Recoil Momentum of Molecular Ions in Collisions ofAr6+ + N2 at Energies below 300 eV/u
Y Inoue, K Ishii* and H Ogawa*
Graduate school of Humanities and Sciences, Nara Women's University, Nara, 630-8506, Japan*Dept. of Physics, Nara Women's University, Nara, 630-8506, Japan(bay.inoue@cc.nara-wu.ac.jp)
ABSTRACTTo clarify collision energy dependence of recoil momentum for molecular ions after electrontransfer, we have measured recoil momentum of non-dissociated molecular ions for single
electron capture (SC) collisions of Ar6+ with N2 at collision energies from 7.5 to 240 eV/u.
Collision energy dependence of the experimental recoil momentum was compared with thoseof the theoretical one calculated using an deflection function with the polarization potential. We
have obtained fairly good agreement between experimental and calculated results.

INTRODUCTIONRESULTS and DISCUSSION

 For the molecular-fragmentation, it is understood that the diatomic
molecular target undergoes Coulomb explosion after the electron
capture and then the two fragments fly in opposite directions with thekinetic energies given by the dissociation energy [1].
 In low energies below 1 keV/u, the recoil momenta of non-dissociatedmolecular ions depend on collision energy [2,3].
 In this work, in order to investigate collision energy dependence of recoil
momentum, we have measured recoil momenta of the target for Ar6++N2
collision system at energies from 7.5 to 240 eV/u.

EXPERIMENTAL SETUP

We have measured
time difference between
projectile
andtarget ions.

Digitizer [ Yokogawa WE7116 ]T=0Trigger ( T )V1

PSD1 ( V1 )MCP–PSD

V2
PSD2 ( V2 )V3PSD3 ( V3 )V4

PSD4 ( V4 )TA

SEMB ( TB )TB

Measured processes are given as follows:

Target gas nozzle

Ar 6 + + N 2 → Ar 5 + ( nl ) + N 2+ + Q1

Electrostatic Parallel
Plate Analyzer

Time of flight
Analyzer→ Ar 4 + * ( nl n ′l ′ ) + ( N 2 ) 2 + + Q 2

SEMBTime of flight
Analyzer

MCI from Mini-EBIS→ Ar 4 + + (N 2 ) 2 + + e − + Q 3

SEMAVVV→ Ar 4 + + N + + N + + e − + Q 4

Figure 1 : Schematic diagram of experimental
setup.

NEWTON DIAGRAM

θlabΘCMVr’vf

Fragmentation

before the collision : E CM

Figure 2 : The correlation diagram of the velocity
vectors in the laboratory and the C-M frame.

′ )after the collision : E CM + Q ( = E CM

METHOD of CALCULATIONThe experimental recoil momentum↓ sin Θ CM ≈ Θ CM′ Θ CM
E CMbbdr′ r
1 − b 2 r 2 − V (r ) ECM

PROGRESS REPORTΘ CM : scattering angle in the C - M frameµ : reduced mass

2v r′ : relative velocity

In the case of V ( r ) in powers of r : V ( r ) = C r sb : impact parameterr : internucle ar distance′ )-1
Θ CM ∝ (b s ECMq : charge state of incident ion

α : polarizabi lity of N2(as the internuclear distance of closest approach)R = ( 2 qt + q ) It
by the Classical Over Barrier (COB) model[5]with Atomic Spectra Database of NIST[6]The relations between b and R′ )b 2 = R 2 (1 − V (R ) ECM

In the polarization potential : s =4

V ( r ) = − (α q 2 ) ( 2 r 4 )In the Coulomb potential : s =1V (r ) = C Rb ≅ R′ ) -1
Θ CM ∝ ( R E CM( E′ R )−1

CMAr6+ + N2 → Ar4+ + (N2)2+: 5.5 %→ Ar4+ + N+ + N+
→ Ar3+ + (N2)3+→ Ar4+ + N2+ + N+ + e- : 35.9 %→ Ar2+ + (N2)4+→ Ar4+ + N2+ + N2+ + 2e- : 58.6 %It : t - th ionization potential

The internuclear distance RP⊥ ∝In order to clarify the charge-asymmetry effect between the far and nearfragment ions from incident MCI beam axis, we have measured molecularfragmentation of N2 molecule in electron capture collisions of Ar6+ ions in thecollision energy from 30 to 150 eV/u and have determined branching ratios foreach reaction channel.

TOFA (100 nsec)∞rFigure 3 : The scattering of aparticle in the field of a center
potential[4].

dϕbb 2 V (r )= ± 2 1− 2 −′drECM
rrr

To understand more precisely, coincidence measurements of TOF spectrawith energy gain spectra of the projectile after the collision is necessary.

Θ CMThe experimental recoil momentum (ref. [4])Θ CM = π − 2 ∫In order to clarify the collision energy dependence of recoil momenta for
non-dissociated molecular ions, we have…
 measured recoil momenta of N22+ and N2+ for SC collisions.
 compared with the theoretical calculation with the Coulomb and thepolarization potential for the energy dependence.
It is concluded that the polarization potential
plays a crucial role in low energy region.

FUTURE PLAN

P⊥ = µ v r′ sin Θ CMP⊥ ∝Figure 5 : The experimental and thetheoretical energy dependence of recoilmomenta for N22+ (a) and N2+ (b). The
experimental results are good agreementwith the theoretical one using the polarization
potential rather than the Coulomb potential atenergies below 30 eV/u.

SUMMARY

Vt depend on ECM
Vf & Vs independent of ECM

vt
v1

Figure 4 : TOF spectra of molecular ions andfragment ions in collisions of Ar 6++N2 atenergies from 7.5 to 240 eV/u. The peak
positions of non-dissociated N2+ and N22+
molecular ions depend on the collision energy.

Velocity of Molecular ions: Vｔ
Velocities of Fragment ion: Vf & Vs
Dissociation Velocities : V1 & V2
V1 , V2 >> Vt
Vf=Vt+V1 ≅ V1 , Vf=Vt+V2 ≅ V2vsv2

Qi : reaction energy[′ )b 2 = R 2 1 + ( α q 2 ) ( 2 R 4 E CM(a) 30 eV/u]2⎛⎡
α q 2 ⎤ ⎞⎟′ ⎢1 +
Θ CM ∝ ⎜ R 4 E CM⎥ ⎟4⎜′ ⎦2 R E CM⎣⎝⎠⎛P⊥ ∝ ⎜⎜⎝′E CM: 11.6 %TOFB (100 nsec)(b) 150 eV/u
TOFB (100 nsec)

Figure 6 : Branching ratio of N2 in double electron capture collisions of Ar6+ for double electron capture
process at energies 30 eV/u (a) and 150 eV/u (b).

−1⎡
(α q 2 ) ⎤R 4 ⎢1 +⎥′ )⎦( 2 R 4 E CM⎣

Ar6+ + N2 → Ar4+ + (N2)2+: 49.8 %→ Ar4+ + N+ + N+
: 18.4 %→ Ar3+ + (N2)3+→ Ar4+ + N2+ + N+ + e- : 19.1 %TOFA (100 nsec)

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