q p r
ð Þ ¼ Q gp e
Àβ 1 r
ð3:107Þ
in units of [molecule s
À1 ] where Q gp indicates the parent production at source and β 1
is a reciprocal scalelength. β 1 can be converted into a lifetime for the parent species
by
β 1 ¼
1
v gp τ p
ð3:108Þ
where τ p is the lifetime of the parent and v gp indicates the velocity of the parent.
Close to the nucleus, one can replace r with r-R N where R N is again the radius of the
nucleus thereby eliminating the point source approximation. However, this is usually
an unnecessary finesse and is, in any case, inconsistent if the acceleration of the gas
Table 3.9 Types of reaction possible within the inner coma of a comet (from Schmidt et al. 1988)
Reaction
number
Reaction type
Example
R1
Photodissociation
hϑ + H 2 O ! H + OH
R2
Photoionization
hϑ + CO ! CO
+ + e
R3
Photodissociative ionization
hϑ + CO 2 ! O + CO
+ + e
R4
Electron impact dissociation
e + N 2 ! N + N + e
R5
Electron impact ionization
e + CO ! CO
+ + 2e
R6
Electron impact dissociative ionization
e + CO 2 ! O + CO
+ + 2e
R7
Positive ion-atom interchange
CO
+ + H 2 O ! HCO
+ + OH
R8
Positive ion charge transfer (charge
exchange)
CO
+ + H 2 O ! H 2 O
+ + CO
R9
Electron dissociative recombination
C 2 H
+ + e ! C 2 + H
R10
3-body positive ion-neutral association
C 2 H 2
+ + H 2 + M ! C 2 H 4
+ + M
R11
Neutral rearrangement
N + CH ! CN + H
R12
3-body neutral recombination
C 2 H 2 + H + M ! C 2 H 3 + M
R13
Radiative electronic state deexcitation
O(
1
D) ! O(
3
P) + hϑ
R14
Radiative recombination
e + H
+ ! H + hϑ
R15
Radiation stabilised positive ion-neutral
association
C
+ + H ! CH
+ + hϑ
R16
Radiation stabilised neutral recombination
C + C ! C 2 + hϑ
R17
Neutral-neutral associative ionization
CH + O ! HCO
+ + e
R18
Neutral impact electronic state quenching
O(
1
D) + CO 2 ! O
(
3
P) + CO 2 + hϑ
R19
Electron impact electronic state excitation
CO(
1
Σ) + e ! CO(
1
∏) + e
Table 3.10 Photo-dissociation rates of the major species
at 1 AU (Crovisier and
Encrenaz 1983)
Species
Photo-dissociation rate [s
À1
]
H 2 O
6.5 10
À5
CO 2
2.0 10
À6
CO
6.5 10
À7
3.5 Reaction Chemistry and the Extended Coma
253
ð Þ ¼ Q gp e
Àβ 1 r
ð3:107Þ
in units of [molecule s
À1 ] where Q gp indicates the parent production at source and β 1
is a reciprocal scalelength. β 1 can be converted into a lifetime for the parent species
by
β 1 ¼
1
v gp τ p
ð3:108Þ
where τ p is the lifetime of the parent and v gp indicates the velocity of the parent.
Close to the nucleus, one can replace r with r-R N where R N is again the radius of the
nucleus thereby eliminating the point source approximation. However, this is usually
an unnecessary finesse and is, in any case, inconsistent if the acceleration of the gas
Table 3.9 Types of reaction possible within the inner coma of a comet (from Schmidt et al. 1988)
Reaction
number
Reaction type
Example
R1
Photodissociation
hϑ + H 2 O ! H + OH
R2
Photoionization
hϑ + CO ! CO
+ + e
R3
Photodissociative ionization
hϑ + CO 2 ! O + CO
+ + e
R4
Electron impact dissociation
e + N 2 ! N + N + e
R5
Electron impact ionization
e + CO ! CO
+ + 2e
R6
Electron impact dissociative ionization
e + CO 2 ! O + CO
+ + 2e
R7
Positive ion-atom interchange
CO
+ + H 2 O ! HCO
+ + OH
R8
Positive ion charge transfer (charge
exchange)
CO
+ + H 2 O ! H 2 O
+ + CO
R9
Electron dissociative recombination
C 2 H
+ + e ! C 2 + H
R10
3-body positive ion-neutral association
C 2 H 2
+ + H 2 + M ! C 2 H 4
+ + M
R11
Neutral rearrangement
N + CH ! CN + H
R12
3-body neutral recombination
C 2 H 2 + H + M ! C 2 H 3 + M
R13
Radiative electronic state deexcitation
O(
1
D) ! O(
3
P) + hϑ
R14
Radiative recombination
e + H
+ ! H + hϑ
R15
Radiation stabilised positive ion-neutral
association
C
+ + H ! CH
+ + hϑ
R16
Radiation stabilised neutral recombination
C + C ! C 2 + hϑ
R17
Neutral-neutral associative ionization
CH + O ! HCO
+ + e
R18
Neutral impact electronic state quenching
O(
1
D) + CO 2 ! O
(
3
P) + CO 2 + hϑ
R19
Electron impact electronic state excitation
CO(
1
Σ) + e ! CO(
1
∏) + e
Table 3.10 Photo-dissociation rates of the major species
at 1 AU (Crovisier and
Encrenaz 1983)
Species
Photo-dissociation rate [s
À1
]
H 2 O
6.5 10
À5
CO 2
2.0 10
À6
CO
6.5 10
À7
3.5 Reaction Chemistry and the Extended Coma
253