Desirable situation

- well characterised closed- shell reaction partners

- room temperature (activa- tion energy)

- not too short timescale

- high density of reactants and products

- conditions (p,T) easy to accomplish

- theory easy to handle

Aptitude of interstellar reactions to experimental investigations

Reactions in the ISM

- odd species (radicals, ions long unsaturated chains C-C)

- low T (10-50K in dark clouds)

- Fast, barrierless reactions

- high vacuum (1-106 cm3)

- theoretical investigations challenging

The Astronomer's Periodic Table

H O C N

Mg

Fe

Si Ar S

Ne

He

Cosmic Abundanceof some elements

Element Abundance (relative)hydrogen (H) 1.000.000helium 80.147oxygen 739carbon 445neon 138nitrogen 91magnesium 40Silcon 37Sulfur 19

Abundance of elements in the ISM

Radicals abundant

Small molecules predominant

H, C, O, N, S dominate,metals rare

2 atoms 3 atoms 4 atoms 5 atoms 6 atoms 7 atoms 8+ atoms

AlF PN C3 OCS c-C3H C5 C5H C6H CH3C3N AlCl SO C2H SO2 l-C3H C4H C5O CH2CHCN HCOOCH3 C2 SO+ C2O c-SiC2 C3N C4Si C2H4 CH3C2H CH3COOH CH SiN C2S CO2 C3O l-C3H2 CH3CN HC5N C7H CH+ SiO CH2 NH2 C3S c-C3H2 CH3NC HCOCH3 H2C6 CN SiS HCN H3

+ C2H2 CH2CN CH3OH NH2 CH3 CH2OHCHO CO HF HCO CH2D+ CH4 CH3SH c-C2H4O CH2CHCHO CO+ SH HCO+ HCCN HC3N HC3NH+ CH2CHOH CH3C4H CP FeO HCS+ HCNH+ HC2NC HC2CHO C6H CH3CH2 CN CS HOC+ HNCO HCOOH HCONH2 (CH3)2O CSi H2O HNCS H2CHN l-H2C4 CH3CH2OH HCl H2S HOCO+ H2C2O C5N HC7N H2 HNC H2CO H2NCN C8H KCl HNO H2CN HNC3 CH3C5N NH MgCN H2CS SiH4 (CH3)2CO NO MgNC H3O+ H2COH+ NH2 CH2COOH NS N2H+ NH3 C3H5CHO

NaCl N2O HC9N OH NaCN HC11N

Dominating species H2.

Shielded from UV light, ionisation by cosmic radiation.

Rich chemistry, molecules with long carbon chains and functional groups

Destruction of molecules by

- reaction with radicals: R + X products - ionisation by cosmic radiation and dissociative recombination

AB + X+ AB+ + X AB+ + e- A + B

- ion-neutral reactions AB + C+ products

Molecules in dark clouds

H2/H ratio about 1.

UV light can penetrate.

CO formation by: C+ + OH CO + H+

Destroying of molecules by UV radiation possible

Molecules in diffuse clouds

Neutral-neutral reactions between closed shell molecules ?

- Relatively high activation energy - not feasible at low temperatures !

Neutral-radical and radical-radical reactions

- no activation barrier, feasible down to very low temperatures.

Ion-electron, ion-ion and ion-neutral reactions

- mostly no activation energy.

Important reactions in the ISM

Interstellar or at least very good vacuum has to be achieved. Gas phase, surface reactions

Low temperatures - more difficult

Can we not simply measure at high temperatures and extrapolate ?

A

A

Ek(T) Aexp

RT

E 1ln k ln A

R T

Plot ln k versus 1/T

(Arrhenius plot)

should be linear

often misleading !

Meaurement of interstellar reactions

k /

10-1

1 cm

3 m

olec

-1 s

ec-1

3

4

5

6

7

103/(T/K)

4.03.32.01.0

1000 500 300

T/K

CN + C2H6 products

103 / (T/K)

10.05.03.31.0

k /

cm3

mo

lec-1

se

c-1

10-12

10-11

10-101000 200300

T/K100

CN + C2H6 products

10

3/ (T/K)

50.020.010.01.0

k / cm

3 molec

-1 sec

-1

10-18

10-17

10-16

10-15

10-14

10-13

10-12

10-11

10-10

10-91000 50100

T/K20

CN + C2H6 products

103/ (T/K)

50.020.010.01.0

k /

cm3

mol

ec-1

sec

-1

10-18

10-17

10-16

10-15

10-14

10-13

10-12

10-11

10-10

10-91000 50100

T/K

20

CN + C2H6 products

+ Unstable species (e.g radicals, ions ) can survive

+ No third-body processes

- Probe molecules have to be evaporated into the vacuum

- Rotationally and vibrationally hot species produced

- Low probe densities

Measurements in high vacuum

+ High density of probe species

+ Thermal equilibrium with environment

- Third body processes important

- Radicals and most ions rapidly destroyed in most environments.

- Conditions largely irrelevant for interstellar medium.

Measurements in condensed phase

Expansion through nozzle into vacuum

No heat transfer from gas to environment adiabatic process

In real gases cooling with expansion (intermolecular forces)

Particles with low transversal and high longitudinal cooling

Additionally, longitudinal (in direction of expansion) uniformising ov velocity through collisions in the orifice. Expansion through an adiabatic nozzle

Combinaton of the advantages: Adiabatic

expansion

Cloud formation through adiabatic cooling

Non-supersonic and supersonic velocity distribution

Translational energy: continuum of energies, cooling throughcollisions very efficient. T (translation) = several K

Rotational energy: energy quanta, cooling through

collisions fairly efficient. T (rotation) = 30-90 K

Vibrational energy: energy quanta, cooling throughcollisions very inefficient. T (vibration) > 100 K

Energy in supersonic beams

Supersonic velocities

For studying of reactions one lets two supersonic beams cross

Beam 2Beam 1

Interactionzone

v1v2

vR

2 2 2

R 1 2 1 2v v v 2v v cos

To study, e. g. the following reaction

C + NO CN + O

Supersonic velocities

Relative kinetic energy

2 2

1 2 1 2(v v 2v v cos )2

vCvNO

vR

Centre ofmass

C + NO CN + O

(CN) kin (educts)

CN

1v 2 ( G E )

m

vO

vCN

Supersonic velocities

min (22.5°)

VBC

VA

VR

P = 10-6 mbar

PV1

PV2

266 nm, 10 Hz

BC :O2

NOCxHy

A : C, Si, Al, Ti, Cr...

Ablation laser

:

Molecularreactant source:

Atom source:Molecular source:BC: O2, C2H2, C2H4…

ET = ½(vA2+vBC

2–2vAvBCcos)

Atom source:A: C, B

C + C2H2 C3H + H800-2200 ms-1

800-1200 ms-1

ET = 0.4-25 kJ mol-1

VUV-LIFC(3PJ), H(2S1/2)CO(X1+), O(1D2)

Schematics of a croosed beam machine (Bordeaux)

UV probe laser

ablation laser

Kr Tripling cell

Very small relative kinetic energies possible.

Collision angle variable.

Detection by laser induced fluorescence, restricted to H atoms.

Experiments yield relative reaction cross sections (dependence of cross section over time), not absolute ones.

No information about product angles

Experimental features

3/ 21/ 2

kinkin kin kin

B B0

1 2 Ek(T) (E ) E exp dE

k T k T

kB = Boltzmann constant m = reduced massEkin = relative kinetic energy = cross section

Underlying assumptions

No barrier.

Reaction cross section only dependent on v

Maxwell Boltzmann distribution of velocities.

No additional reaction channels opening at high v.

Rate from cross sections

0.1 1 100.1

1

10C(

3PJ) + C2H2 C3H + H(

2S1/2)

Relative translational energy / kJ mol-1

Inte

gral

cro

ss s

ecti

on

/ arb

itra

ry u

nits

Cartechini et al.J. Chem. Phys.,

116, 5603 (2002)

= A E(c. m.)

C + C2H2 C3H +H

k(T) AT 0.5

Barrierless processes(Langevin)

no barrier exists

process probably leads to linear and cyclic C3H

both species found in the interstellar medium.

Theoretical investigations: predominance of linear product at low collision energies

Linear or cyclic ?

C + C2H2 C3H +H

Adiabatic capture theory calculation of the C + C2H2 cross section (Buonomo + Clary 2001)

C + C2H2 H + C3H

Linear or cyclic ?

C + C2H2 H + l-C3H H0 = -1.5 kJ/mol

C + C2H2 H + c-C3H H0 = -11 kJ/mol

Doppler analysis of C + C2H2

= 0 (1-vH’. u/c)

vH’ = velocity of H productu = unity vector

Angle fixed so that relative velocity is normal to C-beam(projection of c.m vector on laser axis equal C- velocity inc. m frame)

Costes et al.

Faraday Disc. 133 (2006)

= 0 {1-[wC+

wH’cos(-)]/c}

vH’ = vcm + wH’

vH’

Doppler analysis of C + C2H2

l-C3H

l-C3H

c-C3H

c-C3H

E=0.08 eV

E=0.08 eV

Doppler Analysis Differential cross-section

l-C3H

c-C3H

Signal at m/z=37 amu

c-C3Hfrom C(1D)

At low relativ kinetic energy, preferential forming of c-C3H

0.1 1 10

1

vB ms-1 v

C2D2

775 725 1060 725 1060 1135 fit with = -0.97 fit with = -0.97

and Eth = 0.18 kJ mol-1

Relative translational energy ET / kJ mol-1

Inte

gra

l cr

oss

sec

tio

n /

arb

itra

ry u

nit

s

B(2PJ) + C

2D

2(X1

g

+) BC2D + D(2S

1/2)

Evidence forvery small barrier(0.18 kJ/mol)

Geppert et al.,Phys. Chem. Chem. Phys., 2004, 6, 566

Reactions with a very small barrier

Reaction rate B(2PJ) + C2H2

Potential surface of the B + C2H2 reaction

Balucani et al.,J. Comput. Chem 2002, 22, 1359

Reaction slightly endoergic ?

C(3P) + O2 CO +O

C(3P) + O2 CO +O(3PJ) CO +O(1D2) CO +O(1S0)

C(3P) + O2 CO +O(1D2)

Geppert et al.,Chem. Phys. Lett, 2002, 364, 121

Very strong O(1D2) signal

No evidence for O(3PJ)

Weak O(1S0) signal

0.0

109.1

CO2

139.1

149.0

1g+

11

164.6176.3

264.9

O2+C(3P)

125.0

CO+O(3P)

CO2

COO

COO

3A'1

CO+O(1D)

176.1

1+

273.8

Hwang & MebelChem. Phys., 2001, 256, 169

Entrancebarrier

Entrance barriertowards CO+O(3P) No barrier to CO+O(1D)

C(3P) + O2 CO +OLooking at the CO product

CO (v=15) CO (v=16)

CO (v=17)C(3P) + O2 CO(v=0) + O(3PJ) H = -5.98eV

CO(v=0) + O(1D2) H = -4.02eV CO(v=17) + O(1D2) H = 0.07eVThreshold at 0.045 eV for CO(v=17) evidence for O(1D2)

Rebound

Stripping

Differential cross sections

“forward” scattering

“backward” scattering

Formation of stable intermediate complex

isotropic scattering

Determination of the lab scattering angle reaction mechanism

Crossed Molecular Beams Apparatus

(Prof. Casavecchia, Perugia)

Crossed molecularbeamapparatus

radical/atom

source

beam source

detector

TOF disk

o

10 mbar-7

electron impactionizer

quadrupolemass filter

1. Primary reaction products and "branching ratios".

2. Reaction micromechanism: direct or via long-lived complex.

3. Information on product Energy Partitioning and PES.

Observables

• Product Intensity as a functionof lab scattering angle,Ilab(T ).

• Product Intensity as a function of velocity at selected lab angles, Ilab(T,v). [(TOF)]

Lab c.m.

Ilab(T ,v)=(v2/u2)Icm(?,u)

Icm(?,E)=T(?)×P(E)

Casavecchia et al.University of Perugia

“Backward” scattering

rebound

AB AB AB

“Forward” scatteringScattering in both

directions

stripping Long-lived complex

Angle distribution in Lab coordinates

Atom/ radical beam source

• OH, NH, ClO, CN• Cl(2P3/ 2,1/ 2)• O(3P ,1D)• N(4S, 2D, 2P)

Dilute mixtures in He or Ne

of (~1%) CO2 /(0.2%) O2

p=200600 mbarRF power=200350 W

C(3P,1D)

20 40 60 80 1000

500

1000

1500

2000

2500

Inte

nsit

y

CN C(3P,

1D) vpeak = 2480 m/s

Speed ratio = 8.3

numero di canali (2s/ch)20 40 60 80 100

0

500

1000

1500

2000

2500

vpeak = 2580 m/sSpeed ratio = 7.4

numero di canali (2s/ch)

Casavecchia et al.University of Perugia

L.B. Harding & A.F. Wagner,

J. Chem. Phys. 90, 2974 (1986)

O + C2H2

0o

180o

HCCO

VC2H2 VO(3P)

200 ms- 1

HCCO

Conversion to molecular frame

Isotropic distribution stable HCCHO complex

CO shows forward scattering stripping

Casavecchia et al., 2005

Casavecchia et al.

J. Phys. Chem. A 109,

3527 (2005)

O + C2H4

180o

CH2CO CH2CHO

0o0o

180o

CH2CHO

Forward scattering stripping

Forward + backward scattering stripping and bouncing

0o

180o

Casavecchia et al.

J. Phys. Chem. A 109,

3527 (2005)

Investigations into reaction mechanisms possible.

Distribution of product angles: differential cross sections dependent on product angle measurable.

Not possible at low (interstellar) collision energies, since crossed-beam angle fixed to 90o in the present machines.

Advantages and disadvantagesof angular crossed beam apparatuses

Only relative cross sections derived with crossed beams

Supersonic beams have too low density to allow pseudo-first order conditions.

Use of supersonic flows

Absolute rate measurements

Isentropic expansion and uniform supersonic flow

Laval nozzle and isentropic flow

uniform supersonic flowT = 7 – 220 K = 1016 – 1018 cm–3nozzle throat diameter

3 mm – 5 cm

chamber pressure 0.1 – 0.25 mbarmax pumping speed 30000 m3 h-1

Axisymmetric Laval nozzle

50-100 l/min carrier gas(He, Ar) + reagent + precursor

Smith, Sims & Rowe,Chem Eur J, 3[12], 1925-1928

(1997)

to pum ps

Nd:YAGlasers

430 nm

266 nm

beam steering/com bining optics

carrier/reagentgas m ain flow

PM T and optics

m oveablereservoir

Laval nozzle

m aincham ber

supersonicflow

CH Br / H e3

liquid nitrogenjacket (optional)

d iffuser

dye laser/O PO

Schem atic diagram of combined PLP-LIF / CRESU apparatus

Reaction:CH + CO

Smith, Sims & Rowe

Schematic of the CRESU apparatus

French acronym for Cinétique de Réaction en Ecoulement Supersonique Uniforme.

ultra-low temperature environment in thermal equilibrium, temperatures 7 - 200 K dependent on nozzle

supersonic uniform (Mach no, temperature and density) flow

ultra-cold wall-less reaction vessel

cooling rapid without condensation

very strong pumps and loads of gases needed

different nozzle for each temperature

CRESU technique

First-order decay of LIF signal from CH(v=1) in the presence of 4.2 1014 molecule cm‑3 of CO at 44 K in Ar, fitted to a single exponential decay

Pseudo-first order decay constants for CH(v=1) at 44 K in Ar plotted against the concentration of CO.

[CO] / 1014 molecule cm-3

0 1 2 3 4 5

k 1st /

104

sec-1

0

2

4

6

8

Delay Time / sec

0 20 40 60 80 100 120

LIF

Sig

nal /

arb

. un

its

0

1

2

3

4

5

6

Vacuum pumps (16 000 ls-1)

CRESU apparatus

T / K

10 100

k / c

m3 m

olec

ule-1

s-1

10-11

10-10

10-9

C(3P) + O2

D. Chastaing, S. D. Le Picard, I. R. Sims: J. Chem. Phys. 112 (2000) 8466-69.

C + O2 CO + O

Cyanopolyynes (HC2xCN) are important intermediates in building large carbon chains.

Can be formed as follows:

HC2xH + CN HC2xCN + H

Reaction HC2xH + C2H HC2x+2H + H very fast at low T CN isoelectronic with C2H.

Formation of cyanopolyynes

T / K

10 100

k /

cm3 m

olec

ule-1

s-1

10-11

10-10

10-9

C2H + H-CC-H H-CC-CC-H + H

CN + H-CC-H H-CC-C N + H

Typical dense cloud Room T

I. R. Sims et al, Chem. Phys. Lett. 211, 461 (1993).

D. Chastaing et al, Faraday Discuss. 165 (1998)

T / K

10 100

k /

cm3 m

olec

ule-1

s-1

10-11

10-10

10-9

C(3P) + C2H2

C(3P) + C2H4

C(3P) + H2C=C=CH2

C(3P) + CH3CCHD. Chastaing, P. L. James, I. R. Sims, I. W. M. Smith, Phys. Chem. Chem. Phys. 1 2247 (1999).

Reactions of carbon(3P) with hydrocarbons