08 he cd and hcl lasers

8/14/2019 08 He CD and Hcl Lasers

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Dr. Na. Venkat Nathan


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The Helium-Cadmium (He - Cd) laser is one

type of gas lasers using helium in conjunctionwith a metal which vaporizes at a relativelylow temperature.

Other examples are the Helium-Mercury (He -Hg) and Helium-Selenium (He - Se) lasers.

The typical He - Cd laser can produce a high

quality beam at 442 nm (violet-blue) and/or325 nm (UV) depending on the optics.


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Typical power output is in the 10s to 100s

of milli watts range.

Its wavelengths may be highly desirable

for some forms of spectroscopy, non-

destructive testing, and stereo lithography.

But they are not in use for laser shows and

other common hobbyist applications.


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For that reason, as well as the higher

complexity (and cost), these lasers are notused often.

In terms of popularity, the He - Cd laser

probably ranks behind He - Ne, Ar/Kr ion, andCO2 gas lasers.

The He - Cd laser superficially resembles a

more powerful He - Ne laser emitting blue orUV light, but the differences far exceed itswavelength and power.


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In the He - Ne laser, the bulk of the gas mixture

is He, which absorbs energy from an electricdischarge and transfers it to the light-emittingspecies.

However, in the He - Cd, the active species isionized Cd metal.Cd melts at a moderate temperature, so heating

it in a tube can produce the necessary vapor pressure.

Excitation also ionizes the Cd atoms, removingone of their two outer valence electrons.


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He - Cd lasers have strongest emission at

441.6 nm in the blue; output powers can

approach 100 mW.

The strongest UV emission is at 325 nm,

where power can reach about 20 mW.

There also is a weaker UV line at 353.6

nm, which is rarely used.Cd has several red and green lines, but

these are not used in commercial lasers.


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The blue and UV lines are close enough that a

single set of resonator mirrors can function forboth lines.

This allows some He - Cd lasers to emit on both

lines, but most commercial lasers are designedfor operation at one wavelength or the other.Both internal and external mirror He-Cd lasers

are available.External mirrors provide a polarized beam

(Brewster windows) and allow for swapping ofoptics to select 325 nm and/or 442 nm operation.


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The use of an ionized metal vapor creates

a number of complications in He - Cdlasers.

The metal must be heated to about 250C

in order to produce the Cd vapor pressureof several millitorr needed for laser operation.

This requires a separate heater, whichneeds 10 - 30 min to heat the Cd andgenerate the required vapor pressure.


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He pressure typically is several torr, about a

thousand times higher than cadmiumpressure, but only about 1% of atmosphericpressure.

The positively charged Cd ions migrate fromthe anode toward the cathode.

They remain at high temperature in the

discharge bore,But once they emerge from the bore they can

plate out on any cool surface. This effect

poses several problems.


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First, it gradually depletes the reservoir of

Cd metal.Second, the condensed Cd can wind up in

the wrong places, such as on optical

surfaces, where it contributes to increased

resonator loss.

In addition, the Cd film can trap He atomsunderneath it, depleting the tube's He

reserves.


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These considerations make He - Cd tubes

more complex than those for He - Ne lasers.He - Cd tubes must include chunks of Cd

metal and a heater, placed near the anode.

Like He - Ne tubes, they also include a gas

reservoir.

He - Cd tubes have a cold trap near thecathode end of the bore to catch the metal

vapor before it can condense on vital optics.


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These effects generally limit the lifetimes of

He - Cd laser tubes to several thousand hoursrather than the tens of thousands of hours

typical for He - Ne lasers.

Steady blue and UV beams make He - Cd

lasers valuable for a number of laboratory and

instrumentation applications, includinginducing fluorescence, photoluminescence,

and scattering measurements.


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Many of these applications are in

biotechnology.He - Cd lasers also are used in other

instrumentation, in making masters of

optical data disks in both CD and DVD

formats, and in stereo - lithography.

For animation of working of He Cd laserhttp://www.olympusfluoview.com/java/hecd

lasers/index.html.


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The energies of the 21S and 23S states of

He are greater than the ionization energyplus excitation energy of Cd+ ions.

He atoms in 23S, colliding with Cd atoms,

therefore can excite the D or P states of

Cd+.

He*

+ Cd

He + (Cd+

)*

+ eThe excess of He is transformed to the

K.E. of the ejected electron.


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This process is not resonant process and it is

known as Penning Ionization process. It is more effective, since 21S and 23S states

of He are metastable states.

Although excitation of both D and P states of

Cd+ is possible, the excitation cross section

of D states are three times greater than Pstates.

Life time of D states is in the order of 10-7

second.


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The life time of P states is in the order

of 10-9 seconds.

Therefore the P.I. and Laser action is

achieved for the following transition.

2D5/2 2P3/2 (Wavelength 4416 A

o)

2

D3/2 2

P1/2 (Wavelength 3250 Ao

)The out put is a continuous one.


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He+

25

21S

20 23S

Penning

Ionization 2D3/22D5/2

15

Laser Transition2P

3/22P1/2

10

2S1/2Cd+

5

11S

0He Cd


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When He pressure is ~ 1 Torr and Cd

pressure is ~5 x 10-3 Torr, maximum

output of 300 mW on 4416 Ao and 50

mW of 3250 Ao can be obtained.The transition at 3250 Ao is shortest

continuous wave.It is widely used in photochemistry.


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When atomic hydrogen and chlorine

combine, a substantial amount of energy isreleased and Hcl is formed in the excited

state.

H + Cl2 HCl* + Cl

In low pressure flames the vibrational

anomaly are prominent.The reaction of atomic hydrogen with

chlorine at low pressure ~0.02 Torr.


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This leads to high population in the upper

vibrational levels, especially in = 3.P.I. is achieved and laser emission at = 3.77

m takes place.

In the Halogen hydrogen reaction large part

of the energy released by the chemical

reaction.Which is left as vibrational energy in the

product molecules (halogen-hydride).


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At the low pressure condition laser action

can be achieved.But this laser is not purely a chemical

reaction.

To create necessary free H atom from theH2 Cl2 mixture is exposed to either to aflash lamp or by electron impingement.

The process using flash lamp is called asphoto-dissociation.


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But electron impingement is often used

since it can be started in very short time of10-8 to 10-7 s.

The electrical input in this case more than

the laser output.Flash initiated reaction at the pressures

between 3 and 10 torr with high vibrational

excitation can produce P.I. in certainrotational levels of the vibrational states = 1 and = 2.

08 he cd and hcl lasers

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