Quenching and radiation rates of CO (a 3Π)

Volume 9, nun&r 6

CHEMICAL

QUENCHING

AND

PHYSICS

LE’lTERS

RADIATION

RATES

lS June 1971

OF

CO (a

3R)

G. M. LAWRENCE *

Loborafoly forAtmospheric and Space Physics and Joint Instilutc for Laboratory Astrophysics. University of Colorudo, Boulder. Colorado 80302, USA

Received

15 March

1971

A photon-excited afterglow experiment in CO2 and helium yields an average radiative lifctirw of 7.5 t 1 mecc for the CO(n 3n) v’ = 0 level. The quenching rnte in CO2 is 1.2 k 0.2 x 1011 cm3/sec.

1. INTRODUCTION The carbon monoxide a 311- X 1Z Cameron [l] bands [2] are a very strong feature in the dayglow of Mars [3], but are generally weak in laboratory sources because the spin change leads to a long radiative lifetime. As a first step in experiments to determine absolzrte rates of production of the 311 state, its radiative lifetime has been measured with a form of afterglow experjment. Dissociative excitation of CO2 by a 930A photon beam in helium buffer gas was used to produce CO a 3II states without ionization [4] or complications by buffer metastables. This is to be contrasted with say, a discharge afterglow in argon which contains metastable argon, ions and excited states of CO3, etc. In the final section of the paper we discuss the meaning of the average lifetime measured in a buffer gas, and review other investigations of this transition probability. 2. EXPERIMENTAL A repetitive, pulsed discege in pgon provides exciting photons at 930A, 1048A, and other wavelengths with pulse durations of a few microseconds. These photons, dispersed in a 1 meter concave grating monochromator, form a beam which passes through a differentially pumped slit assembly and down the axis of a 16 cm dia ty 25 cm long ‘absorption cell’. The background pressure in the cell was less than 10-5 torr. The absorption cell was pumped only through the optical slit. l

JILA-LASP

Fellow,

lS70-1971.

Detection of Cameron band emission was obtained using an EMR 542-F photomultiplier (‘solar blind CsTe’) with a quartz window and 0.5 cm air path in front of it. The detector views the optical axis at an angle of 55O. The area viewed includes the monochromator exit slit assembly. The peak response of the PMT and window combination occurs near the wavelength of emission associated with transitions from the u’ = 0 Level of the upper state of the Cameron bands. In order to verify the identity of the detected signal, a monochromator scan of the fluorescence was made. With 1048A radiation into CO2, transitions from CO (a 311) v’ = 0 and 1 states were observed with roughly equ+ intensities. Photon absorption in the 930-1048A region is known to lead to strong dissociation [4], and the threshold fo,r photoproduction of CO (a 3lI) tl’ = 0 is at 108OA. The fluorescence output as a function of time was averaged with pulse-counting techniques (20500 countsjsec) in a multiscaler with the sweep triggered by the exciting light pulse. The mean lifetimes of the (single) exponential decays observed ranged from = 100psec at zero helium pressure to = 3 msec at = low4 torr CO2 pressure with 3 torr of helium in the absorption cell. It was possible to measure lifetimes at He pressures up to 20 torr and partial pressures of CO2 under 0.03 torr. Pressures were measured with capacitance and aneroid manometers. The absorption cell was at room temperature. 3. DATA ANALYSIS The decay curves observed at various gas pressures were fitted weli by single exponentials,

e-at,

for times t between 0.5 and 5 msec 575

Volume 9, number G

CHEMICAL PHYSICS LETTERS

after the light pulse. Ninety values of the decay constant a were determined by a weighted least squares fit to decay curves recorded at several pressures. The a values wers then fitted to the following equation which characterizes the effect of radiation, quenching and diffusion out of the field of view of the detector. a = ar + b/J&

+ c PHI + k Nco2

by as much as a factor of five by the pumping action of the helium as it flows through the slit. The Q values were fit to eq. (1) and the paramsters determined. We obtain Qr = 133 X 16 sec’l, c= 14*2torr’l

.

The first term (a+) is the radiative transition probability. The second term represents an effective diffusion where PHe is the helium pressure and b is a constant depending on the geometry and diffusion coefficient. The third term represents the rate of quenching by the helium or by impurities in the helium, with c a constant to be determined. The last term represents quenching of the CO (a 311) states by the CO2 sample gas. Fig. 1 illustrates the variation of zero-C02-pressure decay constant with helium presma.

15 June 1971

b = 299 L 25 torr sec’l, set’ l

and the CO (a 3lI) - CO2 quenching rate coefficient is X = (1.2 f 0.2) X lo-l1

cm3 molecule-l

sec’l

.

If we assume an effective diffusion length of 2 cm, the value of b implies an STP diffusion coefficient of 1.6 cmz/sec. This value is roughly a factor of two larger than the gas-kinetic coefficient for He + CO(X). Given the unknown ‘size’ of the CO(a) state and the uncertainty in the diffusion length, this is sufficiently ciose agreement. The error estimates are standard deviations obtained by means of a conventional least-squares formalism based on the statistical standard deviations in the decay constants and on the estimated uncertainties in the pressure determination. Although each decay curve yielded a values which had uncertainties of about l%, the extrapolations (curvu fitting) have increased the error estimates of the four parameters to lo-15%, including interactions between the parameters in the fitting process. 4. DISCUSSION

Fig. l.‘Decay

constant versus

He pressure.

Points arc

experimental, extrapolated to zero CO;! pressure. De-

scending he is diffusion term, .zscendlng line is helium quenching term, horizontal is radiative term. and

the curve is the sum.

.The problem of determining absolute presaures of CO2 (a 10’3 torr) in the presence of (” 10 torr) helium was by-passed by extrapolating Q to zero values of rfzlative partial pressure of C@ at each He pressure. The decay constants varied from m 2+ to lOor as a function of the CO2 pressure. The relative partial pressure of CO2 was taken to be equal to the pressure of CO2 existing after the He input was turned off. However, to determine the rate of quenching of CO (a 3il) by CO2, true partial pressures were obtained by measurements of UV absorption. The CG2 partial pressure was found to be diminished ., 576

The reciprocal of the radiative decay rate gives tie average radiative lifetime, i- = 7.5 f 1 msec. Table 1 lists this value and compares it with rd?sults of other workers. This lifetime has been measured in the presence of a buffer gas of He which rovides collisions at a rate of the order of 108/sec. Thus the rotational sublevels will be equilibrated much faster than the radiative rate and will thus decay with a common rate. In contrast, the three components (62 = 0,1,2) of a free CO(a 3lT) will exhibit different radiative lifetimes. James [13] has performed a calculation of the CO(a 3lI) lifetimes as a function d D and total angular momentum, J, for the vibrational state v’ = 0. The reciprocal of the average lifetime is the sum of the transition probability over n = 0, I, 2 divided by the degeneracy. This averagc,lifetime is independent of J and is tite lifetime measured in typical buffered gas experiments.

Volume 9, numbor 6

CHEMICAL PHYSICS LETTERS

Table1 Cameron band lifctimcs M%thOd

cient of CO (a %l) in CO2. is k = 1.1 X lO-1l cm3/ sec.

Meyer et al. f73 have measured lifetimes of *Cameron bands’ of GeO, SeS, SnO, and SnS in cryogenic matrices. Their work ineLudes an attempt to extrapolate their findings to obtain CO gas phase Cameron band lifetimes. The result, two orders of magnitude too large, casts doubt on their assumption that the lifetime for the first singlet-to-ground state transition in these molr cules is independent of molecular species.

Average 7(msec)

Ref.

Intensity, es timate 60 (factor f&e

Intensity, estimate

error est.) >+i%O

Matrix Lifetime, TOF

1

Absorption

= 12

Absorption

8.70 * 0.5 a)

l.GZ zt 0.08

Absorption

8.4 L 1")

1.11L 0.15

Emission Theory

4.4 i: I.2 8.75

3.2 +:0.8a) 1.63

7.5 f 1

1.90 f 0.3aj

Lifetime

I.5 Juno 1971

this work

5. CONCLUSION The average Cameron kand Lifetime obtsined in the present experiment (7.5 msec) supports the absorption measurements of NichoUs and Hasson [lo] and Fairbairn [ll], and the theorotical study of James [13]. Some inaccurate values can now be dismissed or corrected.

a) These numhcrs were computed from the gtven data by means of eq. (2).

ACKNOWLEDGEMENTS In table 1, oscillator

for the O-0 lifetimes

strengths

band (fo0) have been related to average using Jsznes’ results

f 133:

f()o = 1.63 x 10-? X 8.75 msecfi-

.

(2)

values for v’ = 1 which are * 1% different than for tr’ = 0. The paper by Fairbairn [Xl] * contains a numerical error which makes the lifetimes of his table 5 a factor of 100 too small. The omission of a Franck-Condon factor makes his reported ‘electronic oscillator strength’ a factor of 280 too large. The foe value listed in table 1 of the present paper in the Fairbairn row has these errors removed. Fairbairn’s calculations of reiative lifetimes for individual rotational levels are in rough agreement with those of James, but are not based on as sophisticated a formalism. Borst and Zipf [S] appear to have obtained a low value for the lifetime. The source of error is not known hut may result from selection cffects in their Auger detector. The method of Slanger and Black fl2] requires an absolute measurement of the intensity of cascades into CO(a %I) and a measurement of decay ‘lifetimes under conditions of high quenching rates. Their value for the quenching coeffi-

This work was supported by NASA Research Grant NGLO6-003-052 and the National Bureau of Standards Visiting Fellow Program. We thank Drs. T. G. Slanger, R. W. Nicholls, D. Rusain, and T. C. James for permission to quote their results prior to publication.

James predicts

* Numerical errors in this paper have been dIscussed with Dr. F&bairn.

REFERENCES (I} W. H. B. Cameron, Phil. Mng. L (L9ZG) 406. [2j P. H. Krupenie, The band spectrum of cabon monoxide, NSRDS-NBS 5 (U.S. Government Printin): OffiCl , Washington, 1966). [3] C. A. Earth, C. W.fiord, J. B. Pearce, K. K. Keiiy,

G. P. Anderson and A.E. Stewart. 5. C?ZOP~YS. _ _ Res..

to be published. (41 R. S. Nakata. K. Watanabe and F. &I,B~~EKuI~~~. Sci. Light 14 (1965) 54. (51 G. E. Hnnache. PhyEi. Rev. 57 (ma) 289. 161 C. Pa~aHoIio~.Ph. D. Thesis. Iiarvard (196%. 27j B. Miyer, J. JLSmithand K. Spitzer. J. Chcrd. Phy8. 53 (1970) 3616. 181%&L, Borst and E. C!.Zlpf, Phys. Rev. A3 (1971)

_.-.

[9J R. J. Doncwauand D. Husufn, Trxns. Faraday Sot. 63 (1967) 28’79. [lo] R. W. Nicholls and V.Hasson. J. Phya. B, to be ill]

published.

A.R. Fairbti, J. Quant. Spcctry. Rzdintive Transfer LO (1970) 1321. [12j ;$Zti;Z7ger rendG. Black, 5. Chem. P&s., to be [13] T. C. Jam;?~, Ames E&search Centor. private

communication.