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The Wouthuysen-Field Effect
The Wouthuysen-Field effect is a coupling of the 21cm hyperfine transition to Ly- radiation.
This is important for possible high-redshift observations of the 21cm line, during the epoch of reionization. The 21cm hyperfine transition is forbidden by normal dipole selection rules, but transitions from one hyperfine state, up to the state, and back down to the other hyperfine state are not forbidden. So, if there is sufficient Ly- radiation to cause this intermediate transition, it will dominate over the direct (forbidden) hyperfine transition. Cosmologically speaking, this happens towards the end of the “Dark Ages” as reionization begins, and the WF effect remains the dominant effect until reionization is complete.
Fine structure of hydrogen
Energy levels in hydrogen atoms are split due to spin orbit interaction (fine splitting), and the smaller effect of electron-proton spin interaction (hyperfine splitting). Anti-aligned spins lead to lower energy levels. The splittings of the lowest energy levels are:
Energy level diagram with spectroscopic notation
Here we have used spectroscopic notation , where is the principal quantum number, are the electron orbital angular momentum. () shall denote the electron (proton) spin (not to be confused with the orbital). Then is the electron total angular momentum, and is the hydrogen total angular momentum. Note these are vector sums.
The splitting between the two levels is of particular interest. It has energy difference , corresponding to wavelength . We also define . In the regime, the Rayleigh-Jeans limit holds and we can define the “brightness temperature”
only serves to redefine , and should be distinguished from the ambient temperature of the CMB .
After recombination , the photons in the cosmic fluid no longer has free charges to interact with and thus free stream through the “dark ages” until the Epoch of Reionization, believed to be around . Studying the 21cm absorption of neutral hydrogen can thus potentially probe the universe during the dark ages and epoch of reionization. The frequency of 21cm undergoes cosmological redshift and hence the actual observed brightness temperature is
Denoting the lower level by 0, the higher by 1, one can define the “spin temperature ” to characterize the relative abundance of these two states:
During the dark ages till the end of reionization, several processes control the relative abundances. Among them are direct radiative transitions (Einstein Coefficients) , collisional excitation , and the Wouthuysen-Field (WF) Effect and . The WF effect will be explained in detail. For now let’s just note that the overall statistical balance gives
where and are the Einstein Coefficients of the 21cm transition. Recall that in thermodynamic equilibrium, we had
In the Rayleigh-Jeans limit, radiative coefficients satisfy
Here is the temperature of the CMB.
Remember that for collisional excitations of the hyperfine transition, in thermal equilibrium, the gas temperature is coupled directly to the spin temperature. In other words, collisions are the dominant effect that sets the hyperfine level population. This means the transition rates and are given by:
Similarly, WF effect rates are given by the “color temperature” :
Plugging the above results into the balance equation eq:balance, we have the relation of the temperatures:
represents the relative rates. It remains to determine these coefficients.
The rest of this article shall concentrate on the WF effect, and in particular the determination of The WF effect involves absorption of an Lyman- photon from the state and subsequent decay into the state. The CMB temperature during the dark ages have temperatures , and is much smaller than the Lyman- transition temperature of , and thus we expect WF effect to be most important during the epoch of reionization, when the first stars provide the abundance of photons. To study the WF effect, we first look at the rules for allowed transitions.
Parity and Selection Rules
Now we need to look at the selection rules for dipole transitions. First of all, there is the selection rule that is forbidden by parity, a symmetry of electromagnetic interactions, so only transitions between and will matter. (In other words, nevermind about the states we wrote down above.) This is the selection rule that forbids the 21cm transition, which is why the WF effect can dominate over the direct 21cm transition so long as enough Ly- photons are around.
Rotational symmetry is where the rest of our selection rules come from. There is a fancy thing in quantum called the Wigner-Eckart theorem which applies to “irreducible tensor operators” in general, and can generate selection rules for any such operator. The selection rule goes like this: a transition from to is only allowed if is in the range to , where is the “order” of the tensor operator. For dipole transitions, : the dipole operator is an “order 1 irreducible tensor operator”, so the rule becomes . (Lowercase is a totally generic angular momentum quantum number, not the same as .)
To put illustrate all of these in simpler language, a photon is a vector and carries one unit of angular momentum. This means that a single photon must be circularly polarized (left or right) . Since the vector does not has any spin dependence, it commutes with the electron spin operator:
Here is the vector sum of the electron and proton spins.
This implies the first dipole selection rule:
The spin along given directions (), however, are not “good” quantum numbers, and can change.
To consider orbital angular momentum, recall the parity operator . It reverses all physical space directions:
If we fix the phase such that , we have parity as a Hermitian and hence observable:
Parity thus acts on other operators via conjugation. For example
More generally, it is clear that quantities such as vector, pseudo-scalar change sign under parity while a pseudo-vector (such as angular momentum or magnetic field) or a true scalar do not change sign. For example, parity commutes with spin , while it anti-commutes with momentum . This means that while a spin state is a good quantum number when paired with parity, a momentum state is not.
So how do states in a hydrogen atom behave under parity? To answer this question recall that such a state is proportional to the spherical harmonics:
Under parity, , , and since we have
The parity of a state does not depend on , as expected since an intrinsic property such as parity should not depend on the orientation of the system.
Thus the matrix element of the vector can be written as
Since a matrix element is a scalar and has even parity, we see that the end state must have different parity from the initial state. In other words, must be odd!
Furthermore, since the photon carries only one unit of angular momentum, any angular momentum change must be of order unity or less:
Thus we have the rule
To see this more explicitly, one can also write out the full expression
Since a dipole interaction, along a direction called has the form
The matrix element can be written as
From properties of associated Legendre polynomials, we can decompose
for some constants and . And since they also satisfy the orthogonality rules
the above matrix element vanishes unless
In particular, the angular momentum along the dipole direction is unchanged. This is expected since the spin of the photon is pointed perpendicular to the field (a transverse wave).
A difference between and is in place when there are more than one electron in the atom. We have seen that a single electron contributes a factor of when acted on by parity. In the multi-electron case, this generalizes to
Since in general the total orbital angular momentum , two states with the same can have different parity, and so one cannot conclude that is not allowed. One can conclude, however, that transitions are not allowed. This is because the orbital has complete spherical symmetry, a symmetry broken by the vector photon, and thus an orbital cannot absorb a photon and stay an orbital. Similarly, considering L-S coupling, we have corresponding selection rules of forbidden or transitions. Physically, these last two rules amounts to the combined effect of angular momentum conservation, and the first rule, that For example, the transition from to is allowed, even though , because in this transition the photon brought , electron flips its spin to keep , and proton also flips its spin, to preserve and , in the process is flipped. The transition to on the other hand, is forbidden, because as , the two conditions and cannot be simultaneously satisfied.
In summary, we have the following selection rules for dipole transitions in a neutral hydrogen atom
The WF effect
We would like to find the coupling coefficient defined at the end of the Cosmological Context subsection. The allowed transitions between the lowest levels are shown in Fig. 2. The transitions relevant to the WF effect are traced with solid lines. Since we have denoted the states by 0 and 1, we shall by convenience denote the 2 states by 2,3,4,5, in order of increasing energy.
Energy level diagram with allowed transitions
Energy level diagram of WF effect
Evidently, the (de-)excitation rate due to WF effect is given by:
In the second and fourth lines I have used the relation from above:
We would like to relate this rate to the total Ly scattering rate
where is the absorption cross section and , with the oscillator strength. The line can be assumed to be a Voigt profile. Thermal broadening leads to Doppler width
where is the Lyman line center frequency.
Since the dipole operator commutes with spin, the transition of to is independent of and . This means that the emission intensity of transition from , summed over all , to , summed over and , is the intensity of transition from a particular , times , the degeneracy of the initial state. This is called the “sum rule” of the transition.
The sum rules, together with the selected transitions in Fig. 2, immediately lead to
The second line follow from the transition into excited states.
Next we shall neglect radiative transfer effect and make the assumption that background intensities and temperatures are constant across all the hyperfine lines, and are given by the CMB values. Under this assumption
Let be the total intensity of de-excitation. Then from the relations on above we get
Furthermore, the intensities are related to the Einstein coefficients by
where and is the total degeneracy of level.
Thus we have
Thus finally and the coupling coefficient becomes
Color temperature and corrections
The effect of Ly absorption on spin temperature also depends on the color temperature . For an environment that’s optically thick, such as the high-redshift IGM, we may take
One commonly considered correction is due to energy loss in spin-exchange collisions. In these collisions, the electron spins of the two colliding hydrogen atoms exchange, while the total spin remains unchanged. This correction to the color temperature is worked out in for example [, section 2.3.3. We shall here just quote the result:
where the spin exchange temperature
To use Eq. eq:colort, the spin temperature must be determined iteratively. The iteration is shown to converge quickly.
Another commonly considered correction are radiative transfer effects. Previously we assumed that the background intensity is constant and given by the CMB intensity. The Ly absorption would decrease the background intensity and hence the estimated scattering rate. The effect is more important for lower kinetic temperature. This leads to a correction to the WF effect coupling coefficient such that
Neglecting spin exchange, the suppression factor is given by
where the Gunn-Peterson optical depth can be written for overdensity as roughly:
 Direct transitions are disfavored by selection rules, as we shall
 In general, a tensor of rank carries units
of angular momentum, and has possible spin states. Due
to the lack of mass of a photon however, the electromagnetic wave is
transverse in all reference frames (easily seen with Maxwell’s
equations), and thus only has two polarization states, with angular
momentum (anti-)parallel to the direction of motion.