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Transcript of Desirable situation - well characterised closed- shell reaction partners - room temperature (activa-...
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