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2 Preparation of Reactive Intermediates for Study by PES Using Gas-Phase Reactions, Taking the ClO and BrO Radicals as Examples

2 Preparation of Reactive Intermediates for Study by PES Using Gas-Phase Reactions, Taking the ClO and BrO Radicals as Examples

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38



J. Dyke



Fig. 3.1 Inlet system used to

study atom-molecule

reactions by PES



with the target molecules, which pass down the central inlet tube, just above the

photon source. By moving the inner inlet tube upwards above the photon beam,

spectra can be recorded as a function of mixing distance (i.e. reaction time), with

1 cm mixing distance corresponding to ca. 0.5 ms reaction time with the simple inlet

system and the pumping system used. The aim is usually first to record a spectrum

of the selected reactive intermediate at low reaction times and then to record spectra

at longer times. This gives information on how the reactive intermediates decrease

with reaction time and what products are formed. The observed behaviour at longer

reaction times provides important information which assists assignment of the

spectra at short reaction times to a particular reaction intermediate. In selecting

a gas-phase reaction to prepare a particular reactive intermediate for study, it is

important to consider the rate coefficient of the reaction that produces the reactive

intermediate as well as the rate coefficient of the fastest reaction that removes it in

the reaction system used. If two consecutive reactions are considered:ACB!CCD



(3.6)



CCE!FCG



(3.7)



where C is produced by reaction (3.6) and removed by reaction (3.7), then in order

to produce C with sufficient number density to be detected by PES (detection limit

with the spectrometer and photon source used is 1010 –1011 molecules.cm 3 ), the

rate coefficient at room temperature for reaction (3.6) should ideally be 10 10 –

10 11 cm3 . molecule 1 s 1 and the rate coefficient for reaction (3.7) at room

temperature should be 10 12 –10 13 cm3 .molecule 1 s 1 or lower.



3 Photoionization Studies of Reactive Intermediates of Importance in the Atmosphere



-1e-



39



3Σ–,1Δ, 1Σ+ States

of ClO+



Fig. 3.2 Valence electronic structure of ClO and the ionic states obtained on ionization from the

outermost occupied orbital



ClO and BrO are both key reactive intermediates in the atmosphere being

involved as intermediates in the catalytic destruction of ozone by chlorine and

bromine atoms respectively. Their first AIEs are important

(i) in determining energies of electron transfer processes in the atmosphere,

(ii) in determining energies of ion-molecule reactions in the upper atmosphere, and

(iii) in thermochemical cycles which can be used to determine bond dissociation

energies.

For ClO, an efficient way to prepare this radical is from the Cl C ClO2 reaction

(3.8). However, as described above, the main removal reaction must also be

considered. The production and removal reactions are:Cl C ClO2 ! ClO C ClO



(3.8)



ClO C ClO ! Cl2 C O2



(3.9)



The room temperature rate coefficients for these reactions are k8 D 6.0 10 11

cm3 .molecule 1 s 1 and k9 D 4.8 10 15 cm3 .molecule 1 s 1 which fit the general

requirements outlined above with k8 being very much greater than k9 . This proved

to be a very good way of preparing ClO for PE study.

Another important point to bear in mind is that reactive intermediates often

have characteristic “fingerprints” which can be predicted before the PE spectrum

of a reactive intermediate is recorded, and these are very important in terms of

associating spectral bands, observed at short reaction times and which disappear at

longer reaction times, with a particular reactive intermediate. For example, ClO and

BrO are valence isoelectronic with O2 ¯. They have three electrons in the outermost

  antibonding orbital and have a 2 … ground state. One electron ionization from

this outermost orbital will give rise to three ionic states (X3 †¯, a1 , b 1 †C ) with a

relative degeneracy ratio of 3:2:1 (see Fig. 3.2). Three bands are therefore expected

associated with this one electron ionization with an intensity ratio of 3:2:1, each with

a vibrational constant, ¨e , which is greater than that in the neutral molecule because

an antibonding electron has been removed. The vibrational constants of the three

ionic states will also be approximately equal. These expectations were confirmed

in the spectra obtained for the first three bands of ClO where the experimental



40



J. Dyke

1000



d

ClO+IΣ+



ClO+X3Σ–

ClO+IΔ



a=Cl2+



c



b=O2+



COUNT. S -1



b



c=H2O+

d=HCl+



a



+b b



1000



13.0



12.5



12.0



11.5



11.0



IP/eV



Fig. 3.3 PE spectra obtained from the Cl C ClO2 reaction, showing the first three PE bands of

ClO. The lower spectrum has slightly better resolution than the upper spectrum, but has a slightly

higher oxygen level



intensity ratios of the first three bands was measured as (2.9 ˙ 0.1): (2.0 ˙ 0.1):1.0.

The first AIE of ClO was measured as (10.95 ˙ 0.01) eV [17]. Spectra obtained for

the Cl C ClO2 reaction showing the first three bands of ClO are shown in Fig. 3.3.

The vibrationally resolved band envelopes were very similar in all three bands and

as expected in each band the vibrational separations led to an ionic state vibrational

constant which was greater than that in the neutral molecule, and approximately the

same in each ionic state.

In the case of BrO, two main production routes were considered Br C O3 and

O C Br2 . For these reactions, the production and removal reactions are:Br C O3 ! BrO C O2



(3.10)



BrO C O3 ! Br C 2O2



(3.11)



3 Photoionization Studies of Reactive Intermediates of Importance in the Atmosphere



Intensity/Arb. Units



a



O3



SiBr4



BrO+ X 3Σ-



b



BrO+ a1Δ



O2

14



41



13



BrO 2Π 3/2



Br2



Br

12



BrO 2Π 3/2



11



10



Ionization Energy/eV



Fig. 3.4 The HeI PE spectrum obtained for the Br C O3 reaction at a mixing distance of 15 cm

above the photon beam. (b) Br atoms were obtained by discharging a flowing Ar/SiBr4 mixture.

(a) was obtained with the discharge off



and

O C Br2 ! BrO C Br



(3.12)



O C BrO ! Br C O2



(3.13)



For Br C O3, the room temperature rate coefficients are k10 D 1.2 10 12 cm3 .

molecule 1 s 1 and k11 < 2 10 17 cm3 .molecule 1 s 1 , and for O C Br2 the

room temperature rate coefficients are k12 D 1.1 10 11 cm3 .molecule 1 s 1 and

k13 D 4.1 10 11 cm3 .molecule 1 s 1 . Inspection of these rate coefficients indicated that although the production route for Br C O3 reaction (3.10) has a rate

coefficient which is an order of magnitude lower than the “target values” for the

production reaction outlined above (10 10 –10 11 cm3 .molecule 1 s 1 ), the removal

rate coefficient is very low with at least five orders of magnitude difference between

k10 and k11 . In contrast, for O C Br2 the production reaction is very favourable

with k12 D 1.1 10 11 cm3 .molecule 1 s 1 but the removal rate coefficient is also

very high (k13 D 4.1 10 11 cm3 .molecule 1 s 1 ). Therefore, the Br C O3 reaction

was selected for initial study as the most favourable preparative route to give a PE

spectrum of BrO [18].

Figure 3.4b shows the u.v. photoelectron spectrum obtained from the Br C O3

reaction, recorded at a mixing distance of 15 cm above the photon beam. Br atoms

were prepared in this study by microwave discharge of a flowing SiBr4 /Ar mixture.



42



J. Dyke



a

b



Intensity/Arb. Units



c



d



Br



e



BrO+ X3ΣBrO+ a1Δ



12.0



11.5



BrO X2Π 3/2



BrO X2Π 3/2



11.0



10.5



10.0



Ionization Energy/eV



Fig. 3.5 Part (e) shows the first two PE bands of BrO. The parts of this figure are as follows:

(a) HeI PE spectrum recorded in the 10.0–14.0 eV IE region for the Br C O3 reaction at 15 cm

mixing distance (b) HeI“ signals arising from O3 and O2 in the 10.0–14.0 eV IE region. (c) The

PE spectrum obtained by subtracting (b) from (a). (d) HeI PE spectrum recorded in the 10.0–

14.0 eV IE region for discharged SiBr4 /Ar showing bands from Br2 and Br. (e) HeI spectrum

obtained by subtracting (d) from (c)



Two bands attributed to BrO are marked on this figure. Increasing the mixing

distance at constant reagent partial pressures resulted in a decrease in intensity

of these BrO bands. Figure 3.4a was obtained for the same reaction mixture as

Fig. 3.4b but with the microwave discharge turned off. It therefore only shows bands

of ozone and SiBr4 . Figure 3.4b also shows bands from Br2 which arise from Br



3 Photoionization Studies of Reactive Intermediates of Importance in the Atmosphere



43



O2 X 3Σg-



a



Intensity/Arb. Units



Br2



b



Br 2P



Br 2P



O2 a1Δg



O3 P



BrO2



13



12



11



10



Ionization Energy/eV



Fig. 3.6 HeI PE spectra recorded in the 10.0–13.8 eV IE region for Br2 reacted with discharged

oxygen at a mixing distance of 10 cm above the photon beam. (a) and (b) were recorded with the

oxygen discharge off and on respectively



atom recombination. When these Br2 bands are subtracted off, the spectrum shown

in Fig. 3.5e in the ionization energy region 10.0–11.8 eV was obtained.

As outlined above, ionization from the outermost  -antibonding orbital of BrO

is expected to give three bands with intensity ratios 3:2:1. Unfortunately, the third

band of BrO, corresponding to the ionization BrOC (b1 †C ) BrO(X2 …), could

not be observed because of overlap with more intense bands in the 11.9–12.6 eV

ionization energy region, notably bands of Br, O2 and O3 . (The bands in the 11.9–

14.0 eV region in Fig. 3.4b were more intense than those in the 10.0–11.9 eV

region; it is important to note that the count-rate used to record the 11.90–14.0 eV

region of Fig. 3.4b was ten times higher than that used to record the 10.0–

11.9 eV region). The relative intensity of the two bands shown in Fig. 3.5e was

measured as (2.6 ˙ 0.2): 2.0 in reasonable agreement with the expected ratio of

3:2. Also, for each band the vibrational constant, ¨e , obtained from the vibrational

separations in Fig. 3.5e was greater than that in the neutral molecule as expected

for ionization from an antibonding orbital. The values of ăe in each band were also

equal within experimental error. The first AIE of BrO obtained from this work is

(10.46 ˙ 0.02) eV.

The O C Br2 reaction was also investigated, although this was not expected to

show evidence of the BrO radical. The spectrum obtained is shown in Fig. 3.6b

recorded at a mixing distance of 10 cm above the photon beam [18]. For this

reaction, atomic oxygen was obtained by discharging flowing molecular oxygen and

a spectrum obtained under the same conditions as used to obtain Fig. 3.6b but with



44



J. Dyke



the discharge off is shown in Fig. 3.6a. A band, associated with a short-lived reactive

intermediate, was observed at (10.26 ˙ 0.02) eV which decreased in intensity as the

mixing distance was increased above 7 cm.

Clearly this band could not be associated with BrO. It was assigned to BrO2 by a

series of ab initio/Franck-Condon calculations [19] on BrO2 and Br2 O with different

structures. Good agreement was obtained in both band position and structure for a

BrO2 C2v structure. The observed vibrational structure was assigned to excitation of

the ž1 mode in the ground state of BrO2 C (see Fig. 3.7). The first adiabatic ionization

energy of BrO2 was obtained as (10.26 ˙ 0.02) eV. It is clear that the BrO2 must be

produced from the O C BrO reaction to give vibrationally excited BrO2 * which is

then collisionally deactivated by third body collisions i.e.

O C BrO ! BrO2



(3.14)



BrO2 C M ! BrO2 C M



(3.15)



3.3 The Use of PES to Measure Reaction Rate Coefficients,

Using the Atmospherically Relevant Reaction Cl2 C DMS

as an Example

PES has recently been developed to allow rate coefficients of bimolecular reactions

to be measured [20–22]. It has an advantage over mass spectrometry in that it

does not suffer from fragmentation problems. Also, it has the potential to observe

reactants, intermediates and products, and to measure branching ratios between

reaction channels which give different intermediates and/or final products.

PES has recently been used by the Southampton group to measure the rate

coefficient of the DMS C Cl2 reaction at room temperature [20]. This reaction was

found to proceed via an intermediate to give monochloro DMS and HCl. The overall

reaction is

DMS C Cl2 ! intermediate ! CH3 SCH2 Cl C HCl



(3.16)



Figure 3.8b shows spectra obtained at different mixing distances. The bands

labelled as “Intermediate” show a distance dependence which is characteristic of

a reaction intermediate.

The presence of a reaction intermediate can be seen from Fig. 3.8. In Fig. 3.8b,

bands of the reagents (Cl2 and DMS), the intermediate and the products

monochloroDMS (MDMS) and HCl are present. As expected the reagent bands

decrease in intensity from 15 to 25 cm and to 45 cm mixing distance whereas the

product bands increase over these mixing distances. In contrast, the bands of the

intermediate are low in intensity at 15 cm, are larger at 25 cm and are low again at

45 cm, as expected for a reaction intermediate. Based on computed vertical IEs for

possible structures, the observed PE spectrum for the intermediate is consistent with



3 Photoionization Studies of Reactive Intermediates of Importance in the Atmosphere



a



o



(2.V2..0) <- (0.0.0)



o



.

(1.V2 .0) <- (0.0.0)



o



10.0



10.2



45



(0.V2..0) <- (0.0.0)



10.4



10.6



10.8



11.0



11.2



eV



b



.

(V1 .1.0) <- (0.0.0)

o



.

(V1 .0.0) <- (0.0.0)

o



Br

O



10.0



10.2



10.4



10.6



O



10.8



11.0



eV



Fig. 3.7 Comparison of the observed (lower) and simulated (upper) first PE band of BrO2 . The

simulated spectrum was obtained using ab initio/Franck Condon calculations



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