Reaction dynamics and molecularspectroscopy
Tutorial
Mats LarssonDepartment of PhysicsStockholm University
Why molecular spectroscopy
Almost all information we have about the universe is carried to us by photons emitted by atoms or moleculesQuantum mechanics is our friend! The quantizednature of atoms and molecules ensures that the photons have specific wavelengths (frequencies) insteadof a broad distributionMolecular spectroscopy can be used to identify the carrier of the spetrumThe spectrum gives information about the chemicalcomposition and the local environment (temperature)
Fraunhofer lines, spectrum of the sun
hν
Sun
Absorption line
In 1904, Hoffman discovered a sharp absorption line due to CaThe line was ”fixed” or ”stationary”. It could not derive from theStar’s atmosphere or from the Earth’s atmosphere. It wasrealized that there must be something between the star and the Earth, an interstellar medium
In the 1920s and 1930s a number of diffuse bands were discovered in addition tothe sharp atomic absorption lines
The diffuse interstellar bands
In 1936, Merril wrote: ”The chemical identification of these lines has not yet been made”Today, in the year 2011, we can conclude that:
The chemical identification of these bands (lines) has not yet been made
These lines, around 4000 Å (400 nm), could not be assigned to atomsThey were identified as due to absorption by the molecules (free radicals) CH and CN, andto the molecular ion CH+
Astrochemistry was born
1937-1941 new sharp absorption lineswere discovered
Absorption by the CN molecule
T=2.3 K
kommer fundamentala atomära och kemiska processer t.ex. sådana som kan vara avgörande för molekylbildningen i universum att studeras....
Herzberg, 1950, wrote in his famous textbook: ”From the intensity ratio of the lines with J=0 and J=1 a rotational temperature of 2.3 K follows, which has of course only a very restricted meaning”McKellar, 1941, wrote in a hard-to-find paper:
”Hence, in interstellar space a somewhat similar situation [to laboratory experiments] might exist, and in any case the ”rotational” temperature found from the molecular lines for interstellar space will have its own, perhaps limited, significance.”
What does this temperature mean
Publ. Dom. Astrophys. Obs. 7, 251-272 (1941)
McKellar was close to discover the cosmicmicrowave background radiation
But the real molecular revolution came with the radiotelescopes
Molecular Motion
Study of how molecules absorb and emit EM radiation (like light) called SPECTROSCOPY
103 106 109 1012 1015 1018 1021
RADIO μ-wave
IR UV X-RAYS /γ-RAYS
frequency
Electron & nuclearmotion
Molecular rotation
Molecular vibration
Change energy ofthe valence electrons
Knock electrons out of the atomKnock neutrons / protons out nucleii
Optical region → microwave regionAbsorption spectroscopy → emission spectroscopyDiffuse clouds → dark cloudsModerate accuracy → very high accuracy
From optical astronomy to radio astronomy
From molecular spectroscopy toreaction dynamics
Molecular spectroscopy is the tool for identifying molecules in space. In some cases, HCO+ is the most famous example, the molecule was discovered in space before its laboratory spectrum was knownReaction dynamics is the tool for understanding how molecues are formed
Universums periodiska system
How dense are the molecular cloudsBetween galaxies 6×10–11 cm–3
The Milky Way, average 1 cm–3
Diffuse molecular clouds 102 cm–3
Dense (dark) molecular clouds 104 cm–3
Vacuum (10–6 Torr) 1010 cm–3
In this room 1019 cm–3
Center of the sun 1026 cm–3
1 miljard år
2 månader
Cosmic ionization
Chemical reactions in the interstellarmedium
Only bimolecular reactions (a third body is never present in the gas phase)Ion-molecule reactionsDissociative recombinationSurface reactions on grains sometimes needed to explain the presence of some molecules (methanol, for example)
Ion-molecule reactions:H3
+ + O → H2 + OH+
OH+ + H2 → H2O+ + HH2O+ + H2 → H3O+ + H
Dissociative recombination:AB+ + e →A + B
H3O+ → H2O + H or OH + H + H
Reactions
Example: ion-molecule reaction
Gordon Dunn et al. 1984: CH3+ + H2 → CH5
+ + hν
2.4 T13 K
Example: ion-molecule reaction
Ion trap fromRoland Wester’slaboratory
H2 + H2+→ H3
+ + H