Experimental Techniques in Reaction Kinetics

Ksrleti mdszerek Experimental techniques in reaction kinetics

Techniques classified Batch reactors vs. flow reactors Online methods: real-time in situ detection Offline methods: aliquot from the batch quenched flow Pseudo-first order (isolation or flooding) method (only if relative concentrations are available) ??1 ?? ??1 ?? ? = ??2 = ??1?2 = ? ?1 1. measurement of both ?1and ?2 2. ? from the above data 1. ?1<< ?2; series of measurements at several ?2 (time dependence of ?1) 2. ?2 dependent ? from each series 3. ? from the above ? (?2) data Biased; lower precision Unbiased; higher precision

classical techniques (time-limit: manipulation time; ~ 1 s) C4H9Cl + H2O Electrode Inlet for C4H9Cl Thermometer C4H9OH + H++ Cl Conductivity meter Reaction mixture PC or chart recorder Voltage Magnetic stirrer Thermostatted jacket Time

Discharge flow (time-limit: mixing time; ~ 1 ms) ( gyors raml sos m dszer ) H + NO2 OH + NO To diffusion pump OH + C6H12 products Flow controller Photomultiplier Microwave discharge To rotary pump Movable injector Monitoring cell Flow tube (reactor) Frequency doubling crystal Excimer laser Sample/hold device Dye laser Trigger pulse

Resonance fluorescence detection (RF) Photo- multiplier tube Microwave discharge cavity Hydrogen Lamp H2 / He mix inlet outlet Processes in Resonance Lamp Processes in the detection cell a) MW radiation dissociatessome H2 b) H atoms absorb energy and become electronically excited c) H atoms relax, emit charac- teristic radiation directed into the cell a) H atoms absorb photons and become electronically excited b) A portion of isotropically emitted fluorescence photons is detected

Laser induced fluorescence detection (LIF) v = 2 Energy v = 0 Fluorescence (308 nm) Excitation pulse (282 nm) etc. 0 v = 1 Excitation laser tuned to one particular rotational level v = 2 v = 0

Mass spectrometry He Quadrupole mass selector Ion buffer gas To pumps detector C2H5+ detected Flow tube (reactor) Movable injector Ion source To pumps To pumps Microwave discharge Cl + C2H6 HCl + C2H5 Cl2/ He C2H5 + O2 C2H6O2 C2H4+ HO2

Continuous liquid flow tube ( folyamatos raml sos reaktor ) Source Reactant A Mixing chamber Motor driven syringe plungers Flow tube Reactant B Moveable detector system (spectrometer) Detector

Stopped flow system ( meg ll tott raml sos reaktor ) Computer control and data acquisition Limit switch Effluent Reactant A Mixing chamber Motor driven syringe plungers Reactor Spectrometer Reactant B

Shock tube Real time detection High pressure inert driver gas Reaction mixture Diaphragm Before diaphragm breaks Pressure After diaphragm has broken Incident shock Pressure

Flash photolysis (time-limit: flash pulse width; ~ 1 s) Nobel Prize: Norrish, Porter; 1967 Photolysis flash Spectrograph Reactor tube Detection flash

Laser photolysis (time-limit: laser pulse width; ~ 1 s down to 1 fs) Beam dump To exhaust Valve to control pressure in reaction vessel Heatable stainless steel reaction vessel Xenon arc lamp Monochromator Slow premixed gas flow PMT Flow controller Gas inlets Digitizer (A/D) Flow controller Computer control and data acquisition Flow controller

idfelbonts Increase in time resolution 1011 times increase within 36 years!! amplified lasers + pulse compression id , m sodperc time, seconds -15 10 -12 10 picosecond lasers (ring lasers) nanosecond lasers (mode locking) -9 10 -6 10 flash photolysis + relaxation -3 10 flow methods v year 1950 1960 1970 1980

Principles of laseroperation (lasing) Light Amplification by Stimulated Emission of Radiation Pumping energy Tuning device Gain medium Auxiliary parts Output coupler (partially transparent) High reflector (non-transparent)

Pulsed lasers Pumping energy Tuning device Saturable absorber Gain medium Output coupler (partially transparent) High reflector (non-transparent)

Colliding Pulse Mode-locked lasers (CPM lasers) Amplifier Output coupler Absorber Pumping laser

idskla 2 Time scale Time window of elementary reactions atomic nucleus - neutrino interaction vibrational relaxation nuclear motion in atomic nuclei molecule - photon interaction appearance of humans solvation the age of Earth lifetime of the singlet excited state electron- and energy- transfer length of a day human lifespan one minut lifetime of the triplet excited state molecular vibration molecular rotation 1015 peta- 1012 tera- 109 giga- 106 mega- 103 1 10-3 milli- 10-6 micro- 10-910-12 nano- 10-1510-1810-2110-24 femto- atto- pico- zepto- yocto- kilo- second CPU clock cycle time

pump-probe Spectroscopy with femtosecond time resolution: experimental arrangement reference detector Nd:YAG laser probe sample Ar- ion laser excitation H2O CPM laser amplifier (1 fs = 0.3 m path length) delay Laser technics: http://femto.chem.elte.hu/kinetika/Laser/Laser.htm

pump-probe 5 Principles of the femtosecond experiment ultrashort pulse coherence and selectivity potential energy 1 fs = 0.3 m path length excitation (Ig) measurement (Im) reaction coordinate time Pulse width: 10 100 fs

pump-probe 6 Experimental results ICN I CN I + CN LIF signal potential energy reaction coordinate delay time, fs

Reaction dynamics Molecular beam Moveable detector Collimators and velocity selectors Heated oven Effusive beam Reaction chamber 10 7 torr Second stage pumping Molecular flow (collision free) Continuum flow (collisional relaxation) First stage pumping High pressure source (> 300 torr)

Angular dependence Scattering angle in a centre of mass system Density map: HCl intensity

Angular and energy dependence F + D2 DF(v) + D v = 1 v = 2 Constant total energy v = 3 0 180 v = 4 Contour lines: equal velocity of DF

State selective optical detection CX molecules Microwave discharge generated A atoms B atoms from microwave discharge Sapphire window Liquid N2 cooled reaction vessel (cryopump) Split mirror optics (White cell) to collect and focus emission onto detector To pumps