Ion{molecule Collisions Bhas Bapat Physical Research Laboratory, Ahmedabad [email protected] August 2012, IUAC Workshop Ion{molecule Collisions (Bapat, PRL) Outline Outline of an atomic/molecular physics experiment • Basics of collisions and molecular structure • Some aspects of molecular collision dynamics • Types of collision investigations • Ion{molecule Collisions (Bapat, PRL) 1 What is experimental atomic and molecular physics? information about the structure and dynamics of atoms and molecules by spectroscopy, • collisions Why study these processes? • { they are of relevance to atmospheric and stellar, even biological processes { atomic and molecular processes occur all around us { everywhere! experimentally tackle the quantum many-body coulomb problem; • significant overlap with other branches: astrophysics physics, materials science, earth • and atmospheric science, chemistry, medical applications etc. Standard chemistry different from interaction of single molecules! • Ion{molecule Collisions (Bapat, PRL) 2 How do we do it? Perturb atoms or molecules (targets) using charged particles or photons Photon interaction • { energy selective (E = ¯h!) { angular momentum selective (∆L = 1) Charged particle interaction • { a range of energy and momentum transfers { no angular momentum selection rules Ion{molecule Collisions (Bapat, PRL) 3 How do we do it? . and study the response Detect charged particles or photons, which carry information about the response of • the target { Ion mass spectrometry (m=q) { Electron energy spectroscopy, angular distributions { Photon spectroscopy Can combine two or more of the above • Focus here is on Ion{Molecule collisions Ion{molecule Collisions (Bapat, PRL) 4 Scattering Basics A beam of particles (ions) crosses a collection of target particles (molecules) in vacuum. • Usually single collision conditions are maintained • Coulomb interaction with electrons in the molecules changes the path of the ions • (ion{nucleus interaction neglected) Perturbed molecule responds leading to rearrangement of electrons and nuclei • The response is detected in the form of • {photon emission (fluorescence) {electron emission (ionisation) {charged or neutral atom or radical emission (dissociation or ionisation) Target{projectile roles may be reversed (i.e. the projectile may be the object of • investigation and target may be the probe or vice versa) Ion{molecule Collisions (Bapat, PRL) 5 Perturbation of a Molecule A molecule may be thought of as a collection of nuclei moving in the mean field of electrons, with overall charge neutrality Large difference in masses of nuclei • and electrons permits decoupling of degrees of freedom (rotation, vibration, electronic) ABC +* ABC + Energy Can be perturbed by a charged particle • or photon meV: rotation ABC sub-eV: vibration few eV: dissociation/ionisation Internuclear distance Ion{molecule Collisions (Bapat, PRL) 6 Collision Regimes Fast ions • { Large velocities { Short interaction times { Vertical transitions { Frozen molecule Slow ions • { Low velocities { Long interaction times { Electron clouds can adjust { Chemistry Ion{molecule Collisions (Bapat, PRL) 7 Dissociative Ionisation The effect of ionisation is a change in the B+ 2+ • mean field seen by the nuclei, causing the (A+B) nuclei to respond to it An excited molecule may dissociate A+ • producing charged and neutral fragments Ionisation and dissociation occur on differing time scales • AB + γ (ABn+)∗ + e− [ fs] −! ∼ (ABn+)∗ Ap+ + Bq+ (n = p + q)[ ps] −! ∼ Dissociation patterns expected to depend on the type of electronic excitation • Fragments carry information about the excited molecular ion state • Ion{molecule Collisions (Bapat, PRL) 8 Dissociative ionisation example: CO2 Ionisation followed by dissociation of the molecular ion Ionisation CO CO2+ + 2e− 2 ! 2 Concerted fragmentation CO2+ O+ + CO+ 2 ! CO2+ C+ + O+ + O 2 ! CO2+ C + O+ + O+ 2 ! Sequential fragmentation CO2+ CO2+ + O C+ + O+ + O 2 ! ! CO2+ CO+ + O+ C+ + O + O+ 2 ! ! Ion{molecule Collisions (Bapat, PRL) 9 Experiment with CO2 KER and molecular energy levels Energy diagram of dissociative ionisation KER and molecular energy levels Kinetic Energy released depends on • { the participating excited state { the overlap of the ground-state with the excited state Also to be taken into account • { fragmentation sequence { metastability { explosion geometry Bhas Bapat (PRL, Area Seminar) Photo-triple-ionisation of CO2 19 June 2006 21 / 23 Ion{molecule Collisions (Bapat, PRL) 10 Molecular ions. dissociation dynamics ABC ABCn+ Ap+ + Bq+ + C(n−p−q)+ ! ! fragments give us clues : their kinematic • properties are the leads for an N-body break-up, there are 3N 4 • free parameters in the momentum space− N-particle continuum:3 N 4 (= k) • free phase space coordinates− Quantum-mechanically Need kinematically complete • measurements q Tfi = f i i.e. determine all momentum components h j b ~vp t j i j − j of all fragments 2 k Tfi d σ=dq ... dqk j j , 1 Ion{molecule Collisions (Bapat, PRL) 11 Types of Collision Spectrometry Collision-induced fluorescence • Translational Energy Spectrometry • Time-of-flight Spectrometry • Ion Momentum Imaging • Complete Kinematics : electron & ion coincidence mapping • Ion{molecule Collisions (Bapat, PRL) 12 Collision-induced fluorescence Targets in the form of a cell (gas or • liquid) Spectrograph limited to UV-VIS-IR wavelengths Collection Optics • Gas Cell with windows Ion Beam Targets not dense (to carry out • collisions) so count rates low Not easy to get angular distributions • and wavelength dispersion simultaneously. Ion{molecule Collisions (Bapat, PRL) 13 Charged particle Spectrometry More “efficient”than photons, ability to X • C+ count single particles 1000eV e- Counts Y D CO 2+ E Low number density OK 2 • T Z E CO + Y Charged particles can be manipulated 2 C • T easily, while also dispersing them in CO+ X O velocity, mass etc, and deriving angular R distributions O+ TOF Position and TOF are simultaneously recorded and stored as a list of x, y, t triplets Ion{molecule Collisions (Bapat, PRL) 14 Translational Energy Spectroscopy Technique Insights the states of the participating species A fast ion (a few keV in energy) is made • • to collide with a neutral target gas. dynamics of the collision • lifetimes of species Energy and angular deflection of • • scattered ion is measured. collision cross-sections state populations • Ion{molecule Collisions (Bapat, PRL) 15 Translational Energy Spectroscopy Binary collision between a fast ion and a neutral target Ion{molecule Collisions (Bapat, PRL) 16 Translational Energy Spectroscopy Arrangement of a high resolution energy loss spectrometer Ion{molecule Collisions (Bapat, PRL) 17 Translational Energy Spectroscopy + TES spectrum of N2 colliding with NO, at 3000 eV collision energy. Ion{molecule Collisions (Bapat, PRL) 18 Motivation for other techniques Limitations of TES Not very useful for dissociating species • Low efficiency • Large Equipment • Advantages of TOF Compact, simple apparatus • Ability to collect multiple fragments • Ion{molecule Collisions (Bapat, PRL) 19 Time-of-Flight Mass Spectrometry : Technique A crossed molecular beam and ion beam • arrangement Electric field to extract ions and • transport them to a detector Event trigger • { Pulsed ion beam { Pulsed extraction field { Ejected electron detection using separate detector Time-of-Flight to the detector measured • w.r.t. the event trigger rm A fast response detector to record TOF • successive ion arrivals / q Ion{molecule Collisions (Bapat, PRL) 20 TOF: sample spectrum 10 4 + CO 2 10 3 O+ 2+ CO 2 + C+ CO 10 2 10 1 4000 5000 6000 7000 8000 time-of-flight [ns] Ion{molecule Collisions (Bapat, PRL) 21 TOF: insights into dissociation dynamics Gives fragment ion mass and (to some • 10 4 + CO 2 extent) kinetic energy information Determination of dissociation pathways • (which fragments are formed) 10 3 O+ 2+ CO 2 + Glimpse into dissociation mechanisms + CO C • (sequences, rearrangements etc.) 10 2 10 1 4000 5000 6000 7000 8000 time-of-flight [ns] Ion{molecule Collisions (Bapat, PRL) 22 TOF : kinetic energy information Experiment with CO2 Photoion Kinetic Energy Photoion Kinetic Energy In general, t t(m, q, p ,E~ , s, d) • ≡ jj Assuming equipartitionmomentum, KE of of ions every can ion be • can be estimated:determined from the TOF t T T0 = pz /(qEs) ≡ − pjj (t T0)qE ≈ assuming− isotropic emission of photoions, we get 2 KE 3pjj=2m ≡ K = 3 p2/2m h i h z i Ion{molecule Collisions (Bapat, PRL) 23 Bhas Bapat (PRL, Area Seminar) Photo-triple-ionisation of CO2 19 June 2006 19 / 23 TOF : multi-ion-coincidence Record all ions arising from one event • Experimental Technique Ion-ion coincidence ABC ABC2+ [+2e−] A+ + B+ + C Ion-ion coincidence! ! TOF techniques allow recording in a sequence • Ions are separated in time; detect them as sequence [ microsecond Create correlation map ≈ • duration]. { for each ionNeeds from fast each electronics event, and record smallt1 event, t2... rates [ 1/ ion drift time] { repeat forArrival several times events are stored { list event-by-event, mode as a list { Sort the correlated time pairs and look for patterns Ion{molecule Collisions (Bapat, PRL) 24 Bhas Bapat (PRL, Area Seminar) Photo-triple-ionisation of CO2 19 June 2006 10 / 23 TOF : ion-ion-correlation maps Fragmentation Pathways 7000 O+:CO + 1000 Detect ion pairs from a break-up • 6000 Plot a correlation map of the pairs O+:O + • Patterns in the map: 5000 • C+:O + p2jj q1 slope = tof of[ns] ion second −p1jj · q2 4000 1 Size of blob Kinetic Energy release ≡