Chemistry Dictionary

6.9.13 Types of spectroscopy

11:38 AM Unknown

Types of spectroscopy:

(a) Continuous spectroscopy

(b) Absorption spectroscopy

The absorption spectroscopy means that we observe the spectrum of the light right after it go through the substance, (look at the pic.) black line means the photons which have those specific kind of energy were absorbed by the substance;

emission spectrum means the range of electromagnetic spectra in which a substance radiates (emits). The substance first must absorb energy.

This energy can be from a variety of sources. We observe the emission light

Photo Chemistry / spectroscopy

Spectrum and Spectroscopy

11:27 AM Unknown

Spectrum and Spectroscopy

Spectrum:
(a). Different colors observed when the white light was dispersed through the prism
(b). The changing of light intensity as a function of frequency

Spectroscopy: Study of spectrum, to identify substances

In the past, the word spectrum was introduced into the area of optics at first, referring to the range of colors observed when white light was dispersed through a prism. Soon after that, a spectral density or spectrum is also known as the term referred to a plot of light intensity or power as a function of frequency or wavelength. Spectroscopy is the study of spectrum, study of the interaction between radiation , for identifying the substances through the spectrum emitted from or absorbed by them.

Ref.

Live remote operated Spectrograph controll - Universe's finger prints –Spectroscopy

-Chemical and Biological detection Professor: Nam Sun Wang - Haimo Liu

Photo Chemistry / spectroscopy

Instrumentation of Fluorescence Spectroscopy ( spectrofluorometer ) and process

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The basic instrument is a spectrofluorometer

- It contains a light source, two monochromators, a sample holder and a detector.
- There are two monochromators, one for selection of the excitation wavelength, another for analysis of the emitted light.
- The detector is at 90 degrees to the excitation beam.

- Upon excitation of the sample molecules, the fluorescence is emitted in all directions and is detected by photocell at right angles to the excitation light beam.
- The lamp source used is a xenon arc lamp that emits radiation in the UV, visible and near-infrared regions.
- The light is directed by an optical system to the excitation monochromator, which allows either preselection of wavelength or scanning of certain wavelength range.

Instrumentation of Fluorescence Spectroscopy spectrofluorometer and process

- The exciting light then passes into the sample chamber which contains fluorescence cuvette
- A special fluorescent cuvette with four translucent quartz or glass sides is used.
- When the excited light impinges on the sample cell, molecules in the solution are excited and some will emit light.
- Light emitted at right angles to the incoming beam is analyzed by the emission monochromator.
- The wavelength analysis of emitted light is carried out by measuring the intensity of fluorescence at preselected wavelength.
- The analyzer monochromator directs emitted light of the preselected wavelength to the detector.
- A photomultiplier tube serves as the detector to measure the intensity of the light.
- The output current from the photomultiplier is fed to some measuring device that indicates the extent of fluorescence.

Fluorescence Spectroscopy / Photo Chemistry

Fluorescence Spectroscopy Principles

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Fluorescence Spectroscopy Principles :

- Interaction of photons with molecules results in promotion of valence electrons from ground state orbitals to high energy levels.
- The molecules are said to be in excited state.
- Molecules in excited state do not remain there long but spontaneously relax to more stable ground state.
- The relaxation process is brought about by collisional energy transfer to solvent or other molecules in the solution.
- Some excited molecules however return to the ground state by emitting the excess energy as light.

- This process is called fluorescence.

The emitted light has two important characteristics :
1. It is usually of longer wavelength (lower energy) than the excited light. This is because part of the energy associated with S state is lost as heat energy.
2. The emitted light is composed of many wavelengths which results in fluorescence spectrum.

Fluorescence Spectroscopy / Photo Chemistry