ANALYTICAL CHEMISTRY

LASER SPECTROSCOPYAyesha Abdul Ghafoor

MS Chemistry

LASER SPECTROSCOPY

LASER Principle of Laser Laser System Laser as spectroscopic Light source Spectroscopy LASER +Spectroscopy

Laser Induced Breakdown Spectroscopy Laser Induced Fluorescent Spectroscopy Laser ablation inductively coupled plasma optical emission

spectroscopy (LA-ICP-OES) Raman Spectroscopy

Applications of Laser Spectroscopy

FLASHES OF BRILLIANCETHE HISTORY OF THE LASER

04/11/2023

“A splendid light has dawned on me” – Albert Einstein

In 1917 Einstein published ideas on stimulated emission of radiation. The laser is credited as being invented in 1958 by Charles H. Townes and Arthur L. Schawlow. Townes coined the term “laser” with help from his students. On May 16, 1960, Theodore H. Maiman operated the first functioning laser i.e., a pulse mode operation of solid- state flash lamp -pumped

L.A.S.E.R

Light Amplification by Stimulated Emission of Radiation

BASIC LASER

Light Sources Gain medium Mirrors

R = 100% R < 100%

I0 I1

I2I3 Laser medium

I

R. Trebino

GAIN MEDIUM

E1

E2

BN1I = rate of Stimulated absorption

Einstein Coefficients

E2

E1

E2

E1

BN2I = rate of Stimulated emission

AN2 = rate of Spontaneous emission

E = hν

TO ACHIEVE LASING:

Stimulated emission must occur at a maximum (Gain > Loss) Loss:

Stimulated Absorption Scattering, Reflections

Energy level structure must allow for Population Inversion

E2

E1

OBTAINING POPULATION INVERSION

satI

d NBIN BI N AN A N

dt

2

d NBI N AN A N

dt

1 /

1 /sat

sat

I IN N

I I

Laser Transition

Pump Transitio

n

Fast decay

Fast decay

1

2

3

0

2

1

N2

N1

Laser

Fast decay

Laser Transition

Pump Transitio

n1

23

2-level system

3-level system

4-level system

1 / sat

NN

I I

d NBIN BI N A N

dt

/

1 /sat

sat

I IN N

I I

Population Inversion is obtained for ΔN < 0 (ΔN = N1 – N2)

LASER SYSTEM

Active MediumActive medium can be of following types Liquid Solid gases

Pumping Source Optical pumping Chemical pumping Nuclear pumping Discharge technique Laser pumping Electron beam pimping

Resonators Transverse Mode Longitudinal mode

Laser Transition

Pump Transitio

n

Fast decay

Fast decay

1

2

3

0

Tunnable Laserswavlength of operation can be altered in controlled manner.

Dye lasers use complex organic dyesGas lasers are pumped by current. Solid-state lasers have lasing material distributed in a solid matrix (such The Nd:YAG laser emits infrared light at 1.064 nm. Semiconductor lasers, sometimes called diode lasers, are p-n junctions. Current is the pump source. Applications: laser printers or CD players. Excimer lasers (from the terms excited and dimers) use reactive gases, such as chlorine and fluorine, mixed with inert gases such as argon, krypton, or xenon. Excimers lase in the UV. Free electron Lasers is a laser that shares the same opical properties as conventional lasers such as emitting a beam of coherent EMR radiations which can reach high power

R. Trebino

SPECTROSCOPY

Study of interaction of light with matter all atoms and molecules absorb and emit

light at certain wavelengths so we can identify and read their properties

In essence, every element has a unique atomic "fingerprint" that takes the form of a set of wavelengths, or a spectrum.

LASER SPECTROSCOPY INSTRUMENTATION

LASER as Source of Light Gratings and Monochromators Interferometers

Michelsons Interferometers Fourier Transform Spctrometer

Dtectors Thermal Detectors Flourescent detectors etc.

Recorder

LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS) advanc ing significantly over the last

decade. It can analyze solids, liquids and gases

and can return results rapidly, with very little

damage to the sample. It can do its work from a distance, unlike

some analytical tools that require samples being brought to a lab.

WORKING OF LIBS

The laser, of course, Generally, LIBS systems use a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser at fundamental wavelength of 1,064 nanometers

(but many different lasers have been used. The laser doesn't blast the sample with a nonstop beam)

The laser light passes through a lens, which focuses the energy onto the sample.

"laser spark” produced. Excitation Relaxation The spectrometer contains a prism and a camera to

photograph the spectra for further study.

Fig: LIBS Spectra for identification of different elements in sample

LASER ABLATION INDUCTIVELY COUPLED PLASMA OPTICAL EMISSION SPECTROSCOPY (LA-ICP-OES) The "P" in ICP stands for plasma, an ionized gas

consisting of positive ions and free electrons. The Plasma torch consists of three concentric

tubes of silica surrounded by a metal coil. A nozzle at the end of the torch acts as an exit for the plasma.

Now the instrument is ready to analyze a sample. In the laser-based version of ICP-OES, a

neodymium-doped yttrium aluminum garnet (Nd:YAG) laser is used to cut, or ablate, a few microscopic particles from the sample's surface. The ablated particles are then carried to the pl asma torch, where they become excited and emit light.

LASER-INDUCED FLUORESCENCE (LIF)

Laser-induced fluorescence (LIF) is a spectroscopic method used for studying structure of molecules, detection of selective species and flow visualization and measurements.

Experimental Method The species to be examined is excited with a laser. The

wavelength is often selected to be the one at which the species has its largest cross section . The excited species will after some time, usually in the order of few nanoseconds to microseconds, de-excite and emit light at a wavelength longer than the excitation wavelength. This fluorescent light is typically recorded with a photomultiplier tube (PMT).

RAMAN SPECTROSCOPY

C.V. Raman ,Indian scientist discovered Raman spectroscopy

Raman spectroscopy is a spectroscopic technique used to study vibrational , rotational, and other low-frequency modes in a system

Principle: It relies on inelastic scattering , or Raman scattering, of monochromatic light, usually from a laser in the visible , near infrared , or near ultraviolet range. The laser light interacts with molecular vibrations, phonons or other excitations in the system, resulting in the energy of the laser photons being shifted up or down. This happens because the laser light interacts with phonons. The shift in energy gives information about the phonon modes in the system and ultimately about the molecules present in the sample.

Experimental Procedure: The beam from an argon-ion laser is directed by a system of mirrors to a lens, which focuses monochromatic light onto the sample. Most of the light bouncing off the sample scatters at the same wavelength as the incoming light, but some of the light does scatter at different wavelengths and goes to detector This happens because the laser light interacts with phonons. we use photomultiplier ,CCD detectors etc. and determine vibrations kinds and finally sample molecule.

APPLICATIONS OF LASER SPECTROSCOPY

Medical field Analytical Chemistry Industrial Applications Environmental Applications

LASER SPECTROSCOPY IN MEDICINE AND BIOLOGY

Medical diagnostics by breath trace gas analysis Real-time monitoring of exhaled gases (therapeutic

monitoring, toxicology, occupational health) Tissue analysis Mapping of drug delivery Insect studies Plant physiology

IDENTIFICATION OF BACTERIALCONTAMINATION OF PLATELETS (LIF)

Blood transfusion carries a risk of infection (hepatitis, HIV…) or consequent sepsis

every platelet concentrate should be checked before use (after donation and shortly before transfusion

» Fluorescent stain attaches to the DNA of

bacteria (platelets don’t contain DNA!)» Frequency doubled Nd-laser (532 nm)

toexcite LIF» Scattered light also measured» Certain thresholds for both signals

REAL-TIME MONITORING OF HEMODIALYSISReal-time monitoring of hemodialysis» Hemodialysis is used in treatment of renal failure» Urea, creatinine, etc. removed» Treatment 3 times a week, 2-12 hours» Over million patients worldwide, growing fast

LIF SPECTROSCOPY OF TISSUES

There are cellular or subcellular differences

between normal and tumorous tissues

» LIF can be used to visualize tissue characteristics

and detection of anomalies» Fluorescing compounds or

autofluorescence» Non-invasive procedure,

no photosensitization orphotodestruction

RESPIRATION OF INSECTS

Respiration of insects - real-time, on-line measurement

of CO2 - very small quantities sensitive

detection method, small volume of sample line and cell

photoacoustic spectroscopy - mid-IR should be used if possible

(CO2 at 4.234 μm) - OPO (between 3.9 and 4.8 μm)

continuous-wave, single mode operation

- detection limit 0.7 ppb. - sporadic release of CO2 observed

MOLECULES STUDIED IN BREATH BY LASER SPECTROSCOPY

Molecule MethodsAcetaldehyde LIBS , TDLASAcetone CRDSAmmonia PAS, TDLAS, OFC-CEASCarbon dioxide CRDS, TDLAS, CALOS, OFC-

CEASCarbon monoxide TDLASCarbonyl sulfide TDLAS, CALOSD/H isotopic ratio TDLASEthane LIBS , OA-ICOS, TDLAS, PASMethylamine, CRDS

LASER SPECTROSCOPY OF BREATH IS LIMITED TO SMALL MOLECULES single vibration-rotation

lines are measured -the lines have a certainlinewidth (Voigt profile) -the bigger the molecules,the more congested the

spectrum becomes(lines start to overlap

each other) -typical laser wavelength 1.5

to 10 μm -sensitivity ppt – ppm -normal pressure cannotusually be used (typical p =

0.05 – 0.2 atm)

IN ANALYTICAL CHEMISTRY

Laser Spectroscopy in Analytical Chemistry Chemical Reactions Detection of Atoms Study of Transition States Separation of isotopes (In Nuclear

Reactors) Study of Bond Energies and Angles Type of Material

FIG :RAMAN SPECTRUM OF NATURAL DIAMOND

ARTS ( STUDY OF PAINTING)

LIF SPECTROSCOPY OF INTERNAL COMBUSTION ENGINES

LIF spectroscopy of internal combustion engines

Goals: to improve combustion efficiency to reduce emission of pollutants

how well air and fuel are mixed

chemical intermediates rate constants of key

reactions l = air/fuel ratio ArF ,KrF lasers Molecules: NO, CO, CO2,

hydrocarbons…

CONCLUDING THOUGHTS

The key to managing today’s rapidly evolving technology it to constantly analyze how each advance affects us as individuals and as a society as a whole. “

“Our Advancing Technology , if separated from the human factor, I take to be part of the advance in the evolving quality of existence, something that gives added meaning and higher dimension to the human venture…”

- Roger Sperry Neuroscientist and Nobel Laureate

THANKS FOR LISTNIN

Questions???Glad to Answer your Questions