Tải bản đầy đủ - 0 (trang)
3 The Use of PES to Measure Reaction Rate Coefficients, Using the Atmospherically Relevant Reaction Cl2=+DMS as an Example

3 The Use of PES to Measure Reaction Rate Coefficients, Using the Atmospherically Relevant Reaction Cl2=+DMS as an Example

Tải bản đầy đủ - 0trang

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



46



J. Dyke



a



Open tube:

10



00 cm;



20 cm;



30 cm



Cl2



8



Intensity

(x103 counts/sec)



DMS

6



DMS



4



Intermediate

2



0

13



b



12



2 mm exit hole:



15 cm;



11



10



25 cm;



45 cm



9



10



HCl



Cl2



DMS



Intensity

(x103 counts/sec)



8



MDMS

6



Intermediate

4



2



0

13



12



11



10



9



IE (eV)



Fig. 3.8 PE spectra which show the presence of an intermediate in the Cl2 C DMS reaction. In

this figure, MDMS represents monochlorodimethylsulphide



a covalently bound structure (CH3 )2 SCl2 where the sulphur atom is four coordinate

and bonded to two methyl groups and two chlorine atoms [20]. In terms of the

Valence Shell Electron Pair (VSEPR) repulsion model, this structure has four bond

pairs and one lone pair (from the S atom). The structure, which is similar to that of



3 Photoionization Studies of Reactive Intermediates of Importance in the Atmosphere



47



SF4 , can therefore be viewed as a trigonal pyramidal structure with two long S-Cl

bonds in axial positions, the two methyl groups in equatorial positions, and the third

equatorial position occupied by a S non-bonding lone pair.

In the atmosphere, the interaction of DMS with Cl2 , and other halogens such as

Br2 and I2 , are thought to lead ultimately to SO2 production. If the rate coefficients

of the reactions of DMS with molecular halogens can be measured then these rate

coefficients can be built in to atmospheric models of SO2 production. At present,

the calculated levels of SO2 are lower than the observed levels and including these

extra sources may be a way of reducing the difference between the calculated and

observed levels. The main ways of DMS oxidation in the atmosphere are reaction

with the OH radical during the day and the NO3 radical at night. Subsequent

oxidation in the atmosphere leads to formation of species such as SO2 , H2 SO4 and

CH3 SO3 H (methanesulphonic acid). These species may contribute significantly to

the acidity of the atmosphere and, in the case of sulphuric acid, to cloud formation.

Recently, molecular chlorine has been observed in coastal marine air. It is produced

at night, as well as during the day, from heterogeneous reactions of ozone with wet

sea-salt. Employing a high-pressure chemical ionization mass spectrometer, Spicer

et al. [23] measured Cl2 levels ranging from < 15 to 150 pptv. Night-time mixing

ratios were in the range 40–150 pptv, with the highest value being observed near

midnight which dropped to 15 pptv at sunrise.

In order to measure rate coefficients, a differentially pumped, flow-tube, operated

under laminar flow conditions, has been interfaced to a PE spectrometer as shown

in Fig. 3.9. The basic assumptions of the method are:(i) that the reactants are mixed homogeneously with the carrier gas (helium),

(ii) the carrier gas flow velocity is the transport velocity of the reactants (typically

3–20 m.s 1 ) and

(iii) the reaction distance and time are proportional to each other in the flow-tube.

The kinetics experiments were performed under pseudo-first-order conditions.

Chlorine, the reactant in excess, was added through the central movable injector and

DMS was introduced to the flow-tube through a fixed side-arm, as shown in Fig. 3.9.

Contact times were between 20 and 150 ms. The pseudo first-order rate coefficient

was determined by measuring the relative concentration of DMS, from the intensity

of its first PE band, with the movable injector at several different positions while the

chlorine partial pressure was in excess and held constant. This was then repeated at

different chlorine partial pressures. The flow rates of all the gases were regulated

using mass-flow controllers, which were calibrated for each individual gas mixture

used. Typical first order plots at different excess chlorine partial pressures are shown

in Fig. 3.10, where the intensity of the first band of DMS measured at different times

is used to obtain a plot of loge ([DMS]0 /[DMS]t ) against contact time. The slope of

each of these lines gives the pseudo first-order rate coefficient which can be plotted

against the chlorine concentration used, to obtain the second order rate coefficient

(see Fig. 3.11). The value obtained is k D (3.4 ˙ 0.7) 10 14 cm3 .molecule 1 s 1

at T D (294 ˙ 2) K.



48



J. Dyke

Cl2



A

C F1



He

DMS

1

200m3.hr-1

Pump



2



25m3.hr-1

Pump



+V



3



25m3.hr-1

Pump



–V



I

F2



D



D



Fig. 3.9 Schematic diagram showing a flow-tube interfaced to a PE spectrometer for kinetics

studies



This bimolecular rate coefficient has been used [20–22] to investigate the

atmospheric implications of including the DMS C Cl2 reaction in an SO2 production

model. If it assumed that the [Cl2 ] and [DMS] are 50 pptv at night, and these levels

are replenished at night to maintain these levels, then using the bimolecular rate

coefficient derived in this work, 40 pptv of CH3 SCH2 Cl would be generated over

a 6 h period. During the day CH3 SCH2 Cl would either be photolysed or react with

OH. Photolysis of CH3 SCH2 Cl would lead via several steps to SO2 production.

Therefore, the night-time interaction between Cl2 and DMS may well provide a

mechanism to speed up SO2 production in the day and go some way towards

explaining the discrepancy between DMS decay rates and SO2 production rates

during the day. These results mean that interaction between DMS and halogens

cannot be ignored in climate modelling studies. Related work is currently underway

on the reactions DMS C Br2 and DMS C I2 .



3 Photoionization Studies of Reactive Intermediates of Importance in the Atmosphere



49

1.52E+14



Corrected 1st Order Plots for 1.6 Torr



1.70E+14



2

y = 19.407x



1.88E+14



Corrected ln([DMS]0/[DMS]t)



1.8



4.45E+14



1.6

y = 12.167x + 1E-14



1.4



3.97E+14

y = 8.1626x - 1E-14



1.2

y = 12.157x - 3E-15



1



2.36E+14



y = 8.3015x + 6E-15



y = 7.8549x - 3E-15

y = 6.2362x + 8E-15



0.8



y = 5.5697x - 6E-15



0.6



3.31E+14



3.55E+14



y = 7.0377x + 5E-15



0.4



y = 3.8153x



y = 6.4463x + 3E-15



0.2



2.51E+14



y = 3.4548x + 1E-15



2.17E+14



0

0



0.02



0.04



0.06



0.08



0.1



0.12



0.14



0.16



Contact Time (s)



ln



[A]0

[A]t



= (kw +k'2)t



where kw is the wall loss rate constant(s-1)



Fig. 3.10 Typical first order plots for the Cl2 C DMS reaction



Overall 2nd order plot for 1.6 and 3.0 Torr data



18

16

14

k2' (s-1)



12

10

8

6

4

2

0

0



1E+14



2E+14

[Cl2]



3E+14



(molecules/cm3)



Fig. 3.11 Second order plot for the Cl2 C DMS reaction



4E+14



5E+14



50



J. Dyke



3.4 Photoionization of Reactive Intermediates of Atmospheric

Importance with Synchrotron Radiation, Using Atomic

Nitrogen as an Example

The study of a reactive intermediate with PES using monochromatized synchrotron

radiation should allow more information to be obtained on the molecular ionic states

and the associated photoionization processes than a PES study with a low pressure

discharge of an inert gas as the photon source. In particular

(i) because the photon source is tunable it will be possible to identify autoionization resonances, and, once identified, they can give rise to extra vibrational

structure in a molecular PE spectrum over that observed in a PE spectrum

recorded off resonance,

(ii) because the photon source is polarized, angular distribution measurements

are possible and this allows information on photoionization dynamics to be

obtained,

(iii) a study of the relative band intensities in the valence PE spectra of an atom or

molecule as a function of photon energy can provide valuable information to

assist in band assignment, and

(iv) by sweeping the photon energy and detecting threshold electrons, threshold

photoelectron (TPE) spectra can be obtained. These are higher resolution than

conventional PE spectra and hence provide more information on the ionization

process and the ionic states accessed.

It was the first feature which initially attracted the Southampton group to use

synchrotron radiation because in a number of previous investigations some of

the valence PE bands of the reactive intermediate studied, when recorded with a

inert gas discharge source, showed an intense adiabatic component with very little

intensity in other vibrational components. This is the case for radicals such as

OH, SH, N3 and CH3 O. Such an observation for the first PE band of a reactive

intermediate is particularly disappointing, since measurement of the vibrational

level separations in the ground ionic state is usually one of the main experimental

objectives. Also, the intensity of a PE band from a reactive intermediate, recorded at

a non-resonant photon energy, in a given reaction system may be low and monitoring

it at a photon energy corresponding to an autoionization resonance should enhance

its intensity.

A PE spectrometer has been built in the Southampton group to study reactive

intermediates with synchrotron radiation [24–26]. Three types of spectra can be

recorded with this instrument:(a) angularly resolved PE spectra;

(b) angularly resolved constant-ionic-state (CIS) spectra; a CIS spectrum is obtained by monitoring the intensity of a selected PE band as a function of photon

energy.

(c) threshold PE spectra.



3 Photoionization Studies of Reactive Intermediates of Importance in the Atmosphere



51



Some of the capabilities of this instrument, notably in areas (a) and (b), to

study reactive intermediates of atmospheric importance can be demonstrated by

considering angularly resolved PE spectra and CIS spectra of nitrogen atoms.

Photoionization of atomic nitrogen is an important process in the physics and

chemistry of the upper atmosphere.

Angle resolved PE and CIS measurements were made on N atoms using the

Elettra synchrotron radiation source (near Trieste in Italy) on the beamline BL 4.2R

[27]. The degree of linear polarization (P D 1) of the radiation is well established.

The asymmetry parameter (“) was measured for nitrogen atoms at selected photon

energies and over a photon energy range, by recording CIS spectra at two different

angles ™ (0ı and 54ı 440 ) at each photon energy. The angle 54ı 440 corresponds

to the angle at which the measurement is independent of “, thereby, permitting a

straightforward determination of the relative partial photoionization cross-section.

“ was then determined from “ D R 1, where R D I0 /I54ı 440 is the ratio of the

experimental intensities at these two angles.

Nitrogen atoms were produced by passing a flowing mixture of molecular nitrogen mixed with helium through a microwave discharge in a glass inlet tube attached

to the ionization chamber of the spectrometer. A PE spectrum recorded for discharged nitrogen is shown in Fig. 3.12 (upper panel). This shows the first band of N

atoms at 14.54 eV corresponding to the ionization NC (2s2 2p3 ,3 P) N(2s2 2p4 ,4 S).

The corresponding spectrum obtained with the discharge off is also shown in

Fig. 3.12 (lower panel).

The CIS spectrum of N atoms at ™ D 54ı 440 is shown in Fig. 3.13. This is

the relative cross-section for N atoms plotted as a function of photon energy. This

is obtained by monitoring the intensity of the first N atom PE band at 14.54 eV

as a function of photon energy. The resonances seen in this plot correspond

to excitation to N* (2s2p3 (5 S), np) states from the N(2s2 2p3 ,4 S) ground state

followed by autoionization to the NC (2s2 2p3 ,3 P) state. The resonances seen in this

plot show an asymmetric profile. This is due to interference between the direct

and indirect (resonance plus autoionization) ionization processes and results in a

characteristic Fano profile. Each resonance can be fitted to a Fano profile to obtain

Fano parameters which provide information about the excited state lifetime and

the interaction of the excited state with the continuum. Specifically, the resonance

position En , the natural line-width, €, and line-shape parameters q and ¡2 , and

the discrete oscillator strength f have been determined for the N*(2s2p3(5 S),

np) N(2s2 2p3 ,4 S) autoionization resonances for n D 5–10, and explanations of

the changes in these parameters with principal quantum number have been proposed

[27]. Fitting the resonance energies, En , obtained to the Rydberg energy expression

En D E 1



R

.n



ın /2



(3.17)



where E1 is the ionization energy, R is the Rydberg constant and • is the quantum

defect, gave E1 D 20.39 eV for the (2s) 1 ionization and • D (0.61 ˙ 0.01). This

• value is consistent with the value expected for a p Rydberg orbital of a first row

element.



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

3 The Use of PES to Measure Reaction Rate Coefficients, Using the Atmospherically Relevant Reaction Cl2=+DMS as an Example

Tải bản đầy đủ ngay(0 tr)

×