Fig. 5.2 Photo-ionization
cross-sections for atomic
hydrogen and various
species arising from photoabsorption by the water
molecule. The ionization of
atomic hydrogen is marked
as H ! H
+ . The other lines
are marked with the
products. E.g. H
+ + OH
indicates the cross-section
for the reaction
H 2 O + hϑ ! H
+ + OH
Table 5.2 Photolytic reactions involving water vapour and its daughter products. The threshold
wavelengths are given. Reactions marked in bold would produce pick-up ions in the cometary
environment (Data from Huebner et al. 1992)
Precursors
Product
Threshold wavelength [Å]
Rate coefficient [s
À1
]
Quiet Sun
Active Sun
H 2 O + hϑ
OH + H
2424.6
1.03 10
À5
1.76 10
À5
OH (A
2
Σ
+ ) + H
1357.1
H 2 + O(
1
D)
1770
5.97 10
À7
1.48 10
À6
H 2 + O(
1
S)
1450
H + H + O(
3
P)
1304
7.55 10
À7
1.91 10
À6
H 2 O
+ + e
984
3.31 10
27
8.28 10
27
H + OH
+ + e
684.4
5.54 10
28
1.51 10
27
H 2 + O
+ + e
664.4
5.85 10
29
2.21 10
28
OH + H
+ + e
662.3
1.31 10
28
4.07 10
28
OH+ hϑ
O(
1
D) + H
5114
7.01 10
À6a
1.76 10
À5a
O(
1
S) + H
61,320
8.33 10
À7a
2.11 10
À6a
O(
3
P) + H
2823
1.20 10
À5a
1.38 10
À5a
OH
+ + e
928
2.43 10
27a
6.43 10
27a
H 2 + hϑ
H(1 s) + H(1 s)
2768.85
4.80 10
À8
1.09 10
À7
H(1 s) + H(2 s, 2p)
844.79
3.44 10
À8
8.21 10
À8
H 2
+ + e
803.67
5.41 10
28
1.15 10
27
H + H
+ + e
685.8
9.52 10
29
2.79 10
28
O(
3
P) + hϑ
O
+ + e
910.44
2.12 10
27
5.88 10
27
O(
1
D) + hϑ
827.9
1.82 10
27
5.04 10
27
O(
1
S) + hϑ
858.3
1.96 10
27
5.28 10
27
H + hϑ
H
+ + e
911.75
7.26 10
28
1.72 10
27
a Indicates based on experimental cross-sections. Theoretical values may differ substantially
5.2 The Production of Cometary Ions
403