Journal of

MOLECULAR STRUCTURE

ELSEVIER Journal of Molecular Structure 410-411 (1997) 73-76

Time-resolved laser-induced fluorescence spectroscopic studies of Rb(52S 1) with N20 at elevated temperatures

E. Martineza, J. Albaladejoa2*, A. Notarioa, E. JimCneza, D. Husainb

“Departamento de Quimica Fisica, Facultad de Ciencias Quimicas, Universidad de Castilla La Mancha, Campus Universitario, 13071 Ciudad Real, Spain

bDepartment of Chemistry, University of Cambridge, Lensjield Road, Cambridge CB2 IEW, UK

Received 26 August 1996: accepted 6 September 1996

Abstract

We present a kinetic study of the oxidation reaction of Rb(5’S;) with N20 at elevated temperatures by the technique of pulsed photolysis of a Rb atom precursor, RbCl, followed by time-resolved laser-induced fluorescence spectroscopy of Rb atoms using the resolved Rydberg transition at X = 420.2 nm (Rb(6’P;) * constant of (2.1 t 0.4) X IO-” cm3 molecule-’ s-’

Rb(.52S ;)). A value of the absolute second-order rate has been obtained at 830 K. This result is compared with the previously

reported result obtained by atomic resonance absorption spectroscopy. An estimate of the diffusion coefficient for rubidium in helium of DI1(Rb-He) = (0.27 -+ 0.03) cm’ s-’ at s.t.p. is reported. Finally, the result is considered briefly in terms of the potential surfaces involved in the reaction. Rb(52S ;) + N20(X’Ct) correlates directly with the ground state products RbO(X’C+) + Nz(X’C+g) via a ‘A’ surface. We believe the present type of investigation to constitute the first study of the present process by flash photolysis with laser-induced fluorescence (LIF). 0 1997 Elsevier Science B.V.

Keywords: Laser-induced fluorescence; Oxidation of Rb(52S ;) with N20; Pulsed flash photolysis; Potential energy surface; Time-resolved spectroscopy

1. Introduction

The reaction of the alkali metal atoms with N20 at elevated temperatures is a class of reaction that has received much attention for a variety of both funda- mental and applied reasons. One area of interest has arisen from the fact that these reactions are usually highly exothermic on account of the low bond-disso- ciation energy of this molecule (D(Ng-O(X’C’)) = 1.667 eV) [ 11, and so the metal oxide product is often formed in excited states, sometimes giving rise

* Corresponding author.

to strong chemiluminescence [2-61. Another area of interest has been the observation that the O-abstrac- tion reactions of metal atoms with oxidants like NzO constitutes a basic aspect of combustion chemistry [7]. In order to study reactions of the alkali metal oxides which are of atmospheric interest, the reaction between alkali atoms and N20 is a convenient method for preparing the metal oxides cleanly [8,9]. It is very important to gain an understanding of the chemistry of these alkali metals in the mesopause, where a thin layer of neutral metal atoms exists at an altitude of about 90 km [lo]. Atmospheric models indicate that the principal sink for these metals, immediately below

0022-2860/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved PII SOO22-2860(96)09480-X

14 E. Martinez et al.Noumal of Molecular Structure 410-4/i (1997) 73-76

the layers of free atoms, is the formation of metal superoxides through recombination with O2 [lo]. Thus the relative rates of formation and stabilities of these superoxides may partly account for these phe- nomena [lo]. The metal monoxides, which can be formed in flames by O-abstraction reactions with oxi- dants like N20, are important intermediates in the formation of the metal hydroxides, and are believed to play a central role in the catalytic recombination of major flame radicals such as H and OH [l 11. In this context, we present a kinetic investigation of the col- lisional behaviour of atomic rubidium in its electronic ground state with N20 in the gas phase by direct spectroscopic monitoring of ground state atomic rubi- dium in the time domain.

2. Experimental

Reactions of Rb with N20 are investigated here by the technique of time-resolved laser-induced fluores- cence spectroscopy of Rb atoms, employing the shorter wavelength component of the spin-orbit resolved Rydberg doublet transition at X = 420.2 nm

t? -1.4 3

. 3 -1.6

2 -1.8

L S s

-2.0

54 -2.2

(a)

7 . _ . . . : * L . 1.. C. *.

* . * . .

‘..* *.- ;. q... . ??s .“,- . .

0 20 40 60 80 t/ms

0.0 0.4 0.8 1.2 1.6 t/ms

(Rb(Sp(‘P L 1) - (Rb(Ss( 2S I)) h = 42 1.6 nm and X = ?’ * 420.2 nm, gA = 0.47 x 102s s-’ and 0.96 x lo8 s-l, respectively). This is in preference to the longer wave- length D-line emission (52P;,; - 52S 4, X = 794.8 and 780.0 nm, gA = 1.3 x lo* s-‘*and 3.0 x lo8 s-‘, respec- tively), on account of the sensitivity of the photomul- tiplier tube employed (Thorn EM1 9813 B, Bialk) and the pumping efficiency of laser dyes at these longer wavelengths. The experiments were performed using a newly constructed experimental system that has pre- viously been described [12] and is summarised here. Rb(52S 4) was generated in the central chamber of a stainless steel reactor by pulsed irradiation of RbCl at elevated temperatures, using an externally triggered high-pressure Xe flash lamp system (JML-SPlO, max- imum pulse energy 8 J at 10 Hz). In summary, the reactor is constructed from stainless steel with three orthogonal arms with optical windows for pulsed initiation, laser excitation and laser-induced fluores- cence collection. Standard procedure for construction, heating, temperature-monitoring and control are described elsewhere [12]. A mixture of known rela- tive concentration of N20 in excess He is slowly passed through the reactor and controlled by means

600

500

4ca 7

v1 300

2 200

100

0

5E+5

‘: 4E+5

E 5 3E+5

+ g 2E+5

lE+5

@I

0 2000 4000 6000

Fig. 1. (a) Example of the time-resolved decay of the LIF signal recorded at pHe = 50.0 torr and 810 K. (b) Plot of k’ ( = kdiff) versus I/p”,. (c)

Example of the time-resolved decay of the LIF signal in the presence of low concentration of NZO recorded at,@ = 50.0 torr and 830 K. (d) Plot of k’pT versus pt. f = [N20]/( [NzO] + [He]] = 2.4 x IO-‘.

E. Martinez et al./Joumal of Molecular Structure 410-4 I I (I 997) 73- 76 15

of flow controller (ASM, Model LBlB) at a rate ensuring that the system is a slow-flow system, kine- tically equivalent to a static system. Typically, the total mass-flow rate was varied from 50 to 100 seem. The details of the vacuum system and asso- ciated monitoring equipment have been described pre- viously [12]. A pulsed dye-laser (Continuum ND60, Stylbene 420) pumped by a Nd-YAG laser (Conti- nuum NY 81CSlO) was employed for excitation of the laser-induced fluorescence from the transient atomic rubidium. LIF signals were isolated by means of an interference filter centred at h = 420.2 nm and monitored by a gated photomultiplier tube. The entire experiment is controlled by a pulse- delay generator, in combination with a double-pulse generator used to fire externally the probe laser, and a SR250 boxcar averager unit used to scan the probe laser with respect to the photolysis flash. The timing sequences for the pulsed irradiation, photomultiplier gating, laser probing and the procedure for pulse con- trol, together with signal capture using boxcar integra- tion, are given elsewhere [ 121.

3. Results and discussion

3.1. Diffusion of Rb(5 ‘S 4 in He

Fig. la gives an example of the digitised time- variation of the laser-induced fluorescence, (ILIF), at h = 420.2 nm representing the decay of Rb(5*S 4) at 810 K in the presence of He alone (pue = 50.0 torr). Such profiles are fitted by computer to the standard form:

ILIF = 8, + B,exp( - k’r) (1)

where 8, represents the long-time component of the scattered light and k’ is the first-order decay coeffi- cient of Rb(52S ;). In this instance, the first-order decay coefficient due to diffusional removal (k’ = kdirr) would, of course, be expected to demonstrate an inverse proportionality with the total pressure (kdiff = Plpr). Given the very limited temperature var- iation that can be employed with our experimental system, and the relatively small variation of diffusion coefficients with temperature in general, the tempera- ture-dependence of Diz(Rb-He) could not be deter- mined accurately. Nevertheless, using the ‘long-time

solution’ of the diffusion equation for a cylinder of length L and radius r [ 13,141:

P x2 5.81 kdiff= -= -+ - 012

[ 1 pr L2 r2 (2)

and using a simplified approach of a temperature- dependence of D 12 = T’.5 following gas kinetic theory, it has been estimated an average extrapolated value of D,*(Rb-He, s.t.p.) = 0.27 -+ 0.03 cm2 s-‘, in good agreement with the previous value of D,z(Rb- He,s.t.p.) estimated by the technique of time-resolved atomic resonance absorption spectroscopy (ca. 0.2 cm2 s-l) [ 151. However, in the present measure- ments, we are primarily concerned with the depen- dence of the diffusional loss term on pressure (k’ =

/3/pT) rather than its detailed value. Fig. lb gives an example of the linear relationship K’ versus l/p”, for diffusion at 810 K.

3.2. Reaction of Rb(52S ~)+N20 2

This kind of reaction,

Rb(52S ,)+N,O--, RbO+N2 f

(3)

constitutes a convenient kinetic test of the system, especially in view of the inert secondary reaction pro- duct, NZ. Under the conditions of the experiments, the concentration of N20 was always well in excess of the concentration of Rb resulting from the pulsed photo- lysis of RbCl vapour, therefore, the loss of Rb atoms should be described by the pseudo-first-order decay coefficient k’,

k’ =kdiff +kN20[N201 (4)

where k’ combines the kinetic effects of diffusional removal and chemical reaction. This equation may be rewritten in the form [12]:

k’PT = P +kr+,cdp; (5)

where f = [N,O]/( [N20] + [He]], and the absolute second-order rate constant for the reaction, kNZO.

now involves the appropriate units of pressure. f is generally low (ca 10-5-10-4) because of the high reac- tivity between Rb and N20. Hence, for a given tem- perature, decay profiles of Rb(5 2S 4) were recorded for samples of varying total pressures, pr, for a given mixture of fixed relative composition. Fig. lc shows

16 E. Martinez et al./Journal of Molecular Structure 410-4/l (1997) 73-76

an example for pr = 50.0 torr, where k’ was extracted from the exponential fit to Eq. (1). Then, a plot of k’pT

versus pf yields kN,O from the slope of the linear regression fit (see Eq. (5)). Fig. Id shows this plot with f = 2.4 x 10m5. The averaged value of kNzO

obtained It 2a (uncertainty) was (2.1 f 0.4) x lo-” cm3 molecule-’ s-’ at 830 K, in relative accord with the previous value obtained by time-resolved atomic resonance absorption spectroscopy at 830 K, kN20 = (4.0 t 0.2) x lo-” cm3 molecule-’ s-‘.

The detailed thermochemistry of O-atom abstrac- tion by the alkali atoms from N20 is limited primarily by the accuracy of the bond-dissociation energies of the diatomic alkali oxides (D~(RbO(X2C’) = (2.88 ‘_ 0.1) eV) [16]. All the O-atom abstractions are highly exothermic, in this case, AH = -120 kJ mol-‘, and the rate processes are characterised by measurable but small activation energies. In terms of electronic struc- ture, the nature of the potential surfaces for the reac- tion between Rb and K + N20 are also similar, given the detailed calculations of Langhoff et al. [ 171 on both RbO and KO and hence the symmetry arguments for the reaction depicted by Eq. (3), based on the weak spin-orbit coupling approximation and C, symmetry in the collision complex [18], are analogous to those for K + N20 [9]. Thus, Rb(5*S$ + N20(X’C+) corre- lates directly with the ground state products RbO(X*C+) + N*(X’C+g) via a *A’ surface. We would expect a greater breakdown in the weak spin- orbit coupling approximation than for K + N20 [9] with the heavier Rb atom, and this coupled with the fact that the ‘least symmetrical complex’ [ 181 is not of C, symmetry leads one to conclude significant pro- duction of the low-lying RbO(%) (515 cm-’ above the X*C+ state [ 171) resulting from the highly exother- mic reaction depicted by Eq. (3), presumably with relative populations of the *II and X*C+ states close to those expected on the basis of the statistical weight. Finally, the production of RbO via this reaction is very efficient (see kN20) and it could compete with the process of production of RbO through Rb02 [ 1 l] in fuel-lean flames containing Rb and N20, opening a rapid path to yield RbOH and subsequent reduction to Rb with the resulting increase in the inhibition of this

flames in a catalytic cycle [ 111:

Rb ‘zM RbOz 2 RbO Hz0 - RbOH 2 Rb

Rb y RbO “2 RbOH 2 Rb

Acknowledgements

The authors gratefully acknowledge the financial support provided by the Spanish Direction General de Investigation Cientffica y TCcnica (Project No. PB91-0309).

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