Dirubidium

Dirubidium
Identifiers
CAS Number	25681-81-6 ;
3D model (JSmol)	Interactive image;
ChemSpider	4937536 ;
PubChem CID	6432277;
	InChI InChI=1S/2Rb Key: MQZGYYYBCTXEME-UHFFFAOYSA-N ;
	SMILES [Rb][Rb];
Properties
Chemical formula	Rb2
Molar mass	170.9356 g·mol−1
Hazards
	Occupational safety and health (OHS/OSH):
Main hazards	Flammable
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

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.^[1] 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.^[2] In practice, an oven heated to 600 to 800K with a nozzle can squirt out vapour that condenses into dimers.^[3] 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).^[4]

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.^[5]

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.^[6] In solid matrix traps, Rb₂ can combine with the host atoms when excited to form exciplexes, for example Rb₂(³Π_u)He₂ in a solid helium matrix.^[7]

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.^[8]

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.^[1]

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.^[9]

There is also a dirubidium cation, Rb₂⁺ with different spectroscopic properties.^[1]

Bands

Transition	Colour	Known vibrational bands	Bandheads
A-X	infrared
B-X	red	4-0 5-0 6-0 7-0 8-0 9-0 10-0 11-0 6-1 7-1 8-1 9-2	14847.080 to 15162.002
C-X	blue
D-X	blue-violet
1-C	infrared
C→2	6800–8000 cm⁻¹
1¹Δ_g-X	540 nm quadrupole

Molecular constants for excited states

The following table has parameters for ⁸⁵Rb⁸⁵Rb the most common for the natural element.

Parameter	T_e	ω_e	ω_ex_e	ω_ey_e	B_e	α_e	D_e	R_e Å	ref
3¹Σ_g⁺								5.4 Å	^[10]
4³⁺ _u 5s+6s
3³Δ_u 5s+4d
3³Π_u 5s + 6p	22 610.27	41.4							^[11]
2³Π_u	19805.2	42.0			0.01841			4.6	^[11]
1³Σ_g 5p+5s
1³Σ_u 5p+5s							weak		^[5]
1³Π_u 5p+5s
2_g	13029.29				0.01568			5.0	^[12]
1_g	13008.610				0.0158			5.05	^[12]
0⁻ _g	12980.840				0.0151			5.05	^[6]^[12]
0⁺ _g inner	12979.282				0.015489			5.1	^[12]
0⁺ _g outer	13005.612				0.00478			9.2	^[12]
0⁺ _u									^[6]^[12]
c³Σ_u⁺ (unbound) 5p²P_3/2									^[13]
b³Π_u
b³Π_0u⁺	9600.83	60.10						4.13157 Å	^[14]
a³Σ_u⁺ metastable triplet									^[6]
a³Π_u triplet ground state									^[6]
14¹Σ_g⁺	30121.0	44.9			0.01166				pred^[11]
13¹Σ_g⁺	28 863.0	46.1			0.01673				pred^[11]
12¹Σ_g⁺	28 533.9	38.4			0.01656				pred^[11]
11¹Σ_g⁺	28 349.9	42.0			0.01721				pred^[11]
10¹Σ_g⁺	27 433.1	45.3			0.01491				pred^[11]
9¹Σ_g⁺	26 967.1	45.1			0.01768				pred^[11]
8¹Σ_g⁺	26 852.9	44.6			0.01724				pred^[11]
7¹Σ_g⁺	25 773.9	76.7			0.01158				pred^[11]
6¹Σ_g⁺	24 610.8	46.3			0.01800				pred^[11]
11¹Σ_u⁺	29 709.4	41.7			0.01623				pred^[11]
10¹Σ_u⁺	29 339.2	35.0			0.016 85				pred^[11]
9¹Σ_u⁺	28 689.9	43.6			0.01661				pred^[11]
8¹Σ_u⁺	28 147.3	51.5			0.01588				pred^[11]
7¹Σ_u⁺	27 716.8	44.5			0.01636				pred^[11]
6¹Σ_u⁺	26 935.8	49.6			0.01341				pred^[11]
5¹Σ_u⁺	26108.8	39			0.016 47			4.9	^[11]^[15]
5¹Π_u	26131							4.95	^[15]
4¹Σ_u⁺	24 800.8	10.7			0.00298				pred^[11]
4¹Σ_g⁺	20004.13	61.296			0.01643			^[11]
3¹Σ_u⁺ 5s+6s	22 405.2	40.2			0.015 536				^[11]
3¹Π_u = D¹Π_u 5s + 6p	22777.53	36.255			0.01837		5008.59	4.9 Å	^[16]
2¹Σ_g⁺	13601.58	31.4884	-0.01062		0.013430	-0.0000018924	2963	5.4379	^[17]
2¹Σ_u⁺ 6s+4d								5.5 (vibration causes a large stretching)	^[6]
2¹Π_u = C¹Π_u	20 913.18	36.255			0.01837				^[11]
2¹Π_g	22 084.9	30.6			0.01441				^[11]
1¹Δ_g
1¹Π_u
1¹Π_g	15510.28	22.202	-0.1525		0.013525	-0.0001209	1290 cm⁻¹	5.418	^[13]
B¹Π_u 5s+5p	14665.44	47.4316	0.1533	0.0060	0.01999	0.000070	1.4		^[3]
A¹Σ_u⁺ 5s+5p	10749.742	44.58						4.87368 Å	^[14]
X¹Σ_g⁺ 5s+5s	12816	57.7467	0.1582	0.0015	0.02278	0.000047	1.5/3986 cm⁻¹	4.17	^[3]^[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.^[18]

References

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^ Chase, M. W. (1998). "Dirubidium". webbook.nist.gov: 1–1951.
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