Optics For Focusing And Collecting Light Biology Essay

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The main principle involved in LIF technique is fluorescence emission from gas phase atoms. Fluorescence emission is a two step process which involves excitation of gas phase atoms followed by subsequent relaxation. Excitation of gas phase atoms is caused by the absorption of photon energy from incident laser radiation. The wavelength is selected to be the one at which atoms have their maximum absorption. The excited state atoms then relax by reemitting the absorbed energy in the form of fluorescence emissions. The subsequent fluorescence emission is then detected with a photomultiplier tube. Atomic Fluorescence Spectroscopy is a useful method for quantitative assessments of elements because of its wavelength selectivity, its high signal to noise characteristics and low background noise.

The LIF instrumentation in this project consists of

Nd:YAG laser (Pulsed laser)

Tunable dye laser

Frequency doubling crystal

Raman shift cell

Optics for focusing and collecting light

H2 Flame

Hydride generation system

Monochromator

Photomultiplier tube

Box-car integrator

Oscilloscope and computer to view and record the spectrum

Argon gas system

Nd:YAG Laser:

An Nd:YAG pulsed laser is used as an excitation source in LIF (Continuum, Surelite SLII-10).

Schematic diagram of Nd:YAG laser

Mirror

Flash lamp

Laser Rod

Q-switch

MirrorSchematic Nd YAG Laser.png

An Nd:YAG laser consists of four major components including a flash lamp, laser rod, Q-switch and mirrors. The main function of flash lamp is to optically pump Nd:YAG laser. The optical resonator is a closed cavity with a neodymium-doped yttrium aluminum garnet crystal (Nd:Y3Al5O12) as a lasing medium. Nd3+ ions in the gain medium absorb pumping light from flash lamp and they become excited. These excited ions emit photons with the same energy as the laser atomic transition wavelengths. In this process, when a photon passes through the lasing medium and when the frequency of the photon is equal to that of lasing medium, the photon becomes amplified due to the simulation of the decay of the other ions from the upper state to the lower state. The mirrors are arranged in a way to make the amplified light reflect back into the resonant cavity to increase the amplification.

Nd:YAG lasers are one of the most common types of lasers and are used in LIF measurements because of their consistency, efficiency, compactness, and high laser energy production. The fundamental wavelength for an Nd:YAG laser is 1064 nm in the infra red region, which can be transformed to second, third, and fourth harmonic wavelengths.

Table 1 Wavelengths of Nd:YAG laser

Type

Wavelength (nm)

Pulse width (ns)

Fundamental

1064

7

Harmonics

Second

532

4 - 6

Third

355

4 - 6

Fourth

266

4 - 6

The fundamental wavelength (1064 nm) is not directly useful for many applications. Different birefringent materials like KDP (Potassium Dihydrogen Phosphate) and KD*P (Potassium Dideuterium Phosphate) can be used to convert the fundamental wavelength to other harmonic wavelengths like 532 nm (second harmonic) and 355 nm (third harmonic) which have two and three times the photon energy of the fundamental wavelength and are useful for many applications, including dye laser pumping.

Table 2 Energy of fundamental and harmonic wavelengths of Nd:YAG laser

Wavelength (nm)

Energy (mJ)

1064

532

355

Table 3 Specifications of laboratory Nd:YAG laser (Surelite Laser)

Pulse energy

Pulse duration

Pulse rate

Energy stability

Beam diameter

Beam divergence

Flash lamp lifetime

TUNABLE DYE LASER

Tunable dye laser is a laser which uses an organic dye as the lasing medium and is used to change a laser emission wavelength in a given spectral range. The wavelength of operation of a dye laser can be altered in a controlled manner and hence it can be used over a wide range of wavelengths [Koechner]. Wide bandwidth of Tunable dye lasers makes them particularly suitable for a wide range of wavelengths. Dye molecules absorb pump laser light at one wavelength and reemit at a different wavelength. Organic dyes have broad fluorescence bands. These bands are excited by a pump laser (Nd:YAG laser) and a laser output wavelength is selected using gratings and prisms. The combination of tunable dye lasers and Laser Induced Fluorescence has been shown to provide high sensitivity for many elements [Fassel J D].

FREQUENCY DOUBLING CRYSTAL

Frequency doubling or second harmonic generation is a nonlinear optical process, in which photons interacting with a nonlinear crystal are effectively combined to form new photons with double the frequency and half the wavelength of the initial photons [wiki]. This process is used for the conversion of fundamental wavelength (1064 nm) to the second harmonic wavelength (532 nm), and can be obtained by passing the light wave through a non-linear crystal.

Second Harmonic Wavelength (532 nm)

Fundamental Wavelength (1064 nm)

Nonlinear Crystal

While interacting with the nonlinear crystal, the high intensity fundamental wave generates a nonlinear polarization in the crystal, which provides a new wave at double the fundamental frequency. Frequently used nonlinear crystal materials include lithium niobate (LiNbO3), lithium tantalate (LiTaO3), potassium titanyl phosphate (KTiOPO4) and lithium triborate (LiB3O5) [Rudiger].

RAMAN SHIFT CELL

Raman shift cell is designed to convert visible or deep green pulses generated by a dye laser or frequency doubling crystal to the ultra violet wavelength range. Wavelength conversion is achieved by the Raman Effect, i.e. Anti-stokes shift in a gas cell filled with either nitrogen or hydrogen at high pressure. Raman cell generates multiple orders of Stokes and Anti-stokes beams simultaneously.

The fundamental beam is focused into the entrance window of the gas cell and an adjustable lens is used after the cell's exit window to collimate the emerging beams. Shifted and fundamental beams are collinear and need to be separated. Separation of beams is usually achieved by use of Pellin-Broca prisms.

OPTICAL SYSTEMS

Lenses and mirrors of specific focal length are used for focusing and collecting the laser and fluorescence emissions. Different focal length lens are chosen depending upon the wavelength of the element.

HYDROGEN (H2) FLAME

Hydrogen flame uses hydrogen gas and air present in the atmosphere as fuel-oxidant mixture. Hydrogen flame is an invisible flame and as it burns clear it has very less background noise. A H2-Air flame sustained on a quartz tube (1/4th inch o.d.) was used as the atomizer for LIF spectrometry. The laser beam passed through the flame at a height of 2-3 mm above the outlet of the quartz tube. The height of the flame was adjusted with a micrometer stage to optimize the fluorescence signal.

HYDRIDE GENERATION SYSTEM

The hydride generation system used in these studies is a continuous flow hydride system. The continuous flow hydride generation system consists of the following components: a four-channel peristaltic pump (Model RP-1, Rainin Instrument Co. Inc., Emeryville, CA, USA) with Tygon or PVC tubing, a U-shaped gas - liquid separator made of pyrex glass, a Nafion tube dryer (MD Series, Permapure Inc., Toms River, NJ, USA) and reaction coils of various lengths (1-2 m, 1mm i.d.) of Teflon tubing.

Two channels of the four channel peristaltic pump were used to pump the sodium tetrahydroborate and acidified samples. The flow rates of sodium tetrahydroborate and acidified sample were controlled by use of tubing with different diameters. These solutions were combined in a mixing chamber, and sent through a reactions coil to a U-shaped gas-liquid separator (GLS). The GLS separates the gas from the liquid waste and purges the volatile hydrides towards the atomizer. An argon gas stream was used to carry the volatile hydrides to the flame. The flow rates of all the gasses and solutions used are shown in table

Table Operating conditions for the Hydride Generation system

HG-LIF Bi

HG-LIF Ge

HG-LIF Sn

HG-LIF Sb

Acidified Sample flow rate

NaBH4 flow rate

Argon flow rate (flame)

Hydrogen flow rate (flame)

Air flow rate (Nafion tube)

MONOCHROMATOR

Monochromator is an optical device that helps in selecting a desired wavelength from a range of wavelengths. In these studies a monochromator (1000 µm vertical slits, Spectra Pro-275, f/3.5, Action Research Corp.) was used to isolate fluorescence emissions. Fluorescence emissions were collected at 90o to the laser beam direction and were transmitted to the entrance slit of monochromator. From the entrance slit the fluorescence emissions were directed towards a diffraction grating. The grating disperses the light by diffracting different wavelengths at different angles. By adjusting the angle of the grating, the desired wavelength can be isolated. The isolated fluorescence emission is directed to a focusing mirror, exit slit and to the Photomultiplier Tube (PMT) detector. The slit widths of entrance and exit slits of monochromator were adjusted to provide the best signal to noise ratio.

Table Specifications of Laboratory Czerny Turner Monochromator

Focal length

275 mm

Slit width

Upto 3000 µm

Grating

1200 grooves/mm

Resolution

0.05 nm

Focal plane

27 mm wide * 14 mm high

PHOTOMULTIPLIER TUBE

Photomultiplier tubes are extremely sensitive detectors of light in the ultraviolet, visible and near-infrared ranges of the electromagnetic spectrum [wiki]. In this study the isolated fluorescence emission from monochromator was detected by a Photomultiplier tube (PMT, R955, Hamamatsu Corp.). A photomultiplier tube amplifies a single incident photon to approximately 106 electrons. A typical PMT consists of the following components: a vacuum tube, several dynodes and an anode. The vacuum tube contains a photosensitive metal called a photocathode. When a photon with enough energy strikes the photocathode, it emits a photoelectron due to photoelectric effect. These photoelectrons are accelerated towards a dynode which generates 2-5 secondary electrons for each incident electron. When these secondary electrons hit another dynode, additional secondary electrons are generated. As there are a series of dynodes many secondary electrons are produced and when all these secondary electrons reach the anode it generates an electrical pulse that can be detected by the Oscilloscope.

BOXCAR INTEGRATOR/ GATED INTEGRATOR

A boxcar integrator amplifies and integrates the input signal during a predefined gatewidth, starting at a predefined trigger, ignoring the noise and interference that could be present at other times. Each of these analog input signals can then be averaged by using an analog averager. Boxcar integrator is mainly designed to recover fast analog signals from noisy backgrounds. The boxcar integrator consists of a gate generator, a fast gated integrator, and exponential averaging circuitry. The gated generator, triggered externally, provides an adjustable delay from a few nanoseconds to 100 milliseconds, before it generates a continuously adjustable gate of 2 nanoseconds to 15 microseconds. The fast gated integrator integrates the input signal during the gate. The output from the integrator is then normalized by the gate width to provide a voltage which is proportional to the average of the input signal during the sampling gate. The exponential averaging technique is useful for pulling small signals from noisy backgrounds.

OSCILLOSCOPE

An Oscilloscope is basically a graph displaying device, which converts the waveform of detector signal into an electrical signal and draws a graph of the electrical signal. In most application the graph shows how signals change over time: the vertical (Y) axis represents voltage and the horizontal (X) axis represents time. Oscilloscope has a time base control that adjusts the scale of X - axis in seconds and a vertical control that adjusts the Y - axis in volts. By using an oscilloscope the time and voltages values of a signal can be determined and the frequency of an oscillating signal can be calculated.

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