Strong Electronic Selectivity in the Shallow Core Excitation of the

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The Journal of Physical Chemistry K.F. Alcantara et al., Strong Electronic Selectivity in the Shallow Core Excitation of the CH2Cl2 molecule

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1

Strong Electronic Selectivity in The Shallow Core Excitation of The CH2Cl2 Molecule

2 K. F. Alcantara1, A. H. A. Gomes2, W. Wolff 1, L. Sigaud3, and A. C. F. Santos1*

3 4 1

5 6

2

Instituto de Física, Universidade Federal do Rio de Janeiro - 21941-972 Rio de Janeiro, RJ, Brazil

Instituto de Física Gleb Wataghin , Universidade Estadual de Campinas - 13083-859 Campinas, SP,

7 8 9

Brazil 3

Instituto de Física, Universidade Federal Fluminense - 24210-346 Niterói, RJ, Brasil *

Corresponding author: [email protected]

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The Journal of Physical Chemistry

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K.F. Alcantara et al., Strong Electronic Selectivity in the Shallow Core Excitation of the CH2Cl2 molecule

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract

30 31

The photoexcitation and multiphotoionization of the dichloromethane molecule have been studied for

32

photons with energies from 100 eV up to the Cl 2p edge, using time-of-flight multicoincidence technique

33

and synchrotron radiation. The electronic de-excitation gives rise to one to three electrons and an ionic

34

molecule which decays onto smaller moieties through several fragmentation channels. In order to discern

35

the channels, sets of fragments have been dispersed in time, measured in coincidence, and recorded as a

36

function of the incident photon energy. The chlorine ion, Cl+, has the highest intensity around and above

37

the Cl 2p edge, while the CHnCl+ ion, corresponding to the loss of one neutral chlorine atom, dominates

38

the mass spectra in the valence region. In addition, strong electronic selectivity has been observed for the

39

core-excited molecule.

40 41

Keywords: molecular fragmentation, photoionization, PEPICO, PEPIPICO, CH2Cl2, shallow core

42

excitation

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59

I-Introduction

60 61

Core-level excitation and ionization of molecules is an inviting tool for the study of selective

62

photochemistry. Adjustment of the photon energy allows us to exclusively excite a particular resonance

63

close to a chosen inner-shell edge, giving rise to a selective fragmentation (“molecular knife”)1-3. The

64

ability of breaking one given bond leaving the others unaffected is of particular interest, giving rise to hot

65

states through the excitation a core level, which can cause the ejection of several electrons in a molecule2-

66

3

67

solar irradiation.

. This mechanism has potential influence in the photoionization of atmospheric molecules after soft x-ray

68

There are different mechanisms for producing specific fragmentation of a molecule: site-specific

69

(SE), element-specific (ES) and electronic or resonance-specific (RS). SE excitation involves the choice

70

among atoms of the same element in a distinct chemical environment – “the chemical shift”1-4. ES

71

excitation corresponds to the excitation of electrons from two different atoms. RS (or state-selective)

72

excitation is the excitation of different resonances of the absorption spectrum, corresponding to different

73

bonds in the molecule. Notwithstanding, the selectivity in the excitation process does not guarantee

74

selectivity in the fragmentation pattern. The ability of specificity is determined by the occurrence of fast

75

dissociation in core excited molecules with large lifetimes, causing fragmentation before core hole decay

76

can occur. Thus, the selectivity in the fragmentation routes are attributed to the selected photon

77

wavelength and to the electronic system of the molecular ion originated by the Auger decay.

78

Some papers have been devoted to the fragmentation of the CH2Cl2 molecule, both in the valence

79

and in the inner-shell regions5,6. In a previous work5, measurements of ionic branching ratios for fast

80

proton and VUV photon collisions were carried out. It was shown that the fragmentation pattern for

81

charged products in the proton impact spectra could be compared to the corresponding fragmentation

82

pattern for photon impact. Combining measurements of photon-induced ionic dissociation, x-ray

83

absorption and UV-visible dispersed fluorescence, the dissociation dynamics of ionic and excited neutral

84

fragments of CH2Cl2 , following excitation of Cl 2p electrons to various resonances was investigated6 .

85

Using synchrotron radiation deep core (Cl 1s) fragmentation of the dichloromethane molecule has been

86

described, identifying several singly and multiply charged cationic and anionic species in time of flight

87

mass spectra7.

88

This work describes the valence and shallow core ionization and the selective excitation of the

89

resonances around the Cl 2p edge of the CH2Cl2 molecule. The subjects of interest addressed in this work

90

are: shed some light on the fragmentation dynamics of the shallow core excited CH2Cl2 molecule; learn

91

about the stability of the multiply charged ions; find out whether the fragmentation processes are

92

instantaneous or sequential; ascertain if one can benefit from the initial excitation of a specific resonance

93

in the CH2Cl2 molecule to provoke a particular bond breaking; and determine the final charge state of the

94

core excited/ionized CH2Cl2 molecule after Auger decay3. In addition, it is interesting to compare the

95

photofragmentation pathways for CH2Cl2 with the previously investigated molecules found in the

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literature, CH3Cl8, CHCl39 , CCl410, and CH411, in order to investigate the role of the molecular symmetry,

97

in other words, the role of substituting hydrogen with chlorine on the photofragmentation dynamics.

98 99

II- Experimental Setup

100 101

The experimental apparatus has been described in details elsewhere5, and is only summarized

102

here. Monochromatic synchrotron radiation (E/∆E = 500, for 100 µm slit) from the toroidal grating

103

monochromator (TGM) beamline of the LNLS synchrotron radiation laboratory (Campinas, Brazil), is

104

focused onto an effusive gas jet. The incident photon-energy calibration was accomplished by scanning

105

the monochromator through the Cl L23-edge region, monitoring the total photoion production, and by

106

comparing peak structures in the total ion production to previously measured photoabsorption results6.

107

Throughout the experiment the pressure was maintained around 10-6 Torr. The beam spot size is 2 mm ×

108

500 µm.

109

Electrons and photoions were removed from the interaction region by an intense electrostatic

110

field (750 V/cm) which drives the electrons straight into a microchannel (MCP) detector, and the

111

photoions into a Wiley and McLaren type12 time-of-flight mass spectrometer (TOF) for the mass-to-

112

charge ratio analysis. The Photo-Electron-Photo-Ion Coincidence (PEnPICO) (n= 1, 2, 3) techniques have

113

been used to obtain the TOF spectra, allowing the detection of electrons correlated with one, two, or three

114

photofragments. The coincidence measurements have been executed with the TOF drift tube

115

perpendicular to the plane of polarization of the synchrotron light. The TOF has high efficiency for

116

fragments with energies up to 30 eV and electrons with energies up to 40 eV. In addition, Auger electrons

117

are expected to be emitted more or less isotropically. The high voltage employed on the front of the MCP

118

detector, in chevron configuration, ensures the uniformity of the photoions efficiencies with respect to the

119

ion mass. The number of double coincidence events that falls into the single coincidence spectra, were

120

obtained following the procedures outlined elsewhere5 .

121 122

III- Results and discussion

123 124

In order to study the fragmentation processes following inner-shell excitation and ionization, the

125

total ion yield (TIY) spectrum was obtained around the Cl 2p edge (Figure 1). The TIY spectrum closely

126

resembles the corresponding spectrum of the chloroform molecule (CHCl3)9. At photon energies lower

127

than the Cl 2p ionization potential (IP), conforming to the excitation of a core electron to a virtual orbital,

128

the inner vacancy is filled by means of resonant Auger decays, which usually gives rise to singly ionized

129

states, where the molecule is often unstable and breaks into smaller atomic and molecular fragments. For

130

photon energies above the IP, normal Auger process prevails, producing multiply charged species that are


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even more unstable. The spectrum displays contributions from the underlying continuum due to the direct

132

ionization of the valence levels. The doublet structures with energies 200.7 eV (feature A with width 1.01

133

eV) and 202.3 eV (feature B with width 1.79 eV) in the TIY spectrum, are due to the spin-orbital splitting

134

of the 2p3/2 (L3) and 2p1/2 (L2) levels of chlorine possessing a σ* character, in a diatomic-like picture. The

135

different peak widths may indicate different lifetimes or Franck-Condon factors of the corresponding

136

excited states. Peak A is attributed to a transition from a Cl (2P3/2 → 10a1* - 200.9 eV)6. Peak B is a

137

structure assigned to Cl (2P1/2 → 10a1* - 202 eV) and (2P3/2 → 4b1* - 202.8 eV) transitions. Peak C is

138

allotted to a (2P1/2 → 4b1* - 204 eV) transition. There are two ionization limits due to the spin-orbit

139

splitting of the 2p hole, Cl 2p3/2,1/2 → ∞ (IP), indicated by the vertical lines D (206.4 eV) and E (208 eV)6

140

. After transitions of the excited electron to the lowest Rydberg energy levels, the spectator Auger decay

141

dominates6. On the other hand, for transitions into higher Rydberg levels, the shake up process

142

contributes significantly in resonant Auger decay.

143

144 145

Figure 1 – Total ion yield spectra of CH2Cl2 around the Cl L2,3-edge (full-line) compared to the absorption

146

spectra from K. T. Lu et al. 6 .

147 148

III.1 – PEPICO spectra

149 150

With the objective of thoroughly studying the photofragmentation dynamics, it is advantageous to

151

look at the regions below and above the ionization threshold. Figure 2 shows mass spectra of the CH2Cl2



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after core-excitation, valence and core-ionization. The 160 eV photon energy is enough to open all

153

fragmentation channels associated to the direct single and multiple ionization/excitation of valence-shell

154

electrons, i.e., unoccupied states of low energy of the neutral or ionized CH2Cl2 molecule. For those

155

processes, Auger decay is not energetically allowed and the double ionization of the molecule takes place

156

by shake-off or adiabatic double ionization, also called TS-112,13.

157

Fragments associated with rearrangement reactions – namely HCl+, Cl2+ and H2+ were observable. The

158

formation of Cl2+ is clearly observed only near the Cl 2p resonance. The generation of Cl2+ is possibly due

159

to a direct process in which an excitation to a core-bound state occurs with an accompanying decay to an

160

excited vibrational state that supports the intramolecular atomic rearrangement8. Analogue intramolecular

161

rearrangements associated to photoexcitation have been reported in the formation of F2+ from SiF415, H2+

162

and H3+ from methanol (CH3OH)16, methylamine (CH3NH2) and acetonitrile (CH3CN)17. The C+ cation, a

163

signature of the complete atomization of the molecule, was observed at all photon energies and its relative

164

intensity increases as the photon energy approaches the Cl 2p edge, as already observed for other chlorine

165

containing molecules with a central carbon atom18.

166 167

Figure 2 - Valence (V), core-excitation (CE), and core-ionization (CI) mass spectra of the CH2Cl2

168

molecule.

169

The photoion branching ratios, corrected by the electron detection and ion efficiencies, are

170

presented in Figure 3 as a function of the photon energy and are tabulated in Table 1. For direct valence

171

ionization, the CHnCl+ (n=0,1,2) ion, corresponding to the loss of a chlorine atom, is the most intense

172

fragment ~32 %, followed by the Cl+ ~21 % and CH2+ ~15 % fragments. In the vicinity of the chlorine 2p


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edge, the intensities of the H+ and CHnCl+ ions compete and are the most intense fragments after the Cl+.

174

However, at 207 eV, close to the Cl 2p3/2,1/2 → ∞ transition, the yield of the CHnCl+ fragment presents a

175

minimum. As also observed by Lu et al. 6, a relative gain in the production of CH2+ (associated with a

176

drop in the H+ yield) at the Cl 2p → 10a1* resonance is observed. Lu et al. attributed this enhancement to

177

an ultrafast dissociation, occurring through a strongly repulsive potential curve, which is associated to the

178

spectator electron populating the antibonding orbital. It means, that neutral dissociation of the core-

179

excited molecule comes from the strongly antibonding character of the outer-shell orbital to which the

180

core electron was excited, accompanied by electronic decay of the core-excited fragment. The increase in

181

the bond breaking probability results in an increase in the neutral dissociation of CH2Cl2 to CH2* plus

182

other fragments, where CH2* (Cl 2p3/2-1) is in a core excited state ionizing to CH2+. However, Stolte et

183

al.19 have suggested that the transitions to empty molecular orbitals are dominated by relaxation processes

184

characterized as participator decay and whenever the primary excitation involves Rydberg levels, the

185

core hole relaxes mainly by participator Auger decay, as opposed to spectator as suggested by Lu et al.6.

186

On the other hand, the CH2+ exhibits a maximum near the IP (Rydberg levels) where the orbitals are more

187

diffuse and the spectator Auger decay is more probable to occur19. It should also be mentioned that the

188

energy dependence of the CHn+ (n=0,1,2) fragments are very similar, suggesting that they derive from the

189

many-step neutral H release process observed by Almeida et al.20 for the TMS (Si(CH3)4) molecule.

190

The present mass resolution and excellent signal-to-noise ratio has enabled us to observe a weak

191

presence of some doubly charged fragments (Cl2+ and CHnCl2+), showing the contribution of the electron

192

correlation for direct valence ionization. The molecular dication CHnCl2+ might be associated to excited

193

states with two valence electrons (LVV at Cl 2p edge), where chemical bonding beats the substantial

194

Coulomb repulsion of the charge centers. The CHCl2+ ion is more stable in the linear [H-C-Cl]2+

195

configuration21. As observed in the case of the methyl chloride molecule (CH3Cl)22, the Cl2+ is formed

196

selectively on the A resonance, showing a small drop at the Cl 2p3/2,1/2 → ∞ (IP) and increasing again at

197

the Cl 2p continuum. The observed minima in the Cl2+ (at 207 eV) and CHCl2+ (at 202.8 eV) in the PIY

198

supports the observations of Lu et al.6, who showed that transitions of the Cl 2p electrons to energy levels

199

near the Cl 2p ionization threshold of the CH2Cl2 molecule contribute to a remarkable yield of excited

200

atomic neutral fragments.

201 202 Energy

H+

H 2+

C+

CH+

CH2+

Cl+

C35Cl+

HCCl+

CH2Cl+

CH nCl2+

HCl+

CHnCl2+

Cl2+

Cl2+

103.0

8.73

0.20

2.87

4.74

15.94

20.86

4.59

1.40

33.04

6.62

0.41

0.42

0.19

-

120.0

9.71

0.19

3.15

4.85

14.86

22.28

4.07

1.12

31.98

6.73

0.40

0.40

0.26

-

140.0

9.70

0.20

3.21

4.86

14.37

22.10

4.25

1.10

32.27

6.88

0.40

0.40

0.27

-

160.0

9.71

0.20

3.29

5.05

14.26

22.26

4.26

1.03

31.99

6.93

0.45

0.39

0.18

-

(eV)



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180.0

10.94

0.23

3.90

6.02

15.68

26.32

5.90

1.05

21.01

7.71

0.25

0.47

0.32

0.21

195.0

12.74

0.29

4.47

6.61

15.91

30.81

4.40

1.03

15.39

6.64

0.53

0.54

0.46

0.18

200.7

10.92

0.20

4.38

6.51

22.03

36.97

3.37

0.67

10.52

2.13

0.60

0.41

1.29

-

202.8

12.80

0.23

5.86

8.90

16.49

38.97

3.89

1.17

6.62

2.07

0.94

0.39

0.99

0.67

207.0

13.12

0.33

4.52

5.67

7.66

43.75

5.56

2.02

13.69

0.83

1.02

0.96

0.87

-

215.0

14.06

0.30

4.20

4.73

8.79

43.48

5.71

1.80

13.32

0.35

0.79

1.39

1.07

-

203 204

Table 1 – Photoion branching ratios for the CH2Cl2 molecule as a function of the photon energy.

205

Yield

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

14 20 CH+2 12 H+ 10 15 8 10 + CH 0.2 + 5 H + 0.0 2 C 0 100 120 140 160 180 200 220 100 120 140 160 180 200 220 35 1.5 28 + CHnCl 1.0 21 2+ CHnCl 14 0.5 7 2+ Cl 0.0 0 100 120 140 160 180 200 220 120 180 8 40 + 6 CHnCl2 30 Cl+ + CCl 20 4 + HCl (X5) 4 + 2 HCCl 0 0 100 120 140 160 180 200 220 100 120 140 160 180 200 220

Energy (eV) 206 207

Figure 3 -Partial ion yield of selected CH2Cl2 fragments as a function of the photon energy.

208 209

For evaluation of the specificities in molecular fragmentation process, the asymmetry parameter

210

has been introduced1. Denoting by PIY200.7 and PIY215 the partial ion yields of a given fragment before

211

and after the chlorine 2p edge, the asymmetry parameter, α, for electronic specificity is defined as:

212

α =

PIY 200 .7 − PIY 215 PIY 200 .7 + PIY 215

(1)


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213

This definition of the asymmetry parameter ascertains that if the fragmentation patterns before

214

and after Cl 2p edge are alike, the asymmetry tends to zero. Figure 4 shows the asymmetry parameter (Eq.

215

1) for some fragments of the core-excited CH2Cl2 molecule at 200.7 eV and 215 eV energies. The

216

CHnCl2+ fragment is the one that exhibits larger electronic selectivity, being preferentially formed before

217

the edge (α > 0.7) followed by the doubly charged fragment CHnCl2+ which has α≈ -0.6, being

218

preferentially formed after the edge. Although CHnCl2+ relative intensity in the mass spectra is low (see

219

Table I) and its specificity tends to be hidden by the other fragments. This specificity must be due to the

220

Auger process which results in a doubly charged state where the two holes are localized in the excited Cl

221

atom. This is especially true for a halogen atom, for the two valence electrons, which participate in the

222

primary Auger decay, that could comes not exclusively from bonding σ-like orbitals, but also from lone

223

pair orbitals of the Cl atom. The CH2+ fragment also presents a rather high specificity (α > 0.4), while the

224

C+ and Cl+ fragment are the ones which present almost no specificity (α≈0.02 and -0.08, respectively).

225 226

227 228

Figure 4 - Asymmetry parameter for some fragments of the core-excited CH2Cl2 molecule.

229 230

III.2 –PE2PICO and PE3PICO spectra

231 232

Unstable multiply charged molecular ions are produced due to electron correlation. Because of

233

the Coulomb repulsion between positively - charged, subsequently - produced fragments, the molecule

234

dissociates in a very sudden way. Several fragmentation processes in the core-excited CH2Cl2 molecule,

235

lead to fragmentation into smaller pieces due to the double and triple ionization events originated from a

236

cascade of successive fragmentations, as described by

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238

CH2Cl22+ → m1+ + m2+ + neutrals

(2)

239

CH2Cl23+ → m1+ + m22+ + neutrals

(3)

240 241

The observation of molecular trications is less probable than dications. Due to the intense

242

Coulomb repulsion, nearly all triply charged molecular species are not stable and dissociates ad lib23.

243

Figure 5 shows double-coincidence contours (or islands) in the t2 versus t1 plane for selected fragment

244

pairs of CH2Cl2 taken at 140 eV. The islands formed by fragments carrying Cl atoms come in pairs due to

245

the two chlorine isotopes, possessing as expected, almost identical slopes. Most islands present “cigar”-

246

shaped contours.

247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262

Figure 5- PE2PICO peaks of selected double ionization events after 140 eV photon impact.

263 264

The production of multiply charged species follows the electronic relaxation of a core hole plays

265

an important role in the fragmentation of the molecule. Most of the dissociation channels of CH2Cl22+

266

detected by PE2PICO measurements are associated to the production of Cl+, as expected, since the C-Cl


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267

bond is weaker than the C-H bond. As shown in figure 6, among the main coincidences that have been

268

observed in the PE2PICO spectra, one is associated to a single bond breaking (two-body decay): Cl+ +

269

CH2Cl+. There are also the ones associated to multiple bond breaking: H+ + Cl+, CH2+ + Cl+, CH++ Cl+ ,

270

Cl+ + Cl+, and the H+ + C+ coincidence which corresponds to the full molecular fragmentation.

271

Coincidences associated to triple photoionization Cl2+ + Cl+ and H+ + Cl2+ were also observed. The H+ +

272

Cl+ coincidence dominates the PE2PICO spectra (~ 24-25 % below the Cl 2p edge), increasing in

273

intensity up to ~ 27 % at the Cl (2P1/2 → 10a1*) resonance. In the continuum part of the spectra, an

274

increase of the Cl+ + CCl+ and H+ + Cl2+ branching ratios is observed, which can be ascribed to the normal

275

Auger process. For the yield of the CH2+ + Cl+ coincidence, we notice that the dominant features are the

276

increase in intensity at the Cl (2P3/2 → 10a1*) resonance and at the Cl 2p continuum, which can be related

277

to the resonant and normal Auger processes, respectively. A peculiar behaviour is exhibited by the yield

278

of the Cl2+ + Cl+ coincidence, which decreases above the Cl 2p continuum. This can be tentatively

279

understood in terms of a post-collision interaction (PCI)24, where the core hole is produced for photon

280

energies slightly above the IP. In this picture, the slow photoelectron might be overtaken by a fast Auger

281

electron. As a consequence, the slow-moving electron can be recaptured by the remaining multiply

282

charged ion in a process known as shakedown. Recapture ceases when the photoelectron is fast enough to

283

be overtaken beyond a certain distance from the molecular ion25.

284

It can also be seen from figure 6 that, on the one hand, local maxima are observed in the

285

branching ratios for some ion pairs involving CH+ and CH2+ fragments (CH2+ + Cl+, CH+ + Cl+, H+ +

286

CH+) at the Cl (2P3/2 → 10a1*) resonance (peak A). On the other hand, local minima are observed in the

287

branching ratios of the ion pairs containing one chlorine atom each (Cl+ + CHCl+, Cl+ + CCl+, Cl2+ + Cl+)

288

at this resonance. The coincidence pairs (H+ + Cl+, H+ + CH+, C+ + Cl+, H+ + Cl2+, Cl+ + Cl+) exhibit a

289

maximum at the Cl (2P1/2 → 10a1*) resonance (peak B).

290



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291 292

Figure 6 – Partial ion yield (%) from the PE2PICO spectra of CH2Cl2 fragments as a function of the

293

photon energy. The lines connecting the experimental points were drawn to guide the eyes.

294 295

The slope of the coincidence islands from multi-ion coincidence measurements contains

296

information on the momentum issued in the dynamics of the fragmentation. The many-body reactions can

297

be classified as sequential, where independent two-body dissociation steps take place, and concerted,

298

where two or more bond breakages take place simultaneously. For this reason, the inclination is an

299

important parametric quantity in ascertaining the mechanisms of fragmentation. The momentum p carried

300

by the photoion after fragmentation is a function of the flight time spread (∆t) of a detected fragment

301

through the formula:

302

T (m / q, p) = To (m / q ) ± ∆t

303

∆t =

304

p cosθ qE

(4)

305

where T is the flight time and θ is the angle between the bond axis and the analyzer axis, p is the fragment

306

momentum, E is the electric field, and q the fragment charge state. Mechanisms for three26,27 and four-

307

body28 decay have also been listed.

308

The CH2Cl22+ → Cl+ + CH2Cl+ dissociation channel is a two-body (TB) ion pair process. The

309

momentum conservation law demands anticorrelation between the momenta of CH2Cl+ and Cl+: pCl+ = -


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310

pCH2Cl+ . In the double coincidence spectra, the time of flight of the second ion t2, is usually plotted against

311

the time of flight of the first ion t1. Converting the t2 versus t1 plot into a t2-t1, against t2+t1 plot, transforms

312

the peak into a horizontal section28,29.

313

In sequential reactions, the intermediate molecular ion dissociates in a region far enough from

314

the primary moiety so that the Coulomb interactions among the various fragments can be neglected, and

315

on a time scale longer than the molecular rotation. The Cl+ + CHCl+ + H coincidence is clearly a three –

316

body decay, which may dissociate by either a secondary decay (SD) or a deferred charge separation

317

(DCS) process28. Therefore, due to the limited experimental resolution, the dissociation dynamics cannot

318

be experimentally determined, for both processes give rise to very close numerical slope values. In the

319

SD process, the charge separation takes place in a first stage: CH2Cl22+ → CH2Cl+ + Cl+, releasing an

320

energy U1, followed by a second stage: CH2Cl+ → CHCl+ + H, releasing an energy U2. In this case, the

321

long range Coulomb interaction between the two charged fragments demands several picoseconds before

322

the H atom is released, so that the separation of CH2Cl+ + Cl+ is large enough that the Coulomb

323

interaction is on the order of thermal energies. Neglecting U2, the slope is the ratio of the mass of CHCl+

324

and the sum of the masses of CHCl+ and H: –48/49≈-0.98. On the other hand, in the DCS process, the

325

neutral fragment is released in the first stage: CH2Cl22+→ CHCl22+ + H, followed by the charge separation

326

in the second stage: CHCl22+ → CHCl++Cl+. Neglecting the energy release in the first stage, the expected

327

slope is -1, which is very close to the SD value. As can be seen from Figure 7, the measured slopes are

328

slighter larger than -1, being roughly independent of the photon energy. The small differences between

329

the experimental and theoretical values can be attributed to the kinetic energy of the neutral H fragment.

330

Figure 7 shows the slopes of the contour plots of the PE2PICO spectra as a function of the

331

photon energy. The CH2+ + Cl+ coincidence, evidently a three-body decay, presents a constant average

332

slope. In a three-body decay via DCS, a neutral particle is released in the first step of dissociation,

333

accompanied by a subsequent charge separation:

334

CH2Cl22+ → CH2Cl2+ + Cl…. U1

335

CH2Cl2+ → CH2+ + Cl+ ….. U2

336

Neglecting U1 relative to U2, the momenta of the CH2+ and Cl+ fragments will be anticorrelated and the

337

coincidence island exhibits a slope of -1 similar to the two-body case. In the SD, the observed species are

338

ejected in distinct steps

339

CH2Cl22+ → CH2Cl+ + Cl+…. U1

340

CH2Cl+ → CH2+ + Cl ….. U2

341

(6)

Again, whenever U2 is small compared to U1 the slope will be

slope = −

342

(5)

mCH + 2

mCH + + mCl +

= −0.29

2



Page 14 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

343

The Eland's model27 presents an equation for slope without neglecting the energies in the steps of the

344

process

 mCH + mCl + mCH Cl + 2 2 2 slope = −1 + k 2 ( ) m m  Cl CH 2Cl +

345

  

−1 / 2

−

mCl kmCH Cl + 2

2

346

where k = U 2 U 1 . Considering that the process occurs as described in equation (5), here it only possible

347

to have information on the energy of fragments in the second stage (U2), since none of the fragments

348

liberated in the first step have been measured. Experimentally, the slope value obtained was -0.82 (see

349

Figure 7), corresponding to a value of k = 57.7. This indicates that the energy released in the second step

350

of the process is actually much greater than that released in the first stage, so it makes sense in some cases

351

neglect it. But then the slope is -1, indicating that, although small compared to U2, the U1 energy in this

352

case is not negligible.

353

DCS and SD processes are well-grounded in the case of distinctly sequential steps, giving rise to

354

islands with a parallelogram shape. On the other hand, in the case of non-sequential or concerted

355

dissociation (CD) processes (CH2Cl22+ → CH2+ + Cl+ + Cl) the dispersion of momenta among the species

356

is not uniquely determined. Reactions occurring in a short time, so that the long range interactions

357

between the intermediates cannot be neglected, fall under the category of concerted dissociation. As a

358

consequence, the coincidence island shows on “egg” shape due to the momentum of the third fragment.

359

The “cigar” form of the coincidence islands in Figure 5 paints a picture that CD is not the principal

360

fragmentation process of the CH2Cl2 molecule. On the other hand, the coincidence H+ + Cl+ presents

361

“egg” shape contours which results in a not so well determined slope, as shown in Figure 7, portraying

362

the CD process. The slopes of the C+ + Cl+ coincidence vary with the photon energy from -1 at 100 eV to

363

-0.2 near the Cl 2p edge and then falling to -1.2 after the Cl 2p IP.

364


Page 15 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

365 366

Figure 7 – Slopes of the contour plots of the PE2PICO spectra as a function of the photon energy. The

367

lines represent calculated values assuming a many-body decay model.

368 369

The dynamics and kinematics of a triply charged molecule dissociating into three charged parts +

+

370

A + B + C+ can be determined directly by multicoincidence measurements such as PE3PICO. Figure 8

371

shows the projections of the PE3PICO spectrum of the fragments of the CH2Cl23+ molecule at 215 eV. A

372

small contribution of a quadruple ionization can be observed due to coincidences with the dication Cl2+.

373

However the kinematical data in PE3PICO spectra are not complete, as only information about the

374

projections of momenta in the spectrometer axis is obtained and the neutral fragments are not detected.

375

An exception is the three-body decay H+ + Cl+ + CHCl+. Nonetheless, in many cases important insights

376

about the dynamics of the fragmentation process can be achieved just from visual examination of the

377

projections. For instance, average momenta for the fragments are obtained from the width of the peak.

378

Due to the limited statistics, it was not possible to obtain the momenta and KER for all PE3PICO

379

fragments. At 215 eV, in the H+/C+/Cl+ coincidence, the fragment ions carry 9.2 eV, 7.2 eV, and 2.2 eV,

380

respectively, while for the H+/CH+/Cl+ coincidence, the cations transport 7.1 eV, 5.2 eV, and 2.0,

381

respectively. Adding the values of H+/CH+/Cl+ coincidence it is possible to have the minimum value for

382

the energy released, since the Cl is the heaviest fragment and should carry less energy. Those values are

383

consistent with the ones expected following a pure Coulomb explosion represented by equation

384

U(eV)=14.4q1q2/R(Å), for a typical internuclear distance R≈2Å. The dispersion of initial translational

385

kinetic energies for fragments depends upon the potential energy curves for the associate states. The KER

386

limits range from zero to a maximum value given by the pure Coulomb explosion.



Page 16 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

387

388 389

Figure 8 – Projections of the PE3PICO spectrum of the fragments of the CH2Cl23+ molecule at 215 eV,

390

ordered by the arrival time at the detector.

391 392

Table 2 shows the PIY of the PE3PICO fragments as a function of the photon energy around the

393

Cl 2p edge. Due to the low statistics, the data shown for 190 eV were summed from all spectra taken from

394

100 eV to 190 eV, because behaviour in this region is approximately constant. It can be seen that below

395

the Cl 2p resonance the triply charged CH2Cl23+ molecule decays preferentially to H++ C+ + Cl+ (~31 %)

396

and H++ CH+ + Cl+ (30%). At the Cl 2p resonance and above, the H++ C+ + Cl+ coincidence dominates

397

(~50%) while the H++ CH+ + Cl+ coincidence falls to ~20%. The other coincidences virtually do not

398

depend on the photon energy, with the exceptions of the three-body decay H+ + Cl+ + CHCl+ and the C+ +

399

Cl++ Cl+ coincidence.

400 PE3PICO Fragments Energy (eV)

H+ + C+ + Cl+

H+ + CH+ + Cl+

H+ + Cl+ + Cl+

H+ + Cl+ + +

C+ + Cl+ + Cl+

+

(CCl + CHCl + CH2Cl+ ) 215

50.9 ± 3.6

19.7 ± 1.2

11.0 ± 0.8

21.8 ± 1.0

1.5 ± 0.5

200.7

53.6 ± 9.9

16.9 ± 5.1

11.1 ± 3.9

18.4 ± 7.1

-


Page 17 of 23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

190

31.0 ± 4.9

31.1 ± 5.0

13.3 ± 2.4

24.7 ± 4.0

-

401 402

Table 2 - Partial ion yield (%) from the PE3PICO spectra of CH2Cl2 fragments as a function of the photon

403

energy.

404 405

III. 3- The competition between Coulomb explosion and one particle charge localization

406

When no bound or metastable states of a dication are accessible, the potential curves are purely

407

dissociative. This is due to the fact that the vertical Franck-Condon transitions give rise to dication

408

species whose nuclear configurations are often distant from the separation equilibrium30,31. In this case,

409

the dication dissociates exothermically with charge separation (m2+ → m1+ + m2+), the so-called, in

410

certain contexts, Coulomb explosion, giving rise to an energetic pair of monocations flying apart. On the

411

other hand, when bound states are present, exhibiting potential energy minima, the ion dissociates into a

412

daughter dication plus a neutral fragment (m2+ → m12+ + m2)30,31. The later process is governed by

413

chemical forces originating from the valence electrons. Coulomb explosion processes, which are very

414

usual in small molecules, are weaker in polyatomic molecules like the CH2Cl2, because the Coulomb

415

repulsion becomes feebler due to the larger distances between the positive charges32.

416

Aiming to determine the fractions of the “Coulomb” and chemical forces in the fragmentation of

417

the dication CH2Cl22+, we adopt the analysis described previously33. Let D20 represent the number of

418

doubly charged ions constituted by the sum of all non-symmetric dissociation m2+→m12+ + m2 channels,

419

and D11 the number of dissociative double ionization processes corresponding to the sum of all charge

420

separation processes m2+ → m1+ + m2+ . The fractions for both channels are given respectively as33

D11 D11meas = D11 + D20 D11meas + fi D20meas

421

422 423

(7)

and

D20 D20meas = D D11 + D20 11meas + D20meas fi

(8)

424

where fi = 0.29 is the efficiency for detecting an ion34 and D11meas and D20meas are the raw numbers for m2+

425

→ m1+ + m2+ and m2+ → m12+ + m2, dissociation channels, respectivelly. Figure 9 shows that in the

426

energy range studied in this work there is a strong dominance (~94 %) of the symmetric (A+ + B+)

427

dissociative double ionization for the coincidences presented in Figure 9. The most striking result is the

428

enhancement of the asymmetric (A2+ + B) fragmentation at the Cl (2P3/2 → 10a1*) resonance, mainly due

429

to the increasing contribution of the Cl2+ dication.



Page 18 of 23


430

The generation of dications, in the valence region, takes place via shake-off and two-step

431

autoionization13,14 . In molecules, the electronic decay processes are closely connected to the dissociation.

432

As the ionization limit is reached, an increase in the yield of higher-charged ions is usually observed,

433

ascribable to the normal Auger decay. In fact, above the Cl 2p edge, it is possible to notice a growth in the

434

relative yield of dications.

1.00 2+

0.96

CH2Cl2

---->

+

ABCDE

+

m1 + m2

0.92

fraction

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.88 2+

0.08

CH2Cl2

−−−>

2+

m1 + m 2

0.04 0.00 100

120

140

160

200

210

220

Energy (eV) 435 436

Figure 9 – Relative contributions of the symmetric and asymmetric molecular fragmentation pathways

437

(see text for details) as a function of the photon energy. The labels A, B, C, D, and E have been defined in

438

Figure 1.

439

III.4 – Singly and multiple ionization branching ratios and average charge states

440

Ion yields of the various fragmentation channels expressing the same total ionic charge were

441

summed to yield the fractions of the various charge states of the dissociating molecular ion, as shown in

442

Figure 10. Figure 10 also shows the average recoil ion charge qave of the CH2Cl2 molecule ions from

443

valence to the Cl 2p edge, calculated by weighted average, where the weight assigned to the total ion

444

yield is equal to the recoil ion charge for q up to 3 from the PEnPICo spectra (n=1,2 and 3).

445

The most striking feature in Figure 10 is that, as the Cl 2p edge is reached, it leads to a

446

substantial gain of the relative yields of charge states q=2 and q=3, and consequently qave. As fractional

447

yields are presented, charge states q=1 display a similar relative reduction. This is due to the fact that as

448

the core electron suffers a resonant excitation, the molecule is brought up to a rather unstable potential

449

energy surface. The system then decays prioritarily by ejecting one or several electrons. Therefore the

450

average total charge shows a quick rise with photon energy from the resonance up to the IP.

451


Page 19 of 23


1.4

qave

1.3 1.2

100

1.1

f1

95 90 fi (i=1,2,3) (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

f2

85 80 15 10 5

f3X10

0

100

120

140

160

180

200

220

Energy(eV)

452 453

Figure 10 – Upper graph: Average charge state of the CH2Cl2 molecule. Bottom graph: Fractions of

454

single, double, and triple photoionization of the CH2Cl2 molecule as a function of the photon energy.

455

IV – Conclusions

456 457

We have described charge separation mass spectrometry experiments in the valence and near the

458

Cl 2p edge threshold, far UV and soft x-ray regions, to shed additional light on the photofragmentation

459

dynamics of the CH2Cl2 molecule in the region of the electromagnetic spectrum corresponding to the

460

energetic solar radiation, which are present in the upper levels of the Earth’s atmosphere. The

461

experimental set up was projected to minimize mass and energy discrimination effects, and the accurate

462

determination of branching ratios for more than 30 fragmentation channels was achieved. The

463

determination of the mass, charge, and KER of photoions following inner shell excitation provided

464

complementary information.

465

The ionic fragments produced below and after the Cl 2p excitation are primarily those with

466

charge state +1. We have also specified the electronic specificity and the ionization degree, i.e., the

467

relative shares of the single, double, and triple ionization. A strong electronic-selective fragmentation is

468

observed. By measuring the slopes of different coincidence maps, it is found that double ionization of

469

CH2Cl2 is dominated by stepwise dissociation channels. The fragmentation of the CH2Cl2 molecule near

470

the Cl 2p edge presents common features with other chlorine containing molecules14, which provides key

471

panoramas for understanding the fragmentation of highly chlorinated molecules relevant to stratospheric

472

processes. After excitation of the Cl 2p edge, it is the bond between Cl and the rest of the molecule which

473

is preferentially broken.

474 19 ACS Paragon Plus Environment


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475

Acknowledgements

476

The authors would like to express their gratitude to the staff of the Brazilian Synchrotron National

477

Facility (LNLS) for their valuable help during the course of the experiments. This work was supported in

478

part by LNLS (proposal D05A - TGM - 10651), CNPq, Capes, and FAPERJ.

479 480

References

481

(1) Nagaoka, Shin-ichi; Fukuzawa, H. ; Prumper, G. ; Takemoto, M. ; Takahashi, O. ; Yamaguchi, K.;

482

Kakiuchi, T.; Tabayashi, K. ; Suzuki, I. H. ; Harries, J. R. ; et al, A Study to Control Chemical Reactions

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Using Si:2p Core Ionization: Site-Specific Fragmentation, J. Phys. Chem A, 2011, 115, 8822-8831 .

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(2) Hitchcock, A. P. ; in Chemical Applications of Synchrotron Radiation, Sham, T. K. ; ed. Part I:

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Dynamics and VUV Spectroscopy, Advanced Series in Physical Chemistry, Vol. 12A, World Scientific,

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Singapore, 2002, 154-227.

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(3) Nenner I. and Morin, P. ; VUV and Soft X-Ray Photoionization. Edited by Uwe Becker and David A.

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(4) Miron, C. ; Simon, M. ; Leclercq, N. ; Hansen, D. L. ; and Morin, P.; Site-Selective Photochemistry of

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Core Excited Molecules: Role of the Internal Energy, Phys. Rev. Lett. 1998, 81, 4104-4107

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CCl4 Molecule, J. Elect. Spectr. 2007, 156, 236-240

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Si 2p edge: a New TOFMS Study with Improved Mass Resolution, J. Elect. Spec. 2001, 114-116, 115-

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Coincidence Technique PEPIPICO, Mol. Phys. , 1987, 61 , 725-745.

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573 574 22 ACS Paragon Plus Environment

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TOC GRAPHIC


Strong Electronic Selectivity in the Shallow Core Excitation of the

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