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.^[1] The rubidium trimer has also been observed.

Dirubidium

Identifiers
CAS Number	25681-81-6 Y
3D model (JSmol)	Interactive image
ChemSpider	4937536 Y
PubChem CID	6432277
InChI InChI=1S/2Rb Y Key: MQZGYYYBCTXEME-UHFFFAOYSA-N Y
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

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
11Δg-X	540 nm quadrupole

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	[10]
43+ u 5s+6s
33Δu 5s+4d
33Πu 5s + 6p	22 610.27	41.4											[11]
23Πu	19805.2	42.0			0.01841							4.6	[11]
13Σg 5p+5s
13Σu 5p+5s								weak					[5]
13Πu 5p+5s
2g	13029.29				0.01568							5.0	[12]
1g	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]
c3Σu+ (unbound) 5p2P3/2													[13]
b3Πu
b3Π0u+	9600.83	60.10										4.13157 Å	[14]
a3Σu+ metastable triplet													[6]
a3Πu triplet ground state													[6]
141Σg+	30121.0	44.9			0.01166								pred[11]
131Σg+	28 863.0	46.1			0.01673								pred[11]
121Σg+	28 533.9	38.4			0.01656								pred[11]
111Σg+	28 349.9	42.0			0.01721								pred[11]
101Σg+	27 433.1	45.3			0.01491								pred[11]
91Σg+	26 967.1	45.1			0.01768								pred[11]
81Σg+	26 852.9	44.6			0.01724								pred[11]
71Σg+	25 773.9	76.7			0.01158								pred[11]
61Σg+	24 610.8	46.3			0.01800								pred[11]
111Σu+	29 709.4	41.7			0.01623								pred[11]
101Σu+	29 339.2	35.0			0.016 85								pred[11]
91Σu+	28 689.9	43.6			0.01661								pred[11]
81Σu+	28 147.3	51.5			0.01588								pred[11]
71Σu+	27 716.8	44.5			0.01636								pred[11]
61Σu+	26 935.8	49.6			0.01341								pred[11]
51Σu+	26108.8	39			0.016 47							4.9	[11][15]
51Πu	26131											4.95	[15]
41Σu+	24 800.8	10.7			0.00298								pred[11]
41Σg+	20004.13	61.296			0.01643							[11]
31Σu+ 5s+6s	22 405.2	40.2			0.015 536								[11]
31Πu = D1Πu 5s + 6p	22777.53	36.255			0.01837			5008.59				4.9 Å	[16]
21Σg+	13601.58	31.4884	-0.01062		0.013430	-0.0000018924		2963				5.4379	[17]
21Σu+ 6s+4d												5.5 (vibration causes a large stretching)	[6]
21Πu = C1Πu	20 913.18	36.255			0.01837								[11]
21Πg	22 084.9	30.6			0.01441								[11]
11Δg
11Πu
11Πg	15510.28	22.202	-0.1525		0.013525	-0.0001209		1290 cm−1				5.418	[13]
B1Πu 5s+5p	14665.44	47.4316	0.1533	0.0060	0.01999	0.000070		1.4					[3]
A1Σu+ 5s+5p	10749.742	44.58										4.87368 Å	[14]
X1Σg+ 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]