Appl. Radiat. ht. Vol. 41, No. 8, pp. 151-761, 1990 ht. J. Radial. Appl. Instrum. Part A Printed in Great Britain. All rights reserved

08X3-2889/90 $3.00 + 0.00 Copyright 0 1990 Pergamon Press plc

Formation and Quenching of NH

Fragments in Excited States by High

Energy Electron Irradiation of

Helium-Ammonia Gas Mixture

KOJI MATSUDA,’ IWAO FUJITA,’ TOSHIYUKI KIJIMA,’ HIROHISA YAMAMOTO’ and MOTOYOSHI HATADA’

‘Osaka Laboratory for Radiation Chemistry, Japan Atomic Energy Research Institute, 25-l Mii-Minami- machi, Neyagawa, Osaka 572, *Osaka Electra-Communication University, 18-8 Hatsumachi, Neyagawa,

Osaka 572 and ‘Osaka Institute of Technology, 5-16-l Omiya Asahi, Osaka 535, Japan

(Received 9 September 1989; received for publication 30 January 1990)

A gas mixture of helium and ammonia was irradiated with an electron beam from a Van de Graaff accelerator and emission intensities from excited NH fragments were measured as a function of pressure, concentration of ammonia, and the amounts of additives. The energy transfer process is discussed on the basis of the experimental results and kinetic analysis.

Introduction

Emissions from the excited states of NH (excited by various energetic sources) have been the subject of many experimental studies, and data on quenching rate and lifetime have accumulated in relation to the photochemical and discharge chemistry of the system containing ammonia, either under very diluted condi- tions in space, or under higher pressure in synthetic chemistry (Clerc et al., 1971; Someda et al., 1988; Tokue and Iwai, 1980; Fujita, 1983).

In the radiation chemistry of binary gas mixture systems, it is essentially important to know the energy transfer of absorbed radiation energy from one com- ponent to another. This paper describes some exper- imental results on the change of intensity from a gas mixture of He-ammonia generated by 0.6 MeV elec- tron beam irradiation, as a function of pressure and composition of the mixture, and with the presence of additives. The results are discussed in relation to the energy transfer between the species in the system. The rotational band structure of the emission spectrum is compared with those obtained in previous studies, where experiments were carried out at extremely low pressures in the Ar-ammonia system, using 50 eV electron bombardment; this was because a knowledge of the population among the rotational levels is important in order to understand the thermalization process of radiation-induced energetic species.

Experimental

Materials

All gases were obtained from Seitetsu Kagaku Industries and used as received. The grades and purities of the gases as induced by the manufacturer are: helium (zero-A, 99.995%), ammonia (seg, 99.995%), nitrogen (zero-A, 99.995%), oxygen (zero- A, 99.8%), methane (pure, 99%), ethylene (pure, 99.9%), and sulfur hexafluoride (pure, 99.5%).

Apparatus

Figure 1 shows the experimental arrangement. The irradiation vessel was evacuated to lo-” torr using a diffusion pump, prior to introducing the gas mixture. Helium gas was introduced into the vessel through a l/8 in. bellows valve to the desired pressure, which was measured by a Bouldon type pressure gauge. Ammonia gas of a known amount was introduced using a gas-tight syringe through a silicon rubber stopper of 5 mm thickness, which was inserted in a 5 mm diameter hole on the side wall of the vessel.

The electron beam from a Van de Graaff acceler- ator (High Voltage Eng. Co., 0.6 MeV, 10 PA, spot beam) penetrated through an aluminium foil window (thickness, 0.1 mm) which was positioned on top of the vessel. Light emitted from the gas came out perpendicularly to the electron beam through a

157

758 KOJI MATSLJDA et al.

To vpc”wn

Electron beam t

Grating

Gas inlet (He)

c- Injection port

[NH, and addttfves)

Collision chamber

Lead shield

Spectrometer

Lead shield

Photomultiplier

Fig. I. Experimental apparatus.

quartz window on the side wall of the vessel, and then focused on the slit of a monochromator Jasco CT- 5OC with a mesh (1200 lines/mm) blazed at 300 nm.

The dispersed light from the monochromator was amplified by an EMI 6256B photomultiplier, which was shielded from the X-ray produced by the electron beam in a lead tubing. A wall of 80 mm thickness was necessary to decrease the noise level caused by radia- tion, by 20 dB. The signal-to-noise (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 simulta- neous acquisition of the data.

Results and Discussion

Figures 2(a) and (b) show the emission spectra obtained by the irradiation of a He-ammonia mix- ture at a partial pressure of ammonia of 0.057 and 0.95 torr, respectively. while the pressure of helium was constant at 770 torr. Two emission bands, one due to the ‘II-n’A transition of NH radicals in the singlet state and the other A’lI-X3Zm transition of NH radicals in the triplet state, were observed in both spectra (these bands are abbreviated as NH(c -a) and NH(A -X), respectively, and the fragments in these states are indicated by NH(c). NH(a), NH(A),

and NH(X), respectively). It was observed that the intensities of both bands decreased, especially when the ammonia partial pressure was increased, where- upon the intensity of the singlet bands decreased to a greater extent than that of the triplet bands. The pressure dependence of these bands is more clearly

He : 770 Torr

320 325 330 335 340

Wavelength (nm 1 Fig. 2. Emission spectra of NH(c -a) and NH(A -X) under electron impact; pressure of helium, 770 torr; partial

pressure of ammonia (a) 0.057 torr and (b) 0.95 torr.

Formation and quenching of NH fragments

He : 770 Torr

0 0.1 0.2 1.0 2.0 3.0

Pressure of NH, (Torrl

Fig. 3. Emission intensities of NH(c -a) and NH(A -X) as a function of pressure of ammonia; pressure of helium, 770 torr.

shown in Fig. 3 where the intensities of the bands are plotted as a function of the partial pressure of ammonia. The intensities increased with increasing partial pressure of ammonia, reached maxima at 0.06 torr, and then decreased with pressure monot- onously. The result indicates that both intermediates (singlet and triplet) came from a common source and the singlet species is more readily quenched by colli- sion with the ammonia molecule than is the triplet species.

The emission intensities of both bands are plotted as a function of total pressure in Fig. 4, where the pressure ratio of ammonia to helium is kept constant at 0.000247. The intensities of the both band in- creased with increasing pressure, with a levelhng-off tendency; the curve for the singlet bands levelled-off at a lower pressure than the triplet bands. The results indicate that both species are formed by energy transfer to ammonia from helium, which was primar- ily excited by the radiation, and again, the singlet species is more readily quenched than the triplet one.

The following assumptions are made in order to form a series of reactions which fit the above exper- imental results:

OE 100 200 300 400 500 600 700 600

Pressure of He (Torr 1 Fig. 4. Emission intensities of NH(c -a) and NH(A -X) as

a function of helium pressure; [NH,]/[He] = 0.000247.

759

(1) There are many possible states of excited or ionized helium concerned in the formation of excited NH, but we have not distinguished between them.

(2) Deactivation of the excited helium atom may occur either bimolecularly or unimolecularly, although only the former is considered to occur. However, this does not affect the conclusion.

(3) The NH radical excited to c state deactivates to A state by collision, which is induced by inter- system crossing as reported by Rohrer and Stuhl, (1987).

(4) The quenching rate of NH* by the helium atom is small compared with that by NH,, therefore, only quenching by the ammonia molecule is considered in the scheme.

The reaction scheme is shown in Table 1. At the steady state condition, the concentration of NH(c) is given by

W(c)1 = B[He] . ‘k,[NH,]

{(% + 'kdPW1 + MHelI (1)

x {(‘h + ‘kQ)[NH,l + ‘k,)

where B is a constant. A similar equation is obtained for the steady state concentration of NH(A):

BWI [NH(A)1 = (‘k, + ‘k,)[NH,] + k,,[He]

‘k, + ‘k, [NHJ*

(‘k, + ‘k,)[NH,I + ‘k,

1

’ ‘k,[NH,] + ‘k,

At higher concentrations of ammonia, equation (1) is approximately expressed by

[NH(c)]-’ = const x [NH,]. (1’)

Reciprocals of the emission intensities of the NH(c-a) and NH(A-X) bands are plotted as a function of concentration of ammonia in Fig. 5,

760 KOJI MATSU~IA et ul.

Table I. Reactmn scheme

Rate Type of Number Reactant ProdUCt COnslant reaction

I He+e He’ B Energy absorptmn 2 He* + He He ~+ He k,>, Deactivation 3 He* + NH, NH(<,) + HI or 2H ‘k,, Energy transfer and dissociation

NH(A) + H, or ?H %,, 4 NH(c) + NH, NH(4)+NH, ‘k,, Intersystem crossing

NH(X) + NH, I%, Quenching NH(d) + NH,

5 NH(c) NH(U) $ /II> ILK Rad~atmn emission 6 NH(A) + NH, NH(,Y) + NH, k.l Quenching 7 NH(A) NH(L’) + ho ik, Radiation emission 8 He* + additive He + additive DeXtiWtiOIl

‘Excited state.

where both plots lie on straight lines in the higher ammonia concentration region above 0.8 torr.

In the case where total gas pressure was varied while the ratio of the concentration of ammonia to that of helium was kept constant at 0.000247. one obtains the following equation by substituting [NH,]/[He] in equation (I) with a constant:

[NH(c)] -’ = const x i

‘k, (‘k, + ‘k,) + [NH,I 1 (3)

The plot of the experimental data according to equation (3) is shown in Fig. 6, where the intercept gives the ratio (‘k, + ‘ko)/‘kR = 9.3 torr I. By use of the data, ‘k, = ‘7~’ = (440ns) ’ = 2.3 x 10hsm~’ (Fujita, 1983), (‘k, + ‘k,) is found to be 2.1 x IO’ torr-~‘s- ’ = 6.6 x IO-‘“cm’ molecule ’ SK’, which is in reasonably good agreement with the literature values (9.5 x IO ‘” cm3 molecule -‘s ‘, Clerc et (I/., 1971; 4.4 x IO -‘” cm3 molecule ’ s ‘, Haak et cd..

1984). Under low pressures. NH(c) is reported to be

observed in highly rotationally excited levels, with rotational temperatures of 1500. 1690, and 3200K, depending on the method of excitation and gas pressure (Someda et al., 1988; Sekiya e’t al., 1987; Tokue and Iwai, 1980). Figure 7 shows the emission spectra obtained at different gas pressures with the simulated spectrum, assuming the Boltzmann distri- bution at 300K. The rotational distribution of the NH(c) bands at atmospheric pressure agrees well with the simulated spectrum. This indicates that most of the NH radicals existing in a stationary state

He : 770 Torr

Pressure of NH, (Tom)

Fig. 5. Reciprocais of emission intensities of NH((, -a) and NH(A -A’) as a function of NH, pressure.

during electron beam irradiation have been com- pletely relaxed to room temperature, by collisional relaxation with helium atoms.

The strong emission of the Q-branch of the NH(A ) bands did not allow us to simulate the spectrum and thus to estimate a rotational temperature for this state.

As shown in Fig. 7, a decrease of the gas pressure resulted a decrease of the emission intensities of both bands, and the rotational distribution of the NH(c) radical seems to change to the one that populated the higher levels, indicating a decreasing nrimber of colli- sional relaxations of rotational levels. On the other hand, the spectrum of the NH(A) bands was slightly changed by a decrease in total pressure. This suggests that the rotational relaxation of NH(A) by collision with the helium atom is less favourable than that of NH(c).

In Fig. 8, the emission intensity of NH(c -(I) is plotted as a function of partial pressure of different additives. The emission intensity decreased linearly with increasing amounts of nitrogen, while for methane, oxygen, and ethylene, a large decrease of the emission intensity was observed in the region of low partial pressure of additives, although the de- crease became smaller as the pressure of additives increased. When sulfur hexafluoride was added to the system, a more complex sigmoid change was ob- served. Similar changes of emission intensities were observed for the NH(A -X) band system as shown in Fig. 9. No plot was given for the case where nitrogen was added in the system, because the

. /‘../

/” .A’

i , I 1 J

:: 0 2 4 e CI

CNHJ’ (Tow-‘)

Fig. 6. Reciprocal of emission intensity of NH(c -a) as a function of the reciprocal of [NH,].

Formation and quenching of NH fragments 761

2 5 NH(c-a) d b

NH(A-Xl

(a)

ri (b)

I

320

I I

330 340

Wavelength ( nm 1

Fig. 7. Simulated spectrum of (a) NH(c -a), observed spectra obtained at different total pressures; (b) 770 torr and

(c) 160 torr; [NH,]/[He] = 0.000247.

2 He : 770 T0rr

NH3 : 0.19 Torr

0 0.1 0.2 0.3 0.4 0.5 0.6

Partial pressure at additive (Torr 1

Fig. 8. Emission intensity of NH(c -a) as a function of partial pressure of additive; pressure of helium, 770 torr;

pressure of ammonia, 0.19 torr.

, 2 He : 770 Torr

NH3 : 0.19 Torr

0 0.1 0.2 0.3 0.4 0.5 0.6

Partial pressure of additive (Torr 1

Fig. 9. Emission intensity of NH(A -X) as a function of partial pressure of additive: pressure of helium, 770 torr,

pressure of ammonia, 0.19 torr.

measurement of emission intensity of the NH(A -X) bands was difficult due to overlapping of very strong N2 (0,O) emission. The result that both NH(A -X) and NH(c -a) emission intensities show similar de- pendence on the additives may indicate that the additives compete with ammonia for a common precursor, possibly excited helium atoms (reaction 8 in Table 1).

Quenching of NH(c) and NH(A) by different gases is reported by three authors (Kawasaki et al., 1973; Sasaki et al., 1986; Hofzumahaus and Stuhl, 1985) and quenching rate constant of NH(c) with N, is reported to be small (Kawasaki et al., 1973) thus supporting the importance of reaction (8) mentioned above.

References

Clerc M., Scmidt M., Hagege-Temman J. and Belloni J. (1971) J. Chem. Phys. 75, 2908.

Fujita I. (1983) Z. Phys. Chem. 136, 187. Haak H. K. and Stuhl F. (1984) J. Phys. Chem. 88, 2201. Hofzumahaus A. and Stuhl F. (1985) J. Chem. Phys. 82,

3152. Kawasaki M., Hirata Y. and Tanaka I. (1973) J. Phys.

Chem. 59, 648. Rohrer F. and Stuhl F. (1987) J. Chem. Phys. 86, 226. Sasaki S., Tsunashima S. and Sato S. (1986) Bull. Chem.

Sot. Japan 59, 1671. Sekiya H., Nishiyama N., Tsuji M. and Nishimura Y. (1987)

J. Chem. Phys. 86, 163. Someda K., Kondow T., and Kuchitsu K. J. (1988) Phys.

Chem. 92, 368. Tokue 1. and Iwai M. (1980) Chem. Phys. 52, 47.