 8, the fluorescence lifetime is already

shorter than their laser pulsewidth by a factor of about 35. This being

the case, the temporal shape of the fluorescence signal would be insensitive to the value of the fluorescence decay rate, so that any value deduced

from the temporal shape would at best be inaccurate. It thus follows that

these authors need to improve their temporal resolution by about an order

of magnitude in order to make meaningful measurements of the fluorescence

lifetime of OH at 40 torr of oxygen. For ;,:easure-:ents at higher pressures,

still faster excitation and detection will be necessary. In our o;,inion,

the results of Chan et al represent a corm-vendable attempt at a difficult but

important problem in atmospheric chemistry, but their statement that "a ...

strong ... pressure dependence was found ..." appears to be premature and

unwarranted.

According to the conjecture outlined by Chan et al, resonant

energy transfer with oxygen molecules should lead to enhanced electronic

quenching rate for the excited state OH in the v' = 1 manifold, but not for

the excited state OH in the v' = 0 manifold. In the limit of small radiative decay rate, one would expect from their Eq. (21) that the ratio of the

flunrescence intensities near 3,909A and 3,14 0 should remain the same at

atmospheric pressure as that observed previously at 1 ower pressures. Furthermore, this ratio should also remain the same even in the absence of any
4

-3-

resonant enhancement of the quenching rate. This analysis suggests an

error in their results of Fig. 10, which show a ratio at atmospheric

pressure that is four times larger than the ratio at lower pressures.

The question of pressure dependence for the quenching cross

sections of OH is obviously an important one as it affects the value of OH

concentrations deduced from fluorescence measurements. As was discussed

at length in Ref. 5, the quenching cross sections due to nitrogen and oxygen

are independent of pressure up to 25 torr, and will most probably remain so

up to atmospheric pressure. This conclusion was based on the fact that no

pressure dependence was observed either in direct lifetime measurements at

lower pressures 1-5 or in Stern-Volmer type measurements ? up to atmospheric

pressure. It also agrees well with other theoretical and experimental considerations such as the pressure independence of the observed fluorescence

spectrum, and the estimated magnitude of any possible pressure dependence

at atmospheric pressure. In view of these results and the difficulties

associated with the experiments of Chan et al, we believe it is prudent to

assume that the quenching cross sections of OH are pressure independent at

atmospheric pressure.

The author wishes to thank Dr. S. Japar for bringing to his

attention the publication of Ref. 6. This research has been supported

in part by National Aeronautics and Space Administration through Wayne

State University and by the Department of Energy.
-4-

Note added in proof:

Based on their reply to this comment, it is my understanding that

Drs. Chan et al agreed with me on my position as outlined in my introductory

paragraph, but insisted that the temporal resolution of their detection

system was as good as 0.2 nsec. I have no trouble in agreeing that deconvolution can in principle be employed to resolve fluorescence times shorter

than the response times of the system. However, attainable reproducibility

and accuracy in actual experiments usually set a limit beyond which no meaningful results can be expected from deconvolution processes. Just hoar

restrictive this limit may be is also difficult to evaluate. Prudence thus

dictates that the results of deconvolution be viewed with extreme caution

whenever fluorescence times much shorter thar, the response times of the

system are indicated.
e.

b

-5-

1. K. H. Becker and D. Haaks, Z. Naturforsch. Teil A28, 249 (1973).

2. K. R. German, J. Chem. Phys. 62, 2584 (1975).

3. P. Hogan and D. D. Davis, J. Chem. Phys. 62, 4574 (1975).

4. K. R. German, J. Chem. Phys. 64, 4065 (1976).

5. P. M. Selzer and C. C. Wang, J. Chem. Phys. 71, 3786 (1979).

6. C. Y. Chan, R. J. O'Brien, T. M. Hard, and T. B. Cook, J. Phys. Chem.

87, 4966 (1983).

7. K. H. Welge, S. V. Filseth, and J. Davenport, J. Chem. Phys. 53, 502

(1970), and private communication.