|
Molecular Excitation
 |
Boltzmann distribution of energy level populations: Nj = (gj/Z)
* exp(-Ej/kTex) |
| For a given background radiation field temperature T_bg, equating the
collision rate and radiative relaxation rate (sum of spontaneous and
induced emission rates) allows us to estimate a density n_T above which a molecular cloud will show line emission:
n_T
= 64 PI^4 e^2 |r|^2 / { 3 h lambda^3 sig_c v [1-exp(-h nu / kT_bg)] },
where |r| is molecular dipole matrix element (with |e*r| ~ 10^-18 e.s.u.),
sig_c is collision cross section (~10^-15 cm^-2), v is relative collision
speed (~10^5 cm s^-1), lambda and nu are the transition wavelength and
frequency. Usually, we consider T_bg = 2.7 K. |
| Electron excitation of polar molecules (from
Lunch box talk by Dr. Hasegawa on Mar 03, 2008 at ASIAA) In ionized or
partially ionized regions, excitation by collision with electron will become
important when the electron abundance Xe>10^-4.
The critical electron density (ncr(e)) for the
electron collision excitation is found to be roughly the same for all polar
molecules. The collision rate of polar molecules with electron is
about 10000 times higher than with H2. For most of the polar molecules, ncr(e) ~ 10^2 cm^-3, which is far smaller than the
critical density of H2 (10^4-10^7 cm^-3). Only CO has
a higher ncr(e) ~ 10^4. Therefore, electron collision excitation of
CO is usually difficult, while it is easier for the other polar molecules.
In HI clouds, the electrons are main produced by cosmis ray and X-ray
ionization, while in H II regions, the major electron source is X-ray
ionization. The collision rates between electron and some molecules have
been investigated by Turner in several of his
paper during 1995-1997. The formulas used by Turner are from Dckinson &
richards 1975JPhB....8.2846D
and Dickinson et al., 1977A&A....54..645D. |
| A new way of molecular collision:
 | A new mechanism of molecular excitation: tug-of-war excitation
during collision. (from Greaves et al., 2008,
Nature, 454, 88) |
| Measurement of H-D2 collision angular momentum distribution showes
two collision mechanisms important for chemistry:
head-on and tag-of-war modes. Geometrical phase effect study of H-H2 collision showes that the two modes of collision cancel the geometric phase effect. This would be applicable to other chemical reactions as well.
(from Althorpe et al., 2002,
Nature, 416, 67) |
| |
|
|
ynao.ac.cn
[back
to my Home]