Dirubidium

{{chembox | Verifiedimages = changed | Watchedfields = changed | verifiedrevid = | ImageFile_Ref = {{chemboximage|correct|??}} | Name = | ImageFile = Dirubidium-2D-dimensions.png | ImageFile1 = Dirubidium-3D-vdW.png | ImageSize = | ImageAlt = ball model of dirubidium | IUPACName = | OtherNames = | SystematicName = | Section1 = {{Chembox Identifiers | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | ChemSpiderID = 4937536 | PubChem = 6432277 | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChI = 1S/2Rb | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = MQZGYYYBCTXEME-UHFFFAOYSA-N | CASNo_Ref = {{cascite|correct|CAS}} | CASNo = 25681-81-6 | SMILES = [Rb][Rb] }} | Section2 = {{Chembox Properties | Rb = 2 | Appearance = | MagSus = }} | Section3 = {{Chembox Hazards | MainHazards = Flammable | FlashPtC = | AutoignitionPtC = }} | Section4 = | Section5 = | Section6 = }} '''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.<ref name="spieg">{{cite journal|last1=Spiegelmann|first1=F|last2=Pavolini|first2=D|last3=Daudey|first3=J -P|title=Theoretical study of the excited states of the heavier alkali dimers. II. The Rb molecule|journal=Journal of Physics B: Atomic, Molecular and Optical Physics|date=28 August 1989|volume=22|issue=16|pages=2465–2483|doi=10.1088/0953-4075/22/16/005|bibcode=1989JPhB...22.2465S|s2cid=250784871}}</ref> 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&nbsp;kJ/mol.<ref>{{cite journal|title=Dirubidium|url=http://webbook.nist.gov/cgi/inchi?ID=C25681816&Mask=1#Thermo-Gas|website=webbook.nist.gov|year = 1998|pages = 1–1951|language=en|last1 = Chase|first1 = M. W.}}</ref> In practice, an oven heated to 600 to 800K with a nozzle can squirt out vapour that condenses into dimers.<ref name="cald"/> 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&nbsp;°C it constitutes 1.6% of the pressure, and at 677&nbsp;°C the dimer has 7.4% of the vapour pressure (13.8% by mass).<ref>{{cite journal|last1=Rakića|first1=M.|last2=Pichler|first2=G.|date=March 2008|title=Photoionization bands of rubidium molecule|journal=Journal of Quantitative Spectroscopy and Radiative Transfer|volume=208|pages=39–44|bibcode=2018JQSRT.208...39R|doi=10.1016/j.jqsrt.2018.01.003}}</ref>

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.<ref name="Moro">{{cite journal|last1=Moroshkin|first1=P.|last2=Hofer|first2=A.|last3=Ulzega|first3=S.|last4=Weis|first4=A.|title=Spectroscopy of Rb2 dimers in solid|journal=Physical Review A|date=7 September 2006|volume=74|issue=3|article-number=032504|doi=10.1103/PhysRevA.74.032504|arxiv=physics/0606100|bibcode=2006PhRvA..74c2504M|s2cid=53701950}}</ref>

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.<ref name="huang" /> 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.<ref>{{cite journal|last1=Moroshkin|first1=P.|last2=Hofer|first2=A.|last3=Ulzega|first3=S.|last4=Weis|first4=A.|title=Spectroscopy of Rb₂ dimers in solid ⁴He|journal=Physical Review A|date=7 September 2006|volume=74|issue=3|article-number=032504 |doi=10.1103/PhysRevA.74.032504|arxiv=physics/0606100|bibcode=2006PhRvA..74c2504M|s2cid=53701950}}</ref>

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.<ref>{{cite journal|last1=Shore|first1=Bruce W|last2=Dömötör|first2=Piroska|last3=Sadurní|first3=Emerson|last4=Süssmann|first4=Georg|last5=Schleich|first5=Wolfgang P|title=Scattering of a particle with internal structure from a single slit|journal=New Journal of Physics|date=27 January 2015|volume=17|issue=1|article-number=013046|doi=10.1088/1367-2630/17/1/013046|bibcode=2015NJPh...17a3046S|doi-access=free|hdl=1969.1/180938|hdl-access=free}}</ref>

==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.<ref name="spieg" />

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&nbsp;nm makes the vapour almost opaque from 670 to 700&nbsp;nm, wiping out the far red end of the spectrum. This is the band due to X→B transition. From 430 to 460&nbsp;nm there is a shark-fin shaped absorption feature due to X→E transitions. Another shark fin like effect around 475&nbsp;nm s due to X→D transitions. There is also a small hump with peaks at 601, 603 and 605.5&nbsp;nm 1→3 triplet transitions and connected to the diffuse series. There are a few more small absorption features in the near infrared.<ref>{{cite journal|last1=Vdović|first1=S.|last2=Sarkisyan|first2=D.|last3=Pichler|first3=G.|title=Absorption spectrum of rubidium and cesium dimers by compact computer operated spectrometer|journal=Optics Communications|date=December 2006|volume=268|issue=1|pages=58–63|doi=10.1016/j.optcom.2006.06.070|bibcode=2006OptCo.268...58V}}</ref>

There is also a dirubidium cation, Rb₂⁺ with different spectroscopic properties.<ref name="spieg"/>

===Bands=== {|class="wikitable" !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&nbsp;cm⁻¹|| || |- |1¹Δ_g-X||540&nbsp;nm quadrupole | | |}

===Molecular constants for excited states=== The following table has parameters for ⁸⁵Rb⁸⁵Rb the most common for the natural element. {|class="wikitable" !Parameter || T_e||ω_e||ω_ex_e||ω_ey_e||B_e||α_e||γ_e||D_e||β_e||r_e||ν₀₀||R_e&nbsp;Å||ref |- |3¹Σ_g⁺|| || || || || || || || || || || ||5.4&nbsp;Å||<ref name="yang">{{cite journal|last1=Yang|first1=Jinxin|last2=Guan|first2=Yafei|last3=Zhao|first3=Wei|last4=Zhou|first4=Zhaoyu|last5=Han|first5=Xiaomin|last6=Ma|first6=Jie|last7=Sovkov|first7=Vladimir B.|last8=Ivanov|first8=Valery S.|last9=Ahmed|first9=Ergin H.|last10=Lyyra|first10=A. Marjatta|author10-link=Marjatta Lyyra|last11=Dai|first11=Xingcan|title=Observations and analysis with the spline-based Rydberg–Klein–Rees approach for the 31Σg+ state of Rb2|journal=The Journal of Chemical Physics|date=14 January 2016|volume=144|issue=2|page=024308|doi=10.1063/1.4939524|pmid=26772572 |bibcode=2016JChPh.144b4308Y}}</ref> |- |4³{{Su|Σ|b=u|p=+}} 5s+6s | | | | | | | | | | | | | |- |3³Δ_u 5s+4d|| | | | | | | | | | | | | |- |3³Π_u 5s + 6p||22 610.27||41.4|| || || || || || || || || || ||<ref name=jas/> |- |2³Π_u||19805.2||42.0|| || ||0.01841|| || || || || || ||4.6 ||<ref name=jas/> |- |1³Σ_g 5p+5s | | | | | | | | | | | | | |- |1³Σ_u 5p+5s|| || || || || || || ||weak|| || || || ||<ref name=Moro/> |- |1³Π_u 5p+5s | | | | | | | | | | | | | |- |2_g||13029.29|| || || ||0.01568 || || || || || || ||5.0||<ref name="bellos">{{cite journal|last1=Bellos|first1=M. A.|last2=Rahmlow|first2=D.|last3=Carollo|first3=R.|last4=Banerjee|first4=J.|last5=Dulieu|first5=O.|last6=Gerdes|first6=A.|last7=Eyler|first7=E. E.|last8=Gould|first8=P. L.|last9=Stwalley|first9=W. C.|title=Formation of ultracold Rb2 molecules in the v′′ = 0 level of the a3Σ+u state via blue-detuned photoassociation to the 13Πg state|journal=Physical Chemistry Chemical Physics|date=2011|volume=13|issue=42|pages=18880–18886|doi=10.1039/C1CP21383K|pmid=21909578 |bibcode=2011PCCP...1318880B}}</ref> |- |1_g||13008.610|| || || ||0.0158 || || || || || || ||5.05 ||<ref name="bellos"/> |- |0{{su|p=−|b=g}}||12980.840|| || || ||0.0151 || || || || || || ||5.05 ||<ref name="huang"/><ref name="bellos"/> |- |0{{su|p=+|b=g}} inner||12979.282 || || || ||0.015489 || || || || || || ||5.1 ||<ref name="bellos"/> |- |0{{su|p=+|b=g}} outer||13005.612 || || || ||0.00478|| || || || || || ||9.2 ||<ref name="bellos"/> |- |0{{su|p=+|b=u}}|| || || || || || || || || || || || ||<ref name="huang">{{cite journal|last1=Huang|first1=Y|last2=Qi|first2=J|last3=Pechkis|first3=H K|last4=Wang|first4=D|last5=Eyler|first5=E E|last6=Gould|first6=P L|last7=Stwalley|first7=W C|title=Formation, detection and spectroscopy of ultracold Rb2 in the ground X 1Σg state|journal=Journal of Physics B: Atomic, Molecular and Optical Physics|date=14 October 2006|volume=39|issue=19|pages=S857–S869|doi=10.1088/0953-4075/39/19/S04|bibcode=2006JPhB...39S.857H|s2cid=52970920}}</ref><ref name="bellos"/> |- |c³Σ_u⁺ (unbound) 5''p''²P_3/2 || || || || || || || || || || || || ||<ref name=Amiot/> |- |b³Π_u | | | | | | | | | | | | | |- |b³Π_0u⁺||9600.83||60.10|| || || || || || || || || || 4.13157&nbsp;Å||<ref name="salami">{{cite journal|last1=Salami|first1=H.|last2=Bergeman|first2=T.|last3=Beser|first3=B.|last4=Bai|first4=J.|last5=Ahmed|first5=E. H.|last6=Kotochigova|first6=S.|author6-link=Svetlana Kotochigova|last7=Lyyra|first7=A. M.|author7-link=Marjatta Lyyra|last8=Huennekens|first8=J.|last9=Lisdat|first9=C.|last10=Stolyarov|first10=A. V.|last11=Dulieu|first11=O.|last12=Crozet|first12=P.|last13=Ross|first13=A. J.|title=Spectroscopic observations, spin-orbit functions, and coupled-channel deperturbation analysis of data on the A1sigma+u and b3piu states of Rb2|journal=Physical Review A|date=27 August 2009|volume=80|issue=2|article-number=022515|doi=10.1103/PhysRevA.80.022515|bibcode=2009PhRvA..80b2515S}}</ref> |- |a³Σ_u⁺ metastable triplet|| || || || || || || || || || || || ||<ref name="huang"/> |- |a³Π_u triplet ground state|| || || || || || || || || || || || ||<ref name="huang"/> |- |14¹Σ_g⁺||30121.0 ||44.9 || || ||0.01166 || || || || || || || ||pred<ref name=jas/> |- |13¹Σ_g⁺||28 863.0 ||46.1 || || ||0.01673 || || || || || || || ||pred<ref name=jas/> |- |12¹Σ_g⁺||28 533.9 ||38.4 || || ||0.01656 || || || || || || || ||pred<ref name=jas/> |- |11¹Σ_g⁺||28 349.9 ||42.0 || || ||0.01721|| || || || || || || ||pred<ref name=jas/> |- |10¹Σ_g⁺||27 433.1 ||45.3|| || ||0.01491|| || || || || || || ||pred<ref name=jas/> |- |9¹Σ_g⁺||26 967.1 ||45.1 || || ||0.01768 || || || || || || || ||pred<ref name=jas/> |- |8¹Σ_g⁺||26 852.9 ||44.6 || || ||0.01724 || || || || || || || ||pred<ref name=jas/> |- |7¹Σ_g⁺||25 773.9 ||76.7|| || ||0.01158 || || || || || || || ||pred<ref name=jas/> |- |6¹Σ_g⁺||24 610.8 ||46.3|| || ||0.01800|| || || || || || || ||pred<ref name=jas/> |- |11¹Σ_u⁺||29 709.4 ||41.7|| || ||0.01623|| || || || || || || ||pred<ref name=jas/> |- |10¹Σ_u⁺||29 339.2 ||35.0|| || ||0.016 85|| || || || || || || ||pred<ref name=jas/> |- |9¹Σ_u⁺||28 689.9 ||43.6|| || ||0.01661|| || || || || || || ||pred<ref name=jas/> |- |8¹Σ_u⁺||28 147.3 ||51.5|| || ||0.01588|| || || || || || || ||pred<ref name=jas/> |- |7¹Σ_u⁺||27 716.8 ||44.5|| || ||0.01636|| || || || || || || ||pred<ref name=jas/> |- |6¹Σ_u⁺||26 935.8 ||49.6|| || ||0.01341|| || || || || || || ||pred<ref name=jas/> |- |5¹Σ_u⁺||26108.8||39 || || ||0.016 47|| || || || || || || 4.9||<ref name=jas/><ref name=hava>{{cite journal|last1=Havalyova|first1=I.|last2=Pashov|first2=A.|last3=Kowalczyk|first3=P.|last4=Szczepkowski|first4=J.|last5=Jastrzebski|first5=W.|title=The coupled system of (5)1sigmau+ and (5) 1 Π u electronic states in Rb 2|journal=Journal of Quantitative Spectroscopy and Radiative Transfer|date=November 2017|volume=202|pages=328–334|doi=10.1016/j.jqsrt.2017.08.011|bibcode=2017JQSRT.202..328H}}</ref> |- |5¹Π_u||26131|| || || || || || || || || || || 4.95||<ref name=hava/> |- |4¹Σ_u⁺||24 800.8 ||10.7|| || ||0.00298|| || || || || || || ||pred<ref name=jas/> |- |4¹Σ_g⁺||20004.13 ||61.296 || || ||0.01643 || || || || || || ||<ref name=jas>{{cite journal|last1=Jastrzebski|first1=Wlodzimierz|last2=Kowalczyk|first2=Pawel|last3=Szczepkowski|first3=Jacek|last4=Allouche|first4=Abdul-Rahman|last5=Crozet|first5=Patrick|last6=Ross|first6=Amanda J.|title=High-lying electronic states of the rubidium dimer – predictions and experimental observation of the 51Σu+ and 5 Π states of Rb by polarization labelling spectroscopy|journal=The Journal of Chemical Physics|date=28 July 2015|volume=143|issue=4|page=044308|doi=10.1063/1.4927225|pmid=26233130|bibcode=2015JChPh.143d4308J}}</ref> | |- |3¹Σ_u⁺ 5s+6s||22 405.2||40.2|| || ||0.015 536|| || || || || || || ||<ref name=jas/> |- |3¹Π_u = D¹Π_u 5s + 6p||22777.53||36.255|| || ||0.01837|| || ||5008.59|| || || ||4.9&nbsp;Å||<ref>{{cite journal|last1=Jastrzebski|first1=W.|last2=Kowalczyk|first2=P.|title=Potential energy curve of the D(3) 1Π_u state in rubidium dimer from spectroscopic measurements|journal=Journal of Molecular Spectroscopy|date=December 2016|volume=330|pages=96–100|doi=10.1016/j.jms.2016.06.010|bibcode=2016JMoSp.330...96J}}</ref> |- |2¹Σ_g⁺ ||13601.58||31.4884||-0.01062|| ||0.013430||-0.0000018924|| ||2963|| || || ||5.4379||<ref name=cam>{{cite journal|last1=Amiot|first1=C.|last2=Verges|first2=J.|title=The Rb2 21Σ+g electronic state by laser induced fluorescence infrared Fourier transform spectroscopy|journal=Molecular Physics|date=May 1987|volume=61|issue=1|pages=51–63|doi=10.1080/00268978700100981|bibcode=1987MolPh..61...51A}}</ref> |- |2¹Σ_u⁺ 6''s''+4''d''|| || || || || || || || || || || ||5.5 (vibration causes a large stretching)||<ref name="huang"/> |- |2¹Π_u = C¹Π_u||20 913.18|| 36.255|| || ||0.01837|| || || || || || || ||<ref name=jas/> |- |2¹Π_g||22 084.9||30.6|| || || 0.01441 || || || || || || || ||<ref name=jas/> |- |1¹Δ_g|| | | | | | | | | | | | | |- |1¹Π_u | | | | | | | | | | | | | |- |1¹Π_g ||15510.28||22.202||-0.1525|| ||0.013525||-0.0001209 || ||1290&nbsp;cm⁻¹ || || || ||5.418||<ref name=Amiot>{{cite journal|last1=Amiot|first1=C.|title=The Rb2 1 1Πg electronic state by laser-induced fluorescence infrared Fourier transform spectroscopy|journal=Molecular Physics|date=July 1986|volume=58|issue=4|pages=667–678|doi=10.1080/00268978600101491|bibcode=1986MolPh..58..667A}}</ref> |- |B¹Π_u 5''s''+5''p''||14665.44||47.4316||0.1533||0.0060||0.01999||0.000070|| ||1.4|| || || || ||<ref name="cald">{{cite journal|last1=Caldwell|first1=C.D.|last2=Engelke|first2=F.|last3=Hage|first3=H.|title=High resolution spectroscopy in supersonic nozzle beams: The Rb2 B 1Πu-X 1Σ+g band system|journal=Chemical Physics|date=December 1980|volume=54|issue=1|pages=21–31|doi=10.1016/0301-0104(80)80031-0|bibcode=1980CP.....54...21C}}</ref> |- |A¹Σ_u⁺ 5''s''+5''p''||10749.742||44.58 || || || || || || || || || ||4.87368&nbsp;Å||<ref name="salami"/> |- |X¹Σ_g⁺ 5''s''+5''s''||12816||57.7467||0.1582||0.0015||0.02278||0.000047|| ||1.5/3986&nbsp;cm⁻¹|| || || ||4.17||<ref name="cald"/><ref name=cam/> |}

==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&nbsp;cm⁻¹.<ref>{{cite journal|last1=Nagl|first1=Johann|last2=Auböck|first2=Gerald|last3=Hauser|first3=Andreas W.|last4=Allard|first4=Olivier|last5=Callegari|first5=Carlo|last6=Ernst|first6=Wolfgang E.|title=Heteronuclear and Homonuclear High-Spin Alkali Trimers on Helium Nanodroplets|journal=Physical Review Letters|date=13 February 2008|volume=100|issue=6|article-number=063001|doi=10.1103/PhysRevLett.100.063001|pmid=18352466|bibcode=2008PhRvL.100f3001N}}</ref>

==References== {{Reflist}}

{{Rubidium compounds}} {{Diatomic elements}}

Category:Rubidium Category:Homonuclear diatomic molecules Category:Allotropes