Lecture 7 - Radio Lines, Radiative Trapping, Excitation by Recombination, Optical Pumping Flashcards
How is T_ex defined?
n_u/n_l=g_u/g_l * exp(-hv/kT_ex)
T_ex lies between ___ and ____ .
T_B0, T
What is radiative trapping?
The phenomenon of a photon is trapped in a region by getting emitted by one atom and absorbed by another
What is the effective probability for a photon escaping radiative trapping?
exp(-tau_nu)
What two things is the escape probability approximation based on?
- Photon occupation number is zero (no absorption and stimulated emission)
- Effective A value replaces new old one. A –> beta*A
What are the two consequences of radiative trapping being very important?
- beta =e^(-tau)«_space;1
2. We’re able to thermalize CO rotational levels even at densities well below A/gamma.
Physical processes relevant to low-n levels excitation by recombination
- Radiative recombination (and photoionization)
- Spontaneous decay (and photoexcitation)
- [Collisional de-excitation (and excitation)]
Is the photon occupation number large or small for low-n levels excitation by recombination?
Very small (so ignore radiative excitation and stimulated emission)
Physical processes relevant to high-n levels excitation by recombination
- Radiative recombination (and photoionization)
- Spontaneous decay (and photoexcitation)
- Collisional de-excitation (and excitation)
- Three-body recombination (collisional ionization)
Hydrogenic meaning
(of species) that, like hydrogen, consist of a single nucleus and a single orbiting electron
Is Li^2+ a hydrogenic?
Yes, it is
Franck-Condon principle
Molecular transition probability is proportional to the wave function amplitude at the start and end position.
What’s the opposite process of radiative recombination?
Photoionisation
What’s the opposite process of spontaneous decay?
Photoexcitation
What’s the opposite process of collisional de-excitation?
Excitation
