Dirubidium is a molecular substance containing two atoms of rubidium found in rubidium vapour. Dirubidium has two active valence electrons. It is studied both in theory and with experiment. The rubidium trimer has also been observed.

Synthesis and properties Dirubidium is produced when rubidium vapour is chilled. The enthalpy of formation (ΔfH°) in the gas phase is 113.29 kJ/mol. In practice, an oven heated to 600 to 800K with a nozzle can squirt out vapour that condenses into dimers. The proportion of Rb2 in rubidium vapour varies with its density, which depends on the temperature. At 200° the partial pressure of Rb2 is only 0.4%, at 400 °C it constitutes 1.6% of the pressure, and at 677 °C the dimer has 7.4% of the vapour pressure (13.8% by mass).

The rubidium dimer has been formed on the surface of helium nanodroplets when two rubidium atoms combine to yield the dimer:

Rb + Rb → Rb2

Rb2 has also been produced in solid helium matrix under pressure.

Ultracold rubidium atoms can be stored in a magneto-optic trap and then photoassociated to form molecules in an excited state, vibrating at a rate so high they barely hang together. In solid matrix traps, Rb2 can combine with the host atoms when excited to form exciplexes, for example Rb2(3Πu)He2 in a solid helium matrix.

Ultracold rubidium dimers are being produced in order to observe quantum effects on well-defined molecules. It is possible to produce a set of molecules all rotating on the same axis with the lowest vibrational level.

Spectrum Dirubidium has several excited states, and spectral bands occur for transitions between these levels, combined with vibration. It can be studied by its absorption lines, or by laser induced-fluorescence. Laser induced-fluorescence can reveal the life-times of excited states.

In the absorption spectrum of rubidium vapour, Rb2 has a major effect. Single atoms of rubidium in the vapour cause lines in the spectrum, but the dimer causes wider bands to appear. The most severe absorption between 640 and 730 nm makes the vapour almost opaque from 670 to 700 nm, wiping out the far red end of the spectrum. This is the band due to X→B transition. From 430 to 460 nm there is a shark-fin shaped absorption feature due to X→E transitions. Another shark fin like effect around 475 nm s due to X→D transitions. There is also a small hump with peaks at 601, 603 and 605.5 nm 1→3 triplet transitions and connected to the diffuse series. There are a few more small absorption features in the near infrared.

There is also a dirubidium cation, Rb2+ with different spectroscopic properties.

Bands TransitionColourKnown vibrational bandsBandheadsA-XinfraredB-Xred4-0 5-0 6-0 7-0 8-0 9-0 10-0 11-0 6-1 7-1 8-1 9-214847.080 to 15162.002C-XblueD-Xblue-violet1-C infrared C→26800–8000 cm−1 11Δg-X540 nm quadrupole

Molecular constants for excited states The following table has parameters for 85Rb85Rb the most common for the natural element. Parameter TeωeωexeωeyeBeαeγeDeβereν00Re Åref31Σg+ 5.4 Å43 5s+6s33Δu 5s+4d33Πu 5s + 6p22 610.2741.4 23Πu19805.242.0 0.01841 4.6 13Σg 5p+5s13Σu 5p+5s weak 13Πu 5p+5s2g13029.29 0.01568 5.01g13008.610 0.0158 5.05 012980.840 0.0151 5.05 0 inner12979.282 0.015489 5.1 0 outer13005.612 0.00478 9.2 0 c3Σu+ (unbound) 5p2P3/2 b3Πub3Π0u+9600.8360.10 4.13157 Åa3Σu+ metastable triplet a3Πu triplet ground state 141Σg+30121.0 44.9 0.01166 pred131Σg+28 863.0 46.1 0.01673 pred121Σg+28 533.9 38.4 0.01656 pred111Σg+28 349.9 42.0 0.01721 pred101Σg+27 433.1 45.3 0.01491 pred91Σg+26 967.1 45.1 0.01768 pred81Σg+26 852.9 44.6 0.01724 pred71Σg+25 773.9 76.7 0.01158 pred61Σg+24 610.8 46.3 0.01800 pred111Σu+29 709.4 41.7 0.01623 pred101Σu+29 339.2 35.0 0.016 85 pred91Σu+28 689.9 43.6 0.01661 pred81Σu+28 147.3 51.5 0.01588 pred71Σu+27 716.8 44.5 0.01636 pred61Σu+26 935.8 49.6 0.01341 pred51Σu+26108.839 0.016 47 4.951Πu26131 4.9541Σu+24 800.8 10.7 0.00298 pred41Σg+20004.13 61.296 0.01643 31Σu+ 5s+6s22 405.240.2 0.015 536 31Πu = D1Πu 5s + 6p22777.5336.255 0.01837 5008.59 4.9 Å21Σg+ 13601.5831.4884-0.01062 0.013430-0.0000018924 2963 5.437921Σu+ 6s+4d 5.5 (vibration causes a large stretching)21Πu = C1Πu20 913.18 36.255 0.01837 21Πg22 084.930.6 0.01441 11Δg11Πu11Πg 15510.2822.202-0.1525 0.013525-0.0001209 1290 cm−1 5.418B1Πu 5s+5p14665.4447.43160.15330.00600.019990.000070 1.4 A1Σu+ 5s+5p10749.74244.58 4.87368 ÅX1Σg+ 5s+5s1281657.74670.15820.00150.022780.000047 1.5/3986 cm−1 4.17

Related species The other alkali metals also form dimers: dilithium Li2, Na2, K2, and Cs2. The rubidium trimer has also been observed on the surface of helium nanodroplets. The trimer, Rb3 has the shape of an equilateral triangle, bond length of 5.52 A˚ and a binding energy of 929 cm−1.

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