Experimental results for relative vibrational band transition probabilities for v prime = 0 and 1, and v double prime = 0 to 4 in the A-X electronic system of OH are presented. The measurements, part of a larger set involving v prime = 0 to 4 and v double prime = 0 to 6, were made using spectrally dispersed laser-induced fluorescence (LIF) in the burnt gases of a flame. These Einstein coefficients will be useful in dynamics experiments for quantitative LIF determinations of OH radical concentrations in high v double prime

Copeland, Richard A.

Crosley, David R.

Jeffries, Jay B.

English

NASA Technical Reports Server

TRANSITION PROBABILITIES IN OH A2 Z+ - X2
BANDS WITH v1 = 0 AND 1, v" = 0 TO 4
Richard A. Copeland, Jay B. Jeffries and David R. Crosley
Molecular Physics Department
SRI International
Menlo Park, California 94025
ABSTRACT
Experimental results for relative vibrational band transition
probabilities for v' - 0 and 1, and v" = 0 to 4 in the A-X electronic system
of OH are presented. The measurements, part of a larger set involving v' = 0
to 4 and v" =0 to 6, were made using spectrally dispersed laser-induced
fluorescence (LIF) in the burnt gases of a flame. These Einstein coefficients
will be useful in dynamics experiments for quantitative LIF determinations of
OH radical concentrations in high v".
C' (MSA-CE-182946) 1RAKS1T10K I ICEf lEILITIBS N88-25353
IK OH A 2 SIGKA + - X 2 PI i: EA.KDS WITH. '"V
PEIHE = 0 AND 1, V DOUE1E FE1LE = 0 10 a
(SBI In te rna t ic ta l C Q t p . ) 17 F CSCL 20L Unclas
G3/76 0146677
MP 86-234
11-4-86
https://ntrs.nasa.gov/search.jsp?R=19880015969 2020-03-20T06:45:11+00:00Z
LIF Determinations and Transition Probabilities for OH
The OH radical has been the subject of numerous studies, in part due to
its important role in atmospheric and combustion chemistry, and in part due to
its amenability to state-specific energy transfer and reaction studies.
Laser-induced fluorescence (LIF) forms an especially sensitive means of
detecting OH, and is extensively used for field measurements in the tropo-
sphere and stratosphere and for laboratory determinations in flames and in
kinetics and dynamics experiments. For most such measurements, excitation and
observation of the lower levels by means of the strongest bands — viz.,
(v1,v") » (1,0), (0,0) and (1,1) — has proven adequate.
More recently however, there has been increasing interest in the
detection of OH in higher vibrational levels, most of which require LIF
detection via weaker off-diagonal bands. Such experiments have included
vibrational distributions from both nascent reaction products [1] and
following collisional quenching of the electronically excited A^E4" state [2],
and vibrational and rotational energy transfer in the ground state [3]. A
two-laser scheme for tropospheric monitoring of OH also involves LIF of .
vibrationally excited radicals [4] • In all these experiments, quantitative
determinations require well established transition probabilities for the
vibrational band(s) used in excitation and observation. Absolute values can
be obtained from relative intensity ratios for bands with a given v', combined
with an overall theoretical framework and absolute radiative lifetimes for
each v1.
A series of papers [5], describing experimental measurements, theoretical
calculations and a critical review of previous work, dealt with the stronger
bands having Av = v'-v" • 1, 0 and -1. Relative transition probabilities for
those bands were measured and shown to be well described by an electronic
transition moment, Re(r), which varies linearly with internuclear distance, a
fora first proposed many years earlier [6]. For bands with Av = -2 and -3,
the intensity of only the (2,4) band has been measured; and none of these
bands can be adequately described by means of a linear form of Rg(r).
Although ab initio computations of Rg(r) have been performed [7], there is too
much uncertainty in the calculated values for the weak bands with large r-cen-
troids, and new experimental data are needed to provide a consistent picture.
We have made measurements of the intensities of 26 different vibrational
bands, including several not previously observed, in the A Z+ - X2!!^  system of
OH. The experimental method is spectrally dispersed LIF, using OH in the
burnt gases of a flame. The bands Include v' ranging from 0 to 4, and v" as
high as 6. Many of the bands are extremely weak in fluorescence, due in part
to their low inherent transition probability and in part to the predissocia-
tion of levels with v1 > 2. Nonetheless, with the use of strong lasers which
can approach optical saturation even for weak transtions, it will be possible
to use these weak bands as sensitive monitors of ground state OH populations
in high v". We have, for example, detected OH in v" = 4 via excitation in the
(1,4) band [2].
Many of the bands with high v' and v" have r-centroids beyond the
distance (~1.4 A) where the linear moment becomes more and more negative,
losing its meaning and usefulness. Consequently, they form a stringent test
of the ab initio transition moment, and hence the ab initio wavefunctions, at
large values of internuclear distance. We are currently proceeding with the
data analysis of the bands with v1 =2 to 4, together with fits of various
Re(r) in the same general functional form as those from the ab initio
calculations. The full set of results and comparisons with theory will be
published later [8]. Because of the desirability of measuring OH in v" as
high as 4, we report here the experimental results for bands having v' = 0 and
1, in advance of that full analysis.
Experimental Details and Data Analysis
Most of the measurements are made in a flame, although one set is made in
a low pressure flow system. We describe the flame experiments first. Methane
and oxygen are burned at atmospheric pressure on a small laboratory burner; in
the burnt gases there exist copious quantities (> 10 " cm"-*) of OH radicals.
The OH radical is excited using a frequency-doubled, excimer-pumped dye laser
having 0.6 cm"1 bandwidth (about twice the OH Doppler width in the flame) and
operated at typically 5 to 30yJ per pulse (levels where the fluorescence
intensity was linear in laser power). The fluorescence is collected at right
angles with either a one or two lens system, focussed onto the slit of a 0.35 m
monochromator, and detected with a photomultiplier. In many cases, when
measuring weak bands with high v", a WG335 Schott glass filter is used to
block the strong background signals due to the (1,1) and/or (0,0) bands. The
photomultiplier output is amplified and is measured by a boxcar integrator
controlled by a laboratory computer; the resulting signals are normalized to a
simultaneous measurement of laser intensity. The v' » 1 data are taken
exciting in the (1,1) band either the P^ (6) or Q^ (4) transitions; for the
v' = 0 measurements several different rotational transitions are excited in
the (0,0) band.
Spectrometer scans for the weaker bands, Av < -1, are shown in fig. 1.
As plotted, these are all normalized to laser power and detection system
response. The ratio of Einstein emission coefficients AQ2/AQ1 for the (0,2)
and (0,1) bands is determined with narrower slits than the ratio for (0,3) and
the limit for (0,4); both scans are shown in the upper panel.
The lower part of fig. 1 shows scans for bands with v* = 1. In addition
to emission from this directly excited vibrational level, emission also occurs
from nearby bands having v' =0, a level populated by vibrational energy
transfer in the flame [9]. Some of this emission underlies the nominal v1 = 1
bands and must be accounted for in the data analysis. It is done so in the
following manner. Higher resolution fluorescence scans are made of the (0,0)
and (1,1) region following excitation in v' =1 (see fig. 2). Here, the
intensity of (0,0) and (1,1) is first obtained by taking the areas to each
side of the R^ head marked in fig. 2. The contribution of (0,0) in the region
of (1,1) is estimated by a linear extrapolation of the tail of the P and Q
branches of (0,0) at the R^ head; this is subtracted from the (1,1) intensity
and added to the (0,0) intensity. The average intensity ratio Igo/1!! is
0.55. (Note that this intensity ratio corresponds to a population ratio
NQ/N! = 0.32, lower than the value I^/NQ = 0.6 found in ref. [9]. This
apparent inconsistency is caused by a different flame composition; the
present flame contains no nitrogen, whereas that in ref. [9] was an air flame
containing N2 which causes vibrational transfer but quenches poorly.) We then
use ratios AQ^/AQQ = 0.0045 and &Q2/^ Ql a 0*026 to compute the v1 = 0
contributions to the v' » 1 emission in the other bands. The results are that
in Av = 0, 35% of the emission is from vf - 0; in Av = -1 it is 18% and in
Av » -2 it is 5%. There is no observable contribution in the case of the
(1,4) band.
The spectral response of the entire fluorescence detection system
(including filters where appropriate) is calibrated using both tungsten and
deuterium lamps. Such a calibration, covering a large wavelength region and
thus a large range of lamp output, must be done with care; the procedure will
be described in detail in ref. [8]. We note in particular that measurements
of the ratio A.^/AJQ in the flow system (see below) are the same for either
first or second order monochromator repsonse.
Tables 1 and 2 list the Einstein coefficient ratios determined from these
scans. The quoted value is a weighted average over the number of runs
indicated; the weights are the signal to noise ratio of the weaker band in
each case. The 2-a level errors listed are calcuated from the statistical
deviations in the averaging. They include no systematic uncertainties, but
we estimate those to be negligible in comparison with the statistical
component.
In the flame we measure a ratio AJ^/A^Q significantly higher (~2) than
the flow system measurement reported in ref. [5a] or considered [5] the best
theoretical value. Because of this large discrepancy and the crucial nature
of this ratio for many OH determinations, it was remeasured in a low pressure
discharge flow system where there are considerably fewer OH radicals. Here, a
discharge in ^ 0 vapor at about 50 mTorr serves as the OH source. Experiments
are performed detecting the fluorescence in both the first and second order
response of the monochromator; for second order a UG-5 filter (Schott) is
used. This determination results in A^ /A^ j « 1.64 ± 0.15 (the value given in
table 2), in excellent agreement with the result of ref. [5a].
The original flame measurements for this ratio are in error due to the
optical depth for OH in the v" = 0 level in the flame gases. At the point of
observation in the flame, fluorescence back to this level is absorbed whereas
fluorescence to higher v" undergoes negligible absorption. This effect was
confirmed by a subsequent series of relative measurements as a function of
distance into the flame. Consequently, the ratio AQ^/AQ measured in the
flame is also too high. The present results for that ratio vary between 0.004
and 0.0065, consistent with the value determined in ref. [5a].
Error bounds on the upper limits for the (0,4) and (1,5) bands are
difficult to quantify. Our estimate of noise excursions is at about the 2-a
level; the lack of discernible features in the appropriate wavelength region
can be seen in fig. 1.
Discussion
For only two of the ratios measured here are there previously determined
values. Our result for A^/A^g agrees well with both the experimental value
1.58 ± 0.16 (2-a) and the best [5] theoretical value of 1.75 using a linear
Re(r). That for A12/AH *8 tlie same as determined in ref. [5a]
(0.0104 ± 0.0004, 2-a). However, the error bars for this ratio in ref. [5a]
were determined from only two runs, and are unrealistically low; the present
25% error in table 2 is a better estimate of the real uncertainty in this
ratio. The ratio A^/Ago = 0.0037 t 0.0008 (2-a) from ref. [5a] is preferable
to the current range of measured values due to the optical depth problems in
the flame.
Because of the centrifugal distortion in OH, both the vibrational overlap
and r-centroid for a given band vary with rotational level N'; thus the
transition probabilities depend upon N1. This variation is in general
different from band to band, and is quite marked for the weaker bands [5b].
In most of the present experiments, the OH is excited near the maxiumum of the
rotational distribution at the flame temperatures; although rotational energy
transfer in the flame is not fully complete, the intensity ratios should be
quite close to those for a thermal distribution peaking near N'=5. In the
case of v1 - 0, several different N' levels are pumped and no variation is
observed. The anticipated [5] difference compared with the rotationless case
is about 2%. Quantitative, N'-dependent transition probabilities are best
determined from calcuatlon [5c], using an Rg(r) established from the
vibrational band ratios; this procedure will be discussed for these weaker
bands in ref. [8]. In the meantime, we will ignore the rotational effects,
which are smaller than the experimental errors.
For comparison of populations determined by LIF using bands with dif-
ferent upper state vibrational levels v', we must convert the transition
probability ratios to a series of Einstein coefficients. This can be done
readily for the levels v' = 0 and 1, using the measured absolute radiative
lifetimes (for v' > 2, the levels are predissociated and the observed lifetime
is not entirely radiative). We use the lifetime values determined by German
[10] using LIF at very low total pressure, where collislonal effects are
absent. His value tv<aQ agrees excellently with a later determination under
beam conditions [11], and more recent single-level excited LIF measurements
[12] are also consistent with that result. German found T^ to be 6% longer,
in general accord with theoretical [5] expectations. We use the values for
the rotationless, N' - 0, levels: TQ =• 693 ± 20 ns, t^ - 736 ± 22 ns, with
2-a error bars [10].
The results of this computation are given in table 3. The quoted
uncertainties for the weaker bands arise only from the statistical
uncertainties in the Einstein coefficient ratios given in tables 1 and 2;
when more than one ratio enters into the computation the errors are added in
quadrature. The error for the stronger bands — (0,0), (1,0) and (1,1) —
also includes the 3% uncertainty in the lifetimes.
This set of experimental results thus provides the transition proba-
o
bilities needed for quantitative detection of OH in X^n^ ground state
vibrational levels as high as v" =4. Although some of the bands are very
weak, as can be seen from table 3, the use of powerful lasers can saturate the
transitions and yield strong signal levels per unit concentration in these
quantum states. In a similar way, the results for v1 =2, 3 and 4 [8] will
permit measurements as high as v" = 6. The OH molecule, of considerable
practical importance and the subject of many interesting chemical studies, can
now be detected quantitatively in high vibrational levels of the electronic
ground state.
Acknowledgements
We thank Michael L. Wise for assistance with the later set of flame
measurements. This research was supported by the National Aeronautics and
Space Administration, on Contract NAS1-16956. The computer used for data
analysis was furnished to the Molecular Physics Department by the National
Science Foundation.
REFERENCES
1. (a) G. M. Jurisch, M. Trolier, I. A. Watson and J. A. Wiesenfeld, Bull.
Amer. Phys. Soc. 30, 1852 (1985); (b) G. M. Jursich and J. A. Wiesenfeld,
Chem. Phys. Lett. 119, 511 (1985).
2. R. A. Copeland, M. J. Dyer and D. R. Crosley, unpublished data.
3. R. A. Copeland and D. R. Crosley, J. Chem. Phys. 84, 6400 (1984).
4. J. D. Bradshaw, M. 0. Rodgers and D. D. Davis, Appl. Opt. 24, 2134 (1984),
5. (a) D. R. Crosley and R. K. Lengel, J. Quant. Spectros. Radiat. Transfer
15, 579 (1975); (b) D. R. Crosley and R. K. Lengel, J. Quant. Spectros.
Radiat. Transfer V7_, 59 (1977); (c) I. L. Chidsey and D. R. Crosley, J.
Quant. Spectros. Radiat. Transfer^, 187 (1980).
6. K. E. Shuler, J. Chem. Phys. 18. 1221 (1950).
7. (a) S. Chu, M. Yoshimine and B. Liu, J. Chem. Phys. 61, 5389 (1974); (b)
S. R. Langhoff, E. F. van Dlshoeck, R. Wetmore and A. Dalgarno, J. Chem.
Phys. Tl_, 1379 (1982).
8. D. R. Crosley, R. A. Copeland and J. B. Jeffries, to be published.
9. G. P. Smith and D. R. Crosley, Appl. Opt. 22, 1428 (1983).
10. K. R. German, J. Chem. Phys. 63, 5252 (1975).
11. W. L. Dimpfl and J. L. Kinsey, J. Quant. Spectres. Radiat. Transfer 21,
233 (1979).
12. (a) I. S. McDermid and J. B. Laudenslager, J. Chem. Phys. 76, 1824 (1982);
(b) R. A. Copeland, M. J. Dyer and D. R. Crosley, J. Chem. Phys. 82, 4022
(1985).
10
Table 1: Measured Einstein Coefficient Ratios for v' = 0
Ratio
AOI/AOO
AQ2/A01
AQ3/AQ2
AOS/AOI
W*03
WA
°
2
Number of Runs
-
22
12
6
4
4
Result (2o errors)
0.0037 ± 0.00083
0.026 ± 0.006
0.059 ± 0.037
0.0018 ± 0.0010
<0.4
<0.02
afrom ref. [5a]; see text.
11
Table 2: Measured Einstein Coefficent Ratios For v'
Ratio Number of Runs Result (2u errors)
AU/A1Q
A12/AU
A13/A12
A14/A12
A14/A13
A15/A14
25
8
14
8
8
2
1.64 ± 0.15a
0.0104 ± 0.0026
0.100 i 0.017
0.0087 ± 0.0016
0.086 ± 0.015
<0.2
aflow cell result
12
Table 3: Absolute Einstein Coefficients, sec" (2a errors in parentheses)
AQQ 1.44 (0.04) X 106
5.3 (1.1) x 103
1.4 (0.4) x 102
Ao3 9 (6)
<3
A10 5.11 (0.49) x 105
Au 8.38 (0.81) x 105
A12 8.7 (2.3) x 103
A13 8.7 (2.8) x 102
A14 7.5 (2.5) x 101
A,, <1.5 x 101
13
Figure Captions
Figure 1. Laser-induced fluorescence spectra following excitation of the
v1 = 0 (top) and v* = 1 (bottom) vibrational levels of the
electronic state of OH. The two uppermost traces — that of the
(0,1) band and the (0,2) upper scan — are taken at 2 nm
resolution while all other bands are at 4 nm resolution. The area
of each band corresponds to the relative A-factor for each vibra-
tional transition when the indicated scale factors are taken into
account, except for the v' - 0 contribution to the v1 = 1 bands as
described in the text. The (0,4) and (1,5) bands are not observed
above the noise level. These v' - 0 scans are taken following
(0,0) ?2(5) excitation and the v1 » 1 scans are taken following
excitation of the (1,1) P^ (6) transition.
Figure 2. Fluorescence scan of the (0,0) and (1,1) vibrational bands of OH
following excitation of the (1,1) P^ (6) transition in the burnt
gases of an atmospheric pressure CH^ /C^  flame. The monochromator
slit width is 150 ym which corresponds to a resolution of about
0.5 nm. The spectrum is digitized at 0.06 nm/point, and each
point is the average of 50 laser shots. The large feature labeled
the (1,1) R^ head divides the bands. All features to lower
wavelengths are due to the (0,0) band, while those at larger wave-
lengths are predominantly due to the (1,1) band with a small (0,0)
contribution. Scans such as these are used to determine the amount
of v' =0 emission falling in the region of v' » 1 fluorescence
(see text) .
0, 1
0,2
x1 A.
0,3
x100
1,4
350 400 450
WAVELENGTH (nm)
500
xioo
JA-4653-29
Figure 1
0,0
I
1,1
1.1
I I I 1 I
306 310 314 318
WAVELENGTH (nm)
322
JA-4653-30
Figure 2


Transition probabilities in OH A 2 sigma + - X 2 pi i: Bands with v prime = 0 and 1, v double prime = 0 to 4

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