A&. Rodrat. Isot. Vol. 42, No 12, pp 1223-1229. 1991 ht. J. Radmt. A&. Instrum. Part A Pnnted in Great Bntam All nghts reserved

0883-2889/91 $3.00 + 0.00 Copynght 0 1991 Pergamon Press plc

Formation and Quenching of CO and CO Ion in Excited States by High Energy Electron Irradiation of Helium-Carbon

Monoxide Gas Mixture

KOJI MATSUDA’*, IWAO FUJITA’, TOSHIYUKI KIJIMA3, YOSHIYUKI SATOU’ and MOTOYOSHI HATADA’

‘Osaka Laboratory for Radiation Chemistry, Japan Atomic Energy Research Institute, 25-1 Mit-Minamt- machi, Neyagawa, Osaka 572, ‘Department of Solid State Electronics, Osaka Electra-Communication Universrty, Hatsumachr, Neyagawa, Osaka 572 and )Osaka Institute of Technology, 5-16-I Omiya. Asaht,

Osaka 535, Japan

(Received 4 October 1990; recewed for publicatron 14 March 1991)

Gas mixture of helmm and carbon monoxide was Irradiated with electron beam from a Van de Graaff accelerator and emisston intensities from excited CO and CO ion were measured as a function of pressure, concentration of carbon monoxtde in wide ranges. The results are fitted wtth kinettc analytical formulae derived from sample competition reactions mvolvmg energy transfer process assuming steady state concentrations for intermediates, and reasonable agreements were obtained between the experrmental results and those predicted in the range of reaction conditions studied in this work.

Introduction

The method for precise determination of electron energy absorbed by a component of gas mixture when the mixture is irradiated with electron beam has not been established, because the knowledge of excitation and charge transfer from one component to the other is required for this purpose. The charge transfer has been studied quantitatively and data on this process (known as Jesse effect) have been accumulated. En- ergy transfer from excited helium has attracted inter- est, since it has a number of applications for U.V. lasers, and extensive lifetime studies of excited rare gas atoms in various gases were reported (Parr et al., 1982; Koizumi et al., 1986; Ukai et al., 1988). These studies have been carried out using pulse radiolysis technique, but it seems important to observe overall reactions using continuous steady beam technique. In the radiation chemistry of binary gas mixture sys- tems, it is essentially important to know the energy transfer of absorbed radiation energy in one com- ponent to the other. It is our interest to obtain emission intensities from excited CO molecule or ion excited either directly or through energy transfer under a wide range of reaction conditions under continuous electron beam irradiation, and to see whether the change of emission intensities with exper-

*Deceased 23 June 1991.

imental conditions from simple steady state kinetics involving energy transfer process between helium and carbon monoxide using kinetic data compiled in the past studies.

This paper describes some experimental results on change of intensity from gas mixture of helium- carbon monoxide generated by 0.6 MeV electron beam irradiation as a function of pressure and com- position of the mixture, and the results are discussed in relation to the energy transfer between the species m the system.

Materials

Experimental

All gases were obtained from Seitetsu Kagaku Ind. and used as received. The grades and purities of the gases as indicated by the manufacturer are. helium (ZERO-A, 99.995%), carbon monoxide (SEG, 99.995%).

Apparatus

The experimental apparatus was essentially the same as that used in a previous report (Matsuda et al., 1990), and set the entire optical equipment on a movable bench to keep the optical path at an opti- mum condition under an irradiation window of an electron accelerator. Figure 1 shows the experimental set-up. The irradiation vessel (V) was evacuated to 10m6 torr using a diffusion pump prior to introducing

1223

1224 KOJI MATSWA et ul.

the gas mixture. Helium gas was introduced into the vessel through a I/S inch bellows valve (BV) to a desired pressure. The pressure was measured by a Bouldon type pressure gauge (BG). When a small amount of CO is mtxed with He, carbon monoxide gas of a known amount was introduced using a gas tight syringe through a silicon rubber stopper (RS) of 5 mm thick which was fixed in a 5 mm diameter hole on the side wall of the vessel.

The electron beam (EB) from a Van de Graaff accelerator (High Voltage Eng. Co.. 0 6 MeV, 10 PA, spot) penetrated through an alumimum foil wmdow (AW) (thickness, 0.1 mm) which was equipped on top of the vessel. Light emitted from the gas came out perpendicularly to the electron beam through a quartz window (QW) which was equipped on the side wall of the vessel and then focused on the slit of a monochromator (M) JASCO CT-5OC with a grating (1200 lines/mm) blazed at 300 nm.

The dispersed light from the monochromator was amplified by an EMI 6256B photomulttplier (PM) which was shtelded m a lead tubing (LT) from x-ray produced by electron beam. Wall of 80 mm thickness was necessary to decrease the noise level caused by radiation by 20 db. The S/N ratio of the emission light signal was maximum at 0.6 MeV.

Two microcomputers were used for driving a mo- tor to scan the spectral wavelength and for srmul- taneous acqutsttion of the data

~_----- ______i lm

RS \

LT I BC I ^

Results and Discussion

Figure 2 shows two typical emission spectra ob- tained by the irradiation of helium-carbon monoxide mixture: one at low CO pressure (a) and the other at high CO pressure (b). In the spectrum (a) obtained at (He, 100 torr and CO: 10 torr), five emission bands observed and identified m this spectrum are (Pearse and Gaydon, 1976): one due to CO(A’ U-X’ I: + ) (fourth positive system), CO(B’Z-A’I7) (Angstrom System), CO(C’Z-A’I7) (Herzberg system), CO(b3Zpa317) (third positive system). and CO + (B%X*C) (first negative system). A weak band system of CO + (B’Z-A’I7) (Baldet-Johnson system) IS also found in the spectrum. Spectrum (b) shows the emission spectrum observed for the helium-carbon monoxrde gas mixture (He: 100 torr and CO: 100 torr). The comparison of the two spectra revealed that the intensities of all emission bands become weak when the partial pressure of CO increased but the intensity of the emission bands from CO ’ decreased more among other bands due to excited neutral species. This indicates that quenching cross section of ionic species is larger than that of neutral exerted species.

Intensities of these emission band systems are plotted as a function of partial pressure of carbon monoxide while that of helium was kept constant at 100 torr in Fig. 3. where a sum of intensities of all

PM

Fig 1. Experrmental apparatus: AW, alumimum lrradlatlon window; BG, Bouldon type pressure gage, BV, bellows valve; EB, electron beam; LT, lead tubing for radiation shield, M, monochromator: PM,

photomultlplier. QW, quartz window; RS, silicon rubber stopper. V. reaction vessel

CO+ (B-X)

I I II IllIll

O(A-Xl

ml

FormatIon and quenching of CO and CO ion 1225

CO (B-AI

I I I I 1

co lb-01 COIC-Al

--

(a)

(b)

) I

300

I 400

Wavelength (m-n)

I 500

Fig. 2 Emlsslon spectra of He<0 mixture (a) He: 100 torr, CO: 10 torr; (b) He: 100 torr, CO: 100 torr.

lines which belong to the band system is plotted as the emission intensity of the band system. With increas- ing the pressure of carbon monoxide, the emission intensities of CO(A-X), CO(B-A) and CO(C-A) increased and then reached a constant value above 10 torr, whereas the intensities of CO+(B-X) and CO@-a) increased to maximum values at 1 and 5 torr, respectively, and then decreased monotonously with increasing carbon monoxide pressure.

In Fig. 4, emission Intensities of the band systems are plotted as a function of gas pressure, where the

ratio of CO to helium was kept constant at 1: 1. The intensities of all bands increased with increasing gas pressure and were constant and independent of gas pressure above 40 torr.

The figures both show that the emission intensities from neutral and ionic excited carbon monoxide approach constant values with increasing partial pressure of carbon monoxide up to a sufficiently high pressure. This indicates that more excited species, either ionic or neutral, formed with increasing carbon monoxide pressure, but simultaneously, more

PHe= 100 Torr

CO (b-at .

.

I I

40 60

Partlo\ pressure of CO ITorr)

Fig. 3. Emission intensities of five bands of CO and CO+ as a function of partial pressure of CO. Partial pressure of He = 100 torr.

I226 K~JI MATSUDA CI al

PHB Pco =1 1

r=;y

0 .

CO (6-A) x 0

COIC-Al x

40 60

Partial pressure of CO (Tom)

FIN 4 Emwon mtensltws of CO and CO ’ In the equal partial pressures of He and CO

quenching of exctted spectes due to colhsmn wtth carbon monoxide molecules may occur The reason that the large maxtmum observed for CO’(B-X) at I torr m Ftg 3. may be that more exctted CO ’ are formed by Penning iomzation due to colhson with exctted helium atoms than by direct excitation due to collision with electrons The small maxtmum ob- served for the emisston from CO(b~-a) may be quah- tatively explained by the fact that the formatton of CO(b) state due to collision with high speed electrons IS forbidden, and thus. relattvely high cross section of formation of this state due to collision with exctted helium atoms is expected.

The above discussion leads to the reactton mechan- ism listed m Table I. We made the following two assumptions which arc not constdered m the previous paper (Matsuda rr (11.. 1990) where cmtsston mtenst- ties from ammonta-hehum systems by electron beam trradtation:

(1) dn-ect excnatton of carbon monoxtde by col- hston wtth electrons IS constdered. smce emts- sion was observed for the mtxture contammg carbon monoxtde at htgh concentration above 50%. This process was not constdered m ammoma~hehum system because emtsston of NH(c’I7) was observed only at very low concentratton of ammoma. suggestmg that most of the exctted spectes are produced by

colhston of ammoma wtth exctted hehum atoms;

(2) no cascade, e g CO (B -+ A +X). was as- sumed to occur In thts system as a first approximatton, because too many rate con- stants are requtred for complete analysts Thus. kinettc analyses were made mdepen- dently for each hght-emittmg species.

A steady state approxtmatton on mtcrmedtates leads to the followmg equatton:

[co*] = i

k, &, WI k&He] + k,,[CO] “co I WI

k,[CO] + li, (1)

where CO* represents one of neutral and tome ex- cited spectes. and B, (). X,,, k, and /i, are rate con- stants of direct excttatton. quenching. radtatmn degradation and energy transfer of respecttvc spectes denoted by subscrtpt. For infimtely large carbon monoxide concentratton m equatton (I ). concen- tratton of excited carbon monoxide becomes constant value as gtven by equatmn (2)

]tm [CO*] = -:2 (2) !LI,I I 0

This explams the cxpertmental result that the emts- ston mtensittes from excited spectes approach con- stant values wtth Increasing carbon monoxtdc pressure

Rdk Tvnc 0i

He+r He’ + He - --

CO He*t(‘O -p-

co* + co co*

He* He + He

CO’ He + co*

co+co CO + hlz

Two extreme cases are discussed in equation (1):

(i) in the case where the second term in the braces can be ignored, the steady state con- centration of [CO*] becomes a maximum at carbon monoxide concentration given by equation (3):

Formation and quenching of CO and CO ion 1227

01 02 03 04 05

Pc$Torr-‘1

Fig. 5 Reciprocal of Intensity vs reciprocal of partial pressure of CO. Partial pressure of He was equal to that of CO

[CO] = J kRkD2[W

k&o (3)

This relation seems to be the case of CO+ (B*Z) where direct excttation by electron is ignored;

(it) in the case where the first term is negligibly smaller compared with the second term, which corresponds to the case where the formation of CO* due to collision with ex- cited helium 1s ignored, the steady state con- centration of CO* is expressed by equation (4) and it increases monotonously to a con- stant value with increasing carbon monoxide concentration.

[co*] = & k, + ka/[COl

Equation (5) can be derived from equation (1) by regardmg the ratio of partial pressure

of carbon monoxide to stant.

[“*I = ko be

k,, + k, r

that of helium be

+ Bco 1

k, + k&O1

con-

(5)

where r = [CO]/[He]. This equatron predicts that the reciprocal of [CO*]

is linearly correlated to that of [CO] under the condition of a constant [CO]/[He]. The plots of the data are shown m Fig. 5, where the pomts lie on linear lines. Since the extrapolation to the intercept with the horizontal axis grves -k,/k,, k, can be determined from the extrapolated value if k, values are known. Table 2 summarizes the k, determined using k, reported elsewhere.

Our quenching cross section (u) for CO (b-a) of 4.6 x lo-” cm* agrees well with that of Twist et al.

(1979) (7.7 x 10m’5). Better agreement with the value reported for CO+(B-X) by Ibuki and Sugita (1983) (5 x 10m15) obtained (5.3 x 10-‘5).

In order to determine relatrve rate constants of the reactions relating to the other excited states usmg these values, rate constants were varied so that the calculated curves may fit the observed points with least errors. In order to examine whether the relative values thus estimated predict the data obtained under other conditions, the emission intensity observed is

Table 2 Quenchmg rate constants and cross sections

CO(A) CO-(B)

CO(b) CO(C) CO(B) Umt

kQ 0 T Reference for 7

58 110 10.8 Field er al. (1983) 28 53 53 8 Arqueros er al (1981) 24 46 52 Twst ef al. (1979)

II0 2200 I4 Chornay er a/ (1984) 4.5 88 25 Radzlg al. el (1985)

IO ‘0molecule~‘cm3s~’ AZ ns

I228 KOJI MATSUVA et al.

CO(b-01

0 50 100

Partlol pressure of CO (Tom)

Fig 6. Observed (O,@) and calculated (-_) mtensity as a function of partial pressure of CO

compared with the value calculated using these rela- tive rate constants as parameters As an example, the intensities for CO(ba) at 30 torr partial pressure of helium were calculated, and they agreed well with experimental results as shown in Fig. 6. The calcu- lation predicts the expertmental results that at He = 100 torr a maximum of emission intensity ap- peared at CO partial pressure of about 5 torr, whereas at He = 30 torr no distinct peak was ob- served. A qualitative explanation for this behaviour 1s that the relative probability of CO(b) state due to collision with He* to that due to direct excitation by collision with electron decreases as the partial press- ure of helium decreases. Thts is simply related with that of the value of the first term in the braces in equation (1) becomes smaller than that of the second term as the partial pressure of helium decreases

Result of another validity test of the reaction scheme can be given m Fig. 7, where the emission intensity of CO(A-X) was plotted as a function of CO partial pressure, while [He]: [CO] ratto was kept constant at 2.1. Again the calculated points agree well with the expertmental data.

In order to overlook the agreements between the observed and calculated values in wide ranges of CO and He pressures, the calculated emission intensity of CO(ba) was drawn as contour diagram on an X-Y plane in Fig. 8, where pressure of CO was taken on the ordinate and pressure of He on the abscissa. Experimental data seem to lie well on the contour lines, confirmmg the validity of the reaction mechan-

CO (A-X I Ptie PC0 = 2 1

I 0 50 100

Partial pressure of CO (Torr)

Fig. 7. Observed (0) and calculated (-) intensity of CO(A-X) as a function of partial pressure of CO

- CO (b-al

Partlal pressure of He (Torr)

Fig. 8. Calculated contour diagram of emlwon mtenslty of CO m the I’,,-P,. plane. The small numbers m the figure

and right hand side denote the relatwe mtensltles

tsm. Stmtlar diagrams were obtained for all other five emission band system, which are qualitatively stmilar to one another, but small differences were observed for the coordinate at which a peak appears and hetght of the peak for different emission band systems. A sharp maximum of emission intensity of CO+ (B-X) appears at low helium pressure as shown in Fig. 3. This corresponds that a high peak exists in the region of left down area m the contour diagram.

In Table 3, energy levels of excited hehum and CO are summarized. The fact that the energy level of CO + (B*C) is close to that of He* may be related to high probability of Penning tomzation of CO due to collision with He’.

The reaction scheme discussed above excludes effect of cascade in which an excited state IS popu- lated by a transition from other excited states which exist above the first state. In thts scheme. reciprocal of emission intensity can be expressed as a linear function of reciprocal of carbon monoxide pressure as shown by equation (5) where He/CO ratio IS kept constant. The fact that the experimental data deviate upward from the linear lines for CO+(B-X). CO(C-A) and CO(B-A) m the high pressure region can be explained by cascade effect. Assuming the

Table 3 Enerev levels of CO md Hz

CO c’x+ B’Z _ b’I+ A’JJ n’JJ X’Z’

He ‘P, ‘p,, I ? ‘5, ‘s,

Formation and quenching of CO and CO ion 1229

Table 4. Reactmn scheme mvolvmg cascade from UDD~I excited state CO’

He+e P He* BHC

He’ + He

tote

He’ + CO

co* + co

co*

co

He’ + CO

cop + co

cop

cop

* He+He

-- co*

* HefCO’

-- co+co

-- CO+hu

-- cop

-- He + CO’

-- co+co

___t CO+hu

-- CO’

reaction scheme shown in Table 4 involving a precur- sor, CO’, from which the cascade to lower CO* occurs, the steady state treatment gives the following equation:

[CO*] = k,[He*] + B,, +

WI ’ k,[CO] + k,

where

4, [HeI [He*] = k,,[He] + (k, + k,P)[COl (7)

The third term in the braces m equation (6) is the

correction term to equation (5) for the contribution of cascade from upper levels.

Adjustment of this correction term so that the

calculated curve fits to the experimental points for CO(ba) indicates that contribution of cascade is

estimated to be 26% of total rate of population of CO(b) state at 20 torr He and CO pressures.

The emission intensities of several band systems observed for heliumsarbon monoxide systems under electron beam irradiation depend on gas pressure and gas composition and the approximate general depen- dence covering wide range of these parameters can be explained by a simple reaction mechanism assuming competition reactions and energy transfer without considering cascade from upper excited states. How- ever, the results indicate the cascade to occur in some extent, and therefore, more precise analysis involving excited states at higher levels will be necessary for more accurate predictions.

References

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