Orientation Effects in Ion–Molecule Collisions Bhas Bapat E-mail: [email protected] At NCAMP-21 Jan 2017 Orientation Effects in Ion–Molecule Collisions
Orientation Effects in Ion–Molecule Collisions
Bhas Bapat
E-mail: [email protected]
At NCAMP-21Jan 2017
Orientation Effects in Ion–Molecule Collisions
Collaborators
Deepak Sharma (IISER-Pune)Pragya Bhatt, C P Safvan (IUAC-Delhi)Ajit Kumar (JMI-Delhi)
Orientation Effects in Ion–Molecule Collisions 1
Introduction
• Molecules are not spherically symmetric, so the outcome of a collision between an ionand a molecule should depend on the relative angle between the molecular axes andthe projectile direction
• Diatomic molecule aligned perpendicularto the incident projectile:
? projectile interacts mainly with theelectron cloud of one atom
? low-charge molecular ions expected
• Diatomic molecule aligned parallel tothe incident projectile:
? projectile interacts equally with theelectron cloud of both the atoms
? high-charge molecular ions expected
B
A
A
B
Orientation Effects in Ion–Molecule Collisions 2
Introduction
• For a diatomic molecule there can beAlignment and Orientation
? Alignment implies axis parallel w.r.tthe projectile
? Orientation implies alignment plusspecific pointing
• Homonuclear diatomics:? only alignment is meaningful? Outcome of a collision: anisotropy
possible, but forward-backwardasymmetry not possible
• Heteronuclear diatomics:? orientation is meaningful? Outcome of a collision: there may be
anisotropy as well as asymmetry
a bB
A
v0
Alignment
Orientation
Orientation Effects in Ion–Molecule Collisions 3
A simple theoretical model
Wohrer and Watson (1993 Phys Rev A)
• Assume independent atoms
• Add cross-sections for multipleionisation of the two atoms inperpendicular and parallel orientations
Wang (1989 Phys Rev A)
• Added scattering amplitudes instead ofcross-sections (end-view, along the projectile)
Orientation Effects in Ion–Molecule Collisions 4
A simple theoretical model
Wohrer and Watson (1993 Phys Rev A)
• Ionization cross sections are calculated in the independent electron approximation
• Predicted different cross-sections for Ok+2 (k = 1 ... 12)
Orientation Effects in Ion–Molecule Collisions 5
A simple theoretical model
Caraby et al. (1997 Phys Rev A)
• Applied the Wohrer–Watson model toCOq+ fragmentation and
• Predicted a symmetric distributionaround 90◦ w.r.t. projectile
dσ
dΩ=
σ
4π[1 + βP2(cos θ)]
• β is a measure of enhancementor depletion of yield along theperpendicular direction relative to ananisotropic distribution
Orientation Effects in Ion–Molecule Collisions 6
Experimental Strategy
Difficulty:Molecules in an ensemble (e.g. in a cell or a jet) arerandomly oriented. How do we determine orientationeffects in the interaction?
Way out:Difficulty can be overcome in some processes– viz. multiple ionisation leading to dissociation
Orientation Effects in Ion–Molecule Collisions 7
Experimental Strategy
• Under single collision condition
? direction of fragments can be relatedto the molecule’s orientation
? need mass and velocity vector of eachfragment for every collision
• Assumptions
? collision times are shorter thanrotational times
? initial momentum of the parentmolecule is much smaller than thefragment momentum
? For ionic fragmentationlab-frame ≡ molecular frame
a
P(b,m,v,q)
B
Anominal beam axis
a B+
A+nominal beam axis
pB+
pA+
Orientation Effects in Ion–Molecule Collisions 8
Experimental Strategy
α Project
ile axi
sSpectrometer axis
Target Gas Jete.g. CO
C+
O+
• measure three momentum componentsof each ion for each event
• obtain ejection angle w.r.t. projectileaxis event-by-event
• extract dissociation probability as afunction of angle from the list modedata
• in the present case the angle is referredto ~PC
Orientation Effects in Ion–Molecule Collisions 9
Measurement of ion momentum
• spatial and temporal dispersion of charged particles in a uniform electric field
• simultaneous measurement of flight-time and spatial spread
• requires an internally cold, well-localised source of particles
• For pz = 0
t0 = [8s/E ]1/2 (m/q)1/2
• For ~p 6= ~0
pz ≈ (t − t0)qEpx = m(x − x0)/tpy = m(y − y0)/t
Orientation Effects in Ion–Molecule Collisions 10
Detecting multiple ions in coincidence
• Aim: measurement of momenta offragments in reactions of type
AB→ ABn+ → Am+ + B(n−m)+
• Strategy: Record both ions arising fromone event, build a correlation map
• list mode record of all events
t = 0 tA+
tB+
[tAB+
]
tB+
tB++
tA+
AB++
low
high
ToF ion1
ToF
ion2
tA+
Orientation Effects in Ion–Molecule Collisions 11
Collisions with CO+445V
–1709V –2250V
–342V
12
ElectronDetector
InteractionZone
IonDetector
1013819
Projectiles used:
p+ 25–200 keV q/v=1. . . 0.35Xe9+ 450 keV q/v = 24
Orientation Effects in Ion–Molecule Collisions 12
Collisions with CO
Results for two distinct perturbations:
p+ 50 keV q/v ≈ 0.7Xe9+ 450 keV q/v = 24
Orientation Effects in Ion–Molecule Collisions 13
Collisions with CO
p+(50 keV) + CO
C2+:O+
C2+:O2+
C+:O+
600 800 1000 1200 1400
ToF-Hit1 [ns]
600
800
1000
1200
1400To
F-H
it2 [n
s]
0
5000
10000
15000
20000
Orientation Effects in Ion–Molecule Collisions 14
Collisions with CO
p+(50 keV) + CO→ CO3+
0
200
400
600
800
1000
1200
1400
0 30 60 90 120 150 180
Inte
nsity
[arb
.uni
t]
Angle [deg]
50 keV p+
N(θ)sin(θ)
N(θ) = N0[1 + β1 P1(cos θ) + β2 P2(cos θ)] sin θ
Orientation Effects in Ion–Molecule Collisions 15
Collisions with CO
p+(50 keV) + CO→ CO4+
0 10 20 30 40 50 60 70 80 90
100
0 30 60 90 120 150 180
Inte
nsity
[arb
.uni
t]
Angle [deg]
50 keV p+
N(θ)sin(θ)
N(θ) = N0[1 + β1 P1(cos θ) + β2 P2(cos θ)] sin θ
Orientation Effects in Ion–Molecule Collisions 16
Collisions with CO
Xe9+(450 keV) + CO
C2+:O+
C2+:O2+
C+:O+
600 800 1000 1200 1400
ToF-Hit1 [ns]
600
800
1000
1200
1400To
F-H
it2 [n
s]
0
500
1000
1500
2000
2500
3000
Orientation Effects in Ion–Molecule Collisions 17
Collisions with CO
Xe9+(450 keV) + CO→ CO3+
0 100 200 300 400 500 600 700 800 900
1000
0 30 60 90 120 150 180
Inte
nsity
[arb
.uni
t]
Angle [deg]
50 keV Xe9+
N(θ)sin(θ)
N(θ) = N0[1 + β1 P1(cos θ) + β2 P2(cos θ)] sin θ
Orientation Effects in Ion–Molecule Collisions 18
Collisions with CO
Xe9+(450 keV) + CO→ CO4+
0
100
200
300
400
500
600
700
800
0 30 60 90 120 150 180
Inte
nsity
[arb
.uni
t]
Angle [deg]
50 keV Xe9+
N(θ)sin(θ)
N(θ) = N0[1 + β1 P1(cos θ) + β2 P2(cos θ)] sin θ
Orientation Effects in Ion–Molecule Collisions 19
Collisions with CO
• Xe9+ on CO (q/v = 24)
? Nearly isotropic fragmentation for CO2+, CO3+ and CO4+ channels? β1 ≈ 0,β2 ≈ 0
• p+ on CO (q/v = 0.7)
? CO2+ fragmentation : isotropic fragmentationβ1 ≈ 0,β2 ≈ 0
? CO3+ fragmentation: strong orientation dependenceβ2 = 0.63± 0.01, β1 = 0.14± 0.001
? CO4+ fragmentation: stronger orientation dependenceβ2 = 1.22± 0.03, β1 = 0.33± 0.02
Orientation Effects in Ion–Molecule Collisions 20
Projectile Velocity Dependence
Results for same projectile at different velocities:
p+ 25 keV–200 keV (q/v= 1. . . 0.35 au)
Orientation Effects in Ion–Molecule Collisions 21
Projectile Velocity Dependence – CO3+
0
50
100
150
200
250
300
350
400
0 30 60 90 120 150 180
Inte
nsity
[arb
.uni
t]
Angle [deg]
25 keV p+
N(θ)sin(θ)
0
200
400
600
800
1000
1200
1400
0 30 60 90 120 150 180
Inte
nsity
[arb
.uni
t]
Angle [deg]
50 keV p+
N(θ)sin(θ)
0
50
100
150
200
250
300
350
400
0 30 60 90 120 150 180
Inte
nsity
[arb
.uni
t]
Angle [deg]
100 keV p+
N(θ)sin(θ)
0
50
100
150
200
250
0 30 60 90 120 150 180
Inte
nsity
[arb
.uni
t]
Angle [deg]
200 keV p+
N(θ)sin(θ)
Orientation Effects in Ion–Molecule Collisions 22
Projectile Velocity Dependence – p+ on CO
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
β 1,β
2
(1/v) 10-6 [m/sec]-1
β2 (obs)linear-fitβ1 (obs)linear-fit
Anisotropy, as well as forward–backward asymmetry, increase with deceasing velocity.
Orientation Effects in Ion–Molecule Collisions 23
Previous Experimental Result : CO
Siegmann et al. 2002 Phys Rev A
• Dissociation of COn+ (D+ at 100 keVon CO)
• Near-isotropic distribution for n = 2
• Anisotropic distribution for n > 3
• Slight asymmetry for n > 4
• Observations fitted to the StatisticalEnergy Deposition model
Orientation Effects in Ion–Molecule Collisions 24
Another Result
Mizuno 2007 JPCS : separating capture and loss channels
Figure 3. Production cross sections of the ion pair (C+,O+) as a function of the molecularorientation angle θ measured for 1e-loss (upper) and 1e-capture (lower) collisions. Solid lines arethe fitting results with Eq. (3) using anisotropy parameters β as denoted. Dashed lines showsin θ for isotropic distributions.
Figure 4. The same as in Fig. 3 but for(C2+,O+).
Figure 5. The same as in Fig. 3 but for(C+,O2+).
175
.
Figure 3. Production cross sections of the ion pair (C+,O+) as a function of the molecularorientation angle θ measured for 1e-loss (upper) and 1e-capture (lower) collisions. Solid lines arethe fitting results with Eq. (3) using anisotropy parameters β as denoted. Dashed lines showsin θ for isotropic distributions.
Figure 4. The same as in Fig. 3 but for(C2+,O+).
Figure 5. The same as in Fig. 3 but for(C+,O2+).
175
.
Figure 3. Production cross sections of the ion pair (C+,O+) as a function of the molecularorientation angle θ measured for 1e-loss (upper) and 1e-capture (lower) collisions. Solid lines arethe fitting results with Eq. (3) using anisotropy parameters β as denoted. Dashed lines showsin θ for isotropic distributions.
Figure 4. The same as in Fig. 3 but for(C2+,O+).
Figure 5. The same as in Fig. 3 but for(C+,O2+).
175
.
C+:O+ C2+:O+ C+:O2+
Orientation Effects in Ion–Molecule Collisions 25
Previous Experimental Result : N2
Siegmann et al 2003 NIM(B):
of p+ q up to 12 were observed; in the slower
collisions fragment ion pairs with p+ q 6 10 oc-curred.
For each fragmentation channel the relativecross-section was measured as a function of the
angle h between the molecular axis and the pro-jectile beam. As an example, Fig. 2 shows the
distributions obtained for sixfold and tenfold
ionization of O2 in collisions with 360 keV Xe18þ
and 5.9 MeV/u Xe18þ. The (O2)6þ spectrum con-
tains contributions from the O3þ +O3þ and the
O2þ +O4þ channels; the (O2)10þ data contain con-
tributions from O5þ +O5þ and O4þ +O6þ coinci-
dences. Note, that in all cases the symmetric
charge distribution is clearly more abundant than
the asymmetric one. An isotropic fragmentation
would result in an angular distribution propor-
tional to sin h (dotted curves in Fig. 2). The spectrameasured by 360 keV Xe18þ impact are in good
agreement with an isotropic distribution. The sameholds for all observed Coulomb fragmentation
processes of O2 and N2 in collisions with highly
charged Xe ions in the 0.2–0.3 a.u. velocity range.
In collisions with 5.9 MeV/u Xe18þ, however, the
spectra for the highest degrees of ionization clearly
deviate from the isotropic distribution: the highestdegrees of ionization are more easily achieved if
the molecular axis is aligned along the projectile
beam than for the perpendicular orientation (Figs.
2 and 3). Similar effects have been found in colli-
sions with 100–300 keV Hþ and Heþ at consider-
ably lower degrees of ionization [4,6].
The origin of this orientation effect is directly
related to the anisotropy of the electron densitydistribution in the target molecule. Qualitatively,
the achievable degree of ionization is correlated to
the energy deposited by the impinging ion which
depends on the integrated electron density along
the projectile path. Thus higher degrees of ion-
ization can be expected for trajectories which are
close and parallel to the molecular axis, whereas
less energy is deposited in case of a perpendicularoriented molecule. An extended version [2,7] of the
statistical energy deposition model (SED) [8,9]
0 45 90 135 1800.0
0.2
0.4
0.6
0.8
1.0
1.2
dn
/dθ
0 45 90 135 1800.0
0.2
0.4
0.6
0.8
1.0
1.2
dn
/dθ
0 45 90 135 180
0 45 90 135 180
(N2)6+ (N2)
6+
(N2)10+ (N2)
10+
Fig. 3. Orientation dependence of the cross-section for (N2)6þ and (N2)
10þ ionization channels in collisions with 5.9 MeV/u Xe18þ (left)
and Xe43þ (right), (- - -) SED-UCA model, (� � �) isotropic distribution.
B. Siegmann et al. / Nucl. Instr. and Meth. in Phys. Res. B 205 (2003) 629–633 631
5.9 MeV/u Xe18+ 5.9 MeV/u Xe43+
Orientation Effects in Ion–Molecule Collisions 26
Previous Experimental Result : O2
Siegmann et al 2003 NIM(B):
360 keV Xe18+ 5.9 MeV/u Xe18+
Orientation Effects in Ion–Molecule Collisions 27
Alignment and Orientation q and v dependence
• Some of the previous results are in contrast to our observations
• There has been a lack of clarity about the distinction between orientation effects andalignment effects
• The SED explains some features for homonuclear diatomic molecules
Orientation Effects in Ion–Molecule Collisions 28
Alignment and Orientation – Summary
• Our observations
? Low projectile charge leads to greater anisotropy (for the same velocity)? High velocity projectile leads to greater anisotropy (for the same charge)? High q/v leads to greater degree of ionisation
• Explanation: Owing to the Coulombrepulsion for a given impact parameter,larger q/v implies a larger distance ofclosest approach dmin
dmin =b
[1− 2qU(dmin)mv2
]1/2
• For large dmin, the molecule appears tobe nearly structureless – hence weakorientation effects
• However, even for large dmin, a largeq/v ion can cause multiple ionisation
Orientation Effects in Ion–Molecule Collisions 29