The reaction of (SO2)2 and (CO2)2 with Ba

The reactions of (COa)a and (SO2)2 with Ba have been Investigated usmg A crossed beam arrangement and the laser-induced fluorescence technique. Internal energy in the BaO product was probed m order to study differences between monomeric and dimeric reactions The reaction cross section for the dimers of CO, was found to be between four and eightfold larger than that of the monomers. This can be explained by the change in the reac:ion mechanism due to the positi\+ electron affinity of the dtmers versus the negatrve electron affinity of the monomers. The product BaO from the dtmeric reactions is much colder rotatronally than in the monomeric case. Thts phenomenon can be expiamed based on the kmemaucs

1. Introduction Van der Waals molecules and clusters provide interestmg media for the study of intermolecular forces as well as the transition from the isolated molecule to the condensed phase. Recently much effort has been devoted to the study of the spectroscopy [l] of these species in order to define when and how a cluster becomes a solid or a liquid drop. Using microwave spectroscopy Novick et al. [2] and more recently Legon et al. [3] obtained important information regarding the conformation of some small van der Waals molecules_ Optical spectroscopy made it possible to define how many atoms have to be attached to a molecule before its spectroscopy changed to a ccsolvent molecule spectroscopy” f4,5]. Other aspects that were studied were the symmetry of very large clusters using electron diffraction [6], their ionization potential [7] and electron affinity [8,93. There have been several studies of the reactions of van der Waals (VDW) molecties. Some of these deal with reactions of-alkali dimers.with.halogens [lO,ll]. Both chemihrminescence and angular distribution experiments -gave. detailed - information __ , on the subject. Herschbach and co-workers- [12] studied the‘ reaction of Cl, f Bra and Cl, + HI. With the monomeric halogens -no -product could _be ob-

served, but the authors obtained BrCl and ICl signals as the driving pressure of the molecular chlorine beam was raised so that the emergent Jet contained a substantial fraction of (C12)2. Bemstein and co-workers 1131 looked at the reaction of (CH,I), clusters-with alkalis using surface ionization scattering detection, and recently the reaction of Ba with (NO,), was investigated_ by chemiluminescence [14]. Having VDW molecules ds partners in reactions, three effects can be introduced, that are not found in the isolated molecule case: (a) Dynamic effects - Due to the existence of very low -frequency modes in the van der WaaIs complex, which correlate to the_VDW bond, one might expect that the ex&ss energy -would be deposited in rotations and translations~rather than in vibrations. In addition a lengthening of the collision complex lifetime is expected. (b) Steric effect - Many of the cross molecular beam experiments were done with reactions that include no significant barrier in the entrance coordinate. Thus, these reactions are governed by thermodynamics only. However, as solvent becomes involved i&the dynamics, steric factorscan __ cause changes in the reaction pattern, and the branchingratio between two reaction products will. r be-aitered. _ (c) Electronic effects - Due to electronic inter-

0301-0104/84/$03.00 0 Elsevier Science Publishers B-V. (North-Holland Physics fiblisbing Division) r

action between the reacting molecule and the new reacting channels might be open “solvent”, and others become disallowed_ These effects can result from polarization. changes in ionization potential or electron affinity. and symmetry restrictions introduced by the attached solvent. In the present study, these effects were investig&ted by the laser-induced fluorescence (LIF) technique. so that the internal energy distribution in the product could also be probed. We compared ti\o monomeric reactions and their dimeric andIOgS.

The tmo monomeric reactions are: Ba(!S) +SO,(‘A) n H = -4.1 B&S)

--, BaO(‘Z’)

+ SO(3S-).

-

BaO(‘S+)

(1)

(11)

Both hdve been investigated applying the reactive scattering technique [ 151and laser-induced fluorescence in a beam gas arrangement [16.17]. It has been found that although reaction (I) is spm forbidden it has a rate four times larger than that of (II). The effect was explained [16] by an electron Jump mechanism which takes place in (I) due to the positive electron affinity of S02. while it cannot occur in (II) because of the negative electron

affinity

of the CO,.

These systems hale been chosen for the present investigation because of the large amount of ex15ung data on the monomeric reactions. which provides a good base for comparison and for understanding the effect of the formation of dimers on the reaction

process_ Another

reason

is

that the rotational

hnes m BaO are quite dispersed

50 [hat rotational

as \\eil as vibrational

crm be resolved

and analyzed_

trap. The

background

pres-

sure was 2 x 10mn Torr. and during the experiment 7 x 10m6 Torr. The Ba atomic beam was introduced perpendicular to the molecular and laser beams. It was produced using a stainless steel oven, heated by a thermocoax heating element cable and doubly shielded. The typical operating temperature was = 1090 K \\hich corresponds to a Ba pressure of = 0.4 Tort-. The temperature was measured using a thermocouple and a pyrometer_ A drawing of the experimental apparatus is shown in fig. 1. The low-resolution excitation spectrum of the product was obtained with a flashlamp pumped it never exceeded

+CO(*~‘).

AH = - 7.5 * 1.4 kcal/mole.

The experiments have been carried out applying a crossed beam arrangement. The vacuum system consists of two chambers, each pumped by a 6” diffusion pump. One chamber served as the source of the moiecular beam, with a 2 ms pulsed nozzle (fuel injector from Bosch) with a hole diameter of 0.2 mm_ it was separated by a skimmer from the main chamber. In a second arrangement, a 0.5 mm diameter pulsed nozzle with a pulse width of 200 ps (Quanta-Ray PSV). was used. This nozzle was placed inside the mam chamber, 15 cm away from the reaction zone. The reaction chamber was cooled by a liquid-nitrogen

+ 1.3 kcal/moIe.

+CO,(‘E+)

2. Esperimental

structure

Since the electron

.lffmltv of CO, dlmers IS positive. interesting electronic effects may arise by their formation_ FinJily. the amount of SO, dimers m the beam could

be direcily monitored by a technique developed by us. This allows comparison of the cross section for its reaction with that of the monomers.

and with coumarine 504 dye. The high-resolution spectra were taken using a Lambda Physik 2001 dye laser pu_mped with an excimer laser (Lumonics 430). The fwhm in this case was = 0.3 using coumarine 307 dye. The (5,0). (6,0) cm-‘, and (7,l) bands of the BaO (A’Z+-X*x+) 1181 system were excited and the total fluorescence collected using f = 2 optics and a Hamamatsu R562 photomultiplier fitted with a Schott OG550 filter. The signal was processed by a boxcar averager (PAR 162 main frame with 165 and 164 plug-in units)_ Computer simulation [19] spectra were used to ascertain the rotational distribution of the vibronic transitions probed. The spectrum is obtained by assuming a rotational temperature, and comparing the resulting distribution with the experimental one. The program was modified to include the possibility that the experimental spectrum re-

f- Nknan.

R Nawran / Reaction of (SO,),

and (C02)2

wrh Ba

Laser

I

To Pump

FIN 1. Expenmental

arrangement.

fleeted reaction of at least two species - for example, monomers and dimers. Because these two species, as will be shown, yield different rotational distributions, and since they both coexist under appropriate stagnation pressure conditions, it was found that no single rotational temperature could successfully reproduce the spectra_ Instead the simulated traces were generated using two different temperatures, each weighted by a factor representing the species’ contribution_ We estimate an error of _t50 K for the single temperature simulated spectra, whereas for those composed of two temperatures the error is 5 100 K.

3. Results 3.1. Determination

of the SO,

dimer

concentration

The amount of SO, monomers in the molecular beam was determined using their fluorescence intensity. It was established before 1201 that while SO, monomers produce very intense excitation

spectra, no significant contribution could be obtained from the dimers m the beam. Hence we used a frequency-doubled flashlamp-pumped dye laser to excite ?he E band [21] (at 32850 cm-‘) in the SO? monomer electronic transition. The laser has a bandwidth of = 4 cm-‘; therefore no changes in the spectral lineshape could be observed as the SO, stagnation pressure (the pressure behind the nozzle) was increased. The total emission was monitored as a function of the SO, beam flux and stagnation pressure. The flux was measured with a fast ion gauge. In the pressure region in which the experiments were performed we found a linear relation between the pressure and the flux_ In fig. 2, the monomer fluorescence intensity is presented as a function of the stagnation pressure_ We assumed that the deviation from linearity is due to formation of clusters. The dimer production depends on P2. Therefore the signal measured (S) was fitted in fig. 2 to the formula: S = k,P - k2P’. The

contribution

0) from bigger clusters was found

Reactions of Ba kth

3.2

L/ ,

00 (30

I

I

xl

40

I

I

160

120

200

FxloJ(torrl Thr fIuoreseence intensity of SO2 (monomer) as a function of the stagnatIon pressure. The curve was reproduced usmgcq (1).uherck,=O05andk,=l9~10-~

Fig.

1.

to be below our experimental accuracy which is = 3% of the total flux at pressures below 1200 Torr. The concentration (C) of dimers in the beam could be calculated from the ratio: c = &P’/k,P

= (IL/k,)

P

(2)

and’is plotted in fig. 3. The curve in fig. 2 reproduced the experimental results using X-, = 0.08 and k, = 1-9X 10-s. The asymptotic standard errors are 3% and 8% for k, and k, respectively_ The results obtained could be confnmed by the simulation of the spectra obtained. as will be discussed later. The actual concentration of dimers at low stagnation pressures (below 500 Torr) could actuahy be higher by as much as 108, since vibrational cooling could increase the measured monomeric signal. as was experimentally found (table

SO, and (S02),

In fig. 4 the (5,O) band of BaO is shown. Traces (a) and (b) were produced by reaction (I) with stagnation pressures of 150 Torr and 400 Torr of SO2 respectively_ In fig. 5 simulations of traces (a) and (b) of fig. 4 are presented. The lower trace (a) could be reproduced using a rotational temperature of 400 K. but to simulate the 400 Tot-r SO2 spectrum a “combined” temperature of 100 K (0.10) and 400 K (0.90) had to be used. In fig. 6 a high-resolution spec-mm taken at 150 Torr SO, is presented; it could be simulated by using 400 K (Fig. 7). In fig. 8 the high-resolution spectrum (b) shown, was obtained using 1019 Tot-r of SO,; it was simulated (a) by a spectrum obtained using a mixture of 20% of IO0 K and 80% 400 K. It should be noted that whenever mixtures of temperatures were used, the lower temperature weighing factor always agreed very well with the independently measured percentage of dimers in the mixture, obtained from fig. 3. We examined the possibility that the rotationally colder BaO distribution obtained at high

1). and described in section 4. I

I

I

I

I

I

I

1

I

1

40-

3020IS

/ 0

I

I

19150

,

19100 1905

iTcr7i’

400

600

800

1000

P(torr)

Fig 3. The concentmtmn in percent dlmers m A SO, beam 3s a wnctlon of the stagnation pressure.

Fig 4. The (5.0) band of BaO produced by reactIon of Ba wth SO, beams at different stagnation pressures. In (a) the pressure behind the nozzle was 150 Torr. whereas in (b) it was 400 Torr neat SO,.

-

.

_

l”“~““l”“l”“l”” Rofahm7f ;Temp~rtmtre -

4OOKIO9/. /OOKIZ?//

t?otaiiof70/ Tempefolure:

v(cm-‘1 Fig. 5. Computer slmuiation of the (5.0) band excitation spectrum unth rotattonal temperatures that best reproduced the results of hg. 4. In the lower trace the temperature was 400 K; but in the upper one a composite temperature of 90% 400 K and 10% 100 K was necessary to fit the data.

r

5

P

f?

IO

I I

15

IO

15

i

I

I

20

25

Fig. 6. High-resolution (fwhm = 0 3 cm -‘)

5225

5 220 WAVELENGTH

I

30

I

I

I

5215

was sin&y due to coqling of this reagerit in-the expansion, rat&r ‘than’due to cl&ter-formation. Mixtiires of 150 -Torr S?$ with l!Ie up to total pr&sures of 2052 Torr Were expanded, arid alltiwed- to- reasi with-the Ba. Thk resulting spectra yield tio’difference in rotational temperatl?re as compared w‘ith BaO produced by a bea& of neat-SO, with stagnation pressure of 150 Torr. Population of thk d’ = 1 state of BaO by the reaction was also mvestigated, using the (7,1) transition at 5135 A. Spectra were obtained at different SO, stagnation_pressures. Because of high J levgls of other transitions no clear assignment of alI the tines could be made. Nevertheless, it is obvious that the rotational energy distribution in the u” = 1 state does not change in going from 150 Torr up to 2621 Torr of SO,. An estimation of the rotational temperature was made applying the formula T = (S)‘/(l.lS)*B, where 5 is the frequency of the band envelope maximum, measured relative to the band origin. The result, 124 IS, is the temperature derived from these spectra, as well as from those obtained from mixtures of SO, (150 Torr) and 550 and 1127 Torr He. Thus He also had no effect on the rotational energy distribution in this state. The u” = 0 to u” = 1 relative popula20

I

I 1

IS& pre.&es

6,

spectrum of the (5.0) BaO tranntion produced by reaction of 150 Torr SO, with Ba

P

5

I .R

IO

15

I

I

15

IO

20

I

I

20

2’5

I

Rotational

Temp. = 400K

I

~~~ 5214

5i !I6

f

!I8

5220

5222

WAVELENGTH Fig. 7. Computer

simulation

Roto?~onol Temp

L

of the

BaO spectrum of fi_e 6.

26

t

35

utth n rotstronat

IOOK(O201

400K(O801, 22

1

18 P

.

(A)

/I 1 I/

I! !

;

1:: I1

5230

5229

5226 WAVELENGTH

26 ! 35

_

5224

5222

Gil

22

18 P

31I

I 27

5224

6,

R

temperature

of 400 K

tions were obtained by comparing the band heads of the (5.0) and (7.1) transitions_ The results are presented in table 1. The intensities were first normalized for the different rotational temperatures. The temperature of the u” = 0 level was derived from the computer simulations. and thus for all pressures of neat SO, above 150 Torr, it was composed of two temperatures as described above. As can be seen in table 1 for low pressures of neat SO, (150 and 420 Torr) vibrationally hotter spectra were obtained. These results agree very well with those obtained in ref. [16]. All the higher-pressure data, including those with He. yield approximately the sc?me population in the u” = 1 state. The difference between our low- and high-pressure results is no more than = 10%. Its meaning will be discussed in section 4.

(B)

S. Lower trace IS the experimental high-resolution (fmhm = 0 3 cm-‘) spectrum of the (5.0) BaO transitton resulting from the reaction of SO2 (stagnation pressure 1019 Torr) with Ba. The upper trace IS the stmulated spectrum. where a composite temperature (80% 400 K and 20% 100 K) was used. Rg

I 52k-I

I

I I

5226 WAVELENGTH

5222 (ii,

413 Table 1 pa% (Tort=) -

.-

150 42Om 700 1070 150 150

_ _Relative ?

Ptte -(T-o@

Relative’!

-~ population in -

. population in

.0.64

- 0.36

-

0.62

0.3s

-

0.75 0.76

0.25 0.24

0.75 0.74

0.22 026

550 1127

a>Results are accurate to 3.3. Reactions

of Ba

= _tO 05.

with CO,

and (CO,),

Fig. 9 shows the (5,O) band of BaO produced from reaction (II) with CO, stagnation pressure of 180 Tot-r (lower trace) and 400 Tot-r (upper trace). The latter is shown with half the sensitivity. In frg. 8 the simulated spectra reproduced these traces with rotational temperatures of 2000 K (lower trace) and a mixture of 40% product at 200 K and 60% at 2000 K. Thus, in analogy to the SO, case, a 40% dimer concentration would be implied by these results, as opposed to the SO, data where a 10% dimer contribution was sufficient to simulate the spectrum at basically the same pressure. Moreover, this 10% SO, dimer concentration agrees

,

HEAD I

a

15 I

with the. rest+ obtained from :the decrease ii; monomer SO,~intensity, as a. function of pressure (fig. 3)_ it- is unlikely that CO,, bemg%ss condensable tpa&SO,, should- form -four. times as much clusters -at the same pressure. On the contrary, -the CO? -dimer concemration is probably below - 10% at 400 Tot-r- Therefore -we interpret these data as evidence- that the cross section for (CO& reaction is at least four tunes as large as th3t-of the monomers: Comparing the lower traces of figs. 4 and 9 one immediately concludes that the CO, monomer reaction yields a much warmer rotational distribution than that obtained with SO,. This is confirmed by the temperatures obtained from the simulations, 2CKKlK (fig. 10) and 400 K (fig. 5). Similar results were obtained when the rotational distnbution in both reactions was probed with the (6,0) BaO transition. These results are in complete agreement with the work done by Zare et al. [16.17].

“p

$4

-

38

(5,01

Rofahnd

Temper&we

-w

19200

;y #200

19150

19100

5cni’ Fig 9. The (5.0) band-of BaO produced by reactton of Ba and CO,-beams wtth stagnation pressures of 180 and 400 Torr for the lower and upper spectra, respectively.

/l....l.,.,l,,,,1....I,,,,

19150

19100

IC _ 50

G(cm-‘) Fig. 10 Computer sunulation of the spectra of fig 9. The lower graph was produced with a rotational temperature of 2000 K. The upper one required 60% 2000 K and 40% 200 K to reproduce the experimental results.

414

J- Nieman. R. Naaman / Reactron of (SO,),

It should be mentioned that very high stagnation pressures of either SO, or CO2 (above 2OQO Torr) yielded spectra similar to those of the monomeric signal_ This indicates that either large clusters yield rotationally hot temperatures or that we cannot (or do not) observe the products from their reaction. We cannot distinguish between these two alternatives; however, the last argument can be supported by the results of reference [13]. For Let-y large clusters a complex may be formed in the zeaction which is BaO (CO,),,. similar to the RbI(CH,I),_, suggested by Bernstein et al. to explain their results. If this is the case it may very \\ell be that the complex greatly shortened the fluorescence lifetime of the BaO. or its absorption is shifted_ so that It could not be observed under our experimental conditions_

4. Discussion The crossed beam experiments provide a means to study the dimer’s effect on the reaction, and to probe separately each degree of freedom in the product. However, it introduces difficulties in the mterpretation of the results Jue to the coexistence of monomers. dimers. and perhaps even larger clusters m the molecular beam_ The non-equilibrium in the beam bet\\een the different degrees of freedom (translation. rotation and vibration) add to these compieGties_ In this study t\\o tools have been applied to solve these problems_ By being able to probe the SO, beam with the LIF technique. an estimation of the amount of dmlers m the beam could be obtained_ The expansion conditions were chosen so that only monomers and dimers extsted in the molecular beam m significant concentrations. The amount of dimers deduced from our mcasurements seems much larger than those usually observed usmg mass spectrometry. However. recent measurements done by Buck and co-workers [22) pomt to similar concentrations. even for Ar which is less condensable than SO,. The simulated spectra in which the amount of dimers was introduced AS a vanable parameter support our fmdrngs. since independently they give the same contribution from dimers as the concentration mea-

and (CO,)?

wth Ba

sured directly_ By changing the concentration of the reactant in the beam, using He, we could differentiate between the effect on the reaction due to cooling of the reactants, and that due to dimeric reaction_ Substituting He at high pressures for SO, could not reproduce the lower rotational temperatures observed with high SO, pressures. Therefore it was clearly established that the rotational cooling effect was due to dimerization and not to simple rotational or vibrational cooling of the reactants_ In the computer simulations we used the “rotational temperature” as a parameter. One may argue that a priori there is no reason to believe that temperature will reflect correctly the rotational energy distribution. However. as can be seen by comparing the experimental and simulated results, it is obvious that the agreement is remarkable. It is also evident that for a few states this assumption is not perfect: but temperature does help in obtaining pigeneral overview of the results, as reflected in the surprisingly good fit to the observed energy distnbution. Thts observation IS in good agreement with the previous work an the monomeric reactions obtained by Zare et al. [L6.17]. A dominant observation is that the BaO resultmg from the reactions \\ith the dimers is colder rotationally than the product of the monomertc reaction_ In reaction (I) the BaO has only a small portion of the available energy m the rotation, but even in this case. further reductton of rotational energy is observed in the dimeric analog. In reaction (II). with CO,, the BaO formed has a significdnt amount of energy in its rotation (r, = 2000 K): and again in the dimeric case a sharp decrease can be observed (TR = 200 K). Therefore most of the excess energy is deposited either in the products‘ translational motion or as internal energy in the remamdars SO - SO2 and CO-CO, respectively_ One explanation of this dramatic change can be given by assuming a statistical behavior in which rotational energy is deposited in the different fragments according to the available rotational state density in each. The density of states is proportionzl to (l/B,) for a rigid rotor at a given available energy. where B, is the rotational constant. The BaO produced has a Bevalue of 0.3126 cm-’ [23].

J. Nieman. R Noaman / Reacnon of (SO&

The other products of the monomeric reaction, namely SO and CO have much larger Be values, 0.7089 and 1.9772 cm-’ [23] -respectively, and therefore the BaO carries away most of the angular momentum available in the collision complex.

In the dimeric case, however, the silent products those not probed by the laser - are SO - SO; and CO - CO?. Although we do not know if the SO - SO, and CO - CO, fragments remain attached after the reaction, it is obvious that they can take a larger portion of the rotational energy compared to the analogous monomeric fragments. Hence, reaction of Ba with the respective clusters leads to similar (100 and 200 K) rotational temperatures in the BaO product; temperatures which are much colder than those obtained in the respective monomeric cases. As mentioned before, the CO, dimers show a significant increase in reaction cross section compared to the (SOz)_ Unfortunately, we could not probe directly the (CO,), concentration spectroscopically as was done in the SO, system. However, assuming the (COz)z concentration to be equal or less than that of the (SO,)?, and based on the simulated spectrum (fig. _lo), where a weighing factor four times the presumed concentration of (CO,)z was necessary to reproduce the results, we obtain at least a four-fold increase in the cross section. The increase in reactivity of the COz dimer, can be explained by involving an electron-jump (harpooning) mechanism. Whereas the electron affinity of CO2 is negative, that of (CO,), is -i-O.9 eV [9,24]. Thus, in the latter system an electron jump can occur at a distance of = 3.3 A, which leads to an efficient pathway for the reaction to proceed. The vibrational population results of reaction (I) (table 1) indicate that most of the BaO molecules are produced in the ground vibrational state (65%). The reaction at higher pressures results in a slightly colder vibrational distribution, = 75% m u” = 0, and is constant, regardless of the stagnation pressures (and therefore also of (SO)? concentration). Since in the presence of similar pressures of He, with 150 Tot-r SO,, we obtain the same result, we concluded that this cooling. if

and (CO,), _ _ wr!t Ba _

415

significant, has nothing to- do with cluster formation. Instead, we believe that the effect is probably -due to vibraticnal cooling in the expansion-of SO, as the stagnation p>essure is increased_ The low-pressure data yield approximately a 10% increase in the u” = 1 population. If this change is real it indicates that vibrationally warm SO, correlates with vibrationally hot product. Since the lowest vibration in SO, has a frequency of 517.6 cm-‘, we can calculate that at 300 K = 8% of the molecules are in the u” = 1 state. If, in the expansion, the SO, is vibrationally cooled to, say, 70 K we expect to have negligible population in this excited vibrational state. Hence, it may be that this S-10% decrease in vibrational temperature as we increase the SO, (or He) pressure is the contribution of vibrationally excited SO, to BaO u” = 1 product. We emphasize that this means that SO, monomeric or dimeric reaction with Ba yield, basically, the same vibrational population in the BaO product. We are currently investigating the effect of vibrationai energy in reaction (II) (CO?) and it will be discussed elsewhere [25]. By probing the internal degrees of freedom we have demonstrated in this work that a third species attached to one of the reactants can change quite dramatically the product’s energy distribution, and even in some cases, the mechanistic pathway followed_ From this and other studies made by us [26], one can conclude that our ability to a priori predict “ van der Waals-complex-induced phenomena” is quite limited. The relatively small perturbations introduced by cluster formation are sufficient to provide new channels for the reaction to occur. It is clear that more work in this novel field is necessary, are continuing

and investigations

in our laboratory

to this end.

Acknowledgement

This work was supported by the US-Israel Binational Science Foundation, and by The Fund for Basic Research Administered by the Israel Academy of Sciences and Humanities. The technical assistance of A. Bar-On made this research possible_

J. AGeman. R Naaman / Reacrion of (SO,),

416

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[71 is1

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and (CO,)_. wvrhBa

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Phillips. J.

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257.