Dirubidium
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 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.The rubidium dimer has been formed on the surface of helium nanodroplets when two rubidium atoms combine to yield the dimer:
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 Rb2He2 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
Molecular constants for excited states
The following table has parameters for 85Rb85Rb the most common for the natural element.| Parameter | Te | ωe | ωexe | ωeye | Be | αe | γe | De | βe | re | ν00 | Re Å | ref |
| 31Σg+ | 5.4 Å | ||||||||||||
| 43 5s+6s | |||||||||||||
| 33Δu 5s+4d | |||||||||||||
| 33Πu 5s + 6p | 22 610.27 | 41.4 | |||||||||||
| 23Πu | 19805.2 | 42.0 | 0.01841 | 4.6 | |||||||||
| 13Σg 5p+5s | |||||||||||||
| 13Σu 5p+5s | weak | ||||||||||||
| 13Πu 5p+5s | |||||||||||||
| 2g | 13029.29 | 0.01568 | 5.0 | ||||||||||
| 1g | 13008.610 | 0.0158 | 5.05 | ||||||||||
| 0 | 12980.840 | 0.0151 | 5.05 | ||||||||||
| 0 inner | 12979.282 | 0.015489 | 5.1 | ||||||||||
| 0 outer | 13005.612 | 0.00478 | 9.2 | ||||||||||
| 0 | |||||||||||||
| c3Σu+ 5p2P3/2 | |||||||||||||
| b3Πu | |||||||||||||
| b3Π0u+ | 9600.83 | 60.10 | 4.13157 Å | ||||||||||
| a3Σu+ metastable triplet | |||||||||||||
| a3Πu triplet ground state | |||||||||||||
| 141Σg+ | 30121.0 | 44.9 | 0.01166 | pred | |||||||||
| 131Σg+ | 28 863.0 | 46.1 | 0.01673 | pred | |||||||||
| 121Σg+ | 28 533.9 | 38.4 | 0.01656 | pred | |||||||||
| 111Σg+ | 28 349.9 | 42.0 | 0.01721 | pred | |||||||||
| 101Σg+ | 27 433.1 | 45.3 | 0.01491 | pred | |||||||||
| 91Σg+ | 26 967.1 | 45.1 | 0.01768 | pred | |||||||||
| 81Σg+ | 26 852.9 | 44.6 | 0.01724 | pred | |||||||||
| 71Σg+ | 25 773.9 | 76.7 | 0.01158 | pred | |||||||||
| 61Σg+ | 24 610.8 | 46.3 | 0.01800 | pred | |||||||||
| 111Σu+ | 29 709.4 | 41.7 | 0.01623 | pred | |||||||||
| 101Σu+ | 29 339.2 | 35.0 | 0.016 85 | pred | |||||||||
| 91Σu+ | 28 689.9 | 43.6 | 0.01661 | pred | |||||||||
| 81Σu+ | 28 147.3 | 51.5 | 0.01588 | pred | |||||||||
| 71Σu+ | 27 716.8 | 44.5 | 0.01636 | pred | |||||||||
| 61Σu+ | 26 935.8 | 49.6 | 0.01341 | pred | |||||||||
| 51Σu+ | 26108.8 | 39 | 0.016 47 | 4.9 | |||||||||
| 51Πu | 26131 | 4.95 | |||||||||||
| 41Σu+ | 24 800.8 | 10.7 | 0.00298 | pred | |||||||||
| 41Σg+ | 20004.13 | 61.296 | 0.01643 | ||||||||||
| 31Σu+ 5s+6s | 22 405.2 | 40.2 | 0.015 536 | ||||||||||
| 31Πu = D1Πu 5s + 6p | 22777.53 | 36.255 | 0.01837 | 5008.59 | 4.9 Å | ||||||||
| 21Σg+ | 13601.58 | 31.4884 | -0.01062 | 0.013430 | -0.0000018924 | 2963 | 5.4379 | ||||||
| 21Σu+ 6s+4d | 5.5 | ||||||||||||
| 21Πu = C1Πu | 20 913.18 | 36.255 | 0.01837 | ||||||||||
| 21Πg | 22 084.9 | 30.6 | 0.01441 | ||||||||||
| 11Δg | |||||||||||||
| 11Πu | |||||||||||||
| 11Πg | 15510.28 | 22.202 | -0.1525 | 0.013525 | -0.0001209 | 1290 cm−1 | 5.418 | ||||||
| B1Πu 5s+5p | 14665.44 | 47.4316 | 0.1533 | 0.0060 | 0.01999 | 0.000070 | 1.4 | ||||||
| A1Σu+ 5s+5p | 10749.742 | 44.58 | 4.87368 Å | ||||||||||
| X1Σg+ 5s+5s | 12816 | 57.7467 | 0.1582 | 0.0015 | 0.02278 | 0.000047 | 1.5/3986 cm−1 | 4.17 |