# How come the xenon $6s$ state is metastable?

I'm having some trouble puzzling out some aspects of the electronic excitation spectrum of xenon, and I'd appreciate some help with it.

This classic paper ─ the first one to pioneer the use of laser-induced electron holography (preprint) ─ uses a fairly vanilla-type experiment as far as strong-field physics go, but one notable feature is that they required a target with a low ionization potential, so they use, in their words,

metastable ($$6 s$$) xenon atoms,

which they produced using electron-impact excitation.

I was trying to understand this state (with the vague goal in mind to see if there are states with even lower $$I_p$$'s available), but I ended up getting even more confused, and from looking at the electronic configuration, this feels like it shouldn't be metastable at all.

The NIST ASD lists several states that fit this description,

$$\begin{array}{cccc} \text{configuration} & \text{term} & J & I_p \\\hline 5p^5(^2\mathsf{P}_{3/2}^\mathrm{o})6s & ^2[3/2]^\mathrm{o} & 2 & 8.315\:\mathrm{eV} \\ & & 1 & 8.437\:\mathrm{eV} \\ 5p^5(^2\mathsf{P}_{1/2}^\mathrm{o})6s & ^2[1/2]^\mathrm{o} & 0 & 9.447\:\mathrm{eV} \\ & & 1 & 9.569\:\mathrm{eV} \end{array}$$

the first of which is the first excited state of xenon, and whose $$I_p$$ (given by subtracting its excitation energy from the ground-state xenon $$I_p$$ of $$12.13\:\rm eV$$) matches the one reported in the paper, at $$3.8\:\rm eV$$.

Here's my question, though: I really don't see how this state can be metastable against dipole radiative decay down to the ground-state $$5p^6$$ $$^1\mathsf S$$ configuration, particularly since that transition would ostensibly just involve moving one electron from a $$6s$$ shell to a $$5p$$ one, and, at least in isolation from other electrons, that's a classic example of an allowed transition.

This is further backed up by the lines database at ASD, which quotes the existence of such a transition at a wavelength of $$146.9610\:\rm nm$$ coming from the $$^2[3/2]^\mathrm{o}$$ state, and one at $$129.5588\:\rm nm$$ coming from the $$^2[1/2]^\mathrm{o}$$ state. Both transitions have a nonzero 'relative intensity' (presumably?) means that they are indeed observed in emission, if I understand correctly, and they have transition probabilities of around $$A_{ki}\approx 2.5 \times 10^8 \:\rm s^{-1},$$ corresponding to lifetimes of around $$4\:\rm ns$$, which is pretty short for dipole lines (for scale, it's a bit longer lifetime than the Lyman alpha $$2p$$-$$1s$$ line in hydrogen).

So, what gives? Huismans et al. claim that

The metastable xenon atoms were exposed to a train of 5000 mid-IR laser pulses separated by 1 ns,

and that doesn't square at all with the lifetimes as I've derived them from the ASD.

Can anyone see what I'm missing here?

• “I really don't see how this state can be metastable against dipole radiative decay down to the ground-state...” -- sorry, I may be missing something, but would it involve J=2 to J=0? – kkm Jan 31 '19 at 0:38