ELECTRON SPECTROSCOPY ELECTRON SPECTROSCOPY AND MASS SPECTROMETRIC AND MASS SPECTROMETRIC

STUDY OF PENNING STUDY OF PENNING IONIZATION OF MOLECULESIONIZATION OF MOLECULES

F. VecchiocattiviF. Vecchiocattivi Dipartimento d’Ingegneria Civile ed Ambientale Dipartimento d’Ingegneria Civile ed Ambientale Università degli Studi di PerugiaUniversità degli Studi di PerugiaPerugia - ItalyPerugia - Italy

… … it’s elementary, my dear Watson.it’s elementary, my dear Watson.

The Penning ionization process …The Penning ionization process …

(1894-1953)

F. M. Penning,Naturwissenschaften 15, 818 (1927)

(1894-1953)

A* + M A* + M (A…M)* (A…M)* (A…M) (A…M)++ + e + e--

A + MA + M++

A + MA + M++ + e + e--

A* + MA* + M

quadrupole massquadrupole mass

spectrometerspectrometer

ions

target moleculetarget moleculebeam sourcebeam source

electronelectronenergyenergy

analyzeranalyzer

rare gasrare gasbeam sourcebeam source

electronelectronbombardmentbombardment

exciterexciter

E

electrons

beam crossing volumebeam crossing volume

Perugia molecular beam Perugia molecular beam apparatus for Penning apparatus for Penning ionization studiesionization studies

He*(2He*(211S, 2S, 233S) + Ar S) + Ar He + ArHe + Ar++((22PP3/23/2, , 22PP1/21/2) + e) + e--

0.01 0.1 1

1

10

100

Collision energy (eV)

Ioni

zatio

n cr

oss

sect

ion

(Å2 ) Ar+

HeAr+

He*(2He*(233S) + Ar S) + Ar He + Ar He + Ar++((22PP1/21/2) + e) + e--

He*(2He*(211S) + Ar S) + Ar He + Ar He + Ar++((22PP3/23/2) + e) + e--

He*(2He*(211S) + Ar S) + Ar He + Ar He + Ar++((22PP1/21/2) + e) + e--

He*(2He*(233S) + Ar S) + Ar He + Ar He + Ar++((22PP3/23/2) + e) + e-- 3,3/23,3/2

3,1/23,1/2

1,3/21,3/2

1,1/21,1/2

3,3/2

3,1/2= 2.1±0.2

3.6 3.8 4.0 4.2 4.4 4.6 4.8

Ar+(2P1/2

)

Ar+(2P1/2

)

Ar+(2P3/2

)

Ar+(2P3/2

)

He*(21S)-Ar

He*(23S)-Ar

Electron energy (eV)

B. Brunetti, P. Candori, S. Falcinelli, B. Lescop, G. Liuti, F. Pirani, F. Vecchiocattivi (2005) to be published

He*(He*(33S) + Ar S) + Ar [He-Ar]* [He-Ar]* [He-M][He-M]++ + e + e--

He*(He*(33S)S) ArAr

HeHeArAr++

He + ArHe + Ar++((22PP1/21/2))

He + ArHe + Ar++((22PP3/23/2))

The relative population of the J=1/2 and J=3/2 states of the Ar+ product practically reflects the character of the final He-Ar+ state.

Ne*(Ne*(33PP2,02,0) + Ar ) + Ar [Ne-Ar]* [Ne-Ar]* [Ne-M][Ne-M]++ + e + e--

Ne*(Ne*(33PP2,02,0))

ArAr++

NeNe++ ArAr

NeNe ArAr++

e-

e-

symmetry symmetry

NeNe++ ArAr

NeNe ArAr++

e-

e-

symmetry symmetry

1.8 2.0 2.2 2.4 2.6 2.8 3.0

NeNe**((33PP00))

NeNe**((33PP22))

NeNe**((33PP00))

NeNe**((33PP22))

KrKr++((22PP3/23/2))KrKr

++((22PP1/21/2))

EEcollcoll=0.050 eV=0.050 eV

electron energy (eV)

B. Brunetti, P. Candori, S. Falcinelli, B. Lescop, G. Liuti, F. Pirani, F. Vecchiocattivi (2005) to be published

0.0 0.1 0.2 0.3 0.4 0.50

1

2

3

4

5

6

J=0 (present experiment)J=0 (present experiment) J=0 (Hotop and coworkers)J=0 (Hotop and coworkers)

J=2 (present experiment)J=2 (present experiment) J=2 (Hotop and coworkers)J=2 (Hotop and coworkers)

Qj,3

/2/Q

j,1/2

collision energy (eV)

NeNe**((33PP22))

NeNe**((33PP00))

Penning ionization occurs through the Penning ionization occurs through the transfer of an outer shell electron from the transfer of an outer shell electron from the target particle into the inner shell vacancy of target particle into the inner shell vacancy of the metastable atom. the metastable atom.

The process is therefore governed by the The process is therefore governed by the mutual orientation of atomic orbitals.mutual orientation of atomic orbitals.

In the case of atom-molecule systems, this In the case of atom-molecule systems, this also implies a strong effect of the also implies a strong effect of the orientation of the target molecule.orientation of the target molecule.

+ -hexapole filter

CH3Cl beam

rotatable plate

Ar* beam

high voltage

Ion detector

Ar* + CHAr* + CH33Cl Cl CH CH33ClCl++ + Ar + e + Ar + e--

H.Ohoyama, H.Kawaguchi, M.Yamato, T.Kasai, B.G.Brunetti, F.Vecchiocattivi, Chem.Phys.Lett. 313, 484 (1999).

H. Ohoyama, M.Yamato, S. Okada, T. Kasai, B. G. Brunetti, F. Vecchiocattivi, Phys.Chem.Chem.Phys. 3, 3598 (2001).

B. G. Brunetti, P. Candori, S. Falcinelli, T. Kasai, H. Ohoyama, F. Vecchiocattivi, Phys.Chem.Chem.Phys. 3, 807 (2001).

V. Aquilanti, F. Pirani, D. Cappelletti, F. Vecchiocattivi, T. Kasai, in: “Theory of chemical reaction dynamics”, Kluwer Academic, the Netherlands, p.243 (2004).

V.Aquilanti, M. Bartolomei, F. Pirani, D. Cappelletti, F. Vecchiocattivi, Y. Shimizu, T. Kasai. Phys.Chem.Chem.Phys. 7, 291 (2005)

ArAr**

0-1 +1

cos ()

CHCH33 end end

Cl endCl end

Ar* CHAr* CH33ClCl

cos(cos()=0)=0

cos(cos()=-1)=-1

cos(cos()=+1)=+1

stereo-opacitystereo-opacity

ArAr**

0.8

1.2

Rg* + NRg* + N22O O [Rg…N [Rg…N22OO++] + e] + e--

[Rg…N[Rg…N22OO++] ] N N22OO++

NONO++ + N + N

OO++ + N + N22

RgNRgN22OO++

Rg = He, NeRg = He, Ne

tot

Cro

ss

Se

ctio

n (

Cro

ss

Se

ctio

n (

ÅÅ22 ))

Collision Energy (eV)Collision Energy (eV)0.1 0.2

0.1

1

10 N2O+

NO+

O+

NeN2O+

Ne* + NNe* + N22OO

He*(23S, 21S)-N2O

0.06

0.08

0.10

0.12

Collision Energy (e

V)

LUMOLUMO HOMOHOMO

NitrogenNitrogen““lone pair”lone pair”

OxygenOxygen““lone pair”lone pair”

Molecular orbitals of NMolecular orbitals of N22OO

NN22OO++ ion ionin the ground in the ground 22 state state

N N O

He*

NN22OO++ ion ionin the excited in the excited 22 state state

N N OHe*

He*(2He*(211S) + NS) + N22OO

He*He*…… NNO NNO

NNO NNO …… He* He*

Ne*(Ne*(33P) + HP) + H22O O Ne + HNe + H22OO++ + e + e--

He*(He*(33S) + HS) + H22O O He + HHe + H22OO++ + e + e-- 77.9%77.9%

He + OHHe + OH++ + H + e + H + e-- 17.9%17.9%

HeHHeH++ + OH + e + OH + e-- 0.8% 0.8%He + OH + HHe + OH + H++ + e + e-- 3.2% 3.2%

HeHeOO++ + H + H22 + e + e-- 0.2% 0.2%

1b1b11 12.62 eV12.62 eV

3a3a1113.84 eV13.84 eV

1b1b22 17.18 eV17.18 eV

Adiabatic Ionization PotentialAdiabatic Ionization Potential

He(He(33S) 19.82 eVS) 19.82 eV

Ne(Ne(33PP22) 16.62 eV) 16.62 eV

1b1

3a1 1b2

(2p non bonding orbital)

(sp2 lone pair orbital)(sp bonding orbitals)

1 2 3 40 5 6 7 8 eV

He*(He*(33S) + HS) + H22OO

B B 22BB22 (b (b22-1-1))

A A 22AA11 (a (a11-1-1))

X X 22BB11 (b (b11-1-1))

~~

~~

~~

Electron energy

1b1

3a1 1b2

(2p non bonding orbital)

(sp2 lone pair orbital)(sp bonding orbitals)

1 2 3 40 5 6 7 8 eV

NNe*(e*(33PP22) + H) + H22OO

A A 22AA11 (a (a11-1-1))

X X 22BB11 (b (b11-1-1))

~~

~~

Electron energy

1b1

3a1 1b2

(2p non bonding orbital)

(sp2 lone pair orbital)(sp bonding orbitals)

0,03 0,1 0,230

40

50

60

Ne*-H2O

ion

iza

tion

cro

ss s

ect

ion

(Å

2)

collision energy (eV)

B. Brunetti, P. Candori, S. Falcinelli, B. Lescop, G. Liuti, D. Malfatti, F. Pirani, F. Vecchiocattivi (2005) to be published

1b1

(2p non bonding orbital)

3a1

(sp2 lone pair orbital)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

Electron Energy (eV)

~~X (X (22BB11))

~~A (A (22AA11))

Photoionization Photoionization electron spectrum, electron spectrum, energy scaledenergy scaled

Ne* + HNe* + H22OO

B. Brunetti, P. Candori, S. Falcinelli, B. Lescop, G. Liuti, D. Malfatti, F. Pirani, F. Vecchiocattivi (2005) to be published

Ne* + HNe* + H22OO

The Penning ionization of molecules The Penning ionization of molecules strongly depends on the orientation of the strongly depends on the orientation of the molecule with respect to the approach molecule with respect to the approach direction of the metastable atom. direction of the metastable atom.

The orientation not only affects the ionization The orientation not only affects the ionization probability, but also the specific reaction probability, but also the specific reaction following following the ionization event (dissociation, the ionization event (dissociation, rearrangement, etc.) rearrangement, etc.)

Optical Potential Model (H. Bethe, 1940):Optical Potential Model (H. Bethe, 1940):

and therefore the phase shift is also complexand therefore the phase shift is also complex

== + + ii

The Potential is assumed to be complexThe Potential is assumed to be complex

W(R) = V(R) – W(R) = V(R) – ii/2 /2 (R)(R)

I f A B( ) ( ) ( ) ( ) 2 2 2

Ak

Bk

0

0

( ) ( ) ( ) (cos )

( ) ( ) [ cos( )] (cos )

1

22 1 2

1

22 1 1 2

2

2

e sin P

e P

Qk

2

0

( ) ( ) cos( )]v [ e

22 1 1 22

( ) ( ) ]v [ e

k

20

2 1 1 4

Differential Cross Section:Differential Cross Section:

Integral Cross Section:Integral Cross Section:

Total Ionization Cross Section:Total Ionization Cross Section:

B. Brunetti, F. Vecchiocattivi, B. Brunetti, F. Vecchiocattivi, Current Topic on Ion Chemistry and PhysicsCurrent Topic on Ion Chemistry and Physics , , edited by C.Y. Ng, T. Baer, I. Powis (John Wiley & Sons Ltd, New York, 1993), p. 359edited by C.Y. Ng, T. Baer, I. Powis (John Wiley & Sons Ltd, New York, 1993), p. 359

J. Chem. Phys. 95, 1801 (1991)J. Chem. Phys. 95, 1801 (1991)

Integral scattering Integral scattering cross sectioncross section

Differential elastic Differential elastic cross sectioncross section Total ionization Total ionization

cross sectioncross section

Dipartimento d’Ingegneria Dipartimento d’Ingegneria Civile ed AmbientaleCivile ed Ambientale

F. BiondiniF. BiondiniP. CandoriP. CandoriS. FalcinelliS. Falcinelli

G. LiutiG. LiutiD.MalfattiD.Malfatti

Dipartimento di ChimicaDipartimento di Chimica

V. AquilantiV. AquilantiB. BrunettiB. Brunetti

F. de AngelisF. de AngelisF. PiraniF. Pirani

F. TarantelliF. Tarantelli