Dirubidium is a molecular substance containing
two atoms of
rubidium
Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density higher ...
found in rubidium vapour. Dirubidium has two active
valence electron
In chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, a shared pair forms ...
s. 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 (Δ
fH
°) 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 Rb
2 in rubidium vapour varies with its density, which depends on the temperature. At 200° the partial pressure of Rb
2 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 nanodroplet
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
s when two rubidium atoms combine to yield the dimer:
:Rb + Rb → Rb
2
Rb
2 has also been produced in
solid helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
matrix under pressure.
Ultracold rubidium atoms can be stored in a
magneto-optic trap
A magneto-optical trap (MOT) is an apparatus which uses laser cooling and a spatially-varying magnetic field to create a trap which can produce samples of cold, trapped, neutral atoms. Temperatures achieved in a MOT can be as low as several microk ...
and then photoassociated to form molecules in an excited state, vibrating at a rate so high they barely hang together.
In solid matrix traps, Rb
2 can combine with the host atoms when excited to form
exciplex
An excimer (originally short for excited dimer) is a short-lived dimeric or heterodimeric molecule formed from two species, at least one of which has a valence shell completely filled with electrons (for example, noble gases). In this case, form ...
es, for example Rb
2(
3Π
u)He
2 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
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...
. Laser induced-fluorescence can reveal the life-times of excited states.
In the absorption spectrum of rubidium vapour, Rb
2 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
The diffuse series is a series of spectral lines in the atomic emission spectrum caused when electrons jump between the lowest p orbital and d orbitals of an atom. The total orbital angular momentum changes between 1 and 2. The spectral lines inclu ...
. There are a few more small absorption features in the near infrared.
There is also a dirubidium cation, Rb
2+ with different spectroscopic properties.
Bands
Molecular constants for excited states
The following table has parameters for
85Rb
85Rb the most common for the natural element.
Related species
The other alkali metals also form dimers:
dilithium
Dilithium, Li2, is a strongly electrophilic, diatomic molecule comprising two lithium atoms covalently bonded together. Li2 is known in the gas phase.
It has a bond order of 1, an internuclear separation of 267.3 pm and a bond energy o ...
Li
2, Na
2, K
2, and Cs
2. The rubidium trimer has also been observed on the surface of helium nanodroplets. The trimer, Rb
3 has the shape of an equilateral triangle, bond length of 5.52 A˚ and a binding energy of 929 cm
−1.
References
{{Rubidium compounds
Rubidium
Homonuclear diatomic molecules
Allotropes