Forschergruppe Laboratory Astrophysics. + + + + + + + + Interstellar Molecules

ExperimentsBasic Theory

Modeling

Observations

Forschergruppe Laboratory Astrophysics

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Interstellar Molecules

Schlemmer, Cook , Saykally, Science 265, 1686 (1994)Cook, Schlemmer, Saykally, Nature 380, 227 (1996)Cook, Schlemmer, et al. J.Phys.Chem. A102, 1465 (1998)


Modeling

Observations


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

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

T = 10 K

n = 104 cm-3

Physics and Chemistry of the Interstellar Medium

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


Modeling

Observations


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

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

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.

Temperature Dependence of Ion-Molecule Reactions

M.A. Smith (1994)

qkL 2

Langevin Rate Coefficient:

Details of Potential Energy Surface

Reactants

Association

Products

Transition

StateE

kT

Rotational Energy, Zero Point Energy and Fine Structure Energy

Low Temperature Collisions

in Flow Systems and Traps

(state-of-the-art experiments)

k(T)

AB

T0, p0

z, t

[B]T

const.

Pulsed Uniform Supersonic Expansion

A + B Products

k = ( [B])-1

CRESU: Cinétique de Réaction en Ecoulement Supersonique Uniforme

B. Rowe (Rennes), I. Sims (Rennes), M. Smith (Tucson)

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

22-Pole Low Temperature Ion Trap

primary ions detector

rf IIrf I

sapphire 22-pole

cold head 10 K

shield 50 K

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

Method:

Electrodynamical Trapping

Quadrupole 22-Pole

RF Ion TrapMechanical Model

Trajectories of ions in 22-pole trap

f= 17MHz

V0=20V

+ m=20u

× m=2u

f= 17MHz

V0=20V

+ m=20u

× m=2u

Trajectories of ions in 22-pole trap

r / r0

0.0 0.5 1.0

V*

/ V*(

r 0)

0.0

0.5

1.0

Quadrupole

Octopole

22-Pole

Effective Potential

Low Temperature 22 Pole Ion Trap

Low Temperature 22 Pole Trap

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.


Modeling

Observations


H3+

H2+

H+

He+

H2

He

C+

H2

CO

C

H2

CH+

CH2+

N+

H2, e NH3

CH3+

H2CN+

HCO+

CO

HCN,HNC

H2cosmicrays


+

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




Deuteration of H3+

[HD] = 1.8x1012 cm-3

T = 10 K

H3+ H2D+HD D2H+HD D3

+HD

k1 k2 k3

H3+

H2+

H+

He+

H2

He

C+

H2

CO

C

H2

CH+

CH2+

N+

H2, e NH3

CH3+

H2CN+

HCO+

CO

HCN,HNC

H2cosmicrays


+

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




Example I:Negative Temperature

Dependence

NH3+ + H2 NH4

+ + H

Gas Phase Formation of Ammonia

NHn+ + H2 NHn+1

+ + H

n = 0,1,2,3

Implications of a barrier along reaction path

Arrhenius:k(T) = <v> = A exp (-EA/kT)

EAReactants

Products

k

T / K10 100 1000

Experiment

TheoreticalPrediction

Negative Temperature Dependence

NH3+ + H2 NH4

+ + H

Herbst et al. J.Chem.Phys. 1991

Herbst et al. J.Chem.Phys. 1991

kcal/mol

Lowest energy path forNH3

+ + H2 NH4+ + H

van der Waals binding

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

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

H3+

H2+

H+

He+

H2

He

C+

H2

CO

C

H2

CH+

CH2+

N+

H2, e NH3

CH3+

H2CN+

HCO+

CO

HCN,HNC

H2cosmicrays


+

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




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

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

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+

~ ~

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

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

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

Documents

Forschergruppe Laboratory Astrophysics. + + + + + + + + Interstellar Molecules