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H2 -- hydrogen molecule
 | Vibrational excitation of H2
 | H2 is a homonuclear molecule whose dipole transitions between vibrational levels are
forbidden and only electric
quadrupole transitions can happen with a typical
vibrational transition rate of 10^-6~10^-7
s^-1. The vibrational quantum of H2 is very large (h*nu = 0.51 eV), so that it's vibrational level
excitation in diffuse clouds (with Tk ~ 100 K) or dark clouds (with Tk ~
10 K) is mainly maintained by radiation,
while collisional excitation is possible only in shocked
region where temperature is high. (from the Text book by Duley
& Williams, 1984inch.book.....D) |
| Absorption of photons in electronic transitions B<-X and and
C<-X followed by fluorescence can leave the molecule in excited
vibrational state. This maintains a population of
10^-4 (absorption rate 10^-11 s^-1 in unshielded interstellar UV
field and decay rate of 10^-7 s^-1) in excited states of H2, which
dominates over collisions. However, H2 self-shielding
would limit this pumping only to the regions directly exposed to the
strong UV fluxes of hot stars. (from the Text book by Duley &
Williams, 1984inch.book.....D) |
|
| Rotational excitation of H2
| The rotational energy levels in the
ground state X^1^Sigma_g^+ of H2 are
| J |
I |
E(J)(cm^-1) |
g_J |
| 0 |
0 |
0 |
1 |
| 1 |
1 |
118.49 |
9 |
| 2 |
0 |
354.39 |
5 |
| 3 |
1 |
705.54 |
21 |
| 4 |
0 |
1168.80 |
9 |
| 5 |
1 |
1740.19 |
33 |
| 6 |
0 |
2414.98 |
13 |
where I is total nuclear spin and J is rotational quantum number,
statistic weight g_J = (2J+1) * (2I+1). In diffuse clouds with Tk ~ 100 K, only levels with
J<=2 can be appreciably populated, according to boltzmann
distribution law. (from the Text book by Duley & Williams, 1984inch.book.....D) |
| The J=1 and 0 H2 level populations derived from H2 rotational line
observations is not always a good indicator of gas temperature, because,
as shown in the case of zeta Per, the lower and
higher rotational levels of H2 may show different
Tex and the high Tex in higher levels may be controlled by other
pumping processes (e.g., UV photons with cascading). (from the Text book
by Duley & Williams, 1984inch.book.....D) |
| The pumping rates of rotational levels by
UV photons (followed by casecading) and by formation process of H2 on grain are comparable, because the UV photon absorption rate
(~3*beta*n(H2)) is comparable to the equillibrium H2 formation rate
(~beta*n(H2)). (from the Text book by Duley & Williams, 1984inch.book.....D)
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