J. Chem. SOC., Faraday Trans. 2,1982,78,1221-1229

Studies of BrCl by Laser-induced Fluorescence Part 4.-Lower Levels (v' 3) of the B 3 ~ ( 0 ' ) Manifold

BY THE LATE MICHAEL A. A. CLYNE~ AND Lu CHENG ZAI$ Department of Chemistry, Queen Mary College, Mile End Road, London El 4NS

Received 12th January, 1982

Laser-excitation spectra of BrCl(B-X) with u' s 3 have been assigned at high resolution. The magnitude of the collision-free lifetime for the u' = 1 state of BrCl(B) was T~ = 41.5 * 0.5 ps, in good agreement with the previous value, T~ = 40.2 f 1.8 j ~ s for 3 d u ' d 6. It has been demonstrated further that electronic self-quenching of the B state of BrCl was slow, with a rate constant in the range (2.2-3.9) x cm3 molecule-' s-'. During the course of this study, the quenching rate constants with different bath gases have also been measured; the values ranged from (5.8 f 0.2) x cm3 molecule-' s-l for 0 2 down to (1.3 f 0.2) x cm3 molecule-' s-' for Ar. Vibrational energy transfer plays a more important role than quenching in collisional processes, showing a strong dependence of rate constant upon u f , from (3.0k0.2) X lo-'' cm3 molecule-' s-' for the u ' = 1 state, up to (2.1 *4.1) x lo-'' cm3 molecule-' s-' for the u f = 6 state, according to previous work.

In previous studies from this laboratory (Parts 1-3),'-3 high-resolution laser- excitation spectra of the B 311(O+)-X 'C+ system of BrCl and energy relaxation rates have been reported for the vibrational level 3 d v' d 7. The mean value of the collision-free lifetime for stable levels of 81Br3SC1(B) was reported to be T~ = 40.2 f 1.8 P S . ~ Predissociation of all rotational levels in the v' b 7 manifold, and in the v' = 6 manifold'with J 2 42, was c~nf i rmed.~

It has been suggested that the B-X transition of BrCl might be a promising chemical laser operating at wavelengths not shorter than 600 nm, corresponding to transitions from low v' levels (v' = 0, 1) to high v" levels (v" = 12). The B state of BrCl has the useful property that non-collisional predissociation is absent in its lower levels; also, quenching of BrCl(B) is very ineffi~ient.~ But there has been no dynamical information on the lowest v' states, as the laser-induced fluorescence from these excited states (e.g. v '= 1) is one to two orders of magnitude weaker than that from the higher excited states (e .g . v' = 6) , as a result of the Boltzmann vibrational populations and unfavourable Franck-Condon factors.

However, we note that only limited spectroscopic data on the o' = 0 and 1 levels are available, in this case from a study by Clyne and Toby,4 who reported a medium resolution spectrum of B-X BrCl chemiluminescence from the reaction Br2 + OC10. Coxon' has provided accurate spectroscopic data for the higher v' levels (v '> l), and for a range of 0'' levels of the ground state.

We now describe results of a study of laser-excitation spectra involving low v' levels (u' s 3) and their deactivation kinetics. The object was to extend our previous studies'-3 (with 3 d v' d 6 ) to the lowest levels, which we expected to be very stable. The rate constants k~ at 293 K for different quenching gases were measured, and ranged from (5.8 f 0.2) x cm3 molecule-' s-' for O2 down to (1.3 f 0.2) X

cm3 molecule-'s-' for Ar. f Correspondence to Professor K. W. Sykes, Department of Chemistry, Queen Mary College, Mile

$ On leave from: Fudan University, Shanghai, Peoples Republic of China. End Road, London El 4NS.

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1222 LASER-INDUCED FLUORESCENCE S T U D Y O F BrCl

An advantage of studying the lowest vibrational levels in the B state is that collisional predissociation uia vibrational ladder-climbing3 is inefficient, because of the large energy barriers and multi-quantum jumps involved. Therefore, in principle, it should be easier to determine the rates of other important energy- transfer processes when the o' = 0 and 1 levels are excited, than when levels of higher vibrational energy are excited; in the latter cases, collisional predissociation can be dominant even at relatively low pressure (mTorr range).

E X P E R I M E N T A L

An experimental technique for laser excitation of BrCl has been described previ~usly.~ Bromine monochloride was prepared by reaction between bromine and chlorine:

Br2 + Clz = 2BrC1, K::' = 7.4.6

To avoid interference by Br,(B-X) fluorescence, excess C1, was used in the'mixture (Br2: Clz = 1: 15), allowing equilibration for at least 24 h.

The experimental system consists of a dye laser pumped by a Nd-YAG laser,7 a test chamber' and a detection system. The tunable dye laser was pumped by a frequency-doubled Nd-YAG laser; it is described briefly. The dye laser cavity was formed by a 50% reflecting output mirror and a diffraction grating (600 line mm-', blaze 54") used in the 4th order. A 25 x beam expander and a pressure-tuned etalon (air-spaced, 6 mm) were inserted into the resonator, to narrow the bandwidth of the laser beam. The dyes Oxazine 720 and Cresyl Violet 670 (in ethanol) covered the required wavelength range (630-667 nm). The typical output energy of the dye laser was 1.5 mJ per pulse with a duration of 15 ns and a bandwidth of 1 pm. The repetition rate was 20 Hz.

The tunable laser beam was focussed weakly by a lens and used to excite BrCl molecules in the stainless-steel test chamber.' Fluorescence was detected by a broad-band response photomultiplier (EM1 9558QC; S20 cathode, 20 ns rise-time) at right angles to the laser beam. Two long-wavelength pass filters (Schott nos. 695 and 665) were placed in front of the photomultiplier, giving a cut-off near 690 nm. The configuration of the photomultiplier was such that negligible errors due to diffusion of excited statesg would be expected.

Laser-excitation spectra were recorded by supplying the output of the photomultiplier to a boxcar integrator (Brookdeal); the resulting analogue output was input to the Y-axis of an XY chart recorder.

The fluorescence-decay waveform, following each lasing shot, was captured by a fast transient recorder (32 ns per channel; Biomation 6500). At least 5000 shots were averaged in a mini-computer (Nicolet LABSO). For studies at lower pressures, and using oxygen as quenching gas, 20 000-30 000 shots were necessary, since signals were very weak. Averaged data were stored on floppy discs and analysed under software control to give lifetimes.

RESULTS A N D DISCUSSION

L A s E R - E x c I T A T I o N s P E c T R A o F BrCl (B-X )

The undispersed fluorescence intensity was recorded as a function of laser wavelength at total pressures of 100 to 200 mTorr, comprising ca. 15% BrCl and 85% C12. Laser-excitation spectra of the 1-4, 2-3, 2-5 and 3-5 bands have been obtained. Fig. 1 shows part of the laser excitation spectrum of BrCl(B-X) near the heads of the 1-4 band between 657.1 and 657.3 nm.

Fig. 2 shows part of the laser-excitation spectrum of BrCI(B-X) of the 2-3 band from 631.9 to 632.4 nm. The origins of the 1-4, 2-3, 2-5 and 3-5 bands for

Br3'C1 were identified near 657.3, 631.0, 667.0 and 658.3 nm, respectively. As predicted by the intensity factors Iuf,v~l (table l), the 1-4 band and the 2-5 band 79

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M . A . A . CLYNE A N D LU CHENG ZAI

81 35,,T R I

b

3 5 7 9 11 13 15 7 9 35;

2-5 2-7 1 9

T m -

?l I L q3 I I Y I 1 I

* I

I 1 I I I

657.4 657.5 657.6 657.7 laser wavelength/nm

FIG. 1.-Laser excitation spectrum of the 1-4 band of BrCl near the bandhead.

i

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I 1 I I I

632.2 632.3 632.4 632.5 laser wavelength/nm

632.1

FIG. 2.-Laser excitation spectrum of the 2-3 band of BrCl near 632.1-632.6 nm.

TABLE 1 .-ISOTOPE SHIFTS (79-81Br35C1) AND INTENSITY FACTOR FOR MEASURED BANDS

intensity factor/10-7, band origin, SGbb€../ a a , c . /

band I,..,. = quI.,.eu. vo/cm-' cm-' cm-'

0-4 1-4 2-3 2-5 3-5

1.50 1.08 6.89 1.67 3.80

~~ ~~~~

14 996.01 - 15 208.65 6.58 15 843.31 3.82 14 988.43 6.94 15 185.69 6.29

- 6.61 3.82 6.97 6.28

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1224 LASER-INDUCED FLUORESCENCE STUDY OF BrCl

were much weaker than the 2-3 band. The very weak 0-4 band also was identified, although not fully analysed.

band of natural BrCl consists of four pairs of single P and R branches assigned to the isotopic species 79Br35C1, s1Br3'Cl,

Rotational states in the vibrational bands were assigned up to J = 48 (except for the 0-4 band) by the method of combination differences. The expressions for the rotational energy combination differences are as follows, neglecting centrifugal distortion:

The structure of a B 3~(O')-X

Br37C1 and 81Br37C1 in the approximate intensity ratio 3 : 3 : 1 : 1. 79

R(J) - P(J) = 4Bv4J + $)

R(J - 1) - P(J + 1) = 4B,4J + $). The assigned transitions gave results for Bur and Boll which agreed well with the

direct or extrapolated data of Coxon.' The assignments were checked by measuring the magnitudes of the 79-81Br vibrational isotope shift SG' for each band. The results are shown in table 1.

The isotope shifts were calculated from the differences between the vibrational terms of the upper and lower states for 79Br35Cl and 81Br35Cl, using the expression

SG' = T ( u') - G( u") - [Ti( u') - G'( u")]

where T(v') , G(u") refer to 81Br35C1, and T'(u'), Gi(u") refer to 79Br35C1. For u'> 1 and all relevant U" values, Coxon's data5 were used. It was necessary to extrapolate his T(u') function from U' = 1 level. However, the extrapolation is short and is expected to involve an error of <0.5 em-' in the absolute value of T(1), which,was calculated to be 17 3.70.12 cm-l. The resulting calculated magni- tudes of SG' showed good agreement with experiment (table 1).

MEASUREMENTS OF RATE CONSTANTS FOR ENERGY TRANSFER, k ~ , AND RADIATIVE LIFETIME, 70

INTRODUCTION

In order to determine r o and kM for the excited levels v'= 1,3, measurements of fluorescence decay rates were carried out at total pressures from 2 to 100 mTorr. The rate constant kM refers to the sum of all rate constants for collisional processes which deplete the observed fluorescence.

Analysis of collision deactivation data for BrF(B) has been considered in detail by Clyne and Liddy." In summary, the measured rate constant kM will normally contain a large contribution from vibrational relaxation (k,#) within the excited-state manifold. This is due to the red shift of fluorescent (B-X) emission as the vibrational ladder is descended, which leads to a diminishing quantum efficiency of detection by photomultiplier. As for BrF fluorescence, the relative sensitivity of detection, S,, may be calculated as a function of U' €or BrCl fluorescence. Input data for the calculation of S,., as defined in ref. (lo), are Franck-Condon factors for BrCl (B-X)5 and response factors" for the optical filters and photomultiplier used. The resulting values of Svl (relative to S3 = 1.0) were: So = 0.067, S1= 0.31, Sz = 0.62, S3 = 1.0, S4 = 1.16 and S, = 0.30.

In the present work, studies on BrCl(B) were carried out using initial excitation of levels v' = 1 and v'= 3. Initial excitation of v'= 1 (S1= 0.31), followed by vibrational relaxation to v' = 0 (So = 0.067), leads to almost complete loss of detected fluorescence. Thus, in this case k M = kvl + k~ + k l + kQ, to a good approximation.

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M . A . A . CLYNE A N D LU CHENG ZAI 1225

Initial excitation of v t = 3 (S3 = l .O) , followed by relaxation to v t = 2 (S2 = 0.62), on the other hand, leads to a relatively small loss of fluorescence signal and only a lower limit for k3 can be inferred: k3 2 kM- ko.

The approach in the present work is first to analyse the data assuming k,l= kM - ko, and then to refine the results for k,! through computer modelling.

Computer modelling also is useful in predicting the time for achievement of a Boltzmann vibrational equilibrium in BrCl(B). Because of the long radiative lifetime of BrCl(B) and the inefficient quenching, BrCl(B) may be expected to reach vibrational equilibrium after a period of ps that is dependent on bath gas density. After this time, vibrational transfer has no effect on the fluorescence decay rate. Thus, after sufficiently long times, the collisional part of the decay rate constant of BrCl(B) may be assigned solely to electronic quenching, thus permitting a relatively direct determination of the rate constant ko for a Boltzmann population with v ' = 0 as the dominant state. Analysis of this type has been described previously for C12(B) by Clyne and McDermid."

DATA ANALYSIS FOR LOW PRESSURES

Fig. 3 shows typical fluorescence decay curves, in this case following the excita- tion of the (1,25) level of 79Br35 C1 (B) (time base 100 ps). 20 000 shots were averaged for the measurements which involved pressures below 10 mTorr. The semilogarithmic decay plots appeared essentially linear, confirming that there was

7 6 5

' r 4 - 3

2

1 0

7 i

l I 1 I I I I I

0 20 40 60 80 time/ps

0 20 4 0 6 0 80 time/ p s

FIG. 3.-Fluorescence decay of 79Br35C1(B). Initially-excited state is (1.25) of 79Br35C1(B). (a) Logarithmic decay curve at 100 mTorr total pressure showing linear correlation. ( b ) Logarithmic decay

curve at 500 mTorr total pressure showing curvature.

no depletion of excited molecules through diffusional loss, which causes negative curvature of the decay plot.g The gradients (r = -dlnI/dt) increased linearly with increasing pressure up to 100 mTorr, and the data were treated by the Stern-Volmer formulation

I'= 7i1 + kM[M]. (1) Fig. 4(a) shows typical data for this dependence of fluorescence decay rate constant r on pressure. From the ordinate intercept we get ro= 41.5*0.5 ps (la) for the ut = 1 state. The slope gives the values of the rate constant kM.

Since electronic quenching of BrCl(B) is very s ~ o w ~ * ' ~ it can be neglected its a contribution of kM in experiments at lower pressures. Thus, the quantity kM approximately equals the vibrational rate constant k,,I for the initially-excited level

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1226 L A S E R - I N D U C E D FLUORESCENCE STUDY OF BrCl

20 - I - I I I I I I I

v' (see above). These values of kvl, namely kl and k3, are listed in table 2 together with previous data for k4, k5 and k6. We note that these preliminary values of kl and k3 are lower limits, although kl can be expected to be a close limit.

We may now summarize the above results briefly. Comparing the present value of T~ with the value T~ = 40.2 f 1.8 ps for 3 d v ' 6 6,3 we conclude that there is no significant change in the collision-free lifetime T O of BrCl(B) with variation of v'. However, rapid vibrational-energy transfer was confirmed. The previous observation of a strong dependence of kuJ upon vr3 has now been extended down to v'= 1.

TABLE 2.-RATE CONSTANTS FOR VIBRATIONAL-ENERGY TRANSFER IN 79Br35C1(B)

energy below vibrational energy, predissociation, k,,/cm3 kQ/cm3

V ' G (v ')/cm-' AEfcm-.' molecule-' s-' molecule-' s-'

1 321.30 2 526.83 3 724.08 4 912.35 5 1090.40 6 1256.46

1086.3 (3.0* 0.2) x lo-'* 2.24 x 881.0 - - 683.7 495.4 317.4 (9.6 f 2.8) x 151.3

(4.0 f 0.3) x lo-'' (6.2 f 2.3) x 10-""

(20.7 f 4.1) x lo-"" } 3.9x 1 0 - * 3 a

~

a See ref. (3) for data.

DATA ANALYSIS FOR HIGH PRESSURES

In the case of experiments in the higher pressure range (500-5000 mTorr), logarithmic decay plots of fluorescence intensity showed appreciable positive cur- vature (fig. 3) owing to the occurrence of vibrational relaxation. However, at longer times the plots were linear, in accordance with the achievement of a Boltzmann distribution of vibrational population. Electronic quenching was then the only collisional process manifested by these data. In accordance with the above dis- cussion, the time for attainment of vibrational equilibrium was found to shorten as a function of increasing pressure.

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M. A . A . C L Y N E A N D L U C H E N G Z A I 1227

Stern-Volmer analysis of the longer-time, higher-pressure data [fig. 4(b)] gave the rate constant kQ for electronic quenching of a Boltzmann vibrational distribu- tion. This value of kQ refers to bath gases consisting of ca. 85% Cl2 plus 15% BrCl. As reported, the zero-pressure intercept of the Stern-Volmer plot gives a value close to the radiative decay rate constant ( ~ 0 ~ ) . The electronic quenching rate constant kQ was found to be 2.2 x cm3 molecule-ls-l, in comparison with the larger value of 3.9 x cm3 molecule-’ s-l from our previous work,2 using a similar composition of bath gas. Wright et az.l2 reported kQ = 3.4 X cm3 molecule-’ s-l under similar conditions.

COMPUTER MODELLING OF BrCl(B) FLUORESCENCE DECAY

Computer modelling of fluorescence decay kinetics of BrF(B) has been reported by Clyne and Liddy.” Briefly, using this method, the values for the vibrational- energy-transfer rate constants were obtained by computer modelling of the observed decay curves for BrCl(B) v ’ = 1, 3 at different pressures. An energy-level system comprising 7 stable vibrational levels 0 d ZI’ d 6 was set up (any level with v’ > 6 is predissociated). The variation of population N(u’, t) with time was described by

- G(u’)N(v’ , t ) - ~ Q N ( u ’ , t)[M] (11)

where kut-+ut+l and ku~+u~-l are upward and downward vibrational-energy-transfer rate constants. Thus kul = ku~-+u~+l + ku+ut-l. The population N(v’, t) variation was found from the solution of the group of coupled differential equations.

The observed fluorescence intensity as a function of time I ( t ) was fitted using 6

I(t) = c N ( d , t)S,t u‘=O

where Sul, the relative detective efficiency, was calculated by 15

Sul = c Y3u~,u”qu’,uJDvYv u“=O

were u ~ ~ , ~ ~ ~ is the wavenumber of the (v’ + v”) band origin, neglecting any small variation of electric dipole moment with r-centroid, Q V and Y,, are quantum efficiences for the photomultiplier and percentage transmission of filter at various wavenumbers and quI,utp is the Franck-Condon factor taken from ref. (2).

The modelling vibrational-energy-transfer rate constants kuf+ur-l have been weighted relative to each other by T r ~ e ’ s ~ ~ ’ ~ ~ experimental model, assuming the average amount of energy transfer per deactivating collision Y of 1 kT. The results listed in table 3 are in accord with the trends that the vibrational-energy-transfer rate constants increase with increasing v’. As predicted in the introduction to this section, for the v’ = 3 state, the computer modelling rate constant is significantly larger than the measured one.

The computing modelling semilogarithmic fluorescence decay plots were also investigated at high pressures (>500 mTorr), using these modelling rate constants and the decay plots appear to be curved in the same way as those obtained experimentally (fig. 5).

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1228 LASER-INDUCED FLUORESCENCE STUDY OF BrCl

8 - 6 -

2 -

TABLE 3.-MEASURED AND MODELLED RATE CONSTANTS FOR VIBRATIONAL- ENERGY TRANSFER AND ELECTRONIC QUENCHING

- 2

(a )

2)' 0 1 2 3 4 5 6

kM/ lo-'' cm3

k,f,,.- 1 / 10- ' ' k, t," '+ 1 / 1 0- ' '

molecule-' s-' - 0.3&0.02 - 0.4*0.03 6.2*2.3 9.6k2.8 20.7st4.1

cm3 molecule-' s-' 0 0.40 0.60 0.70 0.80 0.80 0.90

cm3 molecule-' s-l 0.05 0.15 0.20 0.25 4.5 8.5 19.5

Measured rate constant for electronic quenching ka = 2.2 X cm3 molecule-' s-'; modelled rate constant for electronic quenching ka = 2.1 x cm3 molecule-' s-l.

ENERGY TRANSFER WITH 02, C12, Ar

The fluorescence decays from BrCl(B) in the presence of 02, C12 and Ar have been measured for v' = 1. Measurements of the fluorescence decay rate with various bath gases were made by the same method as mentioned above. The pressure of BrCl used was 20mTorr. The bath gas was added to the test chamber to the required pressure (0-140 mTorr) via a needle valve. Then the fluorescence decay was determined following the excitation of the (u' = 1, J = 25) state of BrCl(B).

0 I I I I 0 40 80 120 160

pressure /mTorr FIG. 6.-Stern-Volmer plots with different bath gases. ( a ) 02, ( b ) Ci2, (c) Ar.

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M . A . A . C L Y N E A N D L U C H E N G ZAI 1229

For oxygen efficient quenching was observed, and to obtain an adequate signal-to- noise ratio 20000-30000 shots were averaged in some cases. Rate constants obtained from Stern-Volrner plots (fig. 6) are listed in table 4. It was founf2thaf rate constants changed with the bath gases from (5.8*0.2)~10- cm molecule-' s-' for O2 down to (1.3 f 0.2) x cm3 molecule-' s-' for Ar. Similar to BrF(B), oxygen is the most efficient gas for energy transfer, but rate constants to energy transfer with different bath gases for BrCl(B) are less than those of BrF(B) by a factor of 5 .

TABLE 4.-ENERGY-TRANSFER RATE CONSTANTS IN BrCl(B) WITH DIFFERENT BATH GASES

bath gas rate constant, k * / 1 0 - ' ~ cm3 molecule-' s-'

Ar 0 2

c12

1.3 f 0.2 5.8 f 0.2 2.7 f 0.1

It was noticed using C12 as quenching gas that the energy-transfer rate constant kQ = (2.7 f 0.1) x cm3 molecule-' s-' is approximately equal to the vibra- tional-energy-transfer rate constant kUp=l = (3.0 f 0.2) x crn3 molecule-' s-' of BrCl(B) measured above. The reason is that during the measuring of kul of BrCl(B), excess chlorine was added to suppress the fluorescence from bromine. In fact, chlorine plays a dominant role in that case.

We thank the S.R.C. and the U.S. Air Force Office of Scientific Research (grant no. AFOSR-75-2843) for support of this work.

M. A. A. Clyne and I. S. McDermid, J. Chem. SOC., Faraday Trans. 2, 1978,74, 798. M. A, A. Clyne and I. S. McDermid, J. Chem. Sac., Faraday Trans. 2, 1978,74,807. M. A. A. Clyne and I. S. McDermid, Faraday Discuss. Chem. SOC., 1979,67, 316. M. A. A. Clyne and S. Toby, J. Photochem., 1979,11,87.

JANAF Thermochemical Tables, NBS-NSRDS37 (U.S. Government Printing Office, Washing- ton, 2nd edn, 1971). M. A. A. Clyne and M. C. Heaven, Chem. Phys., 1980,51,299.

M. A. A. Clyne, M. C. Heaven and E. Martinez, J. Chem. SOC., Faraday Trans. 2,1980,76,177. M. A. A. Clyne and J. P. Liddy, J. Chem. SOC., Faraday Trans. 2, 1980, 76, 1569. M. A. A. Clyne and E. Martinez, J. Chem. SOC., Faraday Trans. 2, 1980,76, 1275.

l2 J. J. Wright, W. S. Spates and S. J. Davis, J. Chem. Phys., 1977, 66, 1566. J. Troe and H. G. Wagner, Ber. Bunsenges. Phys. Chem., 1967,71, 937. J. Troe, J. Chem. Phys., 1977,66,4745.

' J. A. Coxon, J. Mol. Spectrosc., 1974, 50, 142.

* M. A. A. Clyne and I. S. McDermid, J. Chem. SOC., Faraday Trans. 2, 1978, 74, 1376.

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