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