Spectroscopy based on particle detection

PrinciplesPhotoion detection

Photoelectron detectionPhotoion imaging

Applications:A. Fundamental

Spectroscopic propertiesReaction dynamics: unimolecular decays, bi-molecula reactions autoionization

Complexes, Solvation dynamicsSurface reactions

B. Application in food industry

Properties

1. Lasers are used to prepare and probe particles2. Mass spectrometric analysis

(electric & magnetic sector, time of flight MS, Quadrupole MS, reflectron-typetime of flight MS

3. Highly sensitive particle detection: Faraday cup, electron multipliers, channeltron,multichannel plate (MCP), Daly detector

4. Possibility for 2D visualization with 3D velocity distribution reconstruction

Very sensitive method for spectroscopic studies of spectroscopicproperties of radicals, complexes and clusters

Studies of reaction dynamics: unimolecular fragmentation, bi-molecular reactions and autoionization

Detection of ultra small molecular concentration

Experimental principles

Two main types of charged molecules are generated in ionization processcations and electrons.

Spectroscopic properties can be studied detecting molecular ion or electron detection

In photodetachment experiment(anions are studied):

-neutral molecule detection-electron detection

Ionization based methods

• Spectroscopy– Photoelectron spectroscopy:

• Ultraviolet photoelectron spectroscopy [Al-Joboury, Turner(1962); Vilesov, Kurbatov, Terenin, 1961)]

• Zero electron kinetic spectroscopy (threshold electrons PE, delayed pulsed extraction field, pulsed ionization field) ZEKE, MATI (Müller-Dethlefs, Schlag, 1984)

• Photodetachment spectroscopy of negative ions– Resonance enhanced ionization (photodetachment) spectroscopy

REMPI (REMPD)

• Mass spectrometry

– Charged particle generation• Photoionization• Electron scattering• Field ionization• Chemi(o) ionization

Photoionization process

1. One-photon photoionization

IP of most small molecules is >~8 eV and highly energetic photons in the VUV are required to induce ionization through one-photon absorption

- synchrotron sources

- non-linear conversion of radiation from visible or UV lasers

2. Multi-photonionization through the resonant stepStewise excitation via a resonant

intermediate state (RIS)

Ionization threshold

Photoionization cross section I

*one-photon ionization:-little state selectivity (ionization of normally several

vibrational and rotational levels of the electronic ground state occurs); photoion encounters a superposition of several vibrational and rotational levels determined by the Franck-Condon factors between the initial and final vibrational states

NAB+ is the number of cations formed for a given absorption path L, and I0t incident laser photons. NAB[1/cm3] is a density of cations confined in trap volume. PkI is the probability that a molecule in level Ek is ionized and Rk is the total relaxation rate of level Ek . The δ

and ψ

are the collection and detection efficiencies.

ψδσ ⋅+

= ++ →kkI

kItotABABtABAB RP

PLINN 0

Photoionization cross sections:example

RIS/REMPI

• Minimum two photons are required, intermediate • state lifetime > or ~ to spontaneous decay

• REMPI= multistep process:– 1 step: population of the intermediate step

AB(V“, J“) AB*(v‘, J‘)– 2 step: ionization process out of intermediate state

• PhotoionizationAB*(V‘, J‘) AB+ (v, J) +e-

• Dissociative photoionizationAB*(V‘, J‘) A+B+ + e-

• Auto-ionizationAB*(V‘, J‘) AB**(Eint>IP) AB+ +e-

The schemes are also valid in studies of the negatively charged molecules(photodetachment experiment)

Photoelectron spectroscopy

• PS involves the ejection of electrons from atoms or molecules following bomardment by monochromatic photons-- photoelectrons

• PS is an extension of the photoelectric effect involving the use of higher energy incident photons and frequently gas phase sample

• One considers atomic or molecular orbitals of a gas phase atom or molecule

Ultraviolet Photoelectron Spectroscopy (UPS)

• Kinetic energy of the recoiling photoelectrons is analyzed revealing the energy levels of the corresponding ion.

• Moderate energy resolution: ~10 meV (= 80 cm-1); laser spectroscopy offers about 103 better, ZEKE spectroscopy offers improvements (Müller-Dethlefs 1984)

• The factor limiting the resolution of classical PS is the inability to measure the kinetic energy of the photoelectrons with sufficient accuracy

Photon of the incident radiation has much more energythan is neccesary to ionize the molecule.

The excess energy is removed as kinetic energy of the photoelectron.

Energy delivered to the molecule

Energy balance in photoelectron spectroscopy

hv=IP+Eion + KE(AB+) + KE(e-) ~ IP+Eion +KE(e-)

adiabatic ionization energy:The energy required to produce an ion with no internal energy and an electron with zero kinetic energy

Internal energyof the cation

Kinetic energy of the cation and an electron

Electron detection in Photoelectron Spectroscopy

Angular distribution of photoelectrons

• Atom which is undergoing photoionization, obeys the Δl=+/-1 selection rule– s-electron removed photoelectron is described by a p wave (angular

distribution given by a p wave function angular distribution I(θ) ~ Y210

=3/4π cos2θ), where q is the angle between incident photons and ejected photoelectrons

• In general the photoelectrons are described by a mixture s, p, d, f, functions. For an unpolarized photon source the angular distribution of photoelectrons is:

I(θ) =σ/4π

[1+β/2(3/2 sin2θ −1 )]-1<=β<=2

β

:-

is likely to have different values for different orbitals one can distinguish one band system from another-

generally intensity will be dependant on angle and β. At magic angle 54.7°, intensities are independant of β

Example: photoelectron spectroscopy of O2

X2B1 = …(2a1 )2(1b2 )2 (3a1 )2 (1b1 )1

X1A1 = …(2a1 )2(1b2 )2 (3a1 )2 (1b1 )0

How the resolution of the UPS can be improved?

The excitation energy should be near resonance energy

One should collect near threshold electrons ( Ek=0)

A scheme

For hv2 >>IP, ions will be produced with a distribution of internal energies (no resonant conditions)

Energy resolved analysis of the photoelectrons can provide some limmited insight into the level structure and possibly dynamics of the molecular ion

B scheme - soft approach

The first (b) or second (c) photon,hv, is tuned into resonance with rovibrational state of the molecular ion at conditions KE(e-)=~0, hv2 ~IP+Eion

- Each time zero kinetic energy (ZEKE) electron is produced the internal structure, population of the molecular ion and dynamics can be obtained

TWO SCHEMES IN PHOTOELECTRON SPECTROSCOPY

• Threshold photoelectron spectroscopy (TPS) or ZEKE-PE– For photon energies slightly above the threshold of ionic eigenstate, threshold

photoelectrons are formed

• Delayed pulsed field extraction: or ZERO-PFE ~1 cm-1

– The photoelectrons with zero kinetic energy are separated by from others by waiting ca 1μs after they were produced by the pulsed lasers

• Pulsed field ionization or ZEKE-PFI ~0.003 with cm-1

– For photon energies slightly below the ionization threshold (2-3cm-1), long lived Rydberg states (~10 μs) of the neutral parent molecule are formed which can be ionized by a pulsed electric field

High-resolution photoelectron spectroscopy:Zero electron kinetic energy approaches

Threshold photoelectron spectroscopy ZEKE PE

Method suffers from interference by electrons which are produced with a small amount of kinetic energy. These electrons result from autoionization process from lowerer lying Rydberg states.

All electrons near resonant conditions are detected

Principles of electron detection in delayed pulsed extraction grid ZEKE

-prompt electrons are first deflected out by a weak electric field (10-100 mV/cm-the ZEKE electrons produced by pulsed field are distinguished from kinetic electronsusing steric and TOF separation principles

ZEKE PE -- threshold photodetachment spectroscopic method

Principles of electron detection in pulsed field ionization ZEKE

• ZEKE differs from both standard and threshold spectroscopy in that molecular ionization is achieved in two successive steps

– PhotopreparationThe molecule is excited to a high-n Rydberg state (principle quantum number n>80).

These states have relatively long lifetimes (scalling low n3, mixing with higher angular momentum states). The electron extends spatially over large distances (ion core + e-)

– Pulsed field ionization An electrical pulse is applied after a time delay of normally a few microseconds

• ZEKE PFI– Resolution: 10-1-10-3 cm-1 and sub-Doppler resolution----cw lasers

Pulsed field ionization ZEKE cannot be used for the study of negative ions(anions do not posses Rydberg states). ZEKE electrons must be collected and

very careful minimization of stray electric fields is neccesary

Conventional photoelectron spectroscopy vs ZEKE

Pulsed field ionization method introducesa small shift to the energy levels: ~few cm-1

Ionization energy has to be corrected.

Molecule can be analyzed within a large energy range.

High resolution spectraand a rather short energycan be scanned range.

Even higher resolution can be obtained using recently developed PS technique based on slow electron velocity imaging-----------------------------SEVI -----------------------------------------------------

ZEKE-MATI

• MATI-mass analyzed threshold ionization:ZEKE states are excited and the corresponding ions rather than the electrons are extracted and analyzed in a mass spectrometer

• Advatages:– Mass resolution enables unambiguous identification of the ionized species– Mass resolution allows for the identification of fragmentation pathways in the ion

Resonance enhanced multiphoton ionization REMPI

• Pump-probe process

• General abbreviations:– (m+n) REMPI one color ionization, e.g.,

(1hv +1hv) scheme– (m+n‘)REMPI two color ionization e.g.,

(1hv+1hv‘) scheme

• The potential for mass selectivity– Useful in the investigation of molecular

reactions or van der Waals complexes (molecular beam experiment)

• Species selectivity: resonant laser excitation to an intermediate level and subsequent mass analysis of the ion

– One can distinguish REMPI spectra of systems that have the same mass: phenol-N2 and phenol-CO

fixed

tunable

REMPI• Sensitive technique• Identification through the excited state

– Energy of photons ~>8 eV, influence of intermediate state dynamics on the signal

– IR energy region is difficult to study• Reguirements:

– high vacuum conditions 10-5 mBar – molecular beam preparation– ion, electron detector systems– mass spectrometry (TOF MS, QMS)

• Extensively used in molecular identification, reaction dynamics, material sciences and environmental studies

REMPI -setup

-Molecules from the bottle-Chemical reactor-Laser ablation source-Microwave or discharge source-Pirolytic source-Other

Molecular beam techniques:-effusion beam-supesonic jet expansion beam-Molecular beam guiding techniques by rf fields-trapping techniques

Laser-molecule interactionPump-probe techniqueLaser ionization-prior to mass analyzer (mol. beam expansion)-inside the quadrupole mass spectrometer-Inside the ion trap, eg., Paul trap, 22-pole ion trap-In an extraction region of time of flight mass -spectrometer

Analysis of mass-time of flight mass spectrometer TOFMS-Quadrupole mass spectrometer QMS-Reflectron time of flight mass spectrometer Re-TOFMS

Mass detection-multichannel detectors-Daly detector

Sources

Example of th setup which canbe used to study the molecular reaction dynamics

Mass analyzers: Quadrupole mass spectrometer

TOF and Re-TOF MS

Particle detectors:

For a typical detector device with gain G=106 and anode pulse width Tp=5ns,The main peak pulse voltage, originating from a single electron charge e=1.6 10-19 C,at the input of a preamplifies-discriminator with Rpad=50 Ω

impedance is

Vin ~Rpad xGx e/ Tp =50x 106 x1.6 x10-19 2 108 = 1.6 mV

Quantifying REMPI signals

)",'(1"2

)","(''' JJSq

JJvNcI vvREMPI +

=

N –the population in the ground state2J“+1 – the energy levels degeneracyq-Franck-Condon factors S-rotational line strength factorsC- a v‘,J‘ dependent constant

Example of a high-resolution REMPI spectrum

Photoion Imaging

1. PI combines laser spectroscopic methods, TOF MS and image based detection for studying chemical reaction dynamics:

-measurement of flux-velocity contour maps for quantum-state- selected products from fragmentation, inelastic and relative collisions processes for which the initial state is also well defined

2. PI is a method that allows for final state resolved analysis, and simultaneous detection of all scattering angles and velocity distributions based on single experimental geometry

Chandler, Houston 1987 Photodissociation dynamics of CH3 I

Complete information on the chemical process can be deduced in favorable cases

REMPI is used to determine product quantumstate distributions.

Two dimensional TOF MS using position-sensitive detector gives access to 3D velocity (speed andangular) distributions.

The ion images are also sensitive to the laserpolarization vector or in a suitable external

electric fields.

Conceptual set-up

Initial 3D product distributions can be reconstructed from 2D projections using inverse Abel transformation.

DI + hv (200-250 nm) D + I (2P3/2 )

D + I* (2P1/2 )Ekin (D)=hv-EBDE -Ekin (I)-Eint (I)

The kinetic energy of the nascent D atoms

Reaction dynamics

Steps

• Measurement of size of the rings and the arrival time of the D atoms- speed of the D atoms

• From velocity measurements and knowledge of the photolysis energy hv the bond dissociation energy can be obtained and the branching ratio into the twoproducts

• The angular distribution of product recoil of the two channels provides the character of the transitionwhether the process was direct or indirect.

Solvation dynamics

• Solvent plays important role in the elementary processes of bond formation and bond breaking

– Enhancement of bond formation by trapping reactive species in a ‚solvent cage‘ on the time scale of the reaction

– It can act as a chaperone that stabilizes energetic species

Dissociation of clustersProton transfer reactions

Dissociation of I2- solvated by CO2

Time resolved dynamicsCan be performed

Laser induced surface reactions

• Two approaches:

– Monitoring of speed, angular distribution, and internal excitation of the photofragments leaving the surface (REMPI, LIF, mass spectrometry)

– Monitoring photoproduct left behind at the surface (IR absorption of

adsorbed species-reflection/transmission spectroscopy)

Substrate adsorbate charge transfer processes

Photofragmentation dynamics of adsorbed molecules: surface-aligned photoreactions

Example: photolysis of HCl adsorbed on LiF(001) studied by Rydberg- atom TOF spectroscopy: localized atomic scattering

Chemical composition of materials, surfaces, and aerosols

• Laser induced breakdown spectroscopy(LIBS)• Laser microprobe mass spectrometry (LMMS/LIMA)• Surface analysis by laser ionization (SALI)• Matrix assisted laser desorption ionization(MALDI)• Laser desorption ionization of atmospheric aerosols in

real time (ATOFMS)

Laser induced breakdown spectroscopy

• Elemental analysis of solid, liquid materials and dilute gaseous samples, including aerosol particles dispensed in air

Matrix assisted laser desorption and ionization MALDI

• Large and non-volatile biopolymers (nuclic acids, peptides,…) and synthetic polymers can be converted into intact ions in the gas phase and mass spectrometrically analyzed Nobel Prize 2002 (partial)--------~pmol

• Suitable matrix :– capable of absorbing the laser

energy and protecting the polimer from fragmentation

– Isolating of the polymer molecules, which otherwise can cluster together

– Ionization of the polymer molecules (protonated or cationated species are formed)

Typical spectrum

Aerosol time of flight mass spectrometry

• Size and qualitative chemical composition– Laser ablation and ionization– Thermal vaporization and electron impact

• Four main sections:– Aerodynamic nozzle concentrates the

aerosol into a collimated beam and reduces the background pressure 1atm 10-5 mbar

– Particle sizing section enables measurement of the particle velocity (and aerodynamic diameter of particle) through the detection of light scattering by the particle as they pass through the two laser beams

– Laser ablation and ionisation of atoms and molecules

– TOF MS analysis of positively and negatively charged particles

Aerosol TOF mass spectrometer

Sea-salt particle that has undrgone change as a result of heterogeneous reactions with gas phase atmospheric pollutants

Particles emitted by a Diesel engine

• Complex particle containing carbonaceous material

Simulation of the laser ablation of aerosol particles

Laser application to study non-volatile compounds in fruits

• Analytical problems with detection and identification of non-volatile organic compounds like phytoalexins produced by plants:

– Thermaly labile samples, decomposing upon heating• Methods applied:

– Fast atom bombardment (FAB)– Field desorption (FD)– Laser desorption (LD) – Plasma desorption (PD)– Secondary ion mass spectrometry (SIMS)

-Electron beam ionization -Chemical ionization under vacuumand atmospheric conditions-Laser multiphoton ionization-VUV ionization

+

REMPI + TOFMS is considered to be the most powerful- selectivity: mass-selective detection with resonant ionization process-sensitivity and resolution-major ionization efficiency-easy control of the molecular fragmentation by laser intensity-simultaneous analysis of different components

Examples

e.g., trans-resveratrol in grapes and vine leavesan antioxidant compound naturally produced in a huge number of plants as a phytoalexinit is accumulated in vine leaves and grape skin in response to various fungal organisms, it is produced in optimum in young leaves to protect the plants to protect them against infections and declines with seasonal evolution of plantUV radiation or chemicals (in varying concentrations depending on viticultural and ecological practices)

natural pesticide properties, fungi toxic at physiological concentration, quantitatively, the major component in phytoalexin response from a grape vine,

Analysis:

-absence of any separation method for sample preparation (cold pressing using hydraulic press)-high sensitivity and resolution giving low detection limits (ppb) due to resonant ionization and spectrometric detection

Fast and direct analysis of non-volatile compounds in fruit and vegetables

Detections

Extra slides

Simple time of flight spectrometer

General: t=am1/2 +b----------two masses must be known for the calibration

Mass resolution t=am1/2 +b m/Δm =t/2Δt

Typical Δt broadening due to the detector

Typical: 300-800

Main problems which cause low resolution:

-Uncertainties in the time of ion formation-Initial location in extraction field- Initial kinetic energy before acceleration-Metastable fragmentation

-nature or the quantity of the sample- laser power (MALDI)

Two approaches in a pump-probe experiment

Uncontrolled ionizationprocess

hv2 is tuned into resonancewith a transition to a specific (v+,

J+) quantum state of the molecular ion

A B