Light source in Photochemistry FROM :SAIMA ALEEM. MPHIL. Course
supervisor: Dr rafia
INTRODUCTION Grotthuss-Draper law. the Grotthuss-Draper law,
states that light must be absorbed by a compound in order for a
photochemical reaction to take place. Stark-Einstein law the
Stark-Einstein law, states that For each photon of light absorbed
by a chemical system, only one molecule is activated for subsequent
reaction. This "photo equivalence law" was derived by Albert
Einstein during his development of the quantum (photon) theory of
light.
INTRODUCTION Photochemists, typically work in only a few
sections of the electromagnetic spectrum. Some of the most widely
used sections, and their wavelengths, are the following:
Ultraviolet: 200400 nm. Visible Light: 400800 nm.
Source of light 1. Sun. 2. High pressure mercury lamp. 3. Low
pressure mercury lamp. 4. Medium pressure mercury lamp. 5. Low
pressure mercury lamp. 6. High pressure mercury lamp. 7. Halogen
lamp. 8. Laser. (a). Light emitting diode.
Sun In the early experiments (and in everyday life), sunlight
was used as a light source. Sun light consist of wide range of
radiation and in the visible region, it comprises the violet, blue
, green, yellow, orange and red components. According to early
investigators observation when the photochemical sample is expose
to sun light, after some period of time few changed.
Sun Examples The first example is photosynthesis, in which most
plants use solar energy to convert carbon dioxide and water into
glucose, disposing of oxygen as a side- product. Humans rely on
photochemistry for the formation of vitamin D. Ozone formation When
oxygen is expose to sun light, it convert in to ozone.
High pressure mercury lamp A high mercury-vapor lamp is a gas
discharge lamp that uses an electric arc through vaporized mercury
to produce light. This lamp operate in high ( 100 atm).
High pressure mercury lamp
High pressure mercury lamp disadvantage It easily damage
because it operate at very high temperature so that quartz or glass
envelope and nitrogen gas for cooling purpose is necessary. It has
short life time due to high temperature and pressure. High pressure
lamp are not commonly used in commercial photochemical
apparatus.
low pressure mercury lamp
low pressure mercury lamp Advantage It operate at very less
temperature and low pressure. Low pressure mercury lamps are highly
efficient in providing short wavelength ultraviolet energy.
low pressure mercury lamp Example: the photo reaction of
benzene hv + 253.7nm benzene fulvene Benzvalene
Medium pressure mercury lamp A medium mercury-vapor lamp is a
gas discharge lamp that uses an electric arc through vaporized
mercury to produce light. In this lamp a discharge is passed
through mercury vapor and the electrically excited mercury atoms
emit radiation. It operate in medium pressure ( 5 atm). The
wavelength range is 250 to 600 nm.
Medium pressure mercury lamp Example: Homolytic decomposition
of iodine I2 + hV (520) 2I-
The Low Pressure Sodium Lamp
The Low Pressure Sodium Lamp Low pressure sodium lamps only
give monochromatic yellow light and so inhibit color vision at
night. When the lamp is turned on it emits a dim red/pink light to
warm the sodium metal and within a few minutes it turns into the
common bright yellow as the sodium metal vaporizes.
Low Pressure Sodium Lamp Advantage It have a lowpressure,
lowintensity discharge source and a linear lamp shape.
High Pressure Sodium Lamp
High Pressure Sodium Lamp A sodium-vapor lamp is a
gas-discharge lamp that uses sodium in an excited state to produce
light. The sodium D-line is the main source of light from the
High-pressure sodium lamp. High-pressure sodium lamps are smaller
and contain additional elements such as mercury. It produce a dark
pink glow when first struck, and an intense pinkish orange light
when sodium is warmed. Some bulbs also briefly produce a pure to
bluish white light in between if the mercury achieves its high
pressure arc discharge characteristic before the sodium is
completely warmed.
High Pressure Sodium Lamp Disadvantage At high temperature
sodium is loss because Sodium is a highly reactive element and is
easily lost by reacting with the arc tube and form sodium oxide and
aluminum.
Halogen Lamp A halogen lamp, also known as a tungsten halogen,
quartz-halogen or quartz iodine lamp, is an incandescent lamp ,
these measure most effectively in the visible region. When Electric
current is supplied to a filament.The filament becomes hot, and
light is emitted. The bulb in a halogen lamp is filled with inert
gas and a small amount of a halogen. While the tungsten used as the
filament evaporates due to the high temperature, the halide causes
the tungsten to return to the filament. This helps create a bright
light source with a long service life.
Halogen Lamp Examples photolysis of iron pentacarbonyl 2
Fe(CO)5 hv Fe2(CO)9 + CO Iron diiron pentacarbonyl
nonacarbonyl
laser It produce light in a narrow frequency and to focus it in
an extremely small area. Laser light is directional and
monochromatic.
Light emitting diode In photochemistry Light emitting diode is
used as a light source. A light-emitting diode is a semiconductor
device that emits visible light when an electric current passes
through it. The output from an light-emitting diode can range from
red (at a wavelength of approximately 700 nanometers) to
blue-violet (about 400 nanometers).
Light emitting diode Advantage It produces light at a single
wavelength, without the need for a monochromator. Lamp life is
almost infinite and Light emitting diode sources are stable with
little variation in bandwidth making them an attractive, low cost
solution for simple applications.
Type of light emitting diode High-power light emitting diode
can be driven at currents from hundreds of milliampere. Light
emitting diodes have been developed by Seoul Semiconductor that can
operate on AC power without the need for a DC converter. For each
half-cycle, part of the Light emitting diode emits light and part
is dark, and this is reversed during the next half-cycle.