Transcript Aptitude of interstellar reactions to experimental investigations

Aptitude of interstellar reactions to
experimental investigations
Desirable situation
Reactions in the ISM
- well characterised closedshell reaction partners
- odd species (radicals, ions
long unsaturated chains C-C)
- room temperature (activation energy)
- low T (10-50K in dark
clouds)
- not too short timescale
- Fast, barrierless reactions
- high density of reactants
and products
- high vacuum (1-106 cm3)
- conditions (p,T) easy to
accomplish
- theory easy to handle
- theoretical investigations
challenging
Abundance of elements in the ISM
The Astronomer's Periodic Table
Cosmic Abundance
of some elements
Element
H
C
Mg
Si
Fe
He
N
O
Ne
S
Ar
hydrogen (H)
helium
oxygen
carbon
neon
nitrogen
magnesium
Silcon
Sulfur
Abundance
(relative)
1.000.000
80.147
739
445
138
91
40
37
19
2 atoms
AlF
PN
AlCl
SO
C2
SO+
CH
SiN
+
CH
SiO
CN
SiS
CO
HF
+
CO
SH
CP
FeO
CS
CSi
HCl
H2
KCl
NH
NO
NS
NaCl
OH
3 atoms
C3
OCS
C2 H
SO2
C2 O
c-SiC2
C2 S
CO2
CH2
NH2
HCN
H3 +
HCO
HCO+
HCS+
HOC+
H2 O
H2 S
HNC
HNO
MgCN
MgNC
N2 H+
N2 O
NaCN
4 atoms
c-C3 H
l-C3 H
C3 N
C3 O
C3 S
C2 H2
CH2 D+
HCCN
HCNH+
HNCO
HNCS
HOCO+
H2 CO
H2 CN
H2 CS
H3 O+
NH3
5 atoms
C5
C4 H
C4 Si
l-C3 H2
c-C3 H2
CH2 CN
CH4
HC3 N
HC2 NC
HCOOH
H2 CHN
H2 C2 O
H2 NCN
HNC3
SiH4
H2 COH+
6 atoms
C5 H
C5 O
C2 H4
CH3 CN
CH3 NC
CH3 OH
CH3 SH
HC3 NH+
HC2 CHO
HCONH2
l-H2 C4
C5 N
Radicals abundant
Small molecules predominant
H, C, O, N, S dominate,metals rare
7 atoms
C6 H
CH2 CHCN
CH3 C2 H
HC5 N
HCOCH3
NH2 CH3
c-C2 H4O
CH2 CHOH
C6 H
8+ atoms
CH3 C3 N
HCOOCH3
CH3 COOH
C7 H
H2 C6
CH2 OHCHO
CH2 CHCHO
CH3 C4 H
CH3 CH2 CN
(CH3 )2 O
CH3 CH2 OH
HC7 N
C8 H
CH3 C5 N
(CH3 )2 CO
NH2 CH2 COOH
C3 H5 CHO
HC9 N
HC11 N
Molecules in dark clouds
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 diffuse clouds
H2/H ratio about 1.
UV light can penetrate.
CO formation by:
C+ + OH  CO + H+
Destroying of molecules by UV radiation possible
Important reactions in the ISM
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.
Meaurement of interstellar reactions
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 ?
 EA 
k(T)  A exp  

 RT 
EA 1
ln k  ln A 
R T
Plot ln k versus 1/T
(Arrhenius plot)
should be linear
often misleading !
CN + C2H6  products
k / 10-11 cm3 molec-1 sec-1
T/K
1000
7
500
300
2.0
3.3
6
5
4
3
1.0
103/(T/K)
4.0
CN + C2H6  products
T/K
1000
300
200
100
3.3
5.0
10.0
k / cm3 molec-1 sec-1
10-10
10-11
10-12
1.0
103 / (T/K)
CN + C2H6  products
T/K
1000 100
-9
-1
sec
-1
molec
/kcm
3
10
-10
10
-11
10
-12
10
-13
10
-14
10
-15
10
-16
10
-17
10
-18
10
1.0
10.0
50
20
20.0
50.0
3
10 / (T/K)
CN + C2H6  products
T/K
k / cm3 molec-1 sec-1
1000 100
10-9
10-10
10-11
10-12
10-13
10-14
10-15
10-16
10-17
10-18
1.0
10.0
50
20
20.0
50.0
103/ (T/K)
Measurements in high vacuum
+
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 condensed phase
+
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.
Combinaton of the advantages:
Adiabatic expansion
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
Cloud formation through adiabatic cooling
Non-supersonic and supersonic
velocity distribution
Energy in supersonic beams
Translational energy: continuum of energies, cooling through
collisions 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 through
collisions very inefficient.
T (vibration) > 100 K
Supersonic velocities
Supersonic velocities
For studying of reactions one lets two supersonic beams cross
To study, e. g. the following reaction
Beam 1
Beam 2
Interaction
zone
v2

vR
C + NO
 CN + O
v R 2  v12  v 2 2  2v1v 2 cos 
v1
Relative kinetic energy
 2
 (v1  v 2 2  2v1 v 2 cos )
2
Supersonic velocities
C + NO
v(CN)
1

mCN
 CN + O
vNO
2(G  E kin (educts) )
vO

vC
Centre of
mass
vCN
vR
Schematics of a: croosed beam
machine (Bordeaux)
Molecular
Molecular
source:
reactant
source:
BC: O
2, C2H2,
BC :
C2H4…
O2
NO
CH
x y
Atom
source:
Atom
source:
A
A: :C,C,B Si, Al, Ti, Cr...







)
22.5°

(
n
i
m
Ablation laser
PV 1
266 nm , 10 H z
PV 2
P=106- mbar
VUV-LIF
C(3PJ), H(2S1/2)
CO(X1+), O(1D2)
P.
M
.T
.
VBC
VA
C + C2H2  C3H + H
800-2200 ms-1
800-1200 ms-1
ET = 0.4-25 kJ mol-1
VR
ET = ½(vA2+vBC2–2vAvBCcos)
ablation laser
Kr Tripling
cell
Experimental features
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
Rate from cross sections
 1   2 
k(T)  

 

k
T

  B 
1/ 2
3/ 2 
 E kin
(E kin ) E kin exp 
 k BT
0


 dE kin

kB = Boltzmann constant
m = reduced mass
Ekin = 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.
C + C2H2  C3H +H
C( PJ) + C2H2 C3H + H( S1/2)
Integral cross section / arbitrary units
3
2
Barrierless processes
(Langevin)
10
= A  E(c. m.)
k(T)  AT 
1
Cartechini et al.
J. Chem. Phys.,
116, 5603 (2002)
0.1
0.1
1
10
-1
Relative translational energy / kJ mol
  0.5
C + C2H2  C3H +H
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 ?
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
Costes et al.
Faraday Disc.
133 (2006)
vH’
Angle fixed so that relative velocity is normal to C-beam
(projection of c.m vector on laser axis equal C- velocity in
c. m frame)
l = l0 (1-vH’. u/c)
vH’ = velocity of H product
u = unity vector
vH’ = vcm + wH’
l = l0 {1-[wC+ wH’cos(-q)]/c}
Doppler analysis of C + C2H2
Doppler Analysis
c-C3H
E=0.08 eV
Differential cross-section
Signal at m/z=37 amu
c-C3H
l-C3H
l-C3H
c-C3H
E=0.08 eV
c-C3H
from C(1D)
l-C3H
At low relativ kinetic energy, preferential forming of c-C3H
Reactions with a very small barrier
2
1
+
2
Integral cross section  / arbitrary units
B( PJ) + C2D2(X g )  BC2D + D( S1/2)
vB
1
Evidence for
very small barrier
(0.18 kJ/mol)
ms
vC2D2
775
725
1060
725
1060
1135
fit with  = -0.97
fit with  = -0.97
-1
and Eth = 0.18 kJ mol
Geppert et al.,
Phys. Chem. Chem. Phys.,
2004, 6, 566
0.1
-1
1
10
-1
Relative translational energy ET / kJ mol
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(1D2)
C(3P) + O2  CO +O(3PJ)
 CO +O(1D2)
 CO +O(1S0)
Very strong O(1D2) signal
Geppert et al.,
Chem. Phys. Lett,
2002, 364, 121
No evidence for O(3PJ)
Weak O(1S0) signal
273.8
264.9
Entrance
barrier
O2+C(3P)
176.3
176.1
164.6
149.0
COO
1+
CO+O(1D)
139.1
1A
1
COO
109.1
CO2
0.0
CO2
3A'
1
1 +
g
125.0
CO+O(3P)
Hwang & Mebel
Chem. Phys.,
2001, 256, 169
Entrance barrier
towards CO+O(3P)
No barrier to
CO+O(1D)
C(3P) + O2  CO +O
Looking at the CO product
CO (v=15)
CO (v=17)
CO (v=16)
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 = 0.07eV
Threshold at 0.045 eV for CO(v=17)
 evidence for O(1D2)
Differential cross sections
Stripping
“forward”
scattering
Rebound
“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 molecular beam apparatus
Observables
detector
quadrupole
mass filter
TOF disk
radical/
atom
electron
impact
ionizer
?
source
????? o
-7
10 mbar
• Product Intensity as a function
of lab scattering angle,Ilab(T ).
• Product Intensity as a function
of velocity at selected lab
angles, Ilab(T ,v). [(TOF)]
Lab
c.m.
2/u2)I (?,u)
ICasavecchia
(T
,v)=(v
etcmal.
lab
University of Perugia
Icm(?,E)=T(?)×P(E)
1. Primary reaction products and "branching ratios".
beam source
2. Reaction micromechanism: direct or via long-lived
complex.
3. Information on product Energy Partitioning and PES.
Angle distribution in Lab coordinates
rebound
stripping
AB
“Backward” scattering
AB
“Forward” scattering
Long-lived complex
AB
Scattering in both
directions
Atom/radical beam source
Dilute mixtures in He or Ne
of (~1%) CO 2 /(0.2%) O2
p=200¸600 mbar
RF power=200¸350 W
C(3P,1D)
2500
2500
3
vpe ak = 2480 m/s
S peed ratio = 8.3
1
C( P, D)
2000
1500
1500
1000
1000
500
500
Intensity
2000
0
20
40
60
80
numero di c anali (2 s/ch)
100
vpe ak = 2580 m/s
S peed ratio = 7.4
CN
0
20
40
60
80
numero di c anali (2 s/ch)
100
•
•
•
•
OH, NH, ClO, CN
Cl(2P3/2,1/2)
O(3P ,1D)
N(4 S, 2D, 2 P)
Casavecchia et al.
O + C2H2
L.B. Harding & A.F. Wagner,
J. Chem. Phys. 90, 2974 (1986)
Conversion to molecular frame
HCCO
VC H
2 2
VO( P)
3
200 ms-1
0o
HCCO
180o
Casavecchia et al., 2005
CO shows forward scattering
 stripping
Isotropic distribution
 stable HCCHO complex
O + C2H4
Casavecchia et al.
J. Phys. Chem. A 109,
3527 (2005)
CH2CHO
CH2CO
0o
180o
0o
180o
Forward scattering
 stripping
CH2CHO
0o
Casavecchia et al.
J. Phys. Chem. A 109,
3527 (2005)
180o
Forward + backward scattering stripping and bouncing
Advantages and disadvantages
of angular crossed beam apparatuses
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.
Absolute rate measurements
Only relative cross sections derived with crossed beams
Supersonic beams have too low density to allow pseudo-first
order conditions.
Use of supersonic flows
chamber pressure 0.1 – 0.25 mbar
50-100 l/min carrier gas
Isentropic expansion and uniformmax
supersonic
pumping flow
speed  30000 m3 h-1
(He, Ar) + reagent +
precursor
Axisymmetric Laval nozzle
nozzle throat diameter
3 mm – 5 cm
uniform supersonic flow
T = 7 – 220 K
 = 1016 – 1018 cm–3
Smith, Sims & Rowe,
Chem Eur J, 3[12], 1925-1928 (1997)
Laval nozzle and isentropic flow
Schematic of the CRESU apparatus
Reaction:
CH + CO
430 nm
Nd:YAG
lasers
dye laser/OPO
266 nm
PMT and
optics
CHBr3 / He
diffuser
Laval nozzle
supersonic
flow
beam steering/
combining optics
moveable
reservoir
carrier/reagent
gas main flow
Smith, Sims & Rowe
liquid nitrogen
jacket (optional)
main
chamber
to pumps
Schematic diagram of combined PLP-LIF / CRESU apparatus
CRESU technique
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
LIF Signal / arb. units
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
6
5
4
3
2
1
0
0
20
40
60
80 100 120
Delay Time / sec
k1st / 104 sec-1
8
6
4
2
0
0
1
2
3
4
[CO] / 1014 molecule cm-3
5
Pseudo-first order
decay constants for
CH(v=1) at 44 K in Ar
plotted against the
concentration of CO.
Vacuum pumps (16 000 ls-1)
CRESU apparatus
C + O2  CO + O
10-9
-1
k / cm molecule s
-1
D. Chastaing, S. D. Le Picard, I.
R. Sims: J. Chem. Phys. 112
(2000) 8466-69.
3
10-10
C(3P) + O2
10-11
10
100
T/K
Formation of cyanopolyynes
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.
10-9
3
-1 -1
k / cm molecule s
CN + H-CC-H  H-CC-C N + H
10-10
C2H + H-CC-H  H-CC-CC-H + H
I. R. Sims et al, Chem. Phys. Lett. 211, 461 (1993).
D. Chastaing et al, Faraday Discuss. 165 (1998)
10-11
10
Typical dense cloud
100
T/K
Room T
Reactions of carbon(3P) with hydrocarbons
k / cm3 molecule-1 s-1
10-9
10-10
C(3P) + CH3CCH
C(3P) + H2C=C=CH2
C(3P) + C2H4
C(3P) + C2H2
D. Chastaing, P. L. James, I. R.
Sims, I. W. M. Smith,
Phys. Chem. Chem. Phys. 1
2247 (1999).
10-11
10
100
T/K