Argon energy level diagram

Energy levels and transitions of neutral argon (Ar I): the 75 strongest classified lines below 2 µm from the NIST Atomic Spectra Database and the 51 levels they connect, each line with its vacuum wavelength, frequency and, for 74 of them, the reduced dipole matrix element derived from the NIST transition rate. Measured lifetimes, transition rates, hyperfine constants and isotope shifts from the literature are included, each with its citation.

Hover a line or a level to isolate it, click to keep it selected. Scroll to zoom, drag to move.

Download: diagram (PDF) diagram (SVG) transitions (CSV) levels (CSV) source code

Key transitions of argon

λ, ν from NIST level energies unless a measured frequency for this isotope is available. A: measured value or NIST. Γ/2π: natural linewidth of the upper level, 1/(2πτ).

³⁹Ar hyperfine structure (I = 7/2)

Measured A, B constants; sources in the data file.

Isotope shifts

Shift = ν(first isotope) − ν(second isotope).

Closed and nearly closed transitions of argon

A transition is closed when its upper level can decay only to the level it was excited from, so the atom keeps scattering photons. “closed”: among the levels NIST lists, no other level below the upper level has the opposite parity and |ΔJ| ≤ 1, so no other electric-dipole decay exists. Otherwise the leak is the sum of the branching ratios of the other decay lines of the upper level (or 1 − branching ratio where that of the line itself has been measured), and the number of photons is 1 / leak. “≥” and “≤”: at least one possible decay path has no listed rate, so the leak is a lower limit. These numbers are derived from the branching ratios, lifetimes and rates of this page, not measured as such; the tag names the weakest of the inputs, * marks theory and ≈ a model or semi-empirical calculation. Listed: lines that start on the ground level or on a metastable level and leak at most 10 %, unless more than three possible decay paths have no listed rate. Fine-structure levels only: hyperfine and Zeeman dark states and forbidden decays without a listed rate are not considered. Select a line in the diagram for the full list of its leak channels.

Listed Ar transitions below 2 µm

Reduced dipole matrix elements follow the convention A = ω³|d|²/(3πε₀ħc³(2J′+1)), with J′ the upper level. Tags show where a number comes from: measured, NIST compilation, high-accuracy theory, a semi-empirical calculation (Kurucz line list, used only where no measurement or NIST value exists; typically within a factor 2), or a model calculation.

Ar energy levels

Ar⁺ ionisation limit 127109.84 cm⁻¹ = 15.75961 eV (78.672 nm)

Sources

• Katori & Shimizu, Phys. Rev. Lett. 70, 3545 (1993) (abstract)
• Klein et al., Nucl. Phys. A 607, 1 (1996) [as quoted in Chu et al. (Ritterbusch, Lu), 'Op…
• Klose, J. Opt. Soc. Am. 58, 1509 (1968), Table II (read through the publisher page, which…
• Klose, J. Opt. Soc. Am. 58, 1509 (1968), Table II (read through the publisher page, which…
• Klose, J. Opt. Soc. Am. 58, 1509 (1968), Table II (read through the publisher page, which…
• Klose, J. Opt. Soc. Am. 58, 1509 (1968), Table II (read through the publisher page, which…
• Klose, J. Opt. Soc. Am. 58, 1509 (1968), Table II (read through the publisher page, which…
• Kurucz, R. L., Can. J. Phys. 89, 417 (2011); semi-empirical line list gf1800.pos of 2011-…
• Lawrence, Phys. Rev. 175, 40 (1968) (abstract; read through the publisher page)
• NIST ASD
• NIST ASD level energies
• Quadfasel and Helbig (unpublished; laser-induced fluorescence, selective laser excitation…
• Small-Warren & Chiu, Phys. Rev. A 11, 1777 (1975) (abstract)
• Traub, Roesler, Robertson, Cohen, J. Opt. Soc. Am. 57, 1452 (1967) [as quoted in Chu et a…
• Velchev, Hogervorst, Ubachs, J. Phys. B 32, L511 (1999)
• Wiese, Brault, Danzmann, Helbig, Kock, Phys. Rev. A 39, 2461 (1989), Sec. III A (pp. 2463…
• Wiese, Brault, Danzmann, Helbig, Kock, Phys. Rev. A 39, 2461 (1989), Table I, p. 2464
• Williams, Lu, Rudinger, Xu, Yokochi, Mueller, Phys. Rev. A 83, 012512 (2011) [arXiv:1012.…
• de Regt, Tas, van der Mullen, van der Sijde, Schram, J. Quant. Spectrosc. Radiat. Transfe…

Isotope-shift keys 'A-B' mean nu(A) - nu(B); heavier argon isotopes lie at higher frequency on 811.5 nm (Table III of Williams et al. is given relative to 38Ar: Delta nu(38,A') = nu(A') - nu(38)), while the 800.8 and 922.7 nm shifts of Chu et al. are printed relative to 40Ar. 39Ar and 37Ar are radioactive (ATTA tracers). Level energies are NIST ASD values from data/ar/levels.csv; Paschen names in the labels. Entries marked '(abstract)' were read on the publisher's abstract page only. Added 2026-10-01 (second pass): the 105 nm measurements of Velchev et al. are from 1999 (older than 20 years), so the NIST energy stays in use; isotope shifts of that line are printed as nu(40) - nu(A). Paschen-labelled lifetimes (Klose) were assigned to NIST levels by energy order. Added 2026-10-02 (third pass, papers opened in full): Wiese et al. 1989 (scanned PDF from the Max Planck repository; tables read from the page images), Klein et al. 1996 (CERN preprint scan), Welte's 2011 thesis, de Regt et al. 1996. Lifetimes of all ten 4p levels are second-hand (Quadfasel & Helbig unpublished, via Wiese Table II, no uncertainties printed). Isotope-shift entries of the 763.5 nm line carry the statistical error in 'unc' and the systematic one in 'unc_sys'. The rates A_s of the 5p[5/2]2 branches are R/tau from one paper, so tau x sum(A) = 1 by construction. Added 2026-10-02 (fourth pass, publisher pages read directly): opg.optica.org abstract pages embed the full tables of a paper in their HTML and jou

Atoms with measured data

• Lithium-632 levels, 85 lines
• Lithium-732 levels, 85 lines
• Beryllium-917 levels, 26 lines
• Sodium-2336 levels, 106 lines
• Magnesium-2418 levels, 61 lines
• Magnesium-2518 levels, 61 lines
• Potassium-3934 levels, 84 lines
• Potassium-4034 levels, 84 lines
• Potassium-4134 levels, 84 lines
• Calcium-4035 levels, 61 lines
• Calcium-4335 levels, 61 lines
• Chromium-5256 levels, 87 lines
• Chromium-5356 levels, 87 lines
• Rubidium-8536 levels, 94 lines
• Rubidium-8736 levels, 94 lines
• Strontium-8440 levels, 70 lines
• Strontium-8640 levels, 70 lines
• Strontium-8740 levels, 70 lines
• Strontium-8840 levels, 70 lines
• Cadmium-11115 levels, 21 lines
• Cadmium-11315 levels, 21 lines
• Cadmium-11415 levels, 21 lines
• Caesium-13331 levels, 79 lines
• Barium-13744 levels, 89 lines
• Barium-13844 levels, 89 lines
• Dysprosium-16139 levels, 55 lines
• Dysprosium-16239 levels, 55 lines
• Dysprosium-16339 levels, 55 lines
• Dysprosium-16439 levels, 55 lines
• Erbium-16673 levels, 90 lines
• Erbium-16773 levels, 90 lines
• Erbium-16873 levels, 90 lines
• Thulium-16972 levels, 73 lines
• Ytterbium-17118 levels, 26 lines
• Ytterbium-17318 levels, 26 lines
• Ytterbium-17418 levels, 26 lines
• Mercury-19927 levels, 47 lines
• Mercury-20127 levels, 47 lines
• Mercury-20227 levels, 47 lines

All elements: periodic table