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ExperimentsBasic Theory
Modeling
Observations
Forschergruppe Laboratory Astrophysics
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+
+ +
+
+
+
+
+
Interstellar Molecules
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Schlemmer, Cook , Saykally, Science 265, 1686 (1994)Cook, Schlemmer, Saykally, Nature 380, 227 (1996)Cook, Schlemmer, et al. J.Phys.Chem. A102, 1465 (1998)
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ExperimentsBasic Theory
Modeling
Observations
Forschergruppe Laboratory Astrophysics
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The Astronomer's Periodic Table
H O C N
Mg
Fe
Si Ar S
Ne
He
Cosmic Abundanceof some elements
Element Abundance
hydrogen (H) 1.000.000helium 80.147oxygen 739carbon 445neon 138nitrogen 91magnesium 40Silcon 37Sulfur 19
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INTERSTELLAR MEDIUMH:1 He: 0.1
O: C: N = 8:3:1 (0.001)
gas phase solid state99 % 1 %
T = 10 K= 102-106 cm-3
119 molecules
grains (< 0.1um)cold molecular cloudsT = 10 K
= 10-9cm-3
8 molecules
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T = 10 K
n = 104 cm-3
Physics and Chemistry of the Interstellar Medium
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Helsinki Winter SchoolThe Role of Gas Phase
Ion – Molecule Reactions-
Laboratory Studies
Stephan Schlemmer
WHAT? Kinetics of ion-molecule reactionsradiative association (one example)
WHY? Identification of Species(Column Densities)Formation and Destruction
HOW? Experimental Techniques (Laboratory work)
December 13, 2006
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ExperimentsBasic Theory
Modeling
Observations
Forschergruppe Laboratory Astrophysics
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Importance of Ion-Molecule Reactions
E. Herbst, The Physics and Chemistry of Interstellar MCs (1993)
Arrhenius:k(T) = <v> = A exp (-EA/kT)
Neutral-Neutral ReactionsA 10-11 cm3/s
EA 2000 KTMC = 10 K
Ion-Molecule Reactions 10-7 cm3/s > A > 10-9 cm3/s
EA 0 K
Cosmic RayIonization
Ion-MoleculeReactions
Recombination
EAReactants
Products
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Ion – induced dipole interactionlong range forces
p
b
Kinetic energyE = p2/2
Angular momentumL = b p = l h
Centrifugal barrierV = + L2/(2r2)
V = - q / r4+++
+
---
2 3 4 5 6 7 8
-0.2
0.0
0.2
~ - 1/r4
l = 18
V /
eV
r / Angstrom
l = 15
~ 1/r2
= 2u, = 0.8 A3
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Ion – induced dipole interactionRate of Reaction
p
b
Kinetic energyE = p2/2
Angular momentumL = b p = l h
Cross section (E):
(E) = 2 * b db = bmax
2
Rate coefficient:
k = < v>T
ion-induced dipole:
kL = const.
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Temperature Dependence of Ion-Molecule Reactions
M.A. Smith (1994)
qkL 2
Langevin Rate Coefficient:
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Details of Potential Energy Surface
Reactants
Association
Products
Transition
StateE
kT
Rotational Energy, Zero Point Energy and Fine Structure Energy
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Low Temperature Collisions
in Flow Systems and Traps
(state-of-the-art experiments)
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k(T)
AB
T0, p0
z, t
[B]T
const.
Pulsed Uniform Supersonic Expansion
A + B Products
k = ( [B])-1
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CRESU: Cinétique de Réaction en Ecoulement Supersonique Uniforme
B. Rowe (Rennes), I. Sims (Rennes), M. Smith (Tucson)
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ln [A]
t
z/v ~ 0.1/500 s ~ 200 µs
[B] 1015 cm-3
k 510-12 cm3/s
Pulsed Uniform Supersonic Expansion
A + B Product
d[A]/dt = - k [A] [B]
k = ( [B])-1
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22-Pole Low Temperature Ion Trap
primary ions detector
rf IIrf I
sapphire 22-pole
cold head 10 K
shield 50 K
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Trap Experiment
A + B Products
d[A]/dt = - k [A] [B]
k = ( [B])-1
1 ms 100 s
108 cm-3 [B] 1015 cm-3
k 10-17 cm3/s
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Method:
Electrodynamical Trapping
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Quadrupole 22-Pole
RF Ion TrapMechanical Model
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Trajectories of ions in 22-pole trap
f= 17MHz
V0=20V
+ m=20u
× m=2u
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f= 17MHz
V0=20V
+ m=20u
× m=2u
Trajectories of ions in 22-pole trap
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r / r0
0.0 0.5 1.0
V*
/ V*(
r 0)
0.0
0.5
1.0
Quadrupole
Octopole
22-Pole
Effective Potential
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Low Temperature 22 Pole Ion Trap
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Low Temperature 22 Pole Trap
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H3+
H2+
H+
He+
H2
He
C+
H2
CO
C
H2
CH+
CH2+
N+
H2, e NH3
CH3+
H2CN+
HCO+
CO
HCN,HNC
H2cosmicrays
HCO+, N2H+, HCS+,H2CN+, H3O+, HCO2
+
CO
CS
H2O
N2
HCN
CO2
C2H7O+
CH3OH2+
CH3CO+
CH5+
CH3NH3+
CH3CNH+
C2H5CNH+
CH3HC3N+ CH3C3N
C2H5CN
CH3CN
CH3NH2
CH4
CH2CO
CH3OH
C2H5OHCH3OCH3
e
e
e
e
e
e
e
e
e
O
N
CH3OH
H2O
CO
H2
NH3
HCN
CH3CN
HC3Nhttp://sift.hyperlink.cz/interste.htm
Initial reactions in dense interstellar clouds
D. Smith and P. Spanel, Mass Spectrometry Reviews, 14 (1995) 255-278.
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ExperimentsBasic Theory
Modeling
Observations
Forschergruppe Laboratory Astrophysics
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H3+
H2+
H+
He+
H2
He
C+
H2
CO
C
H2
CH+
CH2+
N+
H2, e NH3
CH3+
H2CN+
HCO+
CO
HCN,HNC
H2cosmicrays
HCO+, N2H+, HCS+,H2CN+, H3O+, HCO2
+
CO
CS
H2O
N2
HCN
CO2
C2H7O+
CH3OH2+
CH3CO+
CH5+
CH3NH3+
CH3CNH+
C2H5CNH+
CH3HC3N+ CH3C3N
C2H5CN
CH3CN
CH3NH2
CH4
CH2CO
CH3OH
C2H5OHCH3OCH3
e
e
e
e
e
e
e
e
e
O
N
CH3OH
H2O
CO
H2
NH3
HCN
CH3CN
HC3Nhttp://sift.hyperlink.cz/interste.htm
Initial reactions in dense interstellar clouds
D. Smith and P. Spanel, Mass Spectrometry Reviews, 14 (1995) 255-278.
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Deuteration of H3+
[HD] = 1.8x1012 cm-3
T = 10 K
H3+ H2D+HD D2H+HD D3
+HD
k1 k2 k3
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H3+
H2+
H+
He+
H2
He
C+
H2
CO
C
H2
CH+
CH2+
N+
H2, e NH3
CH3+
H2CN+
HCO+
CO
HCN,HNC
H2cosmicrays
HCO+, N2H+, HCS+,H2CN+, H3O+, HCO2
+
CO
CS
H2O
N2
HCN
CO2
C2H7O+
CH3OH2+
CH3CO+
CH5+
CH3NH3+
CH3CNH+
C2H5CNH+
CH3HC3N+ CH3C3N
C2H5CN
CH3CN
CH3NH2
CH4
CH2CO
CH3OH
C2H5OHCH3OCH3
e
e
e
e
e
e
e
e
e
O
N
CH3OH
H2O
CO
H2
NH3
HCN
CH3CN
HC3Nhttp://sift.hyperlink.cz/interste.htm
Initial reactions in dense interstellar clouds
D. Smith and P. Spanel, Mass Spectrometry Reviews, 14 (1995) 255-278.
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Example I:Negative Temperature
Dependence
NH3+ + H2 NH4
+ + H
Gas Phase Formation of Ammonia
NHn+ + H2 NHn+1
+ + H
n = 0,1,2,3
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Implications of a barrier along reaction path
Arrhenius:k(T) = <v> = A exp (-EA/kT)
EAReactants
Products
k
T / K10 100 1000
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Experiment
TheoreticalPrediction
Negative Temperature Dependence
NH3+ + H2 NH4
+ + H
Herbst et al. J.Chem.Phys. 1991
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Herbst et al. J.Chem.Phys. 1991
kcal/mol
Lowest energy path forNH3
+ + H2 NH4+ + H
van der Waals binding
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Negative Temperature Dependence
NH3+ + H2 NH4
+ + H
k << kL 1 in 10000 collisions leads to reaction
Turnover at 100 K
barrier height 4.8 kcal/mol (2400 K)
Tunneling is a dominant mechanism at low temperatures
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Example II: Radiative Association
CH3+ + H2O CH5O+ + h
A. Luca, D. Voulot, and D. Gerlich, WDS'02 Proceedings of Contributed Papers, Part II, Safrankova (ed), Matfyzpress, 294-300 (2002)
TP5
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H3+
H2+
H+
He+
H2
He
C+
H2
CO
C
H2
CH+
CH2+
N+
H2, e NH3
CH3+
H2CN+
HCO+
CO
HCN,HNC
H2cosmicrays
HCO+, N2H+, HCS+,H2CN+, H3O+, HCO2
+
CO
CS
H2O
N2
HCN
CO2
C2H7O+
CH3OH2+
CH3CO+
CH5+
CH3NH3+
CH3CNH+
C2H5CNH+
CH3HC3N+ CH3C3N
C2H5CN
CH3CN
CH3NH2
CH4
CH2CO
CH3OH
C2H5OHCH3OCH3
e
e
e
e
e
e
e
e
e
O
N
CH3OH
H2O
CO
H2
NH3
HCN
CH3CN
HC3Nhttp://sift.hyperlink.cz/interste.htm
Initial reactions in dense interstellar clouds
D. Smith and P. Spanel, Mass Spectrometry Reviews, 14 (1995) 255-278.
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Ion DeflectorPulsed Valve
22-Pole Ion Trap
CH3+ CH3
+
He buffer
Experimental Setup
15±5 K
H2O seeded in Xe
Effective temperature
Teff = 50±30 KStagnation pressure: 365 mbarH2O partial pressure: 26mbar at 295 KH2O 7%Xe 93 %
Instantaneouce densities:[H2O] = 7·108 cm-3
[Xe] = 1·1010 cm-3
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0,0 0,5 1,0 1,5 2,010
-1
100
101
102
103
CH3
+
CH5O
+
Ions
per
filli
ng
t / s
Results: Formation of protonated methanol
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Radiative Association
h
1rad
kc
1Dis
Ecoll kT
C2H2+ + H2
C2H4+
C2H3+ + H
Reactants Products
?
hIR
47.5 meV
-2600 meV
AB+ + C
ABC+
A + BC+
~ ~
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1011
1012
1013
1014
1015
1016
1017
10-13
10-12
10-11
10-10
10-9
10-8
CRESU23 - 72 K
CH+
3 + H
2O + He CH
5O
+ + He
CH+
3 + H
2O CH
5O
+ + h
k* /
cm3 s-1
[He] / cm-3
Density Dependence of Association Reaction
Radiative kr
Ternaryk3
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10 100
10-13
10-12
10-11
10-10
10-9
10-8
[bat83]
MB22PT
[her85]
statistical theories
CH+
3 + H2O CH5O+ + h
k r / c
m3 s-1
T / K
Radiative Association: kr
Comparison to Statistical Theories
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Helsinki Winter SchoolThe Role of Gas Phase
Ion – Molecule Reactions-
Laboratory Studies
Stephan Schlemmer
WHAT? Kinetics of ion-molecule reactionsradiative association (one example)
WHY? Identification of Species(Column Densities)Formation and Destruction
HOW? Experimental Techniques (Laboratory work)
December 13, 2006