Is it possible to measure to measure isotopic abundance of remote astronomical objects - ie measurement without having a sample to feed into a mass spectrometer? Do different isotopes show any differences in absorption or emission spectra which can be used for this purpose?

  • $\begingroup$ I'm gonna go ahead and say no to this. When doing distance spectroscopy, what you're really looking at is emission and absorption of photons from electrons. The number and orbital arrangement of electrons should rely almost entirely on the number of protons. So the number of neutrons won't affect the emission or absorption spectra in any measurable way, especially at astronomical distances. Of course, I could be wrong about how much of an effect the neutrons have on the electrons, which is why this is a comment, not an answer. $\endgroup$ – Phiteros Jul 22 '16 at 7:21

Yes, isotopic ratios can be readily established by spectroscopy.

Molecules where the constituent atoms differ by one mass unit have different vibrational and rotational energy levels and the gaps between them depend on the reduced mass of the molecule.

For example, in a diatomic molecule $$ \mu = \frac{m_1 m_2}{m_1 + m_2}$$ and the vibrational frequencies depend on $\mu^{-1/2}$.

This technique is used for example to look at carbon monoxide feature in the infrared and establish the ratio of $^{13}$C/$^{12}$C.

The isotopic shift for atoms is much smaller in general, but is still detectable. The different mass of the nucleus changes the reduced mass of the electron (a little) and this changes the wavelength of the characteristic transition.

An interesting example of this has been the search for $^6$Li in the spectra of population II stars (e.g. Asplund et al. 2006). This isotope of Li should be produced in the big bang along with the more common $^7$Li but is more readily burned in stellar interiors.

The main optical resonance line of lithium occurs at 670.8 nm. The difference between the wavelength of this line (actually, it is an unresolved doublet) is 0.015nm. This separation cannot be resolved in stellar spectra and instead the shape of the absorption line has to be modelled to estimate the $^6$Li/$^7$Li ratio.

The isotopic shifts become smaller for heavier elements.

  • $\begingroup$ Thanks for the answer. You say "The isotopic shifts become smaller for heavier elements" - Does this imply that above a certain atomic number the measurement becomes impractical? $\endgroup$ – Mike H Jul 22 '16 at 9:35
  • $\begingroup$ @MikeH To be honest I am not familiar with any atomic spectroscopy in astrophysics that looks at isotopic ratios in anything heavier than Li. $\endgroup$ – ProfRob Jul 22 '16 at 11:53
  • $\begingroup$ So I understand from your answers that only three elements that can have their isotope ratios measured (H, He & Li) Are there no methods available for the rest of the periodic table? $\endgroup$ – Mike H Jul 24 '16 at 5:17
  • $\begingroup$ @MikeH You understand wrong. I specifically give the example of carbon. Isotopic shifts in molecular spectroscopy are much larger. $\endgroup$ – ProfRob Jul 24 '16 at 7:12
  • $\begingroup$ Sorry for the confusion. So is it the case that in stars, where no molecules exist, Li is the upper limit? $\endgroup$ – Mike H Jul 25 '16 at 13:57

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