Kinetic investigation of the molecular chemiluminescence SrBr(A2II, B2Σ+ → X2Σ+) and the...

J. Phofochem. Photobioi. A: Chem., 83 (1994) 79-89

Kinetic investigation of the molecular chemiluminescence B2x + + X22 ‘) and the time-resolved atomic fluorescence

79

SrBr(A211,

Sr(53P, + 51SO) following the pulsed dye-laser generation of Sr(53P,) in the presence of CH,Br and CF,Br

F. Castatioa,+, M.N. Sanchez Rayo”, R. Pereiraa, J. W. Adamsb, D. Husainb*+ and J. Schifinob aDepartamenfo de Quimica Fisica, Universidad dei Pais Vasco, Aparfado 644, 48080 Bilbao (Spain) bDeparfmcnt of Chemishy, University of Cambridge, Lensfield Rood, Cambridge CB2 1EW (UK)

(Received December 6, 1993; accepted February 8, 1994)

Abstract

A kinetic study is presented of the vibronic energy distribution in SrBr following the reactions of the optically metastable, electronically excited strontium atom, S~(SSS~(~P,)), 1.807 eV above its Ss’(‘Ss) electronic ground state, in the presence of CH,Br and CF,Br. Sr(53P,) was generated by pulsed dye-laser excitation of ground- state strontium vapour at h=689.3 nm (Sr(S’P,‘ +SiSo)) ai elevated temperature (850 K) in the presence of excess helium buffer gas in a slow flow system, kinetically equivalent to a static system. The decay of Sr(S’P,) is then monitored by time-resofved atomic fluorescence from Sr@P,) at the resonance wavelength following rapid Boltzmann equilibration within the ‘P, spin-orbit manifold using boxcar integration.

Time-resolved molecular chemiluminescence to the ground state was also observed from electronically excited strontium bromide and investigated in detail via the SrBr(AZ&n-+X%+) (Au =0, A=667 nm) and SrBr(B%+ +X2Z+) (Au =0, A= 651 nm) systems. The A% (177.6 kJ mol-‘) and B%* (183.6 kJ mol-‘) states of SrBr are both energetically accessible on collision between Sr(3P) and CH,Br and CF,Br. Both the atomic and molecular (A&-X) chemiluminescence emissions are shown to be exponential in form with both reactants and characterized by first-order decay coefficients which are found to be equal under identical conditions within experimental error. SrBr(A’%) and SrBr(B%+) are thus shown to arise from direct reaction with bath CH,Br and CF,Br.

The results obtained here in the time domain are compared with analogous chemiluminescence studies on Sr(3P) with halides, including bromides, using molecular beams and with previous time-resolved measurements on SrCl(A*II, B’S’-X2x*) that we have reported. The results are also compared with those from a series of investigations we have presented from time-domain investigations of molecular emissions from CaF, Cl, Br, I (A&X) arising from the collisions of t21(4~P,) with appropriate halides.

1. Introduction

Measurements on reactions of electronically excited alkaline earth atoms employing atomic fluorescence and molecular chemiluminescence carried out in the time domain [1,2], complementary to analogous processes using molecular beams in the single-collision condition [3-111, have yielded detailed information on vibronic product distributions [1,2,12,13]. In this context, a series of investigations of the collisional behaviour of electronically excited calcium atoms in the low-lying Ca(4s4p(3P,)) state with the molecules CH,F, CH,Cl, CH3Br and CH,I following the pulsed dye-laser generation of the

‘Authors to whom correspondence should be addressed.

atom have been reported [14-171. Atomic resonance fluorescence investigations in the time domain have been described [1,2,12,13] for the radiative, dflusional and collisional properties of the heavier electronically excited strontium atom itself in the low-lying Sr(5sSp(3P,)) state, 1.807 eV above the 5s2(‘S0) ground state [18], including removal with the relatively inert molecule CF, [19]. Time-resohed studies with reactive molecules such as the methyl halides above, for the purpose of investigating halogen atom abstraction and vibronic distribution in the resulting strontium halides, are restricted by competition with spontaneous emission.

The mean radiative lifetime for the transition Sr(53P,) + Sr(5?S0) +hv (A= 689.3 nm) has been

lOlO-6030/94/$07.00 Q 1994 Elscvier Science S.A. All rights reserved SSDI lOlO-6030(94)03808-S

80 F. Castafio et al. I Molecular chemiluminescence and time-resolved atomic fluorescence in CH,Br and CF,Br

characterized in particular detail, yielding the value of 7, =19.6+0.6-0.5 KS [20,21]. Whilst the 5~5p(~P,,) spin-orbit states are so-called “reservoir states” and emission from them can be neglected [12,22], lengthening the effective radiative lifetime of Sr(53P1), kinetic studies under bulk conditions in the time domain with Sr(53P,) analogous to those reported for Ca(43P,) with CH-,X [14-171 are subject to major experimental constraints. In particular, collisional removal will only constitute a limited fraction of the overall loss of the excited atom, which will be dominated by spontaneous emission. This has indeed been found in our recent investigation on vibronic energy distribution in SrCl(A’II, B’S’) following the collision of Sr(53P,) with both CH,Cl and CF,Cl [23].

Solarz and co-workers [3,24] have monitored the products of the reaction of Sr(53PJ), generated by laser excitation, with species such as hydrogen halides and halogenated methanes in molecular beams, using laser-induced fluorescence to monitor the energy distribution in the diatomic product. Brinkmann et al. [25,26] have employed chemiluminescence measurements on strontium halides to study the product distributions of reactions of Sr(53P,), generated from electrical discharge under molecular-beam conditions, with various chlori- nated species. A similar approach has been employed by Kiang, Estler and Zare [5], which, cou- pled with energy inventory, has been used to determine the bond dissociation energy of SFs. A particularly detailed account has been given by Campbell and Dagdigian [27] of spin-orbit effects within Sr(53P0,,,,) with halogen-containing com- pounds under beam conditions.

In this paper we describe the study of the collisional behaviour of Sr(53P,), generated by pulsed dye-laser generation, with CH,Br and CF,Br, using time-resolved atomic fluorescence and time-resolved molecular chemiluminescence from SrBr(A%, B2Cf-X’Z+). The measurements indicate that the electronically excited SrBr molecules in the Azl13n and B’s+ states arise from direct reaction rather than energy transfer. The results derived from these measurements in the time domain are compared with analogous measurements from time-resolved measurements for halogen atom abstraction by Ca(43P,) [14-171 and for Cl atom abstraction by Sr(53P,), together with the observations obtained for Sr(53PJ) under molecular-beam conditions.

2. Experimental details

The experimental arrangement for monitoring time-resolved atomic resonance fluorescence and

molecular chemiluminescence in the time domain were similar to those described in our recent investigations on the reactions of Sr(53P,) with CH$l and CF,Cl [23], with appropriate modiii- cation for strontium atom studies to the system that we have described for the investigations of Ca(43P,) with methyl halides [14-171. Sr(5s5p(3P1)) was thus generated by the Nd:YAG pumped pulsed dye-laser excitation of strontium vapour at elevated temperatures [28-301 at h =689.3 nm (Sr(5s5p(3P,)) -+Sr(5s2(‘S,,))) in the presence of CH,Br, CF,Br and excess helium buffer gas in a slow-flow system, with time-resolved monitoring of the atomic emission at the resonance wavelength and of the SrBr(A*JI, B%+-X22*) [31,32] chemiluminescence systems.

The spectroscopy of these molecular states, particularly SrBr(B’Z+), has been studied in further detail in more recent times by the combination of microwave spectroscopic data for the X?%* ground state and laser excitation techniques [33,34]. For the broad-band chemiluminescence spectral measurements of SrBr(AZIIl/2,3,Z-XzZ’I:) and SrBr(Bz~f-X2C-t) resulting from the reaction of Sr(53P,) with CH,Br, the 3P atom was generated from a dye-laser (Lambda Physik FL3002, Pyridine 1, 0.6 mJ per pulse) pumped by an excimer laser (XeCl, Lambda Physik). Laser-induced Auroesc- ence of the A-X and B-X systems following excitation of the SrBr(X’I: ‘) ground state employed the same system with a different pump dye (DCM). The time-resolved emission profiles for both the atomic and molecular states using the Nd:YAG pumped dye-laser system were constructed using boxcar integration employing typically 1000 pulses, with computer interfacing and data analysis as described previously [14-17,35,36].

All materials (Sr, He, CH,Br and CF,Br) were employed essentially as described in previous investigations [14-17,37-391. Whilst absolute rate data have not been reported for the reactions of ground-state Sr(SiS,) with the reactants CH,Br and CF,Br, of importance in considering the results reported here in the context of the absolute densities of these two reactants reaching the optical excitation region, the analogous data for Ca(4%) will be employed as a guide. Clay and Husain [37,38] have reported absolute rate data for the temperature regime employed here for the reactions of the ground-state calcium atom with CF,Br and various other reactants by time-resolved atomic resonance absorption spectroscopy following pulsed irradiation.

F. Castalio et al. I.Molecular chemihuninescence and time-resolved atomic fluorescence in CH,Br and CF,Br 81

3. Results and discussion

Figure l(a) shows an example of the broad- band molecular chemiluminescence from SrBr(A*II,,-X22’) and SrBr(B*Z+-X%+) following the pulsed dye-laser generation of Sr(?P,) (h= 689.3 nm, Sr(5s5p(3P,)) +Sr(5s2(1S0)) in the presence of CH3Br and excess helium, taken with relatively broad slit widths in order to record a signal. The spectrum also shows the strong atomic resonance fluorescence at A = 689.3 transition together with emission from the energy-pooled state, Sr(6@,)), arising from self-annihilation of Sr(53P,). This has been studied in detail by Husain and Roberts [21]. Such pooling represents a small “bleed-off from the Sr(53P,) store state, whose first-order kinetic decay is not affected by this second-order removal process [21] (see later). Emission from the energy-pooled state can be seen in more detail when the spectrum is recorded at

630 640 650 660 670 h/run

Fig. 1. Examples of broad-band chemiluminexence spectrum. (a) Atomic resonance fluorescence emission at h=689.3 nm {Sr[5s5p(3P,)]-Sr[.5sZ(‘Sa)]}, atomic energy-pooling emission from Sr[(6s)5,] and molecular chemiluminescence emission from SrBr(A*rI,X’Z+) (Au = 0) and SrBr(B% +-X5*) (Au = 0,1,2) following the pulsed dye-laser generation of S@P,) from atomic strontium vapour in the presence of C!H,Br and excess helium buffer gas at elevated temperature (monochromator slit widths, 1.0 mm). (b) Portion of the broad-band molecular chemiluminescence spectrum from SrBr(A’&-X5*) (Av=O) and SrBr(B% *-XT+) (Av=O,l) at greater resolution (monochromator slit widths, 0.5 mm). (T= 923 K; [CH,Br] = 8.4 X ld’ molecules cm -3, [He]-9.5 x 10” atoms cmm3; boxcar integrator pulse width,

670 680 690

h/m

700

18 fis).

Fig. 2. Example of the atomic resonance fluorescence emission at h-689.3 nm {Sr[5sSp(3PI)]-Sr[Ssz~~S0)]} and atomic energy- pooling emission from Sr[(6s)%,] (~s(~S+~SS~(‘P~,,~) at A = 679.1, 687.8 and 707.0 nm, respectively) following the pulsed dye-laser generation of Sr(5’PJ from atomic stmntium vapour in the presence of CH,Br and excess helium buffer gas at elevated temperature. (T=923 K; [CH,Br] = 8.4 X lOi molecules cmU3, [He] =9.5 X 10” atoms cm->; monochromator slit widths, 0.5 mm; boxcar integrator pulse width, 18 ps).

lower sensitivity (Fig. 2). In the present instance, strong emission from the allowed transition Sr(6s(3S,))-5p(3P,)) does, however, interfere with the chemiluminescence from the lower-lying molecular spin-orbit component, SrBr(A*II,,,-X2x’), and chemiluminescence measurements from SrBr(A) are thus restricted to the A2113,Z component.

This spectrum can be contrasted with the analogous chemiluminescence of SrBr(A,B) following the collision of Sr(53PJ) with CH,BrZ in molecular- beam measurements [27], where no energy-pooled states of Sr are observed under single-collision conditions and where SrBr(A%I,,Z-X2T,+) may be monitored. The spectrum is necessarily very broad on account of the rotational structure, J=78 and 77 being the most populated rotational levels at Boltzmann equilibrium under the conditions of Fig. l(a) from the appropriate spectroscopic data for the A and B states [40], and where the spectrum is dominated by the Azi = 0 sequences on account of the similarity in the internuclear distances in the A, B and X states [40] (see later). Marginally improved resolution with smaller slit widths (Fig. l(b)) indicates some contributions from what ap- pear to be the broad emission arising from SrBr(A21111Z-X2x + , Av = 2) and SrBr(A%,,,- X2x+, Av= l), though these are not monitored in the time domain.

SdWSd 5~W1)l , , Sr[SpW,) 5swo)l

82 F. Castafio et al. I Molecular ckemilumimxence and time-resolved atomic ~7uorescence in CH,Br and CF,Br

Figures 3(a)-3(d) show examples of the digitized X-Y output of the time-resolved atomic emission from Sr(S3P,) at h=689.3 nm (Sr(5~5p(~P,) + Sr(%*(‘S,)) following pulsed dye-laser excitation of strontium vapour at T-850 K in the presence of varying concentrations of CH?Br and excess helium buffer gas. This is then used to construct Fig. 4, which shows the appropriate computerized fitting of the digitized output, indicating the first- order decay of the time-resolved atomic emission which yields the first-order decay coefficients for the 3P, state in the presence of CH,Br. These plots are presented as (ln(Ir -B)) 21s. time (where B is the long time-scattered light component of the atomic fluorescence or molecular chemiluminescence signal, &IF). The accompanying time- resolved chemiluminescence from SrBr(A2TI, TX2Z+) (A==667 nm) and SrBr(B*T.‘-X22+) (A=651 nm), taken under identical conditions to the atomic decays, are shown in Figs. 5 and 6 with the appropriate first-order plots in Figs. 7 and 8. These time-resolved measurements all employed wide slits (600 pm) for both atomic and molecular chemiluminescence profiles in order to obtain sufficiently high molecular chemiluminescence intensities.

The time-resolved chemiluminescence measurements are essentially restricted to the Au=0 se-

4 0 too 200 JOD 400

Time /rs

250

E 200 :

t 150

e 2 100 b \

2 50

0

(cl

\

~ 0 ID0 200 300 400

Time /p

250

Cd)

~ 0 (00 200 300 400

Time /.us

Fig. 3. Examples of the digitized X-Y output indicating the decay of the time-resolved atomic fluorescence emission from Sr(S3P1) at A - 689.3 urn (Sr(f?P,) + Sr(5’SJ + kv} following the pulsed dye-laser excitation of strontium vapour at the resonance wavc- length in the presence of CHsBr and excess helium buffer gas atelc~tedtemperature(T=850~p,,,,~~~,-3OT0~-3.4XlO’~ atoms cm-‘). [~sBrj/10r5 molecules cm-? (a) 0.25, (b) 1.3, (c) 2.2; (d) 5.0.

250 (b)

$ 200 2

t 150 ,D E

~

\

e 1DO

P _L 50

0 0 100 200 300 400

Time /pr

foci-:\-;;: ~~~ 0 IO0 200 300 400 0 too 200 300 400

200

Cc) r ‘5 150 \

f z 100

c

Q

50 _L

0 I-_I_,_: II 100 200 300 400

Time /,us

0 100 200 300 4 Time /s?,

0

Fig. 4. Examples of the digitized X-Y output indicating the first- order decay of the time-resolved atomic fluorescence emission from Sr(53P,) at A = 689.3 nm (Sr(53P,) + Sr(S$J + kv) following the pulsed dye-laser excitation of strontium vapour at the resonance wavelength in the presence of CHsBr and excess helium buffer gas at elevated temperature (T=850 K, pun., tilh u,=30 Torr - 3.4 X 10” atoms cm-‘). [CHH,Br]/lO’s molecules cm-s: (a) 0.25; (b) 1.3; (c) 2.2; (d) 5.0.

0 100 200 300 400 Time /p

z 5 150

F ; ‘00

g ’ 50 _C

0 0 100 200 300 400

Time /p

(b)

0 100 200 300 1 Time /ps

0 100 200 300 Time /w

Fig. 5. Examples of the digitized X-Y output indicating the decay of the molecular chemiluminescence from SrBr (A*II,+X%‘) (Au - 0) (A- 667 run, (Avv-0)) following the pulsed dye-laser excitation of strontium vapour at A-689.3 MI (Sr@P,)) * Sr(5’Sa)) in the presence of varying concentrations of CHsBr and excess helium buffer gas at elevated temperature (T=850 K, we] = 3.4 X lOI atoms cm-‘). [CH&]/lO’s molecules cm-? (a) 0.25; (b) 1.3; (c) 2.2; (d) 5.0.

F. Castaiio et al. / Molecular chemiluminescence and time-resolved atomic fluorescence in CH,Br and CF3Br 83

b 50 lb0 150 0 50 100 150 Time /ps Time /p

Fig. 6. Examples of the computerized fitting of the digitized X-Y output indicating the decay of the molecular chemiluminescence from SrBr (B$‘-+X’Z’) (h-651 nm, (Av=O)) following the pulsed dye-laser excitation of strontium vaponr at h=689.3 nm (Sr(S3P,)) + Sr(S%,)) in the presence of varying concentrations of CHaBr and excess helium buffer gas at elevated temperature (T=850 K, [He] =3.4x1@’ atoms cnm3). [CH3Br]/10” molecules cm-‘: (a) 0.25; (b) 1.3; (c) 2.2; (d) 5.0.

0 50 lb0 150 -0 50 100 150 Time /ps Time //AS

Tim* /ps Time /fis

Fig. 7. Examples of the computerized fitting of the digitized X-Y output indicating the first-order decay of the molecular che- miluminesccnce from SrBr (A*& + X'T+ ) (A = 667 nm, (AU = 0)) following the pulsed dye-laser excitation of strontium vapour at 11689.3 nm (Sr(53P,)) +Sr(5’&,)) in the presence of varying concentrations of CH~Br and excess helium buffer gas at elevated temperature (T= 850 K, [He] = 3.4 X 1Ol7 atoms cm-‘). [CH,Bry 1015 molecules cmm3: (a) 0.25; (b) 1.3; (c) 2.2; (d) 5.0.

~~~ 111:_1 0 50 100 150 0 50 100 150

Time /p.s

(4

s ‘111 ‘_ =*.

50 100 150 Time /ps

Fig. 8. Examples of the computerized fitting of the digitized X-Y output indicating the first-order decay of the molecular cbe- miluminescence from SrBr (B%+ +X%‘) (A = 651 nm, (Av =0)) following the pulsed dye-laser excitation of strontium vapour at A= 689.3 nm (Sr(S”P,))+Sr(S$,)) in the presence of varying concentrations of CH& and excess helium buffer gas at elevated temperature (T= 850 K, [He] = 3.4~ lOI atoms cmm3). [CH,Br]/ 10” molecules cm-? (a) 0.25; (b) 1.3; (c) 2.2; (d) 5.0.

quences for both the A-X and RX systems, as can readily be seen from the appropriate Franck-Condon factors using a modification of the programme for a Morse oscillator described by Tuckett [41]. The following parameters are employed for this calculation: X’S’ w, = 216.657 cm-‘, OJ, =0.5188 cm-l , r, =0.2735 nm; A% w’~ = 222.1 cm-‘, w’&‘~ = 0.53 cn- ‘, r’= = 0.2722 nm; B% o’~ =222.132 cm-‘, w’,x’,=O.578 cm-’ and r,, =0.270 75 nm [31-34,401. Examples of the calculated Franck-Condon factors chosen here for the sequences Av = 0 and f 1 are restricted to the vibrational levels v = 0 and 1 in the X, A and B states. Thus, we obtain for the A-X transition the following values: (O,O), 0.9768; (LO), 2.3319x lo-‘; (l,l), 0.9305 and (OJ), 2.3173 x lo-‘. For the B-X tralisition, the analogous data are (O,O), 0.9023; (l,O), 9.461 x 10-2; (l,l), 0.7275 and (OJ), 9.0922X lo-‘. For the A-X systems, observations are restricted to the A2113,*-X2Z,+ system, as the Av =0 sequences for the A2111,,--X28+ are too close to the energy-pooling transition at A=679.1 nm (6~(%,)-5p(~P,)) which causes distortion to the molecular chemiluminescence by scattered light at the slit widths necessarily employed. The reverse is not the case; the Aliz-X molecular transitions do not distort the atomic profiles. This arises because the weaker molecular chemiluminescence

84 F. Castario et aL I Molecular chemilwninescence and time-resolved aiomic fiorescence in CH3Br and CF,Br

emissions are recorded with photomultiplier volt- ages (v) approximately a factor of two larger than those employed for the intense atomic transition. The published response characteristic for the gain (G) for the photomultiplier tube (EMI 9797B, S20 response) [42] can sensibly be fitted to the form In(G) = 8.71n(V) - 54.4, where G is in arbitrary units and V in volts. Thus the molecular profiles are recorded at sensitivities a factor of about 500 times greater than those for the atomic profiles.

A similar range of measurements to those described above were recorded for the atomic fluorescence and the A&X emissions with Sr(53PJ) +CF,Br at the same temperature, for which only the first-order plots are presented in this instance (Figs. 9-11). These are generally of better quality than those reported for Sr(53P,) +CH,Br, principally on account of the greater chemical inertness between CF,Br and ground-state Sr(SS,J. The first-order decay coefficients are thus obtained for the 53P, atomic and SrBr(A,B) molecular species with this reactant. In the absence of the CH,Br reactant, for example, the first-order decay coefficients derived from plots of the type presented in Fig. 4, for example, would be expressed in the standard form [19-211:

&,/I1 + (I/K,) + IQ + p+ (1)

z 6

(0) :

h5 L

e 4 < 3 I3 2 r

2 ~ 0 50 100

Time /MS

150

E 6

Cc) 5

ES = e 4

Q

=;J L

2 2 ~ 0 50 100 150

Time /ps

? 6

(b) : t

5

e Y= g 4

h I 3

g 2 rl 0 50 100 150

Time /p

Fig. 9. Examples of the computerized fitting of the digitized X-Y output, indicating the first-order decay of the time-resolved atomic fluorescence emission from Sr(53P1) at A=689.3 nm (Sr(Z?P,) + Sr(S$,) +Iw) following the pulsed dye-laser excitation of strontium vapour at the resonance wavelength in the presence of CF,Br and excess helium buffer gas at elevated temperature U- 850 KP,,l,l with He = 30 Torr-3.4~10” atoms cm-‘). [CF,Br]/ lO’$ molecules cm-? (a) 0.44, (b) 1.3; (c) 20; (d) 2.3.

~~~ -‘--; 0 50 100 750 0 50 100 150

Time /LE.

” 50 100 1 Time /fis

Time /#is

50 100 150 Time /crt

Fig. 10. Examples of the computerized fitting of the digitized X-Y output indicating the first-order decay of the molecular chemiluminescence from SrBr (A’II, --) X’I; +) (A = 667 nm, (Av=O)) following the pulsed dye-laser excitation of strontium vapour at A = 689.3 nm (SK(~~P,)) + Sr(S%,)) in the presence of varying concentrations of CF3Br and excess helium buffer gas at elevated temperature (T=850 K, [He] =3.4X 10” atoms cm-“). [CF3Br]/10” molecules cm-? (a) 0.44; (b) 1.3; (c) 2.0; (d) 2.3.

2 v

(0)

x 0 50 100 150 0 50 100 150

0 50 100 150 0 50 100 150

Time /p Time /I.LS

Fig. 11. Examples of the computerized fitting of the digitized X-Y output indicating the first-order decay of the molecular chemiluminescence from SrBr (B%+ +X*Z+) (A = 651 nm, (Au=O)) following the pulsed dye-laser excitation of strontium vapour at A= 689.3 nm (Sr(5’P,))+Sr(S’S,)) in the presence of varying concentrations of CF,Br and excess helium buffer gas at elevated temperature (T= 850 K, [He] = 3.4X 10” atoms cm-‘). [CF,Br]/10i5 molecules cm-j: (a} 0.44; (b) 1.3; (c) 2.0; (d) 2.3.

F. Castario et al. 1 Molecular chemiluminescence and time-resolved atomic fluorescence in CH3Br and CF,Br 85

where K1 and Kz represent the equilibrium constants connecting the spin-orbit states within Sr(5”PJ (3P,, + 3Pi, K1; 3P1 + 3P2, IQ which rapidly reach Boltzmann equilibrium on the time-scales of the present measurements. Emission is observed only from Sr(53P,) [21] and the term flIpHe represents diffusional loss of Sr(S3P,). The detailed time-scale by which this equilibrium has been reached for Sr(53PJ) in helium alone is a matter of some controversy. The application in various publications [19-211 of such rapid equilibration and the standard use of the 53P0 and 53P, states as “reservoir states” leads to a consistently accepted mean radiative lifetime of T= = 19.6 + 0.6 - 0.5 us [21]. Such an approach is also consistent with spectrophotographic measurements made directly on the spin-orbit equilibration on Ca(43P0,1,2) by Mcllrath and Carlsten [43] and, very recently, by this research group using dye-laser induced fluorescence measurements on the individual spin-orbit states within a 40 ns time-scale [44]. Kelley et al. [45], by contrast, describe time-resolved measurements on the Sr(53P0,,,J spin-orbit states using atomic resonance absorption spectroscopic monitoring by means of a hollow-cathode source. That work [45] concludes that spin-orbit mixing is a slower process than considered hitherto [N-21] and yields the surprising result that collisionally induced spin-orbit relaxation between J= 1 and 0 and J= 2 and 1 is a factor of about lo3 times faster for helium than for xenon. The resulting mean radiative lifetime for Sr(53P,) of 7, = 22 + 0.5 us is not significantly different from that reported hitherto [N-21], and will not significantly affect the analysis in that regard in the present paper.

Further, the chemical environment of CH,Br or CF3Br will permit us to use rapid Boltzmann equilibration within the spin-orbit components with confidence. Marginal differences of the above type in the values of T~(~P,) are considerably less im- portant than radiation-trapping effects observed hitherto at elevated temperatures [20,21] and also observed with the measurements for Sr(?P,) fCH3C1 and CF3Cl [23] on account of the higher equilibrium densities of Sr(S’&,) employed at the elevated temperatures [28-301. The functionF= (1 + l/K, + K2), calculated by statistical thermodynamics, takes thevalue of 2.311 at T= 850 K, for example, and reaches a limiting value of three at infinite temperature, being determined solely by statistical weights with Sr(S3P,). In fact, eqn. (1) must be modified to allow for radiation trapping, which is significant in this system as observed hitherto [23].

In the presence of CH,Br, for example, the first- order decay coefficients are given by the expression:

&JU + (l/K,) + KJ+ p+ + kn[CH,Br] (2)

where kR is the absolute second-order rate constant for the reaction of Sr(53PJ) with CH,Br, and where we could write

k’ = K +k,[CH,Br] (3)

for constant total pressure and fixed temperature. An estimate of the rate constant for the reaction of Sr(53P,) with CH,Br or CF,Br cannot be reliably obtained from the relatively scattered variation of k’ with [CH,Br, CF,Br] in view of reaction between ground-state Sr(SlS,) in the flow with these bromides, reducing the initial concentrations of CH,Br and CF,Br admitted to the reactor via the flow. Whilst Campbell and Dagdigian [27] have characterized reaction cross-sections for Sr(S3P,) with CH2Br2 and CH,I,, the work did not include CH,Br or CF,Br. Clay and Husain [37,38] have reported the absolute second-order rate constants for the reaction of Ca(4?S,) with CF,Br at a similar temperature, namely, k(Ca + CF,Br) = (2.5 kO.1) x

lo-l1 cm3 molecule-’ s-r (909 K). We assume similar reactivities for Sr(SISO), reducing the concentrations of CH,Br and CF,Br entering the excitation region of the reactor. For characterizing the mechanism of reaction, we would stress that the principal requirement in this context is that of identical conditions for comparing first-order decay constants for the atomic and molecular emissions.

Figures 12(a)-12(d) indicate the resulting cor- relation between the measured first-order decay coefficients for the A,,X and B-X emissions with those for the atomic emissions taken under identical conditions for CH,Br and CF,Br, as indicated. The slopes of Figs. 12(a)-12(d) are found to be 1.2,, 1.2,, 1.1, and Ll,, respectively, which we conclude to be equal within the significant experimental errors outlined earlier. The lo errors are considered to be about 20% in each case as the plots are placed through the origin, which is physically realistic for the 3P, A3/ZX and B-X profiles. All the decay profiles, atomic and molecular, are exponential in character and the equal- ity between the molecular and atomic first-order decay coefficients is concluded against errors arising from radiation trapping, the control of reactant density in the high temperature reactor resulting from reaction with Sr(SS,,) and the limited kinetic range over which k’ can be varied, given the

86 F. Castario et al. I Molecular chemiiumitmcence and time-resolved atomic fluorescence in CHJIr and CF,Br

3

7

‘0 : 2 . .

b ‘L. 2 1

‘L

0

c_ (a)

0 2 k'('P)/l 0’ s-’

3

T (W I

b 2

. .

5 r’ ’

Y

rL 1 :

0 1-- 0 1 2

k’(‘P)/l 0’ 1-I

0 1 2 3 0 1 2 3 k’(‘P)/lO’ s-’ k’(‘P),‘l 0' I-’

Fig. 12. Comparison of the first-order rate coefficients derived from the intensity profiles for the SrBr (A&&Z’) (Au =O, A=667 nm). SrBr (B%+ +X3*) (Av=O, h=651 nm) molecular chemiluminescence emissions (k’(AsTX) andk’(BX)) with those from the atomic emission profiles for the decay of Sr(53PJ) at A = 689.3 nm (Sr(S’P,) + Sr(5’S,)) (k’(“P)) following the pulsed dye-laser generation of Sr(53PJ) at tbe resonance wavelength (Sr(Wp(‘P,)) + Sr(Ss’(‘&,))) in the presence of varying concentrations of CH,Br, CF,Br and excess helium buffer gas at T=850 K. (a) CH,Br, A’(A,-X) us. k’(3P); (b) CH3Br, k’(B-X) ws. k’(‘P); (c) CF,Br, k’(AsTX) vs. k’(3P); (d) CF,Br, k’(B-X) VS. k’(“P).

B-X (Av=O)

635 640 645 650 655 660 665 670 675

Fig. 13. Sequences of the broad-band laser-induced fluorescence spectrum of SrBr(A%,, + X’% ‘) and SrBr(B%+ -+X5’) following the pulsed dye-laser generation of Sr(5’P,) from atomic strontium vapour at A= 689.3 nm (Sr(5s5p(3P1)) + Sr(5s2(‘S&)) in the presence of CH3Br and excess helium buffer gas at elevated temperature (T= 923 K; [CH,Br] = 4.2~ lOI molecules cmW3, we] =4.8x 10” atoms cm-“).

dominance of A&F in eqn. (3) and where the diffusional loss term is relatively small and, indeed, difficult to measure against radiative loss [20,21].

It is for this principal reason that the plots in Fig. 12 are placed through the physically realistic origin points. Similarly, the decay datum contribution of A,,/F is a further physically realistic contribution to each of the plots in Fig. 12. Attempts to increase the kinetic contribution by an increase in reactant concentrations, and hence to extend the range of these plots, simply result in reaction with the precursor Sr(SISO) and a reduction in the concentrations of these bromides reaching the excitation region.

In the case of both CH,Br and CF,Br, the A211 and B2Z’ states of SrBr are energetically accessible on collision with the electronically excited Sr(S3P,) atom. Thus, the thermochemistry for reaction to ground-state products is as follows:

Sr@P,) + CH,Br - SrBr(XZ’) +CH, (4)

AlY= -210.4 kJ mol-’

Sr(??P,) + CF3Br - SrBr(X%+) + CF,

AH= -207.9 k.l mol-’

(5)

(DoO(SrBr(XzZ+)) =3.41 eV 131,461, (329.0 kJ mol-l); DO,,,(CH,Br) = 292.9 kJ mol-’ [47]; D’&CF,Br)=295.4 kJ mol-’ [47]). Hence, the electronically excited states, A211 and B%’ are accessible in both cases:

Sr(53P,) + CH,Br ---+ SrBr(A%) + CH3 (6)

AH= -32.8 kJ mol-’

Sr(53PJ) + CH,Br - SrBr(B%+) + CH,

AH= -26.8 kJ mol-’

(7)

Sr(53PJ) + CF,Br - SrBr(A%) + CF,

AH= -30.3 kJ mol-’

(8)

Sr(53PJ) + CF,Br - SrBr(B%+ ) + CF,

AH= -24.3 kJ mol-’

(9)

Thus, both SrBr(A211, 21’ G 12 and 10) for reactions (6) and (8) and SrBr(B*Z+, ~‘~10 and 9) for reactions (7) and (9) are energetically accessible initially on collision, though we would presume that vibrational equilibration takes place in the time-scales of the present measurements. Neither SrBr(A211) (177.6kJ mol-‘) norSrBr(B%+) (183.6 kJ mol-‘) can be generated by electronic energy transfer between Sr(53P,) (174.3 W mol-l) and ground-state SrBr(X%+, W” = 0) resulting from the overall reaction. However, these are accessible from the low vibrational energy Boltzmann pop- ulations in the X22’ ground state.

F. Castario et al / Molecular chemiluminescence and time-resolved atomic juorescence in CH,Br and CF,Br 87

The tist-order decay of Sr(53P,) in the presence of CH,Br and CF,Br, which is clearly established, can be expressed as

[Sr(53P,)],= [Sr(S3PJ)],=, exp( -k’t) (10) where the first-order decay coefficient for Sr(53P,) (k’) is given by eqn. (3) and its analogue for CF,Br. The mean radiative lifetimes of SrBr(A’II,,J has been characterized by Dagdigian et al. [48] (7,: A2f13,2, 33.2f 1.6 ns) as has that for the B*Z’ state (7,: 44.2f 1.6 ns) [48]. We may clearly place the concentrations of SrBr in the A*II and B2Z’ states in steady state following their direct pro- duction by processes (6)-(g) within the time-scales employed in the present investigation. The removal of SrBr(A,B) is dominated by emission and is readily given, in the case of SrBr(A) from Sr(53P,) + CII,Br, by

1&4-X) = @c@r(53P,)],_0 exp( -k’t) (11) where @ represents the combination in individual cases of the effects of light collection and electronic amplification of the photoelectric signals. Thus, the first-order coefficients for the decays of the SrBr(A-X) and Sr(53P1 +5lS,) should be equal, as indicated in Fig. 12, for example. Similar con- siderations apply to SrBr(B’f,+) (Fig. 12) and to the A-X and B-X emissions from Sr(53P,) + CF,Br (Fig. 12). Thus, within the experimental errors which are significant for the reasons indicated earlier, the molecular A,,X and RX chemiluminescence emissions can be used as a spectroscopic markers for the 3P state in terms of their time-dependences as found for both the A%-XT + and B’Z’-X3+ systems of CaF (A% only), CaCl, CaBr and CaI following the collision of Ca(43P,) with CH3F, CH,Cl, CH,Br and CH31, respectively [M-17].

Energy transfer from Sr(53P,) to an effectively steady population of vibrationally excited SrBr(X*T.+) ([SrBr(X),,]) resulting from the reaction of Sr(S%,J +CH,Br and CF,Br in the flow would add a term of the form k12[Sr(53PJ)],_0[SrBr(X)],, exp( -k’t) to eqn. (11) and would not affect the exponential form and the decay coefficients for the A-X and B-X chemiluminescence. That contribution is difficult to quantify in the present system. Whilst electronic energy exchange between the close-lying SrBr(A*II) and SrBr(B’C+) (hE = 501 cm-‘) would be expected to be relatively efficient, this could not compete with rapid spontaneous emission from these two states, whose mean radiative lifetimes have been described earlier [48]. By contrast, the relatively large and time-variable concentrations

of SrBr(X*Z+) resulting from reactions (4) and (5) would grow approximately with the form (1 -exp( -k’t)) [16,17]. An (E-E) transfer mechanism from the Boltzmann population within SrBr(X) on this basis would result in molecular chemiluminescence of the form I,,(A,B-X)a (exp( -k’t) - exp( - %‘t)) which would grossly distort the single exponential profiles, which is not significantly the case. Such a bi-exponential molecular chemiluminescence profile is observed, for example, for CaH(A’II-X*Z ‘) derived from energy transfer between Ca(43PJ) and CaH(X*Z+) arising from the initial reaction between Ca(43PJ) and GH,, [34].

Figure 13 shows vibrational sequences of the broad band laser-induced fluorescence of the Ai, 2,3n-X and B-X systems resulting from the pro- duction of SrBr(X’Z+) in this system. The spectral regions were recorded at different sensivities, namely, for 630-637,637-657.5 and 657.5-675 nm, these were, respectively, 400, 100 and 200 mV. As we have stressed earlier, the sequences are closely spaced on account of the similarity in the vibrational spacing in the X, A and B states. Furthermore, the rotational structure cannot be resolved without high-resolution Doppler-free polarization spectroscopic measurements on a steady SrBr density of the type described for SrBr(B’Z+-X*x+ (0,O)) by Schroder and Ernst [33]. The resolution of the dye laser employed for the present measurement (see Section 2) is about 0.15 cm-‘, and very much larger than the rotational spacing [33]. The most populated rotational level is J= 78 for the SrBr(X’Z+) ground state at T=923 K employed here, and the spectrum is further complicated by the role of the Sr79Br and S?‘Br isotopes. Nevertheless, the strong features of the B-X (AU =O,l) and A, -X (AU =O,l) bands are particularly clear, the other features being weaker. We may again estimate approximately the relative yields of SrBr(A*II)/SrBr(B’Z+) as greater than about 2:1, arising from both Sr(53P,) + CH& and CF3Br. These are derived from the observed relative molecular chemiluminescence A-X and B-X intensities taken under similar conditions of photomultiplier voltage and electronic amplification, in close wavelength regions, and also taking into account the values of the mean radiative lifetimes of these states [48] as also found for collision with CH,Cl and CF&l. We may fmally stress that the first-order decays for the 3P state itself and A and B-X chemiluminescence emissions observed here in the time domain are in accord with the ob- servation of SrBr,I(AZIT, B’Z+-X’I;*) chemiluminescence under molecular-beam conditions

88 F. Castatia et al. 1 Molecular chemiluminescence and time-resolved atomic fluorescence in CH,Br and CF,Br

from the reactions of Sr(3PJ ) + CHzBrz and CH,I,, respectively [27], and SrCl(A,B-X) from Sr(3P,‘D) +CC14 under single-collision conditions [25,261.

Acknowledgments

We thank the Defence Research Agency, Fort Halstead, UK, and the SERC for a Research Studentship held by J.W.A., during the tenure of which this work was carried out, and for generous equipment support. We also thank SERC and the U.P.V., C.I.C.Y.T. (PB90-1013) and Ministry of Education (C.A.V.) for the purchase of laser systems. J.S. thanks the Conselho National de De- senvolvimento Cientifico e Tecnologico - CNPq, Brazil for a Visiting Scholarship. Finally, we are grateful for helpful discussions on the spectroscopy of strontium halides with Professor W.E. Ernst of the Deparnnent of Physics of the Pennsylvania State University, who also kindly provided un- published data, and we are also indebted to Dr, George Jones of the DRA, Fort Halstead, for encouragement and helpful discussions.

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Kinetic investigation of the molecular chemiluminescence SrBr(A2II, B2Σ+ → X2Σ+) and the...

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