CH405 Dynamics of Chemical Reactions: Introduction to Modern Experimental Method
James, Terry, Morning Host, Co-Founder has reference to this Academic Journal, PHwiki organized this Journal CH405 Dynamics of Chemical Reactions: Introduction to Modern Experimental Methods Assessment methods Oral presentation 5th December 9:05 11:55 B209 Type of assessment Length % weighting Examinations 1.5 Hours 80 Oral Presentation 20 Assessment methods ORAL PRESENTATION FOR CH405 You have been given a research article or letter relating to material or techniques covered in the module. You are required to critically read the article in addition to prepare a 10 minutes presentation as long as a non specialist audience, containing the following elements (in any order you deem appropriate): 1) The context of the article is explained 2) The main findings are described 3) The methodology used by the investigators is outlined 3-4 minutes of discussion (based on questions asked to you by lecturers or other students) will follow your presentation. You should be able to provide clarification on any aspect you decided to include in the presentation.
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5 lectures VGS The LASER in addition to its properties Laser based techniques Examples of modern techniques through pioneering studies: Photodissociation: Femtochemistry High Rydberg Time of Flight The LASER Reminder Light: Electromagnetic radiation Sinusoidally oscillating electric in addition to magnetic fields LASER light Special properties High directionality High intensity Can be highly monochromatic Can be continuous or very short pulsed Highly polarised (all E vectors aligned) Light Amplification by the Stimulated Emission of Radiation
Interaction of light / matter Absorption Photon lost Sample absorbs energy E1 E2 n1 n2 Rate of absorption ()n1 Rate of absorption = B12()n1 B12 is the Einstein coefficient as long as absorption Interaction of light / matter Spontaneous Emission Photon created Sample emits (loses) energy n1 n2 Rate of spontaneous emission n2 Rate of spontaneous emission = A21n2 A21 is the Einstein coefficient as long as spontaneous emission Interaction of light / matter Stimulated Emission Photon created Sample emits (loses) energy The stimulated emission is monochromatic in addition to in phase with the same polarization as the stimulating photon n1 n2 E1 E2 Rate of stimulated emission ()n2 Rate of stimulated emission = B21()n2 B21 is the Einstein coefficient as long as stimulated emission
Einstein coefficients: It can be shown that actually there is only one independent Einstein coefficient: LASER radiation is dominated by stimulated emission i.e., B21 = B12 = CA21 At eqm, rate absn = rate st. em + rate sp. em B12()n1 = B21()n2 + A21n2 i.e., () = A21 B12 eh/kt – B21 Recall n2 n1 = e-E/kt Yet Plancks law states () = 8h3 . 1 eh /kt -1 c3 Conditions as long as LASER Action Stimulated emission to dominate spontaneous emission Want more photons out than are absorbed Feedback Amplification in a fixed direction These impose requirements : A) If stim. Emission is to dominate absorption, we need rate of stim. Emission >> rate of absorption i.e., But as long as systems in equilibrium, n2/n1 is given by the Boltzmann Law: which as long as all temperatures gives n2 < n1. In other words we require population inversion B21()n2 B12()n1 >> 1 i.e., n2 n1 >> 1
Other requirements : B) If Stim. Emission is to dominate spontaneous emission, we need rate of stim. Emission >> rate of spont. emission i.e., We require the radiation intensity to be as large as possible. B21()n2 A21n2 >> 1 i.e., B21 A21 >> 1 () Typical LASER cavity 100% Reflective mirror Partial mirror Lasing medium (gas, crystal etc.) Schematic LASER Action 100% Reflective mirror Partial mirror 1: Pump system to excited levels
Schematic LASER Action 100% Reflective mirror Partial mirror 2: Initial spontaneous emission Schematic LASER Action 100% Reflective mirror Partial mirror 3: Followed by stimulated emission Schematic LASER Action 100% Reflective mirror Partial mirror 4: Feedback produces amplification light leaks out of the partial mirror each trip
Specific Examples of lasers Different Lasers have: Different lasing media Different, often sophisticated methods of generating population inversion Some are fixed wavelength others tuneable. A. The Helium Neon Laser Very common A gas laser (medium a mixture of He in addition to Ne) The first continuous wave (cw) laser Lasing occurs in excited Ne atoms Most common wavelength 632.8nm (RED) HeNe He Discharge creates metastable, He He +Ne Ne + He Creates population inversion in Ne Ne
B.The excimer / exciplex laser Gas lasers Electric discharge creates ions which recombine to give exotic species (excited dimers) Usually high energy pulsed lasers (up to 1J/10ns) Almost always generate ultraviolet light Common versions include ArF (193nm) produces O3 in lab-pungent KrF (248nm) XeCl (308nm) Very common Excimers Discharge ionizes gas mix Ar+ in addition to F- recombine on excited ionic surface Upon charge transfer the system drop to the covalent surface which is dissociative always population inversion Discharge / reaction collision C. The Nd:YAG LASER Solid state laser (crystalline rods) Nd3+ ions doped in a Yttrium Aluminium Garnate crystal Population inversion achieved by external pumping with flashlamps Can be cw or pulsed Lases at 1064 nm (near IR) but frequency doubling generates harmonics at 532 nm or 355 nm.
Tuneable Lasers: Dye Lasers Use large organic molecules as lasing medium Population inversion is created by pumping with a fixed wavelength laser (e.g., excimer / NdYAG) Each dye has a tuning window determined by its fluorescence spectrum. Dye has a lifetime. Need to replace every so often (Rhodium 6G very popular-Red). Schematic dye laser Common dye: Rhodamine 6G PUMP LASE
Different dyes cover IR-UV Ultrafast lasers time ns 10-9 s ps 10-12 s fs 10-15 s Generating ultrafast laser pulses Mode locking-Revision This technique can produce pulses of picosecond (1 ps = 1 x 10-12 Sec) duration in addition to less. The laser radiates at a number of different frequencies depending on (a) the medium in addition to (b) the number of half wavelengths trapped between the mirrors (resonant modes) n x = L 1 2 n = Integer L = Length of cavity Locking the phases of the different frequencies together, interference leads to a series of sharp peaks (pulse duration).
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