CHEM-5151_S05_L5

CHEM-5151_S05_L5 - Lecture 5: Spectroscopy and...

Info iconThis preview shows pages 1–5. Sign up to view the full content.

View Full Document Right Arrow Icon
1 Required Reading: FP Chapter 3 Suggested Reading: SP Chapter 3 Atmospheric Chemistry CHEM-5151 / ATOC-5151 Spring 2005 Maggie Tolbert & Jose-Luis Jimenez Lecture 5: Spectroscopy and Photochemistry I Outline of Next Two Lectures • Today – Importance of spectroscopy & photochemistry – Nature of light, EM spectrum – Molecular spectroscopy • Thursday – The Sun as a radiation source – Light absorption – Atmospheric photochemistry
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
2 Importance of Spectroscopy and Photochemistry I Most chemical processes in the atmosphere are initiated by photons – Photolysis of O 3 generates OH – the most important atmospheric oxidizer: O 3 + hv O 2 + O( 1 D) O( 1 D) + H 2 O 2 OH – Solar photodissociation of many atmospheric molecules is often much faster than any other chemical reactions involving them: CF 2 Cl 2 + hv CF 2 Cl + Cl (photolysis of CFCs in the stratosphere) HONO + hv OH + NO (source of OH in the troposphere) NO 2 + hv O + NO (source of O 3 in the troposphere) NO 3 + hv O 2 + NO or O + NO 2 (removal of NO 3 generated at night) Cl 2 + hv Cl + Cl (source of Cl atoms) H 2 CO + hv H 2 + CO or H + HCO (important step of hydrocarbon oxidation) etc. Importance of Spectroscopy and Photochemistry II Absorption of solar and earth radiation by atmospheric molecules directly influences the energy balance of the planet – Greenhouse effect (CO 2 , H 2 O, N 2 O, CFCs) – Stratospheric temperature inversion (O 3 photochemistry) Spectroscopy of atmospheric molecules is used to detect them in situ – OH is detected via its electronic transition at 310 nm –NH 3 is detected via its fundamental vibrational transition at 1065 cm -1 , etc.
Background image of page 2
3 Solar Radiation: Initiator of Atmos. Reactions Average thermal energy of collisions: ~ RT = 8.3 J mol -1 K -1 x T RT = 2.5 kJ mol -1 @ 300 K Energy of photons (E = hv ): 300 nm photon = 380 kJ mol -1 600 nm photon = 190 kJ mol -1 Typical bond strengths: D 0 (O 2 ) = 495 kJ mol -1 D 0 (Cl 2 ) = 243 kJ mol -1 C-H, O-H, C-O ~ 400 kJ mol -1 Atmospheric chemistry on Earth is driven by photolysis, not by thermal excitation!!! From S. Nidkorodov What is light? • Dual nature – Photon: as particle • Energy but no mass – As wave: electric and magnetic fields oscillating in space and time • Wavelength, frequency • c ~ 3 x 10 9 m/s From F-P&P Discuss in class: at a fundamental physical level, why are molecules capable of absorbing light?
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
4 The Electromagnetic Spectrum • Units used for photon energies and wavelengths: – 1 eV = 8065.54 cm -1 = 96.4853 kJ/mol = 23.0605 kcal/mol = 11604.4 K – 1 Å = 0.1 nm = 10 -10 m; micron = 10 -6 m = 1000 nm
Background image of page 4
Image of page 5
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 14

CHEM-5151_S05_L5 - Lecture 5: Spectroscopy and...

This preview shows document pages 1 - 5. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online