Partial and differential electron impact ionization cross sections

of acetylene

S. Feil, K. Głuch, S. Matt, P. Scheier and T. Märk

Institut für Ionenphysik

Universität Innsbruck

A-6020 Innsbruck

LEIF – Meeting, Belfast 2003

Introduction

Diploma thesis about:

Physical and technological basics

of Inductively coupled Vacuum Plasma Spraying

(IVPS)

At the Plansee Company in Reutte in Tirol

Hochleistungswerkstoffe

• About me:

My job at Plansee

I) Characterisation of the machine:I) Power measurements

II) Temperatur measurements (in the spot)

III) Particle velocity measurements

IV) Flame temperature measurements

II) Development of new warmstrenthened materialsI) Re10W (mixed crystal hardening)

II) Re3Hf (reference)

III) Re3HfN (dispersion hardening)

IV) Re3HfC (dispersion hardening)

Hochleistungswerkstoffe

Mass spectrometry of molecules and clusters

In April 2003 I started PhD.

… on a double focussing mass spectrometer

Work partially supported by the FWF, ÖAW and ÖNB, Wien, Austria. We also thank the European Commission, Brussels.

…our group

Why C2H2 ?

Importance of electron collisional properties of acetylene for several reasons:

• edge plasmas in fusion reactors (wall-plasma-interaction)• present in planetary and cometary atmospheres

• concentration of C2H2 in the atmosphere of earth is expected to nearly double by the year 2030 due to the increased use of automobiles

• C2H2 is a very simple molecule among hydrocarbons

• radiation chemistry• ………

What was done? Partial cross sections for electron impact ionization of C2H2 were measured from threshold to 900eV

Ion kinetic energy distributions were deduced applying a deflection field method

Determination of cross sections that are differential with respect to the initial energy of the ion

Considering discrimination highly energetic fragment ions could be measured with a double focussing mass spectrometer in good agreement to specially dedicated instruments

Our apparatus

Two sector field MS:

NIER Type ion source

deflectorplates

y

z

x

B-field E-field

electron beam

detector

Discrepancies in the literature for cross sections of fragment ions

Specially dedicated instruments

Correction of data due to kinetic energy effectsa) integration of beam profileb) determination of discrimination in the ion source

-60 -40 -20 0

0

2

4

6

8

10

12

sign

al (

kHz)

z - voltage

C2H

2

+

CH+

Technique for Ekin determination

Measurement: z – profile

What is a z – profile?

electron beam

deflector plates

-60 -40 -20 0

0

2

4

6

8

10

12

sign

al (

kHz)

z - voltage

C2H

2

+

CH+

Work schedule

Smoothing (with gauss)then differentiate

distribution function

Ekin = c×Uz2

c from parent ions

Include discrimination

Measurement

z - profile

Data analysis

Experiment

Example: CH+ out of C2H2

Ion beam profile:

• splits the molecule

C2H2 + e CH+ + CH + 2e

But:

C2H2 + e C2H2++ + 3e

same m/q ratio-80 -70 -60 -50 -40 -30 -20 -10 0

0

2

4

6

8

10

12

ion

yiel

d (k

Hz)

z - voltage (V)

electron energy:

36eV 80eV 100eV 120eV 200eV

CH+

Mass spectrum of C2H2

More than 50% of m/q ratio 13 is C2H2++

Example: CH+ out of C2H2

Measurement and gauss-smoothed curve

Differentiated, squared and rescaled

-80 -60 -40 -20 0

0

2

4

6

8

10

12

sig

na

l (kH

z)

z - Voltage (V)

electron energy was 100eV

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

1.2

gauss3 gauss2 gauss1 sum

dI/

dz

Energy (meV)

Example: CH+ out of C2H2

Corrected due to the discrimination

Doing this analysis and finally integrate thekinetic energy distribution

0 1 2 3 4

0.0

0.5

1.0

1.5

2.0

2.5

3.0 mean energy of about 1.5eV ("process 3") mean energy of about 0.4eV ("process 2") mean energy of about 0.1eV ("process 1") kinetic energy distribution

dI/d

z

Energy (eV)

One can get partial cross sections of ions butwith more information

Final result: CH+ out of C2H2

One can see the 3 different reaction channelsdependent on the electron energy

Partial cross section CH+ & C2H2++

100

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5 mean energy of 1.5eV mean energy of 0.4eV mean energy of 0.1eV partial cross section Tian et al

cro

ss s

ecti

on

(10

-17 c

m2 )

electron energy (eV)

Identification of different channels

100

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5 mean energy of 1.5eV mean energy of 0.4eV mean energy of 0.1eV partial cross section Tian et al

cro

ss s

ecti

on

(10

-17 c

m2 )

electron energy (eV)

Compare the red process with the ionization energy of C2H2++:

Appearance of this reaction channel at ~ 36eV

Partial cross section CH+ & C2H2++

Ionization Energy of C2H2++

One can deduce that the second (red) process comes purely from the doubly charged acetylene

C2H2++ can`t be neglected !

Stability of this di-cation?

check a decay reaction of C2H2++

(with MIKE technique)

Further investigations(C2H2

++ C2H+ + H+)

… with MIKE scan technique (m1 m2 + m3)

Mass analysed Ion Kinetic Energy

y

z

x

B-field E-field

electron beam

detector

Selecting a mass

Scanning the electricsector

fragpar

fragparparfrag zm

mzEE 0*

Identification:

Further investigations(C2H2

++ C2H+ + H+)

2

1

2

321

21

22

16

E

E

mmq

VmqKER

960 965 970 975 980 985 990 995 1000

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

sign

al (

arb.

uni

ts)

ESF (V)

C2H+ is at 982.7 V

The kinetic energy of C2H+ is then ~ 150meV.

2E

The fragment C2H+ at Efrag* = 982.7V

= 3.88eV

Conclusion• it is possible to determine partial cross sections in good agreement with results obtained by using specially dedicated instruments

• The doubly charged fraction in the mass spectrum of C2H2 can`t be neglected

• With the present method it is possible to identify the different channels of the reaction processes

• The cross section of CH+ could be deduced by subtracting the „red process“ from the partial cross section

Final result: H+ out of C2H2

One can see the 2 different reaction channelsdependent on the electron energy

100 1000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

cros

s se

ctio

n (1

0-17 cm

2 )

electron energy (eV)

mean energy of 0.7eV (process 1) mean energy of 3eV (process 2) partial cross section Tian et al

H+ out of C2H2

0 1000 2000 3000 4000 5000

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

dI/d

z

energy (meV)

mean energy of 0.6eV (process 1) mean energy of 3eV (process 2) sum

H+ out of C2H

2

electron energy was 120eV

The NIER – Type source

Example: CH+ out of C2H2

Measurement and gauss-smoothed curve

Differentiated, squared and rescaled

Example: CH+ out of C2H2

Corrected due to the discrimination function

Doing this analysis with different electron energies and finally summing all graphs up

One can get partial cross sections of ions butwith more information