LineTAP

[Gregory MANTELET]

Hello Franck, Margarida, and anybody interested in LineTAP,

Regarding all the discussions that followed the Margarida's talk, James 
and I think there is a clearly a room to continue them in the next DAL 
Running Meeting. No date set currently, and no pressure from us to do 
so. But if you are interested, just tell us and we can start to organize 
that.

Cheers,
Grégory and James



On 28/04/2022 18:21, Franck Le Petit wrote:
> Hello,
>
> Following Margarida nice presentation about LineTAP, here are some 
> comments and questions.
>
> *1 - Choice of units for energy levels*
> I saw that Joule was proposed. Joule is a bit a strange unit for 
> energy levels because 1 Joule is lot of energy whereas rotational 
> levels or hyperfine structures contains very few energy. Using Joule 
> means that we will have to manipulate quantities as 10E-XX where XX is 
> a lot.
>
> That is not an issue if users never see these quantities in Joule, if, 
> for instance, they are converted in other units.
> If users see these values in Joule then, that is a bad choice and 
> another unit should be chosen.
>
> The most important point here, for energy levels and line wavelengths 
> is the precision.
> Spectroscopy is one of the domain of Physics where the precision on 
> data is very large. And, in astrophysics we need this precision on 
> spectroscopic data.
>
> So :
> 1 - The VO interfaces should not introduce a lost of precision for 
> example manipulating very small numbers (energy of a hyperfine 
> transitions in Joule) and very large numbers.
> 2 - A unit conversion system is mandatory but it is difficult to write 
> a reliable one.
> Up to 2019, the Boltzman constant was not very precise. So, any unit 
> conversion done using the Boltzman constant introduced a significant 
> error. For instance, spectroscopists determine rotational level 
> energies in cm-1 because it is convenient and precise. When these 
> energies were converted in Kelvin, because of the imprecision on the 
> Boltzmann constant, these operation introduced significant errors on 
> the energy levels.
>
> In 2019, physical constants have been redefined and this issue has 
> been solved. The Boltzmann constant, as the velocity of light, etc … 
> are now exact. But this means that any unit conversion system must use 
> the latest values of the physical constants and that, the numerical 
> values of energy levels or line wavelengths must not be truncated. The 
> less we have to convert units, the less errors will be introduced in 
> the system.
> We should avoid the situation where each data provider and each client 
> has to implement unit conversion system. We can be sure there will 
> have mistakes.
>
> *2 - Zero point energy*
>
> You mentioned the zero point energy and I do not understand the problem.
>
> The Zero point energy is a theoretical concept that represents the 
> difference of energy between the bottom of the potential well of a 
> molecule and the first level that can be populated. It is very complex 
> to estimate precisely (and I guess it is a hot topic among 
> specialists) because one has to consider many subtle quantum effect 
> leading to correction terms on the zero order value 1/2 hbar omega.
>
> But, this quantity is useful in chemistry not in spectroscopy. So, why 
> do we need it for LineTAP ? It is not an issue if we say that the H2 
> v=0 J=0 level is at 0.00000 K and the HD v=0 J=0 level is also at 
> 0.00000 K even if these two zeros are not the same. Indeed, for 
> spectroscopy, we will never do operations between energy levels of H2 
> and HD. So, each molecule as its 0.000 level energy that is not the 
> same between molecules. But, for our problem of line indentification 
> that is not an issue.
>
> So in my opinion, LineTAP should not deal with zero point level energy.
>
> *3 - Einstein coefficients, Oscillator strengths and others*
>
> In the presentation, it was said that one can go from one of these 
> quantities to the other ones by simple conversions. That is true but 
> for some molecules that can be quite tricky and create debates between 
> specialists.
>
> Moreover, to go from one quantity to another one, one often need 
> another quantity that is the degeneracy of the levels. For example, to 
> convert an oscillator strength in Einstein coefficients :
> Aki = 6.6702E15 / lambda**2 * g_i / g_k * f_ik
>
> So, we need the degeneracies of the levels and those one depends on 
> the physical effects that are considered (ex : hyperfine structure or 
> not).
> To understand a numerical value for the degeneracies, one need to know 
> the quantum numbers.
>
> So, how does LineTAP deal with that or, more precisely, how will it 
> provide enough information to users so that they can use properly the 
> data ?
>
> Best regards
> Franck
>
>
>
>
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