Calcium monohydride explained

Calcium monohydride is a molecule composed of calcium and hydrogen with formula CaH. It can be found in stars as a gas formed when calcium atoms are present with hydrogen atoms.

Discovery

Calcium monohydride was first discovered when its spectrum was observed in Alpha Herculis and ο Ceti by Alfred Fowler in 1907.^{[1] [2]} It was observed in sunspots the following year by C. M. Olmsted.^[3] Next, it was made in a laboratory in 1909 by A. Eagle, and with early research by Hulthèn,^[4] and Watson and Weber in 1935.^[5] It was further observed in red dwarfs by Y. Öhman in 1934. Öhman proposed its use as a proxy for stellar luminosity, similar to magnesium monohydride (MgH), in being more apparent in the spectra of compact, cool, high surface gravity stars such as M dwarfs than in cool, low surface gravity stars such as M giants of non-negligible, or even comparable, metallicity.^[6]

Calcium monohydride is the first molecular gas that was cooled by a cold buffer gas and then trapped by a magnetic field. This extends the study of trapped cold atoms such as rubidium to molecules.^[7]

Formation

Calcium monohydride can be formed by exposing metallic calcium to an electric discharge in a hydrogen atmosphere above 750 °C. Below this temperature the hydrogen is absorbed to form calcium hydride.^[8]

Calcium monohydride can be formed by laser ablation of calcium dihydride in a helium atmosphere.^[9]

Gaseous calcium reacts with formaldehyde at temperatures around 1200 K to make CaH as well as some CaOH and CaO. This reaction glows orange-red.

Properties

The dipole moment of the CaH molecule is 2.94 debye.^{[10] [11]} Spectrographic constants have been measured as bond length R_e=2.0025 Å dissociation energy D_e=1.837 eV and harmonic vibrational frequency ω_e=1298.34 cm⁻¹.^[10] Ionisation potential is 5.8 eV.^[10] Electron affinity is 0.9 eV.^[10]

The ground state is X²Σ⁺.^[10]

The electronic states are:^[12]

• 6σ²7σ X²Σ^+[13]
• 6σ²3π A²Π
• 6σ²8σ B²Σ⁺
• 6σ²4π E²Π
• 6σ7σ² D²Σ⁺

Spectrum

B²Σ, with ν'=0 ← X²Σ with ν"=0 634 nm (or is it 690 nm?)^[14] CaH fluoresces with 634 nm light giving 690 nm emissions.^[9]

B²Σ⁺ ← X²Σ⁺ 585.8 nm to 590.2 nm.^[15]

A⁺²Π ← X²Σ⁺ 686.2 to 697.8 nm^[15]

R12 branch^[15]

J'	J"	N"	ν	nm	THz
3/2	1/2	0	14408.94	694.0135	431.9691
5/2	3/2	1	14421.12	693.4274	432.3343
7/2	5/2	2	14432.92	692.8605	432.6881
9/2	7/2	3	14444.54	692.3031	433.0364
11/2	9/2	4	14455.76	691.7658	433.3728
13/2	11/2	5	14467.20	691.2188	433.71574

R2 branch^[15]

J'	J"	N"	ν	nm	THz
3/2	1/2	0	14480.93	690.5633	434.1274
5/2	3/2	1	14495.08	689.8893	434.5516
7/2	5/2	2	14510.09	689.1756	435.0015
9/2	7/2	3	14525.53	688.4430	435.4644
11/2	9/2	4	14541.43	687.6903	435.9411
13/2	11/2	5	14557.98	686.9085	436.4373

C²Σ⁺ →X²Σ⁺ transition is in near ultraviolet.^[8]

Microwave spectrum

The energy required to spin the CaH molecule from its lowest level to the first quantum level corresponds to a microwave frequency, so there is an absorption around 253 GHz. However, the spin of the molecule is also affected by the spin of an unpaired electron on the calcium, and the spin of the proton in the hydrogen. The electron spin leads to splitting of the line by about 1911.7 MHz, and the spin relative to the proton spin results in hyperfine splitting of the line by about 157.3 MHz.^[16]

molecule spin quantum number		electron spin quantum number		proton spin quantum number		frequency
N	N'	J	J'	F	F'	kHz
0	1	1/2	1/2	1	1	252163082
0	1	1/2	1/2	1	0	252216347
0	1	1/2	1/2	0	1	252320467
0	1	1/2	3/2	1	1	254074834
0	1	1/2	3/2	1	2	254176415
0	1	1/2	3/2	0	1	254232179

Reactions

CaH reacts with Lithium as a cold gas releasing 0.9eV of energy and forming LiH molecules and calcium atoms.^[17]

Extra reading

• Calvin. Aaron T.. Janardan. Smitha. Condoluci. John. Rugango. Réne. Pretzsch. Eric. Shu. Gang. Brown. Kenneth R.. Rovibronic Spectroscopy of Sympathetically Cooled40CaH. The Journal of Physical Chemistry A. 122. 12. 3177–3181. 16 March 2018. 10.1021/acs.jpca.7b12823. 29521505. 2018JPCA..122.3177C. 1853/60241 . free.

Notes and References

1. Barbuy . B.. Schiavon . R. P.. Gregorio-Hetem . J.. Singh . P. D.. Batalha . C.. October 1993. Intensity of CaH Lines in Cool Dwarfs. Astronomy and Astrophysics Supplement Series. 101. 2. 409. 1993A&AS..101..409B.
2. Fowler . A. . Alfred Fowler. 1907. The fluted spectrum of titanium oxide. Proceedings of the Royal Society A. 79. 533. 509–18. 1907RSPSA..79..509F. 10.1098/rspa.1907.0059. free.
3. Olmsted . Charles M.. 1908. Sun-spot bands which appear in the spectrum of a calcium arc burning in the presence of hydrogen.. Contributions from the Solar Observatory of the Carnegie Institution of Washington. 21. 1–4. 1908CMWCI..21....1O.
4. 10.1103/PhysRev.29.97. 29. 1. 97–111. Hulthèn. E.. On The Band Spectrum of Calcium Hydride. Physical Review. 1 January 1927. 1927PhRv...29...97H.
5. 10.1103/PhysRev.48.732. 48. 9. 732–734. Watson . William. Robert . Weber. The E Band System of Calcium Hydride. Physical Review. 1 November 1935. 1935PhRv...48..732W.
6. Öhman . Yngve . October 1934. Spectrographic Studies in the Red. Astrophysical Journal. 80. 171. 1934ApJ....80..171O. 10.1086/143595.
7. Bretislav Friedrich. John M. Doyle . 2009 . Why are Cold Molecules so Hot?. ChemPhysChem . 10 . 4 . 604–623 . 10.1002/cphc.200800577 . 19229896 . Bretislav Friedrich .
8. Frum. C.I.. H.M. Pickett . 1993. High-Resolution Infrared Fourier Transform Emission Spectroscopy of Metal Hydrides: X²Σ⁺ state of CaH. Journal of Molecular Spectroscopy. 159. 2. 329–336 . 10.1006/jmsp.1993.1130. 1993JMoSp.159..329F.
9. Doyle. John M.. Jonathan D. Weinstein . Robert deCarvalho . Thierry Guillet . Bretislav Friedrich. Bretislav Friedrich . 1998. Magnetic trapping of calcium monohydride molecules at millikelvin temperatures. Nature. 395. 6698. 148–150 . 10.1038/25949. 1998Natur.395..148W. 38268509.
10. Holka. Filip. Miroslav Urban . 2006. The dipole moment and molecular properties of CaH: A theoretical study. Chemical Physics Letters. 426. 4–6. 252–256 . 10.1016/j.cplett.2006.05.108. 2006CPL...426..252H.
11. 121. 2. 829–834. Steimle . T. C.. Jinhai Chen . Jamie Gengler. The permanent electric dipole moments of calcium monohydride, CaH. The Journal of Chemical Physics. 2004-07-08. 15260612. 2004JChPh.121..829S. 10.1063/1.1759314 .
12. Ram. R.S.. K.. Tereszchuk. I.E.. Gordon. K.A.. Walker. P.F.. Bernath. 2011. Fourier transform emission spectroscopy of the E²Π–X²Σ⁺ transition of CaH and CaD. Journal of Molecular Spectroscopy. 266. 2. 86–91 . 10.1016/j.jms.2011.03.009. 2011JMoSp.266...86R.
13. Fourier Transform Emission Spectroscopy of the E²Π–X²Σ⁺ System of CaH and CaD . Ohio State University . I.. Gordon. R. S.. Ram. K.. Tereszchuk. K. A.. Walker. P. F.. Bernath. 1 April 2011 . 1811/49445 . free.
14. Berg. L-E. L Klynning . 1974. Rotational Analysis of the A-X and B-X Band Systems of CaH. Physica Scripta. 10. 6. 331–336 . 10.1088/0031-8949/10/6/009. 1974PhyS...10..331B. 250910373 .
15. Pereira. R.. S. Skowronek . A. González Ureña . A. Pardo . J.M.L. Poyato . A.H. Pardo . 2002. Rotationally Resolved REMPI Spectra of CaH in a Molecular Beam. Journal of Molecular Spectroscopy. 212. 1. 17–21. 10.1006/jmsp.2002.8531. 2002JMoSp.212...17P.
16. The millimeter-wave spectrum of CaH (X ²Σ⁺) . Barclay. W. L. Jr. . Anderson. M. A. . Ziurys. L. M.. Lucy Ziurys . Astrophysical Journal Letters . 408. 1. L65–L67. 1993ApJ...408L..65B. 1993. 10.1086/186832. free.
17. Singh. Vijay . Kyle S. Hardman . Naima Tariq . Mei-Ju Lu . Aja Ellis . Muir J. Morrison . Jonathan D. Weinstein . 2012. Chemical Reactions of Atomic Lithium and Molecular Calcium Monohydride at 1 K. Physical Review Letters. 108. 20. 203201 . 10.1103/PhysRevLett.108.203201. 23003146 . 2012PhRvL.108t3201S . free . 20.500.11937/20342 . free .